Logic circuits are necessary to fulfill the vision of low cost, large area organic electronics, made with organic semiconductors (OSC) as the charge transfer layer. However, these circuits have stringent requirements. Primarily, all the thin film transistors (TFTs) participating in the circuit need to have a low variation in charge transfer characteristics (charge carrier mobility, threshold voltage, current, etc.). Additionally, organic circuits should be operated with low power consumption. To this end, research is being performed to pattern OSCs on the organic circuit to reduce parasitic current leakage. These twin requirements of low variability and OSC patterning set up conflicting goals, as the variability increases if each TFT is patterned individually. Moreover, patterning TFTs such that every TFT works, for the numerous TFTs required for logic circuits, is difficult with conventional methods such as ink jet printing due to patterning errors over large areas.
Here, we show a self-patterning method that does not require an extra patterning step to deposit the OSC layer onto the organic circuit. We have developed a surface functionalization procedure for a variety of oxide and metal surfaces, which, when paired with controlled solvent flow, can pattern OSCs in the TFT channel region only. Using this method, we show 100% viability of the patterned TFTs with low variability of charge carrier mobility and current. This method has been used to form logic gates and other organic circuits.

Research into photodetectors has been intensified by the desire to produce complex products such as touch screens with integrated photodetectors [1], stacked colour pixel sensors [2] and photosensitive screens [3]. As a result, there is an ongoing search for photodetectors with a specific set of properties including: colour selectivity, transparency, high responsivity, low-temperature and low-cost manufacturing, and the ability to be integrated with driving devices/circuitry. The combination of research from the field of metal-oxide semiconductors, as applied to transistors, and certain organic light absorbing dyes from the field of dye sensitized solar cells (DSSC), has led to a possible novel solution. Dye-sensitized transistors combine the ultra-high gain mechanisms of a transistor with the colour selectivity of the chosen dye allowing for the modular design of the photoresponse via the selection of an appropriate light-absorbing dye. Previous work in this field has involved the functionalization of SnO₂ nanowire field effect transistors (FET) using chlorophyll and fluorescein [4] and ZnO thin film transistors (TFT) functionalized using the well-known organic light absorbing dye D102 [5].
Here we report the demonstration of a dye-sensitized thin film transistor (DSTFT) consisting of indium oxide (In₂O₃) and the dye D102. The devices have been grown using solution-based processes at temperatures le;200 °C. Control and active devices show the dye producing a colour selective photoresponse around the wavelength of green light (~510 nm) with a maximum photosensitivity of 10⁵ and a responsivity value of 3×10² A/W. It is hypothesised that holes originating from the photo-generated electron-hole pairs in the excited dye assist in the desorption of oxygen from the semiconductor surface [6]. This process leads to the production of excess free electrons that dope the metal oxide channel, increasing conductivity and explaining the very high photoresponse. Furthermore, the active device layers, i.e., the semiconductor/light absorbing dye, are highly transparent with an average transmission level of over 93 % across all visible wavelengths (300-650 nm). This unique characteristic makes this dye-sensitized device a promising candidate for applications such as a highly transparent photodetector.
[1] You, B. H., et al., SID Symposium Digest of Technical Papers, 40, 439, (2009)
[2] Gidon, P., et al., Physica Status Solidi (a), 207, 704-707, (2010)
[3] Jeon, S., et al., Nature Materials, 11, 301, (2012)
[4] Wu, H.-C., et al., Advanced Functional Materials, 21, 474-479, (2011)
[5] Pattanasattayavong, P., et al., Journal of Applied Physics, 112, 074507, (2012)
[6] Verbakel, F., et al., Journal of Applied Physics, 102, 083701, (2007)

Interfacial contact layers (ICLs) improve the performance of organic photovoltaic (OPV) devices, by establishing a drift field within the bulk heterojunction (BHJ) and by blocking exciton quenching and carrier recombination at the electrodes. Metal oxide nanoparticle suspensions that can be solution processed at low temperature into pinhole free thin films (~ 10 nm) represent a promising route for ICLs because of the inherent compatibility with scalable device fabrication processes. However, most synthesis methods of nanoparticle suspensions require separate optimization for each metal oxide to achieve the desired combination of suspension stability, ease of film formation, and superior optical and electronic properties. Here, we present a universal method to fabricate stable suspensions of metal oxide nanoparticles where their stoichiometry and electronic properties were optimized prior to solution deposition, and utilize these nanoparticles as hole contact layers (HCLs) in OPV devices. Using hydrolysis of micron sized metal powders in the presence of excess H₂O₂, we synthesized charge stabilized suspensions of MoO₃, NiO, Co₃O₄, Cr₂O₃ and other metal oxide nanoparticles with average diameter < 5 nm and high yield. Spin coating of the nanoparticle suspensions without any post-processing resulted in uniform thin films with work function and stoichiometry comparable to evaporated films, as determined by Kelvin probe and X-ray photoelectron spectroscopy measurements. We also determined the electronic band diagrams of the metal oxide nanoparticle films using a combination of UV-visible spectroscopy and UV photoelectron spectroscopy in air and in vacuum. When the metal oxide nanoparticles films were used as HCLs in P3HT:PC₆₀BM and PCDTBT:PC₇₀BM devices, we found that the performance corresponded most strongly to work function of the ICL, although conductivity and bandgap of the ICLs also have effects on carrier extraction. Finally, we will discuss the synthesis of doped metal oxides and ternary metal oxides using the same general synthesis method, and examine their effect on electronic properties and OPV device performance. This project is sponsored by National Science Foundation DMR-1305893.

The key processes of charge generation and charge extraction in organic and hybrid solar cells occur at the organic-inorganic interfaces, either in the active layer and/or at the active layer-metal interface. These processes are predominantly influenced by interfacial properties such as chemical composition, physical interactions and dipoles. Efficient photovoltaic performance, therefore, requires control over the non-homogeneous interfacial structure and composition. Here we show that diffusion of small molecules through the polymer film can be used to promote the desired organic/inorganic interfacial structure and composition. This approach is demonstrated in two systems: the active layer/metal interface in organic devises, and polymer/metal oxide donor/acceptor interface in hybrid devices. In the former, a polyethylene oxide (PEG) interlayer is spontaneously formed at the active layer/metal interface by its migration from the active layer to the interface. Using a plethora of techniques we unambiguously show that PEG migration occurs during the metal evaporation process and is driven by the metal-PEG interactions. In the second case, atomic layer deposition is used to process a hybrid bulk-heterojunction with exceptional control over film composition and morphology. The BHJ is prepared by diffusion of a ZnO precursor, diethylzinc (DEZ), into pre-formed P3HT films, followed by its in-situ conversion into crystalline ZnO. Finally, the morphology and interfacial interactions directed by the small molecule diffusion are correlated with photo-excitation dynamics and the macroscopic photovoltaic device performance.

Colloidal semiconductor nanocrystals (NCs) are solution processable semiconductors, which have great potential for optoelectronic device fabrication. Their remarkably broad absorption, their large tunability, their high dielectric constant, and their high stability under ambient conditions, are all qualities that makes of NCs solids ideal for solar light harvesting.
The use of small bi-dentate ligands to substitute the long insulating ligands with which NCs are synthetized, allows charge transport between them, so that high performances field effect transistors [1] and efficient solar cells have been reported [2,3].
In this presentation I will report about the open circuit voltage enhancement, respect the one of devices made with core only PbS NCs, obtained using PbS/CdS NCs. The mechanism determining this increased performances is investigated by mean of photoluminescence time resolved spectroscopy, impedance spectroscopy and transport measurements [4]. Finally, I will show as colloidal NCs together with conjugated polymers can give new possibilities towards broad absorption of the solar spectrum not only in solar cells but also for photocatalytic water splitting.[5]
[1] S. Z. Bisri, C. Piliego, M. Yarema, W. Heiss and M. A. Loi, Adv. Mater. Adv. Mater. 25, 4309 (2013); D. M. Balazs, M. I. Nugraha, S. Z. Bisri, M. Sytnyk, W. Heiss, M. A. Loi, Appl. Phys. Lett. 104, art# 112104 (2014).
[2] Z. Ning , D. Zhitomirsky , V. Adinolfi, B. Sutherland, J. Xu , O. Voznyy , P. Maraghechi , X. Lan , S. Hoogland , Y. Ren, and E. H. Sargent, Adv. Mater. 25, 1719 (2013).
[3] C. Piliego, L. Protesescu, S. Z. Bisri, M. V. Kovalenko, M. A. Loi, Energy Environ. Sci., 6, 3054 (2013).
[4] M. J. Speirs, D. M. Balazs, H-H. Fang, L-H. Lai, L. Protesescu, M. Kovalenko, M. A. Loi (submitted).
[5] L-H. Lai, W. Gomulya, L. Protesescu, M. Kovalenko, M. A. Loi, (submitted).

One of the driving interests into the research of organic electroluminescent devices, and in particular polymer light emitting diodes (PLEDs), is the fabrication of large area, low cost devices for use in applications such as large area lighting. PLED devices are particular suited to this application due to their solution based nature, leading to ease of processing. However, deep highest occupied molecular orbital levels (HOMOs) of the polymer materials require interlayers to assist hole injection into the material in devices. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the standard interlayer used.
However, PEDOT:PSS has several disadvantages, one of the most limiting being its work function of 5.2eV, which is not deep enough to achieve ideal ohmic contact with common light emitting polymers. Metal oxides, in particular Molybdenum Trioxide (MoO₃), have been used as an alternative interlayer to achieve ohmic injection into organic layers. However, MoO₃ is usually evaporated for inclusion in small molecule organic devices, and to retain the advantage of PLED materials a solution processed route is required.
Here, we present an investigation of spray pyrolysis of a MoO_x interlayer for use in PLED devices. Spray pyrolysis involves spraying of a precursor material dissolved in solvent onto a heated substrate. The decomposition of the precursor allows deposition of the target material in a manner with similarities to chemical vapour deposition, and control of the deposition temperature allows investigation of the effect of oxygen vacancies in the MoO_x layer on device performance.
We present a comprehensive study of the various stages of the spray pyrolysis process. Decomposition of the precursor material is discussed. Morphological, optical and electrical measurements of the resulting MoO_x layers are then considered. The injection properties of the layer are then investigated through the fabrication of single carrier devices. Finally, luminescent devices are fabricated using poly(9,9'-dioctylfluorene-alt-benzothiadiazole) (F8BT), which show a more than threefold increase in device efficiency compared to PEDOT:PSS.

Device stability of organic photovoltaic (OPV) devices has proven to be largely determined by the transport layers and electrodes. Integration of transition metal oxides as interface layers has enhanced both device stability and performance compared to their organic and/or inorganic counterparts. This, combined with their compatibility with large-area depositing techniques, motivates their integration in OPVs. Here, we propose the stability assessment of MoO₃ in inverted and packaged polythiophene:fullerene cells with Ag/Al composite electrode in operational conditions combining solar radiation and 65°C. The Ag/Al electrode balances the enhanced air stability, owing to the combined metal thickness of 300 nm, with low-cost material selection. In the over 1000 h ageing study the performance loss of over 50% in the first 200 h is dominated by a drop in the short-circuit current (Jsc). We reveal a concurrent loss in reflectance from 85% to 40% below 1.9 eV, which is below the optical gap of the used photo-active materials, hence excluding any major degradation in the bulk of this layer. Correlating the responses of aged devices to a series of test structures comprised of the bottom ITO/ZnO cathode, the top MoO₃/Ag or MoO₃/Ag/Al anode and their combinations with the active layer allowed us to identify that the combination of Ag and Al reproduces the low reflectance of aged devices. This suggests that Al plays a critical role in the failure independent of the photo-active layer. Systematic single-stress ageing further indicates that elevated heat solely triggers this mechanism. Cross-section transmission electron microscope coupled with elemental analysis revealed the unsuspected role of Al, notably it diffuses through the entire 150 nm thick Ag layer and accumulates at the MoO₃/Ag interface. Depth profiling with X-ray photoelectron spectroscopy advanced our understanding by indicating the oxidation of Al only at the MoO₃/Ag interface hinting that MoO₃ is the source of oxygen and is most probably reduced. Control X-ray diffraction measurement, not relying on sputtering, allowed us to confirm the diffusion of Al and exclude any contribution from metal induced crystallization in MoO₃. Finally, the input from the material characterization to optical simulations allowed to reproduce the reflectance response of aged devices and estimated that 20% of the loss in Jsc is ascribed to optical changes in MoO₃. Concurrent shift in its energy levels could account for the remaining performance loss as well as other failure mechanisms might also contribute. In conclusion, our root-cause failure analysis provides insight into the harmful interaction of two non-adjacent layers and their consequently changed optical properties resulting in overall electrical performance loss. This insight provides guidelines for heat-stable MoO₃/metal contacts, independent of the active layer and illustrates the much needed but still rare root-cause failure analysis in OPVs.

In this presentation new approaches to metal oxide and hybrid materials for unconventional electronic applications are presented. Particularly, we will discuss our latest results in developing amorphous and polycrystalline In-Y-X-O (Y, X = none, Ga, Sn, Zn, Y, La) formulations for combustion synthesis in which the corresponding films can be annealed at temperatures < 300 0C. Depending on composition, ITO workfunction can be changed by several eV enabling their use with various photoactive layers for solar cell. A simila strategy has been employed for tuning the conductivity of ZnO using nanowires of perfluorocupperphthalocyanine. Finally, these interlayes can be use to tune injection ability into organic transistors.

Metal oxide semiconductors that can be processed via solution-based methods at low temperatures in air represent an emerging class of electronic materials for application in a range of existing as well as fast emerging opto/electronics. In this talk I will discuss the development of low temperature solution-processable metal oxide semiconductors and their application in thin-film transistors and integrated circuits. I will describe how low-dimensional metal oxide superlattices can be grown from solution in a highly controlled manner and the unique electronic properties of these multilayer systems can be explored to overcome the intrinsic electron transport limitations encountered in conventional single semiconductor channel transistors. These novel prototype transistors exhibit band-like transport with electron mobility values approximately a tenfold higher than control devices based on single metal oxide semiconductors. The development of other exotic devices, such as double-barrier resonant tunnelling diodes, that exploit the low-dimensional nature of these solution grown oxide superlattices will also be discussed.

