Interplay between Roughness and Haze of CVD Grown ZnO:B Layers for a-Si:H/μc-Si:H Solar Cells

Abstract

A broad use of ZnO:B TCO (transparent conductive oxide) layers as a front contact material for amorphous (a-Si:H) and tandem (a-Si:H/μc-Si:H) silicon thin film solar cells is achieved due to an excellent combination of good optical transparency, electrical conductivity and rough surface structure. The latter provides a light trapping effect within the underlying absorber, which is characterized by the optical haze value, i.e. a ratio between diffuse and total transmittances. The synthesis of such kind of surfaces requires the precise optimization of deposition processes, which in turn may significantly affect the further formation of silicon absorber layers and, as a result, influence the solar cells performance. Following the industrial fabrication, one has to consider both the physical properties of deposited TCO layers (resistivity, optical transparency and haze) and their processing parameters (a deposition rate, adjustable for the enterprise production tact time, and an optimum material consumption for the lowest synthesis and reactor's cleaning costs). In the present study, the growth of ZnO:B layers by CVD (chemical vapour deposition) is discussed. The film properties and their production process characteristics were optimized in order to keep the balance between the optimum parameters of TCO layers, the best chemical conversion efficiency and the fastest deposition rate at various gas flows, pressure and temperatures. The highest attention was paid to obtain ZnO:B films with controllable optical haze, which was varied between 2 and 50% (at 600 nm wavelength) by changing the layer thickness and CVD process regimes. It was found that the crystallinity of subsequently deposited a-Si:H/μc-Si:H structures correlates with the optical haze and surface roughness of ZnO:B substrate layers, which leads to the significant effect on the light trapping and band-gap values of silicon films.