Electronic states of intrinsic layers in n-i-p solar cells near amorphous to microcrystalline silicon transition studied by photoluminescence spectroscopy

Thin film n-i-p solar cells were prepared using decomposition of disilane-hydrogen mixtures by plasma-enhanced chemical vapor deposition. By increasing either the H dilution ratio or the thickness, the i-layer structure showed a transition from amorphous to microcrystalline silicon characterized by x-ray diffraction. The electronic states of the i layer were examined by photoluminescence (PL) spectroscopy, which showed that: (a) below the onset of microcrystallinity, a blueshift of the 1.4 eV PL peak energy along with a decrease of the band width occur as the structural order is improved; (b) above the onset of microcrystallinity, the PL efficiency decreases by a factor of 4–5 and the PL peak energy is redshifted toward 1.2 eV as the μc-Si volume fraction is increased. In addition, the solar cell open circuit voltage shows first an increase and then a decrease, correlating with the PL peak energy position. We conclude that the PL spectroscopy is a sensitive tool for characterizing the gradual amorphous-to-microcrystalline structural transition in thin film solar cells.