Impact of the Band Offset for n-Zn(O,S)/p-Cu(In,Ga)Se₂ Solar Cells

The conduction-band offset (CBO) of the Zn(O, S)/Cu(In,Ga)Se <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">₂ </tex></formula> heterojunction can play a significant role in the performance of solar cells. The individual electron affinities and bandgaps are controlled by the oxygen-to-sulfur and gallium-to-indium ratios, and the resulting offsets can range from +1.3 eV in the “spike” direction to −0.7 eV in the “cliff” direction if the full range of the two ratios is considered. The optimal CBO of near +0.3 eV can be achieved with various combinations of the two ratios. The traditional CdS emitter is near optimal for the commonly used 1.15-eV Cu(In,Ga)Se <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">₂</tex></formula> (CIGS) but less optimal for higher Ga. The flexibility with Zn(O,S) emitters ranging from above 90% oxygen for CIS down to 50% oxygen for CGS allows an optimal CBO over the full gallium range. Assuming that other factors remain constant, the optimal offset should also be able to reduce the loss in cell efficiency between room temperature and temperatures more typical of field conditions by about 1% absolute.