A group of researchers from Beijing University of Technology in China has fabricated a silicon heterojunction (HJT) solar cell featuring a molybdenum oxide (MoOₓ) hole transport layer (HTL) and an ultrathin phosphomolybdic acid (PMA) interlayer. The approach is intended to improve device efficiency and stability while reducing reliance on conventional doped silicon hole-selective contacts.
“MoOₓ can induce strong band bending at crystalline silicon interfaces without conventional p-type doping, but oxygen-vacancy defects can reduce its work function, while interactions at the interface between hydrogenated silicon and MoOₓ are weak,” the researchers explained. “The key innovation is an approximately 1-nm-thick phosphomolybdic acid (PMA) interlayer inserted between intrinsic hydrogenated amorphous silicon (i-a-Si) and MoOₓ.”
The team fabricated rear-junction crystalline silicon heterojunction solar cells on 130-μm-thick n-type silicon wafers. The wafers were chemically textured to form randomly distributed pyramids measuring approximately 1 μm to 3 μm, before being cleaned and polished on the rear side.
The front electron-selective contact comprised an 8-nm i-a-Si layer and a 15-nm n-type nanocrystalline silicon oxide (n-nc-SiOₓ) layer. A transparent conductive oxide (TCO) layer and printed silver grid completed the front contact. The devices were then annealed at 190 C for 30 minutes.
For the rear hole-selective contact, the researchers deposited the PMA interlayer by spin coating, followed by a 10-nm MoOₓ layer and a 200-nm silver layer deposited by thermal evaporation.
The team characterized the films using UV-visible spectroscopy and determined their optical bandgaps using the Tauc method. X-ray photoelectron spectroscopy (XPS) was used to investigate elemental composition and chemical states, while solar cell performance was assessed under standard one-sun conditions at 25 C and through external quantum efficiency (EQE) measurements.
The scientists also performed density functional theory (DFT) calculations to investigate the interface at the atomic level and used SCAPS-1D simulations to model energy-band alignment under illumination.
The PMA/MoOₓ device achieved a champion power conversion efficiency of 24.9%, compared with 23.8% for a control device using MoOₓ without the PMA interlayer. Open-circuit voltage increased from 713 mV to 730 mV, while fill factor rose from 83.7% to 84.9%.
The researchers found that the PMA interlayer had little impact on optical performance, with both contact configurations exhibiting transmittance of more than 95% across the visible spectrum. Similar EQE spectra and short-circuit current densities indicated that the interlayer introduced negligible additional optical losses.
Microscopy showed that PMA formed a continuous interlayer approximately 1 nm thick, which the researchers said was thin enough to enable efficient carrier tunneling. Contact resistivity fell by 24%, from 140 mΩ cm² to 106 mΩ cm².
“The modification introduces negligible optical penalties, with visible-light transmittance remaining above 95%,” the academics said.
The PMA modification also increased the effective minority-carrier lifetime from 1.18 ms to 2.44 ms and reduced saturation current density from 40.0 fA/cm² to 14.9 fA/cm². The built-in potential increased from 722 mV to 741 mV, indicating a stronger built-in electric field.
Further simulations linked the performance gains to reduced interface recombination and contact losses, as well as more favorable band alignment at the PMA-modified interface. Using experimentally derived recombination and contact-resistance parameters, the researchers simulated an efficiency increase from 23.9% to 24.7%.
The novel cell concept was described in “Interface Contact Optimization via Phosphomolybdic Acid Enables 24.9% Efficiency in MoOₓ-Based Silicon Solar Cells,” published in Nano-Micro Letters.
MoOₓ has previously been investigated for several functions across a range of photovoltaic technologies.
In 2020, pv magazine reported on a 23.5%-efficient silicon heterojunction solar cell developed by scientists at EPFL using MoOₓ as a hole-selective contact. In 2022, researchers at Delft University of Technology improved on this approach with a 23.83%-efficient silicon heterojunction cell incorporating an ultrathin MoOₓ hole collector, while another research group used a MoOₓ-based transparent back contact in a 25.5%-efficient four-terminal perovskite-CIGS tandem cell.
MoOₓ has also been deployed as a buffer layer in a 24.3%-efficient perovskite solar cell and in a 24.2%-efficient perovskite-CdTe tandem device. In 2024, TU Delft researchers presented a 21.14%-efficient IBC-HJT solar cell featuring a thin, full-area MoOₓ layer.
More recently, Chinese scientists developed a MoOₓ/Ag/MoOₓ sandwich buffer layer for perovskite-silicon tandem minimodules, while MoOₓ has also been used in the transparent electrode of flexible perovskite solar cells with tunable color and transparency.
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