Plasma-induced electronic defects: formation and recovery kinetics for advanced processing

Silicon heterojunction (SHJ) is one of the promising structures for high-efficiency crystalline silicon (c-Si) solar cells. In this type of solar cells, hydrogenated amorphous silicon (a-Si:H) is widely used as a surface passivation layer, where the c-Si surface defects are terminated with hydrogens. This surface passivation strongly depends on the growth conditions of a-Si:H and postannealing. So, a process optimization of a-Si:H deposition and postannealing is required, according to the knowledge of defect kinetics in the a-Si:H/c-Si heterostructure.

In this talk, we show the defect generation and annihilation kinetics in SHJ solar cells during solar cell fabrication [1]. The cell fabrication begins with (i) silicon wafer cleaning by RCA-based wet process, (ii) surface passivation/ junction formation by plasma-enhanced chemical vaper deposition (PECVD), (iii) transparent conductive oxide (TCO) electrode deposition by magnetron sputtering, and (iv) postannealing. The amount of defects is examined at each step of the cell fabrication, and it is evaluated with the minority carrier lifetime measured by the quasi-steady-state photoconductance (QSSPC) technique. The minority carrier lifetime is inversely proportional to the density of defects. We find that the carrier lifetime, i.e., the density of defects, is highly dependent on PECVD and sputtering process. The lifetime is significantly improved by the surface passivation with a-Si:H layer, strongly deteriorated by the TCO deposition by sputtering, and properly recovered by the postannealing. To study the details of defect kinetics, we perform in-situ photocurrent measurement during PECVD [2, 3]. The details of experimental methods and results are presented in the talk. The defect kinetics will be discussed.