In situ ultraviolet treatment in an Ar ambient upon p-type hydrogenated amorphous silicon–carbide windows of hydrogenated amorphous silicon based solar cells

Abstract

We proposed an in situ postdeposition ultraviolet treatment in an Ar ambient ͑UTA͒ to improve the p / i interface of amorphous silicon based solar cell. We have increased the conversion efficiency by ϳ16% by improving the built-in potential and reducing recombination at the p / i interface. Through spectroscopic ellipsometry and Fourier-transform infrared measurements, it is concluded that the UTA process induces structural modification of the p-type hydrogenated amorphous silicon-carbide ͑p-a-SiC: H͒ window layer. An ultrathin p-a-SiC: H contamination layer formed during the UTA process acts as a buffer layer at the interface. Hydrogenated p-type amorphous silicon-carbide ͑p-a -SiC: H͒ films have been widely used as window materials for amorphous silicon ͑a-Si: H͒ based solar cells due to their wide optical band gap. However, an abrupt heterojunction between a p-a-SiC: H window layer and an intrinsic hydrogenated amorphous silicon ͑i-a-Si: H͒ absorber layer generates a considerable recombination loss of photogenerated carriers due to a highly defective interface with a short carrier lifetime. In addition, the low electrical conductivity of p-a-SiC: H severely limits the overall cell performance.Various buffer layers have been developed to solve the p-a-SiC: H / i-a-Si: H interface problem. [1][2][3] The insertion of these buffer layers at the p / i interface remarkably improves the conversion efficiency by forming a strong electric field in the absorber layer and diminishing electron backdiffusion to p-a-SiC: H. However, the additional buffer design is quite complex due to its various deposition parameters. Alternatively, the cell efficiency has been considerably improved by modifying the p / i interface through H 2 treatments. 4,5 However, since hydrogen plasma and photo-assisted hydrogen radicals significantly etch the surface of p-a-SiC: H, the window layer thickness and treatment time must be carefully controlled.In this letter, an in situ postdeposition ultraviolet-UV treatment in an Ar ambient ͑UTA͒ in a photoassisted chemical vapor deposition (photo-CVD) system has been proposed in order to easily overcome the recombination loss at the p / i interface without affecting the p-a-SiC: H layer thickness. Argon has been known to be a useful dilution gas for enhancing the crystallinity of p-type hydrogenated microcrystalline silicon ͑p-c-Si: H͒ 6 or the stability of i-a-Si: H. 7 In addition, Branz et al. reported improved stability of a-Si: H photosensitivity by UV illumination due to mobile hydrogen atom ͑H͒ migration in the bulk. 8 Using a photo-CVD system, we fabricated pin-type solar cells at 250°C with a reference structure (reference cell) of glass/ SnO 2 :F/ p-a-SiC: H ͑17.5 nm͒ / i-a-Si: H ͑600 nm/ n-c-Si: H͒͑40 nm͒/Al, and an area of 0.092 cm 2 .p-a-SiC: H films were grown by direct decomposition of a mixture of Si 2 H 6 , B 2 H 6 and C 2 H 4 reactant gases. To dissociate the mixture, a low-pressure mercury ͑Hg͒ lamp with resonance lines of 184.9 and 25.7 nm was used as an UV light source. Prior to th...

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