Tailoring electric dipole of hole-transporting material p-dopants for perovskite solar cells

•Efficient doping of PTAA via regulatory molecular dipole•Surface doping of perovskite via oriented dipole•Trap states restrain via the oriented dipole•High efficiency of modules based on PTAA The doping of organic HTMs is significant in a broad range of electronic applications, including photovoltaics, transistors, and organic light-emitting diodes, especially influencing the performance and stability of PSCs. Herein, a metal-free cation united with a fluorine-containing anion is developed as an efficient p-dopant for PTAA, which changed the molecule dipole moment and ionic orientation on perovskite surface compared with Li-TFSI via altered ionic radius and interaction difference of both anion and cation on the perovskite surface. As a result, the PSCs deliver high efficiency of 22.86% (small area) and 19.13% (module, aperture area: 33.2 cm2) with enhanced stability. These results inspire bright futures for efficient p-dopants design within HTMs and open new avenues to stable PSC fabrication. Li-TFSI/t-BP are the most widely employed p-dopants for hole-transporting materials (HTMs) within the state-of-the-art perovskite solar cells (PSCs). The hygroscopicity and migration of these dopants, however, lead to devices with limited stability. To solve this problem, we report here on a diphenyl iodide cation and pentafluorophenyl boric acid anion-based dopant (DIC-PBA) with an oriented interfacial dipole moment as an alternative to Li-TFSI/t-BP. Theoretical and experimental data reveal that DIC-PBA exhibits deep doping of poly[bis(4-phenyl)(2,4,6-triMethylphenyl)aMine] (PTAA) and also creates p-doping of perovskite surface, which originates from ionic interactions-derived dipole arrangement that yields fast interfacial charge transport. The improved intrinsic stability of PSCs originates from the inhibition of dipole moment degeneration on the perovskite surface. Devices prepared with DIC-PBA yielded high efficiency of 22.86%, and the modules (aperture area: 33.2 cm2) efficiency reached 19.13%. Importantly, the storage stability also significantly improved exceeding to 90% after aging 1,200 h under air ambient. Li-TFSI/t-BP are the most widely employed p-dopants for hole-transporting materials (HTMs) within the state-of-the-art perovskite solar cells (PSCs). The hygroscopicity and migration of these dopants, however, lead to devices with limited stability. To solve this problem, we report here on a diphenyl iodide cation and pentafluorophenyl boric acid anion-based dopant (DIC-PBA) with an oriented interfacial dipole moment as an alternative to Li-TFSI/t-BP. Theoretical and experimental data reveal that DIC-PBA exhibits deep doping of poly[bis(4-phenyl)(2,4,6-triMethylphenyl)aMine] (PTAA) and also creates p-doping of perovskite surface, which originates from ionic interactions-derived dipole arrangement that yields fast interfacial charge transport. The improved intrinsic stability of PSCs originates from the inhibition of dipole moment degeneration on the perovskite surface. Devices prepared with DIC-PBA yielded high efficiency of 22.86%, and the modules (aperture area: 33.2 cm2) efficiency reached 19.13%. Importantly, the storage stability also significantly improved exceeding to 90% after aging 1,200 h under air ambient. Organic-inorganic hybrid perovskite-based solar cells (PSCs) have drawn significant attention because of the rapid increase in power conversion efficiency (PCE) since 2009.1Kojima A. Teshima K. Shirai Y. Miyasaka T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Crossref PubMed Scopus (16126) Google Scholar, 2Im J.H. Lee C.R. Lee J.W. Park S.W. Park N.G. 6.5% efficient perovskite quantum-dot-sensitized solar cell.Nanoscale. 