Enhanced electrical properties of Li-salts doped mesoporous TiO2 in perovskite solar cells

•Systemic study of counter anions (TFSI−, CO32−, Cl−, and F−) with Li-salts•High optical and electrical properties with Li2CO3-doped TiO2•Strong chemical bonding with Li2CO3-doped TiO2 for a deep conduction band•Achieved over 25% PCE with Li2CO3-doped TiO2 Perovskite solar cells (PSCs) are promising photovoltaic technology products because of the rapid growth of their performance and a power conversion efficiency (PCE) of up to 25% . The high efficiency of PSCs mainly comes from the perovskite itself and also from its transport layer ability. The TiO2 material is one of the best ETL materials, which has a high charge mobility and a moderate band energy level. However, the ultimate PSCs efficiency—over 25% PSCs needs a higher property of ETL. One of the strategies to improve the characteristics of TiO2 is doping with various dopants. In this research, the effect of counter anions (TFSI−, CO32−, Cl−, and F−) with Li-salts doping in the TiO2 was systemically studied and characterized electrical and optical for achieving over 25% PCE. Li2CO3-doped TiO2 showed excellent electrical and optical properties, which directly connected to high PCE PSCs of over 25%. This doping property can open the ultimate PCE of PSCs field and be widely applied to various purposes and applications. The charge transferability of the electron transfer layers (ETLs) is important for achieving high performance in mesoscopic (mp) perovskite solar cells (PSCs). However, because of the low electron extraction efficiency of TiO2, lithium doping is essentially required. In this work, we compared the electrical properties of mp-TiO2 doped with Li-salts with different anions—LiTFSI, Li2CO3, LiCl, and LiF. Interestingly, we found that the anions of the Li-salt dopants affect the electrical properties of the ETLs and the solar cell performance. The Li2CO3 doping of mp-TiO2 led to conduction bands deeper than those of pristine mp-TiO2 or other doped mp-TiO2. The maximum efficiency of 25.28% and certified efficiency of 24.68% (Newport) was obtained by using Li2CO3 dopant. This optimization of the ETLs properties is expected to greatly contribute to the progress of PSCs by leading to further increases in their efficiency. The charge transferability of the electron transfer layers (ETLs) is important for achieving high performance in mesoscopic (mp) perovskite solar cells (PSCs). However, because of the low electron extraction efficiency of TiO2, lithium doping is essentially required. In this work, we compared the electrical properties of mp-TiO2 doped with Li-salts with different anions—LiTFSI, Li2CO3, LiCl, and LiF. Interestingly, we found that the anions of the Li-salt dopants affect the electrical properties of the ETLs and the solar cell performance. The Li2CO3 doping of mp-TiO2 led to conduction bands deeper than those of pristine mp-TiO2 or other doped mp-TiO2. The maximum efficiency of 25.28% and certified efficiency of 24.68% (Newport) was obtained by using Li2CO3 dopant. This optimization of the ETLs properties is expected to greatly contribute to the progress of PSCs by leading to further increases in their efficiency. Perovskite solar cells (PSCs) are one of the most promising photovoltaic technology products because of their outstanding properties, which include high absorption coefficiencies,1Hao F. Stoumpos C.C. Cao D.H. Chang R.P.H. Kanatzidis M.G. Lead-free solid-state organic–inorganic halide perovskite solar cells.Nat. Photonics. 2014; 8: 489-494Crossref Scopus (1878) Google Scholar excellent carrier transport characteristics with long electron and hole diffusion lengths,2Lee M.M. Teuscher J. Miyasaka T. Murakami T.N. Snaith H.J. 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They found that electrons in the perovskite film can be extracted more efficiently by LiTFSI-doped TiO2 than by conventional TiO2.20Liu D. Li S. Zhang P. Wang Y. Zhang R. Sarvari H. et al.Efficient planar heterojunction perovskite solar cells with Li-doped compact TiO2 layer.Nano Energy. 2017; 31: 462-468Crossref Google Scholar Grätzel et al. demonstrated that LiTSFI-doped TiO2 electrodes exhibit superior electronic properties by reducing the electronic trap states, enabling faster electron transport, and improving the power conversion efficiency (PCE) of the device from 17% to 19%, with negligible hysteresis.35Giordano F. Abate A. Correa Baena J.P. Saliba M. Matsui T. Im S.H. et al.Enhanced electronic properties in mesoporous TiO2 via lithium doping for high-efficiency perovskite solar cells.Nat. Commun. 