Controlling light emission improves organic solar cell performance
ICFO researchers have enhanced the performance of an organic solar cell by optimizing the fluorescence quantum yield (FQY), that is, by favoring light emission instead of heat dissipation after light absorption.
The study, published in ACS Energy Applied Materials, shows that FQY related to light emission inside the solar cell can be improved by engineering the interface between the light-absorbing and the charge-transporting layers, resulting in a measurable increase in the open-circuit voltage. This methodology is potentially applicable to other photovoltaic technologies.
Organic solar cells (OSCs) use carbon-based materials instead of silicon to convert sunlight into electricity. Their flexibility, semi-transparency, and low-cost manufacturing make them attractive for applications such as wearables, smart windows, and building-integrated photovoltaics. But OSCs face a significant efficiency bottleneck due to large losses in open-circuit voltage (Voc), the electrical potential difference between the two terminals of solar cells.
A team of ICFO researchers and members of the SOREC2 project, Dr. Francisco Bernal-Texca, Chiara Cortese, and Dr. Mariia Kramarenko, led by UPC and ICFO Prof. Jordi Martorell, have recently proposed a new strategy to significantly enhance the Voc. The team has addressed the fluorescence quantum yield (FQY), a largely unexplored determinant of Voc, which the study, published in ACS Energy Applied Materials, demonstrates to be critical.
When a solar cell absorbs light, the energy of some electrons increases, that is, they become excited. The FQY reflects how efficiently the electrons’ extra energy is re-emitted as light rather than lost as heat, which often occurs due to chemical defects at material interfaces. ICFO researchers have now demonstrated that optimizing FQY can significantly increase the Voc; its effects, contrary to traditional believe, are not negligible.
“A higher FQY indicates that the defect-pathways to heat have been reduced, allowing a much higher density of charge carriers to accumulate,” explains Dr. Francisco Bernal-Texca, first author of the article. “Since the electrical potential increases as the carrier concentration grows, improving the FQY directly increases the Voc, moving the device closer to its maximum theoretical limit,” he adds.
To reach this conclusion, the team fabricated a fully operational solar cell rather than just testing individual organic ingredients, and showed that the FQY is ten times higher in the former. They then applied an ultrathin (5 nm) lithium fluoride (LiF) layer on top of the electron transport layer made of ZnO, which further enhanced the FQY by 18%, leading to a Voc gain as large as 12 millivolts. Spectroscopic analysis revealed that LiF acts as a protective sealant that passivates chemical defects, preventing electrons from being caught in trapping states and wasted as heat. Finally, the researchers developed a theoretical model that correlates the increase in Voc to the rise in FQY, agreeing with the experimental data.
According to Prof. Jordi Martorell, lead researcher of the study: “The presented methodology has the potential to be applied to any type of solar cell and opens a door to surpassing the established limits of solar energy conversion.” For now, it has already revealed that even small improvements in light emission can have a measurable impact on the overall voltage.
Reference:
Francisco Bernal-Texca, Chiara Cortese, Mariia Kramarenko, and Jordi Martorell, Nongeminate Radiative Recombination and Voc in Organic Solar Cells Enhanced by a Charge Transporter/Absorber Interface Change, ACS Applied Energy Materials 2025 8 (24), 17863-17870
DOI: 10.1021/acsaem.5c02775
Acknowledgements:
The authors acknowledge the financial support by the European Commission through the SOREC2 Project (101084326). The authors also acknowledge the financial support of the AGREEN project (grant TED2021-129728BI00) funded by the Ministerio de Ciencia e Innovación (MCIN) / Agencia Estatal de Investigación (AEI) and by the European Union "NextGenerationEU"/PRTR. The work was also partially funded by MCIN (Grant Nos. CEX2019-000910-S and PID2020-112650RB-I00), Fundació Cellex, Fundació Mir-Puig, and Generalitat de Catalunya through Centres de Recerca de Catalunya. F.B.-T. acknowledges financial support from the AEI (Grant No. PRE2018-084881).