Enhanced efficiency in double junction polymer:fullerene solar cells

Polymer solar cells based on the polyfluorene copolymer poly[9,9-didecanefluorene-alt-(bis-thienylene) benzothiadiazole] (PF10TBT) and the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) exhibit a power conversion efficiency of 4%. However, the optimum thickness of the photoactive layer is only 80 nm, such that these solar cells absorb only half of the photons available in their absorption bandwidth. Thicker cells are subject to electronic losses due to space charge effects and recombination. We demonstrate both from simulations and experiments that the optical and electronic losses can be decoupled in a double junction solar cell. For the PF10TBT:PCBM blend, the integration of two thin cells in a double junction device leads to an enhancement of the measured power conversion efficiency with 13% up to a value of 4.5%.