Proceedings of International Conference Asia-Pacific Hybrid and Organic Photovoltaics (AP-HOPV17)
Publication date: 7th November 2016
Perovskite solar cells (PSCs) have captivated researchers due to high efficiencies and easy fabrication. Hole transport layer (HTL) is key to attaining higher efficiencies in PSCs. Currently the most widely used HTL is spiro-MeOTAD. The addition of LiTFSI and t-BP dopants to spiro-MeOTAD HTL is necessary to achieve desirable hole transport properties. Interestingly, it is common knowledge that several hours of ambient air exposure are needed to reach highest solar cell performance. Using photoemission spectroscopy and mercury drop electrode I–V measurements on hole-only devices, we investigated the influences of H2O vapor (RH 90%, RT 25 ºC), dry O2, and ambient air (RH 50%, RT 25 ºC) on charge transport properties of spiro-MeOTAD films doped with LiTFSI and t-BP. Our results reveal that H2O vapor exposure resulted in a substantial and irreversible enhancement in conductivity. On the other hand, O2 exposure resulted in a reversible enhancement under applied bias in the devices. XPS results reveal that freshly prepared spiro-MeOTAD shows a very low concentration of LiTFSI dopants at the top surface and air exposure causes some characteristic elements (e.g., F, S, and Li) belonging to LiTFSI to migrate to the top surface. Furthermore, XPS reveals that humidity is the main force driving the redistribution of the LiTFSI dopants. The migration of LiTFSI dopants across the bulk film seems to be facilitated by two types of pinholes with different sizes, which is confirmed by AFM, SEM, and optical microscopy. In summary, our results reveal the mechanisms that drive the conductivity enhancement of LiTFSI-doped spiro-MeOTAD HTLs, eventually leading to PCEs’ enhancement in perovskite solar cells. [1, 2]
[1] Hawash, Z.; Ono, L. K.; Qi, Y. B., Moisture and Oxygen Enhance Conductivity of LiTFSI-Doped Spiro-MeOTAD Hole Transport Layer in Perovskite Solar Cells. Advanced Materials Interfaces 2016, 3 (13).
[2] Hawash, Z.; Ono, L. K.; Raga, S. R.; Lee, M. V.; Qi, Y. B., Air-Exposure Induced Dopant Redistribution and Energy Level Shifts in Spin-Coated Spiro-MeOTAD Films. Chem. Mater. 2015, 27 (2), 562-569.
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