Influences of Low Intensity on Diode Parameters of CdTe Solar Cells

This study deals with the CdS/CdTe solar cells under low illumination intensity, with cell #1 for the shunt resistance exceeding 100,000 Ω·cm² and cell #2 for the shunt resistance above 1000 Ω·cm². The diode parameter variations with the decline of the irradiance intensity are illustrated by dividing 0-100 mW/cm^-2 into a number of small intensity ranges for J-V measurements and assuming the diode parameters to be constant within each range, the diode parameters of each range including the series resistance, the shunt resistance, the reverse saturation current density and the ideality factor are then extracted by employing an analytical approach. The mechanism of the cell performance deviations are also investigated by basic theories, reports and experiments. For cell #1 with higher R_sh corresponding to less traps, R_sh shows a upward tendency asthe irradiance declines, n and J₀ exhibit a rise with the irradiance and keep nearly unchanged at the low irradiance values mainly due to recombination and carrier contributions, R_s shows a slight increase when the irradiance intensity goes down because of the resistance of CdTe absorption layer. For cell #2 with lower R_sh corresponding to more traps, with the decrease of the illumination intensity, R_sh increases sharply only for captured carrier reduction, R_s goes steadily up similarly, n and J₀ exhibit a decline with the irradiance due to recombination shift. It should be pointed out that R_s varies much smoother than the traditional approximation of a reciprocal of differential at short circuit, and the distribution of R_sh is diverse, and an average R_sh of for each intensity range can reflect the variation trend.