Investigation of the Microstructure of Underdense Hydrogenated Amorphous Silicon Layers for Silicon Heterojunction Solar Cells by Raman Spectroscopy and Hydrogen Effusion

Benedikt Fischer, Maurice Nuys, Andreas Lambertz, Weiyuan Duan, Kaining Ding, Uwe Rau Forschungszentrum Jülich GmbH, Jülich, Germany

The application of thin underdense hydrogenated amorphous silicon (a-Si:H) films for passivation of crystalline Si (c-Si) and for avoiding epitaxy in silicon heterojunction (SHJ) solar cell technology has recently been proposed and successfully applied. Here, we investigate the microstructure of such underdense a-Si:H films, as used in Jülich solar cell technology. From the refractive index and hydrogen content, the density of our films is estimated to 2.1-2.2 g/cm3. In H effusion experiments (besides effusion at higher temperature) a low temperature H effusion peak near 400 °C shows up which has been attributed to the diffusion of molecular H2 through a void network. The dependence of the H effusion peaks on film thickness is similar as observed previously for low substrate temperature a-Si:H material. By applying different plasma power (0.08 W/cm2 – 0.16 W/cm2) the Si-H microstructure parameter measured by Raman decreases from about 0.5 to 0.25 for about 20 nm film thickness. The nucleation zone is estimated from the Raman results to < 10 nm. The substrate type (HF etched c-Si, c-Si with native oxide, glass) shows no influence on the Raman microstructure parameter. The fact that with such material good passivation of c-Si solar cells was achieved suggests that in the c-Si passivation process molecular hydrogen plays an important role. The second layer of dense a-Si:H as usually required for good c-Si passivation may be primarily a H2 out-diffusion barrier.