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In situ observation of heat-induced degradation of perovskite solar cells

Nature Energy volume 1, Article number: 15012 (2016) Cite this article

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Abstract

The lack of thermal stability of perovskite solar cells is hindering the progress of this technology towards adoption in the consumer market. Different pathways of thermal degradation are activated at different temperatures in these complex nanostructured hybrid composites. Thus, it is essential to explore the thermal response of the mesosuperstructured composite device to engineer materials and operating protocols. Here we produce devices according to four well-established recipes, and characterize their photovoltaic performance as they are heated within the operational range. The devices are analysed using transmission electron microscopy as they are further heated in situ, to monitor changes in morphology and chemical composition. We identify mechanisms for structural and chemical changes, such as iodine and lead migration, which appear to be correlated to the synthesis conditions. In particular, we identify a correlation between exposure of the perovskite layer to air during processing and elemental diffusion during thermal treatment.

Solar cells based on lead halide perovskite composites have become increasingly popular in the past few years owing to a combination of low synthesis cost and high power conversion efficiency, with certified values in excess of 20% (refs 1,2,3,4,5). However, the stability of such devices is a concern—it is well known that heating at or above around 85 ∘C, a temperature close to those reached during normal operation in full sunlight, performance degrades rapidly, and such instability is exacerbated by exposure to moisture6,7,8; systematic thermal and ageing studies are required to understand such degradation processes9. Changes happen in both the organic and inorganic components of the cells; the resilience of the perovskite layer, in particular, is expected to become a limiting factor once different hole-conducting materials (or hole-conductor-free cells) are developed10. To overcome this limitation, it is vital to understand the degradation pathways of the structures involved, which here are observed at nanometre-scale spatial resolution in situ, inside an analytical scanning transmission electron microscope (STEM), while the composition is monitored with elemental mapping through energy-dispersive X-ray analysis (EDX). The analysis of such devices is challenging owing to several factors. The spatial dimensions relevant to the fabrication and the operation of the cells are in the 1–100 nm range, and the materials are easily damaged by exposure to an electron beam in a TEM, requiring careful tuning of the electron dose. The system also includes organic and inorganic components in an assembly with complex chemistry and morphology. Finally, the rapid changes to the devices in air and the low degradation temperatures pose an additional challenge to the experiment, which needs to be timed appropriately and carefully executed.

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Figure 1: Cross-sectional view.
Figure 2: Cross-sectional views under heating.
Figure 3: Thermal cycling.
Figure 4: Photovoltaic properties.
Figure 5: Temperature evolution of elemental migration.
Figure 6: STEM-HAADF signal evolution.

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Acknowledgements

G.D., S.C. and C.D. acknowledge funding from ERC under grant number 259619 PHOTO EM. C.D. acknowledges financial support from the EU under grant number 312483 ESTEEM2. F.M., L.C. and A.D.C. acknowledge funding from ‘Polo Solare Organico’ Regione Lazio, the ‘DSSCX’ MIUR-PRIN2010 and FP7 ITN ‘Destiny’. G.D. and S.C. thank F. de la Peña and P. Burdet for assistance with PCA analysis.

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Authors and Affiliations

  1. Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK

    G. Divitini, S. Cacovich & C. Ducati

  2. Department of Electronic Engineering, C.H.O.S.E. (Centre for Hybrid and Organic Solar Energy), University of Rome “Tor Vergata”, via del Politecnico 1, Rome 00133, Italy

    F. Matteocci, L. Cinà & A. Di Carlo

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  1. G. Divitini
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Contributions

G.D., F.M., A.D.C. and C.D. conceived and designed the experiment. F.M. produced the devices. F.M. and L.C. carried out photovoltaic characterization. G.D. and S.C. carried out sample preparation, electron microscopy and data analysis. All authors contributed to the discussion of the results and to the writing of the manuscript.

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Correspondence to G. Divitini.

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Supplementary information

Supplementary Information

Supplementary Figures 1-9 (PDF 861 kb)

Supplementary Video 1

Sample A: montage of STEM-HAADF images upon heating. (AVI 13147 kb)

Supplementary Video 2

Sample B: montage of STEM-HAADF images upon heating. (AVI 16477 kb)

Supplementary Video 3

Sample C: Montage of STEM-HAADF images upon heating. (AVI 30350 kb)

Supplementary Video 4

Sample D: montage of STEM-HAADF images upon heating. (AVI 26096 kb)

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Divitini, G., Cacovich, S., Matteocci, F. et al. In situ observation of heat-induced degradation of perovskite solar cells. Nat Energy 1, 15012 (2016). https://doi.org/10.1038/nenergy.2015.12

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