Chemical Modifications and Passivation Approaches in Metal Halide Perovskite Solar Cells

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• Dr. Mojtaba Abdi-Jalebi, Wolfson College Research Fellow, University of Cambridge
• Wednesday 22 May 2019, 14:00-15:00
• Mott Seminar Room (531), Cavendish Laboratory, Department of Physics.

If you have a question about this talk, please contact Dr Kaveh Delfanazari.

Metal halide perovskite materials have shown a relatively fast evolution in the power conversion efficiency (PCE) reaching the level exceeding those of CIGS and CdTe and approaching those of crystalline silicon solar cells. However, low luminescence efficiency of metal halide perovskite in a complete device and non-radiative losses originating from sub gap charge carrier trap states on the grain surfaces (e.g. halide vacancies) are the main barriers against reaching the efficiency limit in solar cells. In addition, the long-term stability of perovskite solar cells (PSCs) remains a pressing challenge that hinders their commercialisation. Here, I will detail several new and promising passivation approaches through compositional modification and interface engineering aimed at eliminating the sources of instability and loss processes in metal halide perovskites. We demonstrate substantial mitigation of both non-radiative losses and photo-induced ion migration in perovskite films and interfaces by decorating the surfaces and grain boundaries with passivating potassium halide layers. We find significant enhancement in both micro-photoluminescence and photoluminescence quantum efficiency (e.g. internal yields exceeding 95%) while maintaining high mobilities, giving the elusive combination of both high luminescence and excellent charge transport translating into over 21% PCE of the PSCs with the entire elimination of hysteresis. We found that the main source of instability in PSCs is interfacial defects, in particular, those that exist between the perovskite and the hole transport layer (HTL). We then 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.

References:

[1] Mojtaba Abdi-Jalebi, et. al; Richard H Friend, Samuel D Stranks, “Maximising and Stabilising Luminescence in Halide Perovskite Device Structures Using Potassium-Halide Passivating Layers”, Nature 555, 497-501, (2018). https://www.nature.com/articles/nature25989

[2] Mojtaba Abdi-Jalebi, et. al; Richard H Friend, “Charge extraction via graded doping of hole transport layers gives highly luminescent and stable metal halide perovskite devices” Science Advances, 5, eaav2012 (2019). https://advances.sciencemag.org/content/5/2/eaav2012

This talk is part of the Semiconductor Physics Group Seminars series.

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