Planar perovskite solar cells with long-term stability using ionic liquid additives

Abstract

Solar cells based on metal halide perovskites are one of the most promising photovoltaic technologies1,2,3,4. Over the past few years, the long-term operational stability of such devices has been greatly improved by tuning the composition of the perovskites5,6,7,8,9, optimizing the interfaces within the device structures10,11,12,13, and using new encapsulation techniques14,15. However, further improvements are required in order to deliver a longer-lasting technology. Ion migration in the perovskite active layer—especially under illumination and heat—is arguably the most difficult aspect to mitigate16,17,18. Here we incorporate ionic liquids into the perovskite film and thence into positive–intrinsic–negative photovoltaic devices, increasing the device efficiency and markedly improving the long-term device stability. Specifically, we observe a degradation in performance of only around five per cent for the most stable encapsulated device under continuous simulated full-spectrum sunlight for more than 1,800 hours at 70 to 75 degrees Celsius, and estimate that the time required for the device to drop to eighty per cent of its peak performance is about 5,200 hours. Our demonstration of long-term operational, stable solar cells under intense conditions is a key step towards a reliable perovskite photovoltaic technology.

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Fig. 1: Device architecture and characterization.
Fig. 2: Compositional distribution of BMIMBF4 in the perovskite active layer and its impact on ion migration.
Fig. 3: Film stability and the interaction between PbI2 and BMIM-containing ionic liquids.
Fig. 4: Device stability under combined full-spectrum sunlight and heat stressing.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This work was funded in part by the UK Engineering and Physical Sciences Research Council (EPSRC; grants EP/M015254/2 and EP/M024881/1); the European Research Council (ERC) Starting Grant (717026); the Swedish Research Council Vetenskapsrådet (grant 330-2014-6433); the European Commission Marie Skłodowska-Curie action (grant INCA 600398); the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (faculty grant SFO-Mat-LiU 2009-00971); and the European Union’s Horizon 2020 research and innovation program under grant agreement 763977 of the PerTPV project. S. Bai is a VINNMER Fellow and Marie Curie Fellow. P.D. and Z.Y. acknowledge support from the China Scholarship Council (CSC). C.L. and S.H acknowledge financial support from the Bavarian State Ministry of Science, Research, and the Arts for the Collaborative Research Network ‘Solar Technologies go Hybrid’ and the German Research Foundation (DFG). M.K. acknowledges support from the Swiss National Science Foundation (grant cr23i2-162828). We thank H. Long, Z. Yan, C. Bao, N. Noel, B. Wenger, J. Ball and O. Inganäs for experimental assistance and discussions.

Reviewer information

Nature thanks Aditya Mohite, Shougen Yin and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

S. Bai, F.G. and H.J.S. conceived the idea of the project, designed the experiments, analysed the data and wrote the manuscript. S. Bai performed the fabrication, optimization and characterization of the films and solar cells. P.D. contributed to characterization of the films and stability tests of the devices. C.L. and S.H. characterized the ion-migration process. S. Bai, Z.W. and Z.Y. performed the XRD and SEM characterizations. M.K., F.F. and S. Buecheler conducted the ToF-SIMS measurements and analysed the data. X.L. and M.F. carried out the UPS and XPS measurements and analysed the data. S. Bai, Z.W. and N.S. performed the optical measurements. J.T-W.W. contributed to optimization of the positive–intrinsic–negative device architecture. All authors commented on the final version of the manuscript. F.G. and H.J.S. supervised the project.

Correspondence to Sai Bai or Feng Gao or Henry J. Snaith.

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Competing interests

H.J.S. is a co-founder, Chief Scientific Officer and a Director of Oxford PV Ltd. Oxford University has filed a patent related to the subject matter of this manuscript.

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Extended data figures and tables

Extended Data Fig. 1 Impact of BMIMBF4 concentration on device performance.

ae, Statistics of device parameters for solar cells on NiO/FTO substrates fabricated from perovskite precursors with a BMIMBF4 concentration ranging from 0 mol% to 1.2 mol% (with respect to lead atoms). The PCE (a), JSC (c), VOC (d) and fill factor (FF; e) were determined from the FB-to-SC JV scan curves of 20 cells for each condition. The SPO (b) was measured for 50 s at a fixed voltage near the MPP from the JV curves. The top and bottom stars show the maximum and minimum values, respectively; the open squares show mean values; and the boxes show the region containing 25–75% of the data, obtained from 20 cells for each condition. f, g, Light soaking during JV curve measurements of the control (f) and the device with 0.3 mol% BMIMBF4 (g). h, JV curves of an optimized solar cell with 0.3 mol% BMIMBF4 measured from FB to SC and back again, with a scan rate of 200 mV s−1. The inset shows the SPO curve for the device. i, Hysteresis in the JV curves of devices with increasingly higher concentrations of BMIMBF4 in the perovskite layer.

Extended Data Fig. 2 Characterization of perovskite film.

ae, Characteristics of the control film and the film containing BMIMBF4 (0.3 mol%). a, XRD patterns. b, Top-view SEM images. c, UV-Vis absorption and steady-state photoluminescence (PL) spectra. d, Time-resolved photoluminescence decay curves. e, Photoemission cut-off energy and valence-band region of the UPS spectra. Ef, Fermi level; VBM, valence-band maximum; WF, work function.

