Silver (Ag) substitution in Cu(In,Ga)Se2 solar cell: insights into current challenges and future prospects

As one of the most intriguing absorbers for environmentally friendly and low cost thin-film photovoltaic materials, Cu(In,Ga)Se 2 (CIGS)-based solar cells with high device conversion efficiency, high stability, tunable bandgap, and high optical absorption coefficients have attracted widespread attention. As we know, the last record efficiency of 23.35% for CIGS solar cell devices maintained close to four years to be broken. The main obstacle for this stagnation is ascribed to the narrow band gap of CIGS absorber layer. Simultaneously, the highly efficient CIGS solar cells exceed 22% with the absorber bandgap energy ( E g ) below 1.2 eV, it is lower than that of the global optimum lies of 1.34 eV. Therefore, widening the absorber layer bandgap to match the solar spectrum can be meaningful for the further efficiency improvement. However, the wide-gap CIGS solar cell encounters the current pronounced limitation is the large V OC deficit, which is related to the inappropriate heterogeneous junction interface of CdS/CIGS (cliff-like structure). Excitingly, with silver (Ag) alloying, the wider band gap of CIGS can keep the heterojunction interface always in a spike-like structure and optimize the heterojunction structure of the buffer layer/absorber layer. Simultaneously, with the introducing of Ag, the current record efficiency of CIGS has arrived to 23.6% in 2024. The aim in here is to evaluate recent advances with Ag to widening the bandgap of the absorber layer to match the solar spectrum along with several emerging prospective superiorities, including: (i) suppressing the lattice disorder, (ii) ameliorating crystallization properties, (iii) controlling heterojunction interfaces, (iv) tailoring band gap gradient, and surface modification with alkali elements.