2011; 3: 4088-4093Crossref PubMed Scopus (2710) Google Scholar, 3Heo J.H. Im S.H. Noh J.H. Mandal T.N. Lim C.S. Chang J.A. Lee Y.H. Kim H.J. Sarkar A. Nazeeruddin M.K. et al.Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors.Nat. Photonics. 2013; 5: 486-491Crossref Scopus (2244) Google Scholar, 4Burschka J. Pellet N. Moon S.J. Humphry-Baker R. Gao P. Nazeeruddin M.K. Grätzel M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells.Nature. 2013; 499: 316-319Crossref PubMed Scopus (8074) Google Scholar, 5Cruz D. Garcia Cerrillo J. Kumru B. Li N. Dario Perea J. Schmidt B.V.K.J. Lauermann I. Brabec C.J. Antonietti M. Influence of thiazole-modified carbon nitride nanosheets with feasible electronic properties on inverted perovskite solar cells.J. Am. Chem. Soc. 2019; 141: 12322-12328Crossref PubMed Scopus (56) Google Scholar, 6Kim M. Motti S.G. Sorrentino R. Petrozza A. Enhanced solar cell stability by hygroscopic polymer passivation of metal halide perovskite thin film.Energy Environ. Sci. 2018; 11: 2609-2619Crossref Google Scholar, 7Hou Y. Du X. Scheiner S. McMeekin D.P. Wang Z. Li N. Killian M.S. Chen H. Richter M. Levchuk I. et al.A generic interface to reduce the efficiency-stability-cost gap of perovskite solar cells.Science. 2017; 358: 1192-1197Crossref PubMed Scopus (494) Google Scholar, 8Tan B. Raga S.R. Chesman A.S.R. Fürer S.O. Zheng F. McMeekin D.P. Jiang L. Mao W. Lin X. Wen X. et al.LiTFSI-free spiro-OMeTAD-based perovskite solar cells with power conversion efficiencies exceeding 19.Adv. Energy Mater. 2019; 91901519PubMed Google Scholar, 9Cao Y. Li Y. Morrissey T. Lam B. Patrick B.O. Dvorak D.J. Xia Z. Kelly T.L. Berlinguette C.P. Dopant-free molecular hole transport material that mediates a 20% power conversion efficiency in a perovskite solar cell.Energy Environ. Sci. 2019; 12: 3502-3507Crossref Google Scholar, 10Yoo J.J. Wieghold S. Sponseller M.C. Chua M.R. Bertram S.N. Hartono N.T.P. Tresback J.S. Hansen E.C. Correa-Baena J. Bulović V. et al.An interface stabilized perovskite solar cell with high stabilized efficiency and low voltage loss.Energy Environ. Sci. 2019; 12: 2192-2199Crossref Google Scholar, 11Gao X.X. Luo W. Zhang Y. Hu R. Zhang B. Züttel A. Feng Y. Nazeeruddin M.K. Stable and high-efficiency methylammonium-free perovskite solar cells.Adv. Mater. 2020; 32e1905502Crossref Scopus (111) Google Scholar, 12Kim M. Kim G. Lee T.K. Choi I.W. Choi H.W. Jo Y. Yoon Y.J. Kim J.W. Lee J. Huh D. et al.Methylammonium chloride induces intermediate phase stabilization for efficient perovskite solar cells.Joule. 2019; 3: 2179-2192Abstract Full Text Full Text PDF Scopus (1036) Google Scholar, 13Jeong J. Kim M. Seo J. Lu H. Ahlawat P. Mishra A. Yang Y. Hope M.A. Eickemeyer F.T. Kim M. et al.Pseudo-halide anion engineering for α-FAPbI3 perovskite solar cells.Nature. 2021; 592: 381-385Crossref PubMed Scopus (1579) Google Scholar, 14National Renewable Energy LaboratoryBest research cell efficiency.https://www.nrel.gov/pv/cell-efficiency.htmlGoogle Scholar In state-of-the-art n-i-p PSCs, the π-conjugated polymer poly[bis(4-phenyl)(2,4,6-triMethylphenyl)aMine] (PTAA) and a small molecule 2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (Spiro-OMeTAD) are widely used as p-type organic semiconductors, which facilitate hole extraction from the perovskite absorber.15Kong J. Shin Y. Röhr J.A. Wang H. Meng J. Wu Y. Katzenberg A. Kim G. Kim D.Y. Li T.D. et al.CO2 doping of organic interlayers for perovskite solar cells.Nature. 