Motivated by recent experiments on nanocrystal superstructures, we study theoretically optical and photo-current responses of hybrid complexes assembled from semiconductor quantum dots (QDs), nanowires (NWs), metal nanoparticles (NPs), and photosynthetic molecules. QDs and NWs can be arranged into light-harvesting complexes [1,2]. In these complexes, nanocrystals are coupled via Förster energy transfer (FRET). Consequently, this coupling creates a flow of excitons from QDs to NWs. Excitons harvested in NWs can be ionized and used to create photo-voltage. Using kinetic equations for excitons, we model exciton transport in QD-NW and NP-NW complexes and explain the origin of a blue shift of exciton emission observed in the experiment [3]. Another system of our interest is a complex composed of natural photosynthetic reaction centers, semiconductor QDs, and metal NPs [4,5]. We show that, by using superior optical properties of nanoparticles and involving energy transfer, one can strongly enhance an efficiency of light harvesting in natural photosynthetic systems [6-8]. Potential applications of hybrid exciton-plasmon systems are in photovoltaic devices and sensors. [1] J. Lee, A. O. Govorov, and N. A. Kotov, Nano Letters 5, 2063 (2005). [2] P. Hernandez-Martinez and A. O. Govorov, Phys. Rev. B 78, 035314 (2008). [3] J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature Materials 6, 291 (2007). [4] A. O. Govorov and I. Carmeli, Nano Lett. 7, 620 (2007). [5] A. O. Govorov, Adv. Mater., 20, 4330 (2008). [6] S. Mackowski, S. Wörmke, A.J. Maier, T.H.P. Brotosudarmo, H. Harutyunyan, A. Hartschuh, A.O. Govorov, H. Scheer, C. Bräuchle, Nano Lett. 8, 558 (2008). [7] I. Nabiev, A. Rakovich, A. Sukhanova, E. Lukashev, V. Zagidullin, V. Pachenko, Y. Rakovich, J. F. Donegan, A.B. Rubin, and A.O. Govorov, Angew. Chemie, 49, 7217 (2010). [8] I. Carmeli, L. Lieberman, L. Kraversky, Z. Fan, A. O. Govorov, G. Markovich, and S. Richter, Nano Letters, 10, 2069 (2010).

Dye-sensitized solar cells (DSCs) are inexpensive photovoltaic devices that can convert sunlight to electricity with relatively high efficiency. They are favorable alternatives to conventional solid-state solar cells consisting of materials such as Si, CdTe and CuIn_1-xGa_xSe₂. However, their use of organic liquid electrolytes seriously limits long-term performance and durability because of their inevitable problems of high volatility, leakage, and complex chemistry. Despite extensive studies to replace liquid electrolytes, the efficiencies of the resulting DSCs remain modest. Here we demonstrate that the p-type inorganic direct bandgap semiconductor CsSnX₃ (X = halogens or their mixtures) with high-hole-mobility can be solution-processable at room temperature to form all-solid-state DSCs and replace the problematic organic liquid electrolytes. CsSnX₃ compounds are made of inexpensive and earth-abundant elements. The resulting solid-state DSCs consist of CsSnX₃, nanoporous TiO₃ and the Ru dye, and exhibit conversion efficiencies up to ca. 10 per cent. References 1. I. Chung, B.-H. Lee, R. P. H. Chang, M. G. Kanatzidis, Nature2012, 485, 486. 2. I. Chung, J.-H. Song, J. Im, J. Androulakis, C. Malliakas, H. Li, A. J. Freeman, J. T. Kenney, M. G. Kanatzidis, J. Am. Chem. Soc.2012, 134, 8579.

Due to their unique features, semiconductor quantum dots (QDs) are presented as the ultimate frontier as sensitizers for photoelectrochemical solar cells [1],[2]. Up to now, the most interesting results in terms of device performances have been obtained by using polidisperse, in situ generated QDs by means of successive ionic layer absorption and reaction (SILAR) technique [3],[4],[which allows obtaining naked QDs directly grown on the porous structure of the photoanodes, thus guaranteeing an intimate contact between the two interfaces. Moreover, the deposition of networks of QDs presenting absorption features able to collect a wider region of the solar spectrum is easily possible [5]. This study is focused on the application of spray deposition (SD) [6] to the SILAR technique to generate QDs (CdS and PbS) on TiO2 photoanodes. We demonstrate that the use of SD-SILAR systematically results in higher amount of QDs together with smaller nanocrystals as compared with the classical immersion SILAR. Moreover, a reduced amount of chemicals is needed for the preparation of QDs, thus decreasing the environmental impact of the procedure. SD provides for a highly homogeneous coverage of the TiO2 photoanodes for the whole depth of the substrate. Evaluation of the performances of the quantum dot-sensitized solar cells indicates that devices prepared via SD-SILAR present improved functional properties, especially related to photoconversion efficiency and photocurrent density, both of them being almost two-fold the corresponding prepared by immersion SILAR. [1] S. Rühle et al., Chem. Phys. Chem. 2010, 11, 2290 [2] A. Shabaev et al. Nano Lett. 2006, 6, 2856 [3] Y-L. Lee and Y-S. Lo Adv. Func. Mat. 2009, 19 604 [4] H. Lee H et al., Nano Lett. 2009, 9, 4221 [5] A. Bragaet al., J. Phys. Chem. Lett. 2011 2 454 [6] S. Che et al., J. Aer. Sci. 1998, 29 271

By the year 2050, the world&’s energy utilization will have doubled while fossil fuels will be dwindling. Fortunately, if just 0.2% of the earth&’s surface were covered in 10% efficient solar cells by 2050, our energy needs would be met. The only real barrier to such widespread deployment is cost. There are several third generation solar technologies that could make widespread solar cell deployment economically feasible. Amongst those, Dye-Sensitized Solar Cells (DSSCs) show the most promise. With current DSSC efficiencies peaking at over 12.5%, the focus of research needs to shift to mass production. Our research focuses on using inkjet printing to produce the much cheaper DSSCs in a roll-to-roll manufacturing process on flexible substrates with CsSnI3, CsSnI2.95F0.05, and similar molecules as a solid-state electrolyte (SSE) as well as the Z-907 quasi-SSE. Currently, using an aqueous ink containing 10% TiO2 and a liquid I-/I3 electrolyte, we have produced 3.52% efficient DSSCs with a fill factor of 0.668. While this is a good start, there is room for improvement in both the efficiency and the manufacturability of the cells produced with inkjet printing techniques. To further optimize inkjet printed DSSCs, we are investigating TiO2 layer thickness as a function of print speed, drop size, drop spacing, and ink solvent. By maximizing the TiO2 layer thickness the number of printed layers required to produce the ideal DSSC thickness of around 13mu;m can be reduced. This allows cells to be produced with fewer print heads and accompanying lower capital and energy costs. To further reduce cycle times and energy costs, we are investigating the use of fast drying solvents such as low molecular weight alcohols, acetone, and acetonitrile. Additionally, we are optimizing the printability of our ink through the use of various surfactants and viscosifying agents. To ensure the ink will print, a viscosity of at least 7 mPa.s is ideal to ensure the printability of the inks. Currently we are examining glycerol, diethlyene glycol, and polyvinylpyrolidone as viscosifying agents for aqueous inks and polyisobutylene and cyclohexanol for solvent-based inks. In an effort to reduce the variation in efficiency between different cells, improve TiO2 distribution upon drying, and better control film spreading after deposition we are examining the use of Triton 100X, triethanolamine, and Surfynol 465 as surfactants in aqueous inks. To overcome the durability problems associated with I-/I3 while minimizing efficiency loss, we are currently studying the SSEs to investigate potential transport and recombination issues at the P-N junction, ability of the SSE to interact with the dye based on particle size, and the effects of various methods of depositing the SSE on cell efficiency. Also under examination is the effect of various solvents on the SSEs&’ transport characteristics and the potential for inkjet printing.

