Residual impurities in a process chamber on the characteristics of a-Si:H solar cells

Hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells were grown in a single, non-load-locked chamber by using 13.56 MHz plasma. Residual air impurities were controlled with high vacuum pumping, or not controlled, and the concentrations were measured at the onset, before p-layer deposition. The low pressures (approximately 10−7-10−8 Torr) of N2 and O2 near the lowest vacuum pressure of the chamber, incorporated base contamination levels of nitrogen and oxygen (approximately 1018-1019 atoms/cm3). High pressures of H2O (10−5-10−6 Torr) resulted in extremely high oxygen contamination levels (approximately 1021 atoms/cm3), which was the dominant residual impurity. High concentration un-stabilized H2O showed a rapidly decreasing rate which induced non-uniform oxygen doping and resulted in a non-uniform distribution of the internal electric field in the i-layer. The net loss (ΔQE(0,V)) of quantum efficiency (QE) of the cells between zero and a forward bias showed the increase of QE loss in the long-wavelength region for increasing forward bias resulted in poor performance of the low fill factor and energy conversion efficiency, and a high series resistance of the cells. The removal of residual water vapor from the chamber is a key factor in improving the performance of a-Si:H solar cells.