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Researchers at Beijing University of Technology in China have fabricated a silicon heterojunction solar cell that uses a molybdenum oxide hole transport layer together with an ultrathin phosphomolybdic acid interlayer, according to pv magazine. The work, described in Nano-Micro Letters, aims to raise device efficiency and stability while lessening dependence on conventional doped silicon hole-selective contacts.
The team explained that molybdenum oxide can produce strong band bending at crystalline silicon interfaces without conventional p-type doping, but that oxygen-vacancy defects can lower its work function and that interactions between hydrogenated silicon and the oxide are weak. The central innovation is an interlayer of phosphomolybdic acid roughly one nanometer thick placed between intrinsic hydrogenated amorphous silicon and the molybdenum oxide.
The group built rear-junction crystalline silicon heterojunction cells on 130-micrometer-thick n-type silicon wafers. The wafers were chemically textured into randomly distributed pyramids measuring about 1 to 3 micrometers, then cleaned and polished on the rear side.
The front electron-selective contact consisted of an 8-nanometer intrinsic hydrogenated amorphous silicon layer and a 15-nanometer n-type nanocrystalline silicon oxide layer. A transparent conductive oxide layer and a printed silver grid completed the front contact. The devices were annealed at 190 degrees Celsius for 30 minutes.
For the rear hole-selective contact, the researchers deposited the phosphomolybdic acid interlayer by spin coating, followed by a 10-nanometer molybdenum oxide layer and a 200-nanometer silver layer applied by thermal evaporation.
Films were characterized with UV-visible spectroscopy, and optical bandgaps were determined using the Tauc method. X-ray photoelectron spectroscopy was used to examine elemental composition and chemical states, while cell performance was measured under standard one-sun conditions at 25 degrees Celsius and through external quantum efficiency measurements. The scientists also ran density functional theory calculations to study the interface at the atomic level and used SCAPS-1D simulations to model energy-band alignment under illumination.
The phosphomolybdic acid and molybdenum oxide device reached a champion power conversion efficiency of 24.9 percent, against 23.8 percent for a control device using molybdenum oxide without the interlayer. Open-circuit voltage rose from 713 millivolts to 730 millivolts, and fill factor increased from 83.7 percent to 84.9 percent.
The interlayer had little effect on optical performance, with both contact configurations showing transmittance above 95 percent across the visible spectrum. Similar external quantum efficiency spectra and short-circuit current densities indicated that the interlayer added negligible optical losses. Microscopy showed that the phosphomolybdic acid formed a continuous layer about 1 nanometer thick, which the researchers said was thin enough to permit efficient carrier tunneling. Contact resistivity dropped 24 percent, from 140 milliohm square centimeters to 106 milliohm square centimeters.
The modification also raised effective minority-carrier lifetime from 1.18 milliseconds to 2.44 milliseconds and cut saturation current density from 40.0 femtoamps per square centimeter to 14.9 femtoamps per square centimeter. Built-in potential increased from 722 millivolts to 741 millivolts, pointing to a stronger built-in electric field.
Further simulations tied the performance gains to reduced interface recombination and contact losses, along with more favorable band alignment at the modified interface. Using experimentally derived recombination and contact-resistance parameters, the researchers simulated an efficiency increase from 23.9 percent to 24.7 percent.
Molybdenum oxide has previously been studied for several roles across photovoltaic technologies. In 2020, pv magazine reported on a 23.5-percent-efficient silicon heterojunction solar cell developed by scientists at EPFL using the oxide as a hole-selective contact. In 2022, researchers at Delft University of Technology improved on that approach with a 23.83-percent-efficient silicon heterojunction cell incorporating an ultrathin molybdenum oxide hole collector, while another group used a molybdenum-oxide-based transparent back contact in a 25.5-percent-efficient four-terminal perovskite-CIGS tandem cell.
The oxide has also been deployed as a buffer layer in a 24.3-percent-efficient perovskite solar cell and in a 24.2-percent-efficient perovskite-CdTe tandem device. In 2024, TU Delft researchers presented a 21.14-percent-efficient IBC-HJT solar cell featuring a thin, full-area molybdenum oxide layer. More recently, Chinese scientists developed a molybdenum oxide, silver, molybdenum oxide sandwich buffer layer for perovskite-silicon tandem minimodules, and the oxide has also been used in the transparent electrode of flexible perovskite solar cells with tunable color and transparency.
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