Efficiency of dye-sensitized solar cells (DSC) reached 11 % (certified efficiency of cells with more than 1 cm2). One of problems remaining is encapsulation of the cell. We focused on cylindrical solar cells because the shape allows easy and perfect encapsulation. In addition, it has been reported that CuInGaSeS (CIGS) type cylindrical solar cell has some advantages over flat type solar cells from the view point of the total amount of solar light harvesting in a day and light weight modules. We have reported cylindrical DSCs with 5.6 % efficiency. The cell was back contact type solar cells which do not need expensive and awkward transparent conductive oxide layered glasses (TCO). The TCO-less structure actually made the fabrication of the cylinder DSC possible. The cell consists of round-shaped glass/TiO2-dye layers fabricated on Ti protected metal mesh (working electrode)/gel electrolyte sheet/Pt-Ti working as an counter electrodes, from the outside to the inside. The fabrication process is precisely explained in the presentation. The photo active area was experimentally measured by a laser beam induced current (LBIC) method. It was found that the area where light does not reach directly also caused photoconversion. Projected photoactive area against the total area was calculated to be 66 %. However, the actual photoacitive area obtained the LBIC method was 93% of the total projected area. This was explained by the fact that the glass wall act as an optical wave guide. The optical wave guide effect was simulated by using ZEMAX software and these results were consistent with experimental results. The optical wave guide effect was largely affected by dielectric constant of electrolyte compositions. Finally, we report comparison of totally generated electricity in a day between cylindrical DSC and flat DSC. The former was 1.3 times larger than the latter which proved the effectiveness of the cylinder type DSC.

Among the other existing photovoltaic technologies, dye-sensitized solar cells (DSSCs) have a large potential for low production cost in the manufacturing of photovoltaic panels, although stability issues need to be addressed before to move towards a mass production. This talk will focus on a full solid device replacing the liquid electrolyte, which is one of the critical sub-systems incorporated in the DSSCs, with solid hole transporting materials, addressing the primary advantage to remove the extrinsic instability due to the presence of a liquid phase. All the materials employed in the device can be processed easily from solution, e.g. using spin-coating, doctor-blade coating and higher speed methods such as roll-to-roll printing. The materials we use include novel carbon-based organic semiconductors with a tendency to self-assemble in supramolecur structures, which allow controlling the morphology in the bulk and at the interfaces. Charge transport and interface charge dynamics will be discussed in detail as well as the origin of performance degradation under stressed solar aging conditions.

Understanding free carrier generation in solid-state dye-sensitized solar cells is crucial for optimizing device performance. Here we present a detailed photophysical study that directly elucidates how the commonly used additive Li-TFSI facilitates electron injection into the TiO₂ conduction band and we also demonstrate that reduction of the photoexcited dye prior to electron injection ("reductive quenching") is an important pathway for charge generation in solid-state cells. By comparing two all-organic sensitizers we find that careful design of the dyes to exploit both mechanisms is of utmost importance for obtaining good power conversion efficiencies in solid-state cells. Furthermore, we show that quantum efficiencies for charge generation close to unity can be achieved in particular by exploiting the reductive quenching pathway, while still maintaining broad photon absorption and a high open-circuit voltage[1]. Although charge generation can be very efficient in these cells, we also identify a significant loss channel: By exploiting the Stark effect on the dye molecules caused by the electric field across the interface, we observe that due to the high dielectric contrast between the organic hole transporter Spiro-MeOTAD and the TiO₂ the image charge of the holes is sufficient to attract some holes back to the interface, where they successively undergo recombination. We have developed a drift-diffusion model to extract the movement of charges close to the interface by monitoring the change of the Stark signal[2]. Our results are of general importance for solid-state dye-sensitized solar cells beyond the particular material system presented in this study, as they help to develop design rules for all-organic dyes in the ongoing quest for higher efficiencies. [1] M. Meister, I. A. Howard, N. Pschirer, R. Sens, I. Bruder, B. Baumeier, D. Andrienko, and F. Laquai, in preparation [2] M. Meister, B. Baumeier, N. Pschirer, R. Sens, I. Bruder, F. Laquai, D. Andrienko, and I. A. Howard, submitted

Organic/inorganic junctions play a central role in several promising technologies, including dye-sensitized solar cells, colloidal quantum dot based devices and spintronics. While many studies have been directed at understanding both inorganic/inorganic and organic/organic junctions, only a few investigations have been undertaken to characterize the fundamental processes that occur at hybrid organic/inorganic heterojunctions.^1,2 Here, we adapt the detailed balanced approach for heterotype organic junctions developed by Giebink, et al.³ to a traditional semiconductor device model. We use this to simulate device characteristics for several planar hybrid structures, and compare the model to that proposed by Forrest, at el.¹ that treats the hybrid structure as an inorganic diode in series with a space-charge-limited organic layer. From this model we infer the processes mediating carrier interactions across the heterointerface, and evaluate the role of Coulombically-bound polaron pairs at a high/low dielectric constant interface. The implications of these results on how to control charge dynamics through choice of materials and nanomorphologies will be considered. 1. S. R. Forrest, M. L. Kaplan, and P. H. Schmidt, J. Appl. Phys. 55, 1492 (1984). 2. H. Mendez, I. Thurzo, and D. R. T. Zahn, Phys. Rev. B 75, 045321 (2007). 3. N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest, Phys. Rev. B 82, 155305 (2010).

