Highly crystallized tungsten doped indium oxide film stabilizes silicon heterojunction solar cells in sodium environment

High-efficient silicon heterojunction (SHJ) solar cells are ideal bifacial devices. In this application context, Na+ ions induced degradations from encapsulation soda-lime glass are of significance. We conducted accelerated Na+ ions aging experiments at 85 °C, 85% relative humidity without encapsulation to simulate damp heat test in soda-lime glass for SHJ solar cells. It is found that Na+ ions accelerated the formation of metal In and In(OH)3 or InOOH in damp heat condition, which destroyed the capping metal oxide, i.e., tungsten doped indium oxide (IWO). Interestingly, it demonstrates that the device stability is highly dependent on the crystallinity of IWO films. IWO films with large grains are more resistive to Na+ ions erosion, and consequently, account for the enhanced device stability of SHJ solar cells. We believe this work provides a guide for designing more reliable bifacial SHJ solar cells installed in hot climates and coastal areas.