Donor-acceptor nanocomposite structures for organic photovoltaic applications

We investigated the effect of substrate temperature T(S) on the growth of 80 nm donor-acceptor nanocomposite (DAN) layers consisting of co-evaporated Zn-phthalocyanine (ZnPc) and C(60) as absorber materials in organic solar cells. High temperature devices show lower series resistances R(S) but also reduced V(OC). Both effects are diminished when the upper 20 nm of the blend are deposited at room temperature (RT). Moreover, the temperature profile improves the photocurrent density J(SC). Best efficiencies (up to 2.1%) were achieved applying a 90 degrees C/RT profile. Transmission electron microscopy studies on the corresponding DAN revealed the temperature-dependent formation of crystalline C60 phases (already at low T(S)) and crystalline ZnPc phases (at highest T(S)). The strong phase separation at 90 degrees C results in less compact and mechanical unstable films. Such devices exhibit low parallel resistance R(P). When the cold cover layer is applied R(P) doubles. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim