Graphene-Based Electrodes for Silicon Heterojunction Solar Cell Technology

Abstract

Transparent conductive electrodes (TCEs) based on graphene have been previously proposed as an attractive candidate for optoelectronic devices. While graphene alone lacks the antireflectance properties needed in many applications, it can still be coupled with traditional transparent conductive oxides (TCOs), further enhancing their electrical performance. In this work, new architectures of TCEs incorporating graphene monolayers in different spatial configurations have been explored. The aim is to achieve advanced transparent electrodes specifically designed to minimize surface reflection over a wide range of wavelengths and angles of incidence and to improve electrical performance. Furthermore, these hybrid electrodes are designed to improve the performance of silicon heterojunction (SHJ) solar cell front transparent contacts. These structures consist of combinations of a conventional TCO with a few graphene monolayers. The most suitable strategies for their fabrication have been assessed by testing different approaches that addresses issues such as the protection of the device structure underneath, the limitation of sample temperature during the graphene-monolayer transfer process and the determination of the most suitable stacking structure. The results reveal a strong dependence of the optoelectronic properties of the TCEs on (i) the spatial configuration of the different layers involved and and (ii) the specific TCO material used. The best results were obtained when ITO was used as the TCO and the graphene layers were transferred on top of the TCO. Finally, the effect of combining indium tin oxide (ITO) with between 1 and 3 graphene monolayers as the top electrode in the SHJ technology has been analysed. Each additional graphene monolayer is shown to improve the sheet resistance of the hybrid electrode. In the electrical characterization of the finished solar cells, this translates into a meaningful reduction of the series resistance and into an increase of the device’ fill factor. On the other hand, each additional sheet absorbs part of the incoming radiation, causing the short circuit current to simultaneously decrease. Consequently, additional graphene monolayers past the first one did not further enhance the efficiency of the reference cells. Ultimately, the increase obtained in the fill factor endorse graphene-based hybrid electrodes as a potential concept for improving solar cells’ efficiency in future novel designs.