Organic Solar Cells: How X-ray Scattering Has Improved Our Understanding of Morphology

Abstract

One of the greatest challenges facing humanity in the 21st century is providing the world's population with the energy it needs without significantly raising the concentration of greenhouse gases in the atmosphere [1]. Many studies predict that a significant fraction of our power will have to come from solar cells taking advantage of the sun bathing our planet with an average power of 120,000 TW. Solution processable organic photovoltaics (OPVs) show great promise for providing a cost-effective route for creating lightweight and flexible solar energy conversion devices [2–4]. Polymer-fullerene bulk heterojunctions (BHJ) have been intensely studied over the past decade for use as the active layer in such devices (Figure 1); one of the most popular blends consists of poly(3-hexylthiophene) (P3HT), as the electron donor phase, and phenyl-C61-butyric acid methyl ester (PCBM), as the electron acceptor phase. In OPV devices, optical photons are absorbed in the polymer and create excitons (bound electron-hole pairs). These diffuse to donor/acceptor interfaces, where they dissociate to create free electrons and holes that are then transported through the acceptor PCBM and donor P3HT, respectively, to the electrodes. Thus, current is generated. The optimization of BHJ blend morphologies on multiple length scales is critical in order to achieve high performance in these devices [5]. In an organic solar cell device, incident light is absorbed by the electron donor phase creating an exciton. The exciton must then diffuse to the donor–acceptor interface where charge separation can occur. The molecular packing and crystallite size of both the acceptor and donor within their respective domains must be optimized for exciton transport (diffusion) and maximum charge carrier mobilities for efficient transfer of electrons and holes to their respective electrodes. In addition, the nano-scale phase separation of the donor and acceptor materials must have domain sizes less than the exciton diffusion length in the polymer in order for efficient conversion of absorbed photons into electrical current.