Extended Data Fig. 3 Perovskite solar cells on poly-TPD hole-conductor.

a, PCE statistics for perovskite solar cells on poly-TPD-coated FTO substrates fabricated from precursors without and with 0.3 mol% BMIMBF4. The PCEs were determined from FB-to-SC JV scan curves of 13 cells for each condition. The bottom and top stars represent the minimum and maximum values, respectively; the open squares represent mean values; and the boxes show the regions containing 25–75% of the data. b, JV curves for a device fabricated on poly-TPD/FTO with 0.3 mol% BMIMBF4 in the perovskite film, measured from FB to SC and back again with a scan rate of 200 mV s−1.

Extended Data Fig. 4 Device performance and film stability with different ionic additives.

a, PCE statistics for perovskite solar cells on NiO/FTO substrates fabricated from precursors without and with different ionic additives (0.3 mol%). b, Photographs of the non-encapsulated control and of devices with different ionic additives after ageing for 100 h under full-spectrum sunlight at 70–75 °C. c, PCE statistics for perovskite solar cells fabricated on bare NiO and BMIMBF4-modified NiO. The PCEs were determined from FB-to-SC JV scan curves of 15 or more cells from at least two different batches for each condition. The bottom and top stars represent the minimum and maximum values, respectively; the open squares represent mean values; and the boxes show the regions containing 25–75% of the data. d, XRD patterns of the fresh and aged perovskite films (under full-spectrum sunlight at 60–65 °C in ambient air) without ionic liquids on bare NiO and on BMIMBF4-modified NiO substrates.

Extended Data Fig. 5 Device stability performance under combined full-spectrum light and heat stressing.

a, Light soaking during JV measurements of a non-encapsulated device with 0.3 mol% BMIMBF4 in the perovskite layer after 77 h ageing at 60–65 °C in air. RH, relative humidity. b, PCE statistics for devices before and after encapsulation. The PCEs were determined from FB-to-SC JV scan curves of ten cells for each condition. The bottom and top stars represent the minimum and maximum values, respectively; the open squares represent mean values; and the boxes show the regions containing 25–75% of the data. c, JV curves for one device based on BMIMBF4-containing perovskite film before and after encapsulation. d, Stability performance of solar cells with and without BMIMBF4 in the perovskite film under full-spectrum sunlight at 60–65 °C. e, f, JV and SPO curves for one high-performance device based on BMIMBF4-containing perovskite film before (e) and after (f) ageing under full-spectrum sunlight at 60–65 °C.

Extended Data Fig. 6 Long-term stability performance of perovskite solar cells under combined full-spectrum light and elevated temperature.

ac, Evolution of device parameters for encapsulated perovskite solar cells during stability testing under full-spectrum sunlight stressing at 70–75 °C: a, JSC; b, FF; and c, VOC. The average device parameters and standard errors (error bars) were calculated from ten cells for devices with BMIMBF4 in the perovskite film (top eight cells for the SPO), and seven cells for the other two sets of devices (top four cells for the SPO), determined from the FB-to-SC JV scan curves. d, JV and SPO curves for one device with BMIMBF4 in the perovskite film after 105 h of ageing under full-spectrum sunlight at 70–75 °C.

Extended Data Fig. 7 Performance of the most-stable cell based on BMIMBF4-containing perovskite film.

a, b,Device performance before (a) and after (b) encapsulation. cf, Evolution of JV and SPO curves during a long-term stability test under full-spectrum sunlight at 70–75 °C, after ageing for: c, 360 h; d, 792 h; e, 1,122 h; and f, 1,885 h.

Extended Data Fig. 8 Methods for estimating T80 values.

a, For devices with an early ‘burn-in’ effect, we fit the stability performance data after the burn-in section to a straight line, and extrapolated the curve back to time zero to obtain the T = 0 efficiency. We then determined the lifetime to 80% of the T = 0 efficiency, that is, the T80 (ref. 34). b, For devices with a positive ‘light-soaking’ effect, we fit the stability data from the peak performance after the light-soaking section to a straight line. We calculated the lifetime to 80% of the peak efficiency and added the ‘light-soaking’ time to obtain the total T80 lifetime.

Extended Data Fig. 9 Operational stability of MAPbI3 solar cells under combined light and heat stressing.

ad, Evolution of device parameters during long-term stability testing under full-spectrum sunlight at 60–65 °C: a, PCE and SPO; b, JSC; c, VOC; and d, FF. The average device parameters and standard errors (error bars) were determined from peak FB-to-SC J–V scan curves for two and three different cells for devices with (two cells) and without (three cells) BMIMBF4 in the MAPbI3 perovskite film. During the region marked in blue (100–115 h), the chamber temperature was increased to 70–75 °C to evaluate the device degradation behaviour under elevated temperatures. eh, JV and SPO curves for the MAPbI3 device containing 0.3 mol% BMIMBF4 in the perovskite layer during ageing for different times: e, before ageing; f, after ageing for 115 h; g, after ageing for 210 h; h, after ageing for 405 h.

Extended Data Table 1 Summarized parameters of our perovskite solar cells
Extended Data Table 2 Comparison of our device with published perovskite solar cells

Supplementary information

Supplementary Table 1

This file contains detailed information about the chemicals used in this work.

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