Ideally, combining organic and inorganic materials could result in a synergy of the unique properties of both classes of materials. In practice, however, this can be limited by several factors. In case of zinc oxide/organic interfaces, ZnO surface properties highly affect electrical or photovoltaic performance of hybrid organic-inorganic devices. This motivates us to investigate the influence of ZnO surface modification on the electrical and impedance behavior of the ZnO/organic layer hybrid structures. We studied two kinds of ZnO films, i.e. conducting Al-doped ZnO (ZnO:Al) films serving as transparent electrodes, and semiconducting ZnO films with lower carrier concentrations as n-type partner to organic semiconductors. In the first case, we deposited additional very thin layer of ZnMgO:Al to decrease the work function of ZnO:Al electrodes. In the latter case, we modified ZnO surface by depositing a very thin layer of high dielectric constant oxide (Al₂O₃, HfO₂ or TiO₂) to, on the one hand, passivate ZnO and prevent formation of ZnO surface states, and, on the other hand, to form as low potential barrier for the charge carrier flow as possible. All the oxide films were grown by atomic layer deposition. As organic layers, we chose small-molecule materials having different HOMO and LUMO levels (tetracene and two squaraine derivatives). We will show electrical and impedance characterization results of the unmodified as well as modified ZnO/organic layer junction devices. This work was partially supported by The National Science Centre (NCN, Poland) under decision No. DEC-2012/07/D/ST3/02145.

Ultrathin p-type conductive native metal oxide layers are typically categorized as thin films that are less than 10 nm in thickness and find application in a host of optoelectronic devices. In photovoltaics, ultrathin p-type silver oxides can be used to increase the surface work function of the silver back electrodes while maintaining the electrode&’s high reflectivity. P-type metal oxide films can also be used for lighting devices such as top-emitting LEDs to improve hole injection of transparent electrodes without significantly inhibiting light emission. However, the optical transparency and electrical conductivity of p-type native metal oxide films depend heavily on the thickness of these films. If the films are too thick, they would no longer be transparent, but if they are too thin a significant change in work function may not be observed. Therefore, it is very important to be able to control and accurately measure the thickness of these native oxide films. In this project we have investigated two methods to grow ultrathin native silver oxide films on 100-nm-thick thermally evaporated silver films in a controlled manner: electrochemical oxidation and ultra violet ozone oxidation. Electrochemical oxidation was accomplished by exposing the silver film to a brief, reduction electrochemical potential, and, subsequently, applying an oxidization potential for 400 seconds. Ultra violet ozone oxidation was accomplished by exposing the silver film to an ozone environment for either 1 or 2 minutes to produce an optically-transparent oxide films. The oxide films were characterized with x-ray photoelectron spectroscopy (XPS) and grazing incidence wide angle x-ray scattering (GIWAXS). To measure the film thickness, grazing incidence small angle x-ray scattering (GISAXS), spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) techniques are being adopted. The AFM can measure the change in height of steps in the oxide film created by using a selective chemical etchant that attacks the oxide layer but not the metal layer. Some of the etchants that are being investigated are dilute oxalic acid and dilute ammonium solutions. In the XPS data, the binding energies (E_b) for O1s and Ag3d_5/2 photoelectrons were shifted from E_b,O1s = 531.6 eV and E_b,Ag3d = 368.2 eV for un-oxidized Ag to E_b,O1s = 529.4 eV and E_b,Ag3d = 367.6 eV for electrochemically oxidized Ag and E_b,O1s=530.6 eV and E_b,Ag3d=367.7 eV for UV-ozone oxidized Ag indicating formation of Ag₂O. The shift in GIWAXS scattering vector from 2.5 Å^-1 to 2.24 Å^-1 following ozone exposure of Ag for at least 2 minutes further confirmed ultrathin Ag₂O formation. Future work will determine the electronic workfunction and bandgaps of the oxides using ultraviolet photoelectron spectroscopy (UPS). The implications of the results will be used to inform the optimum p-type native oxide thickness required for organic optoelectronic device applications.

In the present work, we propose the study of the atmosphere conditions on device performance of hybrid organic/inorganic thin-film transistors (TFTs) comprising sol-gel processed indium-zinc oxide (IZO) and poly(3-hexyl thiophene) (P3HT). The electrical properties of amorphous transparent metal oxides films produced by sol-gel process have been found to depend strongly on the ambient conditions during film processing and device characterization. The performance of the TFTs was evaluated from parameters obtained from the characteristic transfer transistor curves: majority charge-carrier mobility, threshold voltage and on/off current modulation ratio. A very rigorous control of the ambient oxygen content (from 10 ppm up to completely oxygen saturated atmosphere) during the metal-oxide film processing and also during the device characterization was carried out in order to find the most appropriate conditions for device preparation and the dynamics of oxygen release by the device during characterization. The results show that device performance is almost atmosphere independent for an oxygen content higher than 5% during the metal-oxide preparation. However, device performance changes dramatically on time for characterization at completely inert atmosphere conditions (below 10 ppm), within a time span of several weeks. A extensive analysis of the device performance parameters is carried out to determine the most appropriate preparation and characterization conditions for the device achieve a stable response after a minimal time interval. These information are valuable to continuosly improve device output and performance. (Acknowledgements to Fapesp Proc. 2013/24461-7)
References
[1] T. Kamiya, K. Nomura, and H. Hosono, Sci. Technol. Adv. Mater. 11, 044305 (2010).
[2] G.H. Kim, H.S. Kim, H.S. Shin, B. Du Ahn, K.H. Kim, and H.J. Kim, Thin Solid Films517, 4007 (2009).
[3] K. K. Banger, Y. Yamashita, K. Mori, R. L. Peterson, T. Leedham, J. Rickard, H. Sirringhaus, Nature Materials DOI:10.1038/NMAT2914 (2011).

The processes that generate current in organic photovoltaics (OPVs) are dependent on the micro- and nano-structure of the devices, especially at the donor-acceptor interface. Light trapping strategies, including the incorporation of plasmonic nanostructures, have been proposed to tailor absorption of incident sunlight and generate more photocurrent at the donor-acceptor interface to improve the power conversion efficiency. These plasmonic nanostructures are known to enhance the optical absorption and current density in an OPV without increasing the thickness of the active layers, but little is known about the detailed structure, chemistry, and bonding between the active layer and the plasmonic nanostructures. The understanding of this interface is vital to understanding why these nanoparticles improve the efficiency of such devices. Electron energy-loss spectroscopy (EELS) may be used for the study of such interfaces in OPV structures. From the collected valence loss spectra, it is possible to extract the energy-loss function and calculate the complex dielectric function from which the frequency-dependent refractive index (eta;), and the extinction coefficient (κ) can be obtained. This knowledge allows peaks associated with single electron transitions to be distinguished from collective excitations. This analysis would result in a signal that is related to the interactions between the acceptor/donor and nanoparticle interfaces, which could be studied and analyzed to try and understand why the insertion of nanoparticles in the active layer of the OPV device improves the efficiency of the devices. However, organic materials are extremely susceptible to electron beam damage, so it is first necessary to determine the allowable electron beam doses that will not change the chemistry of specific organic samples. Using spectroscopic ellipsometry (SE), the optical properties of a material may be determined. Kramers-Kronig analysis can be applied to determine the dielectric function from which the energy-loss function may be calculated. This energy-loss function can then be compared to the energy-loss function measured using EELS at different electron beam doses to determine the maximum beam dose that will not change the chemistry of the sample. Four organic materials have been studied in this manner, including copper phthalocyanine (CuPc), fullerene-C₆₀, [6,6] phenyl C₆₁ butyric acid methyl ester (PCBM), and poly(3-hexylthiophene) (P3HT), in order to determine the acceptable electron beam doses for EELS measurements of these organics and develop standards for EELS measurements.

Photonic crystals are highly ordered structures that exhibit unique light controlling abilities, such as being able to theoretically reduce the group velocity of light to zero. They have been shown to increase light absorption and reduce charge-carrier recombination, which would be desirable for solar conversion processes. We present an organic/inorganic nanocomposite for bulk heterojunction cell applications using a titanium dioxide (TiO₂) 3D photonic crystal coated with poly (3-hexylthiophene) (P3HT). TiO₂ inverse opals are used as the photonic crystal in this application. Inverse opals were fabricated by infiltration of an opal template of polystyrene spheres that were deposited onto a glass substrate via colloidal self-assembly. We fabricated TiO₂ inverse opals with different periodicities, number of layers and wall thicknesses to systematically study the effects of different factors on charge generation. Charge generation is probed by photoinduced absorption spectroscopy (PIA) under different excitation wavelengths and modulation frequencies. The TiO₂/P3HT nanocomposites show a significant increase in charge generation in comparison to conventional P3HT/PCBM films. We elucidate the structural and photonic effects from the nanocomposites and address the potential of charge enhancement by photonic crystals for organic photovoltaics. The proof of concept may serve as blueprint that can be applied to different photovoltaic devices in the future.

Recently, we have studied the photoelectrochemical properties of nanodiamond (ND)-ragioregular polyhexylthiophene (RRPHTh), ND- poly (3-dodecylthiophene), another derivative of polythiophene [1-4]. The photocurrent of 8 to 20 times has been observed in the nanodiamond ND (RRPHTh) nanostructured blend film than most of nanoparticles containing blend films of RRPHTh conducting polymer. Under this investigation, the effect of various sizes of (ND) from micro to nano in the blend structure of ND wirh RRPHTh conducting polymer has been studied. The nano-diamonds (ND)-RRPHTh showed increased photovoltaic efficiency than micro-diamonds (MD)-RRPHTH based blend films.
Further, the photoelectrochemical properties of boron nitride (BN) blend with RRPHTh conducting polymer has been studied. The BN posseses the similar characteristics of diamond and, recently has gained interest in the mechanical and optical fields. The UV-vis spectroscopy, FTIR spectroscopy, and SEM properties of blend films were studied and compared. The electrochemical properties such as cyclic voltammetry, chronoamperometric, impedance spectroscopy with and without light were studied. The ND-RRPHTh and BN-RRPHTh hybrid based photoelectrochemical cells were studied and compared for short circuit current, power conversion efficiency (eta;) and fill factor. Our work shows for the first time the use of boron nitride with RRPHTh conducting polymer based photoelectrochemical photovoltaic devices.
References
MK Ram, H Gomez, F Alvi, E Stefanakos, Y Goswami, A Kumar, Novel Nanohybrid Structured Regioregular Polyhexylthiophene Blend Films for Photoelectrochemical Energy Applications The Journal of Physical Chemistry C 115 (44), 21987-21995
PA Basnayaka, P Villalba, MK Ram, L Stefanakos, A Kumar, Photovoltaic properties of multi walled carbon nanotubes-poly (3-octathiophene) conducting polymer blends, structures, MRS Proceedings 1493, 139-144
MK Ram, P Basnayaka, A Kumar Materials Science & Technology 2013 Photoelectrochemical Properties of Conjugated Polymer and Nanomaterial, Hybrid Organic: Inorganic Materials for Alternative Energy, October 27-31, 2013: Montreal, QC
F. M Abdelmola, M K Ram, A Takshi, E Stefanakos, A Kumar, Y.Goswami Photoelectrochemical cell of hybrid regioregular poly(3-hexylthiophen,2,5,diyl) and molybdenum disulfide film, Electro Chimica Acta, Communicated. (2014).

A new phosphorescent (TMP-AM)₂Ir(acac) with 2-bromo-4-(trifluoromethyl)pyridine and 3-Aminophenylboronic acid was synthesized for organic light-emitting diodes (OLEDs). This material was designed by result of Gaussian modeling program. The ligands have sites of both the electron donor and acceptor in structure. There are pyridyl group which decrease electron density and thiophen group which increase electron. So, it showed Intramolecular Charge Transfer (ICT) property. The dopant was synthesized by Suzuki coupling reaction and Nonoyama reaction. Red dopant was observed with an emission peak at approximately 600nm. The device structures were ITO / NPB / CBP: Ir complex / Bphen / Liq / Al. Electroluminescent properties were observed with devices doped with different doping concentrations.

Bottom-up self-assembly processes for the uniform deposition of monolayers or controlled multilayers have been investigated to develop a basic understanding of the relationship between the molecular structure and ordering at interfaces. At the same time, these studies have led to advances in optical devices, photonics, and electronic devices where structure and ordering at interfaces play a key role in device properties. Among the different strategies to fabricating ultrathin films, such as spin-coating, dip-coating, layer-by-layer deposition, and Langmuirminus;Blodgett (LB) assembly. Of these methods, the LB method offers assembly of the molecules at an interface using compressive forces to drive close packing. LB is traditionally used to prepare ultrathin films of amphiphilic molecules at the air/water interface, which are then transferred on to a variety of substrates such as TiO₂, glass, mica, or SiO₂. Here we outline the use of LB to create ordered monolayers of two metal-bipyridine complexes on SiO₂ and TiO₂. We show through a combination of x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and x-ray reflectivity (XRR). The dye is a rhenium-bipyridine complex where the molecular bridge to the carboxylic acid anchoring group to the oxide is either an aliphatic chain (ReEC) or a bithiophene (Re2TC). Both of these molecules have the potential to form stable LB films, and the change in the bridge characteristics from aliphatic to aromatic alters the driving forces for packing such as cohesive and repulsive forces between the various components of the dye, stiffness of the molecule, and hence the characteristics of the isotherm.
The two rhenium-bipyridine complexes ReEC and Re2TC studied here both form stable LB films. The hydrophilic carboxylic acid group anchors the molecule into the water subphase with the organometallic head group facing the air. The presence of a flexible aliphatic linker in ReEC aids in faster packing of the molecules hence compressing the liquid phase in the isotherm, compared to the more rigid aromatic link in Re2TC. Given that the cross sectional dimensions of the head group (CO-Re-Cl distance is approximately 4.8 Å) is larger than the effective distance for pi-pi interactions, the interactions between the aromatic bridges is minimal in Re2TC even when close-packed. Hence, the observed differences between the two molecules, is consistent with the greater rigidity of the 2TC link in comparison with EC. AFM and XRR characterization shows that the molecules are extended close to their calculated maximum length and are oriented with the long axis of the molecule close to normal to the substrate. These molecules can be assembled in ordered monolayers at the air/water interface and transferred to oxide surfaces, including TiO₂ substrates, hence opening up a path to study the effect of specific molecular orientation or aggregation states on the charge injection dynamics at donor/semiconductor interfaces.

