Vacuum sublimation of Dopant‐Free Crystalline Spiro‐OMeTAD films to enhance the stability of perovskite Solar Cells

Juan Ramon Sanchez-Valencia^a^,^b, Jose Obrero-Perez^a, Lidia Contreras-Bernal^a^,^c, Dario Jumilla^a, Javier Castillo-Seoane^a^,^b, Francisco J. Aparicio^a, M. Carmen Lopez-Santos^a, Ana Borras^a, Juan A. Anta^c, Angel Barranco^a

^aInstituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla), Calle Américo Vespucio, 49, Sevilla, Spain

^bDepartamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla, Avda. Reina Mercedes, E-41012, Seville, Spain

^cÁrea de Química Física, Universidad Pablo de Olavide, Seville, Spain

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 13th Conference on Hybrid and Organic Photovoltaics (HOPV21)

Online, Spain, 2021 May 24th - 28th

Organizers: Marina Freitag, Feng Gao and Sam Stranks

Oral, Juan Ramon Sanchez-Valencia, presentation 059
Publication date: 11th May 2021

Organometal halide perovskites have emerged as one of the most promising photoabsorbent materials for efficient and cheap solar cells.¹ The research in Perovskite Solar Cells (PSC) underwent an inflexion point with the replacement of the liquid electrolyte by solid‐state hole conductors (SSHCs), which increased significantly the Power Conversion Efficiency (PCE) and stability of the devices. The first and still most used SSHC is the organic molecule Spiro‐OMeTAD.²

Whereas the use of Spiro‐OMeTAD is widely spread, its undoped form is reported to have very poor intrinsic hole mobility and conductivity that can be increased by means of dopants.³ Unfortunately, the main handicap still hindering the eventual exploitation of PSCs is their poor stability under prolonged illumination, ambient conditions, and increased temperatures, which is partially hindered by the use of dopants that contribute to the degradation. For instance, Li⁺ cations, a common dopant, are highly hygroscopic and induce moisture absorption.³

However, negligible attention has been paid to the effect of the crystalline structure of Spiro-OMeTAD layers mainly due to the highly disordered nature of solution‐processed films (the most widely spread).⁴ Nevertheless, improved hole‐transporting properties of pristine Spiro‐OMeTAD single crystals have been reported.⁴ Even though these results show the potential of crystalline Spiro‐OMeTAD layers to enhance the photovoltaic properties of PSCs, their implementation is not straightforward due to the antisolvent experimental strategy used to grow this organic molecule in its crystalline form.⁴

In this work,⁵ we report the unprecedented sublimation under vacuum conditions of the Spiro‐OMeTAD, in the form of dopant‐free crystalline layers. In addition, we demonstrated the enhanced stability of these layers acting as SSHC in PSCs in comparison with the solution‐processed counterpart. Our results reveal that the substrate temperature is a critical parameter controlling the microstructure and crystallinity of the layers. On the other hand, the implementation of these vacuum sublimated Spiro‐OMeTAD layers on PSCs have demonstrated two key aspects: i) a considerably increased PCE in comparison to the dopant‐free Spiro‐OMeTAD layers fabricated by a standard wet approach and ii) a significant enhancement of the stability of the cells, which have been tested under continuous illumination during 40 h and after annealing in air up to 200 °C.⁵

References:

[1] Kim, J. Y.; Lee, J.-W.; Jung, H. S.; Shin, H.; Park, N.-G. High-Efficiency Perovskite Solar Cells. Chem. Rev. 2020, 52.

[2] Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338 (6107), 643–647.

[3] Liu, S.; Guan, Y.; Sheng, Y.; Hu, Y.; Rong, Y.; Mei, A.; Han, H. A Review on Additives for Halide Perovskite Solar Cells. Advanced Energy Materials 2020, 10 (13), 1902492.

[4] Shi, D.; Qin, X.; Li, Y.; He, Y.; Zhong, C.; Pan, J.; Dong, H.; Xu, W.; Li, T.; Hu, W.; Brédas, J.-L.; Bakr, O. M. Spiro-OMeTAD Single Crystals: Remarkably Enhanced Charge-Carrier Transport via Mesoscale Ordering. Science Advances 2016, 2 (4), e1501491.

[5] Barranco, A.; Lopez‐Santos, M. C.; Idigoras, J.; Aparicio, F. J.; Obrero‐Perez, J.; Lopez‐Flores, V.; Contreras‐Bernal, L.; Rico, V.; Ferrer, J.; Espinos, J. P.; Borras, A.; Anta, J. A.; Sanchez‐Valencia, J. R. Enhanced Stability of Perovskite Solar Cells Incorporating Dopant‐Free Crystalline Spiro‐OMeTAD Layers by Vacuum Sublimation. Advanced Energy Materials 2020, 10 (2), 1901524.

Acknowledgements:

We thank the AEI-MICINN (PID2019-110430GB-C21 and PID2019-109603RA-I0), the Consejería de Economía, Conocimiento, Empresas y Universidad de la Junta de Andalucía (PAIDI-2020 through projects US-1263142, ref. AT17-6079, P18-RT-3480). We also thank the European Union H2020 program under the grant agreement 851929 (ERC Starting Grant) and 899532 (FETOPEN-2018-2019-01) and through cohesion fund and FEDER 2014–2020 programs for financial support. JS-V thanks the University of Seville through the VI PPIT-US and the Ramon y Cajal Spanish National programs. Beam times at DESY (Hamburg, Germany) and at Elettra ring (Trieste, Italy) are acknowledged. The authors acknowledge DESY, a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA-III, at the P03/MinaXS beamline and the authors would like to thank Dr. Björn Beyersdorff for assistance.

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