Along with efficiency and lifetime, costs are one of the most important aspects for the commercialization of organic solar cells. Thinking of large scale production of organic solar cells by an efficient reel-to-reel process, the materials are expected to determine the costs of the final product. Our approach is to develop functional substrates for organic solar cells which have the potential for cost effective production. The functionality is obtained by combining periodically microstructured substrates with lamellar electrode structures. Such structured substrates were fabricated by cost effective replication from masterstructures that were generated by large area interference lithography. Two cell architectures were investigated - holographic microprisms and interdigital buried nanoelectrodes. A structure period of 20 μm in combination with a 2 μm wide metal grid was chosen for the microprism cells based on the results of electrical calculations. Current-voltage curves with reasonable fill factors were measured for these devices. A significant light trapping effect was predicted from optical simulations. Interdigital buried nanoelectrodes are embedded in the photoactive layer of the solar cell. Separated interdigital metal electrodes with a sufficiently high parallel resistance were manufactured despite a small electrode distance below 400 nm. Experimental results on first photovoltaic devices will be presented. We observe an insufficient rectification of the photovoltaic device which we attribute to partial electron injection into the gold anode.

The fabrication of very large area polymer based solar cell modules with a total aperture area of 1000 cm² has been accomplished. The substrate was polyethyleneterephthalate (PET) foil with a pre-etched pattern of indium-tin-oxide (ITO) anodes. The module was constructed as a matrix of 91 devices comprising 7 rows connected in parallel with each row having 13 individual cells connected in series. The printing of the organic layer employed screen printing of a chlorobenzene solution of the active material that consisted of either poly-1,4-(2-methoxy-5-ethylhexyloxy) phenylenevinylene (MEH-PPV) on its own or a 1:1 mixture (w/w) of MEH-PPV and [6,6]-phenyl-C₆₁-butanoic acid methyl ester (PCBM). Our first results employed e-beam evaporation of the aluminium cathode directly onto the active layer giving devices with very poor performance that was discouragingly lower than expected by about three orders of magnitude. We found that e-beam radiation leads to a much poorer performance and thermal evaporation of the aluminium using a basket heater improved these values by an order of magnitude in efficiency for the geometry ITO/MEH-PPV/C₆₀/Al. Finally the lifetimes (τ_1/2) of the modules were established and were found to improve significantly when a sublimed layer of C₆₀ was included between the polymer and the aluminium electrode. Values for the half life of 150 hours were typically obtained. This short lifetime is linked to reaction between the reactive metal electrode (aluminium) and the constituents of the active layer.

At present, heterojunction polymer solar cells are typically fabricated with an active layer thickness of approximately 80 nm to 100 nm. This active layer thickness has traditionally been chosen based upon convenience and empirical results. However, a detailed mechanistic study of the effects of active layer thickness on the short circuit current and efficiency has never been performed for polymer solar cells. We demonstrate that using the high mobility materials regio regular poly(3-hexylthiophene and [6,6]-phenyl (P3HT) and C₆₁-butyric acid methyl ester (PCBM), that high efficiency solar cells can be fabricated with active layer thickness greater than 100 nm. Devices with an active layer thickness of 200 nm are fabricated with a power efficiency of 4.1% under AM1.5 illumination at and intensity of 80 mW/cm². In addition, we explain the variation in short circuit current density as a function of thickness using calculations of the distribution of the optical electric field intensity as a function of device thickness.

The photophysical properties of a solution processed blend of two semiconducting polymers with electron donating and electron accepting properties, respectively, as used in polymer photovoltaic devices have been investigated. In the binary mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly[oxa-1,4-phenylene-(1-cyano-1,2-vinylene)-(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene)-1,2-(2-cyanovinylene)-1,4-phenylene] (PCNEPV) photoexcitation of either one of the polymers results in formation of a luminescent exciplex at the interface of the two materials. The high energy of this correlated charge-separated state is consistent with the high open-circuit voltage of the corresponding solar cells (1.36 eV). Application of an electric field results in dissociation of the marginally stable exciplex into charge carriers, which provides the basis for the photovoltaic effect of this combination of materials.

By applying the specific fabrication conditions such as postproduction annealing at 150^oC for 30 minutes, polymer solar cells with 5% power conversion efficiency are demonstrated. These devices exhibit remarkable thermal stability. We attribute the improved performance to changes in the bulk heterojunction material induced by thermal annealing. The improved nanoscale morphology, the increased crystallinity of the semiconducting polymer, and the improved contact to the electron collecting electrode facilitate charge generation, charge transport to, and charge collection at the electrodes, thereby enhancing the device efficiency by lowering the series resistance of the polymer solar cells. Also new architectural polymer solar cells with 5% power conversion efficiency have been fabricated using titanium oxide (TiO_x) as an optical spacer. Solar cells with a TiO_x layer (deposited by a sol-gel process) between the active layer and the electron collecting aluminum electrode exhibit approximately 50% enhancement in power conversion efficiency compared to similar devices without the optical spacer. The TiOx layer increases the efficiency by modifying the spatial distribution of the light intensity inside the device, thereby creating more photogenerated charge carriers in the bulk heterojunction layer.

