Design and implementation of an ultrathin dielectric/metal/dielectric transparent electrode for Cu2ZnSnS4 thin-film photovoltaics

Abstract

A proof-of-concept for implementing ultrathin dielectric/metal/dielectric layers of AZO/Ag/AZO (AZO: Al-doped ZnO) as a transparent electrode in the earth-abundant, kesterite-type Cu2ZnSnS4 (CZTS) photovoltaic device, is presented. In a first step, a set of optical simulations for designing the layer sequence and the optimum thickness of AZO/Ag/AZO were performed, resulting in a high CZTS absorption within a device configuration of glass/Mo/MoS2/CZTS/CdS/ZnO/AZO/Ag/AZO. The simulation suggests that a high CZTS absorption in a device can be achieved using an Ag layer with a thickness of 7 nm embedded between a first and second AZO layer, having thicknesses of 50 nm and 40 nm, respectively. The AZO/Ag/AZO films were sputtered on a glass substrate with layer thicknesses based on the optical simulation, showing transmittance of >80% (including the substrate), sheet resistance of 10.5 Ω/sq and figure-of-merit of 29.6 × 10−3 Ω−1. These ultrathin transparent electrode properties were obtained without any substrate heating. The implementation of AZO/Ag/AZO as the transparent electrode in a CZTS photovoltaic device yielded a device efficiency of 8.2% with a short-circuit current density of 24.2 mA/cm2, an open-circuit voltage of 576 mV and a fill factor of 58%. These results suggest an opportunity to reduce the nominal thickness of the front electrode material in CZTS photovoltaics.