A deeper look at hole-transporting materials: How a HTMs ionization potential effects the photovoltaic performance of perovskite solar cells

Michael McGehee^a, Rebecca Belisle^a, Eric Hoke^a, Andrew Bartynski^b, Mark Thompson^b

^aMaterials Science & Enginereing, Stanford University, 476 Lomita Mall, Stanford, CA 94305, United States

^bChemistry, University of Southern California, 3620 McClintock Ave, LA, CA, United States

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

Oral, Rebecca Belisle, presentation 149
Publication date: 5th February 2015

Since the recent emergence of perovskite solar cells and their demonstration as highly promising photovoltaics, much work has been done to tune and improve the perovskite device-architecture in the hopes of achieving even higher power conversion efficiencies. One main effort in changing the architecture of these PIN-type solar cells has been to replace the p-type selective contact, most commonly the organic hole-transport material (HTM) Spiro-OMeTAD, with an alternative material. Though there are several desirable reasons to replace Spiro-OMeTAD, the high manufacturing cost and poor device stability being chief among them, one that is often touted is the potential to achieve a higher Voc in perovskite solar cells by switching to a higher ionization potential HTM. In this study we investigate this last point, probing more deeply the effect of varying the ionization potential of the HTM on photovoltaic performance metrics including Voc. We first study this change by employing the drift and diffusion modeling package PC1D to investigate the behavior of simplified perovskite devices consisting of planar layers of TiO2, CH3NH3PbI3, and HTM. With this model we are able to define potential benefits of an altered HTM, most notably the potential for improved fill factors, as well as define design constraints for an improved HTM (such as the minimum HTM conductivity needed for improved efficiency). We then empirically tested the effect of changing ionization potential, replacing Spiro-OMeTAD (ionization potential of 5.0-5.2eV) with the organic small molecule α-NPD (ionization potential ~5.4eV). By evaluating PL quenching behavior, photovoltaic performance, and the light-dependence of Voc we can further understand the role of the hole-transport material in these perovskite photovoltaics. Overall this work lends useful insights into HTM optimization for devices made with the CH3NH3PbI3 perovskite, and looking forward can help us rationally design good architectures for alternative perovskites of varying band gaps.

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