Researchers from the University of Seoul (UOS) and Joenbuk National University (JNU) in South Korea have developed a novel bilayer tin oxide (SnO2) electron transport layer (ETL), via a simple spin-coating method, that significantly improves efficiency and stability of back-contact perovskite solar cells (BC-PSCs).

“We selected SnO2 for the ETL due to its favorable conduction band alignment with perovskite and superior electron mobility compared to conventional titanium oxide,” Kim explained. “As a result, our bilayer ETL enhances interfacial contact, reduces recombination losses, and optimises energy alignment for electron charge carriers.”

The device is built on a glass substrate coated with patterned indium tin oxide (ITO), the SnO₂ ETL, and a perovskite absober. A line-patterned nickel (Ni) electrode is fabricated using photolithography and then thermally oxidized to form nickel oxide (NiOx), which functions as the hole transport layer (HTL). The SnO₂ ETL and NiOx HTL were arranged side by side in an interdigitated pattern at the rear of the device, enabling lateral charge collection. An aluminum oxide (Al₂O₃) insulating layer was introduced to electrically isolate the electrodes and prevent short-circuiting, while a thin polymethyl methacrylate (PMMA) passivation layer was applied to protect the perovskite surface and reduce recombination.

In this architecture, light directly illuminates the perovskite layer from the top without obstruction by front electrodes, while both electrons and holes are selectively extracted laterally through the back-contact SnO₂ and NiOx electrodes, respectively.

To evaluate the role of ETL engineering, the researchers fabricated three BC-PSC devices with different SnO2-based ETLs: a colloidal SnO2 made of nanoparticles, a sol-gel SnO2, and a bilayer SnO2 consisting of a nanoparticle SnO2 layer combined with a sol-gel layer. Each ETL was spin-coated onto indium tin oxide substrates and patterned via photolithography.

A series of experiments compared the performance of the devices, which showed that the device with bilayer SnO2 yielded the highest average photocurrent of 33.67 picoampere (pA), outperforming the sol-gel SnO2 device at 26.69 pA and colloidal SnO2 device at 14.65 pA.

The bilayer SnO2 device also achieved a maximum power conversion efficiency of 4.52%, was the highest of the three, and improved operational stability, owing to its enhanced 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” Baek said. “We believe our findings will help accelerate the development of practical BC-PSC technologies for real-world applications while advancing sustainable energy solutions.

The “Interface engineering for efficient and stable back-contact perovskite solar cells” study, led by UOS Department of Chemical Engineering Associate Professor Min Kim and JNU School of Chemical Engineering PhD student Dohun Baek, was published in the Journal of Power Sources.

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