  CdTe is one of the leading materials for low cost, high efficiency thin-film
solar cells, because it has a high absorption coefficient and a nearly ideal
band gap of 1.48 eV for solar cell according to the Shockley-Queisser limit.
However, its solar to electricity power conversion efficiency (PCE) is hindered
by the relatively low open circuit voltage (VOC) due to intrinsic defect
related issues. Here, we propose the strategy of improving CdTe solar cell
performance byr reducing the "ideal" band gap of CdTe to gain more
short-circuit current from long-wavelength absorption without sacrificing much
VOC. Alloying CdTe with CdSe seems to be the most appropriate approach to
reduce the band gap because of the large optical bowing and relatively small
lattice mismatch in this system, even though CdSe has larger band gap than
CdTe. Using the first principle hybrid functional calculation, we find that the
minimum band gap of the CdTe1-xSex alloy can be reduced from 1.48 eV at x=0 to
1.39 eV at x=0.32. We also show that the formation of the alloy can improve the
defect property, for example, p-type doping of CdTe by CuCd can be greatly
enhanced by the alloying effects.