Among the class of conjugated polymers, polythiophenes and in particular 3-alkyl-substituted thiophenes seem to focus all the attention in the domain of photovoltaic conversion. At CEA, we are working on the optimization of bulk heterojunction solar cells made of poly-3-hexylthiophene (P3HT)and [6,6]-phenyl C₆₁ butyric acid methylester (PCBM) blend. First we will describe the influence of the ratio of P3HT and PCBM blend on the efficiency of the resulting bulk heterojunction solar cells. Best cells based on 1:1 in weight ratio yield 3.6 % power conversion efficiency under air-mass 1.5, 100 mW/cm² illumination. Then we will compare the efficiency and lifetime of different cells by changing the nature and thickness of cathode (Aluminum or Calcium/Silver). On the optimized cells, we have proceeded to ageing and accelerated lifetime measurements on devices with Ca/Ag cathode. It shows that the current densities decrease less than 3 % and that efficiency is still higher than 1.7 %, after 400 hours under AM 1.5, 100 mW/cm² illuminations and at high temperature (60°C).

The degradation mechanisms of conjugated polymer materials used in organic photovoltaic cells were studied. To elucidate the parts of the degradation mechanisms induced by molecular oxygen, isotopic labeling was employed in conjunction with time-of-flight secondary ion mass spectrometry (TOF-SIMS). Devices that were kept in the dark were compared with devices that had been subjected to illumination under simulated sunlight. It was found that molecular oxygen diffuses into the device causing oxygen-containing species to be generated throughout the active layers. The isotopic labeling combined with TOF-SIMS depth profiling and imaging allowed mapping of the oxidation processes by measuring the vertical and lateral distribution of oxygen-containing species. The exact pinpointing of the parts of the device that are susceptible to oxidation allows for a mechanism to be proposed that partly explains the device failure manifested in the insufficient life times of the organic photovoltaics.

Over the last decade, conjugated polymer-based semiconductors have been developed as a novel class of photovoltaic materials that have the potential to lower costs. Solvent based polymers MEH-PPV, MDMO-PPV, P3HT, and P3OT have been reported as electron donors in photovoltaic devices. In this research, we studied the use of a water soluble polythiophene - Sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate]) [PTEBS] in photovoltaic devices. Solar cells in the configuration of bilayer heterojunctions with TiO₂ were prepared. The water-soluble polythiophene showed significant photovoltaic effect and potential for use in solar cells. The use of this polymer would allow safe, environmentally friendly processing. In addition, due to the covalent bonding of the counterion to the polymer backbone chain simultaneously with electron loss in the doping and oxidation, the water-soluble polymer PTEBS can be self-doped by acids. The appearance and absorption spectra of the self-doped solutions and films have also been investigated. New absorption bands in the ultraviolet and infrared have been observed after acidic doping offering the possibility of improved light harvesting. Experimental results have shown that the polymer can be used as the active layer in photovoltaic applications. These photovoltaic devices had an energy conversion efficiency of 0.23% and a fill factor of 0.41 under the illumination of an 80 mW/cm² solar simulator. A simple mechanism has also been proposed to fit the open circuit voltage found in the devices.

A surface alkylated and metal nano-dotted n-Si electrode yields an efficient and stable photovoltaic characteristic in an aqueous redox electrolyte. It generates a high photovoltage due to a unique effect of metal nano-contact and is stabilized by surface alkylation. In the present work, we have prepared a composite electrode, composed of the surface methylated and Pt nano-dotted n-Si single crystal electrode and a tungsten trioxide (WO₃) particulate thin film, to decompose water into oxygen and H⁺ ions under solar irradiation. The onset potential of the oxygen evolution photocurrent for the composite electrode shifts to the negative by about 0.2 V compared with that for the WO₃ electrode alone, indicating that the two-step, Z-scheme mechanism operates in the composite electrode, leading to generation of a high photovoltage that comes from a series sum of the photovoltage in the Si and that in the WO₃. It is discussed that a composite "polycrystalline Si / visible-light responsive metal-oxide thin-film" electrode is a promising approach to high-efficiency and low-cost solar water splitting.

