Solution-Processable Ionic Liquid as an Independent or Modifying

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Solution-Processable Ionic Liquid as an Independent or Modifying Electron Transport Layer for High-Efficiency Perovskite Solar Cells Qiliang Wu, Weiran Zhou, Qing Liu, Pengcheng Zhou, Tao Chen, Yalin Lu, Qiquan Qiao, and Shangfeng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12683 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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ACS Applied Materials & Interfaces

Solution-Processable Ionic Liquid as an Independent or Modifying Electron Transport Layer for High-Efficiency Perovskite Solar Cells Qiliang Wu,#a Weiran Zhou,# a Qing Liu,a Pengcheng Zhou,a Tao Chen,a Yalin Lu,a Qiquan Qiao,b and Shangfeng Yang* a a

Hefei National Laboratory for Physical Sciences at Microscale, Key Laboratory of Materials

for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China (USTC), Hefei 230026, China b

Center for Advanced Photovoltaics, Department of Electrical Engineering and Computer Science, South Dakota State University, Brookings, SD 57007, USA

* Corresponding Author. E-mail: [email protected]. #

These authors contributed equally to this work.

ABSTRACT: Inorganic metal oxide especially TiO2 has been commonly used as an electron transport layer (ETL) in regular-structure (n-i-p) planar heterojunction perovskite solar cells (PHJ-PSCs), but generally suffers from high electron recombination rate and incompatibility with low-temperature solution-processability. Herein, by applying an ionic liquid (IL), 1ethyl-3-methylimidazolium hexafluorophosphate ([EMIM]PF6), as either a TiO2-modifying interlayer or an independent ETL, we investigated systematically IL interface engineering for PHJ-PSCs. Upon spin-coating [EMIM]PF6-IL onto TiO2 ETL as a modification layer, the average power conversion efficiency (PCE) of CH3NH3PbI3 PHJ-PSC devices reaches 1 ACS Paragon Plus Environment


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18.42±0.65%, which surpasses dramatically that based on commonly used TiO2 ETL (14.20±0.43%), and the highest PCE(19.59%) is almost identical to that of the record PCE for planar CH3NH3PbI3 PSCs (19.62%) reported very recently. On the other hand, by applying [EMIM]PF6-IL as an independent ETL, we achieved an average PCE of 13.25±0.55%, and the highest PCE (14.39%) approaches that obtained for PHJ-PSCs based on independent TiO2 ETL (14.96%). Both IL interface engineering methods reveal the effective electron transport of [EMIM]PF6-IL. The effects of [EMIM]PF6-IL on the surface morphology, crystallinity and optical absorption of the perovskite film and the interface between the perovskite layer and substrate were investigated and compared with the case of independent TiO2 ETL, revealing the role of [EMIM]PF6-IL in efficient electron transport.

Keywords: perovskite solar cells, interface engineering, ionic liquid, electron transport layer, dipolar interaction

Introduction Among thin film solar cells developed so far, organic-inorganic hybrid perovskite solar cells (PSCs) have attracted much attention since 2009 because of simple fabrication, large absorption coefficients, tunable bandgaps, high carrier mobility, and especially long charge carrier diffusion lengths,1-13 and the record efficiency exceeding 22% has been achieved.14 Planar heterojunction (PHJ) PSCs developed by Snaith et al. in 2013 have been popularly used as one of the most efficient configuration of PSCs,14 for which interface engineering has been demonstrated to be determinative for efficient charge transport. In particular, interfacial layers employed between organo-lead halide such as CH3NH3PbX3 (X= I, Br, Cl) and electrodes play important roles in adjusting energy level and electrical conductivity, passivating trap states in perovskite and improving the long-term stability.6,16-22

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In regular-structure (n-i-p) PHJ-PSCs, an electron transport layer (ETL) clings to the bottom transparent electrode while the perovskite layer is coated onto ETL. So far the most commonly used ETLs are inorganic metal oxides such as TiO2, ZnO, SnO2,WO3, SrTiO3 and Zn2SnO4.5,6,23-31 Despite that high efficiency of PHJ-PSCs has been obtained by using TiO2 ETL owing to the fast electron injection rate from the perovskite layer to TiO2 ETL, the low electron mobility and relatively high density of electronic trap states of TiO2 usually cause a high electron recombination rate.27,28,32,33 Modification of TiO2 layer by other materials, including alkali metal salts,34 conjugated polyelectrolytes,35 and small organic molecules such as functional self-assembled monolayers (SAMs),36-39 has been demonstrated to be effective in improving the electron transport property of TiO2 ETL.27,28 In particular, small organic molecules are advantegeous in solution-processability, and thus can be applied as ideal modifiers of TiO2 ETL.36-39 For instance, Snaith et al. used self-assembled fullerene monolayers to modify TiO2 compact layer, achieving a PCE of 17.3% due to the passivation effects of trap states formed at the TiO2/perovskite interface.36,37 Ogomi et al. inserted a HOCO-R-NH3+I- monolayer between a porous TiO2 layer and the perovskite layer and obtained an increase of PCE from 8% to 10%, and this was attributed to the promoted electron injection from perovskite into TiO2 due to the passivated surface traps of TiO2.38 More recently, Han et al. introduced an organic silane self-assembled monolayer to modify the TiO2/perovskite interface, leading to optimized interface band alignments and enhanced charge lifetime beneficial for improved PCE from 9.6% to 11.7%.39 Noteworthy, in these reports the interfacial monolayers were prepared by immersing the TiO2 films into the solutions containing small organic molecule materials for a relatively long time (up to 24 hours for the case of self-assembled fullerene monolayers36,37). Obviously, such treatments would considerably prolong the fabrication process, thus it is preferable to develop a more facile interface engineering route to modify TiO2 ETL. On the other hand, a high temperature (> 450 oC) sintering is generally required to improve the crystallinity of TiO 2 ETL,27,28 which 3 ACS Paragon Plus Environment


