Efficiency of Thin-Film CdS/CdTe Solar Cells (2010)

The findings of this paper give further insight into the problems and ascertain some requirements imposed on the CdTe absorber layer parameters in a CdTe/CdS solar cell, which in our opinion could be taken into account in the technology of fabrication of solar cells. The model taking into account the drift and diffusion photocurrent components with regard to recombination losses in the space-charge region, at the CdS-CdTe interface and the back surface of the CdTe layer allows us to obtain a good agreement with the measured quantum efficiency spectra by varying the uncompensated impurity concentration, carrier lifetime and surface recombination velocity. Calculations of short-circuit current using the obtained efficiency spectra show that the losses caused by recombination at the CdTe-CdS interface are insignificant if the uncompensated acceptor concentration Na ­ Nd in CdTe is in excess of 1016 cm­3. At Na ­ Nd 1016 cm­3 and the thickness of the absorbing CdTe layer equal to around 5 µm, the short-circuit current density of 25-26 mA/cm2 can be attained. As soon as Na ­ Nd deviates downward or upward from this value, the short-circuit current density decreases significantly due to recombination losses or reduction of the photocurrent diffusion component, respectively. Under this condition, recombination losses in the spacecharge region can be also neglected, but only when the carrier lifetime is equal or greater than 10­10 s. At Na ­ Nd 1016 cm­3, when only a part of charge carriers is generated in the neutral part of the p-CdTe layer, total charge collection can be achieved if the electron lifetime is equal to several microseconds. In this case the CdTe layer thickness d should be greater than that usually used in the fabrication of CdTe/CdS solar cells (2-10 m). However, in a common case where the minority-carrier (electron) lifetime in the absorbing CdTe layer amounts to 10­10­10­9 s, the optimum layers thickness d is equal to 3­4 m, i.e., the calculations support the choice of d made by the manufacturers mainly on an empirical basis. Attempts to reduce the thickness of the CdTe layer to 1­1.5 m with the aim of material saving appear to be unwarranted, since this leads to a considerable reduction of the short-circuit's current density Jsc and, ultimately, to a decrease in the solar-cell efficiency. If it will be possible to improve the quality of the absorbing layer and, thus, to raise the electron lifetime at least to 10­8 s, the value of Jsc can be increased by 1­1.5 mA/cm2. The Sah-Noyce-Shockley theory of generation-recombination in the space-charge region supplemented with over-barrier diffusion flow of electrons provides a quantitative explanation for all variety of the observed I-V characteristics of thin-film CdS/CdTe heterostructure. The open circuit voltage Voc significantly increases with decreasing the resistivity of the CdTe layer and increasing the effective carrier lifetime in the space charge region. At = 10­10-10­9 s, the value of Voc is considerably lower than its maximum possible value for 0.1 cm and > 10­8 s and the calculated efficiency of a CdS/CdTe solar cell with a CdTe layer thickness of 5 m lies in the range 17-19%. An increase in the efficiency and an approaching its theoretical limit (28-30%) is possible in the case when the electron lifetime n 10­6 s and the thickness of CdTe absorber layer is 20-30 m or more. The question of whether an increase in the CdTe layer's thickness is reasonable under the conditions of mass production of solar modules can be answered after an analysis of economic factors.