The fill-factor limit of organic solar cells

Huotian Zhang^a, Jun Yuan^b, Rokas Jasiu̅nas^c, Vidmantas Gulbinas^c, Feng Gao^a

^aDepartment of Physics, Chemistry and Biology (IFM), Linköping University, Linköping 58183, Sweden

^bCentral South University, College of Chemistry and Chemical Engineering, China, Yuelu District, Changsha, China, 410083, Changsha, China

^cCenter for Physical Sciences and Technology, Vilnius 10257, Lithuania

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)

London, United Kingdom, 2023 June 12th - 14th

Organizers: Tracey Clarke, James Durrant and Trystan Watson

Oral, Huotian Zhang, presentation 203
DOI: https://doi.org/10.29363/nanoge.hopv.2023.203
Publication date: 30th March 2023

Organic solar cells (OSCs) have enabled a high power conversion efficiency (PCE) of around 20%^[1]. The ever-increasing efficiency has been accompanied by a deeper understanding of the dominant performance parameters, including the open-circuit voltage (V_OC), short-circuit current (J_SC) and fill factor (FF). However, the relationship between these parameters is complex and often involves trade-offs. In particular, the FF in OSCs appears to compete with the V_OC, which differs from other semiconductors^[2,3].

For an efficient solar cell, the requirements of V_OC and FF are consistent in terms of reducing recombination losses. Previous analytical expressions have shown that a higher FF is achievable with a higher V_OC. Indeed, for solution-processed perovskite solar cells, suppressed V_OC losses are associated with increased FF^[4,5]. It seems conceivable that reducing the V_OC loss is always beneficial to the maximum FF available in a solar cell. However, this does not apply to OSCs. Reports show that increasing V_OC can be detrimental to FF in OSCs, contradicting what has been observed in inorganic and hybrid semiconductors. This raises two important questions: i) what causes this limit and ii) how it can be avoided.

In this work, we explore the FF limit in OSCs. We analyze over 100 sets of OSCs based on two classical NFAs: rylene-diimide-based and linear-fused-ring electron acceptors. These OSCs have voltage loss from 0.5 eV to 1.1 eV and FF from 0.27 to 0.8. We find that transport limit cannot explain the unique FF limit in organics. We select four representative systems based on Y6-series NFAs with low V_OC loss. We investigate the charge generation process in these systems and observe an emissive bound state that is suggested to be a hybrid state of local excited (LE) and charge transfer (CT) states. This weakly bound state shows field-dependent charge generation, which is a unique feature in OSCs that causes FF loss besides transport-limit. Moreover, the weakly bound state can influence the transport-limit by reseparating into free charge carriers. The reseparation would reduce recombination coefficient, and its reduction degree is proportional to the dissociation probability of the bound state.

References:

[1] Zhu, L.; Zhang, M.; Xu, J.; Li, C.; Yan, J.; Zhou, G.; Zhong, W.; Hao, T.; Song, J.; Xue, X.; Zhou, Z.; Zeng, R.; Zhu, H.; Chen, C. C.; MacKenzie, R. C. I.; Zou, Y.; Nelson, J.; Zhang, Y.; Sun, Y.; Liu, F. Single-Junction Organic Solar Cells with over 19% Efficiency Enabled by a Refined Double-Fibril Network Morphology. Nat. Mater. 2022, 21 (6), 656–663.

[2] Kirchartz, T.; Kaienburg, P.; Baran, D. Figures of Merit Guiding Research on Organic Solar Cells. J. Phys. Chem. C 2018, 122 (11), 5829–5843.

[3] Classen, A.; Chochos, C. L.; Lüer, L.; Gregoriou, V. G.; Wortmann, J.; Osvet, A.; Forberich, K.; McCulloch, I.; Heumüller, T.; Brabec, C. J. The Role of Exciton Lifetime for Charge Generation in Organic Solar Cells at Negligible Energy-Level Offsets. Nat. Energy 2020, 5 (9), 711–719.

[4] Yang, G.; Ren, Z.; Liu, K.; Qin, M.; Deng, W.; Zhang, H.; Wang, H.; Liang, J.; Ye, F.; Liang, Q.; Yin, H.; Chen, Y.; Zhuang, Y.; Li, S.; Gao, B.; Wang, J.; Shi, T.; Wang, X.; Lu, X.; Wu, H.; Hou, J.; Lei, D.; So, S. K.; Yang, Y.; Fang, G.; Li, G. Stable and Low-Photovoltage-Loss Perovskite Solar Cells by Multifunctional Passivation. Nat. Photonics 2021, 15 (9), 681–689.

[5] Wang, R.; Xue, J.; Wang, K.-L.; Wang, Z.-K.; Luo, Y.; Fenning, D.; Xu, G.; Nuryyeva, S.; Huang, T.; Zhao, Y.; Yang, J. L.; Zhu, J.; Wang, M.; Tan, S.; Yavuz, I.; Houk, K. N.; Yang, Y. Constructive Molecular Configurations for Surface-Defect Passivation of Perovskite Photovoltaics. Science. 2019, 366 (6472), 1509–1513.

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