Phase Engineering of Perovskite Materials for High-Efficiency Solar

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Phase Engineering of Perovskite Materials for High-efficiency Solar Cells: Rapid Conversion of CH3NH3PbI3 to Phase-pure CH3NH3PbCl3 via Hydrochloric Acid Vapor Annealing Post-treatment Weiran Zhou, Pengcheng Zhou, Xunyong Lei, Zhimin Fang, Mengmeng Zhang, Qing Liu, Tao Chen, Hualing Zeng, Liming Ding, Jun Zhu, Songyuan Dai, and Shangfeng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15008 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 24, 2017

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ACS Applied Materials & Interfaces

Phase Engineering of Perovskite Materials for High-efficiency Solar Cells: Rapid Conversion of CH3NH3PbI3 to Phase-pure CH3NH3PbCl3 via Hydrochloric Acid Vapor Annealing Post-treatment Weiran Zhou,a Pengcheng Zhou,a Xunyong Lei,b Zhimin Fang,aMengmeng Zhang,a Qing Liu,a Tao Chen,a Hualing Zeng,b Liming Ding,*c Jun Zhu,d Songyuan Dai,e 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

ICQD, Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China (USTC), Hefei 230026, China c

d

National Center for Nanoscience and Technology, Beijing 100190, China

Key Lab of Special Display Technology, Ministry of Education, National Engineering Lab

of Special Display Technology, State Key Lab of Advanced Display Technology, Academy of Opto-Electronic Technology, Hefei University of Technology, Hefei 230009, China e

Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power University, Beijing 102206, China

* Corresponding Authors. E-mail: [email protected] (S.Y.); [email protected] (L. D.)

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ABSTRACT: Organometal halide such as CH3NH3PbI3 (MAPbI3) has been commonly used as the light absorber layer of perovskite solar cells (PSCs), and another halide element especially chlorine (Cl) has been often incorporated to assist the crystallization of perovskite film. However, in most cases a predominant MAPbI3 phase with trace of Cl- is obtained ultimately, and the role of Cl involvement remains unclear. Herein, we develop a low-cost and facile method named as hydrochloric acid vapor annealing (HAVA) post-treatment, and realize a rapid conversion of MAPbI3 to phase-pure MAPbCl3, demonstrating a new concept of phase engineering of perovskite materials toward efficiency enhancement of PSCs for the first time. The average grain size of perovskite film after HAVA post-treatment increases remarkably through an Ostwald ripening process, leading to a denser and smoother perovskite film with reduced trap states and enhanced crystallinity. More importantly, the generation of MAPbCl3 secondary phase via phase engineering is beneficial for improving the carrier mobility with a more balanced carrier transport rate and enlarging the bandgap of perovskite film along with optimized energy level alignment. As a result, under the optimized HAVA post-treatment time (2 min), we achieved a significant enhancement of power conversion efficiency (PCE) of MAPbI3-based PHJ-PSC device from 14.02% to 17.40% (the highest PCE reaches 18.45%) with greatly suppressed hysteresis of current-voltage response. Keywords: Perovskite solar cells, phase engineering, organometal halide, vapor annealing, gas-solid reaction

Introduction Due to advantages of organometal halide perovskite materials in terms of simple fabrication, large absorption coefficients, tunable bandgaps, high carrier mobility, and long charge carrier diffusion lengths, organometal halide-based perovskite solar cells (PSCs) have been attracting ever-increasing attention as an emerging thin film solar cell technology.1-4 During the past few years, continuous efforts devoted into optimizations of device structure, 2 Environment ACS Paragon Plus