Thin film-photovoltaics, especially organic photovoltaics (OPV) as a low-cost technology require alternative solutions to ITO as transparent electrode. Driven by the desire to manufacture on flexible substrates several alternative technologies and ideas have emerged. Several of these are investigated and integrated into standard OPV cells using our established pin technology with evaporation of small molecules, doped transport layers and bulk-heterojunction geometries. Dielectric/Metal/Dielectric (DMD) layers with ultrathin-metal films are used as transparent top contacts, achieving equal performance as standard devices on ITO (Transmission >85% around 20 #8486;/sq, equal PCE of 2.5%). As bottom electrodes, random metal-nanowire network electrodes, from silver and copper are studied. The nanowire electrodes were deposited by spray or dip coating and achieve highly competitive properties, better than 85% transmission and sheet resistances below 20 #8486;/sq, with copper being slightly worse then silver. In addition, alternative materials like PEDOT:PSS with enhanced conductivity or carbon nanotubes are investigated. Most of these can also be combined with each other for transparent cells, demonstrating the applicability of all technologies to OPV, depending on the properties achieved by the electrode technology and the requirements of the product. All materials and technologies are studied and optimized for application as transparent electrode in OPV and show distinct advantages and drawbacks, which are discussed in a comprehensive summary.

Organic photovoltaic (OPV) cells still show significantly higher conversion losses than most of their inorganic counterparts. The study of the underlying physical limitations to the performance of OPV (and hybrid organic/inorganic) solar cells requires answering a multivariant question: a cell&’s performance, and in particular, its open-circuit voltage (Voc) depend on several inter-dependent parameters, such as the geometry of the junction, morphology and band gaps of the organic materials, the alignment of the donor and the acceptor energy levels, and the intra-gap density of states near the band edges. In order to highlight the contribution of the energy level alignment, we use a heterojunction between a wide band gap inorganic semiconductor, SiC, and various organic semiconductors. This simplified system allows us to manipulate the energetic alignment at the donor\acceptor interface with minimal affect on the other parameters. Furthermore, because of the 3 eV bandgap of SiC, optical absorption of solar photons is mainly limited to the organic semiconductor. Thus, we can modify the inorganic layer and its interface with the organic layer without changing the overall cell&’s absorption and bulk relaxation mechanisms. Thus, we modified the surface of single-crystal 6H-SiC with self-assembled alkyl silane monolayers. When different polarity head groups are attached to the molecules, a dipole is created and the local vacuum level surface is shifted, followed by a relative shift between the conduction band of the SiC and the LUMO of the organic absorber layer. The energy alignment was studied using electron photoemission spectroscopy and contact potential difference measurements. The Voc values of series of solar cell fabricated with different organic dyes all show a common trend of increased Voc as the dipole of the molecular layer becomes more negative. The dipole that is measured at the inorganic surface is, however, not fully manifested in the full device performance. To enhance the effect, we used a binary monolayer of short silanes with the desired head-group and a long alkyl silane. The two-step assembly procedure resulted in a homogenous mixed monolayer, where the long molecules act to protect the shorter dipolar molecules from interacting with the thermally evaporated organic over-layer. The effect of the molecular dipole on the Voc of the series of cells, made with the binary monolayer, was now significantly stronger, which suggests this approach as a viable one for molecular interface engineering of a photovoltaic junction.

In this report, we will present two new D-π-A porphyrin dyes, coded Y350 and Y486. The dyes are designed with a motivation to inhibit the recombination by introducing insulating alkoxyl chains on the dyes and yield higher open-circuit voltage (Voc) in a dye-sensitized solar cell (DSC). The synthetic procedure for the dyes will be described, and the photophysical and electrochemical properties of the dye will be demonstrated. Those results combined with the theoretical calculation enable us to rationalize the structure-property relationship of the porphyrin dyes. Furthermore, we investigate the photovoltaic properties of the dyes in conjunction with a cobalt complex redox electrolyte in a mesoscopic TiO2. The specific structure of the porphyrin Y350 greatly retards the interfacial electron recombination from TiO2 to cobalt electrolyte. The cell shows a very high Voc of 980 mV and a power conversion efficiency of 10.8 percent under simulated air mass 1.5 global sunlight.

Aerogels are attractive structures due to their high surface area, high porosity and particle interconnectivity. These structures are made via a sol-gel synthesis. The gel is dried supercritically to avoid the collapse of the structure as the solvent is withdrawn. Previous work has employed aerogels as an inert support for a thin TiO2 film. We have synthesized a conducting aerogel of fluorine-doped tin oxide (FTO). The 3D structure is intended to transport electrons faster than the traditional TiO2 film. TiO2 is deposited over the aerogel by atomic layer deposition (ALD) forming a favorable interface with dye molecules and protecting the conductive FTO from recombination with redox species in the electrolyte. We will report on characterization of the FTO layer and sintering temperatures. Additionally, iodide/triiodide and ferrocene/ferrocenium will be used as electrolytes in the FTO-TiO2, FTO-only and TiO2-only films to study photovoltaic cell performance.

We have designed and synthesized four new promising D-π-A conjugated organic sensitizers (RPT 9, NAT-440, NAT 622 and SFO-346) with a 4H-pyran-4-ylidene as a donor, a thiophene ring in the bridge and 2 cyanoacrilic acid as acceptor. The choice of this donor moiety relies on its proaromatic character, that is expected to improve the charge transfer process through the gain in aromaticity experienced by the donor fragment. The deliberate use of proaromatic donors for solar cells (Dye sensitized solar cells and organic solar cells) has not yet been explored, although the photovoltaic properties of some benzothiazolylidene merocyanines and other pyran derivatives have been recently reported. The aim of this work is the study of the effect of the side-chain modification of these dyes on the photovoltaic properties of complete devices. Thus, we have modified the donor and the thiophene groups in these dyes. The donor group has been exchanged between a phenyl and a tert-butyl group, anchored to the positions 2 and 6 of the 4H pyran-4-ylidene-unit, RPT-9 and SFO 346 dyes respectively. The thiophene group have been modified by the addition of two bulky hexyl groups, the NAT 440 and NAT-622 dyes respectively. The dyes can be easily synthesized in moderate yields by well-known organic reactions including Wittig-Horner, Knoevenagel and formylation reactions.

Dye-sentisized solar cells (DSCs) have attracted much attention as one of the most promising next-generation photovoltaics of low-cost fabrication. A crucial issue in this field is the development of the excellent organic dyes to achieve higher light-to-electricity energy conversion efficiency. Although many organic dyes have been developed so far, most of them have been limited to the compounds, in which the anchoring group having strong electron accepting ability, such as cyanoacrylic acid, is just introduced to the known organic dye skeleton. The development of organic dyes based on a new molecular design concept is necessary for the dramatic improvement of efficiency. We here disclose our new molecular design concept for the organic dyes in DSC, in which an intramolecular boron-nitrogen (B-N) coordination bond formation is utilized as a key scaffold for the π-electron accepting unit. In this system, we envisioned several effects; 1) the B-N coordination would not only constrain the π-frameworks in a planar fashion leading to effective π-conjugation, 2) but also enhance the electron accepting ability by lowering the LUMO level. 3) Fine-tuning of electron accepting ability is possible by the electronic effect of the substituents on the boron center. These effects would have many advantages in the development of DSC dyes that need improvement of their optical absorption properties with fine-tuning of their electronic structure. As the model compounds of this molecular design concept, we designed and synthesized a series of boryl-substituted thienylthiazole derivatives and demonstrated their photophysical and electrochemical properties as well as their performance as DSCs

