Cite as: N. Rolston et al., Science 10.1126/science.aay8691 (2020).

TECHNICAL COMMENTS

Publication date: 17 April 2020 www.sciencemag.org 1

Tsai et al. report a light-induced lattice expansion that leads to improved efficiency in metal halide perovskite solar cells (1). Upon illumination, the perovskite film was reported to exhibit a lattice expansion on the basis of wide-angle x-ray scattering (WAXS) data. A conductive heating stage was used to control film temperature; however, because of thermal insulation by a glass substrate, the measured perov-skite film temperatures likely differed from the actual film temperature (Fig. 1, A and B). According to their study, the perovskite film does not thermally expand under illumina-tion or with changing temperature, which does not agree with previous reports (2–7). This discrepancy in their meas-ured perovskite film temperature may provide a mechanism for their reported findings. The fundamental thermal ex-pansion properties of these films require that an increase in temperature cause a lattice expansion, and we show that the reported light-induced lattice expansion can be explained as a direct consequence of radiative heating.

We used a convective heating scheme in which the per-ovskite film temperature could be accurately controlled and measured (Fig. 1C) and would not be influenced by a ther-mally insulating substrate. We performed in situ film stress measurements that result from lattice expansion on an identical triple-cation pure-halide perovskite composition, as reported below, under illumination and in the dark. We observed an identical response in the perovskite film when heated convectively or by illumination, indicating a heat-induced lattice expansion from the thermal expansion of the perovskite. We repeated the WAXS experiment on a similar

FA0.76MA0.15Cs0.09PbI3 perovskite film (FA, formamidinium; MA, methyl ammonium) while controlling the film tempera-ture—instead of the substrate temperature—and detected a peak shift while heating in the dark but observed no peak shift while illuminated at a constant temperature.

The coefficient of thermal expansion (CTE) of hybrid perovskites has been measured as nearly an order of magni-tude larger than their glass substrates (2, 3). Recent work has shown that this CTE mismatch generates lattice strain in the film (4) and leads to a resulting elastic stress during film formation (5) and after annealing (6). Further, a large tensile film stress can develop in the perovskite film upon cooling to room temperature during processing as the low-er-CTE substrate constrains the perovskite from contracting (7). This tensile film stress accelerates degradation when exposed to environmental stressors such as light, heat, and moisture. Alternatively, compressive film stresses have been shown to inhibit degradation. We note finally that any heat-ing after fabrication—such as that induced by inefficiencies in photon absorption (8, 9)—would induce a lattice expan-sion in the perovskite and a resulting compressive stress.

We used an in situ film stress technique that uses a ca-pacitance measurement of the deflection of a glass cantile-ver with perovskite film where the film stress is directly related to the film lattice expansion (Fig. 1D). A controlled-temperature flow of N2 gas was used to accurately maintain the temperature and thermal equilibrium between the per-ovskite film and glass substrate (Fig. 1C). The resulting inert environment prevented degradation caused by moisture

Comment on “Light-induced lattice expansion leads to high-efficiency perovskite solar cells” Nicholas Rolston1*, Ross Bennett-Kennett2*, Laura T. Schelhas3, Joseph M. Luther4, Jeffrey A. Christians4, Joseph J. Berry4,5,6, Reinhold H. Dauskardt2† 1Department of Applied Physics, Stanford University, Stanford, CA, USA. 2Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA. 3Applied Energy Division, SLAC National Accelerator Laboratory, Menlo Park, CA, USA. 4National Renewable Energy Laboratory, Golden, CO, USA. 5Renewable and Sustainable Energy Institute, University of Colorado, Boulder, CO, USA. 6Department of Physics, University of Colorado, Boulder, CO, USA.

*These authors contributed equally to this work.

†Corresponding author. Email: dauskardt@stanford.edu

Tsai et al. (Reports, 6 April 2018, p. 67) report a uniform light-induced lattice expansion of metal halide perovskite films under 1-sun illumination and claim to exclude heat-induced lattice expansion. We show that by controlling the temperature of the perovskite film under both dark and illuminated conditions, the mechanism for lattice expansion is in fact fully consistent with heat-induced thermal expansion during illumination.

