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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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.
The codes used for this study are available from the corresponding author upon reasonable request.
Hawash, Z., Ono, L. K. & Qi, Y. B. Recent advances in spiro-MeOTAD hole transport material and its applications in organic–inorganic halide perovskite solar cells. Adv. Mater. Interfaces 5, 1700623 (2018).
Bach, U. et al. Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature 395, 583–585 (1998).
Green, M. A., Ho-Baillie, A. & Snaith, H. J. The emergence of perovskite solar cells. Nat. Photon. 8, 506–514 (2014).
Tan, H. R. et al. Efficient and stable solution-processed planar perovskite solar cells via contact passivation. Science 355, 722–726 (2017).
Saliba, M. et al. Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy Environ. Sci. 9, 1989–1997 (2016).
Lee, C. P., Li, C. T. & Ho, K. C. Use of organic materials in dye-sensitized solar cells. Mater. Today 20, 267–283 (2017).
Cho, A. N. & Park, N. G. Impact of interfacial layers in perovskite solar cells. ChemSusChem 10, 3687–3704 (2017).
Cappel, U. B., Daeneke, T. & Bach, U. Oxygen-induced doping of spiro-MeOTAD in solid-state dye-sensitized solar cells and its impact on device performance. Nano Lett. 12, 4925–4931 (2012).
Abate, A. et al. Lithium salts as “redox active” p-type dopants for organic semiconductors and their impact in solid-state dye-sensitized solar cells. Phys. Chem. Chem. Phys. 15, 2572–2579 (2013).
Wang, S., Yuan, W. & Meng, Y. S. Spectrum-dependent spiro-OMeTAD oxidization mechanism in perovskite solar cells. ACS Appl. Mater. Interfaces 7, 24791–24798 (2015).
Nguyen, W. H., Bailie, C. D., Unger, E. L. & McGehee, M. D. Enhancing the hole-conductivity of spiro-OMeTAD without oxygen or lithium salts by using spiro(TFSI)2 in perovskite and dye-sensitized solar cells. J. Am. Chem. Soc. 136, 10996–11001 (2014).
Tan, B. et al. LiTFSI-free spiro-OMeTAD-based perovskite solar cells with power conversion efficiencies exceeding 19%. Ad. Energy Mater. 9, 1901519 (2019).
Boyd, C. C., Cheacharoen, R., Leijtens, T. & McGehee, M. D. Understanding degradation mechanisms and improving stability of perovskite photovoltaics. Chem. Rev. 119, 3418–3451 (2019).
van Reenen, S., Vitorino, M. V., Meskers, S. C. J., Janssen, R. A. J. & Kemerink, M. Photoluminescence quenching in films of conjugated polymers by electrochemical doping. Phys. Rev. B 89, 205206 (2014).
Bard, A. J. & Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications (Wiley, 2000).
Cardona, C. M., Li, W., Kaifer, A. E., Stockdale, D. & Bazan, G. C. Electrochemical considerations for determining absolute frontier orbital energy levels of conjugated polymers for solar cell applications. Adv. Mater. 23, 2367–2371 (2011).
Xu, B. et al. Tailor-making low-cost spiro[fluorene-9,9′-xanthene]-based 3D oligomers for perovskite solar cells. Chem 2, 676–687 (2017).
Benson, E. E., Kubiak, C. P., Sathrum, A. J. & Smieja, J. M. Electrocatalytic and homogeneous approaches to conversion of CO2 to liquid fuels. Chem. Soc. Rev. 38, 89–99 (2009).
Speight, J. Lange’s Handbook of Chemistry 16th edn, Ch. 2 (McGraw-Hill Education, 2005).
Chase, M. W. NIST-JANAF Thermochemical Tables 4th edn (American Institute of Physics, 1998).
Macdiarmid, A. G., Mammone, R. J., Kaner, R. B. & Porter, S. J. The concept of doping of conducting polymers: the role of reduction potentials. Philos. Trans. R. Soc. A 314, 3–15 (1985).
Forward, R. L. et al. Protocol for quantifying the doping of organic hole-transport materials. ACS Energy Lett. 4, 2547–2551 (2019).