Hybrid organic-inorganic photovoltaics (HOPVs) based on a planar heterojunction combine a solution processed, stable inorganic acceptor layer with a highly absorbing organic donor material. In HOPVs, transition metal oxides (TMOs) such as titanium oxide (TiO_x) and zinc oxide (ZnO) can act as an electron acceptor material to replace the more commonly used fullerene based acceptors. The use of a solution derived TMO layer offers the long term potential of lower manufacturing costs, better device stability and allows the morphology of the material to be tuned from a dense material to a nanoridged structure. The field of HOPVs has predominately focused on the use of polymers as the donor material. In planar architectures these devices result in a J_SC commonly below 1 mA cm^-2 due to their low exciton diffusion lengths (typically < 10 nm). Here, we demonstrate the use of the small molecule organic semiconductor, boron subphthalocyanine chloride (SubPc) as a promising new donor material for the fabrication for inverted HOPV devices utilising TMOs as the electron acceptor. SubPc offers well controlled film growth using organic molecular beam deposition, a longer diffusion length and improved light harvesting at longer wavelengths compared to P3HT. The optimised TMO/SubPc hybrid devices give a relatively high photocurrent > 1.60 mA cm^-2 and an external quantum efficiency (EQE) peaking at 22 % which leads to a power conversion efficiency of greater than 0.5 % under AM1.5 solar illumination. Sufficient exciton dissociation at the hybrid interface was confirmed by comparing the EQE of TMOs/SubPc devices to standard SubPc/C₆₀ devices. Both devices show a similar contribution from the SubPc, with the organic donor being the only current contributor in the hybrid devices, due to the high optical band gap of the TMOs used. The effect of inserting a molybdenum oxide (MoO_x) optical spacer layer between the SubPc and the aluminium electrode is also demonstrated, with optical modelling used to maximise photon absorption within the device. This clearly shows the potential of this new small molecule based hybrid interface which is suitable for TiOx and ZnO based HOPV devices.

Solid-state dye-sensitized solar cells (DSC) are promising next generation solar cells, yet they are limited by their ability to only absorb a small amount of the solar spectrum. Broad spectrum dyes are being developed to tackle this issue; however these tend to have weak extinction coefficients. Solid-state DSCs, which are thinner than liquid DSCs due to issues with the infiltration of the hole conductor in thick films, require strongly absorbing dyes in order to absorb enough light to be efficient. Organic dyes have higher extinction coefficients and their absorption spectrum can easily be tuned making them interesting alternatives to conventional metal-based dyes. However, organic dyes have been designed to reduce charge recombination with TiO2 electrons and little has been done to improve hole injection into the hole transporter. We present here a strong, broad absorbing donor-acceptor-donor based oligothiophene containing a barbiturate group, termed TTB, as a sensitizer in a solid-state DSC with 2,2prime;7,7prime;-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9prime;-spirobifluorene Spiro-MeOTAD or poly(3-hexylthiophene) P3HT as the hole conductor. The thiophene units have alkyl side chains making it a potentially good compatibilizer with P3HT. These devices show a broad spectral response extending from 400 nm to 800 nm with external quantum efficiencies reaching 30 % and an open-circuit voltage of 650 mV. Ultrafast transient absorption spectroscopy is performed on this sensitizing oligomer to determine its interaction with the titania and hole transporter, along with charge generation times and loss mechanisms within these DSCs. These results show rapid hole transfer from TTB to the hole-transporting material occurring within 50 ps, demonstrating this oligothiophene is a good compatibilizer with P3HT making it a potential sensitizing dye for dye-sensitized solar cell applications.

The performance of solar energy conversion devices employing mesoscopic junctions depends critically on their nanostructure [1,2]. Here we describe our latest efforts to improve the photon harvesting and the charge carrier collection transport in these mesoscopic solar energy conversion systems. For dye sensitized solar cells, only triodide/iodide based redox electrolytes had attained power conversion efficiencies (PCEs) over 10%. However recent studies combine Co(II/III) complexes with organic donor -acceptor dyes [3,4]. Porphyrin-sensitized systems of this type have now attained PECs of 12.3 % [5]. At the same time we have witnessed several breakthroughs in the development of new light harvesters, in particular semiconductor quantum dots and perovskite nanoparticles. These latest advances forebode well for realizing mesoscopic solar cells with even higher performance in the near future. Literature: 1. Grätzel, M. Nature 2001, 414, 338#61485;344 2. Grätzel, M. Acc. Chem. Res. 2009. 42, 1781-1798. 3. Feldt S. M.; Gibson E. A.; Gabrielsson, E. ; Sun L.; Boschloo, G.; Hagfeldt, A. J. Am. Chem. Soc. 2010, 132, 16714-16724. 4. Yum J-H, Baranoff,E ; Kessler F; Moehl T; Shahzada A; Bessho T, Machioro M; Ghadin E; Moser J-E; Yi C; Nazeeruddin Md-K; Grätzel M. Nature Comm. 2012,3, 1655/1-1655/8. 5. Yella A.; Lee H.-W.; Tsao H. N.; Yi C.;Kumar Chandiran A., Nazeeruddin Md. K.. W-G Diau W- G I E, Yeh, C.-Y. Zakeeruddin S. M.; Grätzel M. Science 2011, 334, 629 - 634.

Dye-sensitized solar cells (DSSCs) are a promising third generation photovoltaic technology that uses relatively inexpensive materials. In DSSCs, photons are absorbed by a ruthenium-based dye with a bandgap of ~1.6eV meaning most of the solar spectrum&’s energy is lost due to incomplete absorption. This makes DSSCs ideal candidates for upconversion, a process that converts two, or more, lower energy photons into a single higher energy photon. NaYF₄ doped with Er³⁺ and Yb³⁺ ions is one of the most efficient upconversion materials known to convert infrared light to visible light. In this work, NaYF₄:Er³⁺,Yb³⁺ is synthesized using a hydrothermal method, with sodium citrate as a crystal modifier to control particle morphology. By optimizing the particle morphology and annealing conditions, we have recently observed marked improvement of green light output, where the dye is best able to absorb. The upconversion process is highly sensitive to particle size and shape, which is controlled by the concentration of sodium citrate and synthesis temperature and time. Annealing the powder at elevated temperatures following hydrothermal treatment also drastically increases the upconversion fluorescence intensity. The purpose of the present work is to exploit the increased upconversion response for spectral engineering of DSSCs. For integration into DSSCs, the upconversion powder is made into a paste and applied as an external rear layer acting simultaneously as reflector to emit upconverted light back into the cell. DSSC integration by employing the NaYF₄:Er³⁺,Yb³⁺ as an internal upconverting/scattering layer is also investigated. The upconverting material is made into a paste and applied as an additional layer in contact with the nanostructured titania. A full host of photovoltaic metrics are employed to evaluate the impact of the additional upconversion layer on device performance including efficiency measurements under simulated solar light, external quantum efficiency across the visible and infrared spectrum, and electrochemical impedance spectroscopy.

