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CO2 doping of organic interlayers for perovskite solar cells

Abstract

In perovskite solar cells, doped organic semiconductors are often used as charge-extraction interlayers situated between the photoactive layer and the electrodes. The π-conjugated small molecule 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9-spirobifluorene (spiro-OMeTAD) is the most frequently used semiconductor in the hole-conducting layer1,2,3,4,5,6, and its electrical properties considerably affect the charge collection efficiencies of the solar cell7. To enhance the electrical conductivity of spiro-OMeTAD, lithium bis(trifluoromethane)sulfonimide (LiTFSI) is typically used in a doping process, which is conventionally initiated by exposing spiro-OMeTAD:LiTFSI blend films to air and light for several hours. This process, in which oxygen acts as the p-type dopant8,9,10,11, is time-intensive and largely depends on ambient conditions, and thus hinders the commercialization of perovskite solar cells. Here we report a fast and reproducible doping method that involves bubbling a spiro-OMeTAD:LiTFSI solution with CO2 under ultraviolet light. CO2 obtains electrons from photoexcited spiro-OMeTAD, rapidly promoting its p-type doping and resulting in the precipitation of carbonates. The CO2-treated interlayer exhibits approximately 100 times higher conductivity than a pristine film while realizing stable, high-efficiency perovskite solar cells without any post-treatments. We also show that this method can be used to dope π-conjugated polymers.

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Fig. 1: Gas-assisted doping of the hole-conducting material and optical properties of reaction products.
Fig. 2: Electronic structures of pristine and doped spiro-OMeTAD, and analysis of the precipitate produced during the CO2-assisted doping process.
Fig. 3: The proposed doping and precipitation reaction.
Fig. 4: Performance of perovskite solar cells using pristine and gas-treated hole conductors.
Fig. 5: Performance of perovskite solar cells using pristine or CO2-doped polymer interlayers.

Data availability

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

Code availability

The codes used for this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We acknowledge the National Science Foundation NSF-PECASE award (CBET-0954985) and New York University for partial support of this work. J.K. acknowledges support from the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2021R1A2C1008968, 2020R1A6A1A03038697); J.M. from the China Scholarship Council and F.A. from the NSF Fellowship program. This research used resources of the NYU Tandon Nanofabrication Cleanroom & Shared Instrumentation Facilities, and the Center for Functional Nanomaterials (CFN), which is a US DOE Office of Science Facility at Brookhaven National Laboratory under contract no. DE-SC0012704.

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Contributions

J.K. conceived the project, and it was supervised by A.D.T. J.K. and Y.S. designed the experiments. J.K. and J.A.R. discussed and analysed data. J.K. prepared samples and devices for XPS, CV, JV and conductivity studies, and conducted the measurements. J.K. and J.M. fabricated and measured solar cells. Y.W. and Hailiang Wang conducted GC-FID measurements. J.K. and Hang Wang prepared the samples for TGA–MS analysis. Y.S. and D.Y.K. carried out the DFT calculations. G.K., J.K., S.K. and K.L. set up the MPP tracking experiments and J.K. conducted them. A.K., T.S. and M.A.M. patterned electrodes for conductivity measurements. E.C. and J.R.K. set up the photoluminescence measurement system, and J.K., Hang Wang, E.C. and F.A. conducted photoluminescence measurements. T.-D.L. performed ToF-SIMS analysis. Hang Wang provided details for the reactions involving O2 or CO2. J.K., J.A.R. and A.D.T. wrote the draft of the paper, and all authors read and approved the paper.

Corresponding author

Correspondence to André D. Taylor.

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Competing interests

J.K., J.A.R., and A.D.T. have filed a PCT patent application.

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Peer review information Nature thanks Jianfeng Lu and Tracy Schloemer for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Possible doping routes.

a, Energy band diagram of spiro-OMeTAD and gas reduction potentials of O2 and CO2. Reduced or negatively charged gases will react with lithium ions, producing lithium oxide and carbonate. The HOMO level (−5.13 eV) of spiro-OMeTAD was estimated from the equation EHOMO = –(Eox + 4.5 eV), where the vacuum level (E = 0 eV) is equivalent to –4.5 V vs SHE15,16,17. The LUMO level (−2.05 eV) was estimated by adding the HOMO level and optical band gap (Egap)17. b, c, Possible detailed reaction paths for lithium oxide from the O2 bubbling process (b) and for lithium carbonate from the CO2 bubbling process (c). The redox potentials and Gibbs free energy (ΔfG°) of each reaction are calculated using values from the thermodynamic database27.

