This PDF file contains the front matter associated with SPIE Proceedings Volume 7416, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Thin metal films are a desirable alternative to indium tin oxide for utilization in organic solar cells (OSC).We describe background and processing parameters for thin metal films and show examples of top-illuminated OSC employing metal electrodes. Simulations are introduced as tool for OSC fabrication; several pitfalls are presented which must be considered for successful numerical treatment of thin-film layers at coalescence thicknesses and for coherent treatment.

Poly-3-AlkylThiophenes (P3ATs) with an n-alkyl chain length varying from C3 till C9 were synthesized by using the Rieke method. Subsequently, these materials were used to make P3AT/PCBM blends which were investigated in bulk heterojunction (BHJ) solar cells. The phase diagram of a P3H(exyl)T:PCBM blend was measured by means of standard and modulated temperature differential scanning calorimetry (DSC and MTDSC). A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing PCBM concentration: from 12 °C for pure P3HT to 131 °C for pure PCBM. The observed range of Tg's defines the operating window for thermal annealing and explains the long-term instability of both morphology and photovoltaic performance of P3HT:PCBM solar cells. All regioregular P3ATs allow for efficient fiber formation in several solvents. The fibers formed are typically 15 to 25 nm wide and 0.5 to >4 μm long and mainly crystalline. By means of temperature control the fiber content in the casting solution for P3AT:PCBM BHJ solar cells is controlled while keeping the overall molecular weight of the polymer in the blend constant. In this way, fiber isolation and the use of solvent mixtures are avoided and with P3HT nanofibers, a power conversion efficiency of 3.2 % was achieved. P3AT:PCBM BHJ solar cells were also prepared from P3B(utyl)T, P3P(entyl)T and P3HT using the good solvent o-dichlorobenzene and a combination of slow drying and thermal annealing. In this way, power conversion efficiencies of 3.2, 4.3, and 4.6 % were obtained, respectively. P3PT is proved to be a potentially competitive material compared to P3HT.

ZnO nano rods were grown by aqueous chemical growth technique over ZnO seeds in mesh form deposited on ITO coated glass substrate. The structural and morphological analysis of the seed layer and the nano rods were studied, both the seed and the nano rods are crystalline in nature and majority of the planes are oriented along the C-axis. Over the nano rods layer P3HT (poly 3-hexylthiophene)/PCBM ((6, 6)-phenyl C61 butyric acid methyl ester) blend, PEDOT: PSS (Poly ethylenedioxythiophene doped with poly styrene sulphonic acid) were successively spin coated and Gold was coated as the top electrode by electron beam evaporation. The Current (I) vs. Voltage (V) characteristics of the device was measured without illumination and with illumination. The device showed an ohmic behavior instead of rectifying behavior.

Dendrimers are branched macromolecules in which multiple functions can be engineered into a single entity. Highly efficient dendrimer-based solution processed organic light emitting diodes have been demonstrated, but to date, there have been fewer reports of their use in organic photovoltaics. In our paper we describe a new family of dendritic light harvesting molecules based upon ruthenium-centered charge transfer complexes specifically designed for use in dye sensitized solar cells (DSSCs). These materials possess enhanced thermal and solution stability, reduced aggregation, and light harvesting efficiencies similar to the gold standard N3 ruthenium complex. We also describe a new method for measuring dye uptake in-situ and show that the adsorption kinetics of the dendritic molecules onto nanoporous titanium dioxide (the photoanode) obeys pseudo 2nd order kinetics with chemisorption being the rate determining step. This architectural approach allows simultaneous tuning of several key molecular properties for light harvesting and may point the way towards higher efficiency organic solar cell materials.

Polymer solar cells have many advantages such as light weight, flexibility, environmental friendliness, low thermal budget, low cost and most notably very fast modes of production by printing techniques. Production experiments have shown that it is highly feasible with existing technology to mass produce polymer solar cells at a very low cost. We have employed state-of-the-art analytical techniques to address the challenging issues of degradation and stability of R2R manufactured devices. We have specifically studied the relative effect of oxygen and water on the operational devices in regard to degradation.

We report on latest progress in the field of p-i-n type tandem solar cells. An optimized tandem cell architecture with two complementary absorbing bulk heterojunctions leads to a certified power conversion efficiency of 5.9% on 2 cm² active area. Moreover, we show that p-i-n type tandem solar cells can be extremely stable: Extrapolated lifetimes corresponding to more than 30 years of sun illumination have been achieved. Furthermore, we show that efficiency and stability only slightly decrease when transferring the cell architecture to large serially interconnected modules of more than 100 cm² area.