ZnO/polymer heterojunction has been studied by many groups for its potential application in solar cell, LED, UV photodetection and other applications. However, there are few studies on ZnO/polymer heterojunction by synthesizing ZnO using pulsed laser deposition. Comparing with other methods, PLD has the advantage of congruent evaporation, and being able to grow high quality thin films at relatively low temperature. In our previous work in CdTe/CdS based thin films we have seen correlations between the pulsed laser deposition parameters and the electrical performance of the thin film solar cells.
In this work, we report our studies on pulsed-laser-deposited (PLD) ZnO/polymer heterojunctions based on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). We studied how the barrier height and built-in-potential of (PLD) ZnO/PEDOT:PSS heterojunctions depend on the conductivitiy of PEDOT:PSS and the pulse-laser deposition conditions of ZnO, such as deposition temperature, background pressure of oxygen, and ZnO film thickness. X-ray diffraction (XRD) and scanning electron microscopy were used to characterize the pulsed-laser-deposited (PLD) ZnO film. Current-Voltage and Capacitance-Voltage measurements were used to characterize the (PLD) ZnO/PEDOT:PSS heterojunctions.

In this research, transferable graphene oxide (GO) serves as an electron transport layer (ETL) in bulk heterojuction solar cells. The GO is inserted by stamping nanotechnology, with the help of a transfer film, on poly[N-9"-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT):[6,6]-phenyl C₇₁ butyric acid methyl-ester (PC₇₁BM). The BHJ solar cells with the GO ETL exhibits improved short circuit current, Jsc, and improved Power Conversion Efficiency, PCE, because of efficient electron transport from the BHJ layer to the Al cathode resulting in reduced series resistance and better charge extraction compared to similar devices fabricated without the GO interlayer. Moreover, solar cells with sequentially coated GO/titanium oxide (TiO_x) ETL show a synergistic effect of increased electric field amplitude as inferred from optical simulation. Addition of the GO/titanium oxide (TiO_x) ETL leads to the highest PCE of 7.5 %. BHJ solar cells fabricated with the GO layer by stamping transfer are promising as candidates for high performance devices because of the new ETL architecture which serves to reduce the electronic charge barrier from the UPS analysis.

Nanocomposite hybrid materials based on inorganic dielectric nanoparticles or nanorods are rapidly attracting increasing attention due to their potential as solution processable high-k dielectrics for organic electronics. Although there are several reports claiming the successful implementation of such dielectrics in organic-field-effect-transistors (OFETs), very little has been done on the in-depth characterization of the dielectric (impedance spectroscopy) and electric (leakage current) response of such hybrid systems. Here we present one of the first works addressing this matter by investigating and discussing the dielectric and electric behaviour of several functionalized titanium oxide nanorods films. Both anatase and rutile nanorods were prepared modifying an existing technique found in the literature. The anatase titanium oxide nanorods have an average diameter of 5 nm and length of 20 nm while the rutile nanorods have a mean diameter of 5 nm and length of 40 nm. Ligands of different types, including oleic-acid, phosphonic-acid and photocrosslinkable phosphonate terminated coumarin, were successfully used to functionalize the nanorods. The use of photocrosslinkable ligands is particularly attractive because it enables the fabrication of 1) devices (OFETs) without using orthogonal solvents as well as 2) devices based on stacks of hybrid thin films allowing thus to further tune the properties of the final dielectric stack. The functionalized nanorods can be solution processed in several common solvents and spin coated producing uniform thin films with RMS surface roughness of the order of 1-2 nm. Metal-insulator-metal (MIM) and metal-oxide-semiconductor (MOS) devices were fabricated to assess the dielectric and electric properties of the hybrid materials. In those samples incorporating the crosslinkable material, ultraviolet light irradiation was used to make insoluble films prior to depositing the top electrode. The highest value of the relative dielectric constant is of the order of 30 at 1 MHz and the dielectric loss is of the order of 0.01. The nature of the ligand shell around the titanium oxide core is very important in determining the final dielectric and electric properties of the hybrid film. For this reason, particular attention is devoted to the analysis of the data in order to extract information that allows us to design functionalized nanorods with desired dielectric properties (in terms of total dielectric constant and leakage current of the film). We will discuss the differences in the dielectric and electric behaviour of devices based on 1) nanorods functionalized with different ligands, 2) nanorods with anatase or rutile crystalline phase and 3) stacks of hybrid titanium oxide films (bilayer approach). It should be stressed that the material preparation, device fabrication and measurements are all carried out in air showing the huge potential of this approach in the organic electronic field.

Measurements of magneto-thermopower (S(H,T)) of interfacial delta doped LaTiO₃/SrTiO₃ (LTO/STO) heterostructure by an iso-structural antiferromagnetic perovskite LaCrO₃ are reported. The thermoelectricpower of the pure LTO/STO interface at 300 K is asymp; 118 mu;V/K, but increases dramatically on δ-doping. The observed linear temperature dependence of S(T) over the range 100 K to 300 K is in agreement with the theory of diffusion thermopower of a two-dimensional electron gas. The S(T) displays a distinct enhancement in the temperature range (T < 100 K) where the sheet resistance shows a Kondo-type minimum. We attributed this maximum in S(T) to Kondo scattering of conduction electron by localized impurity spins at the interface. The suppression of S by a magnetic field, and the isotropic nature of the suppression in out-of-plane and in-plane field geometries further strengthen the Kondo model based on interpretation of S(H, T).

Bulk heterojunction (BHJ) organic photovoltaic (OPV) based solar cells hold the promise of achieving the highly desirable objective of low cost, clean renewable electrical power via solar energy collection in a flexible device architecture. Recent developments in polymeric OPV device technology have increased power conversion efficiencies (PCE) into the 10% range, pushing OPV based devices closer to being viable low-cost, environmentally friendly alternatives to contemporary inorganic based solar cells. Extending OPV performance significantly beyond the 10% PCE barrier is a critical challenge. Factors such as the amount of light absorption, efficiency of photo-generation of electrons and holes, and their collection efficiency at the respective electrodes must be optimized in order to improve the device PCE. The development of light trapping strategies such as the utilization of surface plasmon polaritons (SPPs) in metal nanostructures can be an effective method of improving the amount of light absorption inside the active layer of a thin-film solar cell [4]. While incorporation of plasmonic nanostructures into thin-film PV cells is an attractive solution for enhancement of the optical absorption and current density in an OPV, little is known about the detailed structure, chemistry and bonding between the active layer and the plasmonic structure.
High resolution electron energy loss spectroscopy (EELS) can be used to probe the interfaces in the blended film active layer containing plasmonic nanostructures. In particular, monochromated scanning transmission electron microscopy (STEM) valence loss spectra can probe the complex dielectric function of a material with high spatial resolution yielding information about the effect of the interface between the organic phase and metal nano-particle on the observed resonances. Knowledge about the changes in the chemical bonding at this interface can be gained from studying the single electron transitions and collective excitations in the organic phase resulting from Kramers-Kronig analysis of these spectra. This analysis would then result in a signal that is related to the interactions between the acceptor/donor and gold nanoparticle interfaces. A successful Kramers-Kronig analysis requires as input the energy-loss function (ELF) and the extraction of this function from the collected valence loss data is not a trivial process. In this presentation we will explore the application of monochromated STEM and high resolution EELS to the topic of OPV characterization and, in particular, issues related to the removal of the monochromated zero-loss peak (ZLP) from the low loss EELS spectra prior to Kramers-Kronig analysis will be addressed.
The authors acknowledge the Air Force Office of Scientific Research and the Air Force Research Laboratory Materials and Manufacturing Directorate for funding as well as The Ohio State University Center for Electron Microscopy and Analysis for technical support.

The interfacial region between surface-modified semiconducting nanoparticles and polymers remains difficult to characterize experimentally in atomic resolution and contributes to the limited efficiency of hybrid photovoltaic cells and luminescent devices. Therefore, molecular dynamics simulation was employed to investigate the structure of cadmium sulfide nanoparticles capped with 3-mercaptopropyltrimethoxysilane (MPS) in contact with four substituted poly(phenyleneethynylene)s using a new force field for CdS and the polymer consistent force field. The results show that polymers with long alkyl side chains tend to wrap around the nanoparticles, and reduce backbone bending as well as polymer diffusion. The absence of alkyl side chains decreases the distance of conjugated backbones from the surface. Differences in the preferred location of functional groups of the polymers on the nanoparticle surface and of covalent versus non-covalent bonding were also monitored. Polymers containing terminal hydroxyl groups on alkyl side chains approach the surfactant corona and the core of the CdS-MPS nanoparticles. Close contact supports the formation of silyl ether cross-links although the interfacial structure upon bond formation remains similar to that of the non-covalently attached polymers. Ester groups bound to aromatic rings in the poly(phenyleneethynylene) backbone did not closely approach the nanoparticle surface. The results are the first step to understand nanoparticle-
polymer interfaces at length scales of 10 nm and explore correlations with photovoltaic performance.

Despite their great potential as transparent electrodes in organic electronics and as integral, active components in hybrid (opto-)electronic devices, full control over the surface properties of transition-metal oxides has remained elusive. Atomistic details of their surface structure have proven hard to assess, rendering application-relevant surface properties, such as electronic defect states or work function, highly dependent on environment and preparation conditions.
Here, on the example of zinc oxide (ZnO), we present a novel approach to theoretical ab-initio methods for the prediction of the atomistic surface structure as a function of environmental conditions, specifically temperature and atmospheric composition. Covering both, steady-state structures and kinetic barriers in reaching them, predicted ZnO structures will also be discussed in terms of their experimental signatures in core-level spectroscopy (XPS). The possibility of tuning the properties of ZnO surfaces via self-assembled monolayers (SAMs) of small organic molecules will subsequently be explored and, further extending our methodology, the environmental stability of such SAMs will be elucidated.

The fabrication of organic polymer-based light-emitting diodes (LEDs) is investigated as an option for more environmentally-friendly lighting and displays. These polymer light-emitting diodes (PLEDs) can be a sustainable alternative to incandescent and fluorescent light sources, inorganic light-emitting diodes and liquid-crystal displays. The main problem hindering the use of PLEDs in lighting and display applications is the electroluminescence efficiency of blue light-emitting PLEDs, which is due to the electronic properties of the materials used. A conventional PLED device features the following layers: an ITO on glass anode, a hole transport layer composed of (poly(3,4-ethylenedioxythiophene);poly(styrenesulfonate)) (PEDOT:PSS), an active layer of (poly(9,9-di-n-dodecylfluorenyl-2,7-diyl)) (PFO), an electron transport layer of lithium fluoride, and an aluminum cathode. In this device, the work function of PEDOT:PSS does not provide a good match to the energy of the blue light-emitting PLEDs due to the low HOMO energy level of the conjugated polymer [1]. Furthermore, PEDOT:PSS exhibits poor stability over time resulting in a low operational lifetime. These factors traditionally limit the use of blue light-emitting PLEDs.
In this work, we aim to investigate alternative device architectures such as top-emitting and inverted PLED devices that eliminate the PEDOT:PSS hole transport layer and compare their photoluminescence and electroluminescence lifetime to those of conventional blue PLED devices. Alternative hole transport layers that are under investigation are ultra-thin (< 10 nm) MoO₃ and p-type metal oxides (CuO_x, NiO_x and AgO_x). For device construction, polymer spin coating, annealing, and metal thermal evaporation processes are employed. Bright- and dark-field optical microscopy as well as atomic force microscopy is employed to characterize and optimize the homogeneity and thickness of the individual layers present in the device. Device operational lifetime testing will occur through time dependent electroluminescence testing to determine the operational lifetime of the devices (where its lifetime is considered to end when its luminance reaches 50% of its initial value). Furthermore, photoluminescence and electroluminescence testing of the devices will be carried out at both room temperature (25 ^oC) and elevated temperatures (70 ^oC) in both inert and ambient environments to compare the relative stabilities of the various device architectures.
[1] Deng, Xian-Yu. "Light-Emitting Devices with Conjugated Polymers." International Journal of Molecular Sciences 12 (2011): 1575-594. Web. 28 Dec. 2013.

Storing electricity for portable electronics, electrical vehicles and grid-scale storage calls for next generation of batteries. Inorganic nanostructures functionalized with organic molecules are a powerful materials platform to enable high energy, long cycle life, high power and good safety. Here I present several exciting examples on inorganic-organic hybrids: 1) In-situ polymerization of conducting hydrogel on Si nanoparticles as high capacity cathodes; 2) Self-healing batteries; 3) Polymer-encapsulated hollow S nanoparticle as high-capacity cathodes. 4) Amphiphilic polymer modification for trapping polysulfides.

The properties of interfaces between organic and inorganic materials in heterojunctions are relevant to a variety of electronic applications. The performances of such devices strongly depend on the kinetics of the charge transfer reactions and the thermodynamic properties (e.g., energetic difference at the junctions) at the heterogeneous interfaces.^1-2 Heterogeneous interfaces, especially in iodine-free dye-sensitized solar cells (DSCs),^3-5 can be controlled by modifying the interfacial properties to improve the photovoltaic performance. Herein, we present a novel synthesis of well-defined conducting block copolymers,⁶ which are formed on the surfaces of nanocrystalline TiO₂ particles. This approach permitted the selective positioning of lithium ions on the PEDOT-TB blocks⁷ only and the realization of an ideal device, thereby affording an improvement in the photovoltaic performances in iodine-free DSCs.
References
1) T. Leijtens, J. Lim, J. Teuscher, T. Park, H. Snaith, Adv. Mater. 2013, 25, 3227.
2) S.-H. Park, J. Lim, I. Y. Song, T. Park, Adv. Energy Mater. 2014, 4, 1300489.
3) S.-H. Park, I. Y. Song, J. Lim, Y. S. Kwon, J. Choi, S. Song, J.-R. Lee, T Park, Energy Environ. Sci. 2013, 6, 1559.
4) S.-H. Park, J. Lim, Y. S. Kwon, I. Y. Song, J. M. Choi, S. Song, T. Park, Adv. Energy Mater. 2013, 3, 184.
5) Y. S. Kwon, J. Lim, H.-J. Yun, Y.-H. Kim, T. Park, Energy Environ. Sci. 2014, 7, 1454.
6) I. Y. Song, Y. S. Kwon, J. Lim, T. Park, ACS Nano2014, DOI: 10.1021/nn5016083 (ASAP).
7) I. Y. Song, S.-H. Park, J. Lim, Y. S. Kown, T. Park. Chem. Commun.2011, 47, 10395.