2021; 594: 51-56Crossref PubMed Scopus (76) Google Scholar,16Song S. Park E.Y. Ma B.S. Kim D.J. Park H.H. Kim Y.Y. Shin S.S. Jeon N.J. Kim T. Seo J. Selective defect passivation and topographical control of 4-dimethylaminopyridine at grain boundary for efficient and stable planar perovskite solar cells.Adv. Energy Mater. 2021; 112003382Crossref Scopus (59) Google Scholar Both of these hole-transporting materials (HTMs) are commonly doped with bis(trifluoromethane)sulfonamide lithium salt (Li-TFSI) and 4-tert-butylpyridine (t-BP) to improve their hole mobility and work function (WF). Despite the significant improvement in PCE achieved with these dopants, some drawbacks exist that prejudice the long-term stability of PSCs. The stability of PSC is now the most challenging issue that limits their commercialization. The Li-TFSI is a hydrophilic ionic compound, which adsorbs water from ambient air, resulting in aggregation of the lithium salt and a concurrent rapid degradation of the hole transportation layer (HTL) and decomposition of the perovskite layer via molecular diffusion from the HTL.17Caliò L. Salado M. Kazim S. Ahmad S. A generic route of hydrophobic doping in hole transporting material to increase longevity of perovskite solar cells.Joule. 2018; 2: 1800-1815Abstract Full Text Full Text PDF Scopus (112) Google Scholar,18Wang K. Liu X. Huang R. Wu C. Yang D. Hu X. Jiang X. Duchamp J.C. Dorn H. Priya S. Nonionic Sc3N@C80 dopant for efficient and stable halide perovskite photovoltaics.ACS Energy Lett. 2019; 4: 1852-1861Crossref Scopus (41) Google Scholar In addition, the interfacial t-BP will also cause chemical decomposition of the perovskite by forming a [PbI2-t-BP] coordinated complex over time, which further diminishes the long-term stability of PSCs.19Xu B. Zhu Z. Zhang J. Liu H. Chueh C. Li X. Jen A.K.-Y. 4-tert-butylpyridine free organic hole transporting materials for stable and efficient planar perovskite solar cells.Adv. Energy Mater. 2017; 71700683Crossref PubMed Scopus (104) Google Scholar,20Luo J. Xia J. Yang H. Chen L. Wan Z. Han F. Malik H.A. Zhu X. Jia C. Toward high-efficiency, hysteresis-less, stable perovskite solar cells: unusual doping of a hole-transporting material using a fluorine-containing hydrophobic Lewis acid.Energy Environ. Sci. 2018; 11: 2035-2045Crossref Google Scholar Thus, several groups have now reported on alternative p-dopants based on ionic liquids,17Caliò L. Salado M. Kazim S. Ahmad S. A generic route of hydrophobic doping in hole transporting material to increase longevity of perovskite solar cells.Joule. 2018; 2: 1800-1815Abstract Full Text Full Text PDF Scopus (112) Google Scholar,21Zhang J. Zhang T. Jiang L. Bach U. Cheng Y. 4-tert-Butylpyridine free hole transport materials for efficient perovskite solar cells: a new strategy to enhance the environmental and thermal stability.ACS Energy Lett. 2018; 3: 1677-1682Crossref Scopus (82) Google Scholar acid and inorganic salts,22Li Z. Tinkham J. Schulz P. Yang M. Kim D.H. Berry J. Sellinger A. Zhu K. Acid additives enhancing the conductivity of spiro-OMeTAD toward high-efficiency and hysteresis-less planar perovskite solar cells.Adv. Energy Mater. 2017; 71601451PubMed Google Scholar,23Jiang L. Wang Z. Li M. Li C. Fang P. Liao L. Flower-like MoS2 nanocrystals: a powerful sorbent of Li+ in the spiro-OMeTAD layer for highly efficient and stable perovskite solar cells.J. Mater. Chem. A. 2019; 7: 3655-3663Crossref Google Scholar fullerene derivatives,18Wang K. Liu X. Huang R. Wu C. Yang D. Hu X. Jiang X. Duchamp J.C. Dorn H. Priya S. Nonionic Sc3N@C80 dopant for efficient and stable halide perovskite photovoltaics.ACS Energy Lett. 2019; 4: 1852-1861Crossref Scopus (41) Google Scholar,24Jeon I. Ueno H. Seo S. Aitola K. Nishikubo R. Saeki A. Okada H. Boschloo G. Maruyama S. Matsuo Y. Lithium-ion endohedral fullerene (Li+@C60) dopants in