2016; 7: 10379Crossref PubMed Scopus (619) Google Scholar Although it has been known that Li doping is helpful in improving the efficiency of PSCs, many studies have not been conducted for various types of Li-salts such as interstitial Li doping into the TiO2 or chemical Li-O-Ti bonding doping. Furthermore, the effect of the counter anions (TFSI−, CO32−, Cl−, and F−) with Li-salts doping in the mp-TiO2 was not systemically studied on the electrical properties of mp-TiO2 used as an ETLs.20Liu D. Li S. Zhang P. Wang Y. Zhang R. Sarvari H. et al.Efficient planar heterojunction perovskite solar cells with Li-doped compact TiO2 layer.Nano Energy. 2017; 31: 462-468Crossref Google Scholar,34Salazar-Villanueva M. Cruz-López A. Zaldívar-Cadena A.A. Tovar-Corona A. Guevara-Romero M.L. Vazquez-Cuchillo O. Effect of the electronic state of Ti on M-doped TiO2 nanoparticles (M=Zn, Ga or Ge) with high photocatalytic activities: a experimental and DFT molecular study.Mater. Sci. Semicond. Process. 2017; 58: 8-14Crossref Scopus (20) Google Scholar,35Giordano F. Abate A. Correa Baena J.P. Saliba M. Matsui T. Im S.H. et al.Enhanced electronic properties in mesoporous TiO2 via lithium doping for high-efficiency perovskite solar cells.Nat. Commun. 2016; 7: 10379Crossref PubMed Scopus (619) Google Scholar In this study, we investigated the effect of various counter anions of Li-salt dopants on the ETLs and the device characteristics by doping LiTFSI, Li2CO3, LiCl, and LiF into mp-TiO2 used as an ETLs in PSCs. The morphology, crystallographic properties, optical absorption characteristics, photoluminescence properties, electrochemical impedance spectra, current density-voltage (J–V) curves, and stability of the ETLs fabricated by using mp-TiO2 doped with various Li-salts were investigated. Interestingly, the doping of various Li-salts into ETLs strongly affected their electrical properties and device performance, depending on the counter anion with the Li-salt. This is greatly affected by whether only a pure Li-doped mpTiO2 is formed or residual anions remain together. When LiTFSI, Li2CO3, LiCl, and LiF were doped into mp-TiO2, Li2CO3-doped TiO2 showed excellent electrical and optical properties, which are especially directly related to the performance of the solar cells. Only Li2CO3 formed pure Li-doped mp-TiO2 without residual anions, and this purely Li-doped bond structure could generate deep conduction bands compared with the other counter anions (TFSI−, Cl−, and F−).36Mizuno M. Tanaka I. Adachi H. Chemical bonding in titanium-metalloid compounds.Phys. Rev. B. 1999; 59: 15033-15047Crossref Scopus (55) Google Scholar,37Shen Y. Søndergaard M. Christensen M. Birgisson S. Iversen B.B. Solid state formation mechanism of Li4Ti5O12 from an anatase TiO2 source.Chem. Mater. 2014; 26: 3679-3686Crossref Scopus (51) Google Scholar The highest-performing solar cells were fabricated with Li2CO3 and yielded a maximum PCE of 25.02%, with a certified PCE of 24.68%. After the device was fabricated, we confirmed that the efficiency as high as about 25% was maintained even after 500 h. This study provides a promising direction for the development of high-quality ETLs, and we expect this study to advance the development of PSCs. Figure 1A shows a schematic diagram of the PSCs based on mp-TiO2 doped with various Li-salts with different counter anions (TFSI−, CO32−, Cl−, and F−). To prepare Li-salt-doped mp-TiO2, we deposited mp-TiO2 as an ETLs and then coated an Li-salt solution onto it. Details of the procedures are described in the experimental procedures section. First, the morphologies of the mp-TiO2 films prepared with Li-salts with different anions were observed using scanning electron microscopy (SEM) images, as shown in Figures S1A–S1E. These SEM images show that the morphologies of the doped mp-TiO2 as an ETLs were not affected by Li-salt doping. Although no change in morphology was observed, we confirmed the change in the elemental composition due to the anions of Li-salts through X-ray photoelectron spectroscopy (XPS) images. The spectra in Figure S2A shows that the intensity of Ti 2p slightly decreased, regardless of the type of anion. The reason for this was considered to be because of Li ions being doped on mp-TiO2. These results are very similar with those of a previous study.38Qin J. Zhang Z. Shi W. Liu Y. Gao H. Mao Y. The optimum titanium precursor of