Transparent conductive oxide film with antireflective properties for Cu<sub>2</sub>ZnSnS<sub>4</sub> solar cells

Abstract

At present, there are several kinds of broadband antireflection coatings (ARCs). For the flat multilayer ARC, it usually contains double, triple, or up to 4 layers. It has been demonstrated that the performance of a single layer coating is not good enough across the desired spectral range. Multiple layer ARCs have much better performance for broadband solar cells (SCs). When inspecting the antireflection structure of Cu<sub>2</sub>ZnSnS<sub>4</sub> solar cells (CZTSSCs), it is shown that the transparent conductive oxide (TCO) of traditional CZTSSCs does not have an satisfactory antireflective performance. This paper aims to investigate a way to increase the incident light transmitted into CZTSSCs, and thus improving the efficiency of solar cells by studying the use of the antireflective effect of a TCO film. It introduces a new type of TCO film with better antireflective properties across a wide wavelength range. An SiO<sub>2</sub>/ZnO antireflective TCO (ATCO) is designed under AM1.5 illumination. In order to measure the antireflective effect over the 300–800 nm wavelength range, an effective average reflectance method (EAR) is introduced. Considering the effect of the refractive index dispersion and the coupling of the TCO or ATCO films with the active layer, in this paper we use a multi-dimensional transfer matrix to optimize the thickness of each key layer to accurately confirm the best antireflective effect. In addition, the optimized TCO film and the optimized ATCO film in CZTSSCs are compared and analyzed by means of EAR. The result shows, through the comparison of the antireflection between conventional TCO CZTSSCs and ATCO CZTSSCs, that there are considerable differences in final optimal reflectivity between TCO layer and ATCO film. For the conventional CZTSSC, the optimal effective average reflectance of TCO layer is 5.6%, and the lowest reflectivity in the waveband from 400 nm to 500 nm is 6.9%. In addition, the corresponding values obtained in the new ATCO CZTSSC are 3.8% and 1.6% respectively. These apparent changes in reflectivity are appealing in that the new ATCO films can effectively reduce light loss and improve the efficiency of photovoltaic conversion.