stable perovskite solar cells induce instant doping and anti-oxidation.Angew. Chem. Int. Ed. Engl. 2018; 57: 4607-4611Crossref PubMed Scopus (79) Google Scholar and metal-organic frameworks25Dong Y. Zhang J. Yang Y. Qiu L. Xia D. Lin K. Wang J. Fan X. Fan R. Self-assembly of hybrid oxidant POM@Cu-BTC for enhanced efficiency and long-term stability of perovskite solar cells.Angew. Chem. Int. Ed. Engl. 2019; 58: 17610-17615Crossref PubMed Scopus (80) Google Scholar,26Li M. Xia D. Yang Y. Du X. Dong G. Jiang A. Fan R. Doping of [In2(phen)3Cl6]·CH3CN·2H2O indium-based metal-organic framework into hole transport layer for enhancing perovskite solar cell efficiencies.Adv. Energy Mater. 2018; 81702052Google Scholar to improve the stability and PCE of PSCs. However, these reports focused primarily on improving the hydrophobicity of the p-dopants, which determined the external stability of devices. In contrast, the Li+ ion always migrates to different layers of PSCs due to low migration barriers through perovskite, causing random doping and alteration of the charge equilibrium in PSCs.23Jiang L. Wang Z. Li M. Li C. Fang P. Liao L. Flower-like MoS2 nanocrystals: a powerful sorbent of Li+ in the spiro-OMeTAD layer for highly efficient and stable perovskite solar cells.J. Mater. Chem. A. 2019; 7: 3655-3663Crossref Google Scholar,27Ran C. Xu J. Gao W. Huang C. Dou S. Defects in metal triiodide perovskite materials towards high-performance solar cells: origin, impact, characterization, and engineering.Chem. Soc. Rev. 2018; 47: 4581-4610Crossref PubMed Google Scholar, 28Xiao Z. Yuan Y. Shao Y. Wang Q. Dong Q. Bi C. Sharma P. Gruverman A. Huang J. Giant switchable photovoltaic effect in organometal trihalide perovskite devices.Nat. Mater. 2015; 14: 193-198Crossref PubMed Scopus (1242) Google Scholar, 29Li Z. Xiao C. Yang Y. Harvey S.P. Kim D.H. Christians J.A. Yang M. Schulz P. Nanayakkara S.U. Jiang C. et al.Extrinsic ion migration in perovskite solar cells.Energy Environ. Sci. 2017; 10: 1234-1242Crossref Google Scholar As a consequence, the efficiency of PSCs decreases during long-term aging. To alleviate the Li+ migration problem, metal sulfides have been introduced into the HTL, thus inhibiting the Li+ ion motion via the strong interaction of the Li–S bond, thereby improving the stability of PSCs.23Jiang L. Wang Z. Li M. Li C. Fang P. Liao L. Flower-like MoS2 nanocrystals: a powerful sorbent of Li+ in the spiro-OMeTAD layer for highly efficient and stable perovskite solar cells.J. Mater. Chem. A. 2019; 7: 3655-3663Crossref Google Scholar Despite this significant progress in HTL p-dopant and additive research, the relevant contact interface is rarely explored in p-dopant design. The interface is compromised by interfacial barriers and uncoordinated perovskite surface defects that result in interfacial charge accumulation and recombination, thereby severely diminishing the intrinsic stability and performance.30Ansari F. Shirzadi E. Salavati-Niasari M. LaGrange T. Nonomura K. Yum J.H. Sivula K. Zakeeruddin S.M. Nazeeruddin M.K. Grätzel M. et al.Passivation mechanism exploiting surface dipoles affords high-performance perovskite solar cells.J. Am. Chem. Soc. 2020; 142: 11428-11433Crossref PubMed Scopus (75) Google Scholar, 31Kim H. Pei M. Lee Y. Sutanto A.A. Paek S. Queloz V.I.E. Huckaba A.J. Cho K.T. Yun H.J. Yang H. et al.Self-crystallized multifunctional 2D perovskite for efficient and stable perovskite solar cells.Adv. Funct. Mater. 2020; 301910620Google Scholar, 32Xia J. Luo J. Yang H. Zhao F. Wan Z. Malik H.A. Shi Y. Han K. Yao X. Jia C. Vertical phase separated cesium fluoride doping organic electron transport layer: A facile and efficient “bridge” linked heterojunction for perovskite solar cells.Adv. Funct. Mater. 