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 ∼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 ∼25%, which substantially improves the device efficiency.

Phys. Rev. Lett.
Ultrafast Spectroscopy

Vaynzof, Y., Bakulin, A., Gélinas, S., & Friend, R. (2012). Direct observation of photoinduced charge-transfer states at organic-inorganic interface. Phys.Rev.Lett., 108(24, Article number: 246605), 1–5. doi:10.1103/PhysRevLett.108.246605