Rear‐Passivated Ultrathin Cu(In,Ga)Se 2 Films by Al 2 O 3 Nanostructures Using Glancing Angle Deposition Toward Photovoltaic Devices with Enhanced Efficiency

In this work, for the first time, the addition of aluminum oxide nanostructures (Al 2 O 3 NSs) grown by glancing angle deposition (GLAD) is investigated on an ultrathin Cu(In,Ga)Se 2 device (400 nm) fabricated using a sequential process, i.e., post‐selenization of the metallic precursor layer. The most striking observation to emerge from this study is the alleviation of phase separation after adding the Al 2 O 3 NSs with improved Se diffusion into the non‐uniformed metallic precursor due to the surface roughness resulting from the Al 2 O 3 NSs. In addition, the raised Na concentration at the rear surface can be attributed to the increased diffusion of Na ion facilitated by Al 2 O 3 NSs. The coverage and thickness of the Al 2 O 3 NSs significantly affects the cell performance because of an increase in shunt resistance associated with the formation of Na 2 Se X and phase separation. The passivation effect attributed to the Al 2 O 3 NSs is well studied using the bias‐EQE measurement and J–V characteristics under dark and illuminated conditions. With the optimization of the Al 2 O 3 NSs, the remarkable enhancement in the cell performance occurs, exhibiting a power conversion efficiency increase from 2.83% to 5.33%, demonstrating a promising method for improving ultrathin Cu(In,Ga)Se 2 devices, and providing significant opportunities for further applications.