The recent progress of semi-transparent electrodes based on thin metal layers or metal nanowires and their application in solar cells are reviewed.
Thin-film solar technology is the subject of considerable current research. The classical material platform of amorphous silicon (a-Si) has been complemented by organic solar cells and more recently by solar cells based on quantum dots or organo-metal-halide perovskites. The majority of effort is focused on the synthesis, characterization and optimization of the photo-active components as well as on the invention of novel device architectures. Low-cost, low-weight, flexibility and the opportunity to create semi-transparent devices are among the most frequently claimed selling points of thin-film solar cells. It is clear that the full potential of this technology and the ability to fulfill its promises are intimately linked with tailored concepts for transparent electrodes beyond established avenues. Transparent electrodes, that can be realized at a large area, at low costs, at low temperature, which are flexible (or even elastic), and which afford a conductivity and transmittance even better than those of indium-tin-oxide, are still vigorously pursued. Even though metal based semi-transparent electrodes have a notable history, there is an ever increasing effort to unlock the full potential of metal nano-structures, especially ultra-thin films (2D) or metal-nanowires (1D) as semitransparent electrodes for thin-film solar cells. This article will review the most recent advances in semitransparent electrodes based on metal-nanowires or metal thin-films. Aside from providing general considerations and a review of the state of the art of electrode properties like sheet resistance and optical transmittance, we aim to highlight the current efforts to introduce these electrodes into solar cells. We will demonstrate that by the use of metal based semitransparent electrodes not only a replacement for established transparent conductors can be achieved but also novel functionalities can be envisaged.