Probing the Correlation of Twin Boundaries and Charge Transport of CdTe Solar Cells Using Electron Backscattering Diffraction and Conductive Atomic Force Microscopy

The effect of grain boundaries (GBs) especially twin boundaries (TBs) on the performance of polycrystalline CdTe thin film (1-2 mu m thickness) solar cells remains largely controversial. Understanding the GB's microstructure and physical property and their impact on the photovoltaic efficiency is critically important to the optimization of the solar cell performance. In this work, we present a systematic study of GBs in CdTe thin film solar cells of different efficiencies in the range of 1.97-9.68%, aiming to elucidate a quantitative correlation of GBs especially TBs with the solar cell charge transport by integrating several advanced approaches including electron backscattering diffraction, transmission electron microscopy, conductive atomic force microscopy, and scanning Kelvin probe force microscopy. Importantly, we have confirmed {111} Sigma 3 TBs form in the PLD CdTe solar cells and are beneficial by providing an efficient channel for charge transport. The percentage of {111} Sigma 3 TBs was found to be correlated quantitatively with the solar cell power-conversion efficiency and a higher percentage led to a higher efficiency. In addition, a built-in field was observed between the random-angle GBs with respect to the grain interior, which may enhance exciton dissociation. This study provides a critical insight into the role of GBs in the polycrystalline CdTe thin film solar cells and is important to further improvement of their performance.