Ionic Space Charge Driven Organic Photovoltaic Devices

Abstract

Most all-organic solar cells rely on undoped electron donor–acceptor heterojunctions. Power-conversion efficiencies depend critically on the photoinduced charge generation at these interfaces such as the charge transport through the layers and collection at the electrodes. Hence, the ability to regulate and control these processes would offer advanced device functionality. Mobile ions are able to create internal electric fields similar to conventional, electronic p-n junctions without having the inconvenience of doping, which often leads to carrier recombination and excited state quenching. We demonstrate that at organic heterointerfaces these ionic junctions can shift the electronic orbital energy level, which allows the direction of electron transfer processes to be controlled. Cationic cyanine dyes offer a convenient model system to study the effect of mobile ions systematically. In conjunction with usually strong electron acceptors such as the Buckminsterfullerene C60, and donors such as the poly(p-phenylenevinylene) derivative MEH-PPV, we fabricated bilayer photovoltaic devices to reveal exciting effects due to ionic interfacial space charge. For example, we show that C60 can be turned into an electron donor. Furthermore, oxidative or reductive electron transfer processes can simply be switched on and off with an applied bias, thereby drastically altering device performance and spectral sensitivity.