Understanding Triplet Formation Pathways in Bulk Heterojunction Polymer: Fullerene Photovoltaic Devices

Triplet exciton (TE) formation pathways are systematically investigated in prototype bulk heterojunction (BHJ) super yellow poly(p-phenylene vinylene) (SY-PPV) solar cell devices with varying fullerene compositions using complementary optoelectrical and electrically detected magnetic resonance (EDMR) spectroscopies. It is shown that EDMR spectroscopy allows the unambiguous demonstration of fullerene triplet production in BHJ polymer:fullerene solar cells. EDMR triplet detection under selective photoexcitation of each blend component and of the interfacial charge transfer (CT) state reveals that low lying fullerene TEs are produced by direct intersystem crossing from singlet excitons (SEs). The direct CT-TE recombination pathway, although energetically feasible, is kinetically suppressed in these devices. However, high energy CT states in the CT manifold can contribute to the population of the fullerene triplet state via a direct CT-SE conversion. This undesirable energetic alignment could be one of the causes for the severe reduction in photocurrent observed when the open-circuit voltage of polymer:fullerene solar cells is pushed to 1.0 V or beyond.