In this talk, we will discuss the physics of organic solar cells in terms of simple equivalent circuit models and emphasize the effects of parasitic resistance effects on the current-voltage characteristics of these devices. We will show how novel device fabrication strategies based on transfer lamination can be used to minimize common limitations caused by low parasitic shunt resistance. We will discuss the important role played by interfaces and present strategies to control and stabilize the value of the work function of electrodes. A new method to produce air-stable low work function electrodes as a substitute for Ca will be presented. We will show that these advances in interface modification can be used to design organic solar cells and modules with novel architectures that can overcome some of the economic hurdles of current approaches and accelerate the deployment of these technologies. Finally, we will discuss strategies to reduce the environmental footprint of photovoltaic technologies even further.

While the efficiency of bulk heterojunction polymer solar cells increases into the 8-9% range, it is still clear that there is much room for improvement in efficiency, lifetime, and cost-effective synthetic and fabrication approaches. We are investigating ternary blend solar cells based on two donor components and one acceptor component (or one donor and two acceptors), which have been recognized as a potential route to increase the absorption breadth of a solar cell and consequently the short-circuit current density. Recently, using a three-component system, we demonstrated for the first time that the open-circuit voltage of ternary blend solar cells is composition dependent and can be tuned across the full range defined by the corresponding limiting binary blends without negatively impacting the fill factor or the short circuit current. As a result, with judicious choice of components, the attainable product of short circuit current and open circuit voltage (and by extension the efficiency) in a single-layer ternary blend solar cell could be higher than is achievable with a standard binary blend solar cell. We have successfully demonstrated higher efficiencies in ternary blends based on two donor polymers and one fullerene acceptor than could be achieved in either of the limiting binary blends. Efforts toward developing a deeper understanding of the mechanism of operation in these ternary blends will also be discussed.

We have measured the photodegradation of several encapsulated, high-efficiency organic solar cells and found that there is an exponential drop, or “burn-in”, of the open-circuit voltage during the first 200 hours of aging that appears to depend on morphology. The burn-in affects amorphous donor-acceptor blends about three times as much as aggregated blends. Although a similar amount of photo-induced traps are formed in amorphous and more aggregated photoactive layers, there is a larger voltage loss for more amorphous materials. The trap density and depth are thoroughly characterized using transient photocurrent measurements. One mechanism that is consistent with these observations relies on the idea that a polymer-fullerene bulk-heterojunction has a three phase-morphology comprised of an aggregated polymer domain, an aggregated fullerene domain, and a mixed, polymer-fullerene, amorphous domain. An exciton generated in the mixed amorphous region quickly dissociates. Then, the electrons funnel into the aggregated fullerene while the holes funnel into the aggregated polymer. In the context of the three-phase morphology, photo-induced traps formed in mixed, amorphous regions of the photoactive layer cause more trap-assisted recombination than traps formed in the aggregated domains because both electrons and holes are abundant in the amorphous regions. By choosing an electron donor that does not mix with the electron acceptor, we can obtain a power conversion efficiency of 7.5% and nearly eliminate the burn-in loss.
The burn-in loss results from a combination of light-induced and thermal degradation. Most of the thermal degradation portion can be reversed by peeling off and reapplying the electrode. The photo-degradation is largely irreversible. The thermal degradation occurs if the glass transition temperature of the polymer in the photoactive layer is lower than the operating temperature of the solar cell at one-sun illumination. The short-term thermal degradation near the glass transition temperature is attributed to the formation of an electron-blocking layer at the top, metal electrode because the degradation is greatly reduced in an inverted geometry. To a lesser extent, the thermal degradation near the glass transition temperature can be slowed by using a fullerene that is highly soluble in the bulk heterojunction or by cross-linking the fullerene. Photoluminescence, sub-band-gap external quantum efficiency, and x-ray diffraction are used to confirm that the short-term degradation near the glass transition temperature is, to first-order, an interface issue. XPS measurements are used to identify the nature of the electron-blocking layer. These results imply that conjugated polymers with a glass transition less than 100C will be unstable in an outdoor, photovoltaic module.

Organic photovoltaic devices have progressed a lot in the past decade of research. Many polymer electron donors which yield high photovoltaic performance when combined with PCBM have been identified, such as poly-3(hexylthiophene) (P3HT) (4.4% ), poly[N-9”-hepta-decanyl-2,7-carbazole-alt-5,5-(4&’,7&’-di-2-thienyl-2&’,1&’,3&’-benzothiadiazole) (PCDTBT) (6.9% ), thieno[3,4-b]-thiophene/benzodithiophene (PTB7) (7.9% ) and 4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4,5-bprime;]dithiophene- thieno[3,4-c]pyrrole-4,6-dione (PBDT-TPD) (7.3% ).
However, although P3HT is a more classic polymer with lower efficiency than the other, newer polymers mentioned (mainly due to a mismatch in spectral overlap with the AM1.5G solar spectrum and open circuit voltage losses), it still exhibits the highest free carrier lifetime as observed in time-resolved microwave conductivity (TRMC), when compared to the other polymers which exhibit higher power conversion efficiency. The exact reason is currently unknown and subject to investigation, although it is highly likely that the degree of crystallinity in regioregular P3HT plays a large role. The grand challenge is to find a polymer which has better spectral overlap and less open circuit voltage loss, like PCDTBT, PTB7 and PBDT-TPD have, and also have a high free carrier lifetime as observed in P3HT. This way, power conversion efficiencies could be further improved.
Here, we demonstrate that by varying the solubilizing side chains of copolymers based on BDT and the cyclopenta[c]thiophene-4,6-dione (CTD) monomer units, the free carrier lifetime can be significantly improved. Implications for device performance will be discussed.
1. G. Li et. al., Nature Materials 2005, 4, 864 - 868
2. J.S. Moon et. al., Advanced Energy Materials 2012, 2, 304 - 308
3. H. Zhou et. al., Advanced Materials 2013, 25, 1646 - 1652
4. J. A. Bartelt et. al., Advanced Energy Materials 2013, 3, 364 - 374

The bulk-heterojunction nanostructure of non-crystalline polymer:fullerene blends has the tendency to rapidly coarsen when heated above its glass transition temperature, which represents an important degradation mechanism. We demonstrate that fullerene nucleating agents can be used to thermally arrest the nanostructure of photovoltaic blends that comprise a non-crystalline thiophene-quinoxaline copolymer and the widely used fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). To this end, C60 fullerene is employed to efficiently nucleate PCBM crystallization. Sub-micrometer-sized fullerene crystals are formed when as little as 2 wt% C60 with respect to PCBM is added to the blend. These reach an average size of only 200 nanometers upon introduction of more than 8 wt% C60. Solar cells based on C60-nucleated blends indicate significantly improved thermal stability of the bulk-heterojunction nanostructure even after annealing at an elevated temperature of 130 °C, which lies above the glass transition temperature of the blend. Moreover, we find that a variety of other compounds including C70 fullerene, single-walled carbon nanotubes and sodium benzoate as well as a number of commercial nucleating agents -commonly used to clarify isotactic polypropylene- permit to control crystallization of the fullerene phase.

Bulk-heterojunction (BHJ) polymer solar cells (PSCs) based on mixing polymer donor and fullerene acceptor networks are a proven and promising approach to efficient, cost-effective solar energy conversion. Although PSC performance has improved steadily their metrics are still far away from those of inorganic cells. Multi-junction solar cell configurations, where two or more sub-cells are stacked and connected in series or parallel, offer an exciting approach to overcome the single junction limitations of organic solar cells and further improve their power conversion efficiency (PCE). To improve the efficiency of PSC researchers have focused on maximize current or increase voltage through optimized polymer design[1] or light harvesting.[2] However, raising the fill factor offers an alternative strategy to enhancing PCE.[3] As for the tandem structures the average values of FF reported are around 60%. And in few occasions the FF of the tandem solar cell is higher than any of the FFs of the individual sub-cells.[4]
In this work we report tandem solar cells made of the well-known, low bandgap, high efficiency polymer PTB7 where the FF obtained in the tandem solar cell is not only larger than the FF of the identical sub-cells but it is also, to the best of our knowledge, the highest FF reported for organic tandem solar cells. PTB7:PC71BM was used in both top and bottom photoactive layers that were connected in series by a novel recombination layer comprising of molybdenum oxide, poly [(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene) (PFN) and a thin layer of silver in between. The use of this configuration allowed us to obtain a tandem solar cell with FF up to 76% while slightly increasing the current and doubling the voltage yielding a PCE of 8.2% for the tandem cell. The use of this recombination layer with PFN has other important advantages. For instance, there is no need of any thermal annealing that in some cases may be detrimental to the bottom photoactive layer. Moreover, PFN showed to be robust with high transparency and environmental stability and also remarkable solution processability.
Also reported in this work are morphology studies performed by means of atomic force microscopy (AFM) in order to understand the role of the silver layer and PFN thicknesses on the electrical performance of this new interconnection layer. These morphology studies together with optical simulations showed to be interesting tools in order to reach a better understanding of the role the recombination layer has on organic tandem solar cells.
[1] Y. Liang et al. Adv. Mater. 2010, 22, E135
[2] A. Martínez-Otero et al. Adv. Optical Mater. 2013, 1, 37
[3] X. Guo et al. Nat. Photon. 2013, 7, 825
[4] T. Ameri et al. Energy Environ. Sci. 2013, 6, 2390

Organic solar cells are the subject of an extensive research effort due to their potential application as cheap, light-weight and flexible energy sources. An important step towards the realization of commercialization is attainment of higher efficiencies and improvements of the scale-up process. Both objectives are possible only through a more complete understanding of factors limiting the device performance; one of the limiting factors is the presence of chemical impurities. In this talk, I will discuss effects of chemical impurity and doping on bulk heterojunction solar cells. I will demonstrate how photoconductive atomic force microscopy (PC-AFM) in conjunction with transient absorption spectroscopy and charge transport study can be used to investigate the effect of impurities on the solar cell performance of low band gap conjugated polymer and fullerene devices to gain insight into whether differing impurity shapes may lead to different loss mechanisms. This study provides an example of the importance of material purity in organic solar cells, that identifying the loss pathway is not always trivial, and that PC-AFM is a powerful technique to understand device performance. Alternatively, some “chemical impurities” with the right LUMO level can be used to dope the donor material to improve hole mobility and hence the device performance. A greater understanding regarding the effects of impurities may lead to higher solar cell efficiencies.

One of the major obstacles for a successful introduction of organic photo-voltaic (OPV) technology in the market is represented by the faster degradation of organic materials under operational conditions, compared to traditional inorganic semiconductors. A long lifetime is essential for solar cells to be introduced on a large-scale production, and a proper and unambiguous understanding of the main degrading phenomena is required to both prevent and mitigate them. Uncertainty still exists on what is the main detrimental agent for OPV devices, among water vapor and oxygen.
In this work the PV parameters of a flat hetero-junction solar cell (FHJ) with TAPC (3nm) as small molecular donor (D) and C60 (40 nm) as archetypical acceptor (A) have been analyzed, after exposure of the photoactive layer (PAL) to various gaseous ambiences. Different setups have been tested. The donor side has been exposed to O2 after being broken down in two sub-portions: a region 2nm thick (1 nm far from the D/A interface, setup A) and a region 3 nm thick (setup B). The PV parameters for both setups A and B barely showed any difference with the PV parameters of a fully pristine control device. TAPC was found to be not affected by O2 degradation over the time. The acceptor side too has been broken down in two sub-portions: a region 2nm far from the D/A interface (setup A&’) and a region 38 nm far from it (setup B&’). Each of the two sub-portions has been exposed over time to N2, O2, H2O and air, then completed with the remaining pristine materials. We found that even small O2 amounts lead to more detrimental results compared to H2O exposure, for the same timeframe. Devices stressed in setup B&’ showed no Voc variation and the photo-conversion efficiency (PCE) loss was mainly attributed to a Jsc loss. For devices in setup A&’ the PCE loss could be mostly attributed to Voc and Jsc loss. We argue that the PCE decrease for setup A&’ devices is due to creation of recombination centers at the interface, which induce exciton diffusion and charge generation losses, while for setup B&’ devices traps at outer C60 layer affect the free carrier transport efficiency. For setup B&’ devices we also relate the PV parameters degradation with a consistent C60 conductivity loss. We lastly find a direct connection between O2 pressure and degradation. In-situ XPS measurements on the above mentioned setups are in good agreement with the findings obtained via electrical measurements.

Organic photovoltiacs (OPVs) have demonstrated solar power conversion efficiencies in the regime of 10-12% from several classes of materials, including conjugated polymers and small-molecules. Of note, in each of the classes, the electron-accepting molecule is based on C60. While C60 is a very effective electron-acceptor and transporter, it has several non-optimal properties, including low optical absorption strength in the visible spectrum as well as the difficulty required to tune its optoelectronic properties through changes in chemical composition or substituents. Here we present results on a family of small molecule acceptors based on a core unit of benzothiodiazole (BT). The optoelectronic properties, specifically optical band gap and Lowest Occupied Molecular Orbital (LUMO), of these small molecules are readily tunable through linear conjugation with different donor and acceptors moieties. A library of these small molecule acceptors has been synthesized with broad absorbance bands of roughly 200 nm, which vary in peak absorbance from 400 nm to 640 nm. Additionally, the higher energy LUMO level of these materials, 0.3 eV higher than the C60 derivatives, results in OPVs with open-circuit voltages (Voc) close to 1 V. For device fabrication, both bulk heterojunction (BHJ) and bilayer architectures are explored through use of a variety of co-solvent mixtures and orthogonal solvents. The studies presented investigate overall device performance and compare the efficiencies of the two mechanisms for charge generation: channel 1 (light absorbed in donor polymer with sequential electron transfer to acceptor molecule) and channel 2 (light absorbed in the acceptor with sequential hole transfer to donor molecule).

Previous studies have shown that the morphology at the Donor-Acceptor interfaces plays an important role in the formation of charge transfer (CT) states in organic bulk heterojunction solar cells which mecdiate the exciton-dissociation process. Recent models have shown that excitons in higher manifolds of CT states (or “Hot CT“ states) dissociate into polarons in femtoseconds while the excitons in lower CT states (or “Cold CT“ states) result in CT recombination which is detrimental to the solar cell perfomance. Therefore, understanding the energy levels of the CT states and especially the effect of the morphology on the Hot CT states is important in developing new polymers for high efficiency organic solar cells.
In this report, we study the CT energy evolution of the blend of poly[N-9prime;-hepta-decanyl-2,7-carbazole-alt-5,5-(4prime;,7prime;-di-2-thienyl-2prime;,1prime;,3prime;- benzothiadiazole)] (PCDTBT) : [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) due to the morphological changes via thermal annealing. It has been reported that annealing of PCDTBT:PC70BM blend films leads to changes in the π-π stacking. Using charge modulated electroabsorption spectroscopy (CMEAS), we correlated the Hot CT energy evolution to the morphological changes as a result of thermal annealing. CMEAS showed an lowering of the Hot CT threshold from 1.7 eV to 1.4 eV after thermal annleaing. This decrease in the Hot CT energy results in a decrease in photoluminescence due to the CT emission intensity after thermal annealing corresponding to a lower CT recombination rate. Finally, an increase in the CT peak intensities in CMEAS measurements suggest an higher density of Hot CT states, which facilitate the exciton dissociation process.

While great strides have been made in the study of semiconducting polymers, with record-setting mobilities being reached in 2013, a fundamental understanding of morphological control is still not complete. One product of every polymer synthesis that is often completely neglected or even discarded is the short-chain oligomers. These materials are often considered defects which lower the overall performance of the polymer under study. However, we have found that it is possible to tune the morphology via blending well-defined oligomers with the parent polymer, resulting in an increase in charge transport. Specifically, by blending oligomers of poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) with the polymer, we can create exceptionally smooth films with high mobility. Devices containing a majority of oligomer (80%) and a relatively small amount of polymer (20%) exhibit mobilities several orders of magnitude higher than the neat oligomer, comparable to or even surpassing those of the neat polymer. The electronic, morphological, and thermal properties of these blends were characterized and correlated to determine the role that oligomers play in semiconducting polymer thin films.

Understanding the impact of both the organic semiconductor design and processing conditions, on both molecular conformation and thin film microstructure has been demonstrated to be essential in achieving the required optical and electrical properties to enable high efficiency organic solar cell bulk heterojunction devices and organic transistors fabricated from solution. Organic solar cell efficiencies are currently increasing rapidly based on organic bulk heterojunction devices fabricated from solution. Central to these device efficiency improvements are the development of new photoactive semiconducting donor and acceptor materials, designed at the molecular level to optimise both absorption of the long wavelength region of the solar spectrum and generation of high cell voltages. This presentation will examine some of the key design strategies to control the molecular orbital energy levels and microstructure of donor polymer semiconductors and illustrate with examples and their characterisation. Specifically, the systematic modification of the bandgap in a series of bridged ladder type indacenodithiophene copolymers through tuning molecular orbital energies with atom substitutions in the bridge position, will be illustrated. The impact on lowering the HOMO energy level on cell Voc, and the dependence of the Jsc on bandgap and morphology will be demonstrated. Analogues of these polymers, along with DPP copolymers also exhibit high charge carrier mobilities, and transistor data will be presented and the influence of morphology on the electrical properties will be discussed.

In this talk I will present our recent work to develop conjugated polymers with enhanced backbone rigidity for field effect transistor and photovoltaic applications. I will discuss different strategies to enhance backbone rigidity and reduce torsional disorder whilst maintaining good solubility and processability. I will detail the effects of systematic chemical changes to the conjugated backbone on the thermal, optical and electronic properties of a range of semi-crystalline co-polymers for transistor and photovoltaic applications. Polymers demonstrating high p-type and ambipolar transistor performance will be presented.

We previously reported an effective strategy to facilitate π-π interaction and enhance charge transport in conjugated polymers via moving the branching site in side chains away from conjugated backbones.1 This side-chain engineering approach sparked immediate interest from the research community and record high hole mobilities (up to 12 cm2v-1 s-1) in conjugated polymers have been recently achieved via side-chain engineering.2 In this presentation, I will talk about our latest systematic investigation on side-chain engineering-moving the branching site one carbon at a time in the siloxane hybrid side chains. From this set of seven isoindigo-based conjugated polymers, we are able to provide nice correlations between side chain lengths and charge transport properties.
1) a)Mei, J.; Kim, D. H.; Ayzner, A. L.; Toney, M. F.; Bao, Z. J. Am. Chem. Soc. 2011, 133,20130- 20133; b) Li, H.; Mei, J.; Ayzner, A. L.; Toney, M. F.; Tok, J. B. H.; Bao, Z. Org. Electron. 2012, 13, 2450- 2460.
2. a) Lei, T.; Dou, J.-H.; Pei, J. Adv. Mater. 2012, 24, 6457- 6461; b) Lee, J.; Han, A. R.; Kim, J.; Kim, Y.; Oh, J. H.; Yang, C. J. Am. Chem. Soc. 2012, 134,20713- 20721; c) Lee, J.; Han, A. R.; Yu, H.; Shin, T. J.; Yang, C.; Oh, J. H. J. Am. Chem. Soc. 2013, 135, 9540. d) Kang, II, Yun, H., Chung, D., Kwon, S., Kim, Y., J. Am. Chem. Soc., 2013, 135, 14896-14899.

The completion of the color palette has yielded a family of electrochromic polymers each able to absorb in unique regions across the visible spectrum.[1] Synthetically, our group has recently shown that by varying the content and type of dioxythiophene moieties coupled with the acceptor BTD, the shape of a dual band absorption can be modulated. Thus, in synthetically widening or narrowing the trench of a dual band absorption, we were able to attain numerous shades of green.[2] Using this synthetic approach to tune specific absorptions in a discrete region of the visible spectrum, our group has sought to develop a family of electrochromic polymers that possess sharpened or broadened absorption spectra relative to electrochromic materials we have previously produced. By varying the steric hindrance of dioxythiophenes along the conjugated backbone, we have been able to achieve new hues of magenta and blue. Through progressively adding more steric hindrance and twisting the polymer backbone, the absorbance of a polymer can be pushed towards higher energy, allowing more red light and less blue light to pass through a film. This unequal passing of long and short wavelengths reduces the overall purple color that is normally exhibited by our ECP-Magenta polymer, thereby giving brighter, truer magenta colored materials. By reducing steric hindrance and relaxing the polymer backbone, the opposite can be achieved: pushing the absorbance of a polymer to lower energy allows more blue light and less red light to pass. These polymers also exhibit highly transmissive oxidized states that are attainable at low potentials.
[1] Dyer, A. L.; Thompson, E. J.; Reynolds, J. R. ACS Appl. Mater. Interfaces, 2011, 3 (6), 1787-1795
[2] Beaujuge, P. M.; Vasilyeva, S. V.; Liu, D. Y.; Ellinger, S.; McCarley, T. D.; Reynolds, J. R. Chem. Mater. 2012, 24, 255minus;268

Polymer-based devices, such as transistors and photovoltaics, are promising candidates in organic electronics. However, their efficiencies are dwarfed by their inorganic counterparts, and the design and synthesis of the polymers are often arbitrary and laborious. It remains a key challenge to rationalize the factors that affect device performance, and use these factors to strategically understand state-of-the-art materials and design novel materials. In the past decade, connecting an individual molecule into a circuit has been realized. One means of achieving this is the scanning tunneling microscope break junction (STM-BJ) technique. Our group is interested in how this tool can be used to measure the conductance of molecular analogues of semiconducting polymers, with the aim of relating single molecule electronic properties to those in the bulk. Synthesizing a family of small molecules is rapid and straightforward compared to polymer synthesis. Furthermore, this allows us to isolate and analyze key functional groups. By exploiting the STM-BJ technique, we have measured the conductance of several moieties that are common in polymer semiconductors, and, remarkably, these values correlate well with their polymeric device performance, indicating that molecular conductance could be a predictive tool for device performance. In addition we have used the technique to characterize novel n-type molecules. The latest results of our studies will be discussed in this presentation.

The performance of donor (D) /acceptor (A) structure based organic electronic devices, such as solar cell, light emitting devices etc., relays on the charge transfer process at the interface dramatically. In organic solar cell, the photo-induced electron-hole pair is tightly bonded and will form a charge transfer (CT) state at the D/A interface after dissociation. There is a large chance for them to recombine through CT state and thus is a major loss that limit the overall performance. Here, we report three different strategies that allow us to completely suppress the exciplex (or charge transfer state) recombination between any D/A system. We observe that the photocurrent increases by 300% and the power conversion efficiency increases by 4-5 times simply by inserting a spacer layer in the form of an a) insulator b) Oliogomer or using a c) heavy atom at the donor-acceptor interface in a P3HT/C60 bilayer device. By using those different functional mono layers, we successfully suppressed the exciplex recombination in evidence of increased photocurrent and open circuit voltage. Moreover, these strategies are applicable universally to any donor-acceptor interface. And we demonstrated such strategies in a bulk-heterojunction device which improved the power conversion efficiency from 3.5% up to 4.6%.

The role of aggregation on the opto-electronic properties of the semiconducting polymers has been extensively studied in past few years. The change in relative intensity of 0-0 to 0-1 peaks and the considerable shift in the absorption spectra is used as the tool to distinguish the H-like and J-like aggregation (i.e. to address the interchain and intrachain coupling). In order to elucidate whether only absorption can account for this or not, we studied the solid state absorption, photoluminescence (PL) and the transient photomodulation spectroscopy of three different regioregular poly(3-hexylthiophene) (rrP3HT) systems: We compare the photophysical behavior of neat P3HT films with blends of this semiconductor with the insulating and polar additive poly (ethylene oxide) (PEO,1:1 weight ratio) with and without DI water treatment. As we reported, these processing conditions can lead to a variable and some cases extreme red-shifted absorption spectra and vibrational peak ratios (0-0/0-1) indicating strong J-like aggregation [1]. Surprisingly, the relative intensity of 0-0 to 0-1 peaks in the PL spectrum are essentially the same for all samples with overall H-characteristics and the time resolved PL shows the same long time decay dynamics suggesting all three samples show H-like aggregation behavior in photoluminescence. Furthermore, very small relaxation and Stokes shift is observed for the most J-like aggregated P3HT as observed in absorption and emission spectra. We address these contradictory observations in PL and absorption by time resolved absorption measurements and observe that the interchain electronic coupling is a dynamic process evolving from J-like to H-like in all samples within a few hundred picoseconds.
This work is supported by U.S. Office of Naval Research, EPSRC and a KAUST CRG fund.
1- C. Hellmann, F. Paquin, N. D. Treat, A. Bruno, L. X. Reynolds, S. A. Haque, P. N. Stavrinou, C. Silva, N. Stingelin, Controlling the Interaction of Light with Polymer Semiconductors, Adv. Mater., 2013, 25, 4906-4911.

Organic opto-electronics is a new technology envisioning the fabrication of opto-electronic which can be fabricated at lower cost compared to current technologies or impossible with silicon. In this presentation I will describe the design rationale, synthesis, characterization, and electrical and optical properties of several new semiconducting materials which can be deposited/patterned using complementary methodologies such as thermal evaporation (vapour phase), printing (inkjet, gravure, slot-dye, screen), and spin-coating/photolithography. The combination of these techniques enable the fabrication of devices such as transistors, solar cells, light emitting stacks, sensors, and CMOS circuits.

Over recent years there has been tremendous progress in discovering new organic semiconductors that provide high charge carrier mobilities for both n-type and p-type device operation, good operational stability and other functionalities such as efficient electroluminescene, sensing or memory functions. These materials allow addressing an increasingly broad range of flexible and printed electronic applications based on controlled manufacturing of flexible plastic substrates by a combination of solution processing and direct printing. One of the sources of improvement in performance has been the versatility of organic chemistry to provide a broad range of new molecular structures and the ability to assemble these molecules into ordered structure with minimum degree of disorder. We will review recent insights into the device and charge transport physics of solution-processible small molecule as well as conjugated polymer organic semiconductors, with a particular focus on the microscopic processes that limit the field-effect mobility in these systems. We are also interested in understanding the spin transport physics of these materials and the relationship between molecular structure, microstructure and spin diffusion. Organic semiconductors may enable realisation of long spin relaxation times and long spin diffusion lengths due to the weak spin-orbit coupling in these carbon-based materials. We will present recent measurements of spin-transport in different molecular and polymeric semiconductors. We will also discuss our view of the outlook for the field of organic semiconductors including remaining scientific challenges and future scientific opportunities.

Organic light-emitting diodes (OLEDs) are now in widespread use in display applications, but OLEDs have not achieved success as lighting sources. Currently, high efficiency OLEDs cost 50 times more than inorganic LEDs and 100-1000 times more than compact fluorescent sources. To address this challenge, we are developing low-cost alternatives to conventional OLED devices, particularly, light-emitting electrochemical cells (LEECs) from ionic transition metal complexes (iTMCs). These devices operate efficiently as single layer devices from solution processable materials. We have addressed the challenge of achieving the stringent luminance and response time benchmarks required for lighting applications with iTMC devices. We demonstrated substantially improved performance of iridium ITMCs by blending alkaline additives into the active layer to control ionic space charge effects. For lithium additives, the maximum luminance was increased to practical lighting levels, the turn-on time was drastically reduced from hours to seconds, and the lifetime was preserved. This low-cost approach is generalizable to any iTMC, complementing efforts to develop novel metal complex emitters. These observations show that iTMCs from LEECs have the potential to serve as bright, long-lasting light sources.

A family of dyes having [5]helicene derivative as a core structure was engineered to present high fluorescence quantum yield,Phi;_f. By attaching electron donating groups at two end of the [5]helicene derivative, i.e. at position 3 and 12, and strong electron withdrawing group(s) at the center of the derivative, i.e. at position 7 and 8, the electronic push-pull systems that enable high fluorescence quantum yield were established. Beside, conjugation length of the molecule can be tuned by wisely selecting a match of electron donating and withdrawing groups in order to maintain high Phi;_f. Blue, green and red emitters from [5]helicene derivative were successfully synthesized. Their physical and optical properties were determined and compared for structure-properties relationship purpose. The blue emitter, 3,12-dimethoxy-5,6,9,10-tetrahydro-7,8-dicyano-[5]helicene, has a bandgap of 2.91 eV and Phi;_f (in chloroform) of 0.96. The green emitter, 3,12-dimethoxy-5,6,9,10-tetrahydro-[i]-furan-1,3-diono-[5]helicene, has a bandgap of 2.66 eV and Phi;_f (in chloroform) of 0.49. The conjugation length of the system was further extended in case of 3,12-bis(phenylmethylamine)-5,6,9-10-tetrahydro-[i]-(1-phenyl-pyrrolidine-2,5-diono)-[5]helicene, the red emitter, that has a bandgap of 2.26 eV and Phi;_f (in ethyl acetate) of 0.61. Organic light-emitting diodes (OLEDs) fabricating from these emitters showed promising results as the current efficiency were 2.38, 1.86, and 4.51 cd/A for the blue, green, and red emitter, respectively.

Organic lights Emitting Diodes (OLEDs) already have efficiencies comparable to current commercal lamps. However, the chemical degradation of the materials during operation is still a major obstacle for the development of economically feasible devices. The development of highly stable phosphorescent emitters and complemantary materials is therefore of high interest. Thus, investigating the mechanisms of the degradation of these materials is highly important, as the insights from these investigations guide the development of new materials with improved stability.
In this study we focus on fac-tris(N-diphenyl-benzimidazole-carbene)iridium(III): (Ir(dpbic)3), a blue phosphorescent emitter and stable hole transport material. We introduce a differentiating lifetime measurement setup for probing of material stability toward different stress mechanisms. Temperatur dependent IV characteristics are measured for each mechanism at three aging stages. We use a one-dimensional numerical model based on the Extended Gaussian Disorder Model (ECDM) with ohmic boundry conditions at steady state for a quantitative parameter extraction from the measured IV curves.
We show that the influence of the aging on the IV curves is well described by our model with additional charge carrier traps. The trap density and depth are determined as functions of aging time and can be used for degradation prediction. The observations from this study bring a deep know-how of the degradation processes occurring during the operation of devices and thus help improving the OLEDs lifetime.

Recently, organic light-emitting field-effect transistors (OLEFETs) have attracted enormous interest since they possess the electrical switching functionality and the light-emitting capability in a single device [1]. The OLEFET, in principle, has a high aperture ratio and high light emission efficiency compared to an organic light-emitting diode which requires an additional switching element [2]. However, a typical OLEFET structure, in which a source electrode and a drain electrode are laterally constructed, suffers from a relatively small light emission area due to the intrinsic line emission characteristics [3].
In this work, we demonstrate a quasi-surface emissive vertical OLEFET employing a mesh-type source electrode. For the fabrication of such OLEFET, the mesh-type source electrode (Au) together with the source insulator was patterned on the gate insulator. A p-type organic semiconductor (pentacene), an emission layer which consists of hole/electron transport layers [α-NPD (N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine) and Alq3 (tri-(8-hydroxyquinoline)aluminum)], and the drain electrode (Al) were sequentially deposited onto the source electrode. It was found that the light emission through the mesh-type source electrode occurs indeed in a quasi-surface manner. The brightness can be varied with the gate voltage and the relative light emission area depends on the size of apertures in the mesh-type source electrode.
The approach described here is expected to play a critical role in developing advanced optoelectronic devices for next generation displays.
References
[1] M. A. McCarthy, B. Liu, E. P. Donoghue, I. Kravchenko, D. Y. Kim, F. So, and A. G. Rinzler, Science 332 (2011) 570.
[2] R. Capelli, S. Toffanin, G. Generali, H. Usta, A. Facchetti, and M. Muccini, Nat. Mater. 9 (2010) 496.
[3] M. C. Gwinner, D. Kabra, M. Roberts, T. J. K. Brenner, B. H. Wallikewitz, C. R. McNeill, R. H. Friend, and H. Sirringhaus, Adv. Mater. 24 (2012) 2728.
Acknowledgments
This work was supported by the National Research Foundation of Korea under the Ministry of Education, Science and Technology of Korea through the grant 2011-0028422.