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composition and morphology of the perovskite light absorber layer, and perovskite/electrode interfaces contribute to the improvement of the PCE exceeding 22% already, which becomes competitive with the commercialized crystalline-Si and inorganic semiconductor thin film solar cells.5-11 For the state-of-the-art PSCs devices, methylammonium lead triiodide CH3NH3PbI3 (MAPbI3) has been commonly used as the light absorber layer, which is sandwiched between the electron and hole transport layers (ETL and HTL) comprising of the so-called regular (n-ip) or inverted (p-i-n) structure planar heterojunction (PHJ) device.2 While currently much attention has been paid on interface engineering of PSCs which is determinative for efficient charge extraction,7-9,12,13 composition engineering of perovskite material via optimization of the composition of organometal halide perovskite material itself is also essential for efficient charge carrier generation and transport within the perovskite layer.5,6,14-16,18 In addition to tuning the composition of the organic cation of MAPbI3 extensively studied recently, substitution of the anion with another halide element especially chlorine (Cl) has been often applied to assist the crystallization of perovskite film.14-25,

28

Typically, Cl is incorporated

during fabrication of MAPbI3 film (i.e., in-situ) affording the mixed-halide form MAPbI3-xClx, and stiochiometric control of the Cl-doping is essential for obtaining the stable mixed-halide phase.16-20, 22-29 For instance, Yang et al. obtained the stable MAPbI3-xClx phase by blending PbCl2 with CH3NH3I (MAI) in a 1:3 molar ratio in a precursor solution dissolved in N,Ndimethylformamide (DMF), and the final perovskite film contained only a trace of Cl element, suggesting the loss of excess Cl- via evaporation during the drying process.24 Later on, Niwano et al. used the same stiochiometric ratio of PbCl2:MAI and studied the compositional and structural evolution of the MAPbI3-xClx perovskite film during thermal annealing. They found that, although both MAPbI3 and MAPbCl3 crystals were initially formed, upon further annealing the perovskite film was mainly composed of large MAPbI3 grains with a trace of Cl- due to its slow evaporation.22 Noteworthy, in these reports a predominant MAPbI3 phase 3 Environment ACS Paragon Plus


with a trace of Cl- is obtained ultimately, and the role of Cl involvement on the perovskite film and device performance remains ambiguous.17 Recently, Taghavinia et al. reported the fabrication of MAPbI3-xClx perovskite film by exposing a PbI2 film to MACl vapor, which was composed of mixed phases of MAPbI3 and MAPbCl3 with a ratio of about 2:1. However, although a higher open-circuit voltage (Voc) of the resultant MAPbI3-xClx-based mesostructured PSC devices was obtained due to the increased bandgap of MAPbI3 induced by Cl incorporation, the PCE (11.6%) was actually lower than that of the control device based on MAPbI3 pure phase (13.5%).14 Very Recently, by applying a gas/solid reaction between CH3NH2 gas and HPbI3-xClx (x=0-1), Zhao et al. fabricated a series of MAPbI3-xClx perovskite films with various Cl content including the pure phase of MAPbCl3 coexisted with MAPbI3xClx,

and concluded that MAPbCl3 was helpful to improve the thermal stability of MAPbI3-

xClx

despite that PCE of the MAPbI3-xClx-based PSC devices decreased with increasing Cl

content (9.57% for MAPbI2Cl1 vs 17.44% for MAPbI2.95Cl0.05 film).18 Given that in these cases the existence of pure phase of MAPbCl3 within MAPbI3 is detrimental to the PSC device performance, improving device performance via Cl incorporation appears quite challenging. Herein, we develop a low-cost and facile method named as hydrochloric acid vapor annealing (HAVA) post-treatment, and realize phase engineering of perovskite materials in terms of rapid conversion of MAPbI3 to MAPbCl3 secondary phase via post-treatment for the first time. The whole HAVA procedure was conducted at low temperature (below 50 ºC) and ambient atmosphere within minutes via a facile gas-solid reaction between MAPbI3 and hydrochloric acid (HCl), avoiding stiochiometric control of the Cl-doping ratio for the mixedhalide form MAPbI3-xClx commonly used for previous methods. The effects of HAVA posttreatment on the grain size, phase structure, crystallinity, bandgap and energy level of perovskite material and film are investigated systematically, revealing that HAVA posttreatment is beneficial for increasing the average grain size of perovskite film with reduced 4 Environment ACS Paragon Plus