Electronic ratchets can rectify AC signals that are extracted from unpredictable energy fluctuations. Such rectification is needed in radio frequency identification tags (RFIDs) and in other energy harvesting modules. However, currently available electronic ratchets suffer from poor output currents and voltages, and are generally not able to rectify efficiently random noise signals. We present an ionic-organic ratchet device that delivers record high electrical currents of 2.6 and 1.7 mu;A when driven with an AC signal of square wave and random amplitude, respectively. The device is based on a poly(3-hexylthiophene-2,5-diyl):salt blend that acquires rectification properties after a voltage stress in a transistor configuration. The frequency response of the ratchet can be tuned by the geometry of the contacts to obtain peak response above 5 MHz, making it an attractive candidate for use in RFIDs. The working mechanism of the ratchet is a charge pump with current generation efficiency of 62%. We show that the device performance can be increased even further when ZnO is implemented to the active layer due to formation of the rectifying junctions at ZnO-metal interface. The experimental results are supported with a Monte Carlo simulation of charge transport.

The cathode/electrolyte interface is the location of vital exchange reactions during charging and discharging cycles and the location of degradation processes such as cation dissolution and cathode surface layer (CSL) formation; however very little is known about the chemical nature of this crucial interface or the modification of its properties during use. Investigations into its properties are made difficult by complicated cathode chemistries and exacerbated further by intricate surface geometries. In this work, a model system is proposed with the deposition of epitaxial spinel cathode films (LiMn₂O₄ and LiNi_0.5Mn_1.5O₄) on conducting lattice matched substrates, Nb-doped-STO. The low roughness (<1 nm) of these films and the choice of material makes them an ideal system for several in-situ surface characterisation methodologies. The creation of these model systems will be discussed and results presented on their application for investigating the role of the interface in performance limitation and lifetime reduction in lithium-ion battery systems.

Polymers with stable pendant radical groups are a unique class of redox-active materials emerging as potentially the next generation energy storage breakthrough. These polymers facilitate apparently remarkably rapid, efficient and reversible multi-step charge-transfer processes. The focus of our work is to advance the fundamental understanding of the structure-property relationships associated with the mechanism(s) of electron transfer and ion transport, along with associated interfacial mass-transfer processes that impact the charge-transfer processes of a unique class of organic free-radical polymeric redox active materials. A greater understanding of the inherent charge transfer limitations in such systems will ultimately be paramount to further advancements in the understanding of both inter-film and interfacial ion and electron transfer reactions.
The project involves an integrated approach of chemical synthesis, electrochemistry, spectroscopy and theoretical modeling of a series of stable organic radical materials. Initially, we have focused on the 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) organic radical moiety incorporated into a complex materials set of poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (PTMA). Our initial synthetic efforts focused on developing strategies to alter the fundamental charge transport phenomena in the nitroxide radical polymers through structure control. For example we systematically changed the radical density within the polymers by introducing “blanks”, or monomers not bearing any nitroxide radical. Several series of copolymers and oligomers bearing 20, 40, 60, 80, and 100 mol % of the radical moiety were synthesized. The electrochemical, spectroelectrochemical, EPR analysis and theoretical models then coalesced on a unique property of the 20 mol % PTMA derivative where the change in electron-transfer rates, ionic mobility and observed overpotentials correlated to some of the changes in predicted molecular packing.
Controlled radical polymerization was used to also grow brushes of PTMA on an indium tin oxide (ITO) surface. Subsequently we evaluated the 1-dimensional, intra-molecular charge transport properties along the polymer chain within these brushes, especially across the hard-soft inorganic-organic interfacial barrier.
In our presentation we will discuss in detail the synthesis, electrochemical and spectro-electrochemical properties of these unique PTMA materials, and discuss how variation in oxidation states of nitroxide radical in the films affects electrochemical properties of electrodes.

The correlation between molecular scale morphology and charge generation across hybrid photovoltaic interfaces made of metal oxides (ZnO and TiO₂) and a prototypical electron donor polymer, P3HT, is investigated. Device characterization and UV-NIR transient absorption spectroscopy are used to demonstrate that the local disorder of the polymer chains on the surface of the metal-oxide film provides better electron injection efficiencies than the crystalline phases¹, though the latter are essential for energy and charge transport. Charge injection dynamics from the polymer to the metal-oxide have been largely investigated based on the excitation dynamics in the polymer that include both bulk and interface phenomenon ^2,3. Here, an unambiguous spectroscopic tool is demonstrated to probe the occupation of the conduction band of ZnO following the electron injection from the polymer through the ultrafast tracking of the Burstein-Moss effect⁴.
References:
[1] Kandada et al, Adv. Func. Mater. 24, 3094-3099 (2014).
[2] Meister et al, J. Phys, Chem. Lett. 3, 265-2670 (2012).
[3] Bansal et al, Sci. Rep. 3, 1-8 (2013).
[4] Burstein, Phys. Rev. 93, 632 (1954).

Hybrid solar cells (HSCs) have the potential to combine the advantages of organic solar cells and solid-state dye-sensitized solar cells (SS-DSCs) by implementation of an organic hole transporter instead of the transparent Spiro-OMeTAD in SS-DSCs. High light absorption is possible due to the use of monolayers of dye molecules or thin coating with highly absorbing semiconductors as sensitizers on the high surface area TiO₂ electrodes. Additional light absorption becomes possible if high performance OPV materials with high extinction coefficients are implemented as hole transporters. In many instances throughout literature, however, this polymer light absorption is not efficiently utilized in order to generate additional photocurrent.^[1]
This project focusses on the polymer photocurrent contribution in different hybrid solar cell systems. Our results show how the choice of the dye directly influences how efficiently photocurrent is generated upon photon absorption in the polymer and how charge carrier recombination can be suppressed.^[2] Furthermore, the interfacial modifier 4-mercaptopyridine is found to enhance the polymer contribution in systems where electron or energy transfer from the polymer to the dye is energetically allowed.^[3] We also show a combined electronic and optical study outlining the higher efficiency of hybrid solar cells with an energy transfer mechanism at the polymer-metal oxide interface. This is inferred from photoinduced absorption experiments that are in accordance with external quantum efficiency spectra. Our investigations are based on a model system of three different dye molecules in combination with the conjugated polymer poly(3-hexylthiophene) (P3HT). A squaraine dye is employed in order to allow energy transfer from the polymer to the dye, while a fullerene derivative serves as a charge separation model system. An indoline dye with a high lowest unoccupied molecular orbital further is used in devices where the polymer only serves as hole transporter and cannot contribute to the photocurrent.
[1] J. Weickert, L. Schmidt-Mende, APL Materials 2013, 1, 020901.
[2] J. Weickert, F. Auras, T. Bein, L. Schmidt-Mende, Journal of Physical Chemistry C 2011, 115, 15081.
[3] J. Weickert, E. Zimmermann, J. B. Reindl, T. Pfadler, J. A. Dorman, A. Petrozza, L. Schmidt-Mende, APL Materials 2013, 1.

Electrodeposited porous ZnO shows excellent electron transport properties without high-temperature treatment, and therefore presents a promising alternative to TiO₂ as photoelectrode material in dye-sensitized solar cells (DSCs). Combined with the organic dye D149, a conversion efficiency of around 6 % has been attained. While this result comes close to the top efficiency of 7.5% of ZnO-based DSCs, it remains below the record of 12-13 % among TiO₂-based cells. In order to fully exploit the potential of electrodeposited porous ZnO as a low-temperature alternative to TiO₂, new strategies to improve the photovoltaic performance by modifying the oxide-dye-electrolyte interface need to be developed. Presently, the best cells based on electrodeposited ZnO rely on co-adsorption of cholic acid to prevent aggregation of adsorbed D149 molecules, as photoelectrochemical studies have found that the presence of D149 aggregates accelerates recombination and thereby lowers the fill factor. While cholic acid is effective in reducing aggregation, it does not absorb visible light, and therefore does not actively contribute to the photovoltaic operation of the cell.
The approach of the present study was to combine D149 with light-absorbing co-adsorbates, i.e. co-sensitizers, in order to investigate whether they are capable of preventing D149 from aggregating on the ZnO surface, while simultaneously participating in the conversion of sunlight. Three organic co-sensitizers with different spectral light absorption were employed: the indoline dye D131, a sulfonated zinc phthalocyanine as well as the squaraine dye Sq2. Moreover, the influence of additional adsorption of the co-adsorbates cholic acid and octanoic acid was investigated. Sandwich-type solar cells with an iodide/triiodide redox electrolyte were built from the sensitized ZnO films, and were analyzed in their optical properties, global device performance and microscopic charge transport and transfer processes by means of absorption spectroscopy, current-voltage characterization, quantum efficiency measurements, time-resolved photocurrent and photovoltage measurements, intensity-modulated photovoltage and photocurrent spectroscopy (IMVS and IMPS) and electrochemical impedance spectroscopy (EIS).
Combination of D149 with both D131 and Sq2 led to efficient sensitization of electrodeposited ZnO films and improved performance compared to reference cells with D149 only. For all dye combinations, an additional improvement of the fill factor and the open-circuit photovoltage was found upon co-adsorption of cholic acid or octanoic acid. The observed dye-dye, dye-co-adsorbate and dye-ZnO interactions will be discussed in detail in conjunction with the resulting effects on interfacial charge transfer processes and charge transport. Particular emphasis will be put on correlations between the properties of the dye/co-adsorbate layer and electron recombination across the ZnO-dye-electrolyte interface.

Charge and energy transfer across interfaces are fundamental processes in semiconductor electronics and optoelectronics. Using model organic/inorganic semiconductor interfaces and femtosecond nonlinear laser spectroscopies, we provide real time views on how exciton disscociation across the interface sets up transient electric fields on femtosecond time scales, how Coulomb attraction leads to the formation of charge transfer excitons, and how energy relaxation competes with charge separation. These measurements reveal fundamental mechanisms responsible for electronic and optoelectronic devices.

We demonstrate the use of ultra-thin (0.5-3 nm) titanium dioxide (TiO₂) films for the application of hole-blocking interlayers in inverted organic photovoltaics (OPVs) using plasma-enhanced atomic layer deposition (PE-ALD). In order to evaluate the hole blocking capability of the ultra-thin TiO₂ films, we fabricated diodes from TiO₂/p-doped Si heterojunction devices with aluminum and silver contacts on front and backsides to isolate the rectification effect of the TiO₂ layer. Subsequently, we integrated these films as a modifier of ITO electrodes in organic photovoltaics to create inverted solar cells. This was done using poly(3-hexylthiophene):[6,6]-phenyl-C₆₁ butyric acid methyl ester (P₃HT:PC₆₁BM) bulk-heterojuction OPV devices with TiO₂ interlayers on ITO to evaluate OPV performance. The TiO₂/Si diode device showed remarkable current flow density being more than 6000 mA/cm² at 0.5 V under forward bias while less than 50 mA/cm² under -0.5 V backward bias, which suggests excellent rectification performance without losing electron transport rate. The fabricated OPVs showed 2.7-3.0% power conversion efficiency, 9.2-9.7 mA/cm² of short-circuit current density, and 0.61-0.64 V of open-circuit photopotential under light illumination. X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS) of TiO₂ films suggested that fabricated TiO₂ films are titanium dioxide with low density of defects, and has a deep valence band energy that can block holes effectively. Based on these results and the environmental stability of TiO₂, these results show the thinnest ALD films used to create rectifying contacts in organic solar cells. In addition, the aqueous stability of these films also suggest that these interfaces may present an effective way of creating robust, environmentally stable low workfunction contacts for OPV as compared to more traditional methods using ZnO, thus producing enhanced reliability.

Coupling the properties of electronically active organic materials with those of inorganic semiconductors has long been recognized as an advantageous device design, combining low processing temperatures, tunable structure and properties, and low surface state density given by the first with high carrier mobility, low exciton binding energy and long photo-excited charge carrier lifetime of the other. In many cases, though, this approach results in a system where the materials&’ limitations, rather than their advantages, dominate device performance. By systematically studying and modifying the interfaces of a well-controlled Si/PEDOT:PSS model system we explore ways to mitigate the effect of these limitations on photovoltaic device performance. We find that the Si surface passivation is the most crucial parameter in determining the devices electronic behavior. Well passivated surface by H termination, molecular monolayer or an oxide film results in high performance devices.
Inversion layer photovoltaic devices rely on majority carrier inversion at a semiconductor surface to form a p-n homojunction. The junction formation occurs by charge transfer upon interface formation between the semiconductor and a specifically selected metal (or a metal-like polymer). Using single crystal (sc) n-Si/PEDOT:PSS as we study the effects of interfacial energy level alignment, bulk doping level and mobility, and surface and interface passivation on device performance. With H termination, known to passivate Si for a limited time only, a close to maximal V_bi can be achieved. To obtain more enduring stability we modified the Si surface with several dipole bearing molecular monolayers. The molecular layers serve to provide more lasting passivation and to allow work function tuning in order to vary the V_bi of the junction with PEDOT-PSS. The V_bi can be derived from capacitance-voltage (CV) and current-voltage (IV) measurements. Although the CV results show that the molecular surface modification is preserved at the interface, if the modification implies an increase in V_bi, then only very small effects in the IV characteristics are seen. This can be understood by assuming inhomogeneity of the monolayer. Higher capacity comes from densely covered areas (high V_bi), dominating the CV measurement. Higher current passes through loosely covered area (low V_bi) areas, dominating the IV curve. Alternatively, we used ultra-thin film of highly charged Al₂O₃ to induce inversion electrostatically and stably passivate the Si surfaces.In this approach charges collection is done via small areas from which Al₂O₃ was removed, and Si/PEDOT:PSS interface is formed.
Even though the (sc) n-Si/PEDOT:PSS system has been well-studied, we find it to be quite useful as a model to investigate hybrid systems, not only with PEDOT:PSS. In addition, with PCE up to ~12% and room temperature processing of the wafer, the system is interesting in itself for real applications.