The Al electrode on P3HT:PCBM blended thin films modified the nano structure of P3HT crystals during thermal annealing. The presence of an Al layer induced less preferred distribution of P3HT crystals after thermal annealing. In the surface region, the lateral growth of amorphous like (0.7 nm thick in [010] direction) face-on P3HT crystals was also affected by the inter diffusion of Al atoms into the active layer during thermal annealing in the presence of the Al layer. The inter diffusion of Al atoms produced an intermediate layer between the electrode and the active layer. By the real time measurement using synchrotron x-rays, we could confirm interfacial changes during annealing process. To understand the relation of structures and the device performance, we fabricated devices using pre and post annealing processes. The J-V characteristics show that more randomly distributed P3HT crystals are more advantageous to form the interpenetrating networks in the active layer. The short circuit current seems to be affected by nano structure of P3HT crystals in the bulk region, while the series resistance is more affected by the interfacial properties between the electrode and the active layer.

We report the fabrication and measurement of solar cells with 6% power conversion efficiency using the alternating copolymer, poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole), PCDTBT, in bulk heterojunction (BHJ) composites with the fullerene derivative [6,6]-phenyl C₇₀-butyric acid methyl ester (PC₇₀BM). The PCDTBT/PC₇₀BM solar cells exhibit the best performance of any BHJ system studied to date; J_sc = 10.6 mA/cm², V_oc = 0.88 V, FF = 0.66 and η_e = 6.1% under air mass 1.5 global (AM 1.5G) irradiation of 100 mW/cm². The internal quantum efficiency (IQE) is close to 100%, implying that essentially every absorbed photon results in a separated pair of charge carriers and that all photogenerated carriers are collected at the electrodes.

State-of-the art polymer-fullerene solar cells reach power conversion efficiencies of up to 6%, featuring low polaron recombination rates. In order to identify limiting factors, we investigated the photocurrent of poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) solar cells experimentally. From our investigations, we find the photocurrent to be determined mainly by polaron pair dissociation and charge extraction. Focussing on the polaron pair dissociation, we apply Monte Carlo simulations in order to understand the unexpectedly high internal yield of this separation process. We find that a long effective conjugation length of the polymer chains leads to delocalisation of the positively charged constituent of polaron pairs, a hole, making it easier to escape the Coulomb attraction to the electron. However, we identify an additional loss mechanisms, which our Monte Carlo simulations show to be significant: losses of polaron pairs at the semiconductor/electrode interfaces due to diffusion of the pairs. We discuss how the different processes influencing the photocurrent can be accounted for analytically.

We explore the optimization of organic photovoltaic (OPV) devices based on a thin-film blend of the polymer poly(3-hexyl thiophene) [P3HT] and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester [PCBM]. In particular, we address the preparation of the PEDOT:PSS device anode, the composition, thickness and thermal treatment of the active semiconductor layer along with the effect of presence of self-assembled P3HT nanofibres. We also investigate the use of materials such as glycerol and oleic acid dispersed into the PEDOT:PSS and P3HT/PCBM layers respectively to improve the power conversion efficiency (PCE) of the device. Our optimization protocol results in the creation of OPV devices having power conversion efficiencies (PCEs) of up to 4.5%.

The present paper discusses the strategy for improvement of efficiency of dye-sensitized solar cells based the equivalent circuit model. The influence of these elements upon cell efficiency in areas such as short circuit current density (JSC), open circuit voltage (VOC), and fill factor (FF) was examined. It was demonstrated that the haze factor of TiO2 electrodes is a useful index when fabricating light-confined TiO2 electrodes to improve JSC, and that blocking the TiO2 surface with molecules is an effective way of reducing interfacial charge recombination at the TiO2 surface and of improving shunt resistance and VOC. FF was also improved by reduction of the internal series resistance, which is composed of the redox reaction resistance at the platinum counter electrode, the resistance of carrier transport by ions in the electrolyte, and resistance due to the sheet resistance of the transparent conducting oxide. Finally, the highest efficiency scores of 11.1% and 10.4% (aperture illumination area of 0.219cm² and 1.004cm², respectively) were confirmed by a public test center.

