Organic photovoltaic devices are poised to fill the low-cost, low power niche in the solar cell market. Recently measured efficiencies of solid-state organic cells are nudging 5% while Grätzel’s more established dye-sensitized solar cell technology is more than double this. A fundamental understanding of the excitonic nature of organic materials is an essential backbone for device engineering. Bound electron-hole pairs, “excitons,” are formed in organic semiconductors on photo-absorption. In the organic solar cell, the exciton must diffuse to the donor–accepter interface for simultaneous charge generation and separation. This interface is critical as the concentration of charge carriers is high and recombination here is higher than in the bulk. Nanostructured engineering of the interface has been utilized to maximize organic materials properties, namely to compensate the poor exciton diffusion lengths and lower mobilities. Excitonic solar cells have different limitations on their open-circuit photo-voltages due to these high interfacial charge carrier concentrations, and their behavior cannot be interpreted as if they were conventional solar cells. This article briefly reviews some of the differences between excitonic organic solar cells and conventional inorganic solar cells and highlights some of the technical strategies used in this rapidly progressing field, whose ultimate aim is for organic solar cells to be a commercial reality.

Single crystalline and polycrystalline CuInS2 samples prepared by different methods are characterized by Raman spectroscopy. The measured spectra are fitted according to the phonon confinement model. Correlation lengths were obtained, which correspond to the size of domains of perfect crystallinity. These correlation lengths are in good agreement with distances between twin defects observed by transmission electron microscopy in polycrystalline CuInS2. Additionally, the strain present in the samples was determined from the Raman spectra. A tensile strain was obtained for the polycrystalline CuInS2 thin films, which agrees well with published values for the same material.

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y Contributed equally.To identify optimal deposition conditions and morphologies for ZnO nanorod arrays (NRA) for photovoltaic applications, ZnO NRA were electrochemically grown onto transparent conducting oxide (TCO) substrates pre-covered with an intrinsic zinc oxide (i-ZnO) seed layer (SL) at different cathodic potentials, deposition times and temperatures. The morphology and the optical properties of the ZnO NRA were investigated with respect to application in thin film solar cells. The NRA morphology was found to be strongly correlated with the amount of electrochemically deposited ZnO, irrespective of the reaction conditions for a wide range of parameters. It was demonstrated that also the optical reflectance and transmittance of the NRA were closely connected to the electrochemically deposited amount of ZnO and the NRA morphology. Also the relative amount of defects determined by room temperature (RT) photoluminescence (PL) measurements correlated with the amount of electrochemically deposited ZnO, higher defect densities were however found for high cathodic potentials and low temperatures presumably due to excessive deposition rates and preferential formation of Zn(OH) 2 , respectively. A range of reaction conditions was identified yielding both fast growth and ZnO NRA with morphological and optical properties suited for thin film solar cell applications.

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