As crystalline silicon photovoltaics continue to dominate the solar market, efficient, low-cost, and dopant-free carrier-selective contacts are increasingly important for pushing device performance higher. Molybdenum oxide (MoO_x) is an attractive hole-transport material because of its high work function, but oxygen vacancies and weak interfacial interactions with hydrogenated amorphous silicon can limit carrier selectivity and transport. Researchers from Beijing University of Technology, led by Professors Qian Kang, Zilong Zheng, and Yongzhe Zhang, developed an ultrathin phosphomolybdic acid (PMA) interlayer that simultaneously improves defect passivation, energy-level alignment, and charge transport, enabling 24.9% power conversion efficiency in MoO_x-based silicon solar cells.

Why This Silicon Solar Cell Matters

MoO_x can induce strong band bending at crystalline silicon interfaces without conventional p-type doping, but oxygen-vacancy defects can reduce its work function, while weak interactions at the i-a-Si:H/MoO_x interface hinder carrier transport. The researchers introduce PMA as a molecular-scale bridge between the two materials. Rather than acting simply as an additional layer, PMA strengthens interfacial bonding, passivates oxygen-vacancy defects, increases the effective work function, and enhances band bending for more selective hole extraction.

Innovative Design and Mechanism

The key innovation is an approximately 1-nm-thick PMA interlayer inserted between i-a-Si:H and MoO_x. First-principles calculations show that PMA forms stronger interactions with both components, with bonding energies of 3.1 eV with i-a-Si:H and 2.6 eV with MoO_x, compared with only 1.4 eV for the pristine MoO_x/i-a-Si:H interface. This molecular bridge enhances interfacial coupling and facilitates hole transport. Meanwhile, PMA reduces the Mo⁵⁺/Mo⁶⁺ ratio from 0.18 to 0.14, indicating oxygen-vacancy passivation. The work function increases from 5.0 to 5.1 eV, while the interfacial dipole rises dramatically from 1.63 to 6.34 D, strengthening band bending and field-effect passivation.

Outstanding Performance

The optimized PMA/MoO_x device increases the open-circuit voltage from 713 to 730 mV and the fill factor from 83.7% to 84.9%, achieving a champion efficiency of 24.9%, compared with 23.8% for the MoO_x control. The modification introduces negligible optical penalties, with visible-light transmittance remaining above 95%. Electrical improvements are equally significant: minority-carrier lifetime increases from 1.18 to 2.44 ms, saturation current density decreases from 40.0 to 14.9 fA cm^-2, and contact resistivity falls by 24%, from 140 to 106 mΩ cm². The built-in potential also increases from 722 to 741 mV, while simulations predict efficiency improvement from 23.9% to 24.7%.

Applications and Future Outlook

The PMA/MoO_x architecture provides a promising route toward dopant-free passivating contacts for high-efficiency crystalline silicon photovoltaics. Its solution-processable, ultrathin PMA layer preserves optical transparency while addressing both defect-induced recombination and inefficient interfacial charge transport. More broadly, the molecular-scale interfacial bridging concept offers a strategy for engineering transition-metal-oxide interfaces by simultaneously tuning chemical bonding, defect chemistry, work function, band bending, and carrier transport. This approach could support scalable and cost-effective photovoltaic manufacturing while extending to other optoelectronic devices requiring precisely engineered interfaces.