The optimal band gap for plastic photovoltaics

Abstract

Solar cells are currently attracting much attention as potential energy sources. Those made from thin plastic films are particularly attractive because they are relatively easy to produce, structurally flexible, and can be applied to large areas at low cost. Despite recent improvements and considerable effort, the efficiency of plastic solar cells—the proportion of sunlight energy that they successfully convert into electric energy—is currently limited to 4–5%.1 To identify how to improve these cells, we have developed a numerical model that describes their electrical characteristics, as outlined in a previous SPIE Newsroom article.2 Here we argue that the optimal value of one of the key material parameters—the band gap of the light-absorbing plastic—is significantly different from that predicted for inorganic siliconbased solar cells. Plastic (or polymer) solar cells consist of two materials, the polymer and an acceptor, to facilitate generation of free charge carriers. When a photon is absorbed, a bound state of an electron and a hole (or complementary positive charge) called an exciton is created (see Figure 1, process 1). Figure 1 shows the ionization potential (IP) and electron affinity (EA) of both the polymer and the acceptor phase. A small difference (∼0.4eV) between the EA of the acceptor and the polymer is necessary to ensure efficient exciton dissociation (Figure 1, process 2). As Figure 1 shows, the maximum voltage that a plastic solar cell can supply, the open-circuit voltage Voc, is limited to the difference between the IP of the polymer and the EA of the acceptor. In fact, Voc is significantly smaller than this limit.3 Voc can therefore be increased by reducing the difference between the EA of the acceptor and the polymer (typically 1eV). Another key parameter of solar cells is the current they can supply: the so-called short-circuit current, Jsc. The more photons are absorbed by the polymer, the higher Jsc can be. The Figure 1. The scheme illustrates the relation between the energy levels of polymer/acceptor solar cells and the processes of exciton creation (1) and electron transfer (2).