Origin of Different Dependences of Open-Circuit Voltage on the Electrodes in Layered and Bulk Heterojunction Organic Photovoltaic Cells

Experimental results show that the V _OC of layered heterojunction (HJ) organic photovoltaic (PV) cells behaves with a very weak dependence on the electrodes. However, the V _OC of bulk HJ PV cells behaves with a strong dependence on the electrodes. In this paper, an explanation for the different behaviors of V _OC on the electrodes is proposed. It is found that the V _OC 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 V _OC 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, V _OC 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 V _OC on the electrodes for bulk HJ PV cells is observed.