We report a low cost device for performing chemiluminescent (CL) assays in a miniaturised format. The device comprises a poly(dimethylesiloxane) microfluidic chip for performing the CL assay coupled to a polymer photodiode based on a 1:1 blend by weight of poly(3-hexylthiophene) [P3HT] and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C₆₁ [PCBM]. The integration of organic photodiodes with microfluidic chips offers a promising route to low cost fully integrated diagnostic devices for point-of-care applications.

We present time-resolved photoluminescence studies in conjunction with device characterization of a variety of heterojunctions with poly-(3-hexylthiophene), or P3HT, as a means to understand how exciton dynamics affect device performance. We find that blends of P3HT with the electron-transporting polymer CN-ether-PPV and with the fullerene derivative PCBM result in ~4-fold and ~15-fold improvements in short-circuit currents, respectively, over neat-film P3HT on TiO₂ solgel. Despite efficient charge-transfer in P3HT:PCBM films, as evidenced by enhanced device performance and quenched steady-state luminescence, we observe only moderate reduction of the excited state lifetime, due to the already efficient non-radiative pathways in P3HT. We observe evidence for a new state that we assign to an exciplex in blends of P3HT with the electron-transporting polymer CN-ether-PPV. The exciplex state, which confirms the existence of charge-transfer between the two polymers, may account for the enhanced device performance of these blends by acting as a scavenger for excitons that would otherwise decay rapidly via non-radiative pathways. The long-range order of P3HT is disrupted when spin-cast on rough TiO₂ nanoparticles, and this results in a blueshift of the PL spectrum and a new long-lived decay component that we attribute to long-lived intrachain polarons. P3HT on smooth TiO₂ solgel films shows little or no quenching of the excited state, despite known charge transfer from P3HT to TiO₂.

In hybrid polymer photovoltaics, conjugated polymers are combined with wide bandgap metal oxide semiconductors like TiO₂ or ZnO. Reported maximum power conversion efficiencies (PCE) at AM1.5G conditions for a hybrid polymer bulkheterojunction device are up to 1.6 %. In this paper we report on the current-voltage characteristics of bi-layer devices consisting of TiO₂ and a conjugated polymer. Several polymers with different optical bandgap were studied. The maximum External Quantum Efficiency (EQE) of the devices is comparable, but the PCE differs considerably (0.2-0.5%). The differences can for a large part be explained by the differences in optical bandgap of the polymers. It is shown that a low band gap is beneficial for the short circuit current, but does not automatically result in a high PCE as relative shifts of the highest occupied molecular orbital (HOMO) energy levels of the polymers reduce the open circuit voltage (V_oc). The calculations show that a PCE up to ~ 19 % can be achieved using the maximum possible V_oc and a fill factor of 80%. Judicious engineering of material combinations is required to achieve such a power output, and it expresses the need for a continuing search on potentially low cost, efficient metal oxide/polymer BHJ structures.

We describe a simple and new method to create hybrid bulk heterojunction solar cells consisting of ZnO and conjugated polymers. A gel-forming ZnO precursor, blended with conjugated polymers, is converted into crystalline ZnO at temperatures as low as 110 °C. In-situ formation of ZnO in MDMO-PPV leads to a quenching of the polymer photoluminescence. Positive charges on the MDMO-PPV are formed after photoexcitation, indicating electron transfer from the polymer to ZnO. Results without full optimization already give photovoltaic cells with an estimated performance over 1% under AM1.5 illumination. The large effect of the processing conditions on the photovoltaic effect of the solar cells, indicate that there are several parameters that require control. The choice of solvent, type of atmosphere, and the relative humidity during spin coating, are important for optimization of the photovoltaic effect. These solar cells are made from cheap materials, and via simple processing and can be regarded as promising for further research.

We report on highly efficient organic solar cells based on polycrystalline thin films of pentacene, a material that has been widely investigated for p-type transport layers in organic field-effect transistors (OFET). The spectral measurement of external quantum efficiencies (EQE) shows that these high efficiencies are due to the efficient light-harvesting occurring throughout the visible spectrum. In particular, the peak EQE of 69% has been measured at a wavelength of 668 nm where most of the excitons are generated inside the pentacene layer. This suggests that the polycrystalline nature of pentacene films leading to high field-effect mobilities in OFET also results in relatively large exciton diffusion lengths which are desired in multilayer organic solar cells. In an effort to understand these devices, we model the external quantum efficiencies as a function of wavelength based on the exciton diffusion model using the complex indices (n, k) of participating materials. This study provides information on the correlation of optical properties of photoactive materials to the spectral responses and allows one to estimate exciton diffusion lengths. Based on this information, we discuss the optimization of layer structures that can lead to maximization of the photocurrent under standard illumination condition.

