Highly Luminescent and Stable Metal Halide Perovskite Devices via Graded Hole Transport Layers
Mojtaba Abdi-Jalebi a, M. Ibrahim Dar b, Satyaprasad P. Senanayak a, Henning Sirringhaus a, Michael Grätzel b, Richard H. Friend a
a Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
b Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Oral, Mojtaba Abdi-Jalebi, presentation 083
DOI: https://doi.org/10.29363/nanoge.hopv.2019.083
Publication date: 11th February 2019

Despite rapid improvements in power conversion efficiency (PCE) over the past few years, the long-term stability of perovskite solar cells (PSCs) remains a pressing challenge that hinders their commercialisation. One source of instability in these devices is interfacial defects, in particular, those that exist between the perovskite and the hole transport layer (HTL). Here, we demonstrate that thermally evaporated dopant-free tetracene on top of the perovskite layer, capped with a doped Spiro-OMeTAD layer and top gold electrode offers an excellent hole-extracting stack with minimal interfacial defect levels.  However, we and others find that dopant-free organic semiconductor HTLs introduce undesirable injection barriers to the metal electrode. By capping 120 nm of tetracene with 200 nm solution-processed lithium TFSI - doped Spiro-OMeTAD, we demonstrate a graded hole injection interface to the top gold layer with enhanced ohmic extraction. For a perovskite layer interfaced between this graded HTLs structure and a mesoporous TiO2 electron-extracting layer its external photoluminescence yield reaches 15%, compared to 5% for the perovskite layer interfaced between TiO2 and Spiro-OMeTAD alone. For complete solar cell devices containing tetracene/Spiro-OMeTAD as the HTL with graded doping profile, we demonstrate PCEs of up to 21.5% and extended power output over 550 hours continuous illumination at AM1.5 retaining more than 90% of the initial performance, validating our approach. Our findings represent a breakthrough in the construction of stable PSCs with minimized non-radiative losses.

M.A.-J. thanks Cambridge Materials Limited for their funding and technical support. The authors thank the Engineering and Physical Sciences Research Council (EPSRC, grant number: EP/M005143/1) for support. M.I.D. and M.G. thank the King Abdulaziz City for Science and Technology (KACST) for financial support. S.P.S. acknowledges Royal Society London for the Newton Fellowship.

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