Silicon heterojunction (SHJ) solar cells employ nanometer-thin stacks of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) films as carrier-selective contacts. To achieve excellent carrier selectivity, the a-Si:H must be carefully optimized to guarantee an atomically sharp a-Si:H/c-Si interface. In this work, by combining experiments with molecular dynamics and ab initio calculations, we unveil that H atoms bonded to internal-void surfaces in a-Si:H broaden its optical band gap via a filamentary effect near the valence-band maximum. The photovoltaic performance of rear-emitter SHJ solar cells can be significantly improved by tailoring the Si−H bonding state in the front a-Si:H passivation layer, resulting in a power conversion efficiency (PCE) of 23.4% on a 6-in cell. By implementing double antireflection coatings (ARCs) of SiNx and SiOx, the PCE is further improved to 23.9%. More importantly, the ARC devices show prominently improved damp-heat stability without encapsulation in 1,000-h aging at 85°C, 85% relative humidity. The last few years have seen the photovoltaic market gradually evolve from aluminum back surface field (Al-BSF) solar cells to passivated emitter and rear (PERx) solar cells. Currently, we are looking at full-area passivating-contact c-Si solar cells on their way to mass production. Amorphous/crystalline silicon heterojunction (SHJ) solar cells hold the world-record power conversion efficiency (PCE; 26.7%) among c-Si solar cells, when integrated with an all back-contact design. Here, we present a roadmap to gaining high-efficiency SHJ solar cells, whose PCE is pushed to 23.4% on 6-in devices. However, such high-PCE solar cells are susceptible in damp-heat environments. The feasibility of mass production of long-term, stable, high-efficiency (23.9%) SHJ solar cells has been successfully demonstrated by capping with SiNx/SiOx antireflection coatings (ARCs). The ARCs have dual functions: (1) antireflection and (2) preventing moisture oxidizing amorphous silicon. Silicon heterojunction (SHJ) solar cells hold the power conversion efficiency (PCE) record among crystalline solar cells. However, amorphous silicon is a typical high-entropy metastable material. Damp-heat aging experiments unveil that the amorphous/crystalline silicon interface is susceptible to moisture, which is potentially the biggest stumbling block for mass production. By capping SiNx and SiOx dielectrics, the absolute PCE degradation is predicted to be only ∼0.6% after a 30-year installation. This demonstrates the SHJ solar cell is a highly promising candidate for next-generation photovoltaics.