This PDF file contains the front matter associated with SPIE Proceedings Volume 7052, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Synchrotron induced photoelectron spectroscopy on in situ co-evaporated blends of CuPc and TCNQ and on TCNQ/CuPc interfaces is applied for monitoring the electronic interaction indicated in systematic shifts of the CuPc and TCNQ HOMO and core orbitals. These shifts correspond to a movement of the Fermi level within the HOMO LUMO energy gap. The shifts in CuPc induced by the interaction with TCNQ are similar in the composites and in the interface model experiment. At the interface an additional induced dipole potential can be measured. The interface-dipole plus the Fermile level shift add up to the work function difference of pure CuPc and TCNQ. We conclude that TCNQ forms a separate phase in CuPc rather than single isolated acceptor molecules. Charge transfer at the bulk hetero-junction induces the Fermi level variations, which may be called doping.

Large area organic photovoltaic modules have been fabricated with 232.8 cm² of total area with 108 cm² of photoactive area. Efficiencies up to 2.3% by active area (1.1% by total area) demonstrate operation of a multicell module with configurable voltage and current output. NREL certification is reported and is an example of the largest OPV module certified by NREL, as well as the only polymer:fullerene-based module tested. Module lifetime data were collected and with ca. 550 h of light-soaking, > 2000h of lifetime is expected based on extrapolation. The conditions of the test were 100% duty cycle, ~ 1 Sun Xe-arc lamp, KG5-Si reference, ~25 °C controlled temperature. The lifetime data were normalized with respect to the variations and fade of the Xe-arc lamp source.

We discuss the measurement and analysis of current vs. voltage (I-V) characteristics of organic and dye-sensitized photovoltaic cells and modules. A brief discussion of the history of photovoltaic efficiency measurements and procedures will be presented. We discuss both the error sources in the measurements and the strategies to minimize their influence. These error sources include the sample area, spectral errors, temperature fluctuations, current and voltage response time, contacting, and degradation during testing. Information that can be extracted from light and dark I-V measurement includes peak power, open-circuit voltage, short-circuit current, series and shunt resistance, diode quality factor, dark current, and photo-current. The quantum efficiency provides information on photo-current nonlinearities, current generation, and recombination mechanisms.

Organic solar cells (OSCs) have been extensively studied and significant improvements have been demonstrated in recent years. Along with the excitement in technology development, the accurate measurement of OSCs has become critical for the healthy development of this promising technology. The limited absorption and spectral response of organic based solar cells could lead to significant derivation in solar cell measurement. In this paper, we will discuss several issues in the measurement of organic solar cells, including spectral mismatch factor, elimination of the mismatch by proper selection of reference cell, external quantum efficiency testing, device area issue etc. Results on both polymer based bulk hetero-junction solar cell and small molecule based solar cell will be presented.

Performance characterization of the dye-sensitized solar cells (DSC) and organic polymer solar cells (OSC) has been investigated, in order to clarify how to accurately determine their performance. Accurate characterization of DSC requires consideration on the very slow temporal response, and variation of the quantum efficiency spectrum for the bias light. The I-V curves of the DSC are clearly dependent on the voltage sweep direction, even when the sweep time is the order of seconds. The quantum efficiency spectra shows irradiance-dependence in the wavelength range of <500 nm. Also, the temporal response is dependent on the wavelength of the incident light. On the other hand, the characterizationrelated features for measuring the I-V curves and quantum efficiency spectra of OSC are rather similar to conventional crystalline Si or amorphous silicon devices. Although their characteristics can be strongly dependent on the device design, the present results can be utilized for precisely characterizing the I-V curves and spectral responses of DSC and OSC.

Fullerene (C60) samples purified to seven-nine level by single-crystal formed sublimation were incorporated into i-layer of p-i-n organic solar cells. Fill factor hardly decreased even for the very thick C60: metal-free phthalocyanine (H2Pc) codeposited i-interlayer reaching 1.2 µm. For 1 µm thick i-layer, short-circuit photocurrent density of 18.3 mAcm-2 and photo-electric conversion efficiency of 5.3% was observed. Very large photocurrent density is mainly due to nearly 100% utilization of solar light in the visible region by 1 µm-thick i-layer.