Recently, perovskite photovoltaic technique has attracted an intense interest and great progress has been made for this kind of organolead trihalide based solar cells. Nowadays, efficiency higher than 17% has been realized for this hybrid solid state devices. Different from other high performance thin film solar cells, perovskite soalr cells are solution processable, and processing temperature higher than 150 ^oC is not needed. All those special properties make perovskite solar cells one of the most promising techniques as future energy sources. However, most of the high efficiency perovskite devices are fabricated on substrate with crystalline TiO₂ as electron transport layer, which needs to be sintered at temperature higher than 450 ^oC. This extreme sintering condition limits the choices of substrate and increases the production cost and embedded energy.
Here in our presentation we aim for replacing high-temperature sintered TiO₂ electron transport layer by anatase TiO₂ nanoparticle, which is synthesized by hydrolysis of titanium tetraisopropoxide in oleic acid. The room-temperature processed TiO₂ nanoparticle based perovskite solar cells shows promising efficiency of 11.5%. Further improvement of the device performance can be made by treating this oleic acid-capped TiO2 nanoparticle with adequate ligand exchange methods to eliminate the surface traps of the nanoparticles. Our study demonstrate sintering-free TiO₂ nanoparticle electron transport layer for perovskite solar cells by solid state ligand exchange method. This work relax the substrate choices and enables the fabrication of perovskite devices on flexible polymer substrate, thus opens the door for roll-to-roll solution-precessed perovskite devices.

Organic field effect transistors (OFETs) have been developed for over a decade with various benefits of light weight, low cost, flexibility. The development of electrical property has been of particular concern and nowadays the mobility of several organic semiconductors becomes to exceed that of amorphous Si based FET which is used in display industry. What makes practical application of OFETs still problematic, however, is operational-bias and environmental stability. The factors to determine the stability are generally categorized into four parts; semiconductor channel, dielectric, channel/dielectric interface, and semiconductor channel surface. For the cases of dielectric and interface, the most detrimental factor is thought to be charge trapping phenomena originating from hydroxyl functional group or other ionic impurity. Those trapping effect can be dramatically reduced using polymer dielectric such as fluoropolymer or self-assembled monolayers treatment on conventional inorganic oxide. For the case of organic semiconductor and its surface, on the other hand, the vulnerability of charge carriers to penetrated ambient oxygen and water molecules induces detrimental effects on device stability. To protect OFET from such molecule-induced instabilities, several strategies are also introduced: formation a kinetic barrier by more compact packing, lowering the level of molecule orbitals below such as oxidation energy level, and increasing the band-gap for defending even from energetic photons. However, although finding proper passivation layer can be the most promising strategy to protect the organic semiconductor from penetrated atmospheric species, passivation related works have not been reported as much as other methods, particularly for n-type organic semiconductors.
Here we propose DNA-base small molecule guanine and Al₂O₃ bilayer as a unique passivation layer to solve above issues at once, simultaneously covering the aging and bias stress stabilities of both n- and p-channel OFETs in an air ambient. Although the Al₂O₃ layer shows very low oxygen and water permeability, direct deposition of such an inorganic insulator on organic material usually cause serious damage because the deposition process contains several chemical impurities from precursor and water. So we adopted an organic layer to buffer/or minimize the damaging effects; 20 nm-thin layer by guanine, one of DNA-base small molecules as extracted from DNA polymer, was deposited on n- and p-channel OFETs before atomic layer deposition of thin Al₂O₃ layer, since unintentional doping of hydrogen is efficiently prohibited by guanine. Using guanine/Al₂O₃ passivation layer, both p- and n-type OFETs demonstrated dramatic endurance under gate-bias stress and 30 days-long aging, showing little threshold and mobility change. As a result, a highly stable complementary-type logic inverter was even accomplished by combining p- and n-type FETs, successfully operating at 5 V as a minimum low voltage.

We demonstrate the use of polymeric zwitterions, namely, poly(sulfobetaine methacrylate) (PSBMA), as solution-processable work function reducers for inverted organic electronic devices. A notable feature of PSBMA is orthogonal solubility relative to solvents typically employed in the processing of organic semiconductors. A strong permanent dipole moment on the sulfobetaine moiety was calculated by density functional theory. PSBMA interlayers reduced the work function of a broad range of electrodes [indium tin oxide (ITO), Au, Ag, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), Cu, Al, and even graphene] by over 1 eV. By employing an ultrathin interlayer of PSBMA, one can reduce the electron injection barrier between ITO and C₇₀ by 0.67 eV. As a result, the device performance of OPVs with PSBMA interlayers are significantly improved, and enhanced electron injection is demonstrated in electron-only devices with ITO, PEDOT:PSS and graphene electrodes. This work makes available a new class of dipole-rich, counterion-free, pH insensitive interlayers for use as strong work function reducers for any electrode.

The field of organo-lead perovskite absorbers for solar cells is developing rapidly, with open circuit voltages of some of the reported devices approaching the maximal achievable theoretical voltage. Obtaining such high voltages on spun-cast or evaporated thin films is intriguing, and calls for detailed investigation of the source of photovoltage in those devices. We present a study of the roles of the selective contacts to methyl ammonium lead iodide (MAPbI₃) and methyl ammonium lead iodide chloride (MAPbI_3-xCl_x) using surface photovoltage spectroscopy. By depositing and characterizing each layer at a time, we find that the electron-extracting interface is more than twice as effective as the hole-extracting interface in generating photovoltage, for several combinations of electrode materials. We find indications for the existence of strong internal fields, which we attribute to some band bending in the MAPbI_3-xCl_xat both the electron-extracting and hole-extracting interfaces. We further observe the existence of an electron-injection related spectral feature at 1.1 eV, which might be significant for the cell&’s operation. By varying the granularity of the MAPbI₃ we demonstrate that this IR feature is related to grain boundaries. Our results show how SPV spectroscopy helps pinpoint processes that limit the important charge injection processes in perovskite-based solar cells, thus providing an additional tool to improve cell performance based on understanding.

Photovoltaic (PV) solar cells promise to be a major contributor to our future energy supply. The ramping up in production and affordable global uptake of solar energy requires a significant reduction in materials and manufacture costs. Furthermore, a solar cell industry on the TW scale must be based on sustainable materials and manufacturing techniques.
Hybrid organic -inorganic based perovskite cells prepared by low cost material and technique have shown impressive progress in solar to electric power conversion efficiency in last two years. These types of cells have potential to show even better power conversion efficiency in near future if the growth of perovskite films is further optimized for solar cell application. Though many techniques are used to prepare trihalide-organo-lead perovskite thin films, spin coating method appears to be an attractive option due to the low fabrication cost. Planar heterojunction cells (i.e. without mesoscopic TiO₂/Al₂O₃ layer) prepared in spincoating method shows power conversion efficiency ~12%. These solution processed perovskite thin films suffer from non-uniformity, partial coverage and band gap states. Moreover, the defects at the TiO₂ and perovskite interface can also play a major role in cell performance. In order to improve the performance of these types of cells, it is required to have appropriate passivation of defects, interface engineering for better coverage and higher crystallinity. To this end, we have used silane based organic molecular monolayers to modify compact TiO₂ surfaces using a simple, yet powerful wet chemical method. Perovskite thin films prepared by spincoating on these modified TiO₂ surfaces show better crystallinity than the unmodified surface, observed by Xray diffraction and scanning electron microscope (SEM) measurements. Photothermal deflection spectroscopy (PDS) suggests that perovskite films prepared on modified surfaces has lower density of bandgap (tail) states, which often causes non-radiative recombination that lowers the power conversion efficiency. Time resolved photo-luminesces measurement and other photo-physical measurements also show that higher crystallinity translates into higher carrier life time which is essential for planar heterojunction cells. Solar cells prepared with modified interfaces show best power conversion efficiency over 15% compared to 12% for cells with unmodified interfaces. We attribute the enhancement in efficiency to the better crystallinity in perovskite thinfilm and passivation of defect states at the interfaces which in turn are achieved by interface engineering with small organic molecules.

Organic-inorganic metal halide perovskite absorbers have rocketed to the forefront of PV research as efficient solar cell materials, which seem to be both simple to process and promise to reach the highest efficiencies. Great advances have been made in performance and this appears to be largely driven by improved control of thin film formation and crystalisation. Thin films from the archetypical perovskite, CH₃NH₃PbI₃, can be fabricated in many ways, ranging from one-pot solution casting to sequential deposition to vacuum based sublimation. There are many open questions concerning the mechanism for film formation and crystallisation and the ensuing impact on electronic quality of the perovskite absorber. Here I will present a detailed study understanding how crystallisation proceeds when the film is cast from a common solution of the precursor salts and correlate materials properties to optoelectronic and device characteristics. I will elucidate the role of halide within the precursor solution and the role of excess organic component, finally shedding light in the critical role Cl plays in the mixed halide perovskite CH₃NH₃PbI_3-xCl_x.

Recently, a number of interlayers and chemical interface modifications - ranging from NiOx to the use of self-assembled monolayers to the use of chemical dopants- have been used to tailor the properties of the transparent conductive oxide (TCO) interface with the active layer in organic semiconductor devices. Here, we show how chemistry at the buried interface can affect carrier recombination lifetimes in organic bulk heterojunction solar cells. Furthermore, we show how the heterogeneity inherent in many chemical interfaces can manifest as heterogeneity in local carrier recombination lifetimes, as probed via intensity-modulated scanning kelvin probe microscopy (IM-SKPM).

Crosslinked azobenzene liquid crystal (LC) polymer films are bent by irradiation of UV light. Utilizing the feature, a plastic motor [1], a robotic arm [2] etc. have been developed. The overall mechanism was understood as the surface region of the film was contracted. Although the mechanism has not been fully understood yet on what kinds of interactions between molecules, molecular chains, domains are involved and which time scale they happen. On the other hand, we have developed a new type of the time-resolved technique, called the heterodyne transient grating (HD-TG) method [3,4], which features a highly sensitive detection of the refractive index change and wide temporal response from nanoseconds to seconds. In this study, we studied the molecular dynamics in an azobenzene LC polymer film by using the HD-TG technique, combined with the transient absorption (TA) technique. By observation of the refractive index change induced by a pulse laser, contraction of the film was observed on the order of several hundreds of nanoseconds, and the subsequent reorientation and molecular rotation dynamics was observed from a few microseconds to a hundred milliseconds. Finally, the cis isomer of azobenzene was thermally returned back to the trans isomer about ten seconds because the film could not be bent in the liquid crystal cell. Since the contraction, reorientation and molecular rotation took place before the cis to trans back-transformation, these processes correspond to the preliminary molecular motion preceding the macroscopic bending of the film. [5]. Furthermore, we studied the structure and ordering change in the domains in the LC polymer film by using a phase microscopy with a depth slicing function, which helped understanding how the LC molecules behave for the bending.
1. M. Yamada, M. Kondo, J. I. Mamiya, Y. L. Yu, M. Kinoshita, C. J. Barrett and T. Ikeda, Angew. Chem. Int. Ed., 2008, 47, 4986-4988.
2. M. Yamada, M. Kondo, R. Miyasato, Y. Naka, J.-i. Mamiya, M. Kinoshita, A. Shishido, Y. Yu, C. J. Barrett and T. Ikeda, J. Mater. Chem., 2009, 19, 60-62.
3. K. Katayama, M. Yamaguchi and T. Sawada, Appl. Phys .Lett. 2003, 82, 2775-2777.
4. M. Okuda and K. Katayama, Chem. Phys. Lett., 2007, 443, 158-162.
5. T. Fujii, S. Kuwahara, K. Katayama, K. Takado, T. Ube and T. Ikeda, Phys. Chem. Chem. Phys., 2014, 16, 10485-10490.

The mechanisms of charge transfer (CT) in organic electronics are of fundamental importance for device functionality. Especially a more detailed understanding of interactions at organic/organic and organic/inorganic interfaces is needed to improve device performance and to develop new device architectures. Overall, its basic principles are still under strong investigation and subject of a severe discussion.
Here we show that in-situ infrared (IR) spectroscopy is a powerful tool to investigate CT effects at such interfaces. The prototypical interfaces between hole transport materials, such as 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) and N,N'-Di-[(1-naphthyl)-N,N'-diphenyl]-1,1'-biphenyl)-4,4'-diamine (alpha-NPD), and the hole injection layer material MoO₃ were investigated. When the organic molecules are deposited on MoO₃ charged and non-charged molecular species are formed which can be spectroscopically identified and quantitatively analyzed with in-situ IR-spectroscopy. The different species can be distinguished by their specific mid IR absorption features. For the inverted layer structure, MoO₃ deposited on a thin organic layer, a significantly different behavior was observed. The organic layer is highly doped, due to a strong diffusion of the deposited MoO₃ into the organic layer.
Financial support by BMBF via MESOMERIE Project (FKZ 13N10724) is gratefully acknowledged.

Organometallic molecules, such as porphyrin, are being investigated as circuit elements for organic electronics. Porphyrins are highly conjugated aromatic molecules that have shown electronic properties that exhibit high and low conductance states. These conductance states may be related to conformational changes in the molecule. To investigate these conductance states we plan to perform inelastic electron tunneling (IET) spectroscopy of porphyrin molecules in an electromigrated nanogap. IET spectroscopy is an important tool in determining the vibrational modes of a molecule adsorbed to a metal oxide and is found by identifying peaks in the second derivative of the junction characteristics (d²I/dV²).
We form nanogaps via electromigration of a 20x100 nm gold wire, producing gaps of ~2-3 nm, which is approximately the length of a porphyrin molecule. Each end of the porphyrin molecule is functionalized with a thiol group (-SH) which will covalently bond to the gold, forming a molecular junction. We simultaneously measure I-V, dI/dV, d²I/dV² of the junction at 4.2 K and 300 K. Electron transport will be compared between an empty nanogap and a molecular junction. The size of the junction will be calculated using Landauer theory, and then compared to scanning electron microscopy (SEM) images of the gap.

