Abstract
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 Rsh corresponding to less traps, Rsh shows a upward tendency as the 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, Rs shows a slight increase when the irradiance intensity goes down because of the resistance of CdTe absorption layer. For cell #2 with lower Rsh corresponding to more traps, with the decrease of the illumination intensity, Rsh increases sharply only for captured carrier reduction, Rs goes steadily up similarly, n and J0 exhibit a decline with the irradiance due to recombination shift. It should be pointed out that Rs varies much smoother than the traditional approximation of a reciprocal of differential at short circuit, and the distribution of Rsh is diverse, and an average Rsh of for each intensity range can reflect the variation trend.
Highlights
CdTe has established itself as the most promising material for thin film solar cell application on the market due to its direct bandgap with the value of about 1.45 eV close to the maximum of the solar spectrum the absorption coefficient of about 104 –105 cm−1 in the visual part of the solar spectrum.Recent state-of-the-art CdTe solar cell has been fabricated in the lab with the efficiency surpassing that of polycrystalline Si at over 22% [1,2]
The intensity is normally lower than 100 mW/cm2, as confirmed by the data of the National renewable energy lab (NREL) [4] and the short circuit current density Jsc, the open voltage Voc, the fill factor FF, and the conversion efficiency η depend on the irradiance, it is essential to investigate the relationship between the diode parameters and the intensity
As J0, n, Rs and assumed to be constant, while we focus on parameter variations of a CdTe solar cell
Summary
CdTe has established itself as the most promising material for thin film solar cell application on the market due to its direct bandgap with the value of about 1.45 eV close to the maximum of the solar spectrum the absorption coefficient of about 104 –105 cm−1 in the visual part of the solar spectrum. Materials 2020, 13, 2194 the comparisons of various solar cells [5,6], but the thorough analysis of the mechanism was missing, Shen et al discussed the performance deviation with light irradiance in detail [7], and the ideality factor n and the reverse saturation current density J0 were assumed to be constant for simplicity, the series and shunt resistances were obtained by the reciprocal of differential at points of open and short circuits in the J–V curve, which are apparently not practical. For CdTe solar cells, the parameter variation trends of n, J0 , Rs and Rsh with illumination intensities are discussed, and the corresponding underlying mechanisms of carrier transportation and recombination are analyzed. Traps and the body resistance of a CdTe solar cell
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