We present multi-photon emitting OLEDs where white light emission was achieved by stacking blue and yellow OLEDs utilizing an intermediate charge carrier generation layer (CGL) for monolithic device interconnection. With respect to future printing processes for organic optoelectronic devices, all functional layers were deposited from solution. The CGL comprises a low-work function zinc oxide layer that was applied from solution under ambient conditions and at moderate processing temperatures, and a high-work function interlayer that was realized from various solution processable metal oxides or PEDOT:PSS derivatives. Since every charge carrier generates two photons, the luminance and the current efficiency of the stacked OLED at a given device current density exceed the luminance and current efficiency of the single OLED devices, hence reducing the electrical device stress.

Stick-and-display system can provide electroluminescence to unconventional surfaces including furniture, a gift card, curvilinear surfaces, and even a human tissue that direct fabrication is extremely difficult. In order to enable such system, it is highly desirable to generate extremely thin structure of a display device and place it onto a target surface. When using ultrathin plastic substrate with a thickness of tens of micrometers or less, it is necessary to use a rigid handling substrate to guarantee stability in device fabrication with no significant deformation due to the low Young&’s moduli of the plastic substrate. Naturally, this approach comes along with the contradictory requirement in adhesion property between the two substrates that should be strong for stable fabrication but weak for releasing process. Introduction of a sacrificial layer in the interface and chemical etching process should be one of solutions. However, because the chemical etching process is based on mass transport, inhomogeneous adhesion can easily occur if the substrate becomes large. This study addresses this issue by introducing additional 2-D arrays of posts, which survive during etching process but undergo fracture during releasing process, between the substrates. The post arrays also prevent undesired wrinkle formation and floating behavior that easily occur for the case of using simple sacrificial layer. Evaluation of the interfacial adhesion force and mechanical simulation confirms the utility of the post arrays. Under the optimized conditions, we successfully developed flexible and resizable sticker-type OLED onto unconventional surfaces such as a glass vial, a paper, and an edge of a table.
[1] S. O. Yun, Y. Hwang, J. Park, Y. Jeong, S. H. Kim, B. I. Noh, H. S. Jung, H. S. Jang, Y. Hyun, S. -H. Choa, H. C. Ko, Adv. Mater. 2013, 25, 5626.

Near-infrared (NIR) organic light-emitting devices (OLEDs) would be promising NIR light sources offering advantages of light weight, low power consumption and compatibility with large area and flexible substrates. However, sophisticated multilayer structures and low-work-function cathodes are generally required for NIR OLEDs to optimize device efficiencies, influencing their competitiveness with other solid-state NIR emitting technologies. In contrast with conventional NIR OLEDs, solid-state NIR light-emitting electrochemical cells (LECs) possess several superior advantages such as simple device structure, low operation voltages and high power efficiencies. Therefore, they would be potential candidates of NIR organic light-emitting sources. In this work, we report tandem NIR LECs composed of two stacked single-layered LECs via a thin connecting layer. The peak external quantum efficiency (EQE) obtained in these tandem NIR LECs was up to 2.75%, which showed a 2.64X enhancement as compared to that achieved in single-layered NIR LECs based on the same NIR dye. The device efficiencies achieved are among the highest reported for NIR LECs and thus confirm that a tandem device structure would be useful for realizing highly efficient NIR LECs. Furthermore, effects of emissive-layer thickness on device performance were studied. We found that output electroluminescence spectra of tandem NIR LECs can be modified by adjusting the thicknesses of the emissive layers due to altered microcavity effect. By fitting the output EL spectra with simulated model concerning microcavity effect, temporal evolution of recombination in the emissive layer can be extracted. These results depict that device lifetimes can be improved in thicker devices due to reduced exciton quenching near electrodes.

Techniques of probing for temporal evolution of recombination zone in sandwiched light-emitting electrochemical cells (LECs) would be highly desired since they can offer direct experimental evidence useful for clarifying device physics of LECs. However, direct imaging of recombination zones in thin emissive layers (<1000 nm) of sandwiched LECs could not be obtained easily. In this work, we propose an alternative way to extract temporal evolution of recombination zone in sandwiched LECs by utilizing microcavity effect. Recombination zone positions can be estimated by fitting the measured electroluminescence spectra to simulated output spectra concerning microcavity effect and moving of emissive zone positions. With this tool, effects of modified carrier transport and carrier injection on performance of LECs are studied and significantly altered carrier balance can be measured. For LECs doped with a low-gap hole trapper, the recombination zone was shown to be closer to the anode as compared to the neat-film devices, leading to severer exciton quenching and deteriorated device efficiencies. Similarly, adding a hole-transporting layer with a high ionization potential to impede hole injection also resulted in a recombination zone closer to the anode. Exciton quenching took place in the recombination zone in the proximity of the anode and lower device efficiencies were obtained as well. These results provide direct experimental evidence to confirm that carrier balance of LECs can be modified by adjusting carrier transport or carrier injection. Furthermore, microcavity effect has been shown to be useful in extracting time-dependent evolution of recombination zone of sandwiched LECs.

There are two key factors to build highly performed phosphorescent OLED (PHOLED) devices. One of them is to render energy level of carrier transport material similar with that of host material in EML minimizing energy barrier for moving electrons from ETL to EML. The other is to select carrier transport materials with higher triplet energy blocking triplet excitons diffused from EML to ETL.
We fabricated blue PHOLED with mixed electron transport layer (M-ETL) using TPBi, BCP and 3TPYMB to investigate their role for electron transport and triplet exciton confinement under the device structure of NPB(700Å)/mCP:Firpic-8%(300Å)/M-ETL(TPBi:BCP or TPBi:3TPYMB, 300Å)/Liq(20Å)/Al(1200Å). LUMO energy levels of mCP, FIrpic, TPBi, BCP and 3TPYMB were 2.4,2.9,2.7,3.0 and 3.3eV, and their triplet energy levels were 2.9,2.75,2.7,2.6 and 3.0 eV respectively.
PHOLED with M-ETL1(TPBi:BCP) shows higher luminance as ratio of TPBi is increased in ETL at same voltage because electrons in TPBi are moved into EML easier than BCP according to difference of LUMO energy level between mCP host in EML and TPBi is relatively smaller comparing to that between mCP and BCP. The luminance of device with different mixing ratio are 538.9,299.7,245.4 and 230.9cd/m2 for 10:0,5:5,2:8 and 0:10 at 6.5V, respectively. PHOLED with M-ETL2(TPBi:3TPYMB) performs generally similar current density-voltage-luminance characteristics but there is exceptional behaviors in the region of low voltages, which PHOLED device shows higher luminance although LUMO energy gap between TPBi and 3TPYMB is larger and electrons has higher energy barrier moving from ETL to EML. The luminance of device with different mixing ratio are 876.5,1084 and 1128cd/m2 at 6.5V and 6063, 5168 and 3415cd/m2 at 9V for 8:2,5:5 and 2:8, respectively. This result can be explained that electron transport from 3TPYMB is more difficult than TPBi to generate exciton in EML because high triplet energy of 3TPYMB prohibits diffusion of triplet exciton from ETL to EML as well as hole mobility is higher than electron and then recombination occurs at the interface of EML and ETL at low voltages. However, recombination zone is shifted to the interface of EML and HTL as driving voltage is increased, which can diminish triplet exciton confinement and LUMO energy level becomes major factor to influence on luminance. Two M-ETLs with TPBi:BCP and TPBi:3TPYMB showed different luminance trend according to the mixing ratio at low voltage because of BCP&’s triplet energy lower than 3TPYMB, which result in more triplet exciton diffusion from EML to ETL.
The effect of newly designed M-ETL contributed to improve the electrical and optical performance of PHOLED device because their capability of electron transport and triplet exciton confinement were confirmed. We also have completed additional experiments for various mixed layer such as partially mixing in EML or ETL to show the improvement of PHOLED&’s luminous efficiency eventually.

Phosphorescent organic light-emitting diodes (PHOLEDs) with lower dopant concentration for efficient energy transfer and color stability using host-dopant system in single or multi-emissive layer have been demonstrated in recent research works. In order to perfectly confine the triplet excitons to phosphorescent dopant for higher luminous efficiency, energy transfer processes between host and dopant molecules play an important role and optimization of doping condition is a key factor to determine the phosphorescence by triplet excitons.
In this study, we fabricated white PHOLEDs with single emissive layer using three different phosphorescent dopants generating three primary emission of blue, green and red each under the device structure of NPB(700Å) /mCP:Firpic-x%: Ir(ppy)3-y%: Ir(piq)3-z%(300Å) /TPBi(300Å)/ Liq (20Å)/Al(1200Å). Concentration of three dopants Firpic, Ir(ppy)3 and Ir(piq)3 was adjusted with considering energy transfer process to obtain best luminous efficiency and white color coordinates as observing triplet excitons&’ behavior in EML.
Applying phosphorescent dopant concentration of Firpic-8%: Ir(ppy)3-0.5%: Ir(piq)3-0.5% in mCP host of the single emissive layer, white PHOLED device showed the maximum luminescence and luminous efficiency of 37,490cd/m^2 at 11V, 48.7cd/A at 4.5V with CIEXY color coordinates of (0.34, 0.40). Obviously concentration of Ir(ppy)3 and Ir(piq)3 as green and red dopant should be lower than Firpic as blue dopant considering more efficient triplet energy transfer between dopants based on their triplet energy levels.
In conclusion, triplet excitons were properly transferred from host to dopant and/or from dopant to dopant generating well-balanced emission of three primary colors for white PHOLED without loss of charge carrier energy through optimizing concentration of three phosphorescent dopants in single emissive layer of white PHOLED devices.

Although organic light emitting devices (OLEDs) have the advantages of high brightness, and a wide range of colors, reliability and lifetime are still crucial factors for OLEDs. This study compares the degradation tendencies in blue organic light emitting devices with two different electrical stresses: DC constant voltage and the AC voltages with different amplitudes, frequencies and offset DC voltages. With different stress conditions, IV, capacitance-voltage(C-V) and impedance spectroscopy are identified for examining the electrical and dielectric properties of OLEDs. Firstly, IV measurements show current level and threshold voltage, clearly indicating the change of the fitting parameters (ex. Ideality factor). Second, the dynamic responses of the charge carriers are examined by C-V measurement, correlated with the interface and the information on each layer. Equivalent circuit modeling analysis based on the impedance spectroscopy is useful for understanding the contribution of each layer on the device performance. As a result, AC electrical stresses influence the degradation of OLEDs more than the DC electrical stress. These OLEDs is provided by Samsung display.

The operation of organic opto-electronic-devices depends on the uncharged and charged energy levels of the organic molecules. The addition of charge carriers leads to the formation of polarons within the material. In general only the uncharged energy levels are investigated optically. But for a complete understanding of the processes within the material in the presence of charge carriers also a characterization of the charged energy levels is necessary. Because of the shift of the energy levels in presence of charge carriers new absorption bands appear and they influence the charge-mobility of the material. In general the polaron-absorption bands denote a possible optical loss channel for opto-electronic devices like organic lasers and organic LEDs. For instance, this lost channel can prevent an electrically driven organic laser since an increase of charge density results in an increase of laser threshold. Furthermore the interaction of polarons and light or polarons and excitons causes degradation in many OLED-materials. For organic solar cells the low charge carrier mobility reduces the transformation efficiency.
We present an optical method to measure transitions of a polaron in a simple device for positive and negative charges. These transitions were measured by means of a lock-in-amplifier and a tunable monochromatic light source in a wide spectral range in the visible and infrared regime. An exemplary study of 2,2',7,7',-tetrakis(N,N-diphenylamine)-9,9',-spiro-bifluorene (Spiro-TAD) reveals a polaron absorption band between 460 nm and 560 nm and a second band in the infrared.

The optimization of organic thin film opto-electronic devices requires fundamental understanding and control of excitonic transport. The nature of exciton transport strongly depends on morphology and is related to either temperature activated hopping [1] or band-like coherent motion as, e.g. in single crystals [2]. We will address this issue by temperature dependent Photoluminescence (PL)-Quenching measurements on thin films of the archetypical molecular semiconductors Diindenoperylene (DIP) and C60. Bilayer solar cells consisting of DIP as donor material and C60 as acceptor have proven to be promising candidates for organic photovoltaics [3].The rather poor light absorption of DIP due to the unfavorable orientation of its transition dipole supports the necessity of investigating the material inherent exciton transport for improved solar cell performance. Our experimental approach yields insight into the respective exciton diffusion lengths and activation energies as well as their dependence on morphology. DIP thin films exhibit high crystalline order along the direction of exciton transport, whereas C60 thin films occurred to be X-ray amorphous. As a result, polycrystalline DIP layers exhibit an exciton diffusion length (EDL) of 60 nm at room temperature and exhibit a temperature independent exciton motion below 80 K for films consisting mainly one crystalline domain along the transport direction. Above 80 K the exciton transport becomes thermally activated, especially for film thicker than 80 nm where the exciton has to cross grain boundaries within the layer [4]. In contrast, C60 layers show an EDL of 5 nm and a thermally activated exciton motion which can be related to incoherent hopping. Furthermore, discontinuities in the temperature dependent PL-quenching data reveal structural information on the local C60 film morphology. A continuous phase transition at 80 K [5] and a first order phase transition at 180 K are detectable which illustrate the sensitivity of PL measurements to probe the influence of the local structural order on excitonic processes. Our results demonstrate the correlation between exciton transport and morphology on molecular length scales in organic thin films. These results are of great importance to improve the exciton harvesting in planar hetero-junction solar cells and thereby to further enhance their efficiencies.
[1] H. Bässler and A. Köhler, Top. Curr. Chem. 1 (2012).
[2] P. Stallinga, Adv. Mater. 23 (2011), 3356-3362
[3] A. Steindamm, M. Brendel, A.K. Topczak and J.Pflaum, Appl. Phys. Lett. 101, (2012), 143302
[4] A.K. Topczak, T. Roller, B. Engels, W. Brütting and J. Pflaum, arXiv:1207.1036 [cond-mat.mtrl-sci] (2012)
[5] W. David, R. Ibberson, T. Dennis, J. Hare and K. Prassides, Europhys. Lett. 3(1992), 219

We demonstrate how solution-processed metal oxides can be used to make efficient and stable inverted organic solar cells. Furthermore, stability and lifetime of the solar cell is improved by modification of the metal oxide surface with chemisorbed self-assembled monolayers (SAMs). Phosphonic acid SAMs pacify defects at the surface of the non-stoichiometric metal oxide and also enhance the open-circuit voltage by shifting the effective work-function of the collection layer. Degradation studies lasting >1000 hours reveal solar cells utilizing SAM-modified metal oxide collection layers maintain efficient performance, whereas devices without SAM-functionalization show catastrophic failure after several hundred hours. We show how careful engineering of the chemistry and energetics at the interface of the photoactive and charge-collecting layers in an organic solar cell device provides a pathway to longer lifetimes.

The state of art on interface engineering combined with low band-gap materials in inverted solar cells has promoted organic photovoltaic efficiencies towards 10%. It has been realized the importance of interfaces at electrodes as well as interfaces between donor and acceptor, and both of them may be coupled in the generation of photocurrent and photovoltage under device-operating condition. As a new strategy of efficient energy conversion, the dielectric interlayer effects on light harvesting, charge recombination and collection at the electrodes have been explored. However, few attention is paid to the dielectric interface effects on the internal photovoltaic processes at donor:acceptor (D:A) interface, especially for the dissociation of charge-transfer states at the D:A interface, which is mainly responsible for the efficient photocurrent generation as well as the minimization of energy loss.
It is well known that the binding energy at D:A interface is a critical parameter for the charge dissociation and recombination; however, its measurement is always being a challenge. Based on our previous work, the magnetic field effects on photocurrent change together with an external electric field can be used as an experimental tool to investigate the binding energy at the D:A interface due to two main principles: (1) magnetic field effects of photocurrent can be used to monitor the charge-transfer states at D:A interface through the generation of photocurrent; (2) an external electric field can compete with the binding energy at D:A interface, leading to the dissociation of charge-transfer states. In order to explore the dielectric interface effects on the binding energy at D:A interface, the inverted solar cells based on PTB7 and PC71BM with two dielectric interlayers—ZnO and a polyeletrolyte PFN were studied. The experimental results demonstrate that the solar cell with PFN interlayer exhibiting better photovoltaic performance has a much lower binding energy at D:A interface as compared to the similar device with ZnO interlayer. Our integrated investigation of binding energy, surface polarization, built-in potential, and charge accumulation shows that the overall built-in electrical potential can be enhanced to decrease the binding energy at D:A interface through two pathways: induced interfacial dipoles and reduced space charges. This experimental finding presents a new understanding on the dielectric interlayer effects towards further improvements of photovoltaic efficiencies in organic solar cells.

In this study we demonstrate the synthesis of novel hybrid materials based on metal oxide (MO)-graphene in a core-shell structure. We use low cost, metal oxides (TiO2 and ZnO) with facile availability, compatibility with roll to roll deposition, well-defined crystal structure and high transparency. We hybridize them with reduced graphene oxide (RGO) via chemical modification to tailor their electrical and optical properties to entail the optimal performance for OPVs. We utilize these hybrid materials as electron transport layers (ETLs) in polymer solar cell devices and study their effects on the interface morphology and performance of solar cells with non-inverted architecture. Using these hybrid materials as ETLs, we achieved performance enhancement of 8.38% compared to solar cells utilizing pristine metal oxides as ETLs both in single devices and large modules with active area size 28.3 mm2 and 3920 mm2 respectively. Record high power conversion efficiencies were achieved from large solar cells sufficient to power up a commercial LED on plastic substrate. The advantages of using these hybrid materials are compatibility with solution processes and high performances equivalent to thermally evaporated ETLs. Advanced electrical and optical characterizations were performed on the ETLs the findings are analyzed with theoretical models to determine the interfacial mechanisms which occur with these hybrid materials and improve the efficiency of the solar cells.

The power conversion efficiency (PCE) in organic solar cell devices is controlled by the absorption of light and the collection of charges at the electrodes [1]. Due to low carrier mobilities of the conducting polymer [2] and short lifetime of the excitons [3], the thickness of the active layer is often limited to ~100 nm or less with a consequent reduction of the number of the photons absorbed [4].
Increasing the optical path length of the light in organic solar cells is a crucial problem to be addressed in order to increase the PCE. The easiest way to increase the number of the photon absorbed can be achieved with structures that are similar or smaller in size to the wavelength of the light of interest and with specific materials that interact strongly with the light. In particular, the light can couple with metallic nanoparticles at a specific frequency leading to a strong absorption or scattering of the light as function of particle size [5]. We report a 10% enhancement of PCE in a bulk heterojunction solar cell by depositing a thin layer of gold (1-5 nm) on the ITO electrode and annealing it in order to create gold islands and gold nano particles on the ITO surface [6]. We also performed numerical simulations that confirmed the increasing of the light scattered and how the size and distance between the gold nano particles influence the light absorption in the device.
[1]P. Schilinsky et al., Applied Physics Letters, vol. 81, pp. 3885-3887, 2002.
[2]R. A. Street et al.,Physical Review B, vol. 81, 2010.
[3]M. Theander et al., Physical Review B, vol. 61, pp. 12957-12963, 2000.
[4]T. Kirchartz et al., Journal of Physical Chemistry Letters, vol. 3, pp. 3470-3475, 2012.
[5]K. R. Catchpole et al., MRS Bulletin, vol. 36, pp. 461-467, 2011.
[6]M.Notarianni et al., Solar Energy, in press.

Title:
Microscopic investigations of ZnPc:C60 blends with varying mixing ratio by TEM and pcAFM
Abstract:
The optoelectronic properties of organic thin film solar cells are closely related to the microstructure of the self-organized, nanostructured network of donor and acceptor molecules. Using electrical scanning probe microscopy and transmission electron microscopy it is possible to probe local (photo-)currents, crystallinity, and phase-separation on a nanometer-scale. The goal is to investigate the critical gap between microstructure and electrical transport properties.
We use photoconductive atomic force microscopy (pcAFM) and transmission electron microscopy (TEM) to investigate the phase-separation of in-situ heated ZnPc:C60 blend layers with varied mixing ratio (1:1, 2.1, 3:1, 6:1 by volume; thickness 40 nm). Furthermore, we use the results from TEM and pcAFM as an input for optoelectronic simulations in order to bridge nanoscale experiments and macroscopic organic solar cells.
As shown in literature, ZnPc crystallizes at room temperature (substrate temperature) in the alpha-phase (island like growth) and at temperatures >250 °C in the beta-phase (nanorod like growth).
While varying the mixing ratio of ZnPc:C60 blend, we also observe the presence of ZnPc nanorods at a substrate temperature of 150 °C. On the one hand we can deduce from our TEM investigations that at a substrate temperature of 150 °C the ZnPc beta-phase is present (1:1 and 2:1 sample), but at the same time the ZnPc nanorods are lying parallel to the surface, which is not the optimal geometry for organic solar cells. On the other hand, we observe the disappearance of ZnPc nanorods at the 3:1 sample, but a return at the 6:1 sample. Furthermore, in the pcAFM measurements we can observe that the density of ZnPc nanorods at the surface increases with increasing ZnPc content while the conductivity decreases. This effect is only observable in the pcAFM measurement and not in the topography.

The use of massive inert gas cluster ions (GCIB) as a sputter beam for both XPS and TOF-SIMS has made it possible to acquire molecular depth profiles on a wide variety of polymer and organic systems. The purpose of this study was to combine XPS and TOF-SIMS depth profiling for the purpose of achieving quantitative analysis of organic photovoltaic (OPV) and organic light emitting diode (OLED) devices. TOF-SIMS depth profiling provides a means for 3D molecular imaging and trace-level analysis of the devices, but XPS depth profiling is required to achieve a quantitative benchmark.
A difficulty is encountered when using a GCIB sputter beam, e.g. 5 keV Ar_2,500⁺, in the course of profiling a mixed composition device such as an OLED that contains metallic films or heterogeneous organic/inorganic structures. The GCIB sputtering fails because the sputter rate of the organic phase(s) is 10² - 10⁴ times greater than that of the inorganic phase(s). Hence, the chemical distributions observed in 1D profile or 3D image data sets are highly distorted.
We have found that a C₆₀ ion beam often provides the best solution for characterization of OPV and OLED devices that contain metallic films or heterogeneous inorganic structures. In other cases a combination of sputter beams is required, either simultaneously or in series, to achieve uniform depth profiling and representative chemical distributions. Advantages and disadvantages of each approach for quantitative and 3D analysis will be discussed using examples of real-world OPV and OLED structures.

Power conversion efficiencies of organic photovoltaics (OPV) increased dramatically in the last few years until 11 %.[1,2] These values were mainly attained by improving the design and synthetic pathways of polymers and small molecules with suitable band gaps, molecular weights and desired solubilities as well as device optimizations with various methods including thermal annealing, solvent annealing and solvent mixtures and additives.
Since the charge carriers recombine at the donor/acceptor (D/A) interface, how to fabricate BHJ devices with desired morphological properties becomes one of the key issues to achieve efficient photovoltaic devices.D/ A ratio is one of the significant parameter in fabrication process of the BHJ solar cells in order to have a balanced transport and facilitate this transport during photoelectric conversion. Therefore, when a new polymer is introduced to OPV field this ratio should be fully screened to find the optimized performance.
Photoluminescence (PL) is known as a fast, easy and powerful tool which is contactless and provides complementary and injection level dependent information about radiative recombination. Usually, the efficient quenching of the polymer PL is often reported as a finely intermixed morphology where essentially all excitons generated on the polymer are able to reach BHJ interface. It is generally assumed that CTPL is a good probe of geminate recombination of the excitons and this recombination does constitute a significant limitation for IQE.
In this talk, the applicability and versatility of a new method using steady-state-photoluminescence to unfinished solar cells, without the need for a complete solar cell structure, will be demonstrated firstly on PCDTBT: PC60BM mixtures that are prepared after various processing steps to estimate the efficiency potential of the blends.[3] The experimental results show that, there is a strong relation between the intensity of the singlet emission of pristine material in the blend (IS1-S0) and the intensity of charge transfer emission (ICT). Using this relation, we can predict the efficiency potential of BHJ solar cells via defining a new figure of merit (FOM), which is the ratio of the intensity of singlet emission of the polymer in the blend to charge transfer emission from photoluminescence (CTPL) (ICT/ IS1-S0). The higher FoM resulted in more efficient solar cells in PCDTBT: PC60BM mixtures. Moreover, the method is applied to four other well-known systems including P3HT and PTB7. The striking trend was observable for all systems predicting the best combination in PCBM blends.
References
1 Green, M. A., Emery, K., Hishikawa, Y., Warta, W. & Dunlop, E. D. Progress in Photovoltaics: Research and Applications 21, 827-837,
2 Li, G., Zhu, R. & Yang, Y. Nat Photon 6, 153-161 (2012).
3 Baran, D. Brabec, C. J. PCT/EP2013/058730.

One of factors determining the device properties of organic electronic devices such as organic field-effect transistors (OFETs) is an interface between metallic electrodes and organic semiconductors. An energy barrier for charge injection from the electrode into the organic semiconductor determines the device operation, and the barrier height can be effectively tuned by modifying the electrode surfaces with self-assembled monolayers (SAMs). However, if the SAMs have a disordered structure, anomalous hystereses are observed in device characteristics of OFETs [1], which is possibly due to the structural change of the SAMs. In this presentation, we introduce an idea exploiting the disordered SAM, which is contrary to generally accepted wisdom of using a well-ordered SAM. The disordered SAM is found to impart a switchable nature to metal/organic-semiconductor interfaces [2].
[1] R. Nouchi and Y. Kubozono, Org. Electron. 11, 1025 (2010).
[2] R. Nouchi et al., submitted.

A lot of work has been done to optimize organic photovoltaic (OPV) cell performance by modifying the buffer layers of the metal electrode/organic interfaces. Besides, in order to study the working mechanism, a few attempts have been made by modifying the donor/acceptor (D/A) interface, yet leading to lower power conversion efficiency (PCE). In our recent transient photovoltage studies on tris-(8-hydroxyquinolinato)aluminum/silver and N,Nprime;-Di-[(1-naphthyl)-N,Nprime;-diphenyl]-1,1prime;-biphenyl)-4,4prime;-diamine/silver interfaces, we proposed that the interfacial exciton dissociation may be enhanced by introducing a thin layer of lithium fluoride (LiF)-an insulating material-with optimum thickness to the organic/metal interface. Furthermore, we boldly introduced a thin LiF layer to the D/A interface (copper phthalocyanine/3,4,9,10-perylenetetracarboxylic bis-benzimidazole, CuPc/PTCBI) of both regular OPVs and inverted-OPVs. For a regular OPV cell, the effect of LiF is undoubtedly negative. As the LiF layer thickness increased, both short circuit current (Isc) and open circuit voltage (Voc) dropped. However for an inverted OPV cell, we observed two phenomena. First, when the LiF layer was thin enough (0.1 nm), Isc was surprisingly enhanced, leading to a higher PCE than the device without LiF. Second, as the LiF layer thickness was increased from 0 to 0.1 nm, Voc dropped, while when it increased from 0.1 nm to 0.5 nm, Voc was pinned at a certain value which is irrelevant to the LiF layer thickness. To study the physics behind those phenomena, the effects of LiF on device behaviors, i.e., current density-voltage (J-V) and capacitance-voltage (C-V) characteristics, were investigated under dark condition in regular and inverted OPV cells, respectively. Charging under positive bias and discharging under negative bias were clearly observed in inverted cells, yet not in regular cells. These results imply that the mechanisms of charge separation and collection of inverted and regular OPV cells are possibly different.

Organic photovoltaics (OPVs) represent a promising route to large-scale renewable electricity generation. Several challenges currently impede widespread OPV deployment, including relatively low power conversion efficiencies (PCEs), suboptimal device lifetimes, and issues with device reproducibility. Significant inroads have been made in addressing the first challenge via synthetic customization of novel organic materials with lower optical band gaps and higher carrier mobilities. Encouraging progress has been made regarding device lifetime challenges with the application of inverted device geometries, where a high work function metal collects holes, and an indium tin oxide electrode collects electrons. In contrast, little work has been done to address the device reproducibility challenge.
The current metric for new OPV evaluation typically consists of reporting the “champion” PCE from a single device as opposed to reporting statistics taken from multiple devices. However, reporting limited statistics may not accurately depict whether a new OPV actually outperforms the current standard. Moreover, addressing device reproducibility is essential for scaling laboratory devices to the module level, and to better understand basic photophysical phenomena. Here we report that a (3-aminopropyl)triethoxysilane (APTES) cathode interfacial treatment significantly enhances device reproducibility in inverted high-efficiency PTB7:PC71BM OPVs. First, APTES alone is shown to function as an electron transport layer (ETL) in inverted PTB7:PC71BM OPVs. Next, the beneficial effects of APTES on device reproducibility are demonstrated by fabricating 100 APTES-treated devices and 100 untreated controls in high efficiency inverted PTB7:PC71BM OPVs. A hybrid zinc oxide (ZnO)-C70 ETL is employed, where the fullerene layer is thermally evaporated onto the APTES-modified ZnO. The exploratory implementation of APTES is motivated by its appreciable binding affinity for carbon nanomaterials. The 100 APTES-treated devices achieve a PCE of 8.08 ± 0.12% with histogram skewness of -0.291. In contrast, the 100 untreated controls achieve a PCE of 7.80 ± 0.26% with a histogram skewness of -1.86. Furthermore, analysis of the device dark data indicates that APTES inclusion results in a perturbation to shunt characteristics. Fitting the dark data to a space charge limited current conduction model reveals that the APTES interfacial treatment tends to reduce to magnitude of the shunting current coefficient. By addressing the interfacial origins of underperforming devices, the APTES treatment offers a route toward OPV modules with high spatial performance uniformity.

In previous studies, solution processing methods such as ultrasound irradiation or addition of poor solvents has been used to induce nano-scale structure formation in poly(3-hexylthiophene) (P3HT) thin-films. Concomitantly, P3HT charge carrier mobility has been reported to be enhanced, which indicates that the features assembled during these processes are likely to be ordered P3HT chains that are stacked via pi-pi interactions, resulting in improved interchain charge carrier transport. Here, we present a comprehensive study of the thin film structure-charge carrier transport property relationships within P3HT thin-films by precisely controlling development of highly ordered P3HT nanostructures. When utilized separately, ultrasound irradiation and poor solvent addition are limited in their ability to assemble P3HT nanostructures with various dimensions. However, in a synergistic approach, the combination of both methodologies, i.e., ultrasound irradiation coupled with poor solvent addition, P3HT nanostructures with controlled but varied length, width and quantity have been fabricated. Corresponding filed-effect hole mobility data have been correlated with AFM morphology data and x-ray crystallography results for P3HT thin-films composed of nanostructures having differentiated dimensions and quantities. The systematic investigation has uncovered a general trend that relates nanostructure dimensions and quantity with the charge carrier transport characteristics of the corresponding P3HT thin-films.