We have explored hybrid interfaces comprising organic dyes as sensitizer monolayers on metal-oxide mesostructures films, which have been highly successful when implemented in dye-sensitized solar cells (DSSCs). High-performance dye-sensitized solar cells are usually fabricated using nanostructured TiO₂ as a thin-film electron-collecting material. However, alternative metal-oxides are currently being explored that may offer advantages through ease of processing, higher electron mobility, or interface band energetics. We have conducted a comparative study of electron mobility and injection dynamics in thin films of TiO₂, ZnO, and SnO₂ nanoparticles sensitized with Z907 ruthenium dye. Using time-resolved terahertz photoconductivity measurements, we show that, for ZnO and SnO₂ nanoporous films, electron injection from the sensitizer has substantial slow components lasting over tens to hundreds of picoseconds, while for TiO₂, the process is predominantly concluded within a few picoseconds [1]. These results correlate well with the overall electron injection efficiencies we determine from photovoltaic cells fabricated from identical nanoporous films, suggesting that such slow components limit the overall photocurrent generated by the solar cell. We conclude that these injection dynamics are not substantially influenced by bulk energy level offsets but rather by the local environment of the dye nanoparticle interface that is governed by dye binding modes and densities of states available for injection, both of which may vary from site to site. For SnO₂ in particular, such effects appear to be influenced by a “light-soaking” effects, i.e. we observe a monotonic speeding-up of electron transfer from the photoexcited dye into the semiconductor following exposure by a pulse train at 550 nm wavelength [2]. We postulate that these effects are caused by photoinduced charging of the SnO₂ inducing a rearrangement of charged species or loss of surface oxygen at the dye-sensitized heterojunction. In addition, we explore how surface coating and percolation pathway formation occurs when a solid-state hole transporting material (spiro-OMeTAD) is infiltrated into the pores of a nanoporous TiO₂ network used in solid-state dye-sensitized solar cells [3]. We find that as the hole-transporter coats the surface of the dye-sensitized TiO₂, the yield of hole transfer from dye sensitizer to hole transporter increases with pore-filling fraction and saturates around ~30%. Using a simple model of random infiltration of spiro-OMeTAD into the TiO₂ porous network, we hence show that that charge diffusion through the dye monolayer network must precede transfer to the hole-transporting material. We further observe sharp onsets in photocurrent and power-conversion efficiencies with increasing pore-filling fraction correlate well with percolation theory predicting the points of cohesive pathway formation in successive spiro-OMeTAD layers adhered to the pore walls.
[1] P. Tiwana, P. Docampo, M. B. Johnston, H. J. Snaith, and L. M. Herz, ACS Nano 5, 5158 (2011).
[2] P. Tiwana, P. Docampo, M. B. Johnston, L. Herz, and H. Snaith, Energy Environ. Sci. 5, 9566 (2012).
[3] C. T. Weisspfennig, D. J. Hollman, C. Menelaou, S. D. Stranks, H. J. Joyce, M. B. Johnston, H. J. Snaith, and L. M. Herz, #8232;Adv. Func. Mater. 24, 668 (2014).

ZnO is currently attracting significant interest as a candidate for hybrid photovoltaic and light-emitting devices. We studied - in an all-ultrahigh vacuum approach - the interfacing of ZnO with various conjugated organic molecules, including perylene derivatives, oligo-phenylenes as well as ladder-type oligo-phenylenes whose fundamental optical excitation is resonant to the ZnO band gap. The morphology and electronic structure of the hybrid interfaces were determined by in-situ electron diffraction, scanning probe microscopies, photoemission and reflection spectroscopy, complemented by ex-situ transmission electron microscopy and X-ray diffraction analyses. By appropriate interfacial design, we are able to tune electron-hole separation at the ZnO/organic interface and to achieve excitonic energy transfer with efficiencies of up to 80 %.

Cyanine dyes, often utilized in dye-sensitized solar cells (DSSC), are prone to aggregation forming a range of molecular species from monomer to H- and J- aggregates in both solution, and when adsorbed on an oxide semiconductor photoelectrode. The relative efficiency of the different adsorbed dye species to inject photo-excited electrons into single crystal zinc oxide conduction band and produce photocurrent was determined by simultaneous attenuated reflection (ATR) UV-vis absorption and incident photon current efficiency (IPCE) spectroscopy measurements. ATR measurements enable identification of the dye species populating the surface, while IPCE spectroscopy pinpoints the species contributing to the photocurrent.
We report studies of the sensitization of the dicarboxylate cyanine dye, R8 (2,2&’ carboxymethylthiadicarbocyanine bromide), adsorbed on a zinc oxide (ZnO) single crystal surface. Sequential sensitization photocurrent measurements over a range of R8 dye concentrations from 10^-6 M to 10^-5 M on the single crystal ZnO produced a mixture of adsorbed species from monomer to aggregate. Previous work by the Parkinson group where ZnO was sensitized with G15 (2,2&’ carboxymethylthiacarbocyanine bromide) dye revealed strong agreement between ATR UV-vis absorption spectra and IPCE spectrum indicating all absorbing species were equally contributing to the photocurrent.¹ On the contrary, for R8 we observe a disproportionately greater contribution to photocurrent from the R8 aggregates compared to the monomeric species as measured by ATR dye absorption spectra.
(1) Rowley, J. G.; Parkinson, B. A. Simultaneous Measurement of Absorbance and Quantum Yields for Photocurrent Generation at Dye-Sensitized Single-Crystal ZnO Electrodes. Langmuir2013, 29, 13790-13796.

The orientation of organic semiconducting molecules within electronic and optoelectronic devices has been shown to have significant effects on device performance [1]. It is known that the molecular orientation of organic films on a surface can be influenced by the complex interplay of the intermolecular (molecule-molecule) interactions and interactions between the molecules and the surface [2]. The use of substrates to independently control molecular orientation and charge injection has previously received little attention.
Ferroelectric materials possess a permanent surface dipole moment and as such can be expected to behave as a type of interacting substrate. The effect of ferroelectric materials on the molecular orientation of organic films has not been previously investigated.
Electrodes modified to have a surface dipole moment have previously been shown to improve device efficiency in OLEDs by decreasing the energy barrier to charge injection [3]. The influence of this surface dipole on device efficiency and the potential for influence over molecular orientation makes the use of ferroelectrics as an interacting substrate very attractive.
In this work the interaction between ferroelectric substrates and organic films has been studied for the first time. Two different molecular systems similar in chemical structure but with different dipolar characteristics, vanadyl phthalocyanine (VOPc) and metal free phthalocyanine (H₂Pc), have been investigated. Diffraction and microscopy studies have been conducted to elucidate the effects of the permanent dipole moment of the substrates on the morphology, texture and molecular orientation of the deposited organic layers. Ultra-thin films (1-5ML) of VOPc and H₂Pc have been grown on single crystal ferroelectrics and the effect of the surface on the molecular structure has been studied using STM and LEED. To gather a more cohesive picture of our system thicker organic films (up to 50 nm) have also been examined using AFM and XRD. The potential for incorporation of ferroelectric thin films into devices will also be discussed.
[1] B. Rand et al, Adv. Fun. Mat.22, 2987 (2012)
[2] A. Cruickshank et al, J. Am. Chem. Soc. 35, 14302 (2012)
[3] S. Khodabaksh et al, Adv. Fun. Mat. 14, 1205 (2004)
[4] W. Chen et al, Adv. Fun. Mat. 21, 410 (2011)

The nature of the donor/acceptor interface in an organic heterojunction solar cell plays a critical role in determining functional properties, but relatively little is known experimentally about how the interfacial arrangement of donor and acceptor molecules determines the photoexcited electron dynamics and carrier localization. We address this question by using resonant Auger electron spectroscopy on bilayers of copper phthalocyanine (CuPc) as the donor and C60 as the archetypal acceptor. We find that the ET rate depends strongly on the relative molecular arrangement: the interface where the model donor CuPC is oriented face-on with respect to C60 yields a rate that is approximately 20 times faster than that of the edge-on oriented interface.
We explore the implications of these results for free charge carrier generation in blend organic photovoltaic devices.

This study demonstrates a charge recombination layer(CRL) composed of zinc oxide (ZnO) and self-assembly monolayers(SAMs) for organic tandem photovoltaic (OTPV). This thin ZnO layer was fabricated with spray pyrolysis method. SAMs were passivated on both sides of the ZnO to generate “ambipolarity“ for charge recombination. Afterwards, a simple transfer-printing approach was utilized to transfer such layer to the OTPV. Various techniques have been applied to characterize this CRL, and the results shown that the CRL is crystalline structure with improved electrical properties and high transmittance. As a consequent, this novel CRL provides better charge recombination and higher photon absorption of the rear cell, which enhancing the Jsc, Voc, and thus the overall power conversion efficiency of the OTPV.

Given that the Earth&’s reserve of indium is only approximately 6000 Tons, alternative transparent conducting oxides (TCOs) to indium-tin-oxide (ITO) constituted from Earth abundant elements will be critically important to satisfying the growing demands in organic light emitting diodes (OLEDs), solar cell windows, displays, and other applications. The hole injection capability of ZnO based TCO thin films can be improved by increasing their workfunction to better align with the HOMO levels of adjacent organic layers in OLEDs. Oxygen plasma treatments, introducing nanoscopic large workfunction interfacial layers (MoO_x), and developing p-type ZnO were investigated as methods for achieving this goal. The experimental results along with density functional theory (DFT) studies indicated that plasma treatments result in an electronegative surface which provides an associated dipole moment that reinforces the original surface dipole moment, leading to increase in workfunction. The workfunction of rf magnetron deposited ZnO films increased from 3.74 eV to 4.21 eV with oxygen plasma treatment. The workfunction of AZO films increased from 3.96 eV to 5.23 eV upon deposition of nanoscopic (2 to 4 nm) MoO_x interfacial layers. It is expected that such thin layers will also facilitate current injection by tunneling. In parallel p-type ZnO:Sb films were obtained by annealing pulsed laser deposited ZnO:Sb films in an oxygen environment at 460°C for 1 hour. The typical hole concentration and resistivity obtained were in the 5.35x10¹⁸ to 1.0x10¹⁹cm^-3 and 1.07 x 10^-1 to 7.13 x 10^-1 Omega;.cm ranges, respectively. Based on x-ray diffraction, x-ray photoelectron spectroscopy and Density Functional Theory modeling, it is proposed that as-deposited ZnO:Sb films are n-type due to native, and donor defects, and that p-type conductivity in ZnO:Sb films is due to the formation of acceptor complexes, where Sb sits on Zn sites and forms defect clusters with two adjacent zinc vacancies. The aforementioned ZnO variants are being evaluated in hole-only and complete OLEDs devices to quantify their hole-injection efficiencies, and impact on device power efficiency.

The introduction of nanostructured metal oxides as electron acceptor resulted, among others, in the concept of extremely thin absorber cells. Thus, low cost fabricated inorganic semiconductors like Sb₂S₃ can be utilized as sensitizer, which typically possess promising properties, such as a tunable band gap and a high extinction coefficient. These absorbers are then combined with an organic hole-transport material (HTM).
We have fabricated planar TiO₂:Sb₂S₃:hole-transporter hybrid solar cells with high efficiencies (over 4% in maximum) to investigate the role of different HTMs. Ideally the Sb₂S₃ is combined with a HTM, which also contributes to the charge generation via disjunctive absorption of light and electron transfer over the Sb₂S₃ to the TiO₂ film. However, we did not observe additional current generation by the HTM. Contrary, our investigations revealed a disadvantageous influence on charge generation due a filter effect resulting in parasitic absorption of light by the HTM. We therefore conclude that in principle matching energy levels do not necessarily allow charge transfer and special care has to be taken in the selection of the right HTM. However, our study shows the high potential for increased efficiencies of such planar extremely thin absorber cells, if a suitable HTM can be found.

Hybrid indium tin oxide (ITO)/organic compounds interfaces were formed by modifying the ITO substrate with environmentally friendly biomaterials of amino acids and peptides. With the modification, the work function of the ITO reduced from 4.7 eV to 2.5 eV. With the hybrid interfaces, organic solar cells exhibited a power conversion efficiency (PCE) of 8.6%, and organic photodetectors showed a highest detectivity up to 5.78×10¹³ Jones at zero bias. This work provides a way of hybrid indium tin oxide (ITO)/organic compounds interfaces for high performance organic based devices.

Organic light-emitting diodes (OLEDs) have attracted much interest because they can be applied to flexible displays. Flexible displays are expected to be used in mobile and/or wearable terminals and to produce sheet-shaped displays that can cover large areas in the future. Several mass-produced flexible displays have launched last year. The tide still remains today. However air-sensitive electrode and electron injection layer, which require a strict encapsulation, in those flexible devices have been employed, and also induce the fabrication cost. Practical inverted OLED without a strict encapsulation is eagerly anticipated.
Therefore inverted OLEDs having air-stable two-layered buffer layers, which consist of both metal oxide layers (MOLs) and organic buffer layers (OBLs), have been proposed as a promising structure, in order to prevent degradation under oxygen and moisture for long-term device stability. We have already started the development of the novel practical inverted OLED device structure since the mid of 2011. Several types of organic buffer layer materials and metal oxide layer materials have been developed. Recently, some similar inverted OLEDs were published. S.Höfle et.al. reported the inverted polymer light-emitting diode by combined the solution-processed polyethylenimine interlayer on the top of zinc oxide layer. However, both long-term air stability and operational stability are not so clear. This paper focuses on these device stability as well as initial EL characteristics of the inverted OLED with some two-layered buffer layers in the viewpoint of practical use.
We report our evaluations of the stability of an inverted OLED including published materials and structures. In the inverted OLED, the OBL was placed on the MOL, which deposited on the top of bottom electrode (cathode). In the conventional OLED, we used poly(3,4- ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS) as the hole injection layer on the bottom electrode (anode). The phosphorescent material used in both OLEDs was the red dopant Ir(piq)₃ (tris[1-phenylisoquinolinato-C2,N]iridium(III)). Both OLEDs, which were used to measure the device characteristics and operational stability, were encapsulated using a UV-epoxy resin and a glass cover in nitrogen atmosphere after upper electrode formation. The initial EL characteristics and the operational stability under 1000cd/m² of both the conventional OLED and our inverted OLEDs were evaluated, and we found that they were similar to or greater than that of the conventional OLED. The air stabilities of a conventional OLED and the inverted OLED were evaluated by encapsulating them using a barrier film with a water vapor transmission rate of 10^-4 g/m²/day. Dark spot formation was clearly observed in the conventional OLED after 15 days, whereas it was not observed in the inverted OLED after 250 days. We will discuss the detailed EL characteristics including the operational stability.