2020; 302001418Crossref PubMed Scopus (38) Google Scholar, 33Wang B. Li H. Dai Q. Zhang M. Zou Z. Brédas J.L. Lin Z. Robust molecular dipole-enabled defect passivation and control of energy-level alignment for high-efficiency perovskite solar cells.Angew. Chem. Int. Ed. Engl. 2021; 60: 17664-17670Crossref PubMed Scopus (50) Google Scholar Furthermore, these surface and interfacial problem will be more seriously within the upscaling module devices due to the amplification effect. Thus, the influences of p-dopant on the interfacial contact and uncoordinated perovskite surface defects should be explored and taken into consideration in the design of efficient and stable p-dopants. Herein, a diphenyl iodide cation and pentafluorophenyl boric acid anion-based dopant (DIC-PBA) is developed as an efficient and stable p-dopant for PTAA. The dopant of DIC-PBA exhibits a tuned, molecular dipole moment and a preferential ionic orientation on the perovskite surface compared with Li-TFSI. This arises from an altered ionic radius and interaction difference of both anion and cation on the perovskite surface. Compared with Li-TFSI, the DIC-PBA exhibits excellent hydrophobicity and does not easily migrate, which diminishes the ingress of moisture and maintains the surface dipole stable during aging. Both n-i-p-type small area and series-interconnected modules (aperture area: 33.2 cm2) realized with DIC-PBA dopant achieved efficiencies of 22.86% and 19.13%, respectively. We believe that both module and small cell efficiency values are the one of the highest reported to date for n-i-p structured PSCs based on PTAA. The cells also display much higher long-term stability up to 1,200 h under ambient conditions without encapsulation (retain over 90%; however, the Li-TFSI based degenerated to 45% of initial efficiency). We believe that this work will provide new insights and indicate a new direction in the development of stable and efficient HTM dopant design for PSCs. As illustrated in Figure 1A, in addition to the notorious hydrophilicity of the Li-TFSI dopant, which always dominated the external stability of PSCs, the small radius of Li+ (0.7 Å) can easily migrate to other functional layers and damage the intrinsic stability of devices due to the resulting random doping (Figure 1Aii). Conversely, if a Li-free large radius ionic dopant is developed for PSCs, it may significantly improve the PSC’s intrinsic and external stability (Figure 1Ai). The large van der Waals radius of methyl diphenyl iodide cation (16.4 Å) not only exhibits the advantages of hydrophobicity and possible higher ion migration barrier (Figure S1) but also will increase the molecular dipole moment due to the larger distance between the ionic charge centers, which may result in strong electron acceptance and p-doping when in contact with PTAA. To enhance the dopant dipole moment and coulombically compensate for PTAA radicals after doping, the large van der Waals radius of the highly electronegative fluorine-containing anion (pentafluorophenyl boric acid anion; 13.75 Å) was selected and combined with DIC-PBA in this work (Figure 1B). To thoroughly analyze the hypothesis, the PTAA was first blended with DIC-PBA in toluene (TL) solvent, which may further improve the stability due to the absence of t-BP and acetonitrile. When the DIC-PBA was added into the PTAA solution (Figure 1F), the solution immediately turned reddish-brown, and an additional absorption peak in the range of 450–550 nm was observed. This peak is characteristic of the [PTAA]·+ radical via the charge transfer between polymer and dopant.20Luo J. Xia J. Yang H. Chen L. Wan Z. Han F. Malik H.A. Zhu X. Jia C. Toward