Dark-degradation of high efficiency silicon heterojunction solar cells stemming from p-type hydrogenated silicon emitter

Silicon heterojunction (SHJ) solar cells have emerged as one of the most promising crystalline silicon solar cells, owing to their exceptional power conversion efficiency (PCE). Nevertheless, the PCE degradation observed during the transition from individual solar cell to complete photovoltaic module initiates a critical challenge for expanding of the extensive application into power generation. This study investigates the dark-state stability of SHJ solar cells with varying microstructure and boron (B) doping concentration (C_B) in p-type emitters. It was found that the nanocrystalline silicon emitter undergoes a structural transition to amorphous phase at C_B of 2 %, demonstrating an improved dark-state stability attributed to the surroundings and moderate strain of the bonds in silicon network. Furthermore, we found that the reciprocating activity of micro-voids characteristics such as the diameter and quantity follow a quasi-harmonic variation during dark-state process. The formation and broken of B-H bonds are induced by the relaxation of non-bonded hydrogen atoms (NBHs), involving the two actions of inserting into and escaping from the micro-voids. The migration of NBHs results in the relaxation of the thin-film structure and instability of the PCE. Experimental evidence proves that B atoms exhibit significant retarding effects on migration within p-type emitters. As a result, the emitter films with transition phase from nanocrystalline to amorphous combining with impeding of moderate C_B on NBHs migration make PCE of SHJ solar cells more stable during dark-state process.