We have measured charge carrier transport and recombination in bulk-heterojunction solar-cells. Time-of-flight, carrier extraction by linearly increasing voltage and double injection techniques which are complementary to each other have been used to study the solar-cells of different thicknesses and conductivities. We show the importance of carrier mobility and bimolecular recombination coefficient for testing the suitability of materials in bulk-heterojunction solar-cells. The reduced bimolecular recombination coefficient at zero electric field and its electric field dependence are measured directly. The bimolecular recombination coefficient and Langevin-type coefficient values β/βL ≈ 10^-4 are in the good agreement when measured with presented techniques.

There has been great interest in recent years in using solution processible conjugated organic polymers as the active layers in semiconducting devices including field-effect transistors (FETs), light emitting diodes (LEDs), and photovoltaics (PVs). The structure of the polymer film is very important to device performance at the interfaces where charge separation and collection occur, as well as in the bulk of the film, where it has been shown that alignment of the polymer backbone can increase the pi-orbital overlap, thus enhancing charge carrier mobility. In this work, we report on the temperature-dependent alignment of liquid crystalline fluorene-thiophene copolymer thin film surfaces using the near-edge X-ray fine absorption structure (NEXAFS) technique. Partial electron yield spectra were recorded over a range of temperatures to observe directly the bond orientation in various polymer phases. In addition, samples were annealed under varying processing conditions and spectra were taken at room temperature on these heat-treated samples. The NEXAFS data shows: a) in thin polyfluorene films, the polymer backbone lies flat in the plane of the substrate, b) that along the main chain axis, biphenyl and thiophene rings have a preference to lie flat in the plane of the surface, c) the orientation of the polymer backbone can be controlled using a rubbed polyimide alignment layer as a template for liquid crystal orientation, and d) under proper annealing conditions, there is strong temperature-dependent alignment of the copolymer main-chain axis to the rubbing direction with dichroic ratio (R) reaching R=0.7.

The synthesis of conjugated low band-gap copolymers based on thiophene and benzothiadiazole is described. The synthesis was carried out by oxidative ferric chloride polymerization or Stille cross coupling polymerization. The solubility of the polymer based on quarterthiophene and benzothiadiazole was tested with hexyl, 2-ethyl-hexyl and dodecyl as side chains on the thiophene. It was found that 2-ethyl-hexyl substituents gave high molecular weight polymer products with good film forming ability and good solubility. The polymers based on di-thiophene and benzothiadiazole were applied in photovoltaic devices and the coupling of the alkylthiophene showed no effect on the maximum photovoltaic performance. Band-gaps were estimated to be 2 eV for polymers based on di-thiophene and benzothiadiazole and 1.8 eV for polymers based on quarterthiophene and benzothiadiazole. Attempts to synthesize the polymers with a benzo-bis-thiadiazole unit are also described.

A series of oligo phenylene vinyles (OPVs) have been prepared using a generic step-wise and uni-directional synthesis from stilbene type monomers containing masked aldehyde and benzylic phosphonate ester functionalities. In the course of this investigation six different monomers with alkyl or alkoxy substituents and with benzene, thiophene or benzothiadiazole groups were developed and prepared. Trimer OPVs were assembled and their optical spectra investigated. Systematic end-group modification gave a series of donor-wire-acceptor OPVs that were used to prepare simple large area photovoltaic cells (3 cm²) without any fullerene derivatives. The efficiency of the devices were measured and compared based on the short circuit current I_SC. Two materials were found to perform 10-100 fold better than standard PPV materials and the other OPVs investigated. Another type of end capping with a terpyridine moiety was realized to prepare an example of an OPV-ruthenium dye. A series of devices with mixtures of OPVs with the soluble fullerene derivative PCBM were made and characterized. A maximum efficiency of 0.8 % under AM1.5 conditions were found for a thiophene containing OPV trimer mixed with PCBM (1:4).

In this paper we report on an attempt to substitute the liquid-electrolyte in Dye Sensitized Solar Cells (LC) by quasi-solid-state constructions (SC) adopting organic/inorganic gels as well as a novel dye comprised of a conjugated polymer covalently linked to a ruthenium complex that can be bound to a TiO₂ anatase electrode. Gel polymer electrolytes are prepared by incorporating liquid electrolytes into a polymer matrix such as poly methyl methacrylate (PMMA) using a gelling solvent such as propylene carbonate (PC). Dye Sensitized Solar Cell (DSSC) fabricated using the former gel electrolytes and standard sensitizing dye such as cis-bis(thiocyano) ruthenium(II)-bis-2,2'-bipyridine-4,4'-dicarboxylate (N3) exhibit an encouraging short circuit current densitie (J_sc) of 4.45 mA cm^-2 with open circuit voltages (V_oc) of 495 mV. In the novel dye the conjugated polymer provides light harvesting and hole conduction while the ruthenium complex binds to the anatase electrode providing efficient charge carrier separation and injection into the anatase electrode.