We demonstrate a new device that combines a light-field effect and an electrical-gate effect to control the drain current in a dual-gate transistor. We used two organic layers—photochromic spiropyran (SP)-doped poly(triarylamine) (PTAA) and pristine PTAA—as top and bottom channels, respectively, connected to common source and drain electrodes. The application of voltage to the top and bottom gates modulated the drain current through each layer independently. UV-light irradiation produces an open-ring isomer of SP in the top channel, which has ionic polarized structure and works as a scattering site of carrier transport. As a result, UV irradiation suppressed the drain current through the top channel. The suppressed current was then maintained even after the UV light was turned off because of an optical memory effect induced by photoisomerization of SP. In contrast, UV irradiation did not change the drain current in the bottom channel. Our dual-gate transistor thus has two organic channels with distinct photosensitivities: an optically active SP-PTAA film and an optically inactive PTAA film. These features make it possible to manipulate the drain current by three parameters: the top and bottom gates and light irradiation. This device configuration allows multi-level switching via top- and bottom-gate electrical fields with an optical-memory effect. Our findings will enable the development of new device applications such as optically and electrically controllable electric circuits, and lead to the integration of new functionalities into conventional organic transistors.

As the performance of organic transistors and sensors improve, it becomes possible to expand their applications beyond active matrix arrays, and the devices can be applied to build independent sensor systems. There are new challenges with integrating multiple devices into systems, because the system designs should take into account the characteristics of organic transistors, such as bias stability and device variations, that are distinct from conventional silicon technologies. Here we took a systematic approach to model printed organic devices, and from the modeling and simulations we have designed and fabricated prototypes of temperature-sensing tags based on printed organic complementary transistors.
The temperature sensor tag is consisted of a printed thermistor bridge, a threshold control circuit using 44 transistors, and non-volatile memories. When the thermistor temperature exceeds a pre-set threshold, the control circuit is triggered to generate a pulse to write into the memory. The threshold set point of the tag can range from 8 C to 72 C and is calibrated with accuracy up to ±1C. The sensor circuit is scalable to write to more than one bit of memory. This temperature sensor tag is self-contained and will be applicable to packaging or environmental monitoring applications.
In the course of developing this sensor platform, there are some design rules we learned for printing integrated circuits. Our approach to improve device variations and stability in printed devices will be presented, in order to improve the reliability of the fabrication processes and accelerate the development of printed electronics.

Ferroelectric-gate field effect transistors (Fe-FETs) employing ferroelectric layers as gate insulators have received attention due to their benefits, including a nondestructive readout capability, scalable feature size of 4F2, and low operating voltages. In a Fe-FET, the polarization state of the ferroelectric layer is set by the polarity of the program/erase gate voltage, which controls accumulation or depletion of carriers in the semiconducting channel between the source and drain electrode.
Polymer Fe-FETs have also attracted much attention owing to their ability to store data of more than 2 states in a single device, i.e., multi-level cell (MLC) operation. The distributed domain polarization of a ferroelectric layer controlled by the applied gate voltage precisely defines the discrete interstate levels of the source-drain current, giving rise to a MLC Fe-FET with highly reliable data retention and read/write endurance cycle performance compared with other organic or polymer-based non-volatile memory. The MLC operation is extremely important, especially for polymer non-volatile memories, most of which are fabricated by printing technologies hardly applicable for the high definition, nanometer scale pattern formation necessary for high density data storage devices.
Among a variety of technological issues that limit the performance of multilevel polymer Fe-FETs, the requirement of high voltage for cell operation is one of the most urgent problems that still remains unresolved. Most bistable, 1-bit (program/erase) polymer Fe-FETs require a gate voltage greater than 50 V due to the use of thick ferroelectric gate insulators to minimize the electrical leakage arising from many structural defects, including grain boundaries of semi-crystalline polymers, pinholes and residual solvent trapped in the film.
In this work, we present a simple but robust route to fabricate a low voltage operating MLC polymer Fe-FET memory with top gate/bottom contact architecture. Our method is based on precise control of capacitance of a ferroelectric gate insulator layer by solution-blending a ferroelectric polymer with high-k materials. Binary blending of PVDF-TrFE (k~8) and poly(vinylidene-fluoride-trifluoroethylene-chlorotrifluoroethylene) (PVDF-TrFE-CTFE) (k~18) enabled us to control the capacitance of the films ranging from 9 to 13 without significant deterioration of ferroelectric switching characteristics of PVDF-TrFE. The enhancement of capacitance of a ferroelectric hybrid film successfully led to the reduction of operation voltage of MLC Fe-FET memory when the insulator was employed in a Fe-FET device. For instance, a hybrid insulator with a PVDF-TrFE/PVDF-TrFE-CTFE blend composition of 10/5 allowed discrete six-level multi-state operation of a MLC Fe-FET at a gate voltage sweep of 18 V with excellent data retention and endurance of each state of more than 10000 seconds and 120 cycles, respectively.

The development of high mobility organic semiconductors has proven to be challenging but crucial for numerous applications such as light-emitting transistors and solar cells. Transport is limited by both energetic disorder, resulting from variations in the energies of charge states, and structural disorder resulting from variations in electronic interactions between neighboring transport sites. Since both types of disorder are normally present, until now it has been difficult to isolate the contribution of energetic disorder to charge carrier mobility. In this study, we use a multi-scale modeling approach that combines semi-empirical quantum-chemical calculations, coarse-grained molecular dynamics and kinetic Monte Carlo modeling to investigate the role of energetic disorder in two material systems of current interest for applications. In the case of amorphous poly(9,9-dioctylfluorene) (PFO) we show that time-of-flight mobilities are controlled by the presence or absence of energetic traps to a particular conformer, rather than to intermolecular structural disorder. In the case of a series of fullerene adducts we show that the low electron mobilities observed for higher adducts result primarily from energetic disorder due to multiple isomers, rather than from the influence of side chains on molecular packing. The results confirm that control of energetic disorder is critical to control charge mobility in a range of organic materials.

We report a low-temperature processed, hydroxyl-free poly(vinyl phenyl) (PVP) dielectric layer crosslinked using thiol-ene chemistry. This new dielectric material exhibited a high dielectric constant as compared to conventional hydroxyl-free polymer dielectrics, e.g. polystyrene, and allowed for cross-linking at 80 degree, which is lower than the glass transition temperature of most commonly used plastic substrates, e.g. poly(ethylene terathalate) (PET). Due to the absence of hydroxyl groups, the dielectric layer displayed more stable performance than other previously reported crosslinked PVP dielectrics. The low-temperature processing, high air stability and low current-voltage hysteresis while retaining high device performances are important advantages of this new dielectric material.

The device performance of conjugated polymer semiconductors has improved greatly over the last 25 years through exploration of a wide range of molecular structures and detailed understanding of the relationship between structure and physical properties. However, several fundamental limitations of these low-temperature, solution-processible and mechanically flexible materials have not been overcome. Most importantly, their transport properties including charge carrier mobilities and exciton diffusion lengths remain limited by pronounced energetic disorder associated with spatial variations of the backbone conformation and intermolecular packing of the polymer occurring inevitably in thin polymer films.
Within this framework, we present results on the new high mobility donor-acceptor copolymer indacenodithiophene-co-benzothiadiazole (IDTBT) [1] showing field effect mobilities of > 2 cm2/Vs despite a lack of long range crystalline order. By means of electrical characterization and optical measurements (e.g. photothermal deflection spectroscopy, photoluminescence measurements) we present independent proof for an exceptionally low degree of energetic disorder in IDTBT, which is lower than in any polymer reported up to date and is manifested in textbook-like transfer characteristics down to low temperatures of 200K. We attribute these exceptional features to backbone co-planarity and well defined molecular orientation across the entire film facilitating both, the fast intrachain transport along the polymer backbone as well as the occasional interchain hopping in locations where adjacent chains come close. On these grounds we argue, that Instead of long range crystalline order, long range energetic uniformity seems key for good charge transport properties.
[1] Zhang, X. et al. Molecular origin of high field-effect mobility in an indacenodithiophene-benzothiadiazole copolymer. Nat. Commun. 4, 2238 (2013)

As organic field-effect transistors (OFETs) begin to find novel applications in commercial products, the major concern for the industry is environmental stability, and performance in real-life scenarios.
This talk will highlight the most recent studies performed using Charge Accumulation Spectroscopy (CAS) [1][2], which is used to probe the evolution of the energetic landscape of the accumulation layer in OFETs during stability measurements. This technique is applied principally, alongside electrical measurements and other forms of spectroscopy to reach a well-rounded understanding of the effects observed.
A CAS study is performed on indacenodithiophene-co-benzothiadiazole (IDTBT), to investigate the environmental and current-stress stability of this low-disorder, high mobility, ambipolar copolymer [3][4]. In particular, its stability during current-stress in air and vacuum is investigated. Whilst there is a wealth of literature reporting voltage-stress effects on OFETs [5][6], reports on current-stress remain very scarce. We are able to correlate the noticed change in threshold voltage with spectroscopy. We will discuss the use of CAS to investigate whether trapped charges are located in states in the semiconductor, dielectric or chemical impurities. Moreover, we observe subtle changes in the bandgap of the semiconductor upon air exposure. The stress conditions used are relevant in the context of implementing organic backplanes for OLED displays.
We also investigated the effect of ozone on the high crystallinity, high mobility small molecule, 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES ADT) [7]. We present the severe detrimental effect on OFET electrical behaviour. Through optical spectroscopy, XPS, NMR and FTIR, we also suggest a mechanism for the degradation observed in the molecule. Our results are in close agreement with an established mechanism of ozonolysis.
[1] Di Pietro, R. et al, Adv. Mater. 24, 3367 (2012).
[2] Di Pietro, R. et al, J. Am. Chem. Soc. 134, 14877 (2012).
[3] Bronstein, H. et al, Macromolecules 44, 6649 (2011).
[4] Zhang, X. et al, Nature Commun. 4, 2238 (2013).
[5] Bobbert, P. A. et al, Adv. Mat. 24, 1146 (2012).
[6] Sirringhaus, H. et al, Adv. Mat. 21, 3859 (2009).
[7] Subramanian, S. et al, J. Am. Chem. Soc. 130, 2706 (2008).

Organic field-effect transistors (OFETs), in which the active semiconducting layer is an organic material, allow the simple fabrication of ultra-thin, compact devices. Here we demonstrate, for the first time, an OFET based on an asymmetrically-pure helically-shaped chiral semiconducting molecule known as a helicene. OFETs with a helicene semiconductor layer are solution-processible and have well-behaved device characteristics. Importantly, we find a highly specific photo-response to circularly polarised (CP) light, which is directly related to the handedness of the helicene molecule (Nature Photonics vol. 7, p. 634 (2013)). Chiral, circularly polarized (CP) light is central to many photonic technologies, including CP-ellipsometry based tomography, optical communication of spin information, and quantum-based optical computing and information processing. To develop these technologies to their full potential would require the miniaturization and integration of suitable chiral photo-detecting devices. We believe the helicene OFETs open up a unique possibility for CP detection in highly integrated photonic technologies.

Unlike alkane thiols/dithiols that can readily self-assemble into vertically standing monolayers (SAMs) on conductive substrates (e.g., gold thin films) for implementation into electronic devices, complex molecules pose a significant challenge to form similar monolayers, let alone the related electrical measurements. Moreover, very few reports have studied molecules beyond the typical 1 ~ 3 nm length, with most electron transport falling into the tunneling regime. Recently we employed a “bottom-up” approach to synthesize conjugated polymers that are covalently tethered to conductive substrates. With activated 3-methylthiophene as the monomer, we were able to successfully grow conjugated polymers with considerable length (up to 30 nm) using a surface initiated Kumada catalyst-transfer polycondensation (SI-KCTP). A “grafting from” method was used to grow poly(3-methylthiophene)s of various lengths from the conductive oxides ITO and LSMO. Linear growth of the polymer with respect to time is observed, allowing systematic control over polymer brush length. IV curves of the polymer brushes are measured using conducting AFM on the polymer surface in order to characterize the electron transport of the system. Electrical measurements are also taken through permanently attached electrodes deposited by transfer printing directly on to a thiolated polymer brush surface. Electrical measurements of these two types have been made on P3MT brushes of lengths between 2-15nm. We will discuss the possible transition between electron tunneling and activated electron transport as the length of the polymer brush increases as well as the behavior of these structures in spintronic and thermoelectronic devices.

Due to its low-cost and excellent electrical properties, copper (Cu) is an attractive electrode metal for use in large area organic electronic devices. However, surface instability of Cu and its undesired interactions with the organic overlayers in plastic electronics have made reliable and reproducible Cu based devices so far elusive. Here we describe a simple and robust methodology for achieving stable oxidized Cu thin films with work function of 5.5 eV. The high work function Cu oxide film is stable in ambient and nitrogen atmospheres. We demonstrate efficient injection of holes into conjugated polymers with a range of ionisation potentials from 4.7 eV to 5.9 eV such as poly(diketopyrrolopyrrole) (DPP) polymer, poly(benzodithiophene-alt-[3,4-c] thienopyrrole-4,6-dione) and poly(fluorene). Organic field effect transistors (OFETs) fabricated with these Cu films show low contact resistance owing to the more efficient hole injection into these polymers.

We will present hybrid processing of complementary, sub-micron organic devices for integrated circuits on flexible plastic substrates. The techniques that will be described are useful to both those fabricating organic electronics and are relevant to photonic, bio-sensing, smart packaging and security applications.
The oft-touted advantages of organic materials, namely solution processing, roll-to-roll and print capability, are frequently little more than footnotes to headline mobility figures. In order for organic electronics to become truly viable, we must go beyond organic-on-wafer analogies, beyond optimising single processing parameters, and focus on integrating the best of materials, technologies and designs.
Here we will outline multiple techniques for robust processing on flexible plastic foils, encompassing ultraviolet nano-imprint lithography (UV-NIL), rotogravure printing and self-aligned photolithography. We have developed the ability to pattern sub-micron structures for short channel lengths; to structurally print and pattern active and passive organic layers at high-speed (~40m per minute); and a method to achieve sub-200nm electrode self-alignment, all on flexible plastic foils. We will demonstrate this platform with data from sub-micron channel length, high frequency OFETs.

Imagination of scientists has been attracted to the idea of extracting useful work from a scattered energy field for over a century, as it puts to a test the second law of thermodynamics. Numerous attempts have been undertaken in order to build so-called ratchet devices. In particular, electronic ratchets produce DC current when oscillating voltage is applied. In practice such devices could be used for energy harvesting applications. However, previously proposed electronic ratchets are far from practical applications due to small output current, operation at cryogenic temperatures, impractical device complexity, and inability to work with unpredictable noise-like electrical signals. We present a simple ratchet device that is capable of transforming electrical noise signals into large DC currents, >2mu;A, at room temperature. Our three terminal devices are made of organic semiconductors via solution-processing methods and therefore have potential for low cost, flexible, and light-weight energy harvesting applications. The working principal of the device will be discussed.

Organic semiconductor thin films are promising building blocks for next-generation electronics as they are flexible, lightweight, and have low material and production costs. However, more work is needed to achieve strong device performance while maintaining cost effective device fabrication. Solution processing is a promising, highly cost effective fabrication technique but it leads to complex morphologies with, for example, multiple domains of varied crystal orientation and size. This variation in local morphology impacts the nature of the excitonic states of the material, and may impact device performance. A better understanding of the correlation between morphology and electronic structure is needed for directed design.
Ultrafast spectroscopies such as transient absorption are typically used to determine the dynamics of the excitonic states of materials, but they average over morphology effects. In contrast, we employ transient absorption microscopy, which uses a more tightly focused excitation volume in order to obtain spatiotemporal maps of exciton dynamics that can be correlated with local morphology.
Utilizing transient absorption microscopy, we report on the exciton dynamics within single domains of 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-Pentacene), an organic semiconductor material with high hole mobility and chemical stability. By tracking the polarization dependent optical response of the probe beam within a single domain we are able to assign dynamical processes with greater clarity than could be achieved in a bulk measurement. Supported by a simple kinetic model, we assign an ultrafast vibrational relaxation of the initially excited singlet state occurring within the first 100 fs, followed by a singlet fission process taking place over 3 ps. This more complete understanding of local exciton dynamics within one domain is used as a foundation to understand the spatial heterogeneity in the transient absorption over multiple domains in TIPS-Pentacene films and can also be used as a foundation to explore the heterogeneity in exciton dynamics at interfaces between domain boundaries.

Electrochromic Windows(ECWs) have the potential to save energy through dynamic control of light and solar energy entering a room. ECWs have been developed as an optical shutter in airplane, building and automobile applications. An ECW is composed of three components, a working electrode based on electrochromic materials, a counter electrode based on ion storage materials and the electrolyte as an ion conducting layer. Organic ECW has been gaining popularity due to easy processing, room temperature synthesis, availability of wide range of colors, high optical contrast and flexibility in design. However there are challenges in commercialization and application of organic ECWs. The application of ECW as a sunroof in automobiles demands operation in harsh environment conditions like elevated temperature. Consequently the University of Washington, Center for Intelligent Materials and Systems has been developing a heat resistant organic ECW that can be operated at elevated temperatures maintaining high optical contrast, fast switching speed, optical color memory and electrochemical stability. The proposed design is an ECW based on poly (3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine),PPRODOT-Me2 as a working electrode, V2O5-TiO2 composite materials as a counter electrode and polyethylene imine based electrolyte. The ionic conductivity of the electrolyte was calculated through complex impedance method and temperature dependence of the electrolyte was determined using environment test chamber to control a temperature range of 15 to 80 degree Celcius for 30 days. Differential Scanning Coulometry(DSC) has also been utilized to monitor the thermal stability of the electrolyte. A 76 × 76 mm2 ECW was developed and the optical transmittance change was observed by Chronoamperomerty and Time course measurement. The electrochemical stability of the window was monitored using cyclic voltammetry. The developed electrochromic window showed good optical contrast, electrochemical stability and fast response time after testing at elevated temperatures for 30 days.

Organic thin film transistors (OTFTs) have promising applications due to two distinct advantages: low cost over a large area and low temperature fabrication on a flexible substrate. The substitution of gold electrodes with a conductive polymer is a significant step forward for reducing material cost and for fabrication on flexible substrate. An effective and scalable patterning technique is critical for the low cost fabrication of OTFTs. Stamping is one of the promising technique gives very high resolution patterning features. In this study, a conducting polymer poly(3,4-ethylelenedioxythiophene)/p-toluenesulfonate(PEDOT:PTS) (conductivity: 250 S/cm) was patterned as the source and the drain electrode by using stamping technique. An OTFT channel length of 5 mu;m has been achieved. Electrical characteristics of bottom contact pentacene TFTs with PEDOT:PTS electrode is superior that gold electrode due to a lower carrier injection barrier. Extracted contact resistance shows that the channel length of bottom contact OTFTs can be further reduced to increase the drain current. To achieve higher field effect mobility, higher on/off ratio and lower threshold voltage of OTFTs self-assembled multilayers (SAMTs) have been applied.

Organic Thin Film Transistors (OTFTs) are key elements for low-cost flexible electronics[1]. The replacement of conventional gate dielectrics (e.g. SiO2) with electrolytes as the gating medium is an effective approach to induce high charge carrier densities in the transistor channel upon application of relatively low electric biases, in the order of 1.5 - 4 V [2]. Further lowering of the operating voltage of electrolyte-gated transistors is possible by incorporating in the device structure low-cost, high-surface area carbon gate electrodes.
We report on electrolyte-gated transistors based on light-emitting thin films of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) as the organic semiconductor, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIm][TFSI] as the ionic liquid gating medium, and high-surface area carbon gate electrodes operating at voltages lower than 1 V [3].
Electrochemical impedance spectroscopy and cyclic voltammetry measurements show the electrochemical stability of the high-surface area carbon gate electrode during transistor operation. Such a stability dispenses the use of reference electrodes, permits to monitor the electrical potential in the transistor channel, and ensures the compatibility of the applied voltages with the electrochemical stability window of the electrolyte. [1] H. Klauk, Organic Electronics II, Wiley, 2012. [2] a) S. H. Kim, K. Hong, Wei Xie, K. Hyung Lee, S. Zhang, T. P. Lodge, C. D. Frisbie 25, 1822-1846, 2013. b) G. Tarabella, F. M. Mohammadi, N. Coppedé, F. Barbero, S. Iannotta, C. Santato and F. Cicoira, Chemical Science, 4, 1395-1409, 2013.
[3] M. Lazzari, C. Arbizzani, F. Soavi, M. Mastragostino, in Supercapacitors, Wiley, 2013.

Much work has been accomplished toward the incorporation of organic molecules into molecular electronic devices and accomplishing the goal of utilizing unique single or few molecule effects for electronic applications. Despite this, a need still exists for techniques that can be adapted beyond analytic capabilities into practical electronic applications. While the body of work in molecular electronics has generated many unique techniques toward such a goal, comparatively little work has gone into understanding the effects of these fabrication techniques on device output or on incorporating interesting molecules into practical device architectures. This presentation will show our work incorporating various organic molecules into molecular electronic devices using nanotransfer printing (nTP) and will highlight our studies of these unique systems. First, we see significant effects on the measured molecular properties of molecules in devices prepared with nTP versus other techniques, highlighted by the differing charge transport properties observed in identical phenylenedithiol monolayers when characterized via nTP and conductive atomic force microscopy. We show that understanding fabrication technique and its effects on a system is paramount to our as well as other studies of molecular electronic systems. In addition, we will show our work incorporating various porphyrin based molecules as monolayers into nTP devices. The meso-to-meso ethyne-bridged (porphinato)metal molecules we examined form sparsely packed monolayers due to large solubilizing side chains; despite this, we will show our ability to incorporate them into electrically accessible bulk devices with nTP that can be scaled for macroscopic examination. The unique charge and spin-dependent transport properties that these molecules show in our nTP architecture will be discussed.

Near Infrared (NIR) absorbing organic materials have been applied in a broad range of applications such as OLED, defense, optical recording and heat absorbers due to their unique electronic and photophysical properties. However, the availability of organic chromophores, especially polymers, which exhibit absorption maxima over 1 um remains limited due to complicated synthetic process and finite functional groups that responsive in the NIR range. Here, a family of azulene-containing conjugated polymers designed for NIR absorption were prepared, and the relationship between their spectral, electrochemical and spectral properties were investigated. These azulene-containing polymers show tunable absorption maixima from 380 to 540 nm by using different conjugated systems in the polymer backbone such as electron-rich thiophene and phenylene units and electron-deficient benzothiadiazole. More importantly, tunable and broad absorption bands from 1.5 to 2.5 um in the NIR region are observed when these polymers were treated with trifluoroacetic acid (TFA). Density function theory revealed that the strong NIR absorption was attributed to the intramolecular charge transfer of the polymer backbone. The in-situ spectroeletrochemical study revealed the good contrast and stability of protonated polymers, indicating the promising potential applications of these polymers in NIR optoelectronic devices. In addition, the broad absorption of protonated polymers which covers the whole visible and NIR region is outstanding among the reported NIR absorbing polymers, which could be further applied in NIR sensor, organic solar cells and NIR imaging.

Organic electronics has been widely investigated to fabricate flexible devices using soft materials. However, the performance of organic electronics showed some limitations, especially at the nano-scales, due to materials&’ problems. For example, soft lithography has been employed in organic electronics to transfer small features from the original maskers to substrates at a low cost. Soft lithography employs commercially available PDMS silicon elastomers as a stamping material; however, those commercial PDMS materials often result in collapse and mergence due to their low mechanical strength, especially in the nano-scale regime (< 100 nm). Since the resolution of soft lithography significantly relies on the elastomeric elements, the limitation has motivated us to develop a new stiff, photocured silicon elastomers, which satisfy a set of demands, such as enhanced physical toughness, low linear polymerization shrinkage, photocurability, and freedom from stress. In a molecular modification, we inserted photocurable and rigid functionalities into the PDMS prepolymer networks. The resulting silicon elastomer showed an improved stamping performance at the nano-scales without any mechanical failure. Furthermore, we also demonstrated a photopattening capability of the new PDMS prepolymers with a 5 micrometer of resolution. This is a new version of PDMS prepolymers, which would improve the performance of soft lithography as well as organic electronics.

Large-area metal-molecule-metal junctions (LAMJ), where a self-assembled monolayer (SAM) is either chemically or physically bound between two metal contacts, are of interest for many applications in electronics, spintronics, thermoelectronics, etc. The charge transport in these systems occurs mainly intramolecularly, thus both the chemical structure of the SAM, and the interfaces between the SAM and the metals will have a large effect on the transport characteristics. However, because of the fragility of the SAM, it is very difficult to deposit the top metal contacts onto these devices without a high percentage of shorts, and most current devices rely on some sort of protective layer between the SAM and the top contact. We design and pattern large numbers of microscopic LAMJs with gold contacts and alkanedithiol SAMs, using a kinetically-controlled transfer printing process. We then use photolithography in order to generate individually-addressable permanent molecular junctions. The junctions are air stable and easy to handle, and have direct contact between the SAM and the top gold layer. The charge transport behavior exhibited is non-ohmic, and the junction resistances are exponentially dependent on molecular length. This is consistent with the coherent tunneling mechanism seen in similar single molecule studies, as well as in other large-area molecular electronic devices.

Conjugated polymers based on thiophene unit have been studied for years in organic electronics because of its highly electron rich and moderate charge transport characteristics. Polythiophenes containing rigid and planar backbone, in particular, have been known to show relatively high charge carrier mobility because of their superior pi-orbital overlap property. Recently, 2,2&’-(1,2-ethenediyl)bisthiophene (TV) which has vinyl group between its two thiophene units is widely used for such purpose that the vinyl group suppress the single bond torsion between two thiophene units. Also, dithieno[3,2-b;2&’,3&’-d]thiophene (DTT) is another kind of thiophene derivatives with three fused thiophene units which make the structure of DTT rigid and planar. For this reason, we copolymerize TV with DTT unit and synthesized high mobility conjugated polymer, poly[2,2&’-(1,2-ethenediyl)-3,3&’-didodecylbisthiophene -alt- dithieno[3,2-b;2&’,3&’-d]thiophene] (PTV-DTT). With this polymer, hole mobility exceeding 3 cm2v-1s-1 was achieved in top gate/ bottom contact device configuration. By annealing conjugated polymer to increase crystallinity of polymer backbone, the mobility of OTFT increased gradually. Overall characterization of the polymer including thermal analysis, UV-Vis absorption, cyclic voltammetry, x-ray diffraction measurement were done and the it was found that crystallinity of the polymer was indeed important for high charge carrier mobility of the polymer.

Dinaphtho[2,3-b:2prime;,3prime;-f]thieno[3,2-b]thiophene (DNTT) is a promising organic material for organic based field effect transistor (OFET) because it shows better air-stability and high mobility. We studied DNTT thin films grown by the vacuum evaporation with substrate temperatures ranging from 77K to 400 K. Structural properties of DNTT thin films were investigated by atomic force microscopy, high-resolution x-ray photoelectron spectroscopy, angle-depended near-edge x-ray absorption of fine structure and grazing-incident x-ray diffraction. Also, we fabricated OFET device on SiO2 substrate (300 nm thickness) to determine the dependence of mobility on substrate temperature. We found that the mobility increased from 2x10-4 to 0.6 cm2/Vs as substrate temperature rises. We will discuss about the relationship between grain size, molecular orientation, mobility and substrate temperature.

We report on novel implementations of the vertical organic field effect transistor (VOFET). Instead of a typical capacitor below the organic diode, a carbon nanotube (CNT) based ionic-liquid supercapacitor (or ionic gate) is on top. Carbon nanotubes have been demonstrated to function as transparent anodes with 3-D bulk type collection of charges. The present work has been motivated by the discovery that the conductivity and work function of carbon nanotubes can be strongly modified by electric double layer charging (EDLC) in an electrolyte. The work function shifts as much as ±0.7eV in an aqueous electrolyte and is stable; the effect persists when the charged CNT are physically removed from the electrolyte. The conductivity of EDLC CNT is enhanced by as much as an order of magnitude in mixed single-walled CNT and can shift by a factor of two in multi-walled CNT. By coupling the ionic gate with an organic diode, charge injection into the diode can be controlled via modulation of the workfunction of the CNT electrode, resulting in transistor-like behavior. Additionally, the high capacitance of the supercapacitor will enable the VOFET to be operated at low voltages. Our VOFET components are entirely flexible with part of the capacitor decoupled from the diode - ideal for flexible electronics applications. Additionally, the entire device is processed under ambient conditions with no vacuum equipment used. We have tested VOFETs with two different materials, p-type P3HT and n-type PC₇₀BM. The polarity of the charge transported in the material determines the charge injection rate and whether the device is a normally-ON or a normally-OFF transistor. Both devices have high current transport, excellent output characteristics, good on-off ratios and low operation voltages. We believe that these novel VOFETs will have exciting potential for various future electronic applications.

Gravure printing is a roll-to-roll compatible intaglio process that involves the transfer of ink from a patterned array of pits etched into a metal cylinder, or cliché, onto a flexible substrate. It is used for the large volume manufacturing of magazines, postcards, wrapping paper and packaging. Gravure printing allows fine control over printed layer thicknesses, gives highly uniform coatings with very low surface roughness and has a throughput of up to 60 m² per second. Gravure printing is therefore ideally suited to low-cost, large-area manufacturing of organic electronic structures and devices, where the control of layer properties is crucial for high performance and uniformity. Currently, the application of gravure printing to organic electronics is hindered by the low feature resolution (>10 µm), the high cost of cliché manufacture, and the poor layer-to-layer alignment.
Nanogravure consists of the development of a new set of tools and processes to overcome the limitations of conventional gravure printing. In this talk we will describe a simple and low-cost cliché fabrication methodology, which when combined with a novel inking strategy allows the selective filling of nanoscale pits with a variety of functional inks. We demonstrate the application of nanogravure for direct patterning of sub-micron organic field-effect transistor (OFET) gate electrodes, and the subsequent fabrication of high performance organic transistors with self-aligned source-drain electrodes on plastic foils. We believe that nanogravure offers a route towards high-speed, large-area roll-to-roll compatible assembly of short channel OFETs and complementary logic circuits on flexible substrates.

