A re-evaluation of transparent conductor requirements for thin-film solar cells

The needs of thin-film solar cells are a significant driver in transparent conductor research, making it vital that these requirements are properly understood. Here we demonstrate that the oft-quoted need for sheet resistances less than 10 Ω sq^-1 arises only when the addition of a metal grid is unfeasible, and for cells of a particular size. In addition we show that the performance of a highly transparent layer with a metal grid is generally superior to that of a single layer fulfilling the 10 Ω sq^-1 requirement without a grid. In order to clarify these issues we introduce simple measures of electrode performance which correspond directly to cell efficiency. These specialized figures of merit can be applied to electrodes with or without a metal grid, and also to those embedded in a tandem cell where good electrode performance is often imperative. By comparison we show that the ratio of DC to optical conductivity, the most widely used figure of merit for transparent conductors, is a bad predictor of performance in a solar cell. Our work jointly motivates the development of scalable techniques for incorporating thin metal wires into front-side electrodes, and research into transparent conductors which prioritize transparency over sheet resistance.