Band Offset Reduction at Defect-Rich p/i Interface Through a Wide Bandgap a-SiO:H Buffer Layer

Abstract

In single junction p-i-n solar cells, the optical losses can be mitigated by inserting the wide band gap amorphous silicon oxide layer at the defect-rich p/i interface. In this paper, a simulation and experimental study on the performance of p-i-n solar cells by inserting the intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) buffer layer at the p/i interface is reported. The i-a-SiO:H film has been deposited by radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) (13.6 MHz) at a low substrate temperature of approximately 230°C. The p/i interface is crucial to solar cell performance because the first few nanometers of the intrinsic layer are a defect-rich layer, having the band gap discontinuity, resulting in band offset. Thus, the carrier recombination probability increases in the vicinity of p/i interface because of high defect density and short carrier lifetime. Aided by optically calibrated simulations and with the support of experimental results, this study shows that a wide band gap thin undoped a-SiO:H buffer layer with higher photoconductivity reduces the band-gap offset and minimizes the recombination of photogenerated charge carriers at the defect-rich p/i interface. It has also been found that the a-SiO:H buffer layer augmented the electric field inside the device. As a result, the overall performance of the a-Si:H-based single junction solar cell has significantly improved. By employing a ∼5 nm thick a-SiO:H buffer layer, the blue response of the cell has been improved, resulting in 7.34% and 18.62% enhancement in fill factor (FF) and power conversion efficiency ( η ), respectively, as compared to the buffer-less cell.