Direct Observation of Photoinduced Bound Charge-Pair States at an Organic-Inorganic Semiconductor Interface

It is generally considered that photoinduced charge transfer at the organic-inorganic interfaces in hybrid photovoltaic devices immediately results in a pair of free charge carriers. We extend a novel interface-selective ultrafast "optical pump-push photocurrent probe' technique to study hybrid photovoltaic systems and observe bound electron-hole pair states at the organic-inorganic interface formed between electron-accepting zinc oxide and electron-donating conjugated polymers. We estimate that similar to 50% of photogenerated charges stay bound and later recombine, thus hindering the photovoltaic performance of polymer/ZnO cells. We further demonstrate that interface modification with a fullerene derivative decreases the fraction of bound charges to similar to 25%, which substantially improves the device efficiency.