Distinguishing bulk and surface recombination in CdTe thin films and solar cells using time-resolved terahertz and photoluminescence spectroscopies

Understanding the nature of recombination and its dependence on defects and interfaces is essential for engineering materials and contacts for a higher open-circuit voltage (V-oc) and power conversion efficiency in photovoltaic (PV) devices. Time-resolved photoluminescence (TRPL) has conventionally been used to evaluate recombination, but carrier redistribution often dominates the response at short times. Here, we report on the quantification of carrier dynamics and recombination mechanisms by complementary use of both time-resolved terahertz spectroscopy and TRPL combined with numerical modeling of the continuity equations and Poisson's equation. We have demonstrated this approach using CdTe thin films. A thin-film stack with CdTe fabricated by vapor transport deposition and treated with CdCl2 exhibited a bulk lifetime of 1.7 +/- 0.1 ns, a negligible CdTe/CdS interface recombination velocity, and a back surface recombination velocity of 6.3 +/- 1.3 x 10(4) cm/s. In contrast, a film stack without CdCl2 treatment had a bulk lifetime of only 68 +/- 12 ps and a higher interface recombination velocity of 4 +/- 2 x 10(8) cm/s. By determining the locus and mechanisms of performance-limiting recombination, we can accelerate the development of thin-film PVs with higher V-oc and efficiency. While the method has been demonstrated here using CdTe, it is also applicable to perovskites, Cu(InGa)Se-2, Cu2ZnSn(S,Se)(4), and emerging technologies.

Automated summary

Understanding the nature of recombination and its dependence on defects and interfaces is crucial for improving open-circuit voltage and power conversion efficiency in photovoltaic devices. The use of time-resolved terahertz spectroscopy and TRPL combined with numerical modeling allows for the quantification of carrier dynamics and recombination mechanisms, demonstrating the potential for developing thin-film PV technology with enhanced performance.