We have fabricated organic photovoltaic cell using tetrabenzoporhyrin (BP) as a donor material. Tetrabenzoporhyrin is formed by thermal conversion of the soluble precursor that has four bicyclo rings. Upon heat treatment above 150°C, the precursor molecule is converted to semiconductive tetrabenzoporphyrin, which is insoluble against conventional organic solvents. Taking advantage of this insoluble character of BP, p-i-n bulk heterojunction photovoltaic cell is successfully fabricated from solution, with BP/BP:fullerene/fullerene trilayer, where p-layer is crystal BP, i-layer consists of both BP and fullerene, and fullerene acts as n-layer. We have reported the p-i-n photovoltaic cell using [6,6]-phenyl-C61-butyric acid n-butyl ester (PCBNB) as an acceptor. This solution-processed p-i-n device has achieved power conversion efficiency as high as 3.4% (Jsc=9.8mA/cm2; Voc=0.62V; FF=0.56). The performance of BP device with p-i-n junction is further improved by introducing a new fullerene for the i- and n-layers. Taking into consideration LUMO level, solubility and thermal properties, a novel fullerene derivative, 1,4-bis(dimethylphenylsilylmethyl)[60]fullerene (SIMEF), is designed and synthesized. This new di-adduct fullerene , SIMEF, improves power conversion efficiency up to 4.1% (Jsc=9.1mA/cm2; Voc=0.76V; FF=0.59). This improvement is largely due to an increased Voc. We have analyzed BP:fullerene composite films to find that formation of meta-stable phase of BP is fully suppressed by SIMEF. Depth profile of this p-i-n cell is measured by means of TOF-SIMS method to observe the expected vertical distribution of the donor and acceptor that has been designed.

Studying polymer:fullerene solar cells, we find that the bimolecular recombination is much lower than the anticipated Langevin recombination. In our study, we investigate the performance of annealed P3HT:PCBM bulk heterojunction solar cells by current-voltage characterisation, and complement these measurements with photo-CELIV experiments. We find that at room temperature the bimolecular recombination rate is reduced by a factor of about 50 as compared to the Langevin rate. We discuss the implications of this reduction on the solar cell performance, using a one-dimensional numeric simulator to analyse the experimental results, and compare the reduced recombination rates with the Langevin rates.

In this study we present research results on interdigitated organic / small molecule photoconductors. We investigated photoconductivity in interdigitated lateral photoconductors with aluminum contacts, using (1) a spin-coated organic blend and (2) evaporated organic multilayers as the active layer. The spin-coated devices were made with a blend of a poly[2-methoxy-5-(2-ethylhexyl-oxy)-1,4-phenylene-vinylene] [MEH-PPV] and {6}-1-(3-(methoxycarbonyl) propyl)-{5}-1-phenyl-[6,6]-C61 [PCBM]. In spin-cast devices, the quantum efficiency was limited by the dissociation of the excitons. The field dependence of the dissociation of the excitons was explained using a modified Onsager model for charge dissociation. The evaporated devices were made from layers of alpha-sexithiophene [α-6T] and C60. In the evaporate devices, trap sites in the active layer limited the quantum efficiency. By modeling the quantum efficiency based on exciton diffusion to the interface and the dissociation of the excitons, the experimental quantum efficiency was explained by a trapping model with a charge carrier lifetime of 0.002s.

In this article, the polymer photovoltaic devices based on the poly(3-hexylthiophene)/TiO₂ nanorods hybrid material is present. An enhancement in the device performance can be achieved by removing or replacing the insulating surfactant on the TiO₂ nanorods surface with a more conductive ligand, which can play the role to assist charge separation efficiency or also to prevent from back recombination, giving a large improvement in the short circuit current and fill factor. The relatively high power conversion efficiency of 2.2 % under simulated A.M. 1.5 illumination (100mW/cm²) can be achieved, providing a route for fabricating low-cost, environmentally friendly polymer photovoltaic devices by all-solution processes.