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ingress. The substrates used were glass cover slips (9 mm × 35 mm × 0.145 mm) with a thin evaporated 50-nm-thick conducting Cr-Au film on the backside for capacitance measurements. A 500-nm FA0.7MA0.25Cs0.05PbI3 perovskite film identical to that used in the Tsai et al. study was spin-coated on the topside of the glass. The perovskite film stress σf was measured using a modified Stoney’s equation (10):

2

s sf 2

s f

σ δ1 3

E tv L t

=−

(1)

where Es, vs, and ts are the Young’s modulus, Poisson’s ratio, and thickness of the glass substrate, respectively, tf is thick-ness of the perovskite film, and δ is the cantilever displace-ment from the detector at length L.

The perovskite film stresses were measured in situ in the dark at selected temperatures of 23°C and 30°C (Fig. 1F). After turning on the heated N2 flow, a rapid increase in the film compressive stress to –8 MPa was observed in the first several minutes as the chamber temperature heated and equilibrated at 30°C. This elastic film stress resulted from expansion of the perovskite lattice. Upon flowing room temperature N2 to cool the specimen to 22°C, the stress de-creased back to its initial value as the perovskite and sub-strate cooled to room temperature over similar times of several minutes. This process was repeatable and fully re-versible when cycled twice. When the same setup was used but with 1-sun illumination as the source of heating—which resulted in a perovskite film temperature of 29.5°C (Fig. 1E)—the same response involving a repeatable increase in film stress was observed (Fig. 1G). The identical stress re-sponse when heated to ~30°C by either illumination or con-vective heating indicates that the same mechanism (thermal expansion) was responsible.

To prove that thermal lattice expansion was responsible for the expansion, we performed WAXS at beam line 2-1 of the Stanford Synchrotron Radiation Lightsource on a simi-lar FA0.76MA0.15Cs0.09PbI3 perovskite film. Measurements were taken in a heated chamber held at a controlled temperature and purged with He to reduce beam damage. The thermo-couple was placed directly on the specimen to ensure that the temperature of the perovskite was accurately measured during heating. A shift in the (110) perovskite peak of 0.002 Å–1 was observed when heating from 25°C to 50°C in the dark to lower Q values, indicative of thermal expansion of the perovskite lattice (Fig. 2). Additionally, when the WAXS measurements where repeated under 1-sun illumination after an hour of light soaking with continuous He flow to control the temperature, no shift in the (110) peak was ob-served, indicating that the presence of light does not con-tribute to lattice expansion in the absence of heating. These

results indicate that thermal expansion can readily explain the previous results without the need to invoke any photo-carrier-based mechanism.

REFERENCES 1. H. Tsai, R. Asadpour, J.-C. Blancon, C. C. Stoumpos, O. Durand, J. W. Strzalka, B.

Chen, R. Verduzco, P. M. Ajayan, S. Tretiak, J. Even, M. A. Alam, M. G. Kanatzidis, W. Nie, A. D. Mohite, Light-induced lattice expansion leads to high-efficiency perovskite solar cells. Science 360, 67–70 (2018). doi:10.1126/science.aap8671 Medline

2. J. Feng, Mechanical properties of hybrid organic-inorganic CH3NH3BX3 (B = Sn, Pb; X = Br, I) perovskites for solar cell absorbers. APL Mater. 2, 081801 (2014). doi:10.1063/1.4885256

3. T. J. Jacobsson, L. J. Schwan, M. Ottosson, A. Hagfeldt, T. Edvinsson, Determination of Thermal Expansion Coefficients and Locating the Temperature-Induced Phase Transition in Methylammonium Lead Perovskites Using X-ray Diffraction. Inorg. Chem. 54, 10678–10685 (2015). doi:10.1021/acs.inorgchem.5b01481 Medline

4. J. Zhao, Y. Deng, H. Wei, X. Zheng, Z. Yu, Y. Shao, J. E. Shield, J. Huang, Strained hybrid perovskite thin films and their impact on the intrinsic stability of perovskite solar cells. Sci. Adv. 3, eaao5616 (2017). doi:10.1126/sciadv.aao5616