fabricating TiO2 compact layer for perovskite solar cells.Nanoscale Res. Lett. 2017; 12: 640Crossref PubMed Scopus (23) Google Scholar Besides, in the case of anions such as TFSI, Cl, and F, the binding energy of Ti 2p showed a slight shift from 459.58 → 460.19 eV. However, in the case of Li2CO3, there was no peak shift in the binding energy of Ti 2p compared with the undoped mp-TiO2. Whereas, clear peaks corresponding to Cl 2p, F 1s, and S 2p emissions were observed for the TiO2 doped with LiCl, LiF, and LiTFSI, respectively, as shown in Figures S2B–S2D. This indicated that the anions of Li-salts were incorporated into the mp-TiO2 ETLs. Li2CO3-doped mp-TiO2 indicates that only Li is bound to mpTiO2 purely without anions. Furthermore, we found that the Li ion was doped to the entire mp-TiO2 layer from the depth profiling through XPS, as shown in Figure S3. Ultraviolet photoelectron spectroscopy (UPS) measurements demonstrate that the energy level of mp-TiO2 can be effectively modified by doping with Li-salts containing various anions. Figure 1B shows the energy-level diagrams extracted from the UPS data (Figure S4; Table S1) and the ultraviolet-visible (UV-vis) spectrum (Figure S5; Table S2). The vacuum levels of the samples were determined by linear extrapolation of the secondary electron cut-offs on the high-binding-energy side of the UPS spectra (15–18 eV). The valence-band maxima (VBMs) for the pristine mp-TiO2 and doped mp-TiO2 were extracted from the onset in the low-binding-energy side of the UPS spectra.20Liu D. Li S. Zhang P. Wang Y. Zhang R. Sarvari H. et al.Efficient planar heterojunction perovskite solar cells with Li-doped compact TiO2 layer.Nano Energy. 2017; 31: 462-468Crossref Google Scholar,35Giordano F. Abate A. Correa Baena J.P. Saliba M. Matsui T. Im S.H. et al.Enhanced electronic properties in mesoporous TiO2 via lithium doping for high-efficiency perovskite solar cells.Nat. Commun. 2016; 7: 10379Crossref PubMed Scopus (619) Google Scholar,39Heo J.H. You M.S. Chang M.H. Yin W. Ahn T.K. Lee S.-J. et al.Hysteresis-less mesoscopic CH3NH3PbI3 perovskite hybrid solar cells by introduction of Li-treated TiO2 electrode.Nano Energy. 2015; 15: 530-539Crossref Scopus (223) Google Scholar The conduction band minimum (CBM) level was estimated from the VBM and the measured optical band gaps extracted from the Tauc plots corresponding to the UV-vis absorption spectra (Figure S5B; Table S2). Compared with the CBM and VBM levels of pristine mp-TiO2, the mp-TiO2 doped with Li2CO3 showed the most significant change from −3.92 to −4.17 eV. As shown in Figure 1B, the mp-TiO2 doped with Li2CO3 exhibited the deepest CBM among the investigated samples. These results clearly show that the anions of the Li-salt dopants can change the energy level. Therefore, the doped mp-TiO2 was used as an efficient ETLs with the energy band alignment.36Mizuno M. Tanaka I. Adachi H. Chemical bonding in titanium-metalloid compounds.Phys. Rev. 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Systematically optimized bilayered electron transport layer for highly efficient planar perovskite solar cells (η = 21.1%).ACS Energy Lett. 2017; 2: 2667-2673Crossref Scopus (135) Google Scholar, 42Jeong J. Kim H. Yoon Y.J. Walker B. Song S. Heo J. Park S.Y. Kim J.W. Kim G.-H. Kim J.Y. Formamidinium-based planar heterojunction perovskite solar cells with alkali carbonate-doped zinc oxide layer.RSC Adv. 2018; 8: 24110-24115Crossref Google Scholar This method is helpful in elucidating and comparing the charge transport behavior of doped mp-TiO2. For the SCLC measurement, an electron-only diode comprising mp-TiO2 with and without the Li2CO3 dopant on fluorine-doped tinoxide (FTO) substrates was used to enable electron injection (inset Figure 2A). The log-log plot of the current-voltage curve in Figure 2A shows that current increases linearly with voltage in the low-bias-voltage region. In the high-bias-voltage region, current increases with a much higher slope. The transition point is the ohmic-to-trap-filled-limit voltage (VTFL) transition point, and is related to the trap density.20Liu D. Li S. Zhang P. Wang Y. Zhang R. Sarvari H. et al.Efficient planar heterojunction perovskite solar cells with Li-doped compact TiO2 layer.Nano Energy. 2017; 31: 462-468Crossref Google Scholar,43Luntz A.C. Viswanathan V. Voss J. Varley J.B. Nørskov J.K. Scheffler R. et al.Tunneling and polaron charge transport through Li2O2 in Li–O2 batteries.J. Phys. Chem. Lett. 2013; 4: 3494-3499Crossref Scopus (135) Google Scholar,44Jin M.