This work reports photo-electrochemical activity towards the reduction of dissolved oxygen in aqueous solutions exploiting novel hybrid organic-inorganic systems, based on the coupling of various photoactive conjugated polymers with nanostructured palladium oxide (PdO) as semiconductor photocathodes.
Polymer-based thin films on conductive glass (ITO) clearly showed negative photocurrents depending on the presence of the oxygen in the aqueous solution. Such photo-effect can be related to the excitation by light of the neutral form of the organic materials. Among all investigated pristine and blended bulk-heterojunctions, APFO-3:PCBM attained the highest photocathodic currents. In the next step, the alluded blend has been spin-coated in the matrix of hyperbranched nanostructured palladium oxide (PdO), realizing a photo-electrochemical amperometric dissolved oxygen sensors in aqueous solutions.
Proper choice of materials and device optimization in parallel with suitable selection of electrochemical parameters resulted in an optimal condition under which linear calibration plots were derived, with R²_adj of 0.987 and a sensitivity of -5.87(mu;A/cm²⁾/ppm.
Analysis at different pH, acid, neutral, and basic was also performed, revealing in all studied cases high reproducible sensing performance, making the possible use of the sensor in various fields of research.

The working principle of electrolyte-gated transistors (EGTs) is analogue to that of traditional field-effect transistors, but takes advantage of the use of a convenient electrolyte (a solid, a liquid or a gel) instead of the gate dielectric. Application of a voltage to the gate electrode causes accumulation of ions at the electrolyte-semiconductor interface, and the formation of a nanometrically thin electrical double layer (EDL). As the capacitance of an EDL is orders of magnitude greater than that of traditional insulators, EGTs show high drain current and high transconductance at very low driving voltage. These characteristics make EGTs very promising in flexible and printed electronics as well as in bioelectronic sensors. In this paper we present solution-processed EGTs employing semiconducting metal oxide nanoparticles as the channel materials. Thanks to the environmental stability of metal-oxide, these devices can be driven in aqueous electrolyte without compromising the device performances. Our EGTs show low turn-on voltage (<0.5 V), a high on/off ratio (>10³) and transconductance exceeding 0.1 mS. Furthermore, the use of metal oxide nanoparticles allows for low temperature processing and straightforward surface functionalization, leading to selective and flexible biosensors. The application of MOx nanoparticle EGTs in simple ion-selective and enzymatic sensors is demonstrated.

In an organic photovoltaic (OPV) device exciton dissociation, charge separation, and charge recombination all predominantly occur at interfaces between electron donor (D) and electron acceptor (A) molecules. The nature of these interfaces, including the intermolecular arrangements between D and A molecules along with the resulting energetic landscapes, are therefore pivotal factors in determining PV performance. First, the D-A interface energetics determine the energy of the charge-transfer (CT) state, and the energy of the CT state (E_CT) limits the maximum open-circuit voltage (V_OC). Second, the intermolecular arrangement at the D-A interface determines the relative rates of charge separation and charge recombination. In polymer-fullerene based bulk-heterojunction (BHJ) devices the intermolecular arrangement between polymer and fullerene appears to have a large influence on PV performance. Specifically, for donor-acceptor type polymers, where “donor” and “acceptor” refer to the covalently linked electron rich and electron deficient moieties that compose the donor polymer, it appears preferable for the fullerene to dock with a specific part of the polymer. Through a literature survey combined with a study of specifically designed polymers we show that PV performance tends to be higher when the fullerene docks with a specific cite on the polymer, and this specific site tends to be with the electron deficient moiety of the polymer.

The processes that generate current in organic photovoltaics are highly dependent on the micro- and nano-structure in the semiconductor layers, especially at the donor-acceptor interface. Elucidating film properties throughout the thickness of the devices is therefore key to their further development. In this contribution we report a new approach that has been optimised for cross sectional analysis of organic films in solar cells. This process involves the removal and thinning to electron transparency of a TEM foil from the bulk of the sample using a focussed ion beam (FIB) instrument. We have developed a methodology to minimise the damage induced by the Ga ions and demonstrate that this allows the acquisition of high quality imaging and spectroscopy data.
Utilisiation of this methodology has enabled us to gain unprecedented insights into the structure and composition of the molecular layers within the depth of device structure using high resolution transmission electron microscopy (HRTEM). The technique has been applied to three archetypical solar cell configurations consisting of copper phthalocyanine (CuPc) and C₆₀, which have been cross-sectioned using a focused ion beam method optimized to minimize sample damage. The HRTEM images exhibit lattice fringes in both CuPc and C₆₀, confirming the crystallinity and texture of both materials, and offering novel insight into the growth of C₆₀ onto molecular materials. The donor-acceptor interface morphology was further studied using scanning transmission electron microscopy (STEM) in combination with energy dispersive X-ray (EDX) spectroscopy, extending the scope of our methodology to amorphous heterostructures.
Electron energy-loss spectroscopy (EELS) in the STEM is arguably the only technique that can provide infromation on chemistry and bonding with molecular spatial resolution. In a monchromated STEM-EELS system it is possible to achieve spectral energy resolution that is sufficient to be able to identify molecular functional groups with high spatial resolution. We will demonstrate how monochromated valence loss EELS can be used to directly investigate the donor-acceptor interface in cross-sections of organic solar cells, opening up new approaches to engineer the electronic properties of interfaces in OPV devices.

Organic and hybrid semiconductor devices are attractive for a range of thin film and flexible electronic and optoelectronic applications because specific physical and material properties can be carefully tailored through chemistry. Disentangling the influence of materials design, process dependent morphology, composition, and microstructure, and the device architecture on the physical properties such as charge generation, transport, lifetime, recombination pathway, and injection/extraction efficiency of the contacts remains extremely challenging given their complex interplay. In this presentation I will discuss the use of impedance spectroscopy and transient techniques to probe fundamental charge properties and their evolution with device bias or other stimulus. This combined, more nuanced approach is found yield greater understanding about the role of mobile and immobile charge on electronic properties, as well as accumulated charge at critical buried device interfaces that are typically difficult to interrogate and accurately assess by other measurement methods.

Printed organic electronics, a technology based on organic semiconductors that can be processed into thin films using conventional printing and coating techniques, has been the subject of active research and development over the past decades. Due to their ability to be processed at low temperature, over large areas, at low cost, organic semiconductors are experiencing an accelerated development that will lead to a new generation of products with thin and flexible form factors. While the organic semiconductor layer plays a central role, the interfaces that are formed between the organic semiconducting layer and adjacent oxide layers are very critical and often determine the overall electrical performance of the device.
In this talk, we will discuss novel device architectures that incorporate organic semiconductors and insulating or semiconducting oxide thin films processed by atomic layer deposition (ALD). The performance of a range of solid-state devices, including organic field-effect transistors (OFETs), sensors, and solar cells, will be presented. Our results show that ALD offers unique advantages over alternative thin-film deposition techniques that can yield devices with higher performance and longer lifetime. We will show that these advances are likely to accelerate the deployment of flexible printed electronic technologies.

Bathocuproine (BCP) is one of the most frequently used materials for interfacial layers in organic solar cells. It is inserted between electron-acceptor layer (e.g. fullerene and its derivatives) and metal cathode (Ag or Al) to improve power conversion efficiency [1]. Although much effort has been devoted, its mechanism has not yet been clarified. It is widely believed that BCP works as an exciton blocking layer because the HOMO (highest occupied molecular orbital) level of BCP lies 6.5 eV below the vacuum level (VL) which is significantly lower than the hole conduction levels of the other materials in the photovoltaic cell. By adding the optical gap of 3.5 eV to the HOMO level, the LUMO (loweset unoccupied molecular orbital) level is often assumed to lie at 3.0 eV below VL. Since this value is substantially higher than the workfunction of the cathode and the LUMO level of the fullerene derivatives, the electrons is believed to conduct through the gap state of BCP. On the other hand, recent photoemission studies suggest that the LUMO of BCP aligns with the electron conduction levels of the cathode and acceptor owing to the vacuum level shift [2]. In the latter scenario, electrons conduct through the LUMO level of BCP. Such confliction arises from the lack of reliable data on the LUMO levels of BCP because there were no proper experimental techniques.
Recently, we have developed a new experimental technique, low-energy inverse photoemission spectroscopy (LEIPS), to examine the LUMO levels of organic semiconductors [3]. In this method, electrons having kinetic energy below 4 eV is introduced to sample films. By detecting near-ultraviolet photons emitted due to the radiative transition, the density of unoccupied states including the LUMO levels are precisely examined. In contrast to the previous inverse photoemission spectroscopy (IPES), the damage to the organic samples is negligible because the electron energy is below the damage threshold. Since the near-ultraviolet photons can be analyzed using the interference bandpass filters, the energy resolution is improved to 0.25 eV which is a factor of two better than the conventional IPES.
Using LEIPS, the LUMO level of BCP was precisely examined. Surprisingly, the determined LUMO level is found to be more than 1 eV higher than that believed before urging to reconsider the previous discussions. LEIPS spectrum shows gap states when Ag is deposited on the BCP layer. With the aid of DFT calculation, the reaction product between BCP and Ag is identified. From these findings, we conclude that the electrons in the BCP layer conduct through the gap state which is formed by chemical reaction between the metal and BCP through the diffusion of the metal into the organic layer.
[1] Peumans, et al. Appl. Phys. Lett. 76, 2650 (2000).
[2] Sakurai et al, J. Appl. Phys. 107, 043707 (2010).
[3] H. Yoshida, Chem. Phys. Lett., 539-540, 180 (2012); Anal. Bioanal. Chem.406, 2231 (2014).14).

The presence of interlayers between the active layer and the electrode are known to modify the metal work-function and enhance OPV device performance. Spontaneous formation of interlayers eliminates separate processing steps and hence is technically advantageous and cost effective. However, surface enrichment during film processing of the interlayer material is limited to materials with low surface energy. Here we show that migration of the interlayer molecules to the organic/electrode interface can be induced by interlayer molecule-metal interactions. This is demonstrated by blending polyethylene oxide (PEG), a known interlayer material with a surface energy higher than that of P3HT and PCBM, into the active layer. XPS analysis reveals that, as expected, PEG is not present on the surface of the organic spun film. However, Ca or Al evaporation induces a significant migration of PEG to the organic/metal interface. In contrast, Au evaporation does not induce such migration. The comparison between Al, Ca and Au, metals with significantly different reduction potentials revealed that the driving force for PEG migration is its chemical interaction with the deposited metal atoms. The extent of PEG migration was also found to depend on the type of the underlying substrate, ITO/PEDOT:PSS or ITO. Finally, the PEG interlayer results in a reduction the metal work function confirming that spontaneous additive migration induced by metal-additive interactions could be harnessed for improved charge extraction in organic electronic devices.

Because of the optical transparency and n-type conductivity, ZnO is commonly used as the electron collecting contact interlayer in inverted organic photovoltaics (OPVs). Thin ZnO interlayers can be easily fabricated in the laboratory from sol-gel precursor methods. Diethylzinc (deZn) has been shown to rapidly convert to ZnO (deZn-ZnO) with low thermal annealing requirements (120 °C or less). Zinc acetate (ZnAc) typically requires annealing temperatures above 280 °C to convert to ZnO (ZnAc-ZnO), although some recent reports suggest that temperatures in the range of 130 to 200 °C may be sufficient. We report here on a comparison of the effectiveness of using these two precursors as low temperature routes to forming ZnO interlayers in inverted OPVs.
We found that deZn-ZnO annealed at 120 °C performed similarly to ZnAc-ZnO annealed at 300 °C, when initially exposed to solar simulator irradiation, while ZnAc films annealed below 300 °C did not perform as well. Characterization of these films by X-ray diffraction (XRD) shows that the lower temperatures are insufficient for fully converting ZnAc to ZnO. When the two initially similar device architectures are exposed to long-term illumination under resistive loading the performance diverges significantly, with the deZn-based ZnO being more stable. Photoluminescence (PL) spectroscopy suggests that the two types of ZnO formed with the sol-gel precursors have different defects, and XRD shows different preferential orientation. We attribute the differences in defects and orientation, between the two types of ZnO, to the difference in precursor oxygen coordination to zinc, and the relative rates of reactions in the precursors.
Further, we report on simultaneous enhancements to stability and performance of devices with ZnAc-ZnO that are surface modified with a dipolar phosphonic acid molecule. Differences in PL of unmodified and surface modified ZnAc-ZnO also suggest differences in defect types within or at the surface of the film. We therefore propose that the difference in stability observed by using the two different sol-gel precursors and by using surface modifiers is due to the presence of defects which may form reactive centers for degradation of ZnO/organic interfaces withing OPV devices.

The wide range of work function (WF) tunability of metal electrodes has been desired for high performance organic electronics, in which precise energy level matching at the metal/organic interface is crucial. We report unprecedented WF modifiers based on conjugated polyelectrolytes (CPEs) that can bilaterally shift ‘upward&’ or ‘downward&’ the effective WFs of various metal electrodes with a maximum ‘net&’ variation up to ΔWFsim;1.2 eV (+ 0.4 eV for n-type and minus; 0.8 eV for p-type). The new ‘downward&’ CPEs, p-type CPEs, originate from the reversed dipoles that are induced by a facile oxidative process of conventional ‘upward&’ n-type CPEs, in which ionic side-chains develop molecular dipoles and shift WF of the metal electrodes. Moreover, because our p-type CPEs can be applied to various metals, including indium tin oxide, Ag, Au, Cu, and even graphene, our approach is universal and promises versatile interface engineering in organic electronics (organic solar cells and light-emitting diodes) with substantially enhanced device efficiency and lifetime.

Inorganic semiconductors can contribute in many processes in hybrid solar cells, including selective carrier collection, exciton dissociation, and defining the nano/micro morphology. Here we show the importance of controlling the local electric fields at oxide/polymer interfaces for charge extraction. We demonstrate the advantage of maximizing the potential drop across thin oxide layers in bi-layer heterojunctions and at the interface of an oxide selective contact with a polymer/fullerene bulk heterojunction.
We compare n-type (ZnO) and p-type (CuSCN) inorganic substrate oriented nanorod scaffolds for use with polymer/fullerene solar cells. In both cases the nanorods appear to hinder device performance, likely related to the interfacial electric field. Lastly we present a series of crystalline nanostructures of both ZnO and CuSCN with controlled crystal orientation and morphology by co-electrodeposition with organic dyes.

