Passivation of oxygen-related donors in microcrystalline silicon by low temperature deposition

Low-temperature processing for high-performance solar cells based on hydrogenated microcrystalline silicon (muc-Si:H) has been developed using a conventional rf plasma-enhanced chemical vapor deposition (PECVD) technique at an excitation frequency of 13.56 MHz under a high deposition pressure condition. Among pin type solar cells, it is found that deposition temperature of i-layer at 140 degreesC is effective particularly for improving open circuit voltage (Voc), surprisingly without deteriorating short circuit current or fill factor. Carrier density of undoped muc-Si abruptly decreases for deposition temperatures lower than 180 degreesC, and the improvement of Voc is ascribed to a decrease of shunt leakage current arising from the oxygen-related donors. This implies that oxygen-related donors can be passivated at low deposition temperatures and that hydrogen plays an important role for the passivation. We propose a simple model for the hydrogen passivation of oxygen related donors. We apply this passivation technique to solar cells, and consequently a conversion efficiency of 8.9% (Voc = 0.51 V, Jsc = 25 mA/cm(-1). FF=0.70) has been obtained in spite of an oxygen concentration of 2 x 10(19) cm(-3) in combination with device optimization such as a p-layer. Effect of deposition temperature of i-layer upon other solar cell parameter, short circuit current. and fill factor is also discussed. (C) 2001 American Institute of Physics.