Direct observation of grain boundary PN junction potentials in cigs using photoemission and low energy electron microscopy (PELEEM)

Spectroscopic microscopies with chemical and electronic structure information have become important tools for understanding the complex structure-property-performance relationships of high performing Cu(In_1-xGa_x)Se₂ (CIGS) photovoltaic materials and devices. Here, we describe the application of spectrally resolved photoemission and low-energy electron microscopy (spec-PELEEM) to CIGS. With the ability to map relative electric potentials with high fidelity, a large variation in the built-in pn junction potential was observed at CIGS grain boundaries. In any given 20 μm region, the built-in voltage spanned the range from depletion (~ 0.5 V) to inversion (~ 1.4 V). These grain-to-grain variations could explain the electron collection efficiency of CIGS grain boundaries and devices. These results highlight the potential of spec-PELEEM to solve critical structure-property-performance issues facing compound thin-film materials.