ZnO nanostructures have received much attention for thin film solar cell applications. Their use in organic hybrid solar cells, especially as part of the donor-acceptor junction, promises both optical and electronic advantages. The nanostructure provides an increased specific surface area; this results in a decreased effective organic layer thickness for a given amount of organic material. The reduced effective layer thickness means the path, required for excitons to diffuse, is shortened, which in turn allows an increased amount of organic material per area and thus absorption to be increased. Nanostructures with dimensions in the wavelength range of the incident light exhibit optical properties which may further enhance light absorption and thereby charge generation in solar cells, i.e. increased light scattering resulting in a longer optical path through the absorber and/or reduced reflectance due to the so-called moth eye effect. The optical benefit due to these special properties is however debatable for organic hybrid solar cells, as ZnO and the organic absorber materials typically exhibit similar refractive indices.
A range of ZnO nanorod array (ZNA) morphologies have been deposited using a simple, low temperature electrochemical method scalable to larger areas (10×10 cm² demonstrated). The ZNA were characterized using photoluminescence, optical transmission/reflection, XRD and XPS measurements [1]. Organic hybrid solar cells based on P3HT and ZNA showed an improvement in short circuit current by a factor of ~2 compared to devices based on planar RF-sputtered i-ZnO films. Optical measurements demonstrate that ZNA strongly reduce the reflectance in ZNA/P3HT devices. Angular-resolved optical scattering measurements show also that the light scattering to higher angles is increased in the presence of ZNAs compared to planar ZnO films. This effect is most pronounced for short wavelengths where also the observed increase in EQE is highest. Theoretical simulations are conducted to assess the origin of this optical effect.
[1] W. Ludwig, W. Ohm, Y. Zhao, M. Ch. Lux-Steiner, S. Gledhill, Phys. Stat. Sol. (a), 2013 (210) 1557.

Metal oxide hole transport layer (HTL) greatly improves the performance and stability of organic photovoltaic (OPV) devices. Currently, much research attention has been put onto developing p-type HTL, such as NiO, to replace the often-used n-type MoO₃, V₂O₅ and WO₃, to avoid unwanted recombination at the anode contact. Here, we report for the first time to use Cu(1+)-based delafossite compound, Cu^IM^IIIO₂ (M= Al, Ga, or Cr), as p-type HTL for OPV devices. As a promising p-type transparent conductive oxide (p-TCO) material, CuMO₂ has been previously applied to flat-panel display, ultraviolet light-emitting diode, and recently to replace NiO as photocathode in p-type dye sensitized solar cells (p-DSSCs). Compared with NiO, CuAlO₂ or CuGaO₂ have ~ 0.2 eV deeper valence band edge, 4~7 order of magnitude higher hole mobility, and better optical transparency,¹ making these oxides a p-type HTL candidates for OPV. However, previous reported CuMO₂ film requires high temperature sintering (ge;1100 °C) and hydrothermal synthesis of CuMO₂ nanoparticles are too large (ge; 100 nm).² Here we report successful synthesis of CuMO₂ nanoparticles with size le;10 nm by microwave-assisted heating. We confirm the material phase by X-ray diffraction and selected area electron diffraction. From the valence band position determined by photoelectron spectroscopy, bandgap by UV-Vis, and work function by kelvin probe, we establish that these nanoparticles are p-type. We will discuss the effect of different CuMO₂ (M= Al, Ga, or Cr) nanoparticles as p-type HTL for various OPV device systems. In addition, we found that the interface between CuMO₂ HTL and active layer can be modified by solvent rinsing and greatly affects the OPV device performance. The impact of such an interface, resulted from the CuMO₂ nanoparticle surface ligand, on OPV device performance will also be discussed.
1. M. Yu, T. I. Draskovic, and Y. Wu, Phys. Chem. Chem. Phys., 2014, 16, 5026.
2. B. J. Ingram, G. B. González, T. O. Mason, D. Y. Shahriari, A. Barnabè, D. Ko, and K. R. Poeppelmeier, Chem. Mater., 2004, 16, 5616-5622.
This project is sponsored by National Science Foundation DMR-1305893

In this report, we describe a simple thin film processing process using cuboidal barium titanate nanocrystals (BTO NCs) as building blocks and a layer-by-layer self-assembly technique. Scanning electron microscopy (SEM) images show that BTO NCs thin films are uniform and continuous and their thickness can be finely tuned by controlling the number of ferroelectric layers. Barium titanate nanocube films exhibit high dielectric constant values (ε=230 @ 1 kHz) in tandem with a low dissipation factor at room temperature. The BTO NCs thin films were cast on flexible substrates (Kapton® polyimide) to form dielectric gates, thereby demonstrating that they can be easily integrated into flexible and transparent field effect transistors. Microelectrodes have been attached to these nanocrystals-based thin film structures using a two-step photolithography process and both the output and transfer characteristics of the resulting devices were subsequently tested. The preliminary results indicate that the nanoparticles-based transistors show negligible current leakage coupled with a low-hysteresis and low low-voltage operation.

The characteristics of bulk heterojunction (BHJ) solar cells are largely determined by their electronic structure, in particular the relative HOMO and LUMO levels at the buried interface, the exciton and charge transfer state energies, and the disorder energy. The talk will describe spectroscopic and electronic measurements, focusing on the electronic processes that depend on defects and disorder effects, and the heterojunction interface. Photocurrent spectroscopy is used to give accurate measurements of the HOMO and LUMO energy differences, from which energy band offsets and their influence on exciton dissociation can be explored. The cell materials are disordered and hence are characterized by a distribution of localized band tail states, and in some materials also a significant density of deep defect states. We explore the recombination mechanisms involving band tail states, particularly as they apply to the dark forward bias current-voltage characteristics. A new analytical model shows that the diode ideality factor is directly related to the band tail slope, provided that recombination through deep states is not significant.[1] Such deep state transitions are enhanced by light-induced defect generation that originates from hydrogen dissociation and migration to new bonding sites.[2] Increased optical absorption in BHJ cells can be obtained by forming ternary blends with either two polymers or two fullerenes. Such blends are effective when the materials are highly miscible. Measurements on these blends show that the exciton absorption is molecular in character, retaining the properties of the two blend materials, while the electron and hole states are delocalized and reflect the average composition of the alloy.[3]
[1] S. A. Hawks, G. Li, Y. Yang and R. A. Street, submitted
[2] R. A. Street, J. E. Northrup and B. S. Krusor, Phys. Rev. B.85, 205211 (2012).
[3] R. A. Street, D. Davies, P. P. Khlyabich, B. Burkhartb and B. C. Thompson, J. Am. Chem. Soc., 135, 986 (2013).

Titanium dioxide is a widely studied material which has a tendency for n-type conductivity due to oxygen vacancies. Therefore, the electronic properties of TiO₂ films can be optimized by controlling the type and amount of vacancy defects. In 2005, Yoon et al. demonstrated room temperature negative differential resistance (NDR) in an organic diode with a few nanometer thick interfacial TiO₂ layer [1]. They proposed that the NDR arises due to defect assisted tunneling through the plasma oxidized TiO₂ layer. Thus, TiO₂ defect characteristics should have a great effect on the operation of tunnel diodes exhibiting NDR. More recently, we extended this to anodic oxidation of Ti as a viable method to fabricate thin TiO₂ interfacial layers, with high amount of defects suitable for room temperature NDR [2].
In this work, we compare a variety of potential high volume roll-to-roll compatible TiO₂ fabrication methods to produce thin TiO₂ layers, with varying defect densities, to be used in hybrid oxide/organic tunnel diodes. In addition to anodic [2] and thermal oxidation, atomic layer deposition (ALD) [3] is studied as a way to fabricate high quality interfacial layers. A variety of ALD precursors and deposition temperatures are investigated to correlate their effect on layer properties. Electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS) are used to characterize the layer properties. Furthermore, defect characteristics of the hybrid oxide/organic NDR diodes are examined, as well as their effect on NDR phenomena. The results give insight into NDR diode operation and help to deepen the understanding of this hybrid inorganic/organic device.
[1] W.-J. Yoon, S.-Y. Chung, P. R. Berger, S. M. Asar, Room-temperature negative differential resistance in polymer tunnel diodes using a thin oxide layer and demonstration of threshold logic, Appl. Phys. Lett. 87 (2005) 203506.
[2] P. S. Heljo, K. Wolff, K. Lahtonen, M. Valden, P. R. Berger, H. S. Majumdar, and D. Lupo, Anodic oxidation of ultra-thin Ti layers on ITO substrates and their application in organic electronic memory elements, Electrochimica Acta, in press.
[3] Picosun Oy, 2014, accessed 19 June 2014, http://www.picosun.com/en/products/roll-to-roll+ald+chamber/

Tandem sensitized devices are currently limited by the performance of nickel oxide as p-type hole transport layer. Herein, we present zinc-cobalt-oxide as semiconductor for dye-sensitized photoelectrochemical and solar cells. Thin films deposited by co-sputtering from cobalt oxide and zinc oxide targets result to zinc substituted cobalt oxide (Zn-Co-O) - with homogeneous composition distribution, spinel structure, and p-type conductivity [Zakutayev, et al. Phys. Rev. B 85, 085204 (2012) and MRS Communications 1, 23 (2011)]. The flatband potential, by capacitance measurement, was determined to be ~0.7 V vs NHE. Coumarin 343 sensitized films were used in p-type dye-sensitized solar cell and showed internal quantum efficiency greater than 90% with open circuit voltage around 260 mV with iodide/triiodide redox couple. Intensity modulated photocurrent spectroscopy showed that Zn-Co-O has larger diffusion coefficient than nickel oxide. Using electrochemical impedance spectroscopy, the behavior of the device was fitted to a simplified transmission line circuit model with the major differences in parameters seen in the semiconductor-electrolyte interface. Further investigation of the interface between the electrolyte and the semiconductor is ongoing in order to determine the effect of surface chemistry on hole injection and charge collection efficiency. Understanding the reactions at the interface is essential to application of this material in photoelectrochemical cells.

Transparent Amorphous Oxide Semiconductors (TAOS) [1] represented by IGZO have larger electron mobility (>10cm²/Vs) than that of amorphous silicon, and are feasible to be deposited at low temperatures, by using a conventional sputtering process. Practical applications of IGZO-TFT which was reported in 2004[2], started to drive the high-definition & energy saving LCDs or OLEDs. For application to large-sized OLED backplanes, the above features of TAOS-TFT are advantageous. It is, however, necessary to coordinate the stacks of the OLED suitable so as to fit the feature of TAOS-TFT. Since the TAOS-TFTs operate only in N-channel, an inverted structure with a cathode located at bottom is more favorable than ordinary structure on the basis of the following reasons; (1) simplicity of the circuit. (2) lower electron injection energy from metal Al cathode (workfunction 3.9eV) . The latter is obvious from a view point of the energy alignment. The work function of TAOS is ~4.3 eV which is fairly smaller than that (~4.8eV) of ITO. The situation differs from the ordinary OLED stacked on p-Si TFT with a work function of ~ 5.2eV. For inverted structure, reduction of electron injection barrier is critical because the organic electron transport materials have low electron affinity. Thus, an electron injection layer with chemical stability and a small workfunction is a key
Electride is a material in which electrons serve as anions. Authors reported that 12CaOmiddot;7Al₂O₃ (C12A7) crystalline electride, a first RT stable electride, had a very low work function (phi; =2.4eV) and chemical inertness in 2007[3]. Although this material shows good electron injection properties for n-type organic semiconducting materials [4], it required high temperature to obtain thin films. Recently, we found that amorphous C12A7 electride (a-C12A7:e^-) thin film with anionic electrons of ~1x10²¹cm^-3 could be obtained by a conventional sputtering-deposition at room temperature. The resulting a-C12A7:e thin film is optically transparent (band gap us ~#65301;eV) and chemically inert. Their work function estimated by UPS was ~3.0eV and the sample is semiconducting.
We examined electron injection properties by fabricating the electron only devices with a structure of Al(cathode)/a-C12A7:e^-(10 nm)/Alq3(150 nm)/Al, and found the threshold voltage was ~half of that Al/LiF(0.5 nm)/Alq3(150 nm)/Al.
[1] H.Hosono, J.Non-Cryst.Sol. 352, 851(2006).
[2] K.Nomura,H.Ohta,A.Takagi,TKamiya,M.Hirano,,H.Hosono, Nature 432, 488 (2004).
[3] Y.Toda, H.Yanagi, T.Kamiya, M.Hirano, H.Hosono, Adv. Mater.19, 3564 (2007).
[4].H.Yanagi, T.Kuroda, K.Kim, Y.Toda, T.Kamiya, H.Hosono, J. Mater. Chem., 22, 4278 (2012); K. Kim, M.Kikuchi, M.Miyakawa ,H.Yanagi ,T.Kamiya, M.Hirano, H.Hosono, J. Phys. Chem. C111,8403(2007).

Organic photovoltaics (OPVs) have become an attractive technology that offer a lower cost
alternative to current commercial solar conversion technologies due to their potential for
low temperature, large-area, and high-throughput manufacturing. Further barriers that
must be overcome prior to commercialization lie in the development of OPV materials and
device architectures to result in improved efficiency and stability. To achieve this, we are
developing unique tools, design rules, and new materials for both active layers and
selective contacts. !While the active layer materials are important for determining the ultimate performance,
interfacial contact layers must be optimized both electronically and chemically for new
active layer components. Such interfacial contacts are believed to improve device
performance by a variety of mechanisms such as improved energy level alignment and
charge carrier selectivity leading to improved charge extraction and reduced
recombination. We are investigating the influence of the electronic properties of electron
and hole transport layer (ETL and HTL) contacts on device performance to gain a greater
understanding of the relative contributions of contact properties such as work function and
band alignment. In addition, we are studying the effects of interfacial chemistry on local
band alignment and device performance. The independent control of work function and
band alignment is shown to result in increased performance in both standard and inverted
device architectures. Through this approach we are able to actively tune the contact
properties to the ever changing properties of new active layer materials. !We gratefully acknowledge funding from U.S. Department of Energy under Contract No.
DOE-AC36-08GO28308 with the National Renewable Energy Laboratory for OPV device
development as well as the Center for Interface Science: Solar Energy Materials (CIS:SEM)
and the Center for Energy Efficient Materials (CEEM), Energy Frontier Research Centers
Funded by the U.S. Department of Energy, Office of Basic Sciences, under Award
Numbers DE-SC0001084 and DE-DC0001009, respectively.