Researchers at the University of Queensland and Southwest University have developed a vapor-based fabrication strategy for lead-free tin-based halide perovskite (THP) indoor solar cells, addressing challenges in crystallization control while achieving high efficiency under low-light conditions.
A central challenge in thermally evaporated THP films is their complex and poorly controlled crystallization kinetics, which often leads to defects and performance losses. To overcome this, the team introduced formamidine acetate (FAAc) as a vapor-deposited additive during the formation of FASnI2Br films. FAAc coordinates with SnI2 to form a metastable SnI2-FAAc intermediate phase. This intermediate slows and regulates the solid-state reaction pathway, allowing more controlled crystal growth. At the same time, FAAc reduces the surface free energy of the underlying SnI2 layer, enabling more uniform deposition of the subsequent FABr layer. This dual effect - kinetic regulation and improved film wetting - results in higher-quality perovskite films with fewer defects and significantly suppressed trap-assisted recombination.
The improved film quality translates directly into strong device performance. The resulting indoor photovoltaic (IPV) devices achieved a power conversion efficiency (PCE) of 16.36% under 1000 lx illumination, which is representative of typical indoor lighting conditions.
In addition to efficiency, the devices demonstrated notable operational stability, maintaining performance for over 3000 hours under nitrogen without encapsulation. This highlights the effectiveness of the intermediate-phase strategy in stabilizing wide-bandgap tin-based perovskites fabricated via solvent-free processes.
The work also introduces a manufacturing approach that eliminates both toxic lead and hazardous solvents. The vapor-based process developed by the team enables the fabrication of high-quality perovskite films with fewer performance-limiting defects, while remaining compatible with scalable industrial production.
“Indoor solar cells themselves are not new, but the power conversion efficiency of the commercial silicon-based technology is only around 10 per cent,” University of Queensland's Dr Lyu said. “The technology we developed eliminates those materials while still delivering high efficiency.”
Because the process is entirely solvent-free, it is better suited for large-scale manufacturing and integration into flexible substrates. The resulting panels are thin, lightweight, and can be fabricated on plastic in various shapes.
Tin-based perovskite indoor solar cells are particularly well suited for low-intensity artificial light sources such as LEDs and fluorescent lamps. Their high efficiency under these conditions makes them strong candidates for powering low-power electronics.
“With suitable voltage management, these devices can replace coin cell batteries, reducing the number of small batteries that end up as waste or in children’s toys,” Dr Lyu said.
By combining a controlled crystallization mechanism with a scalable, lead-free fabrication route, this work demonstrates a practical pathway toward high-performance indoor photovoltaics that could replace conventional battery-powered systems in a wide range of applications.
