Diffusion of Rb in polycrystalline Cu(In,Ga)Se2 layers and effect of Rb on solar cell parameters of Cu(In,Ga)Se2 thin-film solar cells

Abstract

The diffusion of the heavy alkali element rubidium (Rb) in Cu(In,Ga)Se2 (CIGS) layers was investigated over a temperature range from 148 °C to 311 °C by outdiffusion from a rubidium fluoride layer. The diffusion profiles were measured by secondary ion mass spectrometry. By using CIGS layers with different grain sizes, diffusion along grain boundaries could be distinguished from diffusion into the grain interior. Rb was found to diffuse from the CIGS surface along grain boundaries but also within the grain bulk. Based on these data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation DV (Rb) = 3.8·10−8 exp(−0.44 eV/kBT) cm2 s−1 and the fast diffusion along the grain boundaries by DGB (Rb) = 5.7·10−9 exp(−0.29 eV/kBT) cm2 s−1. Further, the effect of Na on Rb diffusion was investigated by comparing Rb diffusion into a Na-containing CIGS layer in contrast to Rb diffusion into an alkali-free CIGS layer. This comparison revealed some aspects of the ion exchange mechanism. Finally, the effect of Rb on the solar cell parameters of CIGS thin-film solar cells was investigated. Rb was found to enhance the open-circuit voltage, the fill factor, and charge carrier density in a similar manner as observed for potassium and sodium.

Heavy Alkali Treatment of Cu(In,Ga)Se2Solar Cells: Surface versus Bulk Effects

“…At the grain boundary, Na is the majority alkali with a Gibbsian interfacial excess (Γ) of 1.5 at nm −2 , compared to 0.5 at nm −2 for K. In the RbF‐treated sample Rb and K are below the detection limit of APT inside the grain whereas Na significantly increases to 39 ppm. In another study, Rb has been found at a very low concentration also inside the grains by time‐of‐flight secondary ion mass spectroscopy . At the grain boundary, even if all alkali elements are found to segregate, Rb is now the majority alkali at the grain boundary with Γ = 1.6 at nm −2 .…”

“…In another study, Rb has been found at a very low concentration also inside the grains by time-of-flight secondary ion mass spectroscopy. [77] At the grain boundary, even if all alkali elements are found to segregate, Rb is now the majority alkali at the grain boundary with Γ = 1.6 at nm −2 . Na and K show an equivalent segregation with Γ = 0.4 at nm −2 .…”

Alkali Atoms Diffusion Mechanism in CuInSe2 Explained by Kinetic Monte Carlo Simulations

“…It was shown by ab initio calculations that Cu atoms are the most mobile atoms in CISe, whereas In atoms play no role in the mass transport. [41] Experimental findings indicated that heavier AM atoms tend to segregate next to grain boundaries. The vacancymediated diffusion of Na has a larger migration barrier than those of K and Rb, whereas interstitial Na (Na i ) atoms diffuse faster than heavier AM in the interstitial site.…”

“…[40] On the other hand, the concentration of K and Rb atoms in the GI is low despite the small migration barriers for the diffusion of the substitutional defects in the absorber layer. [41] Experimental findings indicated that heavier AM atoms tend to segregate next to grain boundaries. [40,42,43] Several studies characterized the positive influences of AM atoms on the cell performance but the underlying reasons are still puzzling.…”

“…In addition, the presence of Na atoms in the grain interior (GI) indicates the high mobility of Na atoms (Na interstitial diffusion) . On the other hand, the concentration of K and Rb atoms in the GI is low despite the small migration barriers for the diffusion of the substitutional defects in the absorber layer . Experimental findings indicated that heavier AM atoms tend to segregate next to grain boundaries …”