Emitter Choice for Epitaxial CdTe Solar Cells

High-quality epitaxial CdTe layers with low defect density and high carrier concentration have been demonstrated by several research groups. Nevertheless, one primary challenge for high-performance epitaxial CdTe solar cells is how to choose a suitable emitter partner for the junction formation. The numerical simulations show that a type I heterojunction with small conduction band offset (0.1 eV ≤ ΔEc ≤ 0.3 eV) is necessary to maintain a good cell efficiency even with large interface recombination. Otherwise, a small 'cliff' can assist interface recombination causing smaller Voc, and a large 'spike' (ΔEc ≥ 0.4 eV) can impede the photo current and lead to a reduction of J_SC and FF. Among the three possible emitters, CdS, CdMgTe, and MgZnO, CdMgTe (with ∼30% Mg) and MgZnO (with ∼ 20% Mg) are likely to be a better choice since their type-I junction can tolerate a larger density of interface defects.