Exploring the Potential for High-Quality Epitaxial CdTe Solar Cells

Traditional polycrystalline CdTe solar cell performance is limited by recombination at the grain boundaries, low carrier density (p), compensation from impurities, and a low minority carrier lifetime (τ). The maximum values for these critical parameters in polycrystalling devices are p < 10¹⁵ cm^-3 and τ ∼ 10 ns with open-circuit voltage (V_OC) ∼ 900 mV and η ∼ 20%. Epitaxial CdTe with high-quality, low defect-density, and high carrier density, could yield a higher-efficiency PV device. Using numerical simulation, we investigate the combined effects of minority carrier lifetime τ (0.1 - 500 ns) and carrier density p (1×10¹⁴ - 5×10¹⁸ cm^-3) on device performance, predicting obtainable performance of V_OC > 1100 mV and η > 25% for high τ and high p. While the V_OC is strongly affected by both p and τ, the short-circuit current (J_SC) is mainly dependent on the lifetime τ and absorption losses in the front contact stack. In addition, increasing the thickness of p-CdTe (varied from 0.5 - 20 μm) at different τ (1 - 100 ns) shows an improvement in J_SC due to increased long-wavelength photon collection and then saturates for thicker p-CdTe. In some cases, the cell performance is compromised by the presence of a significant back-contact barrier Φ_b. The simulated results show that the cell performance is not strongly affected until Φ_b exceeds 0.4 eV.