Modulating Nanoscale Defect States in Halide Perovskite Films
Stuart Macpherson a, Andrew Winchester b, Elizabeth Tennyson a, Krzystof Galkowski a, Tiarnan Doherty a, Miguel Anaya a, Christopher Petoukhoff b, Michael Man b, Keshav Dani b, Samuel Stranks a
a Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
b Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, Kunigami-gun, Okinawa, Japan 904-0495
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, Stuart Macpherson, presentation 182
DOI: https://doi.org/10.29363/nanoge.hopv.2019.182
Publication date: 11th February 2019

The photovoltaic performance of world-leading organic-inorganic halide perovskite (OHP) solar cells remains limited by defective electronic states, which introduce non-radiative recombination pathways for charge carriers. In OHP thin films, it is emerging that nanoscale surface defects are the most prevalent and thus have greatest impact on luminescence and device efficiency [1-3].

We employ a state-of-the-art photoemission electron microscopy (PEEM [4,5]) setup to map local surface defect states on triple cation, mixed-halide perovskite ((CsFAMA)Pb(I0.83Br0.17)3) films with 30 nm spatial resolution. We detect a nanoscale population of defective grains which exhibit significant photoemission from intraband trap states. Integrating PEEM with time-resolved pump-probe spectroscopy enables us to monitor the rate and intensity of hole trapping into these defect sites. Confocal photoluminescence maps show a clear anti-correlation between areas of high photoluminescence intensity and the locations of defect-rich grains. We have previously shown the incorporation of potassium halides [6] or light and atmospheric treatments [7] can substantially increase luminescence yields of perovskite films, thereby reducing trap densities.

In this work we utilise light treatments in a variety of atmospheric conditions as a lever to control surface trap distribution. With PEEM, we observe the creation of nanoscale defect states during in situ illumination of the perovskite, in ultra-high vacuum conditions. Conversely, illumination in an oxygen-rich environment leads to a tuneable suppression of the photoemission from defect-rich sites. We show that the photoluminescence heterogeneity previously reported for perovskite films is inherently linked to the distribution of these nanoscale defects and can be similarly controlled.

Finally, we apply Kelvin probe force microscopy (KPFM) to elucidate the nature of defect-rich grains and reveal nanoscale variation in work function, namely local n-type regions which retain a prominent intraband density of states.

This work establishes a clear understanding of what factors impact defects on multiple time and length scales, providing guidelines for improved passivation and ultimately device performance.

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