Metal phthalocyanine, especially copper phthalocyanines (CuPc), is one of the most stable and widely used dyes. Therefore, it has attracted considerable attentions for numerous applications including applied as a p-type semiconductor in organic electronics. The fluorinated version of CuPc, copper hexadecafluorophthalocyanine (F16CuPc), turns out to be an n-type semiconductor and has been widely investigated. Many other metal phthalocyanines, fluorinated and not, have been explored in thin film forms for microelectronics. However, efficient devices have not been reported owing to less attainability of large size single crystals of fluorinated metal phthalocyanines, additionally, the role of different central metal atoms in fluorinated metal phthalocyanines is not well understood. The flexibility in the fluorination of metal phthalocyanines produces many different structures allowing for design of semiconductors with good air, chemical, and thermal stability. However, the relationship between molecular packing and the performance is still an open question.
Here, we report on crystal growth of high-quality centimetre-sized single crystals of F16CuPc, cobalt hexadecafluorophthalocyanine (F16CoPc) and zinc hexadecafluorophthalocyanine (F16ZnPc). Physical vapor transport has been used as a method of choice. Single crystal structures have been determined. New crystal structures of F16CoPc and F16ZnPc have been found and reported here. Organic single crystal field-effect transistors have been fabricated to study the effect of central metal atom on their charge transport properties. The results show that F16ZnPc has the highest electron mobility while F16CuPc has the lowest one. Theoretical calculations indicate that the crystal structure and electronic structure of central metal atom determine the transport properties of fluorinated metal phthalocyanines. In F16ZnPc relative high electron transfer integral and weaker electron-phonon coupling strength cause the electron mobility to be higher than those of F16CoPc and F16CuPc. F16CoPc has larger electron transfer integral, but stronger electron-phonon coupling hinder the mobility. Relative lower performance of F16CuPc is due to both stronger electron-phonon coupling and small electron transfer integral. Both experimental and theoretical results indicate that fluorinated metal phthalocyanines should be considered as stable, high performance n-type semiconductors proper for an inexpensive mass production.

In this contribution, we report on alpha,beta-unsubstituted naphtho[2,3-b:6,7-b']dithiophenediimides (NDTIs) in terms of the synthesis, electronic structure, crystal structure, and device applications. Electrochemical and optical studies on N,N-alkylated NDTIs demonstrate that the compounds ahve a low-lying LUMO energy level (4.0 eV below the vacuum level) and a small HOMO-LUMO gap (ca. 2.1 eV), suitable as n-channel organic semiconductor. The NDTI unit can also be integrated into conjugated polymer backbone to afford low bandgap polymers (Eg ~ 1.2 eV) applicable to ambipolar organic semiconductor. With these interesting electronic and optical properties, NDTI can be a promising and versatile building block for the development of novel pi-conjugated materials in future.

OFETs based on single crystals afford excellent opportunities to examine fundamental connections between charge mobility, molecular structure, and crystal packing. A particularly exciting recent development is the increase in the number of single crystal materials that exhibit OFET mobilities near 10 cm2/Vs and band-like behavior in which mobility increases as the device temperature decreases (up to a point). Observations of such large mobilities suggest that in these systems the intrinsic mobility is approached. The first part of this talk will describe single crystal OFET measurements on a series of rubrene derivatives that have been designed so that the unit cell parameters are systematically tuned. The impact of the crystal structure on carrier mobility and the temperature dependence will be discussed. The second part of the talk will focus on transport in rubrene single crystal OFETs gated with electrolytes (e.g., ionic liquids) in order to achieve extremely large charge carrier densities, on the order of 0.3 holes/molecule. It will be shown that the carrier density can be confirmed by Hall measurements, and further, that the insulator-to-metal transition is very closely approached. Future prospects for understanding transport at high charge densities using electrolyte gating will be discussed.

Solution-processable semiconductors represent an emerging class of active electronic materials that could potentially be used for a variety of existing as well as fast emerging optoelectronic applications. In this presentation I will discuss the development of high charge carrier mobility organic semiconductors based on blends of a small-molecule and an amorphous polymer. By changing the concentration of the small molecule in the blend we are able to monitor, through the use of advanced characterization techniques, the evolution of the film&’s microstructure and elemental composition and its impact on macroscopic charge carrier transport. Despite the polycrystalline nature of the optimized binary blend films, a nearly morphology independent charge transport process, is observed. The phenomenon appears to be unique to blend systems and is attributed to the low resistivity of the grain boundaries formed between the polycrystalline domains comprising these vertically phase-separated small-molecule/polymer blend systems. Ideas on how these blend organic semiconductors can be further improved will also be discussed.

The precise control of the electrical characteristics of organic field-effect transistors is essential for their use in integrated circuits. In addition to the mobility, the threshold voltage, Vth, is a key parameter to control for proper circuit operation. In this work, we demonstrate the controlled tuning of Vth of solution processed, small molecule, organic semiconductor transistors via molecular doping of the solution at multiple different doping levels.
A 1:1 blend solution containing the π-conjugated small molecule 6, 13-triisopropylsilylethynylpentacene (TIPS-pentacene) and polystyrene is used as the baseline solution for the organic transistors. Bottom gate, bottom contact devices are made by spin coating this solution on a SiO2 dielectric patterned with gold electrodes that are functionalized with pentafluorobenzene thiol (PFBT). The organic p-dopant, molybdenum tris-[1-trifluoroethanoyl-2-trifluoromethylethane-1,2-dithiolene] [Mo(tfd-COCF3)3], which is a soluble version of the previously used dopant Mo(tfd)3 [1], is added to the baseline solution at various concentrations up to 0.3% wt, and devices produced similarly. Current-voltage measurements on the resulting devices taken in the saturation regime give baseline transistors with an average mobility of 0.5 cm2/V.s and Vth of -1.5 V, while doped transistors show the same average mobility with the threshold voltage shifted up to an average maximum Vth of +2.5V. Samples were also doped at higher levels of 0.5% wt and 1% wt, but demonstrate a loss of transistor action, remaining permanently in the “on” state due to overdoping. Overall, the various doping levels produce a gradual increase in the threshold voltage which we attribute in part to the filling of trap states that are known to exist in organic semiconductor films [2], and in part to effects related to the organic/dielectric interface. The direct correlation between Vth and doping concentration can be used to tune the threshold voltage in this system.
[1] Yabing Qi, Tissa Sajoto, Stephen Barlow, Eung-Gun Kim, Jean-Luc Bredas, Seth R. Marder and Antoine Kahn, A high electron affinity molybdenum compound for p-doping molecular semiconductors, J. Am. Chem. Soc. 131, 12530 (2009)
[2] H. Sirringhaus, T. Sakanoue. Band-like temperature dependence of mobility in a solution-processed organic semiconductor. Adv. Mater., 2009, 21, 3859-3873

Semiconducting polymers with “donor (D)-acceptor (A)” motifs (D-A polymers), where electron rich (donor) and electron poor (acceptor) units are copolymerized, has recently been the focus of attention in organic electronics. Of our particular interest is to explore A units that possess strong electron affinity and ensure coplanarity of the polymer backbone. The use of such A units, e.g. naphthalenedicarboximide (NDI), diketopyrrolopyrrole (DPP), and isoindigo, in D-A polymers significantly reduces LUMO energy levels (ELUMO) while they possess relatively deep HOMO energy levels (EHOMO), since the energy levels of D-A polymers are resulted from the orbital mixing of the D and A units. The deep ELUMO is crucial for the n-channel/ambipolar characteristics. D-A polymers with such A units could also give rise to strong intermolecular π-π interactions and thus to highly organized polymer chains in the solid state, which facilitates the charge transport property. Hence, regardless of the charge carrier polarity, the exploration of new strong A units is highly important for the development of high performance semiconducting polymers. We here propose thieno[3,2-b]thiophene-2,5-dione (TTD) as a new A unit for semiconducting polymers, which has been of great interest to us since our work on the series of thienoquinoid small molecular semiconductors. With two ketones at the α-position of quinoidal thieno[3,2-b]thiophene, TTD possesses strong electron accepting nature. In this presentation, we report the synthesis, electronic properties, and thin-film structures of TTD-based semiconducting polymers, and their use in OFETs. The polymers were found to have wide absorption range covering 600-1000 nm and a very small band gap of 1.2 eV, along with a low-lying LUMO level of -3.9 eV (from cyclic voltametry). The polymer-based OFETs gave hole-mobilities of as high as 1.4 cm2/Vs in the bottom-gate top-contact configuration, and interestingly showed ambipolar characteristics in the top-gate bottom-contact configuration with hole and electron mobilities of ~0.1 and ~0.2 cm2/Vs. With these quite attractive electronic structure and electrical features, TTD is indeed a fascinating acceptor unit for the development of high-performance semiconducting polymers.

Organic semiconductors have gained increased attention primarily for their potential incorporation in applications that are not compatible with silicon technologies, or where the use of organic materials is economically advantageous. The solution processability or organic electronic materials allows for low-cost manufacturing on flexible substrates such as plastic, paper, or human tissue. But controlling film properties during solution deposition is not a trivial task, given the complexity of film formation. This severely effects the performance and reproducibility of the resulting opto-electronic devices. We developed a simple method to fabricate organic field-effect transistors that outperform similar devices obtained by other solution approaches and reach the fundamental performance limits shown in corresponding single-crystal measurements. The method relies on vibrating the substrate and solution during film crystallization at a controlled amplitude and frequency. Using this method, we fabricated transistors based on 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene (diF-TES ADT) with mobilities as high as 3 cm2/Vs, subthreshold swings of 0.43 V/dec and threshold voltage close to zero. Quantum-mechanical calculations reveal that the observed improvements result from a significant reduction in trap density at the semiconductor/dielectric interface upon gentle mechanical treatment: the vibration-assisted crystallization allows the materials to reach more easily a global potential energy minimum, characterized by reduced fluctuations in molecular spacing and overlap. Results on several small-molecule organic semiconductors will be described and the effect of processing parameters on charge carrier mobilities, on/off ratios and interfacial trap densities will be detailed.

Molecular packing strongly influences charge transport in organic semiconductor (OSC) thin films. Even a slight displacement between adjacent molecules can lead to a significant change in the transfer integral and therefore to the charge carrier mobility. Compared to inorganic semiconductors, OSC crystals are characterized by weak intermolecular interactions, make different polymorphic forms accessible near room temperature and pressure. Despite its importance, structural and thermodynamic information on crystalline phases of an OSC system are scarcely available due to the challenge of characterizing nanoscopically thin films. Therefore, not surprisingly, there have been few strategies reported for controlling OSC polymorphism so far, not to mention methods for controlling both polymorphism and thin film morphology.
In this work, we use in-situ X-ray diffraction techniques to map the phase space of the organic semiconductor. State of the art thin film refinement technique combined with ab initio and molecular dynamic calculations help elucidate the molecular origin of polymorphism observed. We also investigate the impact of molecular packing on the charge transport properties using a combination of experiments and quantum chemical calculations. Furthermore, we discuss strategies for controlling OSC polymorph formation that take advantage of the spatial confinement effect and kinetic trapping.

Additives, including nucleating agents, have been used to regulate the solidification process of (semi-)crystalline polymer solids and thus control both their crystallite dimensions and shape. Here, we demonstrate that minute amounts (0.1 - 1 wt%) of commercially available nucleating agents can be used to efficiently manipulate the solidification kinetics of a wide range of organic semiconductors - including poly(3-alkylthiophene)s, the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13- bis(triisopropyl-silylethynyl) (TIPS) pentacene - when processed from the melt, solution or solid-state, without adversely affecting the semiconductors' electronic properties. We show that heterogeneous nucleation increases the crystallization temperature and rate of poly(3-alkylthiophene)s, permits patterning of crystallites at pre-defined locations in PCBM, and minimizes dewetting of films of TIPS formed by ink-jet printing. Nucleating agents thus enable the fabrication of thin-film transistors with uniform electrical characteristics at high yield.

Nucleation and growth of organic semiconductors govern their crystalline morphology in thin films that directly impact on charge transport characteristics. We will report on our recent attempts to separate nucleation from growth phenomena by applying external constrains to the systems. We will also present new results (unpublished yet) on homogeneous nucleation for extra pure samples in thin film geometry, the necessary purification step being induced by dewetting upon the crystal to melt transition. We will then rationalize our results with non-equilibrium thermodynamics and discuss their implications for charge transport.

The promise of organic semiconductors is to be able to leverage the power of chemical synthesis to produce materials that do not exist in nature having well-controlled optoelectronic properties. While great advances have been made with semiconducting polymers exhibiting unprecedented mobilities and heterojunctions producing photocurrent with a 100% quantum yield, the fundamental structure-properties relationships in this materials family remain largely mysterious. We study the effect of local order and order at the mesoscopic scale to elucidate how the microstructure affects the properties of organic semiconductors. For instance, I will describe the effect of disorder on charge transport, where local order and connectivity govern carrier mobility. Delocalization, governed by the presence of locally ordered regions, may also be the key to understanding why organic heterojunctions can split excitons with nearly 100% efficiency. Understanding structure-property relationships at all length-scales is the key to designing higher-performance new materials.

Organic photovoltaic devices are a promising route to lower costs via roll-to-roll manufacturing. The most promising device architecture involves a bulk heterojunction (BHJ) active layer in which nano-scale phase separation into nominally bicontinuous donor and acceptor rich regions enables both exciton dissociation and charge extraction. The performance of BHJ based devices is a strong function of the active layer processing conditions and the optimized device structure is, in general, not the equilibrium structure. Recently it has become common to optimize BHJ film formation by introducing small amounts of processing additives. However, the mechanisms by which these additives effect the film formation are not known. Photon based techniques, such as spectroscopic ellipsometry and grazing incidence x-ray scattering (both wide angle and small angle) can provide detailed insights into film thickness, composition, and microstructure. We will discuss highlights from real-time studies of the additive effect in film formation of both polymer and small molecule based BHJs. Multiple mechanisms related to both solvent quality and film plasticization are revealed.

A technological bottleneck to widespread use of OSC is the unclear link between processing conditions and how it affects morphology and performance. This is further complicated by the large set of solvents, substrates and fabrication conditions available to fabricate OSCs. This serves as compelling reasons for developing and deploying material informatics tools in conjunction with predictive computational frameworks for accelerated analysis and design. Two topics will be discussed:
1) Exploring structure-property relationships using surrogate models: Fully resolved computational characterization of the photovoltaic performance of morphologies involves the solution of highly-non-linear, tightly coupled set of PDE&’s, or solving complex Monte-Carlo simulations. I will illustrate the power of model reduction by describing a suite of physically motivated morphology descriptors that encode some of the physical processes that affect the photovoltaic efficiency. This is based on formulating the various morphology descriptors as graph based constructs to enable fast computing. This surrogate model enables high-throughput searching, classification and finally the solution of the inverse problem of morphology design.
2) Exploring process-structure relationships: A brief description of our computational multi-scale framework that models morphology evolution during solvent fabrication is presented. Its extension to model roll-to-roll fabrication is discussed.

One of the biggest challenges in the field of semiconducting polymers is the fundamental understanding of crystallization kinetics and thin film growth. The difficulties facing such studies stem from the inherently short crystallization times of these materials and the relatively slow techniques available for characterizing nanoscale microstructures. To overcome these limitations, we chose to investigate the polymer poly(3-ethylhexylthiophene) (P3EHT). A relative of the ubiquitous poly(3-hexylthiophene) (P3HT), P3EHT exhibits similar optoelectronic properties and the same semicrystalline solid-state morphologies in thin film: aggregated, ordered domains coexisting with amorphous, disordered regions. Such similarity allows us to study P3EHT thin films using the numerous models and characterization techniques that have been developed for P3HT over the last few decades.
Due to its low melting point, P3EHT exhibits slow crystallization kinetics when quenched at room temperature from the melt, which is a rare property among semiconducting polymers. We perform quantitative analyses of x-ray diffraction patterns and optical absorption spectra P3EHT thin films and are able to monitor the evolution of aggregates and crystallites as well as the recrystallization kinetics as a function of time. One of the unique aspects of semicrystalline, semiconducting polymer is that crystallites are highly anisotropic: the three directions of molecular order are (1) along the polymer backbone, (2) along the pi stacks and (3) along the alkyl stacking direction. Here we find that the growths of individual aggregates and crystallites along the three packing directions are indeed anisotropic and exhibit different growth rates. Furthermore, we show that crystallization kinetics depend on a number of parameters including film thickness, molecular weights and quenching temperatures, which is all attributed to chain confinement effects. Finally we also correlate the evolution of thin film microstructure to electronic properties and charge transport. From these measurements, we directly observe the onset of charge percolation in thin film and demonstrate that polymer tie chains are responsible for efficient charge transport.

The performance of the next-generation electronic devices based on organic semiconductors is strongly influenced by the structure-function relationship. The local distribution of electron mobility in new semiconductor materials such as pentacene plays a crucial role for the overall efficiency. Single-phase pentacen is well known to be superior in terms of conductivity to polymorph-phase, however the growth procedure is hard to control. Here, we demonstrate an unexpected nanoscale structural coexistence of crystalline polymorphs in chemically pure, 40 nm thick pentacene films by mapping the local infrared response with a scattering near-field optical microscope (s-SNOM). For films grown at elevated temperature, we observe ubiquitous bulk-phase (BP) pentacene nucleating within the dominant thin-film phase (TFP), on a submicron scale and uncorrelated to its grain structure. The BP exhibits a distinctly blue-shifted resonance near 11 µm wavelength, and comes with a subtle topographical depression, but has in the past gone undetected. Our finding of two coexisting structural phases explains why the carrier mobility ceases to increase for pentacene thin films grown at temperatures beyond 50 °C as then the bulk phase begins to obstruct carrier percolation paths within the thin-film phase.

We will use some of our recent results in the area of bulk heterojunction and bilayer solar cells to highlight the complexity of the electronic and optical processes taking place in these devices and the challenges they pose for their reliable theoretical description. In particular, we will disccuss aspects related to charge separation and to polarization effects.

In this presentation, I will discuss an important idea in designing donor polymers for organic solar cell application. Our recent results on investigation of low bandgap polymers lead us to conclusion that not only the energetics but also the internal dipole moment along the polymer chain is critical in facilitating charge transfer to PCBM, which were shown to be partially responsible for the high PCE device made from these low bandgap copolymers.
More than twenty polymers were synthesized and the solar cell performance was found to correlate well with dipolar change between excited and ground state when all of the other parameters, such as morphology and energy level match, are identical.

I will discuss our recent development in my group related to understanding of new materials design for efficient charge transport. Additionally, I will discuss ways to take advantage of processing parameters to pattern large area single crystalline films with controlled molecular packing.

Fused polycyclic aromatic hydrocarbons lie at the forefront of high-mobility organic semiconductors. Functionalization strategies have yielded efficient methods to make these typically insoluble systems more processable, and in some cases to even improve their performance in devices. However, the field of small-molecule semiconductors is still focused on relatively small systems - typically five rings or smaller. Further, in the case of heteroaromatic systems, there is still significant discussion on the importance of isomeric purity to device performance and film morphology. In this talk, I will present methods to prepare a variety of isomerically-pure heteroacenes, discuss the properties of the resulting pure isomers, and will then show how the variety of isomerically pure materials can be used as building blocks for the preparation of liked polycylic system of unprecedented size. The impact of the increase in dimensionality in fused and linked polycyclic systems will be discussed as it relates to intermolecular interactions in the solid state, film formation, and device performance.

Organic solar cells based on the bulk heterojunction architecture have achieved steady increases in device performance over the past two decades, with power conversion efficiencies reaching 10%. Much of this success has come with conjugated polymer/fullerene combinations. More recently, well-defined molecular donor systems have come under intense investigation due to their ease of functionalization, amenability to standard organic purification and characterization methods and reduced batch-to-batch variability compared to polymer counterparts.
Critical properties for efficient small molecule donors are 1) broad optical absorption extending towards the near-IR region to enhance spectral overlap with the solar spectrum, 2) low lying highest occupied molecular orbital energy) to ensure acceptable device open circuit voltages, 3) relatively planar structures to ensure close intermolecular contacts and high charge carrier mobilities, and 4) sufficient solubility to facilitate deposition of thin films of appropriate uniformity and thickness. Ideally, these molecules should be constructed from cost-effective, sustainable building blocks using established, high yielding reactions in as few steps as possible. The structures should also be easy to functionalize to maximize tunability for desired properties.
This presentation will cover a description of our thought process and design strategies used in the development of highly efficient molecular donors. The molecules are based on a modular D1-A-D2-A-D1 architecture, where A is an asymmetric electron deficient heterocycle. Modifications to the D1 and D2 units enable spectral coverage throughout the entire visible region and control of HOMO energy levels, while adjustments to the pendant alkyl substituents dictate molecular solubility, thermal transition temperatures and solid-state organizational tendencies. State of the art materials allow one to deposit the active layer from a single solvent and have a “forgiving” BHJ which provides high PCE values across wide range of donor:acceptor ratios. Efforts to characterize the morphologies of the layers will also be discussed.

There has been considerable interest in designing new organic conjugated materials for the fabrication of efficient bulk heterojunction solar cells. The members of the Victorian Organic Solar Cell Consortium have developed highly efficient solution-processable donor-acceptor polymers for use in combination with fullerene derivatives in inverse configuration bulk heterojunction solar cells. Understanding the contribution of fullerene components has also contributed to improved device efficiencies.
The presentation will review the development of efficient bulk heterojunction solar cells and the efforts to translate these results to large area fully printed modules using a variety of roll-to-roll deposition processes on ITO/PET substrates.

Solution-processable organic semiconductor materials with high thermal stability is crucially important to realize practical printed electronics. Recently, we have reported V-shaped thiophene-containing pi-conjugated materials, dinaphtho[2,3-b:2prime;,3prime;-d]thiophene (DNT-V) derivatives, with high carrier mobility up to 9.5 cm2 V-1 s-1 and high thermal durability in organic field-effect transistor device. We conjectured that the research concept of the V-shaped molecular design could be extended for N-shaped molecular structure, dinaphtho[2,3-d:2&’,3&’-d&’]benzo[1,2-b:4,5-b&’]dithiophene (DNBDT) derivatives by bridging two sulfur atoms. In this work, we present their fundamental properties, single-crystal structures, and charge transporting abilities.
Synthesized DNBDT derivatives exhibit lower ionization potentials than DNT-Vs due to the extension of pi-electron conjugation system, which is beneficial for the p-type FET operation in terms of decreased injection barrier from commonly used gold electrodes. Differential scan calorimetry measurements clarifiedy that C10-DNBDT exhibited much higher phase-transition temperature of 250 °C than C10-DNT-VW (150 °C). The single-crystal structural analyses revealed that the packing motifs as well as the deviation from the planar structure are quite different among the DNBDT derivatives. That is, unsubstituted DNBDT assumes slipped packing motif, in which the molecule is almost planar structure with the dihedral angle of only 0.63°. On the contrary, C10-DNBDT assumes herringbone packing structure with the typical lamella-like layer-by-layer structure. Furthermore, C10-DNBDT molecule possesses deformed zig-zag conformation with the bent angle of 9.65° unlike unsubstituted counterparts.
We evaluated the intrinsic charge carrier mobility of C10-DNBDT in the form of single-crystalline film, which was prepared by solution-crystallization methods, originally developed by our group. The TFT performance of C10-DNBDT was very high with the average and best mobilities of 12 and 20 cm2 V-1 s-1, respectively, with the very high on/off ratio of 10 6 and small threshold voltage of -5 to 0 V. This small Vth value results from the appropriate high HOMO energy level of C10-DNBDT compared with V-shaped molecules. In this presentation, calculated transfer integrals of DNBDT derivatives will be also discussed.

Single-crystalline p-n junctions with contacting two single-crystals of organic semiconductors are expected to show superior charge transport property as well as exciton generation/dissociation ability. Thus, these p-n junctions are promising high-performance materials for opto-electronic applications such as solar cells. However, the single-crystalline p-n junctions are difficult to prepare. Intuitively, crystallization generally requires relatively clean environments and crystallization of “p” and “n” two components together tends to interrupt each other. Here, we describe how to grow, from solutions, organic single-crystalline p-n junctions in a single step from a mixed solution of p-type and n-type molecules. Basically, one crystal (e.g., n-type crystal) grows first and the other (e.g., p-type crystal) with slower growth rate nucleates on the former. Subsequently, both crystals grow simultaneously into single-crystalline p-n junctions. We have prepared single-crystalline p-n junctions based on 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), C60 (n-type) and several other organic molecules. These junctions exhibited ambipolar charge transport characteristics in field-effect transistors (FETs). This work provides a platform to deepen the fundamental knowledge and to fabricate high-performance electronic devices of organic semiconductors.

The current interest on developing molecule-based spin-electronic devices such as switches, memories or spin valves has stimulated the study of responsive bistable molecular materials with magnetic and/or conducting properties. Here it will be presented different electroactive Donor and Acceptor molecules that behaves as switches as well as donor-acceptor (D-A) free radical dyads that have the ability to switch between neutral and charge-separated states via an intramolecular electron transfer (IET) process in response to an external stimulus (temperature, pressure, light, solvent, magnetic, and electrical fields) [1]
When the molecules are immobilized on substrates,[2] highly robust surface-confined switches are obtained. Thus the ones based on an electroactive persistent organic radical derived from polychlorotriphenylmethyl (PTM) [3,4] behaves as an extremely robust redox switch in which an electrical input is transduced, into an optical -absorbance and fluorescence- as well as magnetic and wetability [5] outputs. The switches based on tetrathiafulvalene (TTF) derivatives operate as a ternary redox switches, which would allow for the execution of more complex logic operations and also increasing further the data storage capacity going from 2n memory units in a binary system to 3n in a ternary one.[6,7] Moreover these surfaces operates as switches with an exceptionally high long-term stability and excellent reversibility and reproducibility.
References
[1] a) J. Guasch, et al. Angew. Chem. Int. Ed. 2012, 51, 11024; b) J. Guasch, et al. J. Am. Chem. Soc. 2013,135, 6958; c) M. Souto, et al. J. Phys. Chem. Lett., 2013, 4, 2721.
[2] M. Mas-Torrent, N. Crivillers, C. Rovira, J. Veciana, Chem. Rev. 2012, 112, 2506
[3] a) N. Crivillers, et al., Adv. Mater, 2009, 21, 1177. ; b) M. Paradinas,et al. Chem.Commun., 2011, 4664; c) N. Crivillers, et al., Chem. Commun., 2013, 49, 8145
[4] a) N. Crivillers, et al. Angew. Chem. Int. Ed. 2007, 46, 2215; b) N. Crivillers, et al. J. Am. Chem. Soc. 2008, 130, 5499; c) M. Mas-Torrent, et al., J. Mater. Chem., 2009, 19, 1691
[5] a) C. Simao, et al. Nature Chemistry 2011, 3,359; b) C. Simao, et al., Nano Letters, 2011, 11, 4382
[6] a) C. Simao, et al. J. Am. Chem. Soc., 2011, 133, 13256; b) C. Simao, et al., Chem. Sci. 2013, 4, 307; c) M. Mas-Torrent, C. Rovira, J. Veciana, Adv. Mater., 2013, 25, 462
[7] J. Casado-Montenegro, et al., Chem.cCommun., 2013, 49, 8084

Fundamental studies of intrinsic charge transport properties of organic semiconductors are often hindered by charge traps associated with static disorder present even in optimized single-crystal devices. Here, we report a novel method of surface functionalization using an inert non-conjugated polymer, perfluoropolyether (PFPE), deposited at the surface of organic molecular crystals, that results in accumulation of mobile holes and "trap healing" effect at the crystal/PFPE interface [1]. As a consequence, a remarkable ultra low-noise, trap-free conduction regime characterized by intrinsic mobility and transport anisotropy emerges in organic single crystals, and Hall effect measurements with unprecedented signal-to-noise ratio are demonstrated. This general method to convert trap-dominated organic semiconductors to intrinsic systems may enable the determination of intrinsic transport parameters with high accuracy and make Hall effect measurements in molecular crystals ubiquitous.
[1]. B. Lee, Y. Chen, D. Fu, H. T. Yi, K. Czelen, H. Najafov and V. Podzorov, Nature Materials, Oct. 27, 2013, DOI: 10.1038/NMAT3781.

Thermoelectrics can convert heat to electricity. Optimal thermoelectric materials should possess good electrical conductivity and thermopower, but poor thermo-conductivity. Organic thermoelectrics have advantages over the inorganic counterparts, such as low thermo-conductivity, flexibility, and structural variability. Doping of conjugated polymers via external dopants is a common strategy to tune the thermoelectric properties, but the negative effects from introducing the dopants can&’t be ignored. PEDOT:PSS is widely used as organic thermoelectric materials, and trying to minimize the drawbacks from the insulating PSS layers is one popular solution to improve the thermoelectric performance and has obtained materials with world-record efficiency. Herein, we designed and synthesized a series of new water-soluble conjugated polyelectrolytes with anionic side chains. And we found that they can be doped after dialysis as conducting self-doped polymers, and the formations of radical cations on the polymer backbone were proved by absorptions and electron paramagnetic resonances. The doped materials are stable in air, because the radical cations can be stabilized by the anionic side chains via Coulomb interaction. We studied the electrical conductivities and thermopowers of these self-doped polymers by varying several structural units, such as lengths of side chains, counterions, and bandgaps, in order to identify what kinds of structures are beneficial for the improved thermoelectric performances. Blending these polymers with other materials to achieve synergic improvements of electrical conductivity and thermopower will also be discussed. Our studies of these self-doped polymers will provide a new strategy for designing new organic thermoelectric materials in the future.

Organic semiconductors have moved from a laboratory curiosity to commercial use in organic light emitting diodes. Significant progress has been made to understand their ultimate limits of performance in applications ranging from thin film transistors to solar cells. In comparison to inorganic semiconductors, a remaining challenge for organic materials is the rational control of their electrical conductivity by doping. Doping enables highly conductive solution-processable materials and opens the opportunity for organic thermoelectric devices. Thermoelectric devices can convert thermal gradients to electrical potentials and by reciprocity the converse. Due to the low lattice thermal conductivity of organic materials and their high electrical conductivities, organic semiconductors represent a promising class of processable thermoelectrics. The state of organic thermoelectrics and work from our lab on electrical doping of both p- and n- type semiconducting polymers will be discussed. We find that existing electrical doping methods using molecular compounds are limited by morphological factors rather than the efficiency of charge transfer. Recent work on charge transfer doping with a range of high performance semiconducting polymers and the resulting thermoelectric performance will be presented. Striking relationships are observed between the thermopower and doping method suggesting directions for optimization of organic thermoelectrics.

Using non-chlorinated solvents for polymer device fabrication is highly desirable to avoid the negative environmental and health effects of chlorinated solvents. Here a non-chlorinated mixed solvent system, comprised of a mixture of tetrahydronaphthalene and p-xylene, for processing a high mobility donor-acceptor fused thiophene-diketopyrrolopyrrole copolymer (PTDPPTFT4) in thin film transistors is described. The effects of the usage of a mixed solvent system on the device performance, e.g. charge transport, morphology and molecular packing, are investigated. p-Xylene is chosen to promote polymer aggregation in solution, while a higher boiling point solvent, tetrahydronaphthalene, is used to allow a longer evaporation time and better solubility, which further facilitates morphological tuning. By optimizing the ratio of the two solvents, the charge transport characteristics of the polymer semiconductor device are observed to significantly improve for polymer devices deposited by spin-coating and solution-sheared. Average charge carrier mobility of 3.13 cm2V-1s-1 and a maximum value as high as 3.94 cm2 V-1 s-1 have been obtained by solution shearing. The combination of non-chlorinated mixed solvents and the solution shearing film deposition provide a practical and environmentally-friendly approach to achieve high performance polymer transistor devices.

