Incorporation of Fluorinated Polymer into Spiro-MeOTAD for Highly Efficient and Stable MA-free Perovskite Solar Cells

Grace Dansoa Tabi^a, Wensheng Liang^a, Wenzhong Ji^b, Teng Lu^b, Thanh Tran-Phu^b, Olivier Lee Cheong Lem^c, Yun Liu^b, Kylie Catchpole^a, Klaus Weber^a, Thomas White^a, The Duong^a, Daniel Walter^a

^aSchool of Engineering, The Australian National University, Canberra 2601, Australia

^bResearch School of Chemistry, College of Science, The Australian National University, Canberra 2601, Australia

^cDepartment of Electronic Materials Engineering, Research School of Physics, The Australian National University, Canberra, ACT 2600, Australia

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

Poster, Grace Dansoa Tabi, 124
Publication date: 30th March 2023

Scientific innovation and relevance

Poly(vinylidene fluoride-co-trifluoroethylene) (pvdf-trfe) is an affordable, commercially available and environmentally friendly hydrophobic fluorinated polymer material widely used in biomedical and electrical devices[1,2]. Using such materials as additives in PSC hole transport layers (HTLs) has grown in popularity owing to significant positive influence on film formation, optoelectrical properties and charge transport abilities[3,4]. Here, we show how a straight-forward addition of a pvdf-trfe additive in Spiro-MeOTAD improves both PSC PCE and stability. We fabricate MA-free perovskite devices with an efficiency of up to 24.1% relative to 21.4% for a control device. Long-term ambient and operational stability of the perovskite solar cells are greatly improved, with PCE retention of >90% after 45 days and 1080 hours under ambient in the dark and white LED calibrated under 1-Sun intensity, respectively. FTIR and chemical energetic measurements show the molecular interactions between pvdf-trfe and spiro-based solution, where the pvdf-trfe material can bond with the 4-tert-butylpyridine (TBP) and Li-TFSI materials[5]. Electrical and spectroscopy analyses are consistent with the reduction of non-radiative recombination effects due to reduced trap density at the perovskite-HTL interface. When incorporating pvdf-trfe, HTL film morphology and hydrophobicity improves. These findings suggest hydrophobic fluorinated materials with similar backbones incorporated in HTL materials to improve both PSC performance and stability.

Results

We incorporated 0.7 mol% of pvdf-trfe into the precursor for Spiro-OMeTAD, which improved voltage, series resistance and fill factor in complete PSCs. FT-IR analysis indicates that pvdf-trfe material bond with the TBP additive in spiro solution to limit the fast evaporation and improve the film coverage and morphology. SEM imaging reveal voids in the control films, consistent with other studies[6,7], which are not present in the treated Spiro film. Voids could be linked to poor film formation due to rapid evaporation of the TBP material as opposed to that of the target film[4].

Hole-only samples with a structure of ITO/PTAA/Perovskite/spiro-OMeTAD (pvdf-trfe)/Au were fabricated. From dark J-V data, we extracted lower trap-state density in the target device using the space-charge-limited-current SCLC model. Photoluminescence measurements indicate the reduction of non-radiative recombination in the target device, consistent with the final cell voltage. The target cell voltage increased by 40 mV, from 1.11 to 1.15 mV. In addition, we quantify lower series resistance from light J-V measurements and electrochemical impedance spectroscopy. Addition of pvdf-trfe to Spiro reduces Ohmic series resistance from 4.02 to 1.95 Ωcm² and increases fill factor (FF) from 76.7 to 83.2%.

The target devices also exhibited higher stability. Under dark ambient conditions, PCE exceeded 90% after 45 days. While under light-soaking in N₂, 94% PCE was retained after was retained after 1080 hours. We attribute the improved stability in the target device to the improved film morphology to enhance interface quality and simultaneously prevent the infiltration of water and oxygen (which are agents for fast deterioration in perovskite devices).

Conclusion

This work reports that the additive engineering of a spiro-based HTL with pvdf-trfe material improves film morphology, increases conductivity and suppresses non-radiative recombination, thereby improving the device's efficiency and stability. MA-free perovskite devices containing pvdf-trfe additive exhibited high photovoltaic efficiency of 24.1% compared to control efficiency of 21.4%. Simultaneously, the hydrophobic nature of the pvdf-trfe material acts as an encapsulant to protect the device from oxygen and moisture under ambient conditions, as well as improve long-term operational stability. This work provides a facile approach to utilizing cheap and environmentally friendly polymer material to boost device performance, stability and reliability, opening the possibility for materials with similar polymer backbones to be utilized in photovoltaic and other electronic applications.

References:

[1] Shepelin, N. A.; Glushenkov, A. M.; Lussini, V. C.; Fox, P. J.; Dicinoski, G. W.; Shapter, J. G.; Ellis, A. V. New Developments in Composites, Copolymer Technologies and Processing Techniques for Flexible Fluoropolymer Piezoelectric Generators for Efficient Energy Harvesting. Energy Environ. Sci. 2019, 12 (4), 1143–1176.

[2] Namvarrechi, S.; Dormeny, A. A.; Dargahi, J.; Kahrizi, M. Fabrication and Characterization of Piezoelectric PVDF-TrFE Sensor Fabricated Using Spin Coating Method for Biomedical Device Applications. Proc. Int. Conf. Biomed. Innov. Appl. BIA 2020 2020, 33–36.

[3] Lou, Q.; Lou, G.; Guo, H.; Sun, T.; Wang, C.; Chai, G.; Chen, X.; Yang, G.; Guo, Y.; Zhou, H. Enhanced Efficiency and Stability of N-i-p Perovskite Solar Cells by Incorporation of Fluorinated Graphene in the Spiro-OMeTAD Hole Transport Layer. Adv. Energy Mater. 2022, 12 (36), 1–11.

[4] Ren, G.; Han, W.; Zhang, Q.; Li, Z.; Deng, Y.; Liu, C.; Guo, W. Overcoming Perovskite Corrosion and De-Doping Through Chemical Binding of Halogen Bonds Toward Efficient and Stable Perovskite Solar Cells. Nano-Micro Lett. 2022, 14 (1), 1–13.

[5] Han, Y.; Zhang, G.; Xie, H.; Kong, T.; Li, Y.; Zhang, Y.; Song, J.; Bi, D. Azide Additive Acting as a Powerful Locker for Li+ and TBP in Spiro-OMeTAD toward Highly Efficient and Stable Perovskite Solar Cells. Nano Energy 2022, 96 (2), 107072.

[6] Hawash, Z.; Ono, L. K.; Raga, S. R.; Lee, M. V.; Qi, Y. Air-Exposure Induced Dopant Redistribution and Energy Level Shifts in Spin-Coated Spiro-Meotad Films. Chem. Mater. 2015, 27 (2), 562–569.

[7] Ono, L. K.; Raga, S. R.; Remeika, M.; Winchester, A. J.; Gabe, A.; Qi, Y. Pinhole-Free Hole Transport Layers Significantly Improve the Stability of MAPbI3-Based Perovskite Solar Cells under Operating Conditions. J. Mater. Chem. A 2015, 3 (30), 15451–15456.

Acknowledgements:

This work was supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information, or advice expressed herein is not accepted by the Australian Government. D.W. and T.D. acknowledge the financial support of Postdoc Fellowships from the Australian Centre for Advanced Photovoltaics (ACAP). T.W. is the recipient of an Australian Research Council Future Fellowship (project number FT180100302) funded by the Australian Government.

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