high-efficiency, hysteresis-less, stable perovskite solar cells: unusual doping of a hole-transporting material using a fluorine-containing hydrophobic Lewis acid.Energy Environ. Sci. 2018; 11: 2035-2045Crossref Google Scholar To further confirm the possible electron transfer effect and the formation of this radical, the electron spin resonance (ESR) spectra were measured for DIC-PBA-doped PTAA and dopant-free PTAA solutions (Figure 1G). It was evident that the doped solution exhibited intense paramagnetic peaks at 3,510–3,520 G, which were absent for the un-doped material. These paramagnetic peaks verify the presence of the [PTAA]·+ radical. Furthermore, the calculation of electron spin density indicated widespread unpaired electrons over the entire [PTAA]·+ radical, which means the electrical conductivity will be improved in the presence of the radical.18Wang K. Liu X. Huang R. Wu C. Yang D. Hu X. Jiang X. Duchamp J.C. Dorn H. Priya S. Nonionic Sc3N@C80 dopant for efficient and stable halide perovskite photovoltaics.ACS Energy Lett. 2019; 4: 1852-1861Crossref Scopus (41) Google Scholar Thus, the electrical conductivity was probed by conductive atomic force microscopy (c-AFM), and the result showed that the sample doped with DIC-PBA displays strong current fluctuations compared with the Li-TFSI based film (Figure S2), which further shows the efficient doping by DIC-PBA. A discussion of the mechanism of radical generation by doping follows. First, the electronic properties of DIC-PBA and PTAA (simplified to two monomer units) were studied by density functional theory (DFT) and the calculations and shown in Figures 1C–1E and S3. It is evident from the ground-state orbitals that there is a separated electron-hole pair over the entire DIC-PBA molecule and that the hole (red) is dominantly localized at the pentafluorophenyl boric acid anion components; nevertheless, the electron (blue) is localized on the methyl diphenyl iodide cation, which may derive from the strong molecular dipole moment (23.26 Debye; Figure 1C) due to the non-overlapping B− and I+ charge sites. The PTAA molecule displays an inconspicuous separation of charge, which may originate from the weak dipole (1.74 Debye) around the entire molecule (Figure S3). However, the charge distribution of ground-state orbitals within PTAA become separated when contact is made with the DIC-PBA dopant, and the electron (blue) localized at the methyl diphenyl iodide cation inverse to the hole (red), which accumulates over the entire PTAA molecule. This is attributed to the extra electric field arising from the intermolecular dipole moment (21.34 Debye; Figure S3) between the PTAA core and dopant. It is analogous to the intramolecular charge transfer behavior of DIC-PBA and further validates that the dipole moment is important for the charge transfer process. As shown in Figure S4, the charge density difference (Δρ) is further calculated to directly observe the charge transfer process. The blue and yellow, respectively, are the electron depletion and accumulation. The electron dominantly accumulates at the contact interface and primarlily concentrates on the side of the methyl diphenyl iodide cation, in agreement with the preceding analysis. Therefore, the doping is probability derived from the intermolecular dipole between the PTAA core and the methyl diphenyl iodide cation. However, the pentafluorophenyl boric acid anion components interact with the [PTAA]·+ radical to maintain the charge compensation (Figure S4). In ionic doping systems, the WF of an organic semiconductor is calculated from the sum of μ and χ (Δφ = μ + χ). Here, the μ indicates the electrochemical potential of carriers at the Fermi level and is always governed by the electronic structure of the organic semiconductor core. The χ is the surface dipole potential supplied by surface ionic layering and dipole effects.34Kahn A. Fermi level, work function and vacuum level.Mater. Horiz. 