Characterization of defects in a-Si:H solar cells using sub-band gap photocurrent spectroscopy

Measurements of sub-band gap primary photocurrent spectra on p-i-n devices are used to characterize defects between 1.0 and 1.4 eV above the valence band. The effect of light soaking is to increase the absorption due to light-induced defects centered between 1.2 and 1.3 eV, consistent with the results from photothermal deflection spectroscopy. A degradation in FF (fill factor) is correlated with an increase in light-induced defects measured on the same solar cells. Although increasing the i-layer deposition temperature reduces FF, it is suggested that this reduction is due to p/i interface degradation because bulk i-layer defects decrease with the deposition temperature. The measurement is applicable to any a-Si:H or a-SiGe:H p-i-n solar cell and provides information about fundamental i-layer properties (defect density and Urbach energy) which can be related to the device performance.