Back-contact perovskite solar cells place the perovskite absorber at the top of the device stack so that incoming light reaches the active layer directly, while electron and hole-collection contacts and charge transport materials are located on the rear side. In conventional front-contact perovskite cells, light must pass through these transport and contact layers before reaching the perovskite, which leads to optical losses and lowers the amount of usable light absorbed by the device.
In the back-contact architecture, sunlight generates electrons and holes in the perovskite layer, which then travel to their respective transport layers at the rear to form photocurrent. This geometry reduces optical losses and can increase charge collection and power conversion efficiency, but it also forces charge carriers to move over longer paths, making them more likely to encounter interfacial defects and undergo recombination that decreases efficiency and stability.
To mitigate these losses, a team led by Associate Professor Min Kim of the University of Seoul and PhD student Dohun Baek of Jeonbuk National University developed a bilayer tin oxide electron transport layer deposited by spin-coating. The tin oxide structure combines a nanoparticle SnO2 layer with a sol-gel SnO2 layer to improve interfacial contact and electronic properties at the perovskite - ETL interface in back-contact devices.
The work, published online on July 4, 2025 and appearing in Volume 654 of the Journal of Power Sources on October 30, 2025, explores how this bilayer modifies interface quality and charge extraction. The researchers report that this strategy targets recombination at interfaces and band alignment issues that have limited back-contact perovskite device performance.
"We selected SnO2 for the ETL due to its favorable conduction band alignment with perovskite and superior electron mobility compared to conventional titanium oxide. As a result, our bilayer ETL enhances interfacial contact, reduces recombination losses, and optimizes energy alignment for electron charge carriers," explains Dr. Kim.
To clarify the role of electron transport layer engineering, the team fabricated three types of back-contact perovskite devices using different tin oxide-based ETLs: a colloidal SnO2 composed of nanoparticles, a sol-gel SnO2, and a bilayer SnO2 combining a nanoparticle layer with a sol-gel layer. Each ETL was spin-coated onto indium tin oxide substrates and patterned by photolithography, providing a consistent platform for performance comparison.
Experimental measurements showed that the device using the bilayer tin oxide ETL delivered the strongest photocurrent, with an average of 33.67 picoampere, compared with 26.69 picoampere for the sol-gel SnO2 device and 14.65 picoampere for the colloidal SnO2 device. The bilayer device also reached the highest power conversion efficiency among the three architectures, achieving a maximum efficiency of 4.52 percent while exhibiting improved operational stability due to stronger suppression of charge recombination.
"BC-PSC devices hold great promise for a variety of applications, including flexible devices and large-area solar modules, due to their high efficiency, enhanced stability, and scalable design. We believe our findings will help accelerate the development of practical BC-PSC technologies for real-world applications while advancing sustainable energy solutions," concludes Mr. Baek.
Research Report:Interface engineering for efficient and stable back-contact perovskite solar cells
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