We present photoelectrochemistry and solar cell performance of ITO/TiO2 electrodes sensitized with two (4, 4prime;-difluoro-4-bora-3a, 4a-diaza-s-indacene) BODIPY dyes. BODIPY-1 bears two carboxylic acid groups at its 2, 6 positions and BODIPY-2 is modified with two cyanoacetic acid groups at its 2, 6 positions. The photophysical and photoelectrochemical properties of modified electrodes are studied by steady-state and transient absorption spectra, fluorescence, photocurrent action spectroscopy and temporal photoresponse signals. BODIPY-2 shows a better photostability and higher photocurrent gain due to the quality formation of monolayer on TiO2 surface to allow efficient charge injection as evidence by ultrafast spectroscopy study and photoelectrochemical results. The efficient photocurrent generation of BODIPY-2 cell is also due to the efficient redox reaction with hole transport media I-/I3- in comparison with BODIPY-1.

Molecular photovoltaics based on dye-sensitized solar cells (DSC) have gained significant attention due to the ease of device fabrication and low material cost. Recently, DSC reached a record power conversion efficiency of 12.3% at AM 1.5 G sun conditions using a novel phorphyrin dye sensitizer and single electron redox mediator based on Co(bipyridine) shuttles.[1] The one electron outer sphere redox systems are known for fast electron transfer rates which enhances the electron recombination, affecting the achievable power conversion efficiencies. To circumvent this issue, recently we developed an ultrathin Ga2O3 electron tunnelling layer by atomic layer deposition technique (ALD) to arrest the recombination, leading to a new record open-circuit potential of 1.1V. This was achieved with simultaneous increase in the photo-current density of the DSC. [2] To realize the ALD technology beyond the tunnelling layers, in the current work, we will introduce a new concept of step-wise injection wherein we deposit an oxide ALD layer based on In2O3 which has a conduction band(CB) position lying between the CB of titania and LUMO (or π* orbitals) of the dye sensitizer. With the intermediate layer, the excited electrons in the dye are injected into the TiO2 by taking a step at the bottom of the CB of the In2O3. After injection, the electrons are trapped inside the conduction band of titanium oxide and cannot recombine easily due to the thermodynamic barrier that exists between the TiO2-electrolyte interface. In this work, we will show that the intermediate-band barrier layer enhanced the open-circuit potential of the dye-sensitized solar cell by blocking the electron recombination. The evolution of the recombination rate as the function of In2O3 thickness was investigated using transient photovoltage decay measurements. A systematic study has been done to find an optimum thickness of the ALD In2O3 over layer where a balance of blocking and injection properties are achieved to improve the conversion efficiency of the solar cell. We will also show that the photo injected electrons do not take a path, to the external contact, along the In2O3 layer by studying the similar layers on the insulating silica mesoporous substrates. At the end, we compare the effectiveness of step-wise injection layer with a tunnelling layer in blocking the electron recombination. [1]. Yella. A. et al. Science 2011, 334, 629. [2]. A. K. Chandiran, et al. Nano Letters 2012. in press.

Dye sensitized solar cells (DSSC) have recently reached power conversion efficiencies (PCE) >12%, making them quite attractive for application in low cost solar energy technology.1 Although ruthenium based sensitizing dyes are commonly used, metal-free sensitizing dyes are advantageous due to their higher molar absorption coefficients, ease of chemical synthesis and modification, lower cost, lower environmental impact, and increased performance in solid-state DSSCs (ssDSSCs).2,3 Typically, metal-free sensitizers consist of three moieties: an electron donor (D); an electron-rich conjugated bridging group (π); and an electron acceptor (A) which also serves to chemically bind the dye to the titania surface. These dyes are commonly referred to as D-π-A dyes and typically have a broad visible-light absorption spectrum and the ability to separate charges due to their photoinduced intramolecular charge transfer (PICT) properties.2 To date, the world record sensitizing dye, Y123, has a maximum absorption at 532 nm and a PCE of 7.2%.4,5 Moving forward, overall efficiency gains can be realized in two ways: 1) by lowering the bandgap of the sensitizing dye in order to red-shift the absorption and thereby harvest more solar photons; or 2) molecular-engineering of the dye-sensitizer to provide a more effective recombination barrier between the hole-transport material and titania surface in order to achieve a higher open-circuit voltage. In this presentation we will address the second method through the synthesis of a series of dyes in which the number and position of alkyl chains are methodically varied on the sensitizing-dye in order to block recombination and improve subsequent device performance of ssDSSCs. A fluorene π-group is utilized to allow for the addition of an extra pair of alkyl chains along the π backbone while a donor moiety similar to that of Y123 is employed to add additional chains on the donor. Using this strategy we introduce dyes that to our knowledge have the best recombination properties of any highly performing dyes in ssDSSCs resulting in PCE values near 7%. References 1. Cao, Y. M.; Bai, Y.; Yu, Q. J.; Cheng, Y. M.; Liu, S.; Shi, D.; Gao, F. F.; Wang, P. J. Phys. Chem. C 2009, 113 (15), 6290-6297. 2. Chen, R.; Yang, X.; Tian, H.; Wang, X.; Hagfeldt, A.; Sun, L. Chem. Mater. 2007, 19, 4007-4015. 3. Chen C.; Hsu, Y.; Chou, H.; Thomas, K.R.J.; Lin, J.T.; Hsu, C.P. Chem. Eur. J. 2010, 16, 3184-3193. 4. Tsao, H. N.; Yi, C.; Moehl, T.; Yum, J.-H., Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. ChemSusChem. 2011, 4, 591-594. 5. Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, N.-L.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. J. Am. Chem. Soc. 2011, 133, 18042-18045.

Fullerene systems have to date dominated the acceptor landscape in solution processed and indeed vacuum evaporated organic solar cells. Notably, [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) generates respectable open circuit voltages with low optical gap polymers in high efficiency bulk heterojunctions. The accepted "historical wisdom" is that photoinduced electron transfer (PET) via photoexcitation of the polymeric donor and its subsequent oxidation at the acceptor-donor interface is the main mechanism for current generation. We term this the Channel I process. More recently, the role of acceptor photoexcitation followed by photoinduced hole transfer (PHT) has been recognised as a potentially valuable (and maybe even dominant) current generation pathway (Channel II) [1, 2]. Hence, the concept of designing acceptor-donor pairs with complementary absorption to maximise spectral coverage is a valid and potentially powerful strategy for enhancing photocurrent generation without compromising open circuit voltage. Motivated by this possibility, and also by a desire to understand and quantify Channel II, we have created a family of model non-fullerene small molecule acceptors based upon the benzothiadiazole (BTD) motif variously functionalized with, for example fluorene and dithienosilole [2]. We have engineered and manipulated the optical gaps, electron affinities, solubility and solid-state morphology of these acceptor molecules in order to create complementary absorbers with standard donor polymers such as poly(3-n-hexylthiophene) (P3HT), generating respectable device efficiencies of order 1-2% and clearly demonstrating Channel II photocurrent. In this presentation, we will summarise the current state of non-fullerene acceptors and discuss the design considerations for creating complementary absorbing systems. In addition, the essential transport physics controlling photocurrent extraction in these BTD molecules will be detailed. [1] A. A. Bakulin, J. C. Hummelen, M. S. Pshenichnikov, P. H. M. van Loosdrecht, Adv. Funct. Mater. 2010, 20, 1653. [2] P. E. Schwenn, K. Gui, A. M. Nardes, K. Krueger, K. H. Lee, K. Mutkins, H. Rubinstein-Dunlop, P. E. Shaw, N. Kopidakis, P. L. Burn, P. Meredith, Adv. Energy Mater. 2011, 1, 73.