We will present efficient semi-transparent bulk-heterojunction [regioregular of poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester] solar cells with an inverted device architecture. Highly transparent ZnO and TiO2 films prepared by Atomic Layer Deposition are used as cathode interlayers on top of ITO. The topanode consists of a RF-sputtered ITO layer. To avoid damage due to the plasma deposition of this layer, a sputtering buffer layer of MoO₃ is used as protection. This concept allows for devices with a transmissivity higher than 60 % for wavelengths 650 nm. The thickness of the MoO3 buffer has been varied in order to study its effect on the electrical properties of the solar cell and its ability to prevent possible damage to the organic active layers upon ITO deposition. Without this buffer or for thin buffers it has been found that device performance is very poor concerning the leakage current, the fill factor, the short circuit current and the power conversion efficiencies. As a reference inverted solar cells with a metal electrode (Al) instead of the ITO-top contact are used. The variation between the PCE of top versus conventional illumination of the semi-transparent cells was also examined and will be interpreted in view of the results of the optical simulation of the dielectric device stack with and without reflection top electrode. Power conversion efficiencies of 2-3 % for the opaque inverted solar cells and 1.5-2.5 % for the semi-transparent devices were obtained under an AM1.5G illumination.

Fiber-Based photovoltaic cells are solar collectors that utilize internal reflectors to confine light into an organic absorber, thereby significantly enhancing absorption cross-sections of the device. The performance of the device is particularly sensitive to internal resistivity of the "optical can." Using ITO of differing thicknesses we show that can be controlled and that Jsc's that exceed planar device limits can be achieved. However, the morphology and film quality of the layers must be maintained to achieve maximum performance.

We demonstrate fully solution-processed organic photovoltaic cells on metal foil substrates with power conversion efficiencies similar to those obtained in devices on transparent substrates. The cells are based on the regioregular poly- (3-hexylthiophene) and C61 butyric acid methyl ester bulk heterojunction system. The bottom electrode is a silver film whose workfunction is lowered by Cs2CO3 using spin-coating to serve as a cathode. The transparent top anode consists of a conductive polymer in combination with a solution-processed silver nanowire mesh that is laminated onto the devices. Each layer of the device, including the transparent electrode, is fabricated from solution, giving rise to the possibility of completely printed solar cells on low-cost substrates.

Organic photovoltaic (OPV) devices have attracted much interest in recent decades because they have a great potential for low cost solar cells. Among different kinds of organic solar cells, conjugated polymer/fullerene bulk heterojunction (BHJ) solar cells have exhibited improvements in the power conversion efficiency (PCE) in recent years. The performance of BHJ solar cells is highly dependent on different fabrication processes. To address this issue, we focus on the dependence of different photovoltaic parameters on the fabrication methods. BHJ solar cells fabricated using platinum metallopolyyne (P1) with a low band gap of 1.85 eV as an electron donor and phenyl-C61-butyric acid methyl ester (PCBM) as an electron acceptor have been studied. The fabrication parameters, such as ratios of P1 to PCBM, solvents used, thickness of the active layers and top contact materials, have been systematically investigated. Blend ratio and solvent used had most significant influence on photovoltaic performance with several times higher efficiency of the best condition compared to the worst condition. They affected all photovoltaic parameters [open circuit voltage (V_oc), short circuit current density (J_sc) and the fill factor (FF)]. Top contact materials affected the V_oc and the FF, while thickness of the active layer mainly affected the J_sc and FF. The influence of different fabrication conditions on photovoltaic performance has been discussed.

Here we present our work on development of new solution processable small molecules for efficient organic photovoltaic cells (OPVs). Boron subphthalocyanine derivatives possess unique structural and photophysical properties, i.e. excellent solubility, low tendency to aggregate, and high extinction coefficients, that enable the formation of high quality thin films via solution processing for OPVs application. Both p type (donor) and n type (acceptor) boron subphthalocyanine derivatives have been investigated. Using a soluble 2-Allylphenol SubPc derivative as donor and fullerene as acceptor, we have demonstrated simple planar heterojunction OPVs with power conversion efficiencies of over 1.7%, which represents one of the highest efficiencies for devices with solution processable small molecules to date. The use of fluorinated subphthalocyanines as acceptor and typical poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1, 4-phenylene vinylene] (MDMO-PPV) as donor has led to fully solution processed OPVs with efficiencies over 0.1%. Our work shows that solution processing of light harvesting small molecules has great potential for application in low cost thin film photovoltaic cells and boron subphthalocyanine derivatives are promising new-generation OPV materials.