Porphyrins have attracted a lot of interest as potential light harvesting dyes in polymer solar cells due to a broad absorption range, wherein the porphyrin serves as a synthetic surrogate for chlorophyll. Asymmetric porphyrins are essential building blocks in one of our polymer solar cell projects. Synthesizing these asymmetric porphyrins on large scale, in good yield, with few scrambling byproducts and without the necessity for chromatographic workup proved to be a challenge. Here we present different approaches to the synthesis of asymmetric trans-A₂B₂-porphyrins and trans-AB₂C-porphyrins on a large scale and detail problems associated with the synthetic work. The products were characterized using SEC, MALDI-TOF, UV-vis and NMR.

The synthesis of two new poly(dialkylstilbenevinylene)s obtained through a palladium-catalyzed polymerization with a controlled molecular weight and a terpyridine moiety in the backbone is presented. Assembly using ruthenium complexation led to coordination polymers with a ruthenium complex in the middle. The coordination homo and copolymers were characterized using NMR, UV-vis and were processed into thin films for solar cells applications. The best photoresponse was obtained for the device prepared from the ruthenium homopolymer bearing cyano substitutents with a maximum output power of 0.86 μW cm^-2 and a fill factor of 26% under illumination at 1000 W m^-2 AM1.5. A blend of this compound with a zinc porphyrin was also investigated and gave a lower performance.

Light harvesting and energy transfer in two oligomer-dye assemblies has been investigated. In both cases the oligomer was a poly(terphenylenecyanovinylene) derivative while two different dyes was used, a porphyrin and an ionic dye. It is well known that the efficiency of solar cells consisting of a single homopolymer is limited. To increase overall efficiency different strategies have been used. One possible strategy aims at covalently linking different domains. With careful design, this type of assemblies is envisaged to show improved charge separation and charge transport properties. We have shown how photophysical measurements can be used to determine what happens to an exciton formed on any of the domains. From fluorescence and absorption measurements on the assemblies, along with model compounds, it was possible to quantify the number of excitons that are emitted (fluorescence), transferred between domains or lost in internal transfer processes. Both steady state and lifetime measurements were performed in solution and on solid films. The effect of acid was investigated in the cases of the oligomer-porphyrin assembly. We found that in solution the effect of acid was an increase in the time of energy transfer, probably due to acid induced structural change of the porphyrin moiety. It was possible to make LB-films of the ionic dye-assembly, which made it possible to investigate a monolayer of the assembly.

In this paper we would like to address the key role of fabrication in the performance and lifetime of organic photovoltaics. The realization of a complete process line for the construction of large area organic photovoltaics (250 x 400 mm) is described. Among many of the factors that influence organic solar cell lifetime, oxygen and water exposure is the most important. Multiple processes have to be performed under controlled atmosphere and a glove box (or glove boxes), which involves more volume than commercially available glove boxes, needs to house different instruments. The processes housed in the glove boxes were spin coating, evaporation, lamination/sealing and testing, under an inert atmosphere. The main strategy employed multiply connected glove boxes with one load lock. The first glove box was used for spin coating and lamination/sealing, the second will house a screen printer and the third one accommodate an evaporator completely build in house. The evaporator has 2 thermal evaporation sources and 2 e-beams with 4 and 1 crucibles. The process line should allow the entire device realization from substrate coating, to electrode evaporation including the sealing process avoiding air and water exposure. Organic solar cells from small test cells on ITO glass to big modules (250 x 400 mm) of 91 connected cells on ITO PET substrates were fabricated and characterized.

The design of light harvesting systems based on zinc-porphyrin linked polyphenyleneethynylene systems is demonstrated through two different synthetic procedures. The use of the Sonogashira cross-coupling reaction was found to be problematic in several aspects and it was only possible for one of the synthetic strategies to incorporate a single porphyrin molecule in each polymer chain. The polymer materials were separated into fractions using preparative SEC and subjected to photophysical studies in both the solution and in the solid state. In the solid state light harvesting by the pendent polyphenyleneethynylene and energy transfer to the zinc-porphyrin was found to be high whereas it was low in solution. This observation was confirmed when making electroactive devices. I/V characteristics were measured on fabricated solar cells of the polymers. Material properties for the native polyphenyleneethynylene were determined using ultra violet photoelectron spectroscopy on thin films and carrier mobility studies were performed using pulse radiolysis time resolved microwave conductivity.