We report the synthesis and photovoltaic properties of poly[2,7-(9, 9-di-n-octyl-silafluorene)-alt-5,5''-(4', 7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PSiF-DBT). The polymer heterojunction solar cells fabricated from PSiF-DBT as the electron donor blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the electron acceptor exhibited a high powerconversion efficiency up to 5.4% with an open-circuit voltage of 0.90 V, a short-circuit current density of 9.5 mA cm^-2 and a fill factor of 50.7% under the illumination of AM 1.5 G from a solar simulator (800 W m^-2). A comparative study between PSiF-DBT and its polyfluorene analogous PFDTBT and PFO-DBT demonstrates that the high performance of PSiF-DBT originated from its red-shifted absorption spectrum up to 680 nm and high mobility of 1 × 10^-3 cm² V^-1 s^-1 compared with 645 nm and 3 × 10^-4 cm² V^-1 s^-1 for corresponding polyfluorene derivatives, respectively. These results indicate polysilafluorene derivatives are a promising new class of donor materials for polymer solar cells.

We report the development of a fully regioregular Poly(3-Dodecyl-2,5-thienylenevinylene) (HT-HT PDDTV) using the Horner-Emmons reaction, and studies using proton and carbon NMR spectroscopy, UV-vis absorption spectroscopy, fluorescence spectroscopy, cyclovoltametry, thermal analysis (DSC & TGA) and XRD. The HT-HT PDDTV developed has practically no solubility in boiling hexane, in sharp contrast to the literature PDDTV prepared from the Stille coupling reaction, which is mostly soluble in hexanes (an indication of high content of structural defects). The optical energy gaps are 1.80 eV in chloroform solution and 1.65 eV in film. The HOMO/LUMO of the film were -5.03 eV and -3.63eV, respectively. The electrochemical energy gap in the film is 1.4 eV. XRD study shows that a decent crystalline structure was formed without any annealing of the as-cast films. The lamellar sheets (formed from π. π. stacking) preferentially are oriented in parallel to the substrate surface with an interlayer spacing of 17.6 angstrom.

A composite of polyaniline (PANI) containing iron oxides (Fe₃O₄) with nanometer size was prepared by a chemical method. The electrical properties of (PANI-Fe₃O₄) sandwich structure using ohmic gold and blocking aluminium electrodes were studied. The current density - voltage (J-V) characteristics for the device resemble the typical dark current versus applied voltage characteristic for conventional Schottky diode. Electronic parameters have been calculated using J-V and capacitance-voltage (C-V) measurements.

A series of dye-sensitized solar cells (DSSCs) were fabricated using ZnO nanorod arrays as the anode electrode. The ZnO nanorod arrays were grown on the fluorine doped tin dioxide (FTO) substrates by a hydrothermal method. The scanning electron microscopy (SEM) images indicated that the ZnO nanorod arrays were highly oriented on FTO substrates with an average diameter of ~40 nm and an average length of ~1 μm. After sensitized by Z-907 dye via impregnation in solution, ZnO nanorod arrays changed the color from white to pink. This indicated that the dye had been successfully attached to ZnO nanorods. The high-aspect-ratio (~25) ZnO nanorod arrays are expected to improve charge transport through the formation of continuous channels along the nanorods. We fabricated photovoltaic cells based on these ZnO nanorod arrays and found the deposition time and effective area were two important factors affecting short circuit current densities and cell efficiencies. The device performance (Voc = 0.48 V, Jsc = 5.39 mA/cm², η = 0.73 %) showed a great potential for solar energy conversion.

