Luminescence methodology to determine grain-boundary, grain-interior, and surface recombination in thin-film solar cells

Abstract

We determine the grain-boundary (GB) recombination velocity, SGB, and grain-interior (GI) lifetime, τGI, 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 τGI, SGB, grain size, and the surface recombination velocity, Ssurf. TRPL measurements provide an estimate of Ssurf 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 SGB and τGI values for the samples: SGB(τGI)= 2.6 × 106 cm/s (68–250 ps), SGB(τGI)= 4.1 × 105 cm/s (1.5–3.3 ns), and SGB(τGI)= 5.5 × 105 cm/s (1.0–3.8 ns) for as-deposited, CdCl2-treated, and CdCl2- and 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 SGB and τGI 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(In1-xGax)Se2, Cu2ZnSnS4, and perovskites.