Why Increased CdSeTe Charge Carrier Lifetimes and Radiative Efficiencies did not Result in Voltage Boost for CdTe Solar Cells?

We show that trapping impacts charge carrier dynamics in undoped CdSeTe, undoped CdSeTe/CdTe bilayers, and in As-doped CdSeTe/CdTe solar cells. Trapping has much smaller influence in CdTe-only (no Se, no As) films. Electrostatic potential fluctuation model (amplitude γ > 30 meV) applies to As-doped CdSeTe/CdTe, and defect model (defect activation energy Ea = 0.2 eV) applies to undoped CdSeTe. Unusually, undoped CdSeTe with trap states can have high radiative efficiencies and TRPL lifetimes (10-15 μs). As a result, thermodynamic CdTe solar cell analysis (radiative voltage, implied voltage, external radiative efficiency metrics) needs to be used with caution. As a metric for trap states, PL emission spectroscopy and charge carrier lifetimes at low temperatures provide distinct trap signatures. Trapping impact on devices can be evaluated in modeling and assessed in measurements outlined in this paper, perhaps providing a verifiable hypothesis when we search how to overcome voltage bottleneck in CdTe solar cells.