Four different perylene-based electron-acceptors having similar electron affinities, but different thermotropic phases are blended with nematic liquid crystalline electron-donors with a fluorene-thiophene structure to form single layer photovoltaic devices. Best results are obtained when the nematic donor is mixed with an amorphous acceptor to give a supercooled nematic glass at room temperature. Atomic force microscopy operating in the phase contrast mode reveals phase separation on a nanometer scale with a broad distribution of domain sizes peaking at 26 nm. We correlate the morphology of the different blends with the performance of the photovoltaic devices. Power conversion efficiencies up to 0.9 % are obtained with excitation at 470 nm.

A distribution-controlled bulk heterojunction (dc-BHJ) photovoltaic cell where the distribution of an electron donor and acceptor is controlled across the film was fabricated by Evaporative Spray Deposition using Ultradilute Solution (ESDUS) method, which can build a layered structure of polymer semiconductors soluble in a same solvent, and the energy conversion efficiency, PCE, of the dc-BHJ was significantly improved compared with a conventional BHJ cell. The dc-BHJ cells were fabricated by changing the ratio of a donor, regioregular poly(3-hexyl-thiophene-2,5-diyl) (P3HT), and an acceptor, a fullerene derivative (PCBM), from 2:1 to 1:2. The short circuit current, Jsc, and PCE of a dc-BHJ cell having P3HT-rich BHJ/PCBM-rich BHJ structure was 6.06 mA/cm² and 2.15 %, respectively, while those of a conventional BHJ cell having P3HT:PCBM=1:1 layer were 5.26 mA/cm², 1.71 %. In the dc-BHJ cells, the introduction of composition gradient brought about the increase in conversion efficiency. The transportation of photogenerated charge carriers to the collecting electrodes was improved because the better pathways should be formed in the dc-BHJ cells.

CdSe/ZnS core/shell quantum dots have been decorated on thin multiwalled and singlewalled carbon nanotubes (CNTs) by chemical functionalization and substrate gate-bias control. CdSe quantum dots were negatively charged by adding mercaptoacetic acid (MAA). The silicon oxide substrate was decorated by octadecyltrichlorosilane (OTS) and converted to hydrophobic surface. The negatively charged CdSe/ZnS NCs were adsorbed on the SWCNT surface by applying the negative gate bias. The selective adsorption of CdSe/ZnS quantum dots on SWCNTs was confirmed by confocal laser scanning microscope. Quantum dots decorated carbon nanotubes have been used for effective photogenaration and carrier transport through the organic photovoltaic device which has fabricated using effective polymers. The results clearly indicate the efficient photocurrent generation and carrier transport which effectively increased the efficiency of the device for the next generation organic solar cell applications.

Tandem solar cells, in which two individual cells are stacked on top of each other, offer the potential to increase the efficiency significantly compared to a single cell on the same area. To reach maximum efficiency, each cell in the stack must have a distinctive spectral response and the current in each cell must be similar. This requires smart selection of materials, proper cell design and appropriate layer thickness. Tandem polymer solar cells can be made by processing two individual cells from solvent based liquids, separated by a recombination layer. Potential candidates for the recombination layer are 1) a combination of a ZnO layer and a pH-neutral PEDOT:PSS layer, 2) a TiOx layer combined with a normal PEDOT:PSS layer. We will discuss the properties of the suggested recombination layers. To determine the performance of tandem cells, accurate spectral response measurements are crucial. Spectral response measurements of a polymer tandem cell show that the response of each subcell can be measured only when a bias light with sufficient intensity and suitable spectrum is applied. We will discuss the special requirements for the spectral response set-up that are needed in order to successfully discriminate between the responses of each subcell.

The solution processable poly(3-hexylthiophene)(P3HT)/TiO₂-nanorod hybrid material for solar cells has been successfully demonstrated. A critical issue for using hybrid heterojunction concept is the interface properties which affect the exciton separation efficiency and bi-carrier transport. To improve the interface properties, we replace the insulating surfactant on TiO₂ nanorod surface with a more conductive oligomer, carboxylate terminated 3-hexylthiophene (P3HT-COOH). The enhancement of exciton separation efficiency due to better organic-inorganic interfacial compatibility can be obtained. The electron mobility for transporting in the TiO₂ network is improved. A power conversion efficiency has been increased 3 times by using this new hybrid material without optimization as compared with the hybrid without P3HT-COOH modification.

Poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonic acid) (PEDOT:PSS) is a common material of hole injection layer used in polymer light emitting diodes (PLEDs) and organic solar cells. It can improve the efficiency of the charge collection at the anode. It has been reported that adding glycerol to PEDOT:PSS could increase the conductivity and improve the efficiency of PLEDs and organic solar cells. However, it is less noticed that the conductivity could be improved when the solution of PEDOT was heated before deposition. Here we experimented different concentrations of glycerol into PEDOT:PSS to make G-PEDOT:PSS solution, and heated the G-PEDOT:PSS solution at different temperatures before deposition. The solutions are then spin-coated on the glass and annealed at 140 °C. The conductivity was then measured and compared. The experiments showed that the conductivity of pure PEDOT:PSS slightly increased for 2-3 times, while the G-PEDOT:PSS increased over two orders of magnitudes. The conductivity increased with the heating temperature before deposition. The enhancement of the conductivity of the G-PEDOT:PSS film was higher than that of the pure PEDOT:PSS film. The overall conductivity increase for over three orders of magnitude. The reason is because the high temperature causes the glycerol and PEDOT:PSS to mix evenly. This is helpful for the swelling and aggregation of colloidal PEDOT-rich particles, forming a highly conductive network. When G-PEDOT:PSS resistance is reduced, it may not only increase the hole collection ability, but also replace ITO as the anode layer due to its advantages of low production cost and high work function.

PPV derivatives, polymers with vinylene units, have their tendency to exhibit degradation, after irradiation with white light or operation of the device, resulting in the appearance of a shifted absorption and emission band in the short wavelength regions of the spectra. In order to reduce oxidation of the vinylene group, the vinylene group was cyclized using two 5-membered rings. In this paper, we report the synthesis and properties of new copolymers utilizing poly(5,5,10,10-tetrakis(2-ethylhexyl)-5,10-dihydroindeno[2,1-a]indene-2,7-diyl) (PININE) and benzothiadiazole (BT). PININE copolymers with BT and thiophene units exhibit high PCE for polymer solar cells (PSCs). Under white light illumination (AM 1.5 G, 100 mW/cm²), the cell based on PININEDTBT/PCBM as the active layer has a short circuit current density (Isc) of 5.93 mA/cm², a fill factor (FF) of 43 %, and PCE of 1.88 %. These copolymers have not only good processability due to indenoindene unit, in which four alkyl groups can be incorporated, but also the strong and uniform absorbance in the whole visible region.

We introduce a novel nanostructuring method for bulk heterojunction solar cells which is aimed at overcoming current limitations associated with short exciton diffusion lengths and poor charge transport. By employing a nanosphere templating technique porous interconnected films of copper phthalocyanine (CuPc) have been prepared. Subsequent infiltration of the CuPc structures with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) results in the formation of three dimensionally structured nanocomposites, consisting of interpenetrating and interconnected networks. The lengthscale separation in the composite can be engineered to match exciton diffusion lengths and the interconnectivity is compatible with good charge transport. We propose this templating strategy as a widely applicable solution to the continued development of low-cost organic photovoltaics.

We report a study on solar cells using pentacene derivatives with triisopropylsilylethynyl substitution at the 6,13-position and 1,3-dioxolane substitution to the terminal benzenoid rings of pentacene as the electron donor and C₆₀ as the electron acceptor. A significant increase in the open circuit voltage (V_oc) was obtained in all the pentacene-derivative based cells with the highest Voc as high as 0.90 V, compared to a 0.24 V value for pentacene. The variation in the Voc of the cells is in qualitative agreement with the larger offset between ionization potential of the electron donor and the electron affinity of C₆₀. The power conversion efficiency (η) at 100 mW/cm² of EtTP-5/C₆₀ cells reached 0.74%, which is comparable to that of a pentacene/C₆₀ cell (0.82%).