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.