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.
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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
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