2016; 3: 7-10Crossref Google Scholar,35Tang C.G. Ang M.C.Y. Choo K.K. Keerthi V. Tan J.K. Syafiqah M.N. Kugler T. Burroughes J.H. Png R.Q. Chua L.L. et al.Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts.Nature. 2016; 539: 536-540Crossref PubMed Scopus (167) Google Scholar The large dipole moment of DIC-PBA dopant may create deep doping and a shift of the WF. Ultraviolet photoelectron spectroscopy (UPS) is used to explore the dopant influence on the energy level of PTAA. The onsets (Eonset) and WF regions of UPS based on Li-TFSI and DIC-PBA are compared in Figure S5. The Δφ of PTAA doped with DIC-PBA (4.69 eV) is higher than that of Li-TFSI (4.65 eV) in accordance with the narrower energetic gap between the highest occupied molecular orbital (HOMO) and Fermi levels (EF) as observed from the Eonset region. These experimental and theoretical results confirm the efficient p-doping by the organic DIC-PBA provided by intermolecular electron transfer. The energetic alignment after the DIC-PBA and Li-TFSI doping, which balances EF, is compared in Figure 1H. Similar energy levels are observed for both Li-TFSI doped PTAA. In addition, the devices show a larger split of quasi-Fermi level (△EFq ≈ 0.04 eV) between the TiO2 film and PTAA when the DIC-PBA is employed as the dopant. The higher split of the quasi-Fermi level may be beneficial to the open-circuit voltage (Voc) of devices.36Shao Y. Yuan Y. Huang J. Correlation of energy disorder and open-circuit voltage in hybrid perovskite solar cells.Nat. Energy. 2016; 115001Crossref Scopus (578) Google Scholar,37Xia J. Luo J. Yang H. Sun C. Wan Z. Malik H.A. Zhang H. Shi Y. Jia C. Ionic selective contact controls the charge accumulation for efficient and intrinsic stable planar homo-junction perovskite solar cells.Nano Energy. 2019; 66104098Crossref Scopus (28) Google Scholar The surface electronic properties of the perovskite/HTM heterojunction interface are vital for carrier transport. However, the semiconductor surface contact is usually tuned by the adsorbed organic molecule and ionic charge layers via surface or molecular electron transfer, facilitating or suppressing the interfacial free-charge transport.30Ansari F. Shirzadi E. Salavati-Niasari M. LaGrange T. Nonomura K. Yum J.H. Sivula K. Zakeeruddin S.M. Nazeeruddin M.K. Grätzel M. et al.Passivation mechanism exploiting surface dipoles affords high-performance perovskite solar cells.J. Am. Chem. Soc. 2020; 142: 11428-11433Crossref PubMed Scopus (75) Google Scholar,31Kim H. Pei M. Lee Y. Sutanto A.A. Paek S. Queloz V.I.E. Huckaba A.J. Cho K.T. Yun H.J. Yang H. et al.Self-crystallized multifunctional 2D perovskite for efficient and stable perovskite solar cells.Adv. Funct. Mater. 2020; 301910620Google Scholar,38Gaulding E.A. Hao J. Kang H.S. Miller E.M. Habisreutinger S.N. Zhao Q. Hazarika A. Sercel P.C. Luther J.M. Blackburn J.L. Conductivity tuning via doping with electron donating and withdrawing molecules in perovskite CsPbI3 nanocrystal films.Adv. Mater. 2019; 31e1902250Crossref PubMed Scopus (60) Google Scholar To investigate the influence of the dopant on the perovskite surface, first, the behavior of both DIC-PBA and Li-TFSI adsorption on the perovskite surface was studied and modeled using DFT (Figure 2A). Here, a MAPbI3 perovskite slab as well as the PbI2 and methanaminium iodide (MAI) termination of (001) surfaces with 2 × 1 periodicity in the x-y plane was used for the adsorption models. For the PbI2 termination side, the Li+ of Li-TFSI was prone to fill the absent surface vacancies of MA+ and interact with surrounding I atoms at the center of the tetrahedral halide cage, leading to strong surface coordination,39You S. Wang H. Bi S. Zhou J. Qin L. Qiu X. Zhao Z. Xu Y. Zhang Y. Shi X. et al.A biopolymer heparin sodium interlayer anchoring TiO2 and MAPbI3 enhances trap passivation and device stability in perovskite solar cells.Adv. Mater. 