Camphor sulfonic acid doped conducting polyaniline (PANI:CSA) was synthesized by self-stabilized dispersion polymerization (SSDP). Well ordered polymer chains grow at the interface between aqueous and organic phase at low temperature around -35 °C. Thus, the growing polymer chains act as a stabilizer, producing high quality polyaniline with high electrical conductivity and with low content of structural defects. Moreover, the PANI:CSA thin film shows an apparent Drude peak in the infrared region with a high d.c. conductivity of 550 Scm^-1, and a high transmittance in the visible region. Using this highly conducting polyaniline as a transparent electrode, flexible polymer light-emitting diodes (PLEDs) and flexible polymer solar cells (PSCs) were fabricated on flexible poly(ethersulfone) (PES) substrates. The flexible PLEDs show high performance with a luminance of 2300 cdm^-2 and a luminous efficiency of 1.6 cdA^-1. In addition, flexible PSCs based on composites of regioregular poly (3-hexylthiophene) (rr-P3HT) as an electron donor and phenyl-C₆₁-butyric acid methyl ester (PCBM) as an electron acceptor exhibit a reasonable power conversion efficiency (PCE) of 1.8 %.

set of novel regioregular poly(3-hexylthiophene)-based random copolymers containing varying ratios of ester functionalized alkyl side chains were synthesized using the Rieke method. The percentage of functionalized side chain varied between 10 and 50 mol% for each copolymer. Using post-polymerization reactions, the ester functions in the alkyl side chain were hydrolyzed to yield an alcohol or acid group. These groups are available for further functionalization reactions, so a wide variety of secondary functionalities may be covalently attached to the conjugated polymer. The copolymers were applied in polymer: fullerene bulk heterojunction solar cells (BHJSCs) with [6,6]-phenyl -C61-butyric acid methyl ester (PCBM) as electron acceptor. The influence of side-chain functionalities on absorption, device performance and layer morphology depends on the ratio and nature of the functionalized side chains. For a 9/1 copolymer, containing 10% of functionalized side chains, behaviour and efficiency in BHJSCs comparable to P3HT:PCBM solar cells were observed.

The performance of the polymer/ZnO nanorod hybrid solar cells based on poly(3-hexylthiophene) and methanofullerenes is improved with the enhanced optical absorption by increasing the thickness of the photoactive layer and introducing a solution-processed interlayer. The dependence of the optical absorption on the thickness of the photoactive layer is studied as a function of the spin-coating rate. With the slower spin-coating rate, the photoactive layer is thicker, and the polymer chains have longer time to self-organize and more effectively infiltrate into ZnO nanorod spacing. In addition, a solution-processed fullerene interlayer is introduced to modify the ZnO nanorod surface. With this interlayer, the optical absorption of the photoactive layer increases due to the better ordering of the photoactive layer. Our investigations show that the power conversion efficiency (PCE) is improved from 1.6% to 2.6% with the thickness of the photoactive layer from 240 nm to 350 nm by slowing the spin coating rate of the photoactive layer. Moreover, the PCE is also improved by the fullerene interlayer. The slow-drying method and the solution-processed fullerene interlayer both improve the crystallinity of the polymer and light harvesting.

The surface Plasmon resonance, which occurs between the dielectric material layer and the metal surface, is of great interest in several fields including the materials to fabricate solar cell, because the enhanced electromagnetic field caused by surface Plasmon resonance increases the optical absorption of solar cell by adding metal nanoparticles.[1] The metal nanoparticles used in this paper are spherical gold nanoparticles, which deposit on the surface of device, can enhance the optical absorption via the excitation of surface Plasmon resonance in an organic photovoltaic cell. [2] The shape, size, spacing, and the surrounding dielectric materials of nanoparticles affect the absorption ratio and absorption peak with different wavelengths.[3-4] The gold nanoparticles used in this paper are 5nm, 10nm, and 20nm in diameter, which are deposited on ITO (indium tin oxide) glass substrates, and then another two organic materials deposit on it. The first organic materials are Poly (styrenesulfonate) / poly(2,3-dihydro-thieno-1,4-dioxin) (PEDOT) and poly {2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene} (MEHPPV) with Fullerene(C60) mixtures, the second materials are poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl-C71 butyric acid methyl ester(PC70BM) mixture, and the third materials are poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester(PCBM) mixture act as p-type and n-type organic semiconductor. By using the DDSCAT software, The Discrete Dipole Approximation for Scattering and Absorption of Light by Irregular Particles, the SPR enhancement and absorption peak can be predicted in this software, and the enhancement caused by surface Plasmon resonance within the layers of this device is supposed to be related to the photocurrent in this organic photovoltaic cell with gold nanoparticles. Under halogen illumination, the proposed enhancement is around 20~30% power efficiency in this device agrees well to the result as DDSCAT software simulated.