Extended Data Fig. 2 Optical data for CO2-treated spiro-OMeTAD:LiTFSI.

a, b, Transmittance and reflectance (a) and absorptance (b) spectra of the CO2-treated spiro-OMeTAD:LiTFSI film. The standardized absorptance ratio (SAR) is calculated from the ratio of absorptance at 521 nm and 407 nm.

Source data

Extended Data Fig. 3 JV data for a solar cell with a CO2-treated spiro-OMeTAD:LiTFSI HTL before and after air exposure.

a, b, JV curves of the solar cell before (a) and after (b) air exposure for 120 min.

Source data

Extended Data Fig. 4 JV data for a solar cell with a pristine spiro-OMeTAD:LiTFSI HTL before and after air exposure.

ae, JV curves of the solar cell upon exposure to air for 0 min (a), 10 min (b), 30 min (c), 60 min (d) and 120 min (e).

Source data

Extended Data Fig. 5 JV data for a solar cell with an O2-treated spiro-OMeTAD:LiTFSI HTL before and after air exposure.

a, Summary of evolution of the JV curve of the solar cell upon exposure to air. be, Separate JV data for the solar cell upon exposure to air for 0 min (b), 10 min (c), 30 min (d), 60 min (e) and 120 min (f).

Source data

Extended Data Fig. 6 Changes in the electrical conductivity of spiro-OMeTAD:LiTFSI films fabricated using pristine, O2- and CO2-bubbled spiro-OMeTAD:LiTFSI solutions, and the conductivity changes of pristine films over time upon exposure to air.

a, Electrical conductivities (σ) of the films fabricated with pristine, O2-treated and CO2-treated spiro-OMeTAD:LiTFSI solutions. b, Electrical conductivity evolution for a pristine spiro-OEMeTAD:LiTFSI film in air over time. Central bars or empty circles in the graphs represent average values obtained from 4 samples, and error bars show deviations.

Source data

Extended Data Fig. 7 TOF-SIMS 2D elemental mapping for Li ion.

Green dots represent Li ions. The scale bar for the Li ion signal intensity in each map was adjusted for the best viewing level. a, The density of Li ions is high, forming clusters in the pristine spiro-OMeTAD:LiTFSI film. b, There are few Li ions in the CO2-treated spiro-OMeTAD:LiTFSI film, because Li2CO3 precipitate (the doping by-product) was filtered out before film fabrication.

Extended Data Fig. 8 TOF-SIMS depth profiles.

ac, Solar cells with a pristine spiro-OMeTAD:LiTFSI HTL (a), a spiro-OMeTAD:LiTFSI:Co(III)TFSI HTL (b) and a CO2-treated spiro-OMeTAD:LiTFSI HTL (c).

Source data

Extended Data Fig. 9 MPP traces for solar cells with pristine spiro-OMeTAD:LiTFSI, spiro-OMeTAD:LiTFSI:Co(III)TFSI, and CO2-treated spiro-OMeTAD:LiTFSI as an HTL.

MPP was collected every 350 ms under continuous light illumination (100 mW cm−2, AM1.5G) with neither a UV cut-off filter nor a temperature controller. Raw data, including spikes that might originate from a lamp power instability, are presented.

Source data

Extended Data Fig. 10 Changes in the electrical conductivity of polymer:LiTFSI films with CO2 doping.

ac, CO2 doping enhances the electrical conductivities of polymer:LiTFSI films that are composed of P3HT:LiTFSI (a), PBDB-T:LiTFSI (b), PTAA:LiTFSI (c) and MEH-PPV:LiTFSI (d). Central bars or empty circles in the graphs represent average values obtained from 4 samples, and error bars show standard deviations.

Source data

Supplementary information

Supplementary Information

This file contains Supplementary Figures 1–9 and Supplementary Tables 1–4.

Supplementary Data

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Kong, J., Shin, Y., Röhr, J.A. et al. CO2 doping of organic interlayers for perovskite solar cells. Nature 594, 51–56 (2021). https://doi.org/10.1038/s41586-021-03518-y

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