Shirono, K., Morimatsu, T. & Takemura, F. Gas solubilities (CO2, O2, Ar, N2, H2, and He) in liquid chlorinated methanes. J. Chem. Eng. Data 53, 1867–1871 (2008).
IUPAC. Solubility Data Series Vol. 7, 311 (Pergamon, 1981).
IUPAC. Solubility Data Series Vol. 50, 257 (Pergamon, 1992).
Koppenol, W. H. & Rush, J. D. Reduction potential of the CO2/CO2•− couple. A comparison with other C1 radicals. J. Phys. Chem. 91, 4429–4430 (1987).
Armstrong, D. A. et al. Standard electrode potentials involving radicals in aqueous solution: inorganic radicals (IUPAC Technical Report). Pure Appl. Chem. 87, 1139–1150 (2015).
Zhu, Z., Shi, X., Fan, G., Li, F. & Chen, J. Photo-energy conversion and storage in an aprotic Li–O2 battery. Angew. Chem. Int. Ed. 58, 19021 (2019).
Gittleson, F. S. et al. Raman spectroscopy in lithium-oxygen battery systems. ChemElectroChem 2, 1446–1457 (2015).
Ryu, W. H. et al. Heme biomolecule as redox mediator and oxygen shuttle for efficient charging of lithium-oxygen batteries. Nat. Commun. 7, 12925 (2016).
Edström, K., Gustafsson, T. & Thomas, J. O. The cathode–electrolyte interface in the Li-ion battery. Electrochim. Acta 50, 397–403 (2004).
Deng, X. Y. et al. Surface chemistry of Cu in the presence of CO2 and H2O. Langmuir 24, 9474–9478 (2008).
Favaro, M. et al. Subsurface oxide plays a critical role in CO2 activation by Cu(111) surfaces to form chemisorbed CO2, the first step in reduction of CO2. Proc. Natl Acad. Sci. USA 114, 6706–6711 (2017).
Feng, N. N., He, P. & Zhou, H. S. Critical challenges in rechargeable aprotic Li–O2 batteries. Adv. Energy Mater. 6, 1502303 (2016).
Xie, J. F., Liu, Q., Huang, Y. Y., Wu, M. X. & Wang, Y. B. A porous Zn cathode for Li-CO2 batteries generating fuel-gas CO. J. Mater. Chem. A 6, 13952–13958 (2018).
Ma, S. Y. et al. Tailoring the components and morphology of discharge products towards highly rechargeable Li-CO/CO2 batteries. Chem. Commun. 54, 8072–8075 (2018).
Strehle, B., Solchenbach, S., Metzger, M., Schwenke, K. U. & Gasteiger, H. A. The effect of CO2 on alkyl carbonate trans-esterification during formation of graphite electrodes in Li-ion batteries. J. Electrochem. Soc. 164, A2513–A2526 (2017).
Lamberti, F. et al. Evidence of spiro-OMeTAD de-doping by tert-butylpyridine additive in hole-transporting layers for perovskite solar cells. Chem 5, 1806–1817 (2019).
Yaws, C. L. & Satyro, M. A. in The Yaws Handbook Of Vapor Pressure: Antoine Coefficients 2nd edn (ed. Yaws, C. L.) Ch. 1 (Gulf, 2015).
Wang, S. et al. Role of 4-tert-butylpyridine as a hole transport layer morphological controller in perovskite solar cells. Nano Lett. 16, 5594–5600 (2016).
Liu, B. et al. Recent advances in understanding Li-CO2 electrochemistry. Energy Environ. Sci. 12, 887–922 (2019).
Liu, G. L., Xi, X., Chen, R. L., Chen, L. P. & Chen, G. Q. Oxygen aging time: a dominant step for spiro-OMeTAD in perovskite solar cells. J. Renew. Sustain. Energy 10, 043702 (2018).
An, Y. et al. Perovskite solar cells: optoelectronic simulation and optimization. Sol. RRL 2, 1800126 (2018).
