Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites
Authors: Sofiia Kosar
Giorgio Divitini
Elizabeth M Tennyson
Felix U Kosasih
E Laine Wong
Vivek Pareek
Tiarnan A S Doherty
Keshav M Dani
Andrew J Winchester
Samuel D Stranks
Christopher E Petoukhoff
Paul A Midgley
Aron Walsh
Miguel Anaya
Zahra Andaji-Garmaroudi
Duncan N Johnstone
Mojtaba Abdi-Jalebi
Yu-Hsien Chiang
Ji-Sang Park
Stuart Macpherson
Caterina Ducati
Young-Kwang Jung
Julien Madéo
Michael K L Man
Journal: Nature (2020)
Discipline: Multidisciplinary
Keywords: Halide perovskite materials; optoelectronic applications; photovoltaic devices; power conversion efficiencies; thin films
Affiliations: Cavendish Laboratory, University of Cambridge, Cambridge, UK., Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, Onna-son, Japan., Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK., Institute for Materials Discovery, University College London, London, UK., Department of Materials, Imperial College London, London, UK., Department of Physics, Kyungpook National University, Daegu, South Korea., Department of Materials Science and Engineering, Yonsei University, Seoul, South Korea., Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, Onna-son, Japan. ., Cavendish Laboratory, University of Cambridge, Cambridge, UK. ., Department of Chemical Engineering and Biotechnology, University of Cambridge, Cambridge, UK. .

Abstract:  Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices1,2. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively3) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects4. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance5, perovskite devices still have many states deep within the bandgap that trap charge carriers and cause them to recombine non-radiatively. These deep trap states thus induce local variations in photoluminescence and limit the device performance6. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions7 and with local strain8, both of which make devices less stable9. Here we use photoemission electron microscopy to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor photoluminescence efficiency, we observe discrete, nanoscale trap clusters. By correlating microscopy measurements with scanning electron analytical techniques, we find that these trap clusters appear at the interfaces between crystallographically and compositionally distinct entities. Finally, by generating time-resolved photoemission sequences of the photo-excited carrier trapping process10,11, we reveal a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters. Our approach shows that managing structure and composition on the nanoscale will be essential for optimal performance of halide perovskite devices. 

Retracted: No

PubPeer comments:
 The authors claimed  We took care to minimize the total UV exposure during our experiments, such that we see negligible changes in the perovskite properties as a result of these measurements (see Methods and Extended Data Figs. 3 and 4). However, this is not the case. In Extended Data Fig. 3e 3f, the entire sample has shown multiple spots with intensified signals (for few example red arrows), but a yellow arrow indicates the decrease of intensity. Clearly the sample s properties have been altered during the measurement, rather than what the authors have claimed. The entire article is based on such measurements to identify nanoscale trap clusters, but the significant changes in Extended Data Fig. 3 are in the scale of micrometers. ![] Also there are significant changes in Extended Data Fig. 3g and Extended Data Fig. 4. Especially for Extended Data Fig. 4, they are the data from only few doses, but the changes resulted from these few measurements are already obvious. , 
 Dear Catenulifera Brachyconia, thank you for your comment seeking clarification of our work. Degradation of perovskite samples during measurement is obviously an important topic in the community. It is a subject that we paid close attention to during our study, and also covered extensively during the review process. Indeed, as you state, Extended Data Fig. 3e f shows some changes in the sample, but this is after intense exposure to PEEM conditions ( 7 hours measurement, with over an order of magnitude greater UV dosage than used for the results in the main manuscript, hence the label u2018Intense PEEMu2019 on the figures). In response to the review process, ***the point of Extended Data Fig. 3e f was precisely to increase the UV dosage such that the film begins to get affected***, as weu2019ve discussed in the section u2018Methods: Exposure Controlu2019. You have correctly pointed out some examples of these changes in your comment - increases in PEEM intensity in some regions and decreases in others. Extended Data Fig. 3g is the same dataset from Extended Data Fig. 3e f plotted to show the overall intensity changes more quantitatively. ***In contrast, under u2018normal PEEM conditionsu2019 we do not see changes in our measurement, u2013 obviously within our experimental error.*** For u2018normal PEEM conditionsu2019, we use lower UV intensity and shorter measurement times, e.g. 20 min for the pump probe data shown in Extended Data Fig. 4 (shown below). Again, in response to the review process, we studied this further u2013 Extended Data Fig. 4 not only shows a pump probe curve for the first 20 min, but also how it changes if we kept repeating the pump probe measurements again and again over the next 80 min. In the typical measurement time of under 20 min, we do not see a change in the response of the sample (as seen from the very similar Scan1 and Scan2). ***However, (as you have also correctly observed) for times longer than our typical experimental window (e.g. on the order of 80 min), we start to see changes of ~4% in the trap density***. Linearly extrapolating this, we can quantitatively estimate changes to the sample within one scan, i.e. the change in trap density over our 20 min measurement window for pump-probe is 1% - well within our experimental error. ![file] **Extended Data Fig. 4** In conclusion, although challenging, it is possible to do careful experiments with a low enough UV intensity and in a short enough time such that sample changes are relatively small. However, if you expose your samples to high UV intensity or long periods of UV exposure, your sample indeed begins to change. The reviewers were absolutely right in pushing us to explore the limits of sample change more quantitatively, and this is what you are seeing in Extended Data Fig. 3e, f g and in Extended Data Fig 4. ,