Researchers from Soochow University, Chint New Energy Technology, Wuxi EliTe Solar, Monash University, Suzhou Maxwell Technologies, University of Oxford and Beijing Institute of Technology have demonstrated high-efficiency indium-free perovskite/silicon tandem solar cells enabled by reactive plasma deposited tin oxide (RPD-SnOx), addressing both performance and scalability challenges associated with conventional indium-based transparent conductive oxides.
Indium-based oxides are widely used as electrodes and recombination layers in tandem devices, but their deployment is constrained by the scarcity and cost of indium, as well as sputtering-induced damage to sensitive perovskite layers. To overcome these limitations, the team developed a low-damage reactive plasma deposition (RPD) process for tin oxide films. In this vacuum-based technique, oxygen plasma reacts with evaporated metal atoms to form a metal oxide film, while avoiding the high-energy ion bombardment typical of sputtering. This enables the deposition of dense and conductive oxide layers with minimal damage to underlying materials.
“The research team developed a reactive plasma deposition (RPD) process for tin oxide (SnOx) films to serve as the recombination layer, achieving a certified efficiency of 33.6% on a 1 cm² device,” said Monash University Adjunct Professor Yuan Cheng. Using RPD-SnOx as the recombination layer, the team achieved a certified power conversion efficiency of 33.6%.
The approach was further extended to fully eliminate indium from the device stack. By employing RPD-SnOx as both the recombination layer and the front and rear transparent electrodes, the researchers fabricated fully indium-free tandem solar cells with a champion efficiency of 33.2% on a 1 cm² device. Scaling the technology, they demonstrated a 207.9 cm² mini-module with a certified efficiency of 31.0%, marking a milestone for commercially relevant device sizes.
“We then scaled the technology to a 207.9 cm² mini-module, achieving a certified efficiency of 31.0%,” Cheng added, highlighting the scalability of the process.
At the materials level, the RPD-SnOx films play a critical role in improving both efficiency and stability. The dense and uniform oxide layers enable the formation of high-quality self-assembled monolayers (SAMs), which serve as interfacial layers. This improved interface reduces nonradiative recombination losses and suppresses halide ion migration - two key degradation pathways in perovskite devices. As a result, the tandem cells exhibit enhanced carrier recombination control and improved long-term operational stability.
The devices also demonstrated robust environmental stability. “Indium-free minimodules exhibited high thermal, damp-heat, and outdoor operational stability and retained 65% of their maximum initial efficiency after 105 days of outdoor operation,” the researchers reported.
Beyond performance, the replacement of indium with tin oxide has important cost implications. Tin is significantly more abundant, with a material cost estimated at around 1% that of indium, offering a pathway toward lower-cost tandem photovoltaics without sacrificing efficiency.
“Considering the cost of tin is a mere 1% of that of indium, this breakthrough provides a new materials approach and a viable engineering pathway for low-cost, sustainable and scalable tandem photovoltaics,” Cheng said. “Ultimately, this work is of strategic importance for advancing the industrialization and terawatt-scale deployment of next-generation, ultra-high-efficiency photovoltaic technologies.”