2018; 30e1706924Crossref Scopus (186) Google Scholar, and the Li–I bonds exhibit short lengths of 2.75 and 2.80 Å (Figure S6). Meanwhile, the I+ cation of DIC-PBA coulombically interacted with the surface I− anion. It showed interatomic distances of 3.59 and 3.53 Å, resulting in weaker interactions compared with those of the above Li–I bonds. When the dopant was inverse to the anion side near to perovskite, both dopants of Li-TFSI and DIC-PBA formed Pb–F bonds with the perovskite surface. The formation energies (ΔEf) are defined as ΔEf = Es − Ez − ∑Ex where Es is the total energy of the perovskite supercell interacting with dopants, Ez is the total energy of the surface of the perovskite supercell, and Ex is the sum of the chemical potential of ions or the total molecular energy that is added or removed from the supercell (Figure 2B). The calculated ΔEf of Li+, DIC+, TFSI−, and PBA− sides of the dopants adsorbed on the PbI2 termination of (001) surfaces is −31.92 (Li+), −29.74 (DIC+), −34.8 (TFSI−), and −36.2 (PBA−) kcal/mol−1. This indicates that the anion of DIC-PBA preferentially forms on the surface compared with the Li-TFSI dopant and is prone to form forward orientation (F) of dipole (±) on the PbI2 termination surface. For the MAI termination surface, both sides of DIC+ and PBA− exhibit weak interactions with the perovskite surface via coulombic interactions and the N–H···F hydrogen bond, respectively,40Li N. Tao S. Chen Y. Niu X. Onwudinanti C.K. Hu C. Qiu Z. Xu Z. Zheng G. Wang L. et al.Cation and anion immobilization through chemical bonding enhancement with fluorides for stable halide perovskite solar cells.Nat. Energy. 2019; 4: 408-415Crossref Scopus (699) Google Scholar and they display similar ΔEf of −16.59 and −15.32 kcal/mol. However, the Li-TFSI cannot maintain a stable geometry of TFSI− anion orientation with respect to the MAI termination surface and exhibits a dipole parallel to the perovskite surface plane (Figure 2A). This may originate from the stronger electrovalent bond of Li–I compared with the hydrogen bond with MA+ cation, as well as the minor steric hindrance of Li+ ion, leading it to quickly move to the solid surface. Based on the results of these simulations, the DIC-PBA may display a forward orientation on the perovskite surface, but the Li-TFSI may exhibit a weaker orientation in this direction (Figure 2C). To further confirm the orientation of DIC-PBA, the sum frequency generation (SFG) vibrational spectrum had been employed for different concentrations of DIC-PBA deposited on the perovskite film. From the SFG spectrum (Figure S7), the –CH3 symmetric stretching mode (around 2,875 cm−1) and –CH3 Fermi resonance mode (around 2,960 cm−1) were visible in all the samples of DIC-PBA (different concentrations) deposited on perovskite film.41Zhang M. Chen Q. Xue R. Zhan Y. Wang C. Lai J. Yang J. Lin H. Yao J. Li Y. et al.Reconfiguration of interfacial energy band structure for high-performance inverted structure perovskite solar cells.Nat. Commun. 2019; 10: 4593Crossref PubMed Scopus (181) Google Scholar Specifically, the intensity of –CH3 became lower with the increasing of molecule concentrations, which indicated that the –CH3 group (DIC+ cation) was away from the perovskite surface when the concentrations of molecule