Origin of different dependences of open-circuit voltage on the electrodes in layered and bulk heterojunction organic photovoltaic cells

Abstract

Experimental results show that the VOC of layered heterojunction (HJ) organic photovoltaic (PV) cells behaves with a very weak dependence on the electrodes. However, the VOC of bulk HJ PV cells behaves with a strong dependence on the electrodes. In this paper, an explanation for the different behaviors of VOC on the electrodes is proposed. It is found that the VOC of the two types of PV cells follows the same mechanism and is mainly determined by the light-injected carriers at the donor/acceptor (D/A) interface and the electrodes. However, the distinct device structures make the boundary conditions in layered and bulk HJ PV cells different, which leads to the different dependences of VOC on the electrodes. The layered HJ PV cells have geometrically flat D/A and metal/organic (M/O) interfaces (the interface near the electrode), which makes the effective thickness from the D/A interface to the M/O interface large. Thus, there is a low electric field at the M/O interface and, then, a very small barrier lowering. Under this condition, the light-injected carriers at the D/A interface tend to pin the Fermi level of the electrodes. As a result, VOC shows only a very weak dependence on the work function of the electrodes. However, the formation of the interpenetrating network in bulk HJ PV cells greatly decreases the D and A domain dimensions and induces the ambipolar carrier distribution in the blend layer. This will cause very large barrier lowering at the M/O interface when there is a high barrier. Under this condition, the light-injected carriers at the D/A interface can no longer pin the electrode Fermi level. Thus, a strong dependence of VOC on the electrodes for bulk HJ PV cells is observed. © 2009 IEEE.