Interfacial Segregation in Polymer/Fullerene Blend Films for Photovoltaic Devices

It has recently been shown that surface energy effects can cause selective segregation at the active layer interfaces of a bulk heterojunction (BHJ) organic photovoltaic device. The active layer interface composition has been suggested to impact device performance. In this study changes in the BHJ vertical composition profile of BHJ active layers cast on two hole transport layers (HTL) with significantly different surface energies (gamma) are characterized using spectroscopic ellipsometry and near-edge X-ray absorption fine structure spectroscopy. Changes in the HTL gamma are shown to significantly affect the BHJ interfacial segregation at the buried interface near the HTL while the composition near the free surface (air) of the BHJ is unaffected. Despite the significant differences in vertical segregation at the HTL interface, the performances of the resulting organic photovoltaic devices were relatively similar.