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is however energy consuming and thus unfavorable for commercial low-cost roll-to-roll fabrication of large area devices. Although a few methods toward low-temperature fabrication of TiO2 ETL have been recently reported, such as a nonhydrolytic sol-gel route6 and a magnetron sputtering process at room temperature,40 TiO2 ETL films fabricated at low temperature suffer from inferior electron transport and consequently lower device performance than those based on high-temperature treated TiO2 ETL. Therefore, it is desirable to substitute TiO2 ETL by new materials compatible with low-temperature solutionprocessed fabrication. Rare study on replace TiO2 ETL by organic materials has been reported. Very recently Snaith and Park et al. employed ethoxylated polyethylenimine (PEIE) as an independent ETL, resulting in a decent PCE of 15.0 % due to formation of a strong dipole moment at the interface between perovskite layer and FTO induced by the self-organized PEIE interlayer.35 However, to our knowledge, none of these interfacial materials was reported to function as both an independent ETL and a modification layer of TiO2 ETL. Herein, by applying an ionic liquid (IL), 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM]PF6), as either a solution-processable independent or a TiO2-modifying ETL of PHJPSCs, we investigate systematically IL interface engineering for PHJ-PSCs. First [EMIM]PF6-IL was spin-coated onto TiO2 compact layer so as to modify the TiO2 ETL, and the device performance based on IL-modified TiO2 ETL substantially surpasses that based on sole TiO2 ETL. Furthermore, upon spin-coating [EMIM]PF6-IL onto FTO substrate, an independent [EMIM]PF6-IL ETL was constructed, and its role in electron transport is comparable to the commonly used TiO2 ETL. To unveil the enhancement mechanism, we carried out a series of characterizations regarding the effects of [EMIM]PF6-IL on the optical absorption, crystallinity and morphology of the perovskite film as well as the interface between TiO2/perovskite. Experimental Section 4 ACS Paragon Plus Environment

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Materials. FTO-coated glass substrates with a sheet resistance of 7 Ω.sq-1 were purchased from NSG Group, Japan. CH3NH3I was synthesized following the procedure reported previously.41,42

PbI2,

lithium

bis(trifluoromethylsulphonyl)

imide

(Li-TFSI),

4-tert-

butylpyridine (tBP), hydriodic acid, methylamine solution, 1-hexanethiol, dimethyl sulfoxide (DMSO), chlorobenzene, isopropanol and acetonitrile were purchased from Alfa Aesar. [EMIM]PF6-IL was purchased from Shanghai ChengJie Chemical Co. LTD. Spiro-OMeTAD was purchased from 1M company. All chemicals were used as received. Device fabrication. Our detailed fabrication procedure of the control CH3NH3PbI3 PHJPSC devices has been reported previously.41,42 Briefly, the FTO-coated glass substrate was etched with Zn powder and 6 M HCl diluted in water, then ultrasonicated in detergent, deionized water, acetone and isopropanol for 15 min every time, and subsequently dried in an oven overnight. For PHJ-PSC devices based on independent [EMIM]PF6-IL ETL, [EMIM]PF6-IL was dissolved in methanol in variable concentrations (2-8 mg/mL), and was spin-coated onto FTO substrate at 4000 rpm, followed by annealing at 100 °C for 10 minutes. For the reference device, a compact TiO2 layer was deposited onto FTO by spin-coating a mixture solution of 350 L titanium isopropoxide, 5 mL ethanol and 65 L HCl (2 mol·L-1) at 2000 rpm, followed by annealing at 500 °C for 60 minutes. For the bilayer ETL configuration, [EMIM]PF6-IL was spin-coated onto TiO2 layer, followed by annealing at 100 °C for 10 minutes. CH3NH3PbI3 perovskite layer was fabricated onto FTO/IL or FTO/TiO2/IL substrate by a two-step method reported previously.41,42 A PbI2 solution (dissolved in dimethylsulfoxide (DMSO) with a concentration of 460 mg/mL was then spin-coated on top of the prepared substrate at 4000 rpm for 30 s. The coated substrate was dipped into a solution of CH3NH3I in isopropanol (10 mg/mL) for 10 minutes, and then washed by isopropanol and dried by spinning at 3000 rpm. Subsequently, the as-prepared substrate was heated at 100 oC for 10 minutes. After the substrate was cooled down to room temperature, Spiro-MeOTAD was 5 ACS Paragon Plus Environment


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subsequently deposited by spin-coating at a speed of 3000 rpm for 30 s. The HTL solution was prepared by dissolving 73.2 mg of Spiro-MeOTAD, 28.8 µL of 4-tert-butylpyridine (tBP), 18.8 µL of a 520 mg mL−1 lithium-bis (trifluoromethanesulfonyl) imide (Li-TFSI) in acetonitrile in 1 mL of chorobenzene. Finally, the device was transferred into a vacuum chamber (~10-6 Torr), and a Au electrode (ca. 100 nm thick) were thermally deposited through a shadow mask to deﬁne the effective active area of the devices (0.10 cm2). All device fabrication procedures were carried out in a N2-purged glovebox (

Solution-Processable Ionic Liquid as an Independent or Modifying

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