Boosting near-infrared harvesting in tandem solar cells

Researchers from the National University of Singapore have demonstrated a perovskite–organic tandem solar cell with a power conversion efficiency of 26.4% over a 1 cm² active area, a development that reportedly makes it the highest-performing device of its kind to date. This milestone is driven by a newly designed narrow-bandgap organic absorber that enhances near-infrared (NIR) photon harvesting, a long-standing bottleneck in thin-film tandem solar cells.

This research breakthrough was achieved with guidance from Assistant Professor Hou Yi from the National University of Singapore. The research findings have been published in the scientific journal Nature.

Perovskite and organic semiconductors both offer widely tuneable bandgaps, enabling tandem cells to approach high theoretical efficiencies. According to Hou, their light weight and flexible form factor makes perovskite–organic tandem solar cells suitable for power applications that are run directly on devices such as drones, wearable electronics, smart fabrics and other AI-enabled devices.

However, the absence of efficient NIT thin-film absorbers — which help to capture the sunlight in the NIR region more efficiently and therefore improve the overall efficiency of tandem cells — has kept perovskite–organic tandem cells lagging behind alternative designs.

To overcome this challenge, the researchers developed an asymmetric organic acceptor with an extended conjugation structure, enabling absorption deep into the NIR region while maintaining a sufficient driving force for efficient charge separation and promoting ordered molecular packing. Ultrafast spectroscopy and device physics analyses confirmed that this design achieves high free charge carrier collection with minimal energy loss.

Building on the organic subcell’s performance, the researchers stacked it beneath a high-efficiency perovskite top cell, interfacing the two layers with a transparent conducting oxide (TCO)-based interconnector. This tandem cell achieved a power conversion efficiency of 27.5% on 0.05 cm² samples and 26.7% on 1 cm² devices, with the 26.4% result independently certified. These findings reportedly mark the highest certified performance to date among perovskite-organic, perovskite-CIGS and single-junction perovskite cells at comparable size.

“With efficiencies poised to exceed 30%, these flexible films are ideal for roll-to-roll production and seamless integration onto curved or fabric substrates — think self-powered health patches that harvest sunlight to run onboard sensors, or smart textiles that monitor biometrics without the need for bulky batteries,” Hou said.

The researchers will now focus on enhancing real-world operational stability and advancing towards pilot-line manufacturing, to bring flexible, high-performance solar technology to market.

Image caption: The new perovskite–organic tandem cell (right) designed by NUS scientists can achieve a power conversion efficiency of 26.4%, outperforming the previous benchmark for similar devices. Image credit: National University of Singapore.