Luminescence Methodology to Determine Grain-Boundary, Grain-Interior, and Surface Recombination in Thin-Film Solar Cells

We determine the grain-boundary (GB) recombination velocity, S G B, and grain-interior (GI) lifetime, τ G I, parameters in superstrate CdS/CdTe thin-film solar cell technology by combining cathodoluminescence (CL) spectrum imaging and time-resolved photoluminescence (TRPL) measurements. We consider critical device formation stages, including after CdTe deposition, CdCl₂ treatment, and Cu diffusion. CL image analysis methods extract GB and GI intensities and grain size for hundreds of grains per sample. Concurrently, a three-dimensional CL model is developed to simulate the GI intensity as a function of τ G I, S G B, grain size, and the surface recombination velocity, S surf. TRPL measurements provide an estimate of S surf for the CL model. A fit of GI intensity vs. grain size data with the CL model gives a self-consistent and representative set of S G B and τ G I values for the samples: S G B (τ G I) = 2.6 × 10⁶ cm/s (68-250 ps), S G B (τ G I) = 4.1 × 10⁵ cm/s (1.5-3.3 ns), and S G B (τ G I) = 5.5 × 10⁵ cm/s (1.0-3.8 ns) for as-deposited, CdCl₂-treated, and CdCl₂- A nd Cu-treated samples, respectively. Thus, we find that the CdCl₂ treatment both helps to passivate GBs and significantly increase the GI lifetime. Subsequent Cu diffusion increases GB recombination slightly and has nuanced effects on the GI lifetime. Finally, as a partial check on the S G B and τ G I values, they are input to a Sentaurus device model, and the simulated performance is compared to the measured performance. The methodology developed here can be applied broadly to CdTe and CdSeTe thin-film technology and to other thin-film solar cell materials including Cu(In_1-xGa_x)Se₂, Cu₂ZnSnS₄, and perovskites.