C 60 as an Efficient n‑Type Compact Layer in Perovskite Solar Cells Konrad Wojciechowski, † Tomas Leijtens, †,‡ Svetlana Siprova, †,§ Christoph Schlueter, ∥ Maximilian T. Hö rantner, † Jacob Tse-Wei Wang, † Chang-Zhi Li, ⊥ Alex K.-Y. Jen, ⊥ Tien-Lin Lee, ∥ and Henry J. Snaith* ,† † Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, United Kingdom ‡ Center for Nano Science and Technology@Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, 20133, Milan, Italy § Dipartimento di Fisica, Universita ̀ della Calabria, via Bucci, Rende, 87036, Italy ∥ Diamond Light Source Ltd, Harwell Science & Innovation Campus, Didcot, OX11 0DE, United Kingdom ⊥ Department of Materials Science & Engineering, University of Washington, Seattle, Washington 98195, United States * S Supporting Information ABSTRACT: Organic−inorganic halide perovskite solar cells have rapidly evolved over the last 3 years. There are still a number of issues and open questions related to the perovskite material, such as the phenomenon of anomalous hysteresis in current−voltage characteristics and long-term stability of the devices. In this work, we focus on the electron selective contact in the perovskite solar cells and physical processes occurring at that heterojunction. We developed efficient devices by replacing the commonly employed TiO 2 compact layer with fullerene C 60 in a regular n−i−p architecture. Detailed spectroscopic characterization allows us to present further insight into the nature of photocurrent hysteresis and charge extraction limitations arising at the n-type contact in a standard device. Furthermore, we show preliminary stability data of perovskite solar cells under working conditions, suggesting that an n-type organic charge collection layer can increase the long-term performance. O rganic−inorganic lead halide perovskites have been proven to be excellent materials for photovoltaic applications with certified power conversion efficiency (PCE) exceeding 20%. 1 Over the last three years, research into these materials has exploded, which places perovskites as the fastest growing photovoltaic research area with tangible commercial prospects in the near term. 2−8 Application for these materials extends into light emitting devices, 9−11 lasers, 12−14 and photodetectors. 15,16 Such rapid developments and break- throughs in enhancing efficiency in the solar cells were for a large part achieved by enhancing the crystalline nature of the perovskite films and with suitable interface designing and engineering in a n−i−p solar cell architecture. 5,17−22 The interfaces between the charge selective contacts and the perovskite itself can thus be seen as crucial parameters to explore in attempts to further improve device performance. One pertinent issue for perovskite solar cells is hysteretic behavior commonly occurring during current−voltage charac- terization. Current output measured during sweeping voltage bias across the terminals of the device is dependent on the direction and speed of scanning, which questions the reliability of extracted photovoltaic performance. 20,23−25 The magnitude of that phenomenon varies between different architectures. Numbers of possible origins have been suggested, including ferroelectric properties of perovskite material, 23,25,26 interface trap states 20,23 or ionic displacement. 23,24,27 We recently showed that via careful interface engineering, the contact between perovskite and charge extraction layers can be largely improved and hysteretic behavior significantly reduced. By modifying compact TiO 2 with a self-assembled fullerene monolayer, we increased the efficacy of photogenerated charge extraction. 20 We have also recently demonstrated that the hysteretic effect appears to be caused by a long-lived polarization, temporarily changing the electric field profile and charge accumulation within the device to favor charge extraction at the contacts. 28 Here, we present perovskite solar cells with an n-type selective contact based on a solution-processed fullerene compact layer, entirely removing the compact TiO 2 metal oxide layer as an electroactive component from the device architecture. The use of organic layers facilitates fabricating the entire solar cell at low temperatures (no need for the high- temperature sintering step required to obtain highly crystalline metal oxide layers), suitable for processing involving temper- ature sensitive substrates, including plastic foil and silicon solar cells. Furthermore, we show that replacing TiO 2 with an organic semiconductor improves charge extraction, influences hysteretic behavior of those cells, and has a large positive impact on steady-state efficiency at the maximum power point. Received: May 1, 2015 Accepted: May 28, 2015 Letter pubs.acs.org/JPCL © XXXX American Chemical Society 2399 DOI: 10.1021/acs.jpclett.5b00902 J. Phys. Chem. Lett. 2015, 6, 2399−2405