The sidechains that are added to conjugated polymer materials convey desirable solubility and film forming properties but they generally lead to a low glass transition temperature that is comparable or slightly higher than the operational temperature for the devices. The use of thermo cleavable sidechains allow for solution processing and film forming while a subsequent thermal treatment leads to efficient removal of the sidechains yielding a dense insoluble film with in principle a much higher glass transition temperature making diffusion phenomena much slower. This approach has been shown to improve the operational lifetime using accelerated lifetime testing with an incident light intensity of 1000 W m^-2 (AM1.5) at 72^oC and opens up the possibility for the formation of multilayer structures by sequential film forming and thermal cycles. The synthesis of regiorandom poly(2,5-thienyl-co-3'-(1''-valeryloxy-1''-ethyl)-2',5'-thienyl) (4) a polythiophene based on a copolymer of thiophene and the valeric acid ester of 3-hydroxyethylthiophene was demonstrated to thermo cleave valeric acid efficiently at temperatures above 200^oC leaving vinyl groups on the polythiophene backbone giving regiorandom poly(2,5-thienyl-3'-vinyl-2',5'-thienyl) (5). The thermocleaved film was insoluble in common organic solvents. Thermocleavage experiments using spincoated films of a 1:1 (w/w) mixture of 4 and the soluble fullerene derivative 6,6-phenyl-C₆₁-butyric acid methylester (PCBM) gave films of 5 and PCBM in a ratio of 2:3 (w/w). Photovoltaic devices were prepared and devices based on 5 and PCBM gave significantly improved lifetimes for devices operated in the atmosphere while the efficiency for the devices was lowered by a factor of 20 upon thermo cleavage for this system.

We describe the optimization of bulk heterojunction type photovoltaic devices from blends of ZnO nanoparticles and conjugated polymers. The photovoltaic effect of these devices depends on the choice of solvent, the amount of ZnO, and the thickness of the active blend layer. Optimized solar cells have an estimated AM1.5 performance of 1.6%. Incident photon to current conversion efficiencies (IPCE) show that up to 40% of the incident photons can be collected as charges. At high light intensities the performance of the cell drops, due to a decreasing fill factor (FF).

A new low band gap polymer (pBEHTB) with an absorption onset at 800 nm is reported. When combined with a soluble fullerene derivative (PCBM), efficient electron transfer occurs after excitation of the polymer. Bulk heterojunction solar cells have been prepared with a response up to 800 nm, and an estimated power conversion efficiency of 0.9 %.

We report the fabrication of polarization-sensitive photovoltaic devices made of hetero-junction type vacuum-sublimed multilayer films composed of aligned 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (aligned-PTCBI) and titanyl phthalocyanine (TiOPc). The PTCBI layer was successfully made to be well aligned without losing high photovoltaic power-conversion efficiency. High polarization sensitivity was achieved at around 540 nm. The device configuration was ITO/In/aligned-PTCBI/TiOPc/PEDOT:PSS/Au and the thickness of each layer was optimized for polarization-sensitive photo-detection. The power-conversion efficiencies under the polarized white light parallel and perpendicular to the molecular-orientation axis, through the ITO electrode were 0.78% and 0.45%, respectively. The ratio of short-circuit current, parallel to perpendicular, was 1.66. This device can be used as transparent photo-detectors, because the transmittance of the Au electrode was about 40% at 500-600nm. The short-circuit current ratio was increased to 3.0, when 510nm monochromatic polarized light through the Au electrode was used.

Monte Carlo simulations are used to investigate the dissociation of a Coulomb correlated charge pair at an idealized interface between an electron accepting and an electron donating molecular material. In the simulations the materials are represented by cubic lattices of sites, with site the energies spread according to Gaussian distributions. The influence of temperature, applied external fields, and the width of the Gaussian densities of states distribution for both the electron and the hole transporting material are investigated. The results show that the dissociation of geminate charge pairs is assisted by disorder. When the rate for geminate recombination at the interface is very low (<1 ns^-1) the simulations predict a high yield for carrier collection, as observed experimentally. Comparison of the simulated and experimentally observed temperature dependence of the collection efficiency indicates that at low temperature dissociation of the geminate charge pairs may be one of the factors limiting the device performance. Furthermore, the simulations show that excess exciton energy liberated in the photoinduced charge transfer process enhances dissociation of the geminate pair and can thereby allow for high yields for carrier collection.

In this paper we present a study of thermally interdiffused poly (3-octylthiophene) (P3OT) - C₆₀ photovoltaic devices of varying polymer and fullerene layer thicknesses. It is found that overall device performance and external quantum efficiencies (EQEs) in the wavelength region of peak absorption are enhanced for thinner polymer as well as thinner fullerene layers. We present a simple, first level model for EQE curves that accounts for the effects of absorption throughout the film. The model considers 3 distinct regions; pure donor, interdiffused donor and acceptor, and pure acceptor. It is found that this model reproduces quite effectively the experimentally observed shapes and relative magnitudes for varying donor thickness when compared with a similar experimental study. It also reproduces the shapes of the EQE curves but is not as effective in reproducing the relative magnitudes for varying fullerene thickness devices.