Poor understanding and control of exciton dynamics at the mesoscale-in volumes of material containing a small number of coupled chromophores-is a significant hurdle to developing optimizing materials and device structures for organic photovoltaics. This applies to nominally homogenous materials consisting of a single type of chromophore, either in small-molecular or polymeric form, in which variations in orientation and local environment introduce heterogeneity into the energetic landscape and kinetic rates. It also applies to multi-chromophore systems, in which several different types of chromophores are mixed together. This is done typically for the purpose of achieving a broader spectral response, but it also can be considered as a method for controlling the flow of excitons toward specific “reaction” sites, to follow the example given by natural photosynthetic complexes. Here we have synthesized a multi-chromophore macromolecule, consisting of oligothiophene arms tethered to an hexabenzocoronene (HBC) core moiety, as a test-bed for understanding excitonic dynamics in a system in which the chromophore spacing and alignment is largely determined by the macromolecular structure. Time-correlated single photon counting is used to examine the decay rates of the various excitonic states, and a kinetic model is developed to analyze the inter-chromophore exciton dynamics. These indicate the existence of resonant energy transfer from HBC to oligothiophene, as would be expected from the relative energies of the two moieties, on a nanosecond timescale. We apply the kinetic analysis to investigate the role of intersystem crossing of the HBC on the system dynamics as a whole. Lastly, we calculate the rates of resonant energy transfer for various possible transitions in the macromolecule using DFT and many-body Green function theory. From these, we speculate on the possibility of three-body interactions leading to coherent energy pooling in such systems.

Solution processed small molecule bulk heterojunction solar cells with power conversion efficiencies (PCE) of 8% have recently been reported. This achievement demonstrates that solar cells fabricated from blends of small molecule donors and fullerene acceptors are a viable alternative to polymer:fullerene based systems. Advantages of using small molecules as donors include the ease of synthesis and purification. Moreover, in contrast to polymers, conjugated small molecules do not suffer from broad molecular weight distributions or batch to batch variations. The recent rise in PCE of small molecule based photovoltaics has largely come due to dramatic rises in the fill factor from previous values less than 50% to upwards of 75% for the best performing systems reported to date. However, the origin of this increase in FF is not well understood and has unfortunately come along with increasingly arduous and costly synthetic procedures.
In this work, we employ single carrier diode measurements to gauge the hole and electron mobilities of over 15 different donor molecules comprising a range of different molecular structures and solar cell PCEs. Both pristine donor molecule films and films blended with [6,6] phenyl-C71-butyric acid methyl ester (PC71BM) are investigated. While in almost all cases efficient electron mobilities in the blend films exceeding 10^-8 m2/Vs are established, the blend film hole mobilities range several orders of magnitude depending on donor materials and processing conditions. A strong correlation between blend film hole mobility and solar cell device FF is observed indicating that 10^-8 m2/Vs stands as the minimum mobility needed to obtain FFs greater than 65%. As neat film hole mobilities are found to always exceed the blend film, a similar threshold value for the pristine film hole mobility emerges which may serve as useful metric for screening new donor materials. Further investigation using x-ray scattering techniques demonstrates the importance of structural order for establishing blend film hole mobilities that can approach the pristine film value. Looking at global trends across the collective body of molecules allows for deeper insight into how parameters such as pi-pi stacking distance, planarity, relative degree of crystallinity and molecular weight influence both pristine and blend film mobilities. The insights gained from this work will enable the development of the next class molecular donors for efficient photovoltaics that can be readily synthesized in a cost effective manner.

Singlet fission sensitized photovoltaics have the potential to surpass the Shockley-Queisser limit for a single-junction structure. We investigate the dynamics of triplet excitons resulting from singlet fission in pentacene and their ionization at a C60 heterojunction. To this end we model the generation and diffusion of excitons to predict the spectral response. We find the triplet diffusion length in polycrystalline pentacene to be 40nm. Poly(3-hexylthiophene) between the electrode and pentacene works both to confine triplet excitons and also to transfer photogenerated singlet excitons into pentacene with 30% efficiency. The lower bound for the singlet fission quantum efficiency in pentacene is 180±15%.

Defect formation arising from radiation induced hydrogen rearrangement in polythiophenes has been investigated experimentally and theoretically.[1] Defect formation by irradiation has been proposed to be a source of recombination centers in organic solar cells.[1] Formation of the defects is reversible by annealing. Calculations show that hydrogen related defects indeed give rise to electronic states in the gap, and that such defects can migrate with an activation energy that is in agreement with annealing studies. To fully understand the process of radiation induced hydrogen rearrangement we must determine the minimum energy required to form such defects. We therefore will discuss density functional calculations of the energy barriers that must be surmounted to form hydrogen related defects in polymers such as poly(3-alkylthiophene).[2] Pathways for defect production corresponding to interpolymer and intrapolymer H rearrangement were identified. The calculations indicate that radiation induced gap state production in poly(3-alkylthiophene), via removal of H from the alkyl chain, becomes possible when the energy of the radiation exceeds a threshold value in the range from 2.7 to 3.2 eV. [2] This energy is less than the nominal value for C-H bond breaking (~4.4eV), because bond breaking and bond forming occur concurrently. This work was supported in part by the AFOSR under Grant No. FA9550-13-1-0106.
[1] R. A. Street, J. E. Northrup, and B. S. Krusor, Phys. Rev. B 85 (2012) 205211.
[2] J. E. Northrup, submitted.

Organic stretchable electronics have attracted extensive scientific and industrial interest because the fully elastic matrix can be stretched, twisted, or compressed, providing a path to create the next-generation of organic electronics for human/machine interfaces. These electronic devices have already been described for applications such as field-effect transistors, photovoltaics, light-emitting diodes, and sensors. High-performance stretchable electronics, however, currently still involve complicated processing steps to integrate the substrates, semiconductors, and electrodes. Herein, we describe a facile method to efficiently identify suitable polymers for organic stretchable devices. Briefly, the various polymers investigated are first transferred on elastomeric poly(dimethylsiloxane) (PDMS) film, and subsequently stretched (up to 100 %) along with PDMS. The polymer/PDMS layers are then laminated on source/drain electrode-deposited Si substrates with a PDMS dielectric layer. Using this device configuration, the semiconducting polymer were repeatedly interrogated with laminate/delaminate cycles under different tensile strain. From our obtained electrical characteristics, e.g. mobility, drain current, and on/off ratio, limitation of semiconductors can be derived. With efficient lamination testing approach, we can thus rapidly identify semiconducting polymer for stretchable electronics.

Double-shot inkjet printing (DS-IJP) enables us to manufacture exceptionally uniform thin films of small-molecule organic semiconductors [1]. Specifically, the sequential deposition of an antisolvent and a semiconductor solution can trigger the controlled formation of single-crystal films of C8-BTBT (2,7-dioctyl[1] benzothieno[3,2-b][1]benzothiophene, provided by Nippon Kayaku Co., Ltd.) at arbitrary positions reproducibly, which was extremely difficult by the conventional printing techniques. In the DS-IJP process, a supersaturation state of the organic semiconductor is immediately formed in the intermingled droplet and it results in unique film growth of the organic semiconductor at the liquid-air interface of the droplet. In order to elucidate further details of film growth mechanism through the DS-IJP, we investigated the film growth for several organic semiconductor materials under various printing conditions.
Temporal variation of printed mixed droplets investigated by using high-speed camera microscope revealed that the film formation manners were primarily determined by the mixed state of antisolvent and semiconductor solutions and thus the proper choice of solvents were essential to induce single-crystal film growth at liquid-air interface. We consider that the feature should be most probably associated with the unique mixing mechanism of binary microdroplets, in which the surface Marangoni effect takes crucial roles. It was also found that the film morphology and the single-domain nature were strongly influenced by the crystalline preference of organic semiconductors, as well as substrate temperature. In addition, the effect of molecular structure on the film growth was investigated by using asymmetric mono-alkylated BTBT derivatives. We successfully obtained single-crystal films for these molecules through the crystal growth at liquid-air interface under the same printing condition for symmetric C8-BTBT, however, we found the characteristic film morphology indicating different molecular arrangements from that of symmetric di-alkylated Cn-BTBTs. Based on the results, we discuss the optimal printing conditions and whole characteristics of film growth through the DS-IJP.
1) H. Minemawari, et al., Nature, 475, 364 (2011).

In the past decade, great success has been achieved in bulk hetero-junction (BHJ) solution-processed polymer BHJ solar cells. Despite the high PCEs reported for polymer BHJ solar cells, one finds that for a given polymer structure, batch-to-batch variations in solubility, molecular weight, polydispersity and purity can lead to different processing properties and performance. In contrast, solution-processed small molecule BHJ (SM BHJ) solar cells consist of well-defined molecules. However, despite their recent rapid rise in PCE, there have been few detailed investigations using advanced characterization techniques to link structure and morphology to device performance - a well-known strategy to further optimize the power conversion efficiency. Here we present an in-depth study on the effect of most commonly used processing conditions in OPV optimization viz., additive and annealing, on the photovoltaic performance of the molecular donor (p-DTS(FBTTh2)2), known to be one of the highest performing solution-processed SM BHJ solar cells to date. We reveal and quantify using Resonant Soft X-ray Scattering (RSoXS) for the first time the hierarchical structure in a promising high performance SM BHJ blend system. Two sets of samples were studied - different additive amounts and annealing temperature. The device fill factor within each set was seen to be primarily governed by the average composition variations in a non-monotonic manner. The behavior of median domain size obtained from Lorentz corrected scattering profiles was found to correlate well with the photogenerated current. Further detailed analysis of RSoXS indicated the presence of molecule-rich/PCBM-rich macro-phase separation and a nano-phase separation within the molecule-rich domains. The median length scale corresponding to the population of macro-phase separation length scale was found to be approximately twice compared to the corresponding to the nano-phase separation as measured by RSoXS. The molecule aggregate structure as revealed by grazing incident wide-angle x-ray scattering (GIWAXS) was found to correspond to the R-SoXS revealed nano-phase. The higher recombination rate for annealed devices could then be explained in terms of higher composition variations found for those devices. These results take us to a morphology paradigm in which a suitably mixed phase and percolation are critical in order to optimize performance and counters prevailing assumptions of device operation that are based on a two-phase, interconnected morphology.

Applying conventional printing technologies to fabricate large-area flexible bulk heterojunction (BHJ) solar cells is of great interest. Achieving this task requires (i) large tolerance of the maximum photoconversion efficiency (PCE) to the film thickness, (ii) fast hole transport in both the thickness and lateral directions of the BHJ layer, and (iii) improved stability against bending and heat. This paper demonstrates that a P3HT:PCBM BHJ layer made of long P3HT nanofibrils of almost 100% crystallinity can be an excellent approach to achieve large-area printed solar cells. We applied a cool-and-heat (C&H) process with a P3HT/PCBM m-xylene solution to generate P3HT:PCBM nanofibril composite films. We found that the hole transport of the nanofibril composite was 2.6 times faster in the thickness direction and 6.5 times more conductive in the in-plane direction compared with conventionally annealed composites. The fast hole transport in the thickness direction led to negligible dependence of the PCE on the thickness of the composite layer. The improved conductivity in the in-plane direction prevented the sharp drop of the PCE as the active area increased. Taking advantage of the unique characteristics, we employed a roll-printing method to fabricate large-area unit solar cells in air. In addition, the curved contour path of the nanofibrils provided excellent stability against large bending strains, allowing the production of highly bendable organic solar cells.

The organic photovoltaic blend consisting of, Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-
[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), and the fullerene derivative, [6,6]-phenyl-C71-butyric acid methyl ester (C71-PCBM) has received much attention in the literature recently. Bulk heterojunctions of these materials have been used to produce photovoltaic devices with efficiencies in excess of 9.2% power conversion efficiency[1].
The photovoltaic efficiencies of this blend can be enhanced with the addition of solvent additives, and this improvement is ascribed to an optimization of the blend morphology[2, 3]. The small lengths involved, however, make direct measurement of morphology extremely problematic.
In order for photo-excited states to be harvested to generate free charges, they must first migrate through the film by means of a random walk until the find an acceptor. Thus, exciton diffusion is a key process in the functioning of organic solar cells. In this work we use time-resolved fluorescence and transient-absorption spectroscopy to calculate values of exciton diffusion coefficient for the widely used electron acceptor C71-PCBM. The exciton diffusion coefficient is an important parameter describing the movement of excitons in the acceptor of a broad range of recent high performance photovoltaic blends. This value for the exciton diffusion coefficient in C71-PCBM is then used in conjunction with time-resolved fluorescence decays of the blend to predict the nanoscale morphology in the device blend. Such analysis predicts a much smaller substructure to the previously reported 200 nm C71-PCBM domains in the blend in the absence of DIO. This smaller domain size was then corroborated with high resolution photoconductive AFM.
1. He, Z., et al., Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat Photon, 2012. 6(9): p. 591-595.
2. Lou, S.J., et al., Effects of Additives on the Morphology of Solution Phase Aggregates Formed by Active Layer Components of High-Efficiency Organic Solar Cells. Journal of the American Chemical Society, 2011. 133(51): p. 20661-20663.
3. Collins, B.A., et al., Absolute Measurement of Domain Composition and Nanoscale Size Distribution Explains Performance in PTB7:PC71BM Solar Cells. Advanced Energy Materials, 2013. 3(1): p. 65-74.

The device efficiencies of bulk heterojunction photovoltaics have shown a significant improvement in recent years, with record efficiencies approaching 10%.[1] Further improvements in device efficiency and, importantly, stability will help to make this technology a genuinely competitive alternative to the more established inorganic photovoltaic technologies.
While the design and optimisation of the active layer is very important for device performance, the choice of electrode interlayer can also have a large impact on both stability and efficiency. The use of conducting metal oxides as electrode interlayer alternatives is becoming increasingly popular.[2] A deeper understanding of the processes occurring at the electrode / active layer interface is important for future photovoltaic device design.
For many material systems, it has been shown that non-geminate recombination is the dominant loss mechanism at open circuit.[3] However, for the case of imperfect electrode alignments, surface recombination becomes another important loss mechanism. While there are a number of studies which discuss surface recombination, there are few which try to measure it quantitatively.[4, 5] Therefore, in this work we combined both transient optoelectronic measurements, such as charge extraction (CE) and transient photovoltage (TPV),[6] with device drift-diffusion simulations,[7] to quantitatively understand how changes in the electrode / active layer interface can impact on the observed charge carrier recombination dynamics as a result of surface recombination. The work function of a metal oxide hole transporting interlayer in both conventional and inverted architectures was controlled by an oxygen plasma treatment.
The results from CE showed that an energetic constraint of the quasi-Fermi level splitting is present, which could be mistaken for a change in the active layer density of states. In addition to this, TPV measurements showed a small change in the average charge carrier lifetime, indicating a change in charge recombination dynamics.
1. Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat Photon 6 (9), 591-595 (2012).
2. Y. Sun, C. J. Takacs, S. R. Cowan, J. H. Seo, X. Gong, A. Roy and A. J. Heeger, Advanced Materials 23 (19), 2226-2230 (2011).
3. D. Credgington and J. R. Durrant, The Journal of Physical Chemistry Letters 3 (11), 1465-1478 (2012).
4. K. X. Steirer, P. F. Ndione, N. E. Widjonarko, M. T. Lloyd, J. Meyer, E. L. Ratcliff, A. Kahn, N. R. Armstrong, C. J. Curtis, D. S. Ginley, J. J. Berry and D. C. Olson, Advanced Energy Materials 1 (5), 813-820 (2011).
5. S. Chen, C. E. Small, C. M. Amb, J. Subbiah, T.-h. Lai, S.-W. Tsang, J. R. Manders, J. R. Reynolds and F. So, Advanced Energy Materials 2 (11), 1333-1337 (2012).
6. C. G. Shuttle, B. O&’Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley and J. R. Durrant, Physical Review B 78 (11), 113201 (2008).
7. T. Kirchartz and J. Nelson, Physical Review B 86 (16), 165201 (2012).

Organic solar cells (OSCs) have attracted considerable interest as one of the promising candidates to future photovoltaic applications owing to their potential low cost, highly tunable optical and physics properties, flexibility, and light-weight. Currently, the development of new materials and implementation of new device structures, such as inverted devices, bulk-heterojunction, and tandem structures, as well as interfacial modification, have led to significant improvement in cell performance. To date, the power conversion efficiency (PCE) of 8-9 % has been achieved, which is close to a broad commercialization.
Utilizing wide band gap donor material with a low highest occupied molecular orbital (HOMO) energy level to achieve high open circuit voltage (VOC) has received great attention due to its potential application in tandem solar cells, in which two or more individual cells with complementary absorption were connected via an interlayer. While using a low HOMO energy level material as the donor to gain a high VOC, the device sometimes shows a small fill factor (FF) or even an S-shaped current-voltage characteristic, which limits the cell efficiency. Therefore, a systematic investigation of metal/organic interface and organic layer with a low HOMO energy level is required in order to improve the FF. Also, the enhancement of FF will help us to largely increase the device performance.
Here, planar heterojunction organic solar cells using wide band gap phosphorescent material bis[2-(4-tertbutylphenyl)benzothi azolato-N, C2,] iridium (acetylacetonate) [(t-bt)2Ir(acac)] as electron donor and fullerene (C60) as electron acceptor were fabricated. A large open circuit voltage of 0.94 V was achieved due to low HOMO level of (t-bt)2Ir(acac). It was found that the insufficient extraction of holes due to the low hole mobility of (t-bt)2Ir(acac) was the major limiting factor for the FF, which even caused the accumulation of space charges and the S-shaped J-V curve in device with thicker active layer. The results also showed that improving the charge carrier extraction capability of electrode and increasing the hole mobility of active materials are critical ways to improve the FF of OSC devices. The PCE was improved by ~30% by either using hole transport layer with high carrier mobility or heating the substrate to increase the hole mobility of (t-bt)2Ir(acac). Moreover, it is worth of noting that the large injection barrier at interface can resulted in the decrease of VOC, although it has a negligible effect on the FF. Therefore, the performance of OSCs could be further improved by synthesizing the wide bandgap donor materials with higher mobility and designing the high performance device architecture with proper interfacial layers.

Single-electron devices, where Si and Ge nano-dots are adopted as quantum dots, are a promising candidate for future complementary metal-oxide-semiconductor (CMOS) devices. [1,2] However, the devices have not been realized yet. A major obstacle is the extreme difficulty in controlling the uniformity of the size of quantum dots at the nanometer scale. The poor controllability of size and uniformity of the dots hinders clear operation of single-electron tunneling (SET). To solve these problems, we have proposed to employ unique organic molecules as quantum dots in Si-based double tunnel junction. [3] Organic molecules have many advantages as quantum dots over their inorganic counterparts. First, molecules have uniform size on nanometer scale, leading to large integration of quantum dots. Another advantage of molecules is tunability of molecular orbitals. In particular, photochromic molecules permit the molecular orbitals to be reversibly changed by light irradiation.
In this talk, we present optical manipulation of SET in a Si-based double tunnel junction, where a diarylethene photochromic molecules, 1,2-bis(2-methyl-5-(4-biphenyl)-3-thienyl)hexafluorocyclopentene, was used in the form of optically controllable quantum dots. In our device, the threshold voltage of SET was clearly modulated as a reversible change in the molecular orbital induced by photoisomerization, indicating that diarylethene molecules worked as optically controllable quantum dots.[4] These findings will allow the integration of photonic functionality into current CMOS devices, which is a unique feature of organic molecules that is unobtainable with inorganic materials.
[1] S. Tiwari, F. Rana, H. Hanafi, H. Haratstein, E. F. Crabbe, K. Chan, Appl. Phys. Lett. 68, 1377-1379 (1996).
[2] M. Khoury, A. Gunther, D. P. Pivin, M.J. Rack, D. K. Ferry, Jpn.J. Appl. Phys. 38, 469-472 (1999).
[3] R. Hayakawa, N. Hiroshiba, T. Chikyow, Y. Wakayama, Adv. Funct. Mater. 21, 2933-2937 (2011).
[4] R. Hayakawa, K. Higashiguchi, K. Matsuda, T. Chikyow, Y. Wakayama, ACS Appl. Mater. Interfaces (2013) (in press).

Organic small-molecule semiconductors have been successfully proposed to fabricate different electronic devices because of their potential in large area and low-cost applications. Examples of these applications cover photovoltaic devices, thin-film transistors (TFT) and light-emitting diodes. Nowadays, the electrical performance of the bests organic solar cells fabricated from small-molecule organic semiconductors is already comparable to that obtained with p-i-n hydrogenated amorphous silicon.
The optoelectronic properties of the aforementioned devices depend largely on the density of states (DOS) in the band gap of the active semiconductor layers. Since the DOS distribution in the organic semiconductors determines the electrical transport, photosensitivity, doping efficiency and, at the end, the device performance, it is rational to assume that material with lower DOS will improve the device performance; however, little work has been presented on the evolution of the DOS and its influence on the photovoltaic effect.
In this work we correlate the evolution of the hole mobility and the density-of-states (in the donor semiconductor deposited at different substrate temperature) with the performance of organic solar cells.
To evaluate charge carrier mobility a series of Thin-Film Transistors (TFTs) were fabricated at different substrate temperature (30, 60, 90 and 120 C) using tetraphenyldibenzoperiflanthene (DBP) as donor semiconductor. TFTs electrical measurement at different temperature allows evaluating the spectral density of localized states in the band gap, i.e., the trap densities as a function of energy (trap density-of-states (DOS).
Finally, a series of solar cells with the structure glass/ITO/MoO3 (3nm)/DBP(10nm)/C70(40nm)/BCP(8nm)/Al were fabricated, with the DBP layer deposited at different substrate temperature. Best performance was obtained for solar cells in which the DBP was deposited at 60oC (efficiency 2.5%, Voc:0.89, Jsc:4.8mA/cm2). A correlation was observed with electrical performance of the solar cells (in particular with the open-circuit voltage) and the density of localized states estimated from TFT electrical measurement.
Finally, a heterojunction solar cell (coevaporating DBP and C70) was fabricated at the optimum substrate temperature of 60C, yielding a solar cell efficiency of 4%.

In order for bulk heterojunction (BHJ) polymer solar cells (PSCs) to be a cost competitive option for energy production it is necessary to push their efficiencies as high as possible. A key step in improving the efficiency of PSCs is understanding the device physics which determine how efficiently a BHJ PSC performs. Understanding charge transport in the BHJ is particularly crucial, yet it is also one of the areas in which there is the most debate and the least certainty. Kinetic Monte Carlo (KMC) studies modeling charge transport in BHJ PSCs have predicted that charge separation should be a limiting factor (P. Peumans, S.R. Forrest , Chemical Physics Letters 398, 2004), yet internal quantum efficiencies (IQEs) approaching 100% are observed in a number of BHJ PSC systems. An explanation may be provided by recent KMC and experimental studies (C. Groves, Energy Environmental Science 6, 2013, and S. Shoaee, J.R. Durrant, Advanced Functional Materials 2013), which suggest that the presence of offsets between the carrier energy levels in different morphological phases may be able to explain the high experimentally observed IQEs.
In this work, we use a combination of UV-VIS absorption, ultraviolet photoelectron spectroscopy and electrochemical techniques to characterize the hole energy levels in the polymer phases of polymer:fullerene BHJs. We demonstrate that there are two important possible energetic offsets present in the polymer phases of BHJ PSC. First, there can be an energetic offset between the amorphous and crystalline polymer in semicrystalline polymer systems. This energetic offset can approach 300 meV, and is due to bandgap widening associated with changes in conjugation length. Second, an intermolecular interaction between the semiconducting polymer and the fullerene can affect the polymer energy levels, in some cases resulting in energy level shifts greater than 200 meV. The first effect has been widely observed, but the second effect is often not accounted for when considering the energetic landscape of a polymer:fullerene BHJ. With this work we hope to raise awareness that accounting for energy levels shifts due to both bandgap widening and intermolecular interactions is necessary to properly characterize the energy levels of carriers in a polymer:fullerene BHJ.

Solution-processing in organic optoelectronics offers new opportunities for the large-area, low-cost, and printed manufacturing technologies. The most commonly used materials of the blended film are poly (3-hexythiophene) (P3HT) as the donor in combination with a fullerene derivative (PCBM) as the acceptor, of which the physical and chemical properties have been extensively studied for the use of organic solar cells. Recently extensive study about the formation of the fullerene surrounded by the electron-rich supramolecular complexes such as hexabenzocoronenes (HBCs), dibenzotetrathienocoroenenes (DBTTC) and aromatic compound, shows the co-crystals forming at the interface between these materials [1-3]. Z.Liu. et. al. reported that graphene addition (~ 10.0 wt. %) into polymer/fullerene blends behaves as the electron acceptor and provide percolation paths, which resulted in increasing both the device short-circuit current and power conversion efficiency[4]. Coronene(C24H12), a polycyclic aromatic hydrocarbons (PAHs) of intermediate size with electron-rich, has drawn much attention due to its a special role such as a full zig-zag periphery among the PAHs : its high symmetry and low band gap (<2.3 eV) for a PAH of this size make it a highly promising molecules. Based upon these reports, coronene appears to behave as an electron donor but also provide the P3HT:PCBM / coronene interfaces for exciton dissociation.
In this work, we will show the effect of coronene units on organic devices, resulting in forming a new active layer by tuning the concentrations of coronene in a 1:1=P3HT:PCBM blends. In order to investigate the electronic and structural properties of these films, x-ray/ultraviolet photoelectron spectroscopy (XPS/UPS), atomic force microscopy (AFM), UV-vis and I-V measurements were employed. All of these results will be presented.
[1]. S.Ju. Kang, J.B.Kim, C.-Y. Chiu. Angew. Chem. Int. Ed. 2012. 51. 8594-8597.
[2]. N. J. Tremblay, A. A. Gorodetsky, M. P. Cox. Chem. Phys. Chem. 2010, 11, 799 - 803.
[3]. C. Y. Chiu, B. Kim, A. A. Gorodetsky, W. Sattler, Chem. Sci. 2011, 2, 1480 -1486.
[4]. Z.Liu, D.He, Y.Wang, H.Wu, J.Wang. Synthetic Metals 160 (2010) 1036-1039.

The bulk heterojunction (BHJ) solar cell performance of many donor-acceptor (D-A) copolymers depends strongly on the polymer number-average molecular weight (MW) and the solvent additives used for solution processing. But, the underlying mechanism that causes these dependences is not well understood. We systematically investigate how MW and high-boiling point solvent additives affect the performance of BHJ solar cells based on the D-A copolymer, poly(di(2-ethylhexyloxy)benzo[1,2-b:4,5-bprime;]dithiophene-co-octylthieno[3,4-c]pyrrole-4,6-dione) (PBDTTPD). We show that low MW PBDTTPD devices have large, ~300-400 nm diameter fullerene-rich domains that cause extensive charge-carrier recombination. Increasing the MW of PBDTTPD significantly improves device performance by decreasing the size of these fullerene-rich domains and improving charge-carrier separation and transport. We determine that the propensity of PBDTTPD to aggregate in solution affects the size of the fullerene-rich domains in PBDTTPD BHJ thin-films, and link this effect to the dependence of PBDTTPD solubility on MW. We infer that due to its poor solubility high Mn PBDTTPD quickly forms a fibrillar polymer network during spin-casting, and this network acts as a template that prevents large-scale fullerene phase separation. Furthermore, we show that processing low MW PBDTTPD devices with a high-boiling solvent additive induces polymer aggregation in solution and improves the performance of these devices by preventing large fullerene-rich domains from forming. These findings highlight that the propensity of the polymer to aggregate in solution plays a significant role in determining the thin-film morphology and performance of BHJ solar cells.

The practice of combining electron rich (donor, D) and electron deficient (acceptor, A) moieties to make low-bandgap co-polymers has led to dramatic efficiency gains in organic photovoltaics (OPV). The flexibility afforded by combining different D and A building blocks to make distinct co-polymers allows for the tuning of the optoelectronic properties of the active layer to improve device performance. While the properties of the active-layer material may be estimated with a variety of electronic structure methods, the interactions between polymer chains, which can significantly impact charge migration, requires simulations in order to estimate morphologies.
One avenue for predicting the morphology of candidate materials is via molecular dynamics (MD) simulations using classical force fields. However, accurate force fields for conjugated D-A systems need to be developed. In particular, the torsional potentials in standard force fields have not been parameterized for the sorts of conjugated fused-ring systems typically used in OPV. Incorrect torsional potentials will lead to incorrect amounts of twist along the backbone, and hence incorrect packing between chains. Therefore, torsional potentials for D-A polymers have been explored based on MP2 ab initio calculations. The forms of these potentials as functions of D-A identities and of molecular weight and conjugation length have been investigated and classical force field parameters have been established. We discuss the possibility of creating a transferrable torsional potential for conjugated systems, including what appear to be the limits of such potentials. Atomistic MD simulations of oligomer films for high efficiency D-A copolymers have been performed, allowing for the prediction of packing motifs that have been related to experimental results such as X-ray diffraction, time-resolved microwave conductivity, and device characteristics. Finally, we address implications of these results for extending molecular design of active layer materials beyond isolated-molecule optoelectronic properties to include properties of films.

Fluorine atom has been introduced onto the many kinds of molecules owing to the interesting properties, which are a result of the highest electronegativity and small atomic radius. Among the diverse molecules with fluorine atom, semi-fluorinated alkyl chains exhibit self-organization behaviors as well as thermal stability, rigidity and hydrophobicity due to the intermolecular interaction between the hydrocarbon and fluorocarbon. Furthermore, introduction of semi-fluorinated alkyl chains into molecules leads to closely packed structures via self-assembly. In general, well-ordered structures are known to be favorable for efficient charge transport. Therefore, we synthesized polythiophene derivative containing semi-fluorinated alkyl chains by Grignard metathesis (GRIM) method, and fabricated organic field-effect transistors (OFETs) based on polythiophene with semi-fluorinated alkyl chains. Self-assembly behaviors and morphological changes were analyzed by means of AFM. In addition, thermal, optical and electrochemical properties were measured by using DSC, UV-vis spectroscopy and cyclic voltammetry. Finally, we will discuss how self-organization affects morphologies of thin film and performance of OFETs.

Over the past few decades, extensive efforts in design and synthesis of π-conjugated molecules have made remarkable progress in the field of organic electronics; in particular, organic field-effect transistors (OFETs) exhibited high performance exceeding that of amorphous silicon. In spite of various kinds of conjugated building blocks, such as acenes, heteroaromatics, fused moieties and etc., there still remains the issue of developing new organic semiconducting materials with efficient charge transport. Recently, conjugated small molecules based on quinoid structure have emerged as promising candidates for active semiconductors in OFETs due to their highly planar structure and more delocalized π-electrons. Commonly, well-known synthetic strategy to obtain quinoidal molecules is substitution of dicyanomethylene (DCM) group at both termini. However, quinoidal molecules containing DCM group have some drawbacks such as low solubility and too high electron affinity. Thus the synthetic strategy to attain quinoidal molecules without DCM group is challenging. Here, we designed and synthesized novel type of quinoidal small molecules based on chalcogenophene. Due to the dodecyl alkyl chain on the quinoidal conjugated backbone, these quinoidal small molecules could be processable via not only vacuum deposit but also spin coating. In order to estimate electrical properties of quinoidal small molecules, we fabricated and characterized OFET devices using both vacuum and solution processing.