5. K. A. Bush, N. Rolston, A. Gold-Parker, S. Manzoor, J. Hausele, Z. J. Yu, J. A. Raiford, R. Cheacharoen, Z. C. Holman, M. F. Toney, R. H. Dauskardt, M. D. McGehee, Controlling Thin-Film Stress and Wrinkling during Perovskite Film Formation. ACS Energy Lett. 3, 1225–1232 (2018). doi:10.1021/acsenergylett.8b00544

6. N. Rolston, K. A. Bush, A. D. Printz, A. Gold-Parker, Y. Ding, M. F. Toney, M. D. McGehee, R. H. Dauskardt, Engineering Stress in Perovskite Solar Cells to Improve Stability. Adv. Energy Mater. 8, 1802139 (2018). doi:10.1002/aenm.201802139

7. C. Ramirez, S. K. Yadavalli, H. F. Garces, Y. Zhou, N. P. Padture, Thermo-mechanical behavior of organic-inorganic halide perovskites for solar cells. Scr. Mater. 150, 36–41 (2018). doi:10.1016/j.scriptamat.2018.02.022

8. M. Knudsen, The Path to 25% Silicon Solar Cell Efficiency: History of Silicon Cell Evolution. Prog. Photovolt. Res. Appl. 17, 183–189 (1934).

9. A. Wang, Y. Xuan, A detailed study on loss processes in solar cells. Energy 144, 490–500 (2018). doi:10.1016/j.energy.2017.12.058

10. G. G. Stoney, The Tension of Metallic Films Deposited by Electrolysis. Proc. R. Soc. London Ser. A 82, 172–175 (1909).

ACKNOWLEDGMENTS Supported by the U.S. Department of Energy (DOE) Solar Energy Technology Office (SETO) of the Energy Efficiency and Renewable Energy (EERE) award for the De-risking Halide Perovskite Solar Cells project at the National Renewable Energy La-boratory under contract DE-AC36-08-GO28308 managed and operated by the Alli-ance for Sustainable Energy LLC. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract DE-AC02-76SF00515. 9 December 2019; accepted 18 March 2020 Published online 17 April 2020 10.1126/science.aay8691

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Fig. 1. Illumination heats perovskite and induces thermal expansion. (A to C) Illustration of the conductive heating scheme used by Tsai et al., where a temperature gradient is observed between the perovskite and temperature-controlled stage during conductive heating (A) and illumination (B), compared with the thermal equilibrium achieved in our in situ film stress measurement setup and wide-angle x-ray scattering (WAXS) with convective heating (C). (D) Schematic of the capacitance-based cantilever displacement setup used for in situ measurements of perovskite film stress in an inert environment. (E) Thermal image of the FA0.7MA0.25Cs0.05PbI3 perovskite film on the glass cantilever (dashed black line) under illumination, reaching a 29.5°C temperature while the bottom of the glass as measured by a thermocouple is 24.4°C. (F and G) Measured stress values during cycles of 1-sun illumination with continuous flow of room-temperature N2 (F) and during cycles of heated and room-temperature N2 (G).

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Fig. 2. Heat-induced lattice expansion consistent with known material properties. (A) WAXS data for the (110) perovskite peak on glass during heating in the dark, where each spectrum was taken at a 5°C increase in temperature—from 25°C to 50°C—showing a thermal expansion of the lattice. The top spectra were taken an hour apart under 1-sun illumination, showing no light-induced lattice expansion. A slight broadening of the peak width was observed under illumination. (B and C) Peak positions of the WAXS spectra plotted with temperature (B) and with 1-sun illumination (C).

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Comment on ''Light-induced lattice expansion leads to high-efficiency perovskite solar cells''

H. DauskardtNicholas Rolston, Ross Bennett-Kennett, Laura T. Schelhas, Joseph M. Luther, Jeffrey A. Christians, Joseph J. Berry and Reinhold

DOI: 10.1126/science.aay8691 (6488), eaay8691.368Science 

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