In this article some brief theoretical considerations addressing the potential of single and tandem solar cells, the main experimental achievements reported in the literature so far and finally some design rules for efficient material combinations in bulk-heterojunction organic tandem solar cells are presented.

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research is focussing on further improvements to this effi ciency, equally important fundamental research on the photophysics, photochemistry, and structure/function relationships of organic photovoltaics devices and their constituent materials remains an integral facet of this research area. Such research is vital for investigating all the steps comprising the operation of organic solar cells, from exciton generation, diffusion and dissociation to charge separation, transport and extraction.Optical and opto-electrical methodologies have been successfully applied in order to understand this organic solar cell behavior. Most common are charge carrier mobility studies, [ 4 ] using techniques such as time-of-fl ight, [5][6][7][8] photo-CELIV (photoinduced charge extraction by linearly increasing voltage), [ 5,9,10 ] space-charge limited current [ 11 ] and dark injection. [ 12 ] Other techniques such as electrochemical impedance spectroscopy (EIS), charge extraction and transient photovoltage provide information on charge carrier density, lifetime, and kinetics.In order to produce an organic photovoltaic device with a high power conversion effi ciency, the three relevant parameters to be optimized are the open circuit voltage, V OC , the fi ll factor, FF , and the short circuit current, J SC . The latter parameter, the J SC , is in turn dependent upon the effi ciencies of light absorption, charge carrier generation, and charge carrier collection ( η COLL ). A poor charge carrier collection invariably lowers the J SC and thus the overall device efficiency. The effi ciency of charge carrier collection depends on the distance charges can travel before they recombine. Under short circuit conditions, if the photocurrent response is dominated by drift, effi cient charge carrier collection requires a long charge carrier drift length, L DR . Conversely, if diffusion dominates, then effi cient charge carrier collection relies upon a long charge carrier diffusion length, L . Ideally, L should be at least three times longer than the active layer thickness, d , such that:under the condition of αd << 1, where αd is the optical density. [ 13 ] In either case, one of the most pertinent parameters It is important to accurately measure the charge carrier lifetime, a crucial parameter that infl uences the collection effi ciency in organic solar cells. Five transient and small perturbation experimental techniques that measure charge carrier lifetime are applied to a device composed of the polymer PDTSiTTz blended with the fullerene PCBM: time-resolved charge extraction (TRCE), transient absorption spectroscopy (TAS), photoinduced charge extraction by linearly increasing voltage (photo-CELIV), transient photovoltage, and electrochemical impedance spectroscopy. The motivation is to perform a comprehensive comparison of several different lifetime measurement techniques on the same device in order to assess their relative accuracy, applicability to operational devices, and utility in data analysis. The techniques all produce similar charge...