A series of bi-component single crystals were grown with the silylethynylated N-heteropentacenes. [1] Compared with previously reported cocrystals in which strong donor-acceptor interaction lead to alternating D-A structure, [2] our bi-component crystal has its unique properties with molecules similar in both shape and crystal lattice. Like a solid solution, ratio of the two components could be tuned as any value in a wide range. Hence we apply the term “molecular alloy” to such system for the first time, describing a nature of mixing in molecular level without breaking the pristine ordered π-π stacking motif.
Among the silylethynylated N-heteropentacenes, pristine material could be p-type, n-type or even insulator depending on number and position of N atoms. With different combination and tunable ratio of two components, conducting nature of the molecular alloys could be adjusted.
To study both optical and electronic properties of these novel bi-component semiconductors, organic thin film transistors (OTFTs) were constructed by vacuum-deposition method with the molecular alloys as source material. Besides, by depositing alternating ultrathin films of two components, a multilayer structure with phase separation in nanometer scale in bulk was also created for comparison. Difference in the thin film absorption, photoluminescence spectrum, surface potential landscape as well as I-V transfer property was observed, suggesting that mixing in molecular level has led to new band-gap and properties as well.
References:
[1] a) Z. Liang, Q. Tang, R. Mao, D. Liu, J. Xu, Q. Miao, Adv. Mater. 2011, 23, 5514-
5518. b) Z. He, D. Liu, R. Mao, Q. Tang, Q. Miao, Org. Lett. 2012, 14, 1050-1053.
[2] a) T. J. Kistenmacher, T. E. Phillips, D. O. Cowan, Acta. Cryst. 1974, B30, 763. b) N. J. Tremblay, A. A. Gorodetsky, M. P. Cox, T. Schiros, B. Kim, R. Steiner, Z. Bullard, A. Sattler, W. So, Y. Itoh, M. F. Toney, H. Ogasawara, A. P. Ramirez, I. Kymissis, M. L. Steigerwald, C. Nuckolls, Chem. Phys. Chem. 2010, 11, 799 - 803. c) J. Zhang, J. Tan, Z. Ma, W. Xu, G. Zhao, H. Geng, C. Di, W. Hu, Z. Shuai, K. Singh, D. Zhu, J. Am. Chem. Soc. 2013, 135, 558minus;561. d) S. K. Park, S. Varghese, J. H. Kim, S. Yoon, O. K. Kwon, B. An, J. Gierschner, S. Y. Park, J. Am. Chem. Soc. 2013, 135, 4757minus;4764.

The correlation between morphology on the molecular level and the electrical properties of organic semiconductors and dielectrics such as self-assembled monolayers (SAM) [1] is of great importance for the progress of organic thin-film transistors (OTFT). Tuning the 2D-order by chemical design of molecules leads to improved performances in OTFTs.
X-ray reflectivity (XRR) has been identified as one of the rare excellent surface sensitive techniques, to investigate the in-depth structure of nanometer scaled SAMs or very thin molecular assemblies on smooth substrates [2, 3]. The information provided by XRR is the in-depth electron density profile accompanied by the thickness and roughness of the surface and interfaces.
We show the power of the XRR method for simple, mixed and electroactive SAM molecules. XRR measurements of alkyl-phosphonic acids (PA) provide information on film thickness and tilting. In mixed SAMs of n-alkyl-PAs, doped with C₆₀-PAs, the mixing ratio, the vertical arrangement and the overall order can be estimated from electron density profiles of fullerene moieties in the SAM. (Experiments were performed at ID10, ESRF, Grenoble).
Additionally highly ordered electrical active BTBT-C₁₂-PA SAMs (BTBT: benzothiophene) were investigated in SAMFET devices on rough and flexible substrates. XRR was used to proof the in-depth structure of BTBT-C₁₂-PA SAM on atomic layer deposited (ALD) AlO_x. It was shown that the monolayer is homogenously assembled on the surface with a thickness of approx. 2.55 nm corresponding to the length of the molecule with a surface normal alkyl chain and tilted BTBT chromophore. The device channel is confined to only 0.76 nm in thickness of assembled BTBT π-system, but still exhibit remarkable charge carrier transport [4]. (Experiments were carried out on a Bruker D8 reflectometer).
[1] M. Halik and A. Hirsch, Adv. Mater. 23 (2011), 2689-2695.
[2] C.M. Jäger et al. J. Am. Chem. Soc. 135 (2013) 4893 - 4900.
[3] A.Y. Amin et al. J. Am. Chem. Soc. 134 (2012) 16548-16550.
[4] T. Schmaltz et al. Adv. Mater. 25 (2013) 4511-4514

Complex three-phase morphologies including pure PCBM aggregates, pure polymer aggregates and a mixed phase, which is thought to consists of dispersed PCBM in mostly amorphous polymer, have been observed or inferred in several cases. The structure and importance of the mixed phase have not been fully exploited or understood. We reveal and quantify for the first time the hierarchical structure in a promising and high performance polymer:fullerene blend, which contains polymer-rich/PCBM-rich macro-phase separation and a nano-phase separation within the polymer-rich domains. The detailed structure-performance relationship is established by symmetrically manipulating two characteristic length scales (xi;) which correspond to the macro- and nano-phase separation, respectively, and the average composition variations () at these two length scales using six different solvents combination. Using resonant soft x-ray scattering, we find that the macro-phase separation length scale xi;2 is roughly four times larger than nano-phase separation xi;1. Also, anti-correlated composition variations are observed between the two-length scales of phase separation for the first time. Furthermore, we find that the internal structures of the polymer-rich phases are important for charge separation and charge transport and is intimately correlated with device performance and the partial quantum efficiency of the polymer and fullerene constituents. The total photocurrent generated from PCBM and polymer is considered separately and correlated with length scale of macro- and nano-phase structure. FF shows an interesting correlation with the average composition variations. Both very pure domains and impure domains are not favorable for FF due to either bimolecular recombination loss or the lack percolation pathway. The highest FF is achieved when the suitable mixed phase and the balanced composition variations of macro-phases and nano-phases separation are formed. This is counter to prevailing assumptions of device operation that are based on a two-phase, interconnected morphology. This work is thus an important elucidation of the device operation paradigm of real devices, where morphologies are more complex.

Electrical characteristics of organic field effect transistor (OFET) devices can be improved by using polymer-molecular semiconductor blend thin-films as the active layer. We demonstrated that incorporation of the small molecule, 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT), into poly(3-hexylthiophene) (P3HT) polymer thin-films can enhance charge carrier mobility and lead to approximately a 15-fold decrease in threshold voltage. The performance enhancement was dependent on the small molecule volume ratio, and was maximized at a critical oligomer concentration of about 50%. This thin-film blend combines the high charge transport properties of the crystalline small molecule semiconductor with the facile solution processability of the polymer component. In this investigation, BTTT was selected because of its planar symmetrical fused aromatic rigid frame, i.e., thieno[3,2-b]thiophene, and because relative to P3HT, the thienothiophene is oxidatively more stable. Moreover, the alkyl side chains may facilitate interdigitation and formation of a compact lamellar structure. By taking advantage of these features, the crystalline small molecule might enhance P3HT molecular ordering. Alternatively, the polymer matrix might serve as a bridge between the solution processed small molecule crystalline domains to decrease the impact of crystal grain boundaries. Correlations between molecular ordering and the charge transport characteristics of resultant thin-films with respect to the polymer:small molecule volume ratio will also be presented and discussed.

The greatest challenge facing the development of organic photovoltaics (OPV) is combining high efficiency, stability and processability. The operational lifetime of encapsulated polymer solar cells under solar illumination recently reached a few years. Degradation mechanisms in polymer solar cells are complex and include a variety of inseparable processes which can be instigated by the same factors: exposure of the device to light, heat, water and/ or oxygen.
Concentrated sunlight was recently suggested for accelerated studies of light-induced degradation of OPV . However, heating is very significant under concentrated sunlight. We demonstrate the first experimental realization of independent control of light intensity, the sample temperature and environment during the exposure. We find that heating is caused by absorption of photons at all wavelengths, not only due to IR photon absorption. P3HT degradation mechanisms in a P3HT:PCBM blend in the presence of oxygen are similar under 1-100 suns, with and without heating, though the degradation rates are different. Therefore, combined heating and concentrated sunlight illumination may allow significant acceleration rates for stability testing.
Experiments with well encapsulated P3HT:PCBM films under sunlight concentration up to 4800 suns and dose up to 3600 sun hours, in the absence of heating, revealed no P3HT photobleaching and slight degradation in PCBM absorption. Corresponding exposure of P3HT:PCBM-based, ITO-free cells resulted in significant degradation probably due to deterioration of charge collection in the devices. We conclude that P3HT:PCBM-based cells can be stable if cell heating is limited and current collection degradation issues are solved.

Conductive polymers have enabled the development of many flexible electronic devices, including organic light emitting diodes (OLEDs), organic photovoltaics (OPVs) and organic thin film transistors (OTFTs). However, organic materials are often more mechanically fragile than their inorganic counterparts with a high tendency for adhesive and cohesive failure. While increasing studies have reported on the critical adhesion or cohesion energy of active layers in organic devices, this does not provide information regarding the time-dependent sensitivity to environment parameters. Such debonding kinetics is critical for the device reliability and lifetime.
In this study we demonstrate how moisture, temperature and UV photons accelerate debond propagation at mechanical stresses well below those required for critical failure. We use a thin-film decohesion technique to measure the debonding kinetics under systematically varied environmental conditions with changing temperature, relative humidity, UV intensity and wavelength. For example, in an inverted polymer solar cell, the interface between the bulk heterojuction layer, poly(3-hexyl)thiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM), with the conductive polymer poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS) layer is the weakest interface in the absence of any reactive environmental species. However, when species are introduced, the bulk layers and other interfaces in the OPV structure become more susceptible to debonding. The cohesion of the PEDOT:PSS layer is significantly influenced by moisture along with temperature and mechanical loads. Elucidating the kinetic mechanisms using atomistic bond rupture models supports that decohesion is facilitated by a chemical reaction between water molecules from the environment and strained hydrogen bonds. We also explore the role of UV photons and temperature on the resistance to time dependent debonding in the P3HT:PCBM layer and its interfaces. This extensive series of quantitative analysis provides the impact of the different environmental species and most importantly their synergies, leading us to an in-depth understanding of the debonding mechanisms.

Due to the shape anisotropy inherent to many aromatic small molecules, the orientation of these molecules within thin films and at various interfaces affects optical properties such as absorption and emission as well as electrical properties such as charge transport, charge transfer and energy level alignments. Despite its significant impact on device performance, the effect of molecular orientation in thin film devices has been relatively unexplored, especially in diode structure relevant for organic light emitting diode and organic photovoltaic applications. In our study, the orientation of two important classes of organic molecules, Acenes and Phthalocyanines, is controlled by depositing on appropriate surfaces and its influence on optical and electrical properties is investigated.
First, we will present the results of morphological and structural investigations on epitaxial growth and preferred molecular orientation of vacuum-deposited small molecules on various surfaces. A combination of complementary methods including grazing-incidence wide- and small-angle x-ray scattering, near-edge X-ray absorption spectroscopy, scanning electron and atomic force microscopy shows that these molecules adopt edge-on orientation with relatively smooth morphology on metal oxide surfaces such as (ITO, MoO3, SiO2/Si) as well as on polymer surface such as PEDOT:PSS. In contrast, face-on stacking orientation is observed on the surface of graphene, inorganic layer CuI and organic layer PTCDA. While thin film morphology of these molecules on CuI and PTCDA is smooth, assembly of nanostructured domains were observed on graphene surface. The effect of orientation in co-deposited bulk-heterojunction small molecules system is also studied and we will show that incompatibility of molecular orientation between donor and acceptor molecules could disrupt the packing structure of both molecules.

In the second part of the presentation, orientation-dependent optical and electrical properties of the organic diode and photovoltaic devices, both bi-layer and bulk-heterojunction, will be discussed. Our preliminary results show significant improvements in both optical absorption (x1.5) and mobility (x10) for face-on orientation. The effect of nanostructured D/A interface on efficient exciton dissociation and molecular orientation with respect to charge transport direction on effective charge collection will also be reported.
Acknowledgment: This study is supported by the Energy Frontier Research Center under Award Number DE-SC0001085 with additional support by the New York State Office of Science, Technology, and Academic Research (NYSTAR) and New York State Energy Research Development Authority (NYSERDA). This study was also in part carried out at the Center for Functional Nanomaterials and the National Synchrotron Light Source, Brookhaven National Laboratory supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.

In organic bilayer solar cells an accurate description of the morphology at the donor/acceptor interface is essential to understanding how these cells work. The charge separation process depends on the interfacial energy landscape, which is in part determined by the interfacial morphology. Therefore, the first step to understanding the interfacial morphology is to investigate whether a pure bilayer exists or if an intermixed phase is present. In this work we study the intermixing between fullerene C60 and a set of selected donor polymers and small molecules in a bilayer configuration. Ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy (XPS) are used to probe the intermixing between various donor materials and the fullerene C60 in frozen films and at room temperature. By monitoring the attenuation of the XPS signal from the bottom donor layer upon addition of C60, as a function of film temperature and electron escape angle, we can detect the presence of intermixing between the C60 layer and the underlying film. We observe that no intermixing occurs in the frozen films for most small molecules and for all polymers. However a sizeable amount of intermixing is observed when the fullerene is deposited at room temperature for both polymers and small molecules. In addition for some materials, the mixing occurs during deposition of fullerene C60 on the frozen donor layer while for others the clean interface observed during deposition is maintained even at room temperature. These results indicate that intermixing should not be neglected and should be investigated in bilayer systems when developing models that rely on interfacial morphology. We also study the interfacial molecular orientation and its impact of device performance in the Zinc Phthalocyanine/C60 ( ZnPc/C60) bilayer system, where the orientation is controlled by altering the the surface energetics. Based on Near Edge X-ray Absorption Fine Structure and Grazing Incidence Wide Angle X-ray Scattering results, we observe that molecular orientation at the interface is substantially different from that of the bulk. Furthermore, intermixing occurs at the ZnPc/C60 interface during room temperature deposition, which combined with a change in orientation from the bulk to the interface makes it difficult to draw definitive conclusions on the effect of molecular orientation on OPV performance. Our work demonstrates that orientational disorder and intermixing can occur at the donor/acceptor interface in bilayer solar cells and highlights the necessity of a differentiation between bulk and interface morphology in bilayer systems.

New polymers with high electron mobilities have spurred research in organic solar cells using polymer rather than fullerene acceptors due to their potential of increased diversity, stability and scalability. However, as organic solar cells based on polymer-fullerene blends stride past 10% power conversion efficiencies, all-polymer solar cells have struggled to keep up. We examine this issue by combining advanced structural and photo-physical characterization on poly(3-hexylthiphene) and P(NDI2OD-T2) blend devices, and find that the anisotropy of polymer electronic orbitals may necessitate correlated donor/acceptor molecular orientation for efficient separation of the charge transfer state. Advanced X-ray and electron-based studies reveal the effect of chloronapthalene additives in both reducing domain size and reorienting the acceptor polymer crystals to be coincident with those of the donor. This correlates well with increased photocurrent from devices. In addition, low efficiency in separating directly excited charge transfer states ties the low device efficiency to geminate recombination. Thus, the anisotropic nature of conjugated orbitals in polymers may represent an extra hurtle compared with the isotropic fullerene acceptor in realizing efficient devices.

The power conversion efficiencies of organic solar cells have significantly been improved over the last years. In this context, the application of blends of donor and acceptor type materials as photo-active layers is one of the key concepts to overcome the efficiency limitation by short exciton diffusion lengths in small molecule solar cells. Here, morphology tuning of the bulk-heterojunction (BHJ) is of tremendous importance for an efficient extraction of photo-generated charge carriers.
In this contribution, we demonstrate the impact of substrate heating during vacuum co-sublimation on the efficiency improvement in methylated DCV5T:C60 BHJ solar cells. In particular, the characteristic solar cell parameters are compared under variation of the number and position of methyl side chains attached to the DCV5T core molecule. Here, different critical substrate temperatures Tc in the range of 90...115°C are observed, each characterized by maximal power conversion efficiencies of respective solar cells of up to 5.9%. In comparison, the critical temperature increases with rising number of methyl side chains.
Additionally, grazing incidence X-ray deflection, X-ray reflection, 2D grazing wide angle X-ray scattering and atomic force microscopy (AFM) measurements are performed on pure DCV5T films as well as on blends with C60 evaporated either below or above the critical substrate temperatures. Here, the scattering patterns of samples with TOur findings clearly show the possibility of suitable morphology tuning and are, thus, highly relevant for future efficiency improvement of organic solar cells.

Recently, we demonstrated the double shot inkjet printing process (DS-IJP) which use two kinds of solvent could be one of the promising methods to fabricate organic electronic devices. For example, an organic electrode composed of a charge transfer complex can be fabricated by an alternate deposition of soluble donor and acceptor components[1]. A highly uniform organic semiconductor single crystal thin film can be grown by combining a conventional antisolvent crystallization technique with DS-IJP[2]. However, a binary microfluid mixing mechanism which gives such a high quality organic thin film has not been understood. In order to clarify such a unique film-formation mechanism through the DS-IJP, we made direct observation of the binary microliquid mixing and of the subsequent film formation by using a high-speed camera microscope.
The printing process is composed of following steps: antisolvent ink (A-ink) is first printed, and then semiconductor solution ink (S-ink) is overprinted on top of the pre-deposited microdroplet of the A-ink. So as to investigate the scale effect of the binary fluid mixing, both inks were deposited on a predefined hydrophilic surface designed on a scale length less than capillary length.
Depending on the printing conditions, the fluids dynamics can be classified into the following three categories in terms of the surface transformation of the droplet: One is the “WET” process, where the S-ink immediately covers the droplet surface of the A-ink. The "WET" proceeds with keeping the A-ink droplet surface continuously through the process, which eventually affords films with high uniformity. Second one is the “DEWET” process, where the S-ink microdroplet touches the substrate surface to exclude the A-ink droplet out. "DEWET" process undergoes temporal interruption of A-ink droplet surface which is followed by a turbulent mixing, which results in the films showing typical coffee-ring effect. The last one is the “SINK” process, where the S-ink microdroplet sinks below the droplet surface of A-ink with keeping the mass. Among them, the “WET” process affords uniform organic semiconductor thin films. Based on the results, we discuss the role of interfacial energy of the microliquid to account for the mixing mechanism.
[1] M. Hiraoka, et al. Adv. Mater. 19, 3248 (2007).
[2] H. Minemawari, et al. Nature 475, 364 (2011).

High current densities and high quantum efficiencies are prerequisites for any electroluminescent device and in particular for light-emitting field effect transistors (LEFETs). However, organic semiconductors with high charge carrier mobilities generally do not also exhibit high quantum efficiencies and vice versa. Here we present a thorough investigation of ambipolar charge transport properties and near-infrared photoluminescence and electroluminescence of four representative high mobility, low bandgap semiconducting polymers: two diketopyrrolopyrrole (DPPT-TT and DPPT-BT) copolymers, a thienoisoindigo (IGT-BT) and a benzodipyrrolidone (BPT-T) copolymer. These polymers exhibit relatively balanced hole and electron mobilities between 0.02 and 0.7 cm2/Vs and emit near-infrared light between 870 and 1250 nm. We find that those polymers with larger bandgaps exhibit higher photoluminescence quantum yields (0.008 % to 0.05%) and also higher external quantum efficiencies (1*10^-6 % to 5*10^-4 %) in ambipolar LEFETs. A clear correlation of high carrier mobility with low quantum yield does not arise. We discuss possible structure-property relationships that may explain the observed dependencies.

Polymer-fullerene bulk heterojunction solar cells have been receiving increasing attention in the past years since they have the potential to constitute a low-cost, simple technology for alternative energy generation. This interest has been translated in an increase in device efficiencies that go from 2.5 to 11% in less than ten years, which suggests that the commercialization of OPVs may be possible in the near future, provided we are able to solve some technical issues,[1] an example of this, the on-going discussion on whether triplets lead to degradation pathways, [2] or can contribute to the generation of free charges.[3]
The population of polymer and fullerene triplet states in organic solar cells is commonly believed to be an important charge recombination channel limiting the solar cell performance,[4, 5] and a major cause for a reduction of polymer stability and hence shortening of device lifetime [6, 7]. In addition to these, triplet excitons have recently been used for improving device efficiency via direct charge generation from triplet excitons populated from singlet fission [8]. Our knowledge of these processes and in particular the basic properties of triplet excitons in polymer films, such as intersystem crossing rates, energy of the triplet states and their diffusion lengths are very unsatisfactory, although critical for our ability to improve device stability and possibly device efficiencies
Therefore, in this study, we present our most recent efforts in improving our understanding of triplet excitons in polymer/fullerene solar cells. Using femtosecond to millisecond laser spectroscopy we show that triplet states can be populated via three independent recombination channels, namely direct intersystem crossing, geminate charge recombination and non-geminate charge recombination. The rate constants of these processes and their overall efficiency were estimated for a series of polymers and PC70BM, associating their relevance to device stability.
1. Su, Y.-W., S.-C. Lan, and K.-H. Wei, 2012. 15(12): p. 554-562.
2. Distler, A., et al., Chemistry of Materials, 2012. 24(22): p. 4397-4405.
3. Yang, K., U. Scherf, and S. Guha, The role of triplet excitons in enhancing polymer solar cell efficiency: a photo-induced absorption study. ArXiv e-prints, 2009.
4. Ohkita, H., et al., 2006(37): p. 3939-3941.
5. Westenhoff, S., et al., 2008. 130(41): p. 13653-13658.
6. Hintz, H., et al., 2010. 23(2): p. 145-154.
7. Soon, Y.W., et al., 2013. 49(13): p. 1291-1293.
8. Smith, M.B. and J. Michl, Singlet Fission. Chemical Reviews, 2010. 110(11): p. 6891-6936.

We report p-type bottom-gate organic field effect transistors made of poly(3-hexylthiophene) (P3HT) as channel material and poly(methyl methacrylate) (PMMA) as dielectric layer. The architecture of the device plays an important role on the versatility of transistors in electronics. Bottom-gate transistors, in particular, are well suited for dielectric modification and channel characterization by spectroscopy techniques. Transistors with channel length ranging from 50 µm to 200 µm, and W/L around 20, were produced yielding currents of the order of 1 µA and the hole mobility close to 10-2 cm2/V.s. As an attempt to understand the physical phenomena responsible for the electrical behavior of OFETs, we developed a model for current in transistors based in a generalization of Vissenberg-Matters model. This model is derived by a Poisson equation that is numerically solved taking into account a two dimensional dependence of the voltage in the channel, due to a superposition of source-drain and gate voltages. Finally, the electric current is calculated in terms of a resistor network.

Organic photovoltaics (OPVs) have been fabricated with power conversion efficiencies (PCEs) of approximately 10%, comparable to other thin film photovoltaic technologies such as amorphous silicon. Additionally, OPVs are capable of being solution processed on mechanically flexible substrates using high volume manufacturing techniques such as roll-to-roll coating. Efforts to commercialize this technology, however, may be hindered by the difficulties and expense associated with the production of the functionalized fullerenes utilized in the vast majority of OPV devices. Functionalized fullerenes also exhibit relatively low absorption coefficients within the terrestrial solar spectrum. There then exists motivation for replacing fullerenes in OPVs with materials that likewise exhibit high electron mobility and enable efficient charge generation but that can be inexpensively synthesized and that absorb more solar photons.
Several BHJ OPV systems utilizing fullerene acceptors have been reported to exhibit internal quantum efficiencies approaching unity. In contrast, even the most efficient BHJ OPV systems utilizing non-fullerene acceptors are plagued by significant geminate recombination and therefore low PCEs compared to high performance fullerene-based devices. Additionally, even less is known about charge generation in non-fullerene BHJ OPV systems compared to those utilizing fullerene acceptors. There then exists a need to better understand the charge generation process in non-fullerene BHJ OPVs so that the amount of geminate recombination in these systems can be reduced. The suppression of geminate recombination in non-fullerene systems is crucial to the fabrication of an efficient BHJ OPV that utilizes a readily synthesized, and therefore scalable, acceptor material.
We have thus investigated the photovoltaic performance, morphology and charge generation and transport in a small molecule donor:perylene diimide acceptor solution-processed BHJ OPV system with and without the solvent additive diiodooctane (DIO). This system exhibits a PCE of 3.0% when fabricated using DIO, making it among the highest performing non-fullerene BHJ OPVs.[1] As this system exhibits dramatically different photovoltaic performance when processed with and without DIO, it allows the comparison of differences in blend morphology induced by DIO with various photophysical and electrical processes to produce a structure-property-processing relationship for a non-fullerene BHJ OPV device. UV-Vis and transient absorption spectroscopy, as well as single carrier diodes, reveal that the development of donor and acceptor crystallinity via the use of DIO correlates with a significant increase in charge carrier mobilities, and perhaps more significantly, charge generation efficiency, consistent with the dramatic increase in device PCE when using the solvent additive.
[1] Sharenko, et al. Advanced Materials. DOI: 10.1002/adma.201301167

Solar cells based on the polymer:fullerene bulk heterojunction (BHJ) represent one of the most promising technologies for next-generation solar energy conversion due to their low-cost and scalability. In the last ten years, the highest power conversion efficiency (PCE) obtained from polymer:fullerene BHJ solar cells, has risen from 2.5 to 12% through the development of new materials. However, when organic solar cells are manufactured in large area with high-throughput techniques, the efficiencies are much lower, usually lower than 3.5%. This performance gap is standing in the way of market penetration for this technology. One of the reasons is mainly due to the poor control of morphological changes in the active layer when shifting from spin coating in the laboratory to high-throughput techniques on a commercial scale. The incremental changes in organic photovoltaic (OPV) materials will not get us where we need to go. To close this performance gap, a thorough understanding of the complex processing-structure-performance relationships in OPV devices is required. Among several strategies that are most often used to tune the nanomorphology of polymer:fullerene active layer, the use of processing additives has been proved to be one of the most effective methods. The incorporation of a small percentage of solvent additives results in a nearly doubling of device efficiency. So far, the role of processing additives in improving the OPV performance still remains unclear. In this work, by taking advantage of resonant soft x-ray scattering (RSoXS) and energy-filtered transmission electron microscopy (EFTEM), we have determined that the solvent additives induce the change in the formation mechanism of polymer: fullerene nanomorphologies in the process of film casting. The superior performance of polymer:fullerene BHJ solar cells is attributed to hierarchical nanomorphologies with optimum crystallinity and nanoscale intermixing of copolymers with fullerenes. Progress established in the course of these studies on solvent additive effects outline above will serve as the foundation for further improving the efficiency of polymer solar cells to realize their large-scale commercial use.

Small molecule organic thin films can be manufactured from domestically available, low toxicity materials, making them attractive materials for portable photovoltaic devices. Thin films of these materials are observed to have structure on the micro scale: small microcrystalline domains are observed in these films, and their size, orientation, shape and arrangement define this microstructure. Little is known about how this microstructure affects local photophysics or how that translates into bulk device performance, since measurements on these materials are usually performed without spatial resolution on the bulk film.
In this work, the exciton dynamics of individual domains in thin films of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) were measured using transient absorption microscopy. Owing to its high hole mobility, stability in air and solution processibility, TIPS-Pn is a promising material for high-performance organic semiconductor devices. Thin films of TIPS-Pn consist of long, thin domains, and it is unknown whether the electronic structure of different domains are the same, and how this electronic structure is perturbed at domain boundaries. By utilizing sub-domain sized excitation volumes, transient absorption was measured in individual domains and across boundaries. By use of this approach and a simple kinetic model, the signature of singlet fission as well as vibrational relaxation of the initially excited singlet state is identified. Unique transient absorption signals were measured at domain boundaries, indicating that the electronic structure at the boundary is significantly different from that of the surrounding domains, and that other photophysical processes may occur in these regions. A deeper understanding of the relationship between photophysics and film morphology will inform the selection and preparation of these materials to improve future device performance.

Hybrid silicon/polymer photovoltaic cells (PVCs) with power conversion efficiency close to 10% were realized. To further improve such PVC performance, it is important to know the fundamental mechanism that leads to mobile charges carrier generation, which we elucidate here for junctions formed by n-doped crystalline Si and three p-type polythiophene derivatives [poly(3-hexylthiophene-2,5-diyl) (P3HT), poly-(3-[3,6-dioxaheptyl]-thiophene) (P3DOT), and poly-(3-[2,5,8-trioxanonyl]-thiophene) (P3TOT)]. We determined the energy level alignment at these hybrid interfaces with photoelectron spectroscopy and found evidence for the formation of an inversion layer within Si at the interface. The deposition of the polymers increased the surface band bending in the Si by up to 0.40 eV, as determined from the Si 2p core level shift. The polymer valence band onset was found ca. 0.10 eV below the Fermi level similar, i.e., reminiscent of Fermi level pinned polymer films on high work function substrates. The results are discussed with respect to the increase in open circuit voltage by 150 meV in PVCs in the sequence P3HT, P3DOT, and P3TOT. This increase is attributed to a bandgap widening of P3TOT due to reduced intermolecular coupling within thin films.

Organic photovoltaic (OPV) cells have been a topic of research focus in recent years as they are a low cost renewable energy source due to their compatibility with large scale, flexible and high throughput roll-to-roll production. Even though power conversion efficiencies over 10% have been reached for polymeric solar cells, most reported high efficiency results are based on solar cells processed in nitrogen atmosphere in order to avoid oxygen and moisture which are the main causes for degradation. For this technology to be commercialized, polymers that can withstand oxygen and moisture without significant degradation during processing are desirable.
In this report, polydithienogermole-thienopyrrolodione (PDTG-TPD) was used to demonstrate good air processability while maintaining high efficiency. A bulk heterojunction device was made with P(DTG-TPD) as the donor and PC70BM as the acceptor. Comparing devices made in air and in nitrogen atmosphere, only a 7% decrease in device performance was observed for the devices with the polymer film processed in air. Efficiencies over 6% were achieved with an open circuit voltage of 0.83V and a short circuit current of 12.6 mA/cm2 which to our knowledge is the highest efficiency reported for OPV cells with an active layer processed in air.

Although PEDOT:PSS is highly conductive in its positive oxidation state, its conductivity decreases by more than three orders of magnitude when it is reduced to its neutral state. Several groups have demonstrated that this redox switching behavior of PEDOT can be used as the basis for the modulation of channel current in organic electrochemical transistors (OECTs). When positive ions are driven by a gate voltage into the transistor channel, they interact with the PSS anions, displacing them from the PEDOT, and ultimately reducing the PEDOT to its non-conductive state. In this work, we examine the long-term time response of OECTs, and we demonstrate that there is an irreversible component of the redox switching. Our findings suggest that this irreversibility is due to deeply intercalating ions that change the morphology and electrostatics of the film. We fabricate our OECTs on glass with evaporated gold electrodes. The PEDOT:PSS is spin-cast to form an approximately 100 nm film. OECT channels are defined by subtractively patterning the PEDOT:PSS film using a microcutter, and the solid-state electrolyte is printed from solution using a microcontact printing platform. We characterize these devices with drain voltages ~100 mV and a square wave gate voltage stepped from 0 V up to ~2 V. We observe that the initial ON state drain current is about 50 µA, but that within the first ~10 gate voltage cycles, the ON state current drops to ~40 µA. We examine the effect that different gate voltage magnitudes and frequencies have on the magnitude and timescale of this characteristic decay. Furthermore, we suggest that the characteristic decay can be explained by modeling the ionic circuit in the device as a simple RC-circuit where a net charge is stored in the capacitor due to the asymmetric gate voltage square wave. This capacitive charge storage corresponds to ions being trapped in the OECT channel - thus explaining the reduction in ON state current. Finally, we discuss the implications of tradeoffs between short-term performance and long-term stability for various applications of OECTs.

