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The impact of branching in a diketopyrrolopyrrole polymer on the performance of polymer-fullerene photovoltaic cells is investigated. Compared to the linear polymer, the branched polymer affords a more finely dispersed fibrillar network in the photoactive layer and as a result a large enhancement of the photocurrent and power conversion efficiency.

Solution processed polymer tandem solar cells that combine wide and small bandgap absorber layers reach a power conversion efficiency of 7% in a series configuration. This represents a 20% increase compared to the best single junction cells made with the individual active layers and shows that the tandem configuration reduces transmission and thermalization losses in converting sunlight., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Developing new acceptor materials as alternatives to fullerene acceptors remains a challenge in the field of organic photovoltaics. We report on the synthesis and optoelectronic properties of three acceptor polymers bearing diketopyrrolopyrrole units in the main chain (PA, PB and PC). Their performance as the acceptor material in bulk heterojunction solar cells using P3HT as the donor material has been tested. The solar cells show relatively high open-circuit voltages (≥0.9 V) but low fill factors and short-circuit current densities limit the photovoltaic device performance. Formation of free charge carriers and low electron mobility are identified as the major obstacles. In blends of P3HT with PA or PB charge formation is limited, while for the P3HT:PC blend photogenerated charges recombine into the PC triplet state before they can separate, unless assisted by a reverse electric field., (© The Owner Societies 2011)