MONITORING OF PHOTODEGRADATION AND RECOVERY OF A-SI-H P-I-N SOLAR-CELLS BY CAPACITANCE MEASUREMENTS

In this paper we report an application of forward bias, low frequency capacitance measurements in order to relate the electrical properties of the intrinsic material to the solar cell performance after photodeg radation and thermal recovery. The capacitance vs. frequency technique (C-v-f) is based on the measurement of the variation of the total cha rge trapped in the gap defect states of the intrinsic layer during a l ow frequency voltage perturbation. As expected, after illumination the efficiencies of the tested devices decreased. These variations were f ound to be only weakly related to the initial electrical performance, but strongly dependent on the initial density of states of the intrins ic layers. Efficiency degradation of the investigated solar cells has been quantitatively related to a reduced mobility-lifetime product and , lesser, to a variation of the density of defects. The defects detect ed by the C-v-f technique are positioned around 0.55 eV from the band- edge and do not act as recombination centers but, rather, as free carr ier traps. Thus, due to their energy position they do not interfere wi th steady-state current-voltage (I-V) measurements and with recombinat ion mechanisms. Employing either the I-V or the C-v-f technique, a mor e complete picture of the defect distribution in the gap of a-Si : H h as been drawn and the defects behaviour, after light soaking and annea ling, has been investigated.