Characterizing the charge collection efficiency in bulk heterojunction organic photovoltaic cells

We demonstrate a technique for determining the charge collection efficiency (eta CC) in bulk heterojunction organic photovoltaic cells (OPVs). To extract eta CC, we describe an approach to measure the optical absorption efficiency by examining the external quantum efficiency under reverse bias. Using estimates of the exciton diffusion efficiency obtained from photoluminescence quenching measurements, eta CC is calculated without any input from optical field simulations. For bulk heterojunction OPVs based on the donor-acceptor pairing of boron subphthalocyanine chloride and C-60, a peak eta CC of (79 +/- 2)% is realized for films containing 80 wt. % C-60 under short-circuit conditions. This optimum composition also coincides with a peak in the fill factor and the short-circuit current density. (C) 2012 American Institute of Physics. [doi:10.1063/1.3686909]