A novel cyanine-fullerene dyad is used as active layer in a heterostructure photovoltaic cell based on PEDOT and buckminsterfullerene C₆₀. The photocurrent spectrum of the device matches the absorption spectrum of the film and shows monochromatic quantum efficiencies of 5.5% and fill factors above 0.35 at white light irradiation intensities up to 310 mW/cm². By blending a long wavelength absorbing cyanine with the dyad, the photocurrent spectrum extends to 800 nm with appreciable monochromatic quantum efficiency. Sensitization by cyanine dyes demonstrates the possibility to achieve photovoltaic response in the near infrared.

We present a highly fluorescent polymer poly[2,7-(9,9'-dioctylfluorene)-alt-1,4-bis(1-cyanovinyl-2-thienyl)-2-methoxy-5-(3,7-dimethyloctyloxy)phenylene] (PF1CVTP), that was found to perform exceptionally well as electron acceptor in polymer photovoltaic devices when mixed with poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) as electron donor. The optical and electrochemical properties of the blend were investigated. Both the quenching and the position of the oxidation and reduction waves indicate that charge transfer could take place if the blend is illuminated. Solar cell devices were made of blends containing different ratios of donor and acceptor. Maximum external quantum efficiency of more than 50 % was obtained and a power conversion efficiency of up to 1.5 % was measured under AM1.5 G (100 mW/cm²) conditions.

Stacked devices that consisted of transparent organic photodiodes (TOPDs) and organic electroluminescence devices (OELs) were demonstrated. TOPDs were prepared by poly-(2-methoxy-5- (2'-ethylhexyloxy)-1,4-phenylene vinylene (MEH-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend films as an active layer and transparent Au cathode (10 nm thick). These TOPDs showed about 45 % transmittance on average in visible light region (380-780 nm) and good correlation between incident light intensity and output photocurrent. Based on these results, the stacked devices were prepared by introducing OELs on TOPDs through a SiO insulating layer. The structure of OELs was ITO/Carbon/TPD/Alq₃/LiF/Al. These stacked devices work as light emitting devices and also photo diodes. Since TOPDs have transparency, OELs can illuminate a paper put on the glass substrate through TOPDs and TOPDs can receive reflective light from the paper. Although the TOPDs also absorb light from OELs directly, the output signals from TOPDs changed according to the black and white pattern of the paper. These results show that the devices act as an image sensor having light emitting layer and light receiving layer in a same area.

In this paper two different methods, doctor blade of a paste and glancing angle deposition (GLAD), have been used to fabricate layers of titanium dioxide for use in organic solar cells. Doctor blade TiO₂ consists of a random network of nanocrystals with an average pore size of about 9 - 10nm. Controlled nanometre-scale columnar structures of TiO₂ with column spacing between 50 and 100nm were obtained using the GLAD process. Solar cell based on GLAD films gave performances inferior to doctor blade TiO₂ devices. The optical property of the TiO₂ has been identified as a possible influential factor limiting the solar cell efficiency of GLAD-based devices.

Solar cells fabricated from composites of conjugated polymers with nanostructured metal oxides are gaining interest on account of the stability, low cost and electron transport properties of metal oxides. Zinc oxide (ZnO)/polymer solar cells are promising compared to other metal oxide/polymer combinations, on account of the possibility of low temperature synthesis, as well as the potential for controlling interface morphology through simple processing from solution. Here, we focus on the effect of surface morphology of ZnO films on photovoltaic device performance. We have successfully grown ZnO nanorods standing almost perpendicular to the electrodes on a flat, dense ZnO "backing" layer. We studied structures consisting of a conjugated polymer in contact with three different types of ZnO layer: a flat ZnO backing layer alone; ZnO nanorods on a ZnO backing layer; and ZnO nanoparticles on a ZnO backing layer. We use scanning electron microscopy, steady state and transient absorption spectroscopies and photovoltaic device measurements to study the morphology, charge separation and recombination behaviour and device performance of the three types of structures. We find that charge recombination in the structures containing vertically aligned ZnO nanorods is remarkably slow, with a half life of over 1 ms, over two orders of magnitude slower than for randomly oriented ZnO nanoparticles. A photovoltaic device based on the nanorod structure which has been treated with an ambiphilic dye before deposition of poly(3-hexyl thiophene) (P3HT) polymer shows a power conversion efficiency over four times greater than for a similar device based on the nanoparticle structure. The best ZnO nanorods: P3HT device yields a short circuit current density of 2 mAcm^-2 under AM1.5 illumination (100mWcm^-2) and peak external quantum efficiency over 14%, resulting in a power conversion efficiency of 0.20%.