With growing energy and environmental concerns due to fossil fuel depletion and global warming there is an increasing attention being attracted by alternative and/or renewable sources of power such as biomass, hydropower, geothermal, wind and solar energy. In today's society there is a vast and in many cases not fully appreciated dependence on electrical power for everyday life and therefore devices such as PV cells are of enormous importance. The more widely used and commercially available silicon (semiconductor) based cells currently have the greatest efficiencies, however the manufacturing of these cells is complex and costly due to the cost and difficulty of producing and processing pure silicon. One new direction being explored is the development of dye-sensitised solar cells (DSSC). The SFI Strategic Research Centre for Solar Energy Conversion is a new research cluster based in Ireland, formed with the express intention of bringing together industry and academia to produce renewable energy solutions. Our specific area of research is in biomimetic dye sensitised solar cells and their electrical properties. We are currently working to develop test equipment, and optoelectronic models describing the performance and behaviors of dye-sensitised solar cells (Grätzel Cells). In this paper we describe some of the background to our work and also some of our initial experimental results. Based on these results we intend to characterise the opto-electrical properties and bulk characteristics of simple dye-sensitised solar cells and then to proceed to test new cell compositions.

In this paper, the pore filling of spiro-MeOTAD (2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9, 9'-spirobifluorene) in mesoporous TiO₂ films is quantified for the first time using XPS depth profiling and UVVis absorption spectroscopy. We show that spiro-OMeTAD can penetrate the entire depth of the film, and its concentration is constant throughout the film. We determine that in a 2.5-•m-thick film, the volume of the pores is 60-65% filled. The pores become less filled when thicker films are used. Such filling fraction is much higher than the solution concentration because the excess solution on top of the film can act as a reservoir during the spin coating process. Lastly, we demonstrate that by using a lower spin coating speed and higher spiro-OMeTAD solution concentration, we can increase the filling fraction and consequently the efficiency of the device.

This work reports the study of solvent effects on the morphology for P3HT/PCBM films using chlorobenzene, 1, 2-dichlorobenzene and 1, 3-dichlorobenzene. Although extensive research has been focused on investigating devices using chlorobenzene and 1, 2-dichlorobenzene and it was found that 1, 2-dichlorobenzene led to an improved device performance, little work has been conducted in the morphology by comparing films fabricated via chlorobenzene, 1, 2-dichlorobenzene and 1, 3-dichlorobenzene. Atomic force microscopy (AFM) was performed to study the film morphology using chlorobenzene, 1, 2-dichlorobenzene and 1, 3-dichlorobenzene as solvents. Initial studies showed that the size of nanocrystallites in 1, 2-dichlorobenzene based films is smaller than nanoclusters in the other two films. Kelvin probe force microscopy (KFM) images, which were used to figure out the electron transport pathway, together with AFM images, showed the solvent effect on the morphology of these films. In addition, obvious red shifts were observed in the UV-Vis absorption spectra for the P3HT/PCBM blend from 1, 3-dichlorobenzene and 1, 2-dichlorobenzene compared to the one from chlorobenzene.

Polymer solar cells (PSCs) have attracted increasing attention due to their inexpensive, flexible, light weight and large area device fabrication. However, the efficiency of PSCs is still not yet sufficient for large scale implementation. Many approaches have been proposed to enhance the efficiency of PSCs. In addition to using new materials and new device structure, the performance of polymer solar cells can also be improved using efficient interface layer between the electrode and active layer. Here, we studied the effect of MoO3 as an anode interlayer on both small molecule and polymer photovoltaic cells. Significant improvement has been observed in the performance of PSCs and the power conversion efficiency (PCE) of the cell with a MoO₃ interlayer can be enhanced by ~15% comparing to the cells with a PEDOT:PSS interlayer. This improved device performance is attributed to the combined effect of efficient charge extraction and the reduction in series resistance of device.

We present a study of charge transfer and carrier dynamics in films of zinc phthalocyanine (ZnPc) and buckmisnsterfullerene (C₆₀) investigated by time-resolved terahertz spectroscopy (TRTS). These films are model structures for charge generation layers in organic photovoltaics and their intrinsic properties are therefore of interest. We compare two classes of films: composite films of ZnPc and C₆₀ prepared by co-evaporation and layered ZnPc/C₆₀ films prepared by alternating deposition. We find evidence for a short-lived charge transfer state of C₆₀ that decays within several picoseconds of excitation. In contrast, both composite and multi-layered films have a long-lived THz absorption that depends on the composition and structure of the fims. The optimum composition for charge transfer within composite films is a 1:1 blend of ZnPc and C₆₀. Amongst the layered films, there is an increase in charge photogeneration with decreasing layer thickness with a sample having ultrathin (2 nm) exhibiting the strongest THz absorption. A much stronger THz absorption signal was obtained from the layered structure than for the best composite film, even both structures contain similar fractions of ZnPc and C₆₀ .