Antenna effects and improved efficiency in multiple heterojunction photovoltaic cells based on pentacene, zinc phthalocyanine, and C60

For organic solar cells, effective absorption over a wide wavelength range is important. A simple donor-acceptor pair is usually not sufficient to reach this goal. Thus, it would be desirable to utilize multiple photoactive materials in a single cell. In this work, two hole conducting materials, pentacene and zinc phthalocyanine (ZnPc), and electron conducting C-60 are chosen to construct three-component heterojunctions aiming at improved effective photon harvesting in organic solar cells. It is found that in pentacene/ZnPc/C-60 multiple heterojunctions, part of the excitons in pentacene reach the ZnPc/C-60 interface, where efficient exciton separation occurs and contributes to the photocurrent (PC). Triplet excitons are confirmed to be the major origin of PC by transient PC response measurements, suggesting that triplet-to-triplet energy transfer from pentacene to ZnPc is responsible for the improved PC of pentacene/ZnPc/C-60 multiheterojunctions. Furthermore, exothermic energy transfer from ZnPc to the lower lying triplet levels of pentacene is employed for extending the absorption range and enlarging the absorption intensity. To realize such a structure, an ultrathin ZnPc layer is embedded in the pentacene film in pentacene/C-60 single heterojunctions, leading to an enhanced quantum efficiency in the long wavelength range compared to the reference cell. These findings pave a way to efficient photovoltaic cells with a wide photoresponse ranging from near UV through the visible to the near infrared. (C) 2009 American Institute of Physics. [doi:10.1063/1.3187904]