Efficient organic photovoltaic cells (OPVs) based on vapor-deposited small molecule materials frequently utilize a planar-mixed heterojunction (PMHJ) architecture. In a PMHJ, a mixed layer of electron donating and accepting materials is sandwiched between neat, planar layers of the donor and acceptor. The neat layers contribute to photocurrent, and help to overcome the low rectification behavior of the uniformly mixed layer by making the anode (cathode) selective for hole (electron) collection. This selectivity reduces the dark current and maximizes the device open-circuit voltage. The use of organic semiconductors as the planar layers is not without complication, as the low charge carrier mobility in these materials can hinder efficient charge collection. In this work, we examine OPVs in which the neat donor layer between the anode and the mixture is replaced with a layer of MoO_x. An average device power conversion efficiency of (7.7±0.3)% is realized under AM1.5G (100 mW/cm²) illumination in a uniformly mixed heterojunction OPV based on the donor-acceptor pairing of 2-{[7-(4-N,N-ditolylaminophenylen-1-yl)-2,1,3-benzothiadiazol-4-yl]methylene}malononitrile (DTDCPB)-C₇₀ without any organic semiconductor transport layers. The use of MoO_x versus a neat donor layer is found to reduce the series resistance and facilitate charge collection from the mixed layer, leading to an increased fill factor and short-circuit current. To our knowledge, this result is among the highest single-cell efficiencies reported for vapor-deposited small molecule OPVs. We further find that mixed OPVs based on DTDCPB-C₇₀ exhibit good shelf lifetime, maintaining 98% of initial efficiency after one month. Thus, the removal of the neat donor layer favorably affects device efficiency without adversely impacting stability.

With the popular adaptation of smart devices, many communication methods have been applied. One of them is Near Field Communication (NFC) and it is a wireless technology working on HF band which enables the exchange of data in a short range. Usually, this NFC antenna is made by the same method used in flexible printed circuit board (FPCB). Since the use of direct printing methods for the fabrication of FPCB, a few attempts have been carried out to fabricate NFC antenna by roll to roll process. For perfect adaptation of printing NFC antenna, ideally the printed patterns should have uniformity without voids. To enhance the electrical properties of the printed patterns the conditions of the experiments has to be modified. In order to improve the printability of printed NFC antenna, controlling the pattern structure or pattern geometry which changes the ink transfer behaviors of paste [1] was applied to the gravure offset printing process. [2]
In the present study, various pastes as well as several type of pattern geometry which have been engraved to the pattern roll was investigated. In order to find the effects of pattern geometry and good printability, experiment conditions have been changed to adapt to those patterns. The pattern geometry used in this experiment consists of several types of dot and line patterns with 14 variations in patterns and alignment of patterns. Various printing conditions and pattern geometry have been tested to find good printability for wide printing area about 1mm wide. The printability of pattern geometry will be discussed for NFC antenna application.
Acknowledgement: "This work was supported by Nano-Convergence Foundation[Development of 5Omega;-level NFC antenna for smart device with nano paste fusion printed electronic technology] funded by the Ministry of Education, Science and Technology(MEST, Korea) & the Ministry of Knowledge Economy(MKE, Korea)"
[1] L. W. Schwartz, P. Moussalli, P. Campbell and R. R. Eley, Trans IChemE, Vol 76, Part A, (1998) 22
[2] S. Lee and Y. NA, Int. Journal of Precision Engineering and Manufacturing. Vol. 10, Issue 5, (2009) 123

Printed, flexible sensor systems are promising for smart labels and wearable electronics. In certain high performance applications complex computational functionalities are desirable, such as signal processing and high speed wireless communication, which are readily available in conventional silicon devices. In order to combine the benefits of flexible, distributed sensors and the high performance of silicon electronics we have developed a hybrid fabrication platform that allows for organic/printed electronics alongside pre-formed microelectronic devices using digital fabrication. This approach allows for high performance sense-and-transmit systems to be developed incorporating organic devices with little change in mechanical flexibility due to the use of small, low profile integrated circuits.

This report describes a hybrid sensing platform which reads in data from multiple printed sensors (organic or nanoparticle-based resistive light and heat sensors), processes that information at high resolution and transmits it wirelessly to a separate reading device. Starting with a flexible PEN substrate, printed components are used for sensing, multiplexing, interconnection, the antenna and ancillary passive elements, with low-profile microelectronic devices used for analogue-to-digital conversion, processing and wireless transmission. As the silicon-based components are driven at a potential less than 5 V it is desirable for all other elements to operate at the same voltage. For example, for multiplexing multiple sensor signals we have developed complementary printed field-effect transistors based on organic semiconductors using a solution-processed high-k composite dielectric. These devices can be operated at low-voltage (complementary inverters show a gain greater than 4 at a 2 V input) making them suitable for direct integration with the discrete components. Other relevant issues that will be discussed include impedance matching between the sensors and circuits, robust printed interconnection of the chips, interface electronics between printed and discrete parts, and power constraints.

The solution casting process of organic thin film transistors attracts increasing interest due to the low cost and easy manufacture, depending on the discovery of the new materials, the development of new casting methods and the increasing understanding of the casting process. Beyond the common casting methods like drop-casting and spin-coating, several new casting methods, such as doctor blading, inkjet printing, slot-die coating, etc. have been introduced to improve the device performance through the control of structure and morphology. Doctor blading Balding method, casting films via guiding solutions by a moving blade, is one of the promising methods to implement for future roll-2-roll processing. Moreover, the one dimensional movement of the blade leads in-plane alignment of the molecular deposits which could result in transistors with in-plane anisotropic performance.
Here, we demonstrate the in-plane anisotropy of the blade-cast OTFTs in mobility, structure and morphology. We present the evolution from anisotropic to isotropic films by controlling the blading speeds. Through in situ monitoring the casting process by mu;GIWAXS and high speed microscopy, we describe the transition from homogeneous to heterogeneous nucleation and its relation with anisotropy evolution.

The device performance and microstructure of a series of PTB7-based polymers with varied molecular weight and degree of fluorination are investigated. Although the energy level of the highest occupied molecular orbital is found to increase with degree of fluorination, a strong molecular weight dependence of device performance dominates any underlying fluorination-dependent trend on overall performance. Microstructural investigation using a combination of x-ray techniques reveals a striking effect of polymer molecular characteristics on film morphology, with the size of PC71BM domains systematically decreasing with increasing polymer molecular weight. Furthermore, the relative composition fluctuations of the mixed PTB7:PC71BM domain is found to systematically decrease with increasing molecular weight thus implying a more complex morphology evolution than previously known for this system. Comparing domain sizes with and without the use of the solvent additive diiodooctane (DIO), the effectiveness of DIO in shrinking PC71BM domain sizes is also found to be strongly dependent on the molecular weight of the polymer. It is found that molecular weights of at least 150 kg/mol are required for DIO to be effective in reducing the PC71BM domain size and resulting in high short-circuit current densities. Finally, it is seen that relatively high device efficiencies can be achieved with low degrees of fluorination, with an efficiency of 4.6% achieved for a degree of fluorination of only 5.3%.

Organic materials are very promising for low cost manufacturing of electronic devices including those intended for harvesting of solar energy. Furthermore, organic electronics based on small molecules has the potential to reduce dimensions of active elements to the size of the molecule which is only a few nm. In order to produce better organic devices there is a need to improve (a) nanostructure by enhancing short range order between donor and acceptor molecules and (b) microstructure by enhancing long range order across the length of the device. One possible approach to achieve the former is self organization of the constituent molecules during and after processing. Donor and acceptor molecules that have affinity for each other can be expected to self align at close proximity. Good charge overlap between the molecules enhances charge transport and functioning of the device.
What is explored in this research is enhancement of short range order by self alignment between donor and acceptor molecules. Organic molecules chosen for the study include anthracene, 2,2':5',2"-terthiophene (3T), tetrathiafulvalene (TTF), 1,4,5,8-naphtalenetetracarboxylic dianhydride (NTCDA) and 7,7,8,8-tetracyanoquinodimethane (TCNQ). Given a pair of molecules, optimal positioning of the molecules with respect to each other can be found by cluster type of geometry optimization. Calculations are performed using the DFT method with B3LYP functional and Pople type basis sets augmented with polarization functions. Based on the optimal arrangement of the molecules, proximity at the closest point, degree of overlap between the molecules, and binding energy between the molecules are estimated. Atomic models consist of an electron donating molecule paired with an electron accepting molecule.
Calculated results are as follows:
Anthracene/TCNQ: Proximity 0.42 nm, overlap 90%, binding energy 0.11 eV
Anthracene/NTCDA: Proximity 0.40 nm, overlap 90%, binding energy 0.10 eV
TTF/TCNQ: Proximity 0.36 nm, overlap 100%, binding energy 0.15 eV
TTF/NTCDA: Proximity 0.39 nm, overlap 60%, binding energy 0.16 eV
3T/TCNQ: Proximity 0.38 nm, overlap 80%, binding energy 0.16 eV
3T/NTCDA: Proximity 0.41 nm, overlap 70%, binding energy 0.11 eV
These results indicate that best matching donor/acceptor pairs are TTF/TCNQ, 3T/TCNQ and TTF/NTCDA. Binding energies are 4 to 6 times higher than the thermal energy at room temperature, which might be sufficient for organic devices where there is some degree of redundancy of the active elements. Search for other organic molecules which might provide better alignment is ongoing and additional results will be provided during the presentation.

For poly(3-hexylthiophene) (P3HT), one of the standard donor materials in organic solar cells, we previously presented methods on how to precisely control nucleation density and crystal growth conditions in thin films by swelling and deswelling of thin films in controlled solvent vapor atmospheres.(1,2) Confinement additionally allows us to direct crystallization into transistor channels leading to highly anisotropic structures. One of the main findings was that charge transport along the polymer chain is higher than along the pi-stacking.
We have recently expanded our methodology to semiconducting polymers which are reported to be hardly crystallizable and typically are combined with solvent additives upon processing from solution. Here we focus on the high potential donor-acceptor polymer PCPDTBT (poly{[4,4-bis(2-ethylhexyl)-cyclopenta-(2,1-b;3,4-b&’)dithiophene]-2,6-diyl-alt-(2,1,3-benzo-thiadiazol)-4,7-diyl}).(3) Compared to the standard donor material P3HT donor-acceptor materials show for polymer/PCBM solar cells inherently better quantum efficiencies. Until today no comprehensive crystal structure or macroscopic charge transport investigations of the pristine PCPDTBT have been published, this is mainly due to the low crystalllinity of the system.
In this contribution we will show that PCPDTBT can be nicely crystallized into fibre like structures both by careful choice of processing conditions and solvent vapor treatment. Crystallization under confinement allows to orient the fibres over macroscopic areas up to 20mu;m. Polarized Raman is for example employed to determine the overall polymer chain orientation and correlate it to the macroscopic fiber growth direction. Correlations of these findings are further related to absorption properties.(4)
(1) Crossland, E. J. W.; Rahimi, K.; Reiter, G.; Steiner, U.; Ludwigs, S. Advanced Functional Materials 2011, 21, 518-524.
(2) Fischer, F. S. U.; Tremel, K.; Sommer, M.; Crossland, E. J. C.; Ludwigs, S. Nanoscale 2012, 4, 2138-44
(3) Fischer, F. S. U.; Tremel, K.; Saur, A.-K.; Link, S.; Kayunkid, N.; Brinkmann, M.; Herrero-Carvajal, D.; Navarrete, J. T. L.; Delgado, M. C. R.; Ludwigs, S. Macromolecules 2013, 46, 4924-4931.
(4) Fischer, F. S. U.; Tremel, K.; Kayunkid, N.; Brinkmann, M.; Yager, K.; Alamgir, K.; Ludwigs S. in preparation.

An important step for bringing organic electronics from the lab to the production line is the deeper understanding of coating and printing of solvable functional materials. In the past hundred years engineers have developed a huge variety of techniques. However, while the traditional goal of coating has been achieving films with high surface smoothness and well-controlled thickness - in high quality magnetic tape and color photographic film acceptable tolerances are on the order of 1% - the key to the deposition of functional materials is control of the microstructure at the interface. This encompasses phase selection, grain size, and degree of orientation, all of which contribute to the electronic device performance. Thus the focus in coating technology is shifting from fine-tuning the fluid dynamic properties of the ink to the understanding and control of nucleation, solidification, and self-organization of the functional material. Grazing-incidence x-ray scattering offers a unique view of these complex processes in-situ and in real time.
We have recently shown that the processing-structure relation can be probed on time scales of down to 10 msec and length scales down to 10 microns [1], which is important for the understanding of the solution shearing process [2], as the meniscus passes the x-ray microbeam. We have developed a miniature shearing jig to probe the crystallization of high-performance organic semiconductors such as TIPS-pentacene and difluoro TES-ADT under the relevant conditions of coating speed, concentration, and substrate temperature [3]. The shearing stage provides access to a large variety of coating conditions, from the evaporation regime to the Landau-Levich regime [4]. First tests of coating conjugated polymers, nanoparticles, and block copolymers functional films are under way.
While a judicious choice of deposition parameters is the first step towards microstructure control, there may still be kinetic limitations of obtaining the optimum material. Post-processing steps such as solvent vapor processing or thermal treatment can further enhance the performance. We have recently shown that the grain size in TIPS-pentacene thin films can be controlled by solvent vapor annealing, yielding a performance enhancement, until the grain size grew beyond the percolation limit [5]. Hence developing proper temporal vapor pressure profiles will be crucial. The new materials processing lab at CHESS will facilitate the efficient development of sample environments in which critical steps of material processing from solution can be probed in-situ and in real time.
references
[1] D.-M. Smilgies et al., Phys. Status Solidi - Rap. Res. Lett. 7, 177-179 (2013).
[2] G. Giri et al., Nature 480, 504-508 (2011).
[3] G. Giri et al., submitted.
[4] M. LeBerre, Y. Chen, and D. Baigl, Langmuir 25, 2554-2557 (2009).
[5] H. Ullah Khan et al., ACS Appl. Mater. Interfaces 5, 2325-2330 (2013).

Weak intermolecular interactions and disorder at junctions of different organic materials limit the performance and stability of organic interfaces and hence the applicability of organic semiconductors to electronic devices. We have demonstrated control of donor-acceptor heterojunctions through microphase-separated conjugated block copolymers. When utilized as the active layer of photovoltaic cells, block copolymer-based devices demonstrate efficient photoconversion well beyond devices composed of homopolymer blends. The 3% block copolymer device efficiencies are achieved without the use of a fullerene acceptor. Resonant soft X-ray scattering and grazing-incidence X-ray diffraction results reveal that the efficient performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies with primarily face-on crystallite orientations. We can build on these initial results with the combination of Density Functional Theory, Molecular Dynamics simulations and polymer theory to design donor-acceptor block copolymers with control of charge transfer processes. For example, interfaces in conjugated block copolymers are governed by chain flexibility and the interaction parameter. As such, we can present strategies to design block copolymers with suppression of bimolecular recombination through the molecular composition and microstructure.

The ability to solution process polymers is one of a key advantage for the fabrication of large area organic bulk-heterojunction (BHJ) photovoltaic devices. The majority of the power conversion efficiencies (PCE) have been reported based on small active areas (~0.1 cm2) that are fabricated by spin-coating methods that cannot be directly transferred to device fabrication in an industrial setting. For BHJ devices to realize commercial applications, devices need to be fabricated in a roll-to-roll fashion which requires a deposition process, like blade coating. It is important, therefore to understand the evolution of the morphology and its dependence on processing parameters, like the nature of the solvent, the use of additives, solution concentration and temperature. Here, we developed a mini slot-die coater, to fabricate BHJ devices using mixtures of different low band-gap polymers and phenyl-C71-butyric acid methyl ester (PCBM). The evolution of the morphology was monitored in situ by grazing incidence wide and small angle x-ray scattering methods as a function of solvent choice, processing additives, mechanical shearing, and coating temperature. The final morphologies were also characterized by both scattering and electron microscopy methods. We observed that by controlling the drying time, additive amount, the onset of the polymer chain aggregation could be controlled and the morphology could be tuned to a preferred state, i.e. a polymer fibrillar network, forming the base for a multi-length scale morphology. Efficiencies comparable to lab-setting spin-coated devices were obtained.

We will address the synthesis and optimization of the morphology of solution processed π-conjugated oligomers and polymers, as they are applied in organic electronic devices. We seek to address the processing gap that is holding back developments in organic electronics, wherein laboratory experiments are most often carried out using spin-coating or drop-casting, yielding materials whose interfacial properties and morphologies are distinctly different from what is obtained using more practical roll-to-roll methods. We will discuss the control of morphology via a combination of structure and processing method (focusing on blade and slot-die coating), and use an organic photovoltaic (OPV) platform for materials characterization. Using thiophene-based donors and linkers, we use a Donor-Acceptor (D-A) arrangement with isoindigo, thienoisoindigo, and diketopyrrolopyrrole (DPP) acceptors. We have demonstrated how the mixing of symmetric and asymmetric molecules provide compositions that form films with small crystalline features, elevated charge mobility, and enhanced PV properties. We apply Hansen solubility parameters to the symmetric oligomers and polymers where we seek to separate the effects of molecular size from the structurally controlled dispersion, polarity and hydrogen bonding characteristics of the molecules in order to further our understanding of the parameters controlling morphology on a molecular scale.

There is an increasing interest on alternative n-type materials for substituting the commonly used fullerene-type acceptors in organic photovoltaic (OPV) devices [1]. Hitherto perylene diimide (PDI) derivatives have given OPV cells with power conversion efficiency (PCE) values of 2% - 4% [2]. It is not yet clear which factors limit the PCE values of PDI-based OPV devices below 5%. PDI molecules are rigid planar structures that tend to form aggregates which may lead to photocurrent generation losses due to the formation PDI intermolecular excited states, better known as excimers [3]. A smart way for increasing the efficiency of PDI-based solar cells is to utilize the omnipresent PDI excimers for producing photocurrent [4]. Here we study the photovoltaic blend film of the ethylpropyl-substituted PDI derivative (EP-PDI) after mixing it with the copolymer of poly[4,8-bis-substituted benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-substituted-thieno[3,4-b]thiophene-2,6-diyl] (PBDTTT). The electrical properties of the PBDTTT:EP-PDI OPV devices are correlated with the photophysical properties of the PBDTTT:EP-PDI composite layers. Our study is complemented with charge transport measurements in unipolar devices. The inter-relation between the electro-optical properties of the PBDTTT:EP-PDI system and the multi-length scale morphology features of the PBDTTT:EP-PDI layers is established in the light of optical microscopy and atomic force microscopy imaging studies. Valuable information on the hierarchical organization of the EP-PDI domains in the molecular scale is provided by wide-angle X-ray scattering characterization measurements on macroscopically oriented (extruded) fibers as a function of thermal annealing temperature. Our results provide rational guidelines for the accurate tuning of the layer microstructure in PDI-based photoactive layers of efficient OPV devices. We achieve the realization of PBDTTT:EP-PDI OPV cells that exhibit a PCE~2% (0.81 Suns, AM1.5G). Short and partially disordered PDI columnar aggregates permit the dissociation of the slowly diffusive PDI excimers at the PDI/polymer interfaces and favour electron transport [4].
[1] Chochos, C. L.; Tagmatarchis, N.; Gregoriou, V., RSC Advances 2013, 3, 7160
[2] a) Rajaram, S.; Shivanna, R.; Kandappa, S. K.; Narayan, K. S. J. Phys. Chem. Lett. 2012, 3, 2405, b) Sharenko, A.; Proctor, C. M.; van der Poll, T. S.; Henson, Z. B.; Nguyen, T.-Q.; Bazan, G. C. Adv. Mater. 2013, 25, 4403, c) Zhang, X.; Lu, Z.; Ye, L.; Zhan, C.; Hou, J.; Zhang, S.; Jiang, B.;Zhao, Y.; Huang, J.; Zhang, S.; Liu, Y.; Shi, Q.; Liu, Y.; Yao, J. Adv. Mater. 2013, DOI: 10.1002/adma.201300897
[3] a) Keivanidis, P. E.; Howard, I. A.; Friend, R. H., Adv. Funct. Mater. 2008, 18, 3189, b) Howard, I. A.; Laquai, F.; Keivanidis, P. E.; Friend, R. H.; Greenham, N. C., J. Phys. Chem. C 2009, 113, 21225.
[4] Ye, T; Singh, R.; Butt, H.-J.; Floudas, G.; Keivanidis P. E., ACS Appl. Mater. Interfaces, 2013, accepted

In recent years there has been a great deal of interest in using highly ordered materials and photonic architectures to optimize the performance of organic optoelectronics. Numerous studies have detailed morphology-dependent electronic properties in organic semiconductors, and identified device architectures which maximize performance by considering the presence of strong anisotropies. The origins, strengths, and impacts of morphology-dependent optical properties, in contrast, are considerably less well understood. Here, we report a suite of optical characterization experiments that elucidate the relationship between structure and optical properties in small-molecule and polymer materials, and theoretically demonstrate an organic photovoltaic architecture which exploits optical anisotropies to drastically increase thin-film absorption.
Our experimental studies focus on small-molecule (copper pthalocyanine; CuPC) and polymer (P(NDI2OD-T2)) materials which we can deposit with different morphologies by varying the processing conditions. We perform a suite of optical spectroscopies, including spectroscopic ellipsometry, UV-Vis-NIR transmittance, and photoluminescence measurements. By controlling morphology and using angle-resolved spectroscopies, we demonstrate highly anisotropic optical properties which depend on the morphologies of these materials. Knowledge of how material structure determines optical properties will be vital for our understanding and use of these materials in any optical device architecture. To illustrate this point, we present theoretical studies of plasmon-based light trapping, demonstrating significant improvement in thin-film absorption when materials are properly aligned with the highly anisotropic plasmon fields. Finally, we discuss ongoing efforts to use angle-resolved spectroscopy to measure the plasmon dispersion in oriented molecular films, thus demonstrating the strong interplay between molecular orientations and device geometries.

Small-bandgap polymers are promising materials to serve as electron donors in the photoactive layer of bulk heterojunction solar cells. In combination with a suitable electron acceptor such as PC70BM the photoactive layer covers a broad absorption range spanning from the visible to the near-infrared spectral region leading to increased photon harvesting and thus a higher photocurrent compared to mid-bandgap polymers such as P3HT.
In this study we investigate the impact of morphology and charge carrier mobility on the Charge Carrier generation and recombination in an organic bulk heterojunction solar cell consisting of the small band gap diketopyrrolopyrrole-oligothiophene copolymer PDPP5T and PC70BM. The exciton and charge carrier dynamics in PDPP5T:PC70BM photovoltaic blend films were investigated by time-resolved transient absorption spectroscopy (TAS) and quasi steady-state photoinduced absorption spectroscopy (PIA). Charge carrier mobilities were determined by fitting current-voltage characteristics to a space charge limited current (SCLC) model and the blend morphology was investigated by atomic force microscopy (AFM).
In a simple single layer solar cell device structure a blend of the PDPP5T-copolymer with PC60BM has shown power conversion efficiencies of 5.3%, exhibiting a high fill factor of 0.65.[1] Furthermore, it has been shown that in organic thin film transistors (OTFT) PDPP5T yields hole mobilities of 1.08 cm2 Vminus;1 sminus;1 without annealing and 3.46 cm2 Vminus;1 sminus;1 after annealing at 200°C in air athmosphere[2].
In this work we show how the PDPP5T:PCBM blend morphology, which can be controlled by solvent additives, affects the exciton dynamics and charge carrier generation in organic solar cells under open circuit conditions. We correlate the non-geminate and geminate charge recombination and charge carrier mobility with device efficiency. Futhermore, we investigate the dynamics of triplet states generated in the photovoltaic blend.
References
[1] Gevaerts, V.; Furlan, A.; Wienk, M.; Turbiez, M.; Janssen, R. Advanced Materials 2012, 24, 2130-4.
[2] Yi, Z.; Sun, X; Zhao, Y.; Guo, Y,.;Chen, X.; Qin, J.; Yu, G.; Lu, Y; Chemistry of Materials 2012 24 (22), 4350-4356

Microstructures for organic photovoltaics (OPV) are often achieved by processing thin films of highly interacting semi-crystalline polymer and fullerene derivatives deposited from solution. However, the resulting microstructure is often unstable and very sensitive to processing parameters, thus leading to poor reproducibility and incompatibility with efficient, low cost and large scale manufacturing. Further, morphologies coarsen with time and component diffusion may take place in the temperature range of normal device operation. It is known that fullerene crystallisation has a huge impact on device behaviour. However, the observation of different size and shape of fullerene crystals in different polymer matrices is not yet understood. Therefore, understanding the dynamic self-organisation of macromolecules and microstructure development is required to control and optimise thermal processing and ageing of OPVs. We select as a model system amorphous regiorandom poly(3-hexylthiophene-2,5-diyl) (RRa-P3HT) and crystalline phenyl-C61-butyric acid methyl ester (PC60BM) deposited from chlorobenzene (CB) by spin-coating. We establish the ternary phase diagram of the system using UV-Vis absorption spectroscopy and differential scanning calorimetry (DSC). We then study the nucleation and growth of PCBM crystals into RRa-P3HT matrix as a function of fullerene loading, solution concentration, film thickness, supercooling and surface energy (tuned by UV ozonolysis) using optical and atomic force microscopy, and DSC. We show that the shape of PCBM crystallites can be tuned from needles to spheroidal crystals by increasing undercooling applied to the thin film. We argue that the different behaviour of polymer:PCBM blends can be rationalised in term of undercooling, viscosity and fragility of the composite. We finally evaluate the role of additives semi-crystalline polymer and silica nanoparticles, on the crystallisation of PCBM. By mapping the impact of the processing parameters in this model system, we establish correlations with thin film self-organisation and a simple strategy towards controlling PCBM crystallisation at relevant lengthscales for OPV performance.

Polymers based on alternating benzo[1,2-b:4,5-bprime;]-dithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) units (PBDTTPD) are promising materials for bulk heterojunction organic solar cells on account of the potential to achieve high power conversion efficiencies as well as open circuit voltages approaching 1V. The performance of PBDTTPD in solar cells is strongly affected by the side chains attached to the conjugated backbone; exchanging linear alkyl side chains for a combination of linear and branched chains on the BDT and TPD units leads to a remarkable performance improvement, resulting in 7.9% power conversion efficiency (PCE), via an improvement in all figures of merit. Using transient absorption spectroscopy to monitor the generation and recombination of charges, we find that although the exciton quenching and ultrafast charge generation steps are not strongly affected by side chain modification, the linear-branched polymer exhibits both slower non-geminate recombination and a lower fraction of sub-ns geminate recombination. This leads to an overall doubling of the long lived charge yield and improves both the obtained short circuit current and the fill factor. Both these effects can be ascribed to the increased order in the films obtained when branched side chains are used, with an increase in the π- π stacking intensity and in the alignment of polymer domains with respect to the substrate(as measured using grazing incidence X-ray scattering, GIXS). These results demonstrate a direct link between recombination rates and the extent of ordering in the polymer domains, induced by side chain modification, and will be of significant use to researchers designing new polymers for organic photovoltaic applications.

We present the computational modeling of several configurations of polymer photovoltaic cells with an active organic polymer poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). These cells use the organic polymer poly-(3-hexylthiophene) [known as P3HT] doped with the acceptor phenyl-C61-butyric acid methyl ester [known as PCBM]. This polymer mixture composes the active layer of the solar cell. Organic photovoltaic materials such as P3HT:PCBM are usually cheaper than inorganic materials, but come at the price of lower power conversion efficiency (PCE). A sample P3HT:PCBM solar cell is modeled in COMSOL Multiphysics to determine the PCE of the cell at different thickness of the active layer. The cathode of the cell is aluminum and the anode is indium tin oxide (ITO). A buffer layer of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) ( PEDOT:PSS) is added between the ITO and P3HT:PCBM layers. The modeling describes the thickness vs. PCE relationship for P3HT:PCBM solar cells. This project relates to the Space Technology (STDM) NASA Mission Directorate. We would like to thank The Louisiana Space Consortium and NASA under grant NNX10AI40H for support during this project.

For the past 20 years the standard approach to drug discovery has been the automated computational screening of chemical structure libraries to identify lead systems for further investigation and experimental development. Recent advances in the power of computational resources and the improvements in the efficiency and stability of first-principles simulation packages has made it possible to apply this paradigm to challenges in material science. It is now possible for multi-step property calculations using accurate quantum-based methods to be executed automatically for diverse chemical libraries, with the results collected in a growing data record. This record can then be sorted and mined to identify exemplary candidates and establish critical structure-property limits within a chemical design space. To date very few studies have been reported in which quantum chemical calculations are carried out in a high-throughput fashion to compute properties and screen for optimal materials solutions, however with time virtual screening will become central to advanced materials chemistry research.
In this presentation, the use of high-throughput quantum chemistry to analyze and screen a chemical structure library is demonstrated for key organic semiconductor applications including organic light-emitting diode (OLED) and organic photovoltaic (OPV) materials.

Though the crystal structure of molecular semiconductors is known to significantly impact intermolecular charge transport in organic electronic devices, tuning the crystal structure of a given molecule to access other polymorphic forms is challenging. Typically, known polymorphs are accessed through careful selection of film deposition conditions, film thickness, and substrate chemistry. Without necessitating any specifications in film deposition conditions, film thickness, and substrate chemistry, we have been able to access three polymorphs of contorted hexabenzocoronene (HBC), two of which have not been previously observed. With a thermally-evaporated, amorphous film as a starting point, we have been able to access these distinct polymorphs through post-deposition processing via thermal-annealing and solvent-vapor annealing with tetrahydrofuran. Thermally-annealing an initially-amorphous HBC film induces crystallization in a P21/c crystal structure, which we refer to as polymorph I. If the amorphous film is instead solvent-vapor annealed with tetrahydrofuran vapor, the film adopts a previously unobserved crystal structure denoted polymorph II. By altering subsequent rounds of solvent- and thermal-annealing, we can transform polymorph I to polymorph II; subjecting polymorph II to thermal annealing results in yet another crystal structure, denoted polymorph II&’.

Using our ability to manipulate the crystal structure of thin-films of HBC, we evaluated the effect molecular packing has on device performance in polycrystalline thin-film transistors (TFTs). These results indicate that changes in the crystal structure can affect the field-effect mobility by as much as an order of magnitude, though the mobility extracted also depends critically on the preferred out-of-plane molecular orientation. The preferred molecular orientation is determined primarily by the first post-deposition processing step but the crystal structure may continue to be tuned through subsequent processing. By mapping these processing-structure-function relationships, we can now rationally process HBC films to optimize device performance.

To improve the power conversion efficiency has become the grand challenge of organic photovoltaics. Currently a major bottleneck of these devices is the competition of charge transfer state dissociation and recombination at the donor/acceptor interface. Recent experiments and modeling have shown that the relevant interfacial processes can be tuned by the incorporation of a thin insulating spacer layer (LiF or oligomer), that for certain parameters suppresses charge transfer state recombination more effectively than its formation through exciton dissociation. Hence, for sufficiently thin spacer layers improved device power efficiency is achieved. In this work, we present recent progress from the theoretical perspective. The differences between different spacer-layer materials and their effects on charge-transfer kinetics are explored and the spacer thickness dependence of the short circuit photocurrent between different materials is discussed. For example, LiF spacer layers provide tunnel barriers; consequently an exponential decay of the photocurrent is found for thick layers. Oligomer spacer layers of suitable design on the other hand allow for Förster energy transfer through the layer, resulting in thickness dependence to the -6th power for thick layers. The dependence of these effects on the internal electric field is explored through theoretical studies of the current-voltage characteristics. Under forward bias, depending on the direction of electric field at the interface, formation or dissociation of charge transfer states is facilitated. The theoretical results are compared with experimental data and spacer layer design criteria for the optimization of photovoltaic power conversion are discussed.

Molecular p-doping of organic semiconductor (OSC) films is done by admixing strong molecular acceptors as dopants of an electron affinity (EA) in the range of the ionization energy (IE) of the OSC. However, the standard model for the fundamental doping mechanism assuming direct electron transfer between the frontier molecular orbitals of OSC and dopant entails a number of consequences that are in conflict with well-established concepts, such as polaron formation. Based on a combined experimental and theoretical study on pentacene doped with tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) and various additional OSC/dopant pairs [1], we propose an alternative model that rationalizes molecular doping by weak chemical interaction instead: Intermolecular ground-state charge transfer complexes (CPXs) are formed leading to a substantial energy-level splitting (in the eV range) between a doubly occupied bonding and an empty antibonding supramolecular hybrid orbital. The magnitude of this splitting, which, in a Hückel-like picture, is captured by an intermolecular resonance integral β, is determined by the individual OSC and dopant energy levels, the nodal structure of their frontier orbitals and, finally, by their packing. Upon molecular doping, CPXs are generally formed within the OSC matrix representing low-lying unoccupied states in the fundamental OSC gap. As available states are occupied following Fermi-Dirac statistics, only a fraction of the CPXs is ionized at room temperature, which readily explains the low doping efficiencies and high dopant ratios used in practical applications to date.
Based on these results, we test which strategy is more promising for enhancing the efficiency of future molecular dopants [2]: increasing the dopant EA, as emerging from the standard model, or reducing β, as the model of CPX formation suggests. To this end, we employed TCNQ and its fluorinated derivatives F1-, F2-, F4-TCNQ as dopants of increasing EA, which form isostructural CPXs with alkylated benzothienobenzothiophene (C10-BTBT) as solution-processed OSC. Via absorption spectroscopy, we find the energy-level splitting to scale with the dopant EA for constant β, which is fully in-line with a Hückel-like modelling and supported further by time-dependent density functional theory calculations. In particular, we conclude that even if IE = EA, a substantial energy-level splitting remains and severely limits the doping efficiency. The design strategy emerging from the standard model, that is, increasing the p-dopant EA, is therefore not sufficient and must be supplemented by the criterion emerging from the alternative model, that is, reducing β. Overall, preventing the dopant frontier molecular orbitals from overlapping with those of the OSC by steric shielding emerges as strategy for the chemical design of future improved molecular dopants.
[1] I. Salzmann et al., Phys. Rev. Lett. 108, 035502 (2012)
[2] H. Méndez et al., Angew. Chem. Int. Ed. 52, 7751 (2013)

In bulk heterojunctions (BHJ) devices, electron donor and acceptor materials are blended to form a distributed network of D/A heterointerfaces to facilitate disassociation of optically created excitons into charges. The structural organization of the donor and acceptor molecules at the interface determines the electronic interaction among these molecules and has been shown in model bilayers to affect the charge separation process. However there is little known about the impact of D/A interface molecular orientation in BHJ solar cells. Here we probe for the first time the f-sec charge dynamics in a polymer:fullerene BHJ system in which the polymer orientation has been controlled from face-on to slightly edge on through chemical and solvent processing methods. The ultrafast charge generation dynamics revealed by polarization resolved transient absorption spectroscopy shows that charge generation in face-on structures takes place rather quickly in comparison to edge-on materials, in perfect correlation with the global device performance results as revealed by J-V data. We thus find that device efficiency and charge dynamics depends significantly on the relative orientation of the donor and acceptor molecules, with face-on polymer orientation relative to the fullerene D/A interface resulting in higher efficiency in comparison to edge-on orientation. Control of interface structure is thus a critical step in order to achieve high efficiency polymer based BHJ photovoltaic cells.

Organic photovoltaic (OPV) represents a promising route toward lightweight, flexible, and low-cost renewable energy generation. Detailed understanding of carrier dynamics in OPV devices is critical to improve their performance. However, quantifying the impact of active layer and contact layer on charge generation, transport, and recombination still needs refinement. Here, we applied simultaneous experimental and modeling approaches so that detailed knowledge in carrier dynamics in BHJ OPV devices can be elucidated. Experimentally, we examined the frequency-dependent optoelectronic response using electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS). In addition, we analyzed the current density-voltage (J-V) response, both in the dark and under illumination, using a drift-diffusion model1 that contains a spatially-resolved carrier generation profile calculated based on realistic optical properties of different layers in the OPV devices. EIS results of poly(3-hexylthiophene):[6,6]- phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole):[6,6]-phenyl-C70-butyric acid methyl ester (PCDTBT:PC70BM) BHJ OPV devices taken under different white-light illumination levels are fitted to an equivalent circuit model containing a constant phase element, yielding recombination resistance that strongly correlates to the fill factor from J-V measurement and chemical capacitance that provides information on carrier photogeneration and separation. Carrier transit times and lifetimes derived from IMPS and IMVS measurements exhibits dependence on BHJ crystallinity and thickness, consistent with published results using time-domain techniques. We incorporated spatially-resolved, instead of constant, carrier generation profiles from the transfer matrix method into the drift-diffusion modeling, and observed a significant improvement in the agreement between simulated and measured J-V data. In particular, we will discuss the effect of changing spatially-resolved carrier generation profile, which arises from contact layer thickness, active layer thickness and material, on exciton dissociation rate, exciton decay rate, and carrier generation within the framework of drift-diffusion modeling.
Reference:
1.Koster, L., Smits, E., Mihailetchi, V. D. & Blom, P. Device model for the operation of polymer/fullerene bulk heterojunction solar cells. Physical Review B (2005).

While it has been traditionally assumed that low-energy excitons in organic solids are of a single-molecule character, experiments show clear evidence of charge-transfer (CT) excitons in some organic crystals. The existence of such low-energy CT excitons can significantly affect exciton dynamics, with implications for the performance of organic photovoltaic materials. We introduce a general analysis of exciton wavefunctions aimed at understanding the influence of the solid-state on delocalization: An electron-hole correlation function, computed within highly accurate many-body perturbation theory, quantifies the extent, directionality, and degree of CT character of the solid-state exciton. We apply this analysis to solid pentacene and several other molecular crystals where there is ambiguity as to the degree of CT character in the excited-state. We demonstrate that inter-molecular interactions in the bulk lead to several delocalized and CT-like low-energy states, characterized by weak overlap between electron and hole and an average electron-hole distance approaching 1 nm. Furthermore, through a comparison of calculations with experiments and on different molecular crystals, we identify ways in which the CT nature can be tuned via molecular functionalization or change of conjugation length.

Although organic electronics have made substantial progress in the past, organic devices still lack in efficiency and long term stability. Charge carrier generation in doped transport layers is currently limited by trap distributions in the organic host material [1]. The intended loss-less power conversion in organic solar cells (OSCs) and organic light emitting diodes demands for highly purified materials to minimize non-radiative charge carrier recombination via deep trap states. Moreover, organic semiconductors are required to be inert even under ambient conditions to simplify the application of advanced fabrication techniques like photo lithography [2]. For fully functional, reliable and efficient device operation as well as progress in material design, knowledge on the inherent trap distributions and crucial processing conditions is thus mandatory.
In our work, we systematically investigate the trap response in pentacene Schottky diodes by frequency dependent impedance spectroscopy and thermally stimulated currents. Analyzing and modeling the capacitive response of the charge carrier depletion zones, we identify a deep Gaussian distributed trap center (Et = 0.6 eV, σ = 0.055 eV) whose density is in the range of 10^16 cm-3 and depends on the material purification grade. Employing bulk doping, we can shift the Fermi level towards the hole transport level and measure sequential trap filling, thus saturating the Gaussian trap and populating exponential trap states. Furthermore, we are able to resolve and profile the trap distribution close to the transport level directly by fractional thermally stimulated current method. Additionally to tailing trap states, we observe a pronounced charge carrier emission from two Gaussian shaped trap centers with depths of 110 meV and 160 meV. While these states are not substantial in pristine material, we see a strong increase in density with sample storage time (ambient air) as well as oxygen treatment of the organic films. Most interestingly, similar trends are observed in F4-ZnPc, a donor material relevant for OSCs. Together with previous findings of Schafferhans et al. in poly(3-hexylthiophene) [3], this points towards a generic trap formation in hole transporting layers induced by oxygen exposure - an observation highly relevant for organic films treated under ambient conditions.
[1] M. L. Tietze, L. Burtone, M. Riede, B. Lüssem, and K. Leo, Phys. Rev. B 86, 035320 (2012).
[2] H. Kleemann, A.A. Günther, K. Leo, and B. Lüssem, Small (published online) (2013).
[3] J. Schafferhans, A. Baumann, C. Deibel, V. Dyakonov, Appl. Phys. Lett., (93) 9 (2008).

The energetic offset between donor and acceptor materials in organic solar cells (OSCs) gives rise to a driving energy, which enables photoinduced charge transfer between the materials. The influence of this driving energy on OSC performance is still not fully understood and therefore a topic of interest in the field of organic photovoltaics.[1, 2] It is has been suggested that driving energy influences the ratio between ultrafast generated charge carriers and bound polaron pairs, also known as charge transfer (CT) states.[3, 4] Furthermore, recent results have highlighted the role of delocalised band like states in charge separation [1,5], which call into question classical models of charge separation based on Marcus theory and the Onsager model.
Here we study the role of driving energy on the rate of ultrafast electron transfer from donor to acceptor in a series of model polymer-fullerene systems, using ultrafast (< 30fs) transient absorption spectroscopy. We study films of two different polymers, PCDTBT and MDMO-PPV, each blended with the fullerenes PCBM, bis-PCBM, tris-PCBM, ICMA, ICBA, or ICTA. This allows us to vary the driving energy, the difference between LUMO levels (LUMO(D)-LUMO(A)), between 0.2 and 1 eV and study the effect of this on the rate of electron transfer. We find evidence of varying electron-transfer rates, ranging from 50-750fs and correlate this to the electronic band structure of the fullerenes.
Our results point to the role of electronic coupling and charge delocalisation in ultrafast electron transfer and long range charge separation.
References:
[1] A.A. Bakulin et al., ‘The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors&’. Science 2012, 335, 1340-1344
[2] D.C. Coffey et al., ‘An Optimal Driving Force for Converting Excitons into Free Carriers in Excitonic Solar Cells&’. J. Phys. Chem. C 2012, 116, 8916-8923
[3] F. Etzold et al., ‘Ultrafast Exciton Dissociation Followed by Nongeminate Charge Recombination in PCDTBT:PCBM Photovoltaic Blends&’. J. Am. Chem. Soc. 2011, 133, 9469-9479
[4] S.D. Dimitrov et al., ‘On the Energetic Dependence of Charge Separation in Low-Bandgap Polymer/Fullerene Blends&’. J. Am. Chem. Soc. 2012, 134, 18189-18192
[5] A.A. Bakulin et al., ‘Charge-Transfer State Dynamics Following Hole and Electron Transfer in Organic Photovoltaic Devices&’. J. Phys. Chem. Lett. 2013, 4, 209-215

We have recently explored the photophysics of the high electron mobility (up to 0.85 cm²/Vs) conjugated polymer P(NDI2OD-T2). By comparing the optical properties of P(NDI2OD-T2) solutions obtained using different solvents with the ones of thin films we have been able to determine that, under a wide range of processing conditions, approximately 40% of the polymer chains in thin films exist in an aggregated conformation, with the π-stacking enhanced by the folding of polymer chains [1].
Here, we present a complete analysis of the thin film packing of this polymer using grazing incidence x-ray diffraction revealed a strong correlation between pre-aggregation in solution (which can be tuned by dissolving the polymer in solvents with different polarity) with the formation of highly ordered crystallites in thin films. The choice of solvent proved to have a strong impact also on the orientation of the polymer chains with respect to the substrate, with polymer backbone adopting a face-on or edge-on orientation only due to the employed solvent, while keeping all the other processing conditions unchanged.
We complemented this study with the analysis of a newly synthesised regioirregular P(NDI2OD-T2) with a random linkage between the naphthalenediimide and the bithiophene units. This polymer shows rather unique properties, with a LUMO energy which is very similar to its regioregular counterpart, but a strongly reduced aggregation and the formation of highly anisotropic ordered structures with extended lamellar stacking but no measurable π-stacking.
With this knowledge, by carefully choosing materials and processing conditions, it has been possible to separately tune degree of aggregation, degree of crystallinity and chain orientation over a very broad range, and to understand the role of each of these parameters on the optical and electron transport properties of the polymer films. The chemical structure of the repeating unit proved to be responsible for the very low energetic disorder, with bulk electron mobilities never below 10^-4cm²/Vs, even for the less ordered films prepared from RI-P(NDI2OD-T2). Chain orientation and degree of crystallinity (DoC) on the other hand demonstrate the highly anisotropic nature of electron transport along the lamellar and π-stacking directions, with DoC along the lamellar stacking having very little impact on charge transport, and DoC along the π-stacking direction being very important in achieving the high mobilities reported in field effect transistors.
[1] Steyrleuthner, R. et al J. Am. Chem. Soc. 2012, 134, 18303minus;18317.

The performance of organic light-emitting diodes can be improved by adjusting the position of the carrier recombination zone in the devices. As electrochemical doping increases the electrical conductivity of organic semiconductors, it is a powerful method to influence the position of the recombination zone. We investigated the doping behavior of the transition metal oxide molybdenum oxide (MoO₃) in the charge transport material CBP (4,4prime;-Bis(N-carbazolyl)-1,1prime;-biphenyl) using photoelectron spectroscopy (PES). Sample preparation and analysis was performed under UHV-conditions. The results will be correlated with transmission electron microscopy (TEM) and infrared measurements on this material system.
From TEM-investigations, which reveal the formation of MoO₃ precipitates in the CBP matrix, it can be concluded that internal interfaces exist in CBP:MoO₃ composites. Therefore we performed a PES-interface experiment by stepwise evaporation of CBP on MoO₃ and analyzed the interface band alignment. This experiment shows that a large interface dipole of 1.15 eV reduces the driving force for wide range charge transfer. Also CBP:MoO₃ composite layers were evaporated and investigated. Concerning the Fermi level shift, which is an indicator for creation of charge carriers in CBP, three different regimes can be distinguished: At low doping concentrations (< 9 mol%) a rapid shift towards the HOMO level of CBP is observed. For higher doping concentrations the slope of the Fermi level shift becomes less strong and finally saturates at a maximum shift of 1 eV at a doping concentration of 45 mol %. The electron transfer from CBP to MoO₃ leads to the formation of a reduced Mo⁶⁺-species, which appears as shoulder structure in the Mo3d spectra. Determining the amount of these species we get information about the surface to volume ratio of the MoO₃ clusters in dependence of the doping concentration. Also here three different regimes can be distinguished and can be correlated with the observed Fermi level shift regimes. Finally the amount of transferred charges is calculated regarding the reduced Mo⁶⁺-species. The number of charged dopant molecules is in very good agreement to the number of CBP-cations that was determined by in-situ IR-spectroscopy.

Several organic photovoltaic blends can now operate with internal quantum efficiencies approaching unity, meaning that they can convert almost every absorbed photon into an electron-hole pair that does work in an external circuit. The underlying mechanism is still ill-understood. Here, we combine theoretical and experimental methods to obtain quantitative and general insight in the transient processes in operational organic solar cells. The obtained insight allows us to identify an efficient and universal mechanism for generation of free charges. The physical picture that arises is (a) that initial charge motion is highly diffusive and boosted by energetic relaxation in the disordered density of states, (b) that mobile charge carriers dissociate from and re-associate into Coulombically-associated pairs faster than they recombine, and (c) that the remaining Coulomb interaction between the nearly-free charges can be overcome by small electric fields to give rise to efficient generation of completely free charges.
In more detail, we present a rich multidimensional set of transient absorption observations which provide detailed information (on the ps to µs timescale) about the behavior of mobile, separating charge carriers in the prototypical high quantum efficiency PCDTBT:fullerene derivative blend. The measurements concurrently observe the relaxation of charge carriers within the disordered density of states (DOS), and the recombination of charge carriers at a sequence of different initial charge carrier concentrations. Understood through the lens of kinetic Monte Carlo calculations, these observations reveal how mobile charge carriers hop and relax, dissociating and re-associating potentially multiple times before recombination or extraction by contacts.
We find that a minimalistic kinetic Monte Carlo model, incorporating only exciton and charge motion in a disordered but otherwise homogeneous medium, gives an accurate description of the experimental data. This proves the relevance of the extended Gaussian disorder model, that was developed to describe charge transport in near-equilibrium situations, for gaining insight in the very-far-from-equilibrium processes governing charge generation in organic solar cells. A simple analytical calculation confirms the picture deduced from the MC simulations and hints at sub-Langevin recombination as the cause for quantitative deviations between the MC calculations and the measured concentration dependence of the charge recombination.

The lifetime of organic devices, especially OPV cells and OLEDs is in many cases determined by their susceptibility to ambient humidity. Therefore, understanding their degradation and their encapsulation remains one of the grand challenges in organic electronics.
We have studied typical bottom illuminated, pin-type OPV devices and bottom emitting, pin-type OLED devices. Both types of devices were vacuum processed on glass substrates with doped transport layers and opaque aluminum back electrodes. For these types of devices, the degradation is dominated by the humidity based degradation of the metal/organic interface at the reflective back electrode. We show the metal layer acts as a first barrier to the ambient humidity and can be treated as a series connected barrier, if combined with further encapsulation.[1]
The barrier properties of the metal back electrode are dominated by rather large pinholes (> 0.4 µm). As those pinholes can be easily characterized by optical inspection and their permeation is linear to their perimeter, the barrier performance of the metal back electrode becomes predictable. If cells without further encapsulation are exposed to humidity, the efficiency loss is well proportional to the area loss around the electrode pinholes at the metal/organic interface.
In a comparative experiment, aluminum back electrode layers have been deposited on calcium based water permeation tests as well as on the PV cells and OLEDs mentioned above. All devices have been aged at different climate conditions, to investigate their response to humidity and temperature. By evaluating the water permeation and device degradation in parallel, the crucial water dose for the time to 50% efficiency (T50 dose) for both systems has been determined (around 20 mg/m2 for OPV and 63 mg/m2 for OLED devices). From our measurements, we can show these doses remain constant for different climate conditions and determine climate acceleration factors for permeation and device aging.[2]
[1] Hermenau, M. et al. The effect of barrier performance on the lifetime of small-molecule organic solar cells. Sol. Energy Mater. Sol. Cells 97, 102-108 (2012).
[2] Hannes Klumbies, Markus Karl, Martin Hermenau, Roland Rösch, Marco Seeland, Harald Hoppe, Lars Müller-Meskamp, and K. Leo Water ingress into and climate dependent lifetime of organic photovoltaic cells investigated by calcium corrosion tests. Sol. Cells Sol. Energy Mater. accepted, (2013).

Devices with vertical architectures, such as organic solar cells and organic light-emitting diodes, require efficient out-of-plane charge transport. To investigate the influence of molecular orientation on out-of-plane charge transport, we employed graphene as a surface template to induce face-on molecular stacking in CuPc films [1]. We demonstrate that graphene templation can be used to substantially enhance the out-of-plane mobility of CuPc on substrates relevant for optoelectronic device integration.
Grazing-incidence X-ray diffraction (GIXD) of CuPc on graphene indicates that the organic semiconductor adopts a near face-on orientation with the (01-2) plane of alpha-phase CuPc oriented parallel to the substrate, having a 9^o average tilt of the molecular plane with respect to the substrate. Conversely, on glass and PEDOT:PSS/ITO, CuPc is preferentially oriented edge-on, with its (100) plane oriented parallel to the substrate. Atomic force microscopy (AFM) and angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy corroborate these conclusions. To assess the impact of graphene-templated growth on out-of-plane electrical transport in CuPc, we employed conductive atomic force microscopy (C-AFM). Space charge limited current analysis, with a correction for the tip geometry, was used to quantify the out-of-plane hole mobility, which was found to be (1.9±0.2) x 10^-3 cm²/Vs for CuPc on PEDOT:PSS/ITO, and (1.6±0.2) x 10^-2 cm²/Vs on graphene/PEDOT:PSS/ITO. The enhancement in out-of-plane mobility of CuPc through graphene-templation should be generalizable to other planar aromatic molecules, and applied to benefit organic solar cell and organic light-emitting diode performance.
[1] J. M. Mativetsky, H. Wang, S. S. Lee, L. Whittaker-Brooks, Y.-L. Loo, Chemical Communications, accepted for publication (2013).

High quality charge injection or collection contacts are very important for the realization of efficient organic devices. Interface doping is a well-known method for decreasing interface injection energy barriers, and this method is being actively pursued with vacuum-processed molecular films in which dopants can be selectively introduced within a few nanometers of the interface. However, interface dopant confinement in solution-processed films is a-priori not possible. We present here a novel method to circumvent this problem and to selectively dope the interface region of a polymer film. We use soft-contact transfer lamination of thin polymer films, which permits the positioning of a separately spin-coated doped film as a bottom or top contact to an otherwise undoped film or blend. We demonstrate that soft-contact laminated polymer homojunctions of several polymers are electronically “transparent” [1], making this technique extremely appealing for assembling devices from parts made separately. We then use separately spin-coated doped polymer films to form ultra-thin electrically and morphologically excellent contacts. Applications to solar cells and photodetectors are demonstrated [2].
[1] A. Shu, A. Dai, H. Wang, Y.-L. Loo, and A. Kahn, Org. Electr. 14, 149 (2013)
[2] A. Dai, A. Shu, H. Wang, S. Barlow, S. Mohapatra, T. Sajoto, Y. Zhou, C. Fuentes-Hernandez, Y.-L. Loo, S. R. Marder, B. Kippelen, and A. Kahn, Adv. Funct. Mat. (in press)

It is remarkable that electrons and holes separate efficiently in organic solar cells given that their coulomb attraction is substantially larger than the thermal energy. We have developed a model based on extensive structural characterization and kinetic Monte Carlo simulations that explains how the charge carrier separate. We find that it is critically important to use a hopping rate that is based on charge carrier mobilities measured by terahertz absorption measurements instead of those obtained by diode or transistor measurements because the hopping rate away from the donor-accepter interface is not limited by charges being trapped or held up at grain boundaries. Charge separation is possible because hops away from the interface are substantially faster than recombination. Consequently the charge carriers can attempt to get away from each other thousands of times before they separate or recombine.
We also find that in most highly efficient solar cells there is a three-phase morphology in which there are pure regions of polymer and fullerenes along with a mixed region. There are energetic offsets that push electrons and holes out of the mixed region and into the pure region. One can control the size and composition of the phases by adjusting factors such as the regioregularity and molecular weight of the polymer, the polymers sidechains, the donor:acceptor ratio, the choice of fullerenes, the use of processing additives and annealing conditions.

Organic photovolataic devices have seen remarkable gains in power conversion efficiency (PCE) over the last four years, with reported efficiencies for laboratory-scale devices approaching and exceeding 10% with increasing regularity. Much of the improvement can be attributed to improved active layer materials that simultaneously have low band gaps, so as to absorb more light from the solar spectrum, and large oxidation potentials, so as to produce large open circuit voltages. In order to continue improving PCEs, new active layer materials still must be developed that can absorb large fractions of the solar spectrum, produce charge from each photon with high quantum efficiency, conduct the charges well, and produce large open circuit voltages. To meet all of these criteria is very challenging because many of the desirable characteristics conflict with each other. Moreover, given the large number of possible materials one could synthesize, it is desirable to constrain the possibilities. In this talk, we describe our approach to designing novel materials based on combining combinatorial structure generation with electronic structure calculations, followed by large-scale molecular dynamics calculations of film morphologies to understand packing and charge transport. The range of computations takes us from the single-molecule level to the bulk and allows us to screen materials for desirable properties ranging from good light absorption to high charge mobility, all in advance of deciding whether or not to synthesize a material. Results on a variety of systems will be described and the computational and modeling methods needed for accurate modeling will be discussed.

One of the key parameters for charge transport is the charge carrier mobility which depends on the electrical field, the charge carrier density, and the temperature. Recently, we have shown how the field and charge carrier density dependence can be experimentally determined from a thickness variation of unipolar transport devices made of small molecules using the potential mapping method POEM [1]. For the small molecule blend layers of ZnPc:C60 (1:1), a field activation of µ(F) ~ exp(0.01radic;F cm/V) for fields of (1...5)x10^5 V/cm and hole densities of (1...5)x10^16 /cm^3 was found. A charge carrier density dependence could not be resolved in that range.
However, introducing only the field activation into a numerical drift-diffusion simulation does not reproduce the measured current-voltage characteristics. Here, we show that the current-voltage behaviour can be modeled by an additional charge carrier density dependence of the mobility originating from deep tail states of the density of states. Experimentally, a power law relation between current and voltage J ~ V^m is observed, which is characteristic for an exponential trap distribution. Especially, we observe a thickness dependent increase of m in the experiment, which was not reported before. From simulations, the thickness dependence can be attributed to local diffusion currents in the devices and can be used to characterize the trap distribution. From these results, we suggest the thickness dependence of the power law exponent as a novel way to determine the distribution of exponential trap states.
Finally, the simulations also explain that the charge carrier density dependence is more difficult to resolve by the POEM technique than a field dependence when traps are present, because free and trapped charge carriers cannot be distinguished in this method, whereas only free charge carriers contribute to the current. On the other hand, the field activation is difficult to recognize alone from the current-voltage behaviour. The results show that the combination of simulations and potential mapping gives a detailed and consistent picture of the relevant charge transport processes.
[1] J. Widmer, J. Fischer, W. Tress, K. Leo, M. Riede (2013), Organic Electronics, in press.

Conjugated polymers based on electron-deficient co-monomer units have attracted much attention due to their low optical bandgaps and thus enhanced light harvesting in the near-IR relative to their predecessors. Recently we have shown that by changing the sidechain periphery of an isoindigo-based low bandgap polymer, the thin film polymer microstructure could be significantly altered. In particular, we found that by switching from a branched aliphatic sidechain to a linear one terminated with siloxane groups, the crystallographic texture changed from edge-on to dual edge-on and face-on orientations. In order to understand how these dramatic microstructure changes affect the exciton diffusion length (EDL) of polymer excitations, we have measured steady-state photoluminescence quenching with a crystalline TiO2 layer as the electron acceptor. We estimate the EDL by modeling the experimental quenching efficiency with account of the internal optical mode structure in the thin film stack. In addition, we use X-ray reflectivity to characterize the roughness of the exposed and buried interfaces and use this information to refine our estimates of the EDL. We relate the differences in EDL to microstructure of bare polymer films with aliphatic and siloxane sidechains and to that of blend polymer/fullerene films used in previously-reported photovoltaic devices.

PEDOT, poly(3,4-ethylenedioxythiophene), doped with a variety of anionic species, most commonly the polyanion PSS, poly(styrenesulfonate), is one of the most widely used conducting polymer systems to date. Its high conductivity, low impedance, and electrochemical properties have lead to wide spread use as an electrode, electrochromic material, mechanical actuator, and sensitive bio-signal transducer. Many of these applications require both efficient ionic and electronic transport; such mixed conduction carries additional requirements from a structural perspective. However, this material is challenging to study and requires structural probes at multiple length scales. Cast from dispersion, PEDOT in PEDOT:PSS has ill-defined macromolecular characteristics, the colloidal dispersion and film is generally disordered, the polymeric components have poor contrast, and it is challenging to study the homopolymers individually. We address these issues by employing complementary characterization techniques, including wide and small angle (resonant) scattering, microscopy, and optical probes, and use this information to explain trends in measured electrical conductivity and ionic mobility. Specifically, we show that structural parameters simultaneously describe the evolution of electronic and ionic conductivity upon addition of a co-solvent or crosslinker before film-casting. These studies represent a significant step towards the rational design of organic mixed conductors, crucial for emerging areas or organic electronics such as bioelectronics.

Organic light emitting diodes (OLED) driven by organic transistor are an excellent technical approach to realize the dream of flexible or even rollable OLED devices on virtually any substrate. However, to achieve this goal, highly effcient, at best top-emitting, OLEDs and organic transistors driving very high currents are needed. In this paper, we report on recent progress in outcoupling for OLED devices, using scattering structures and microcavity approaches. In particular, we successfully address the problem of avoiding angular dependencies of the emission in such structures. Furthermore, we will report on progress in novel vertical organic transistor structures which are ideal candidates for all-organic displays. Here, the vertical design allows to realize very short channel lengths even without submicron lithography. Thus, devices which reach an order of magnitude higher current densities compared to conventional lateral FET have been realized. - * Work done in collaboration with Björn Lüssem, Hans Kleemann, Axel Fischer, Malte Gather, Hong-Wei Chang, Jonghee Lee, Simone Hofmann, Chung-Chih Wu

Organic photovoltaics (OPVs) depend on the interface between an electron donating material (D) and an electron accepting material (A) for exciton dissociation and charge separation. The energetic landscape at and around this interface is thus expected to play a significant role in determining charge separation probabilities and OPV device performance. For example, if D or A molecules at the interface are higher energy states for holes or electrons, respectively, then this will provide an energetic driving force for charges to move away from the interface. Furthermore, the energy of the materials can vary based on whether they are in an amorphous or crystalline state, and in a mixed or pure phase. In this work we explore how the material state and film composition affect the energy levels of small molecule donors and electron accepting C60 molecules. This is accomplished through utilizing ultraviolet photoelectron spectroscopy and external quantum efficiency measurements to probe the ionization potential (IP) and the energy of the charge-transfer (CT) state, respectively, where CT states are intermolecular states formed between D and A molecules. These measurements show that the IPs can vary by up to 0.5 eV between molecules in pure and blend states. Correspondingly, the energy of the CT state can shift by up to 0.5 eV depending on whether the device is a bilayer or if a small percentage of donor material is blended with C60. These large energetic shifts between pure and mixed phases will provide a significant driving force for charge separation in bulk-heterojunction devices, where a three-phase morphology consisting of pure phases and a mixed phase has been shown to exist. In some donor material/C60 bilayer devices intermixing occurs to a large extent, which results in a CT state energy that reflects the IP of the donor material in a blend film and not in the pure state. Overall this work highlights that the energy levels of OPV materials are sensitive to intermolecular interactions, a fact that may have significant implications for charge separation, and that in certain material systems the CT state energy is determined by the energy of D and A in the mixed phase, thereby leading to a higher CT energy and open circuit voltage than would be predicted based on the IP of the pure donor material.