The Microstructure of Underdense Hydrogenated Amorphous Silicon and its Application to Silicon Heterojunction Solar Cells

Abstract

The application of thin underdense hydrogenated amorphous silicon (a-Si:H) films for passivation of crystalline Si (c-Si) by avoiding epitaxy in silicon heterojunction (SHJ) solar cell technology has recently been proposed and successfully applied. Herein, the microstructure of such underdense a-Si:H films, as used in our silicon heterojunction solar cell baseline, is investigated mainly by Raman spectroscopy, effusion, and secondary ion mass spectrometry. In H effusion experiments, a low-temperature (near 400 °C) effusion peak which has been attributed to the diffusion of molecular H2 through a void network is seen. The dependence of the H effusion peaks on film thickness is similar as observed previously for void rich, low substrate-temperature a-Si:H material. Solar cells using underdense a-Si:H as i1-layer with a maximum efficiency of 24.1% are produced. The passivation quality of the solar cells saturates with increasing i1-layer thickness. The fact that with such underdense material combined with a following high-quality i2-layer, instead of only high-quality a-Si:H with a low defect density direct on the c-Si substrate, good passivation of c-Si solar cells is achieved, which demonstrates that in the passivation process, molecular hydrogen plays an important role.