Organic/polymeric solar cells can be optimized in both space (morphology) and energy regimes to minimize the 'photon loss', the 'exciton loss' and the 'carrier loss'. In spatial regime optimization, for instance, a set of thin film solar cell devices fabricated from a -donor-bridge-acceptor-bridge- (-DBAB-) type block copolymer containing polyphenylenevinylene (PPV) conjugated donor and acceptor segments exhibited open circuit voltages (V_oc) up to 1.1 volt, which is very impressive for organic/polymeric photovoltaics, though the photoelectric quantum and power conversion efficiencies are still very low due to energy gap mismatch and very small and not yet optimized short circuit current (in micro Amps regime).

We report a novel type of nanocomposite of conjugated polymer (regio-regular polythiophene) with infrared-sensitive, PbSe quantum dots (QD), which have size-tunable lowest-energy absorption bands between 0.3 and 1 eV. Thin film devices show very good diode characteristics and sizable photovoltaic response with an open circuit voltage, Voc, of ~ 0.3-0.4 V and short circuit current density, J_sc, of ~ 0.2mA/cm², which is significantly higher than recently reported in PbS QD-based devices. This is the evidence of a quite efficient photoinduced charge transfer between the polymer and QD, with infrared sensitivity. Photocurrent under reverse bias is significantly enhanced to J_ph ~ 1 mA/cm² indicating that the polythiophene/PbSe QD system can be used as effective infrared photodetectors. Detailed spectroscopic studies of photoresponse over a wide spectral range are presented. Quenching of photoluminescence by PbSe QDs has also been studied to gain more understanding of energy and charge transfer in this system.

We investigate the dependence of the photovoltaic characteristics of organic photocells on the relative concentration of the donor-acceptor molecular complex. The devices were fabricated using a new [MEH-PPV] - co - [phenylene vinylene] blend with C₆₀. We find that the morphology and device performance are strongly influenced by the molar fraction (x) of C₆₀ in the electroactive layer of the device. The best device was obtained with x = 0.6 and manifested V_OC = 0.85 V, J_SC = 2.65 mA/cm², FF = 0.42, and η^P_ext = 1%.

We show that the insertion of thin interfacial layers of pyronin B between the conjugated polymer-methanofullerene blend and Al electrode enhances the open-circuit voltage (V_oc) and short-circuit current density (J_sc). The cells based on blends of poly[2-methoxy,5-(3',7'-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) with the pyronin B layer (~ 1.5 nm) shows V_oc~0.86 V, J_sc~3.36 mA/cm² and the power conversion efficiency of ~1.46 % under white light photoexcitation of about 80 mW/cm², similar to the effect of thin LiF layer. Compared to cells without the Pyronin B layer, the power conversion efficiency increases up to about 40 %. Similar effect is also obtained in poly(3-octylthiophene) (P3OT)-PCBM blends. The increased solar cell performance can be attributed to enhanced carrier extraction efficiency at the polymer blend/Al interfaces when pyronin B was inserted. This effect is considered as the reduced Al work function with the thin Pyronin B interfacial layer.

We investigate the stacked structure of bulkheterojunction (BHJ) organic solar cells based on conjugated polymer and fullerene derivative by the solution processing, especially spin coating method. Poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene], (MDMO-PPV) blended with [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) are used as a photo-active layer. The stacked-cell structure is as follows: ITO/(lower BHJ cell) PEDOT:PSS/MDMO-PPV:PCBM/inter-layer/(upper BHJ cell) PEDOT:PSS/MDMO-PPV:PCBM/Al. We evaluate Ag and ITO as materials of inter-layer to connect the upper BHJ cell with the lower BHJ cell. High open circuit voltage (V_OC) BHJ organic solar cell is obtained by using ITO inter-layer. V_OC of stacked-cell with ITO inter-layer is 1.31 ± 0.04 V under 100 mW/cm² illumination intensity, the AM 1.5 solar simulator. This is about 1.6 times as large as that of single BHJ organic solar cell. By virtue of inserting ITO inter-layer, the upper BHJ cell can be formed without damaging the lower BHJ cell and consequently stacked-cell in which two BHJ cells are connected in series can be fabricated. Moreover, the origin of V_OC of stacked-cell is likely the sum of each V_OC of the upper and the lower BHJ cells, and is different from that of stacked small molecule heterojunction organic solar cells. In the case of Ag interlayer, stacked-cell does not work at all. From the elemental analyses of device cross section, Ag layer seems to be damaged by PEDOT:PSS solution.