Air-Exposure Effects on Spin Coated Spiro-MeOTAD Films and Implication on Perovskite Solar Cell Stability

Michael V. Lee^a, Sonia R. Raga^a, Luis k. Ono^a, Zafer Hawash^a, Yabing Qi^a

^aEnergy Materials and Surface Sciences Unit (EMSS), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Kunigami-gun, Okinawa, Japan

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Zafer Hawash, 109
Publication date: 5th February 2015

Certified power conversion efficiency exceeding 20% has been reported for perovskite solar cells. However, most perovskite solar cells so far degrade rapidly under operation conditions, which results in a short lifetime. To better understand the degradation mechanisms, it is necessary to investigate on each individual layer in a perovskite solar cell. In this work we present insights on a commonly used hole transport layer (HTL) in the perovskite solar cells, which consists of 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) doped with Li-bis(trifluoromethanesulfonyl)-imide (LiTFSI). Doping properties of solution prepared films of spiro-MeOTAD was investigated by X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), and Atomic Force Microscopy (AFM) under air exposure. Our XPS results show that 3 hours of ambient air exposure (37% relative humidity) on doped spiro-MeOTAD films induces the migration of LiTFSI from the bottom of the film to the top surface. Based on UPS results, the highest occupied molecular orbital (HOMO) showed a shift towards high binding energies after the same period of exposure time followed by almost disappearance of the HOMO level after 18 hours of air exposure. High density of pinholes forming channels across the spiro-MeOTAD films were revealed by AFM and Scanning Electron Microscope (SEM). The presence of these pinholes may play a major role in the migration processes of the LiTFSI within the spiro-MeOTAD film as well as on the degradation processes of the active materials (i.e., perovskite). This is further confirmed by the rapid decreasing efficiency of perovskite solar cells with solution prepared doped spiro-MeOTAD layers when exposed to air.
Cross sectional SEM of doped spiro-MeOTAD film on Au. (a) pinholes form channels across the doped spiro-MeOTAD film indicated with arrows in high magnification image, and (b) the pinholes observed from the top surface of the film and from the cross section.
Hawash, Z., Ono, L. K., Raga, S. R., Lee, M. V., & Qi, Y. (2014). Air-Exposure Induced Dopant Redistribution and Energy Level Shifts in Spin-Coated Spiro-MeOTAD Films. Chem. Mater. doi: 10.1021/cm504022q

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