Improved Rear Surface Passivation of Cu(In,Ga)Se $_{\bf 2}$ Solar Cells: A Combination of an Al $_{\bf 2}$

An innovative rear contacting structure for copper indium gallium (di) selenide (CIGS) thin-film solar cells is developed in an industrially viable way and demonstrated in tangible devices. The idea stems from the silicon (Si) industry, where rear surface passivation layers are combined with micron-sized local point contacts to boost the open-circuit voltage (V_OC) and, hence, cell efficiency. However, compared with Si solar cells, CIGS solar cell minority carrier diffusion lengths are several orders lower in magnitude. Therefore, the proposed CIGS cell design reduces rear surface recombination by combining a rear surface passivation layer and nanosized local point contacts. Atomic layer deposition of Al₂O₃ is used to passivate the CIGS surface and the formation of nanosphere-shaped precipitates in chemical bath deposition of CdS to generate nanosized point contact openings. The manufactured Al₂O₃ rear surface passivated CIGS solar cells with nanosized local rear point contacts show a significant improvement in V_OC compared with unpassivated reference cells.