The temperature and incident light intensity dependences of open circuit voltage for hetero- and homojunction a-Si:H pin solar cells have been studied. The temperature coefficient of the open circuit voltage decreases from minus 2. 40 to minus 2. 67 mV/ degree C as the illumination intensity is decreased from 150 to 7 mW/cm**2. This is consistent with the empirically calculated open circuit voltages. The diode quality factor obtained under dark condition shows high values (1. 6-3. 0) and strong temperature dependence. On the other hand, the diode quality factor under illumination is less than 1. 5 and shows slight temperature dependence. The results appear to be due to the long dielectric relaxation time compared with the carrier lifetime in the i-layer.