Fullerenes are currently the most popular electron acceptor materials used in organic photovoltaics (OPV) due to their superior properties, such as good electron conductivity and sufficient charge separation at the donor/acceptor interface. However, low absorptivity in the visible spectrum region is a significant drawback of fullerenes. In this study, we design a Zinc Dipyrrin derivative (ZIPYCl) that absorbs strongly in the visible region (450 600 nm, optical density is sevenfold higher than C60 film) and can transfer energy to C60 in the thin films. Application of ZIPYCl as an energy sensitizer in OPV devices has shown to improve photoresponse of C60 up to 35% without changing other device characteristics such as open circuit voltage and fill factor. While searching for a new electron acceptor remains challenging, our sensitization approach allows improving absorption of the electron acceptor layer and utilizing advanced properties of C60 in OPV devices.

Cyanine dyes exhibit extraordinary high extinction coefficients. For most cyanine dyes, saturated absorption can be achieved with a film thickness of about 35 nm within their respective absorption ranges. On the other hand, the aggregates formed within cyanine films are very favorable for the diffusion of excitons, resulting in an average diffusion length of between 30 to 40 nm. The combination of these two advantages makes cyanine dyes an excellent material class for organic photovoltaics. Here we report an inverted cyanine organic solar cell with planar heterojunction geometry. The device was consisted of an ITO glass as the cathode, a chemically converted graphene oxide layer as the electron transporting layer, a C60 layer of 70 nm thick as the electron accepter, a cyaine film of 35 nm thick as the electron donor, a MoOx layer of 10 nm thick as the hole transporting layer, and a silver anode of 70 nm thick. The graphene oxide and cyanine layers were spin-coated, while the C60, MoOx and silver anode layers were formed by thermal sublimation. The graphene layer established a nice ohmic contact between ITO and C60. The device showed very high rectification ratio. Under simulated AM 1.5 solar spectrum (100 mW/cm2), the short-circuit current was 9.2 mA/cm2, the open-circuit voltage was 0.76 V, and the fill factor reached 0.73. The power conversion efficiency was calculated to be 5.1%.

Efficiencies of organic solar cells have surpassed 10%, yet the vast majority of these solar cells use fullerene derivatives as the electron acceptor. However, devices containing fullerene acceptors are energetically limited to open-circuit voltages of 1.0V or less, thus limiting their potential use as the high-voltage device in a tandem architecture. This is in addition to other drawbacks such as high cost of synthesis, purification and functionalization and relatively poor light absorption. In our recent publications, a new material, HPI-BT, has recently achieved as high as 3.4% efficiency, a record for organic solar cells using poly(3-hexylthiophene), P3HT, as donor material. Through investigation of the dependence of quantum efficiency on applied electric field and light intensity in the most efficient devices, we have determined that the fundamental loss mechanism in these devices is recombination of geminate charge pairs before they have reached a charge-separated state. In addition to P3HT, we have investigated these effects using additional donor materials. In donor materials with ionization potentials more than 0.1 eV larger than P3HT, energy is transferred from the charge transfer state to the excited state on the donor polymer, where it can no longer be effectively separated. Also, devices with PDHTT, a polymer with an ionization potential higher than P3HT by 0.1 eV, and HPI-BT demonstrate a high open-circuit voltage of 1.1V with efficiencies up to 3.4%.

The organic bulk-heterojunction solar cell has a large area donor-acceptor (D-A) interfaces owing to the micro-segregated mixture of donor and acceptor materials. Such a larger D-A interfaces are essential to enhance the dissociation of the photo-generated excitons. However, carrier conduction pathways for photo-dissociated electrons and holes are not sophisticated because of the random orientation and aggregation of donor and acceptor molecules. For further improvement of carrier collection efficiency, we need to enhance the charge carrier transport properties in the D/A mixed thin film: this improvement can be achieved by introduction of molecular order to the film. A few strategies can be considered for the introduction of such the ordered structure. Here, we are focusing on the liquid crystalline material which exhibits self-organization in molecular aggregates. In this study, we have investigated the idea described above with small molecular liquid crystalline pyrrolopyrrole derivative as a donor and C61PCBM as an acceptor. From XRD measurement, we found that molecular orientation of the liquid crystalline pyrrolopyrrole could be oriented parallel to the substrate in the films when the thermal annealing was carried out, which enhanced its self-organization. The resulting molecular orientation appeared in both one-dimensional ordered liquid crystalline material (nematic) and two- dimensional ordered one (smectic A). However, favorable micro-phase segregation was achieved only in the latter case. As a result, power conversion efficiency was improved from 0.028 to 1.1% owing to the improvement of photo-carrier generation and their transport. This difference of self-organization behavior in the nematic and smectic liquid crystals seems to be attributed to the degree of molecular interaction in the materials. These results indicate that such self-organizing ability in liquid crystals is effective to realize the ordered structure in the D/A mixed thin films.

Recent progress in molecular organic photovoltaics (OPVs) revealed the realization of 10% power conversion efficiency (PCE) for single-junction cells, which put them in direct competition with amorphous silicon PVs. Incorporation of plasmonic nanostructures in these thin-film devices for light trapping offers an attractive solution to realize ultrahigh-efficiency OPVs with PCE>>10% . In this talk, we review recent progress on plasmonic-enhanced OPV devices using metallic nanoparticles, and one-dimensional (1-D) and two-dimensional (2-D) patterned periodic nanostructures. A discussion will be given on the benefits of using various plasmonic nanostructures for broad band, polarization insensitive and angular independent absorption enhancement, and their integration with one or two electrode(s) of an OPV device.

