Back-contact perovskite solar cells (BC-PSCs) represent a promising new alternative design to conventional solar cells, offering improved light absorption and higher power conversion efficiency. However, for addressing interfacial defects and recombination losses in BC-PSCs, advanced interface engineering techniques are required. Now, researchers have developed a novel bilayer tin oxide electron transport layer for BC-PSCs, which reduces recombination losses, promotes efficient charge transfer, and improves power conversion efficiency.
As the demand for renewable energy grows, scientists are developing new types of solar cells that are both highly efficient and scalable. The back-contact perovskite solar cell (BC-PSC) is one such innovative architecture, emerging as a promising alternative to traditional front-contact designs. In conventional perovskite solar cells, the electrode contacts and charge transport materials are placed on front of the device – the surface that faces the sun. Because incoming light must first pass through these layers before reaching the active perovskite material, a portion of the light is inevitably lost.
In contrast, BC-PSCs position the perovskite absorber layer at the top of the stack, allowing direct sunlight exposure, while electron and hole-collection contacts are positioned at the back. When light falls on the perovskite layer, it generates holes and electrons, which subsequently migrate to their respective transport layers to produce photocurrent. This design minimizes optical losses, enhances charge collection, and improves power conversion efficiency. However, this design introduces new challenges. Since charge carriers must travel longer distances, they are more likely to encounter interfacial defects, leading to recombination losses. This leads to reduced efficiency and stability, limiting practical application.
In a breakthrough, a research team led by Associate Professor Min Kim from the Department of Chemical Engineering, University of Seoul, Republic of Korea and Mr. Dohun Baek, a PhD student from School of Chemical Engineering, Jeonbuk National University, Republic of Korea, has developed a novel bilayer tin oxide (SnO2) electron transport layer (ETL), via a simple spin-coating method, that significantly improves efficiency and stability of BC-PSCs. The study was made available online on July 04, 2025, and published in Volume 654 of the Journal of Power Sources on October 30, 2025.
“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 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.
The researchers conducted a series of experiments to compare the performance of the devices. The results 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. Furthermore, the bilayer SnO2 device also achieved a maximum power conversion efficiency of 4.52%, highest among 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. 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.
