Evidence of Enhanced Carrier Collection in Cu(In,Ga)Se2 Grain Boundaries: Correlation with Microstructure

Solar cells containing a polycrystalline Cu-(In,Ga)Se-2 absorber outperform the ones containing a monocrystalline absorber, showing a record efficiency of 22.9%. However, the grain boundaries (GBs) are very often considered to be partly responsible for the enhanced recombination activity in the cell and thus cannot explain the registered record efficiency. Therefore, in the present work, we resolve this conundrum by performing correlative electron beam -induced current electron backscatter diffraction investigations on more than 700 grain boundaries and demonstrating that 58% of the grain boundaries exhibit an enhanced carrier collection compared to the grain interior. Enhanced carrier collection thus indicates that GBs are beneficial for the device performance. Moreover, 27% of the grain boundaries are neutral and 15% are recombination-active. Correlation with microstructure shows that most of the Sigma 3 GBs are neutral, whereas the random high-angle grain boundaries are either beneficial or detrimental. Enhanced carrier collection observed for a big fraction of high-angle grain boundaries supports the "type inversion" model and hence the downward band bending at GBs. The decrease in current collection observed at one of the high angle grain boundaries is explained by Cu being enriched at this GB and hence by the upward shift of the valence band maximum.