We demonstrate a tandem organic photovoltaic cell incorporating a combination of sub-cells, one based on solution- and the other on vacuum-deposited small molecules as the active layers. A blue and green-absorbing boron subphthalocyanine chloride (SubPc):C70 graded heterojunction subcell is combined with a green and red-absorbing functionalized squaraine/C70 bilayer heterojunction subcell resulting in a tandem cell with a wavelength response from 350 nm to 800 nm. The efficiency of the cells depends strongly on process conditions such as solvent annealing, resulting in nanocrystalline morphology that leads to improved charge and exciton transport compared with unannealed cells. The incorporation of C70 as the acceptor leads to an increase of short-circuit current in each subcell by at least 30 % compared to analogous cells using C60. The optimized tandem cell&’s power conversion efficiency is 6.5 ± 0.1 % and an open-circuit voltage of 1.97 ± 0.1 V under simulated 1 sun, AM 1.5G illumination. To our knowledge, this is the highest efficiency reported in the scientific literature for a small molecule based tandem cell. We will discuss optimization of this device which employs the unusual combination of solution and vapor deposition, and will consider routes to achieving efficiencies of 8% by the methods and materials described.

Bilayer small molecule organic photovoltaic devices typically have significant current collection losses that occur between the generation of excitons and collection of current. Typically, the external quantum efficiency is 50-75% of the flux absorbed in the donor and acceptor layers; this loss is often associated with an exciton diffusion length that is shorter than the layer thickness. Here, we present results of asymmetric functionalized squaraine1, 2 (DPASQ)/C₆₀ cells. The as-cast cells have results typical of bilayer cells with a peak EQE of ~40% and a short circuit current of J_SC= 4.4 mAcm^-2, open circuit voltage V_OC=1.0 V, fill factor FF=0.75 and a power conversion efficiency of PCE=3.4%. When the bilayer cells are solvent vapor annealed, the average EQE increases to asymp;50% from lambda;asymp;350 nm to lambda;asymp;600 nm with a peak EQEasymp;60%, values that are approximately equal to the absorbed photon flux. From this we infer that nearly all absorbed photons in the active materials are collected at the contacts. These cells have J_SC= 7.1 mAcm^-2, V_OC=0.86 V, FF=0.75 and PCE=4.6%. We associate the unusually high FF and EQE with improved bulk and interface morphologies resulting from solvent vapor annealing.

The use of self-assembling characteristics is one of the most potential candidates for the realization of a prevailing solar cell. We have demonstrated a high-efficient bulk-heterojunction solar cell based on liquid crystalline phthalocyanine, 1,4,8,11,15,18,22,25-octaoctylphthalocyanines (C6PcH2), exhibiting a high carrier drift mobility in excess of 1 cm2/Vs. The device can be fabricated through a spin-coating process from the blend solution of C6PcH2 and 1-(3-methoxy-carbonyl)- propyl-1-1-phenyl-(6,6)C61 (PCBM). For the formation of the optimally phase-separated nano-structure for efficient carrier generation and transportation, the mesogenic properties should play an important role. Solar cells have demonstrated a high external quantum efficiency (EQE) above 60% in the Q-band absorption region of C6PcH2, a high open circuit voltage (Voc) above 0.8V and a high energy conversion efficiency of 3.8%. The tandem organic thin-film solar cell has also been studied by utilizing active layer materials of C6PcH2 and poly(3-hexylthiophene) (P3HT), and a high Voc of 1.27 V has been achieved. C6PcH2 is also available as a dopant for conventional organic thin-film solar cells with a bulk hetero-junction active layer composed of P3HT and PCBM. The improvement of long-wavelength sensitivity in P3HT:PCBM bulk hetero-junction solar cells (EQE>40% in the wavelength range of 650-750nm) has been achieved by doping C6PcH2. The molecular alignment has also been investigated in this mesogen. The coexistence of different molecular packing of C6PcH2 with different electronic states has been observed in a planarly aligned sandwich cell.

Recent progress in small donor molecule BHJ OPVs suggests the potential to overcome the many of the synthetic issues inherent with polymeric donors. The molecular designs follow similar principles to polymer donors in which the “push-pull” concepts are applied to lower the band gap.1 Thus, proximate electron donating and electron withdrawing units are incorporated in alternating positions to extend the electronic conjugation. A common “pulling” element is the electron deficient thiophene-capped diketopyrrolopyrrole (TDPP) unit, which has successfully been used in both polymeric and small molecular systems.2,3 The common “pushing” element benzo[1,2-b:4,5-b&’]dithiophene (BDT) unit has also been used in both polymers4 and small molecules5. Unfortunately, for the majority of OPVs based on small molecule donors, low FFs are observed, and further mechanistic understanding is needed to enhance their efficiencies. We analyze the performance of bulk-heterojunction solar cells fabricated from solution processed small molecule donors based on the TDPP moiety. By using impedance spectroscopy on completed solar cells we are able to separate the electrical processes taking place in a working cell. In particular, we are able to separate recombination processes6 from transport of carriers7 and analyze the interfacial properties8. Each of these factors play an important role towards the low FFs observed. In this paper we will present results on how high FFs can be achieved by optimising each of these parameters independently. 1. P. L. T. Boudreault, A. Najari and M. Leclerc, Chemistry of Materials, 2011, 23, 456-469. 2. F. Silvestri, M. D. Irwin, L. Beverina, A. Facchetti, G. A. Pagani and T. J. Marks, Journal of the American Chemical Society, 2008, 130, 17640. 3. S. Loser, C. J. Bruns, H. Miyauchi, R. P. Ortiz, A. Facchetti, S. I. Stupp and T. J. Marks, Journal of the American Chemical Society, 2011, 133, 8142-8145. 4. H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang, L. P. Yu, Y. Wu and G. Li, Nature Photonics, 2009, 3, 649-653. 5. Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian and Y. Chen, Advanced Materials, 2011, 23, 5387-5391. 6. P. P. Boix, A. Guerrero, L. F. Marchesi, G. Garcia-Belmonte and J. Bisquert, Advanced Energy Materials, 2011, 1, 1073-1078. 7. G. Garcia-Belmonte, A. Munar, E. M. Barea, J. Bisquert, I. Ugarte and R. Pacios, Organic Electronics, 2008, 9, 847-851. 8. A. Guerrero, L. F. Marchesi, P. P. Boix, S. Ruiz-Raga, T. Ripolles-Sanchis, G. Garcia-Belmonte and J. Bisquert, ACS Nano, 2012, 6, 3453-3460.

Organic solar cells have recently achieved much progress and have broken the 10% efficiency barrier. However, for a broad market application, a further significant increase of efficiency seems necessary. Although most research has been performed on polymer solar cells, small molecule cells have recently gained increased attention: Due to the possibility to easily deposit multilayer structures e.g. for tandem cells and the easier purification of the materials, small molecule solar cells are a promising materials choice. However, the control of the key element of efficient organic solar cells, the donor-acceptor bulk heterojunction, is more difficult in small molecule solar cells compared to polymer cells since there are fewer handles to control the morphology, in particular when vacuum deposition is used. In this talk, I will discuss recent results showing that the bulk heterojunction morphology sensitively depends on the molecular structure. By using various structural methods, we could show that the morphology of the donor and acceptor phases vary in a subtle manner e.g. with temperature. Due to a better understanding of these effects and the basic physical effects in the structures, significantly higher efficiencies are in reach. The major obstacles are to find materials with better mobilities in the bulk heterojunction, allowing higher active layer thickness, and better infrared absorbers.

