Modeling Cu migration in CdTe solar cells under device-processing and long-term stability conditions

Abstract

Subscript p denotes the local matrix (CdTe, CdS, etc.) in which Cu is diffusing, and the number of sites per plane is n, . The Cu and matrix mole fractions in plane i are x6u and x~, respectively. For simplicity, the ideal-solution model is used for Jl6u and Jl~. Now a chemical potential, Jlbu-p' is defined for plane i that accounts for both species, subject to the complementarity condition X6u + X~= 1. Differentiating Eq. (1) with respect to the amount of Cu in plane i leads toABSTRACT An impurity migration model for systems with material interfaces is applied to Cu migration in CdTe solar cells. In the model, diffusion fluxes are calculated from the Cu chemical potential gradient. Inputs to the model include Cu diffusivities, solubilities, and segregation enthalpies in CdTe, CdS and contact materials. The model yields transient and equilibrium Cu distributions in CdTe devices during device processing and under field-deployed conditions. Preliminary results for Cu migration in CdTe photovoltaic devices using available diffusivity and solubility data from the literature show that Cu segregates in the CdS, a phenomenon that is commonly observed in devices after back-contact processing and/or stress conditions. A = Llhxbu~bu +hx~~~j. ;(1) Distance (lattice constants) Figure 1. Schematic illustration of the ideal-solution Cu migration potential near a hypothetical material interface.; 8 A ;xbu Jlcu-p = r;: 8 1 v ; = i1H cu_ p + kT I n ;(2) ,~cu 1-X Cu(3) 10 5 o -5 -10

A comprehensive picture of Cu doping in CdTe solar cells

“…22 The possibility of such a metastable defect concentration in excess of solubility has also been shown using a numerical Cu migration model by Teeter and Asher. 17 Recent calculations by Krasikov et al 7 and the experimental work of Jones et al 21 200 C. Hence at 200 C, Cu Cd can diffuse lm distances within minutes inside the CdTe bulk. At room temperature, Cu Cd diffusion over lm distances is slower and takes hours.…”

Controlled activation of ZnTe:Cu contacted CdTe solar cells using rapid thermal processing

“…First, it is expected that high temperature-short time processes should be selective to Cu activation over diffusion based on energetics. The diffusion process is weakly activated, with reported activation energies of 0.3-0.7 eV [16,17,24]. In contrast, the enthalpies of formation for copper doping states range from 1.5-2.5 eV [25].…”

CdTe Photovoltaics in Superstrate Geometry–Back-Contact Materials: A Review

“…Laser-beam-induced current analysis found that Cu tends to migrate deep into the CdTe main body when stress is applied to the device. , Capacitance–voltage analysis of the device with intentionally included Cu indicates that the interdiffusion of Cu increases the carrier density in the metal–semiconductor region. In other words, the shallow defects introduced by Cu increased the carrier density.…”