Polycrytalline, thin-film Cu2S/Cd1−xZnxS heterojunction solar cells with conversion efficiencies of 10% have been prepared on Cd1−xZnxS with 0.1⩽x⩽0.2. Light-generated currents of up to 26 mA/cm2 (prorated to 100 mW/cm2) have been achieved, comparable to the best observed in Cu2S/CdS cells of the same design. The improved performance for Cu2S-based devices is as a consequence of the higher open-circuit voltage achieved with the addition of zinc.
Analysis of high efficiency, thin film, small grain, polycrystalline, heterojunction CdTe and CuInSe2 based solar cells can help explain the high quantum efficiencies and the resulting short circuit current (Jsc) as well as the forward diode current that controls the open circuit voltage (Voc). This analysis shows that minority carrier recombination at the metallurgical interface and at grain boundaries is greatly reduced by the proper “doping” of the window and absorber layers thereby increasing Jsc. Additional analysis and measurements show that the Voc in present state of the art solar cells is controlled by the magnitude of the forward diode current which appears to be caused by recombination in the space charge region of the absorber layer. This also shows that any quantitative modeling of these devices which relates the device performance to the bulk electronic properties of the material should consider the additional geometric dimension introduced by the polycrystallinity because of grain boundary effects.
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