Characterization of stable protocrystallin silicon and its application to light-absorbing layers of amorphous silicon-based solar cells

Abstract

In this thesis, the degradation and the thermal annealing behaviors of amorphous silicon (a-Si:H)-based solar cells incorporating stable $H_2-diluted$ i-a-Si:H films as well as the film characteristics and light-soaking behaviors of $H_2-diluted$ i-a-Si:H deposited at several hydrogen dilution ratios $(R = [H_2]/[SiH_4])$, were investigated. The stability of the protocrystalline silicon (pc-Si:H) and the newly observed instability of the silicon films deposited at the onset of the microcrystalline regime were reported, and a reason for the instability was suggested. Firstly, the characteristics and the light-soaking behaviors of $H_2-diluted$ a-Si:H films prepared at several R values, were investigated. From the investigation, just before the onset of the microcrystalline regime, we found a moderate R = 1.5 where the very stable pc-Si:H was formed. However, for the films deposited at the onset of the microcrystalline regime (R = 2, 2.5), the films were less stable compared with the pc-Si:H (R = 1.5) in spite of the 3 facts: the microcrystallized films have i) more blue-shifted amorphous TO phonon bands in Raman spectrum; ii) microcrystallites; and iii) lower broadening parameters (C), which were suggested as the empirical evidences for the stability of intermediate material between a-Si:H and μc-Si:H. In contrast with these 3 facts, the amplitude parameter A was strongly correlated with the stability of all the H2-diluted silicon films. The film with the highest A, considered as the most dense film, was deposited at R = 1.5, namely the protocrystalline condition, and it exhibited the highest stability. At the onset of the microcrystalline regime where the parameter A decreased, the clustered Si-H bonds were observed. The fact that such bonds enhanced the light-induced degradation supported the instability of the onset regime and the correlation between the parameter A and the stability of the films. Also, the light-soaking behaviors and the thermal annealing k...