-j. Jo J. Yoo J.-W. Impedance spectroscopy analysis on the effects of TiO2 interfacial atomic layers in ZnO nanorod polymer solar cells: effects of interfacial charge extraction on diffusion and recombination.Org. Electron. 2015; 19: 83-91Crossref Scopus (26) Google Scholar The value of the VTFL increases with increasing density of electron traps. From the I–V curves, we estimated that the VTFL value of devices based on pristine mp-TiO2 (0.537 V) is higher than that of devices based on Li2CO3-doped mp-TiO2 (0.455 V). According to the relation between electron traps and the VTFL, the density of electron traps in the pristine mp-TiO2 ETLs is much higher than that in Li2CO3-doped mp-TiO2. Li2CO3 doping clearly reduced the electron trap density. Furthermore, we calculated the electron mobility of the ETLs on the basis of the Mott-Gurney law. As a result, the Li2CO3-doped mp-TiO2 ETLs (extracted electron mobility of 2.24 × 10−5 cm2 V−1 S−1) shows a higher current than the pristine mp-TiO2 ETLs (extracted electron mobility of 5.72 × 10−6 cm2 V−1 S−1). We conducted electrochemical impedance spectroscopy (EIS) measurements to study the effect of charge transport and recombination with doped mp-TiO2 on the PSCs. EIS is a useful method to analyze interfacial properties, such as electron transport and recombination in solar cells.39Heo J.H. You M.S. Chang M.H. Yin W. Ahn T.K. Lee S.-J. et al.Hysteresis-less mesoscopic CH3NH3PbI3 perovskite hybrid solar cells by introduction of Li-treated TiO2 electrode.Nano Energy. 2015; 15: 530-539Crossref Scopus (223) Google Scholar In the EIS analysis, the high-frequency component is the signature of the transfer resistance (Rtr), and the low-frequency component is associated with the recombination resistance (Rrec). In this study, because the perovskite/HTL interface is identical for all devices, the only variable affecting the Rtr is the ETL/perovskite interface. Figure 2B shows the Nyquist plots of the EIS data recorded under dark conditions at 0.8 V. The plots for all of the solar cells show a single semicircle without transmission line (TL) behavior, irrespective of the various dopants. A TL pattern is observable when the transport resistance is lower than the recombination resistance.45Garcia-Belmonte G. Guerrero A. Bisquert J. Elucidating operating modes of bulk-heterojunction solar cells from impedance spectroscopy analysis.J. Phys. Chem. Lett. 2013; 4: 877-886Crossref PubMed Scopus (99) Google Scholar Therefore, the absence of a TL pattern indicates that all of the PSCs exhibit strong recombination, which can be interpreted by using the Gerischer impedance model.45Garcia-Belmonte G. Guerrero A. Bisquert J. Elucidating operating modes of bulk-heterojunction solar cells from impedance spectroscopy analysis.J. Phys. Chem. Lett. 2013; 4: 877-886Crossref PubMed Scopus (99) Google Scholar,46Liang P.W. Liao C.Y. Chueh C.C. Zuo F. Williams S.T. Xin X.K. Lin J. Jen A.K.-Y. Additive enhanced crystallization of solution-processed perovskite for highly efficient planar-heterojunction solar cells.Adv. Mater. 2014; 26: 3748-3754Crossref PubMed Scopus (1257) Google Scholar In addition, the presence of a single semicircle in the plots indicates that the interface contacts between the active layer and the mp-TiO2 layer are not likely to be rectifying contacts.39Heo J.H. You M.S. Chang M.H. Yin W. Ahn T.K. Lee S.-J. et al.Hysteresis-less mesoscopic CH3NH3PbI3 perovskite hybrid solar cells by introduction of Li-treated TiO2 electrode.Nano Energy. 2015; 15: 530-539Crossref Scopus (223) Google Scholar,46Liang P.W. Liao C.Y. Chueh C.C. Zuo F. Williams S.T. Xin X.K. Lin J. Jen A.K.-Y. Additive enhanced crystallization of solution-processed perovskite for highly efficient planar-heterojunction solar cells.Adv. Mater. 2014; 26: 3748-3754Crossref PubMed Scopus (1257) Google Scholar In these plots, the smaller diameter of the semicircle correlates with a lower recombination rate. Notably, the devices with LiCl- or Li2CO3-doped mp-TiO2 exhibit recombination resistances greater than those of devices with pristine mp-TiO2, whereas the device with LiF exhibits a lower recombination resistance. Interestingly, these trends show a strong correlation with both the CBM data obtained by UPS and the solar-cell device data (Figures 1B and S3). The greater recombination resistance in the solar cells with Li-doped mp-TiO2 can be explained by the fact that the doping process effectively modified the CBM level of t