Schloemer, T. H., Christians, J. A., Luther, J. M. & Sellinger, A. Doping strategies for small molecule organic hole-transport materials: impacts on perovskite solar cell performance and stability. Chem. Sci. 10, 1904–1935 (2019).
Hawash, Z., Ono, L. K. & Qi, Y. B. Moisture and oxygen enhance conductivity of LiTFSI-doped spiro-MeOTAD hole transport layer in perovskite solar cells. Adv. Mater. Interfaces 3, 1600117 (2016).
Dawson, J. A. et al. Mechanisms of lithium intercalation and conversion processes in organic–inorganic halide perovskites. ACS Energy Lett. 2, 1818–1824 (2017).
Li, Z. et al. Extrinsic ion migration in perovskite solar cells. Energy Environ. Sci. 10, 1234–1242 (2017).
Stolterfoht, M. et al. Approaching the fill factor Shockley–Queisser limit in stable, dopant-free triple cation perovskite solar cells. Energy Environ. Sci. 10, 1530–1539 (2017).
Le Corre, V. M. et al. Charge transport layers limiting the efficiency of perovskite solar cells: how to optimize conductivity, doping, and thickness. ACS Appl. Energy Mater. 2, 6280–6287 (2019).
Luo, D. Y., Su, R., Zhang, W., Gong, Q. H. & Zhu, R. Minimizing non-radiative recombination losses in perovskite solar cells. Nat. Rev. Mater. 5, 44–60 (2020).
Kim, G. et al. A thermally induced perovskite crystal control strategy for efficient and photostable wide-bandgap perovskite solar cells. Sol. RRL 4, 2000033 (2020).
Lee, J. et al. A printable organic electron transport layer for low-temperature-processed, hysteresis-free, and stable planar perovskite solar cells. Adv. Energy Mater. 7, 1700226 (2017).
Kato, Y. et al. Silver iodide formation in methyl ammonium lead iodide perovskite solar cells with silver top electrodes. Adv. Mater. Interfaces 2, 1500195 (2015).
Almora, O., Aranda, C., Mas-Marza, E. & Garcia-Belmonte, G. On Mott–Schottky analysis interpretation of capacitance measurements in organometal perovskite solar cells. Appl. Phys. Lett. 109, 173903 (2016).
Snow, A. W., Barger, W. R., Klusty, M., Wohltjen, H. & Jarvis, N. L. Simultaneous electrical-conductivity and piezoelectric mass measurements on iodine-doped phthalocyanine Langmuir–Blodgett films. Langmuir 2, 513–519 (1986).
Frisch, M. J. et al. Gaussian 16 Rev. C.01 (Gaussian Inc., 2016).
Martínez, L., Andrade, R., Birgin, E. G. & Martinez, J. M. PACKMOL: a package for building initial configurations for molecular dynamics simulations. J. Comput. Chem. 30, 2157–2164 (2009).
Kim, D. Y. et al. Ni-stabilizing additives for completion of Ni-rich layered cathode systems in lithium-ion batteries: an ab initio study. J. Power Sources 418, 74–83 (2019).
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.
J.K., J.A.R., and A.D.T. have filed a PCT patent application.
Peer review information Nature thanks Jianfeng Lu and Tracy Schloemer for their contribution to the peer review of this work.
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Extended data figures and tables
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.
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.
Extended Data Fig. 3 J–V data for a solar cell with a CO2-treated spiro-OMeTAD:LiTFSI HTL before and after air exposure.
a, b, J–V curves of the solar cell before (a) and after (b) air exposure for 120 min.
Extended Data Fig. 4 J–V data for a solar cell with a pristine spiro-OMeTAD:LiTFSI HTL before and after air exposure.
a–e, J–V 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).
Extended Data Fig. 5 J–V data for a solar cell with an O2-treated spiro-OMeTAD:LiTFSI HTL before and after air exposure.
a, Summary of evolution of the J–V curve of the solar cell upon exposure to air. b–e, Separate J–V 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).
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.
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.
a–c, 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).
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.
Extended Data Fig. 10 Changes in the electrical conductivity of polymer:LiTFSI films with CO2 doping.
a–c, 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.
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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