Photovoltaic (PV) solutions, which are designed to convert sunlight into electrical energy, are becoming increasingly widespread worldwide. Over the past decades, engineers specialized in energy solutions have been trying to identify new solar cell designs and PV materials that could achieve even better power conversion efficiencies, while also retaining their stability and reliably operating for long periods of time.

The many emerging PV solutions that have proven to be particularly promising include tandem solar cells based on both perovskites (a class of materials with a characteristic crystal structure) and organic materials. Perovskite/organic tandem solar cells could be more affordable than existing silicon-based solar cells, while also yielding higher power conversion efficiencies.

These solar cells are manufactured using wide-bandgap perovskites, which have an electronic bandgap greater than 1.6 electronvolts (eV) and can thus absorb higher-energy photons. Despite their enhanced ability to absorb high-energy light particles, these materials have significant limitations, which typically adversely impact the stability of solar cells.

Researchers at the Hong Kong Polytechnic University recently devised a new strategy to improve the stability and efficiency of perovskite/organic tandem solar cells. This strategy, outlined in a paper in Nature Energy, relies on the use of acidic magnesium-doped tin oxide quantum dots.

“Wide-bandgap perovskites in monolithic perovskite/organic tandem solar cells face challenges such as unregulated crystallization, severe defect traps, poor energetic alignment and undesirable phase transitions, primarily due to unfavorable bottom interfacial contact,” wrote Yu Han, JieHao Fu, and their colleagues in their paper.

“These issues lead to energy loss and device degradation. In this article, we synthesize acidic magnesium-doped tin oxide quantum dots to modulate the bottom interface contact in wide-bandgap CsPbI₂Br perovskite solar cells.”

Quantum dots are nanoscale semiconductor particles that exhibit unique optical and electronic properties. The researchers synthesized quantum dots doped with acidic magnesium and then used them to enhance the connection between the perovskite layer and underlying material in perovskite/organic solar cells.

“This design balances physical, chemical, structural and energetic properties, passivating defects, optimizing energy band alignment, enhancing perovskite film growth and mitigating instability,” wrote the researchers. “We also elucidate the instability mechanism caused by alkaline-based tin oxide bottom contact, emphasizing the impact of the tin oxide solution’s acid/base properties on the stability and performance of the device.”

The researchers used their new quantum dot-based design strategy to create a wide-bandgap CsPbI₂Br solar cell and then tested the performance of this cell in a series of tests. Their results were promising, as their approach yielded good power conversion efficiencies, while also boosting the solar cell’s stability under a variety of environmental conditions.

“The wide-bandgap CsPbI₂Br solar cell achieves a power conversion efficiency of 19.2% with a 1.44 V open-circuit voltage,” wrote the researchers. “The perovskite/organic tandem solar cell demonstrates an efficiency of 25.9% (certified at 25.1%), with improved stability under various conditions.”

The recent work by Han, Fu and their collaborators could contribute to the advancement of perovskite/organic tandem solar cells, potentially facilitating their future widespread deployment. The quantum dot-based strategy they developed could soon be improved further and applied to similar solar cells based on other materials.

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