Over the past several years significant advances have been made concerning our understanding of the growth of crystalline small molecule organic thin films consisting of a single component. An important challenge in organic electronics, photonics and photovoltaics is to develop and improve methods to integrate both p-type and n-type small molecule organic semiconductors into the same device microstructure. Thus, developing an understanding of the molecular scale events that lead to heterojunction formation is essential in these systems consisting of multiple components. To this end, we report on our examinations of the nucleation, growth, and dynamics of adsorption of a n-type organic semiconductor, N,N'-ditpentylperlyene-3,4,9,10-tetracarboxylic diimide (PTCDI-C5), on SiO2 surfaces modified by self-assembled monolayers (SAMs) and on pre-deposited layers of pentacene (a p-type semiconductor) using supersonic molecular beam techniques, in situ synchrotron x-ray scattering and ex situ atomic force microscopy. From real-time x-ray scattering we find that PTCDI-C5 exhibits prolonged layer-by-layer growth for approximately the first 10 to 15 monolayers (MLs) of deposition on all SAMs examined, as well as on pentacene surfaces. Concerning the kinetics of growth we find that the adsorption probability of PTCDI-C5 on itself is similar to that observed on two SAMs that possess aromatic endgroups, but it differs significantly to that observed on a relatively short, methyl-terminated SAM and bare SiO2. These differences could reflect mechanisms such as direct molecular insertion of PTCDI-C5 into either the existing PTCDI-C5 film, or the longer chain SAMs with aromatic endgroups. Concerning growth in the submonolayer regime, we find that nucleation is homogeneous, and that the absolute density of islands depends on the nature of the surface, while the relative change of the island density with increasing growth rate is essentially independent of the underlying SAM. Finally, we will discuss our recent results concerning the growth of heterostructures composed of a few to several monolayer stacks of PTCDI-C13 and pentacene. In this work we find that PTCDI-C5 grows in a smooth layer-by-layer fashion on pentacene, but the opposite is not true—pentacene grows in a purely 3D mode when deposited on PTCDI-C5. We will discuss the implications of this observation concerning the growth of organic heterostructures for applications in electronics, photonics and photovoltaics. Additionally, we have performed real time in situ grazing incidence wide angle x-ray scattering (GIWAXS) experiments to probe the evolution of the in-plane structure of PTCDI-C5 films on SAM surfaces during growth.

The LUMO level (the lowest unoccupied molecular orbital-derived levels in solid; unoccupied states) of organic semiconductors is crucial to the charge separation, electron transport, and electron correction in organic photovoltaic cells. In principle, the LUMO levels can best examined by inverse photoemission spectroscopy (IPES), which is a complimentary of photoemission spectroscopy (PES). In the previous IPES, vacuum ultraviolet (VUV; hnu;asymp; 10 eV) photons has been detected following the injection of electrons with energies of 5 - 20 eV into solid materials. The high energy electrons can cause damage on the organic samples. Also, the energy resolution is limited to about 0.5 eV due to the difficulty of detecting VUV photons. Surprisingly, such instruments have been used without any fundamental improvement since the late 1970s. Recently we have developed the IPES in the near ultraviolet (NUV) range. Detection of NUV photons allows us to use high resolution optical bandpass filters that improve the energy resolution to 0.27 eV, which is better than that of the commonly used apparatus by a factor of two. By detecting NUV light, measurements can be made with electrons having a kinetic energy less than 4 eV, reducing the damage to the organic samples by a factor of at least 1/100. This technique is especially suitable for examining the conduction levels of organic semiconductors because of low sample damage and high resolution. This new method has been applied to the several acceptor materials most frequently studied for organic photovoltaic cells, including C60, C70, phenyl-C61-butyric acid methyl ester (PC61BM), and phenyl-C71-butyric acid methyl ester (PC71BM). The determined electron affinities range between 3.6 and 3.9 eV. The values are consistent with those predicted from a series of open circuit voltages in the photovoltaic cells with the same donar material. In the presentation, the values will be compared with the previously reported values measured by the conventional VUV-IPES, the HOMO energy (from PES) and the optical gap energy, and the reduction potentials in the solution, demonstrating the advantages of the present method.

Organic photovoltaics hold promise as a renewable energy source due to their low material and processing costs. The most implemented device framework, a bulk heterojunction (BHJ), consists of a blend of a light harvesting p-type and an n-type electron accepting compound. To date, the most investigated p-type organic semiconductor materials have been conjugated polymers, however they can suffer from pitfalls such as labor intense purification methods and batch to batch variation in device performance. A number of small molecule p-type organic semiconductors have recently been designed that absorb a large portion of the solar spectrum and have ideal energy levels for efficient charge transfer with PCBM. A recent solution processable small molecule has achieved device efficiencies up to 6.7% when a high boiling solvent additive is used. Structural evolution due to the use of different solvent additive amounts are correlated to device performance by examination via GIWAXS, GISAXS, and EF-TEM.

Solution processed small molecule bulk heterojunction solar cells (SMBHSCs) with power conversion efficiencies (PCE) of 7% have recently been reported. This achievement demonstrates that SMBHSCs fabricated from blends of small molecule donors and fullerene acceptors are a viable alternative to polymer:fullerene based systems. Advantages of using small molecules as donors include the ease of synthesis and purification. Moreover, in contrast to polymers, conjugated small molecules do not suffer from broad molecular weight distributions or batch to batch variations. However, the fill factor (FF) of SMBHSCs remains an area for improvement as many of the most efficient systems exhibit FFs below 50% while efficient polymer systems have FFs exceeding 65%. A deeper understanding of the field-dependent recombination and charge transport processes that limit the FF of SMBHSCs could lead to significant improvements in PCE. The nature of the field-dependent recombination losses that determine the FF of polymer:fullerene based solar cells has been the subject of much research. There is evidence that field-dependent exciton dissociation (geminate recombination) and both bimolecular and trap-assisted (nongeminate recombination) mechanisms may all play a role depending on materials and device processing conditions. Initial studies of SMBHSCs have concluded both geminate and nongeminate recombination also influence the FF of small molecule systems, however, it has yet to be seen if this is true for all SMBHSCs. In this work, we report on the use of light intensity measurements and a novel differential resistance analysis technique to probe the voltage dependent recombination mechanisms of SMBHSCs with diketopyrrolopyrrole (DPP) based donor molecules blended with [6,6] phenyl-C71-butyric acid methyl ester (PC71BM). By extracting resistances from impedance measurements, we demonstrate that the field-dependent recombination losses in the DPP based systems studied occur within 1-10µs - a timescale comparable to what has been reported for polymer based systems dominated by bimolecular recombination. Light intensity measurements provide further evidence that these field-dependent recombination losses are primarily bimolecular with only negli