High rate deposition of microcrystalline silicon with silicon oxide doped layers: Highlighting the competing roles of both intrinsic and extrinsinc defects on the cells performances

Hydrogenated microcrystalline silicon (μc-Si:H) has become a material of increasing interest these last years mainly for its use in cost-effective production of tandem and triple junction thin film silicon based solar cells. Lately, the use of novel doped silicon oxide (SiO_x) layers were shown to be very promising for increasing the solar cells efficiency [1,2]. We present in this study a detailed analysis on the possible reasons behind this significant increase of electrical performances. Complete solar cells were developed in an industrial type reactor with their intrinsic layer (i-layer) deposited at a high growth rate of 1 nm/s by VHF-PECVD. Different i-layer material quality and substrate roughness were systematically evaluated during this investigation. We demonstrate conversion efficiency increase of up to 29% when both these p-type and n-type doped SiO_x layers are used instead of the regular microcrystalline ones, while keeping the bulk of intrinsic material unchanged and efficiencies over 8% are achieved for a wider range of plasma parameters and substrate roughness. Extensive material analysis is presented hereafter to understand the physical origins for the improvements observed. XRD, Raman and FTIR spectroscopy, intrinsic stress, FTPS and SIMS measurements were done along with SEM images of the solar cells. It is found that devices with very different efficiencies can lead to similar FTIR and FTPS spectrum. We show that the integration of doped SiO_x layers reduces to some extent the influence of porous regions, i.e. microcracks, on the electrical properties of the solar cells, and the possible physical reasons for this improvement are discussed. The development of these extrinsic defects, not detected by FTPS and FTIR, is becoming especially detrimental on highly textured substrates, required for increased light trapping. This highlights the fundamental nature difference of intrinsic and extrinsic defects which c- n both drive the cells performances.