Local Electronic Structure Changes in Polycrystalline CdTe with CdCl$_2$ Treatment and Air Exposure

TL;DR

This study uses spectroscopic microscopy to map electronic structure variations in CdTe solar cells, revealing how CdCl$_2$ treatment and air exposure affect grain boundary properties and potentially improve photovoltaic efficiency.

Contribution

It provides detailed spatial mapping of electronic structure changes at grain boundaries in CdTe, clarifying effects of CdCl$_2$ treatment and oxygen exposure on electronic properties.

Findings

Grain boundaries show lower work function after air exposure of treated CdTe.

Average depletion width at grain boundaries is 290 nm.

Band bending at grain boundaries is approximately 70 meV.

Abstract

Post-deposition CdCl$_2$ treatment of polycrystalline CdTe is known to increase photovoltaic efficiency. However, the precise chemical, structural, and electronic changes that underpin this improvement are still debated. In this study, spectroscopic photoemission electron microscopy was used to spatially map the vacuum level and ionization energy of CdTe films, enabling the identification of electronic structure variations between grains and grain boundaries. In vacuo preparation and inert transfer of oxide-free CdTe surfaces isolated the separate effects of CdCl$_2$ treatment and ambient oxygen exposure. Qualitatively, grain boundaries displayed lower work function and downward band bending relative to grain interiors, but only after air exposure of CdCl$_2$-treated CdTe. Analysis of numerous space charge regions at grain boundaries (GBs) showed an average depletion width of 290 nm and an average band bending magnitude of 70 meV, corresponding to a GB trap density of 10$^{11}$ cm$^{-2}$ and a net carrier density of 10$^{15}$ cm$^{-3}$. These results suggest that both CdCl$_2$ treatment and oxygen exposure may be independently tuned to enhance CdTe photovoltaic performance by engineering the interface and bulk electronic structure.