The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.

The open-circuit voltage ͑V oc ͒ of polymer:fullerene bulk heterojunction solar cells is determined by the interfacial charge-transfer ͑CT͒ states between polymer and fullerene. Fourier-transform photocurrent spectroscopy and electroluminescence spectra of several polymer:fullerene blends are used to extract the relevant interfacial molecular parameters. An analytical expression linking these properties to V oc is deduced and shown to be valid for photovoltaic devices comprising three commonly used conjugated polymers blended with the fullerene derivative ͓6,6͔-phenyl-C61-butyric acid methyl ester ͑PCBM͒. V oc is proportional to the energy of the CT states E CT . The energetic loss q⌬V between E CT and qV oc vanishes when approaching 0 K. It depends linearly on T and logarithmically on illumination intensity. Furthermore q⌬V can be reduced by decreasing the electronic coupling between polymer and fullerene or by reducing the nonradiative recombination rate. For the investigated devices we find a loss q⌬V of ϳ0.6 eV at room temperature and under solar illumination conditions, of which ϳ0.25 eV is due to radiative recombination via the CT state and ϳ0.35 eV is due to nonradiative recombination.

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High-performance polymer solar cells of an alternating polyfluorene copolymer and a fullerene derivative Svensson,

Renewable and cheap materials in electrodes could meet the need for low-cost, intermittent electrical energy storage in a renewable energy system if sufficient charge density is obtained. Brown liquor, the waste product from paper processing, contains lignin derivatives. Polymer cathodes can be prepared by electrochemical oxidation of pyrrole to polypyrrole in solutions of lignin derivatives. The quinone group in lignin is used for electron and proton storage and exchange during redox cycling, thus combining charge storage in lignin and polypyrrole in an interpenetrating polypyrrole/lignin composite.