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defect states and enhanced crystallinity, and the generation of phase-pure MAPbCl3 improves carrier mobility and enlarges the bandgap of perovskite film affording optimized energy level alignment at perovskite/TiO2 interface. Consequently, the photovoltaic performance and the ambient stability of the devices are significantly improved after HAVA post-treatment. Experimental Section Materials. FTO-coated glass substrates were purchased from NSG Group, Japan with a sheet resistance of 13±1.5Ω•sq-1. CH3NH3I was synthesized following the procedure reported previously.7,8,19 Titanium (IV) isopropoxide, PbI2, lithium bis(trifluoromethylsulfonyl) imide (Li-TFSI), 4-tert-butylpyridine (t-BP), hydriodic acid (HI), methylamine solution, dimethyl sulfoxide (DMSO), chlorobenzene, isopropanol, and acetonitrile were all purchased from Alfa Aesar. 2, 2′, 7, 7′-tetrakis (N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (SpiroOMeTAD) was purchased from 1 M company. The 36 wt% (11.7 M) hydrochloric acid (guaranteed reagent) was purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received with no further purification. Device fabrication. For the two-step fabricated CH3NH3PbI3 PHJ-PSC device, our detailed fabrication procedure has been reported previously. 7,8,43 FTO-coated glass substrate was first etched with zinc powder and 2M HCl (diluted in water), then ultrasonicated in detergent, deionized water, acetone and isopropanol for 15 min before dried at 60 ºC. A TiO2 compact layer was prepared by spin-coating a mixture solution of 350 mL of titanium isopropoxide, 5 mL of ethanol, and 65 µL of HCl (2 M) onto FTO at 2000 rpm, 40 s and dried at 100ºC for 20 minutes, followed by annealing at 550 ºC for 50 min. The MAPbI3 perovskite layer was fabricated by a commonly used two-step method.23,43 A PbI2 film was first spin-coated atop the compact TiO2 layer using the precursor PbI2 solution (dissolved in DMSO with a concentration of 460 mg•mL-1) with the condition of 1000 rpm, 10 s, followed by 3500 rpm for 30 s. For the formation of CH3NH3PbI3, the coated substrate was immersed in a solution of CH3NH3I in isopropanol (10 mg•mL-1) for 10 min and then rinsed with isopropanol and 5 Environment ACS Paragon Plus


dried by spinning at 3000 rpm. The as-prepared MAPbI3 film was annealed at 100 ºC for 10 min, then cooled to room temperature. 73.2 mg spiro-OMeTAD was first dissolved in 1 mL chlorobenzene, then 28.8 µL of t-BP and 18.8 µL of Li-TFSI solution (520 mg Li-TFSI in 1 mL acetonitrile) were added. The spiro-OMeTAD solution was spin-coated onto the perovskite layer at 3000 rpm for 30 s. Finally, the device was transferred into a vacuum chamber (~10-6 torr), and the Au electrode (ca. 100 nm thick) was thermally deposited through a shadow mask to define the effective active area of the device (0.10 cm2). All device fabrication procedures were carried out in a N2-purged glovebox ( 70% in the broad visible light region of 400 - 750 nm, indicating the efficient utilization of the absorbed photons. The overall EQE response of the device after 2 min HAVA post-treatment is much higher than that of the control device, leading to an obvious increase of the integrated photocurrent density. This is consistent with the increase of Jsc obtained in J-V measurements. The hysteresis of J-V curves is then examined by measuring the J-V curves in different scan directions and rates. As shown in Figure 4C, the control device based on the pristine MAPbI3 absorber displays a severe hysteresis of 18.8% between the forward and reverse scans (Table S7, Supporting Information), which mainly originates from the drop of FF. After 2 min HAVA post-treatment, only a 2.2% drop from reverse to forward scan is observed (Figure 4D), revealing a significantly suppressed hysteresis. The suppressed hysteresis after HAVA post-treatment is further confirmed by measuring the J-V curves in different scan rates from 0.01 to 1.0 V/s, indicating that the PCEs of the devices after HAVA post-treatment keep almost constant in different scan rates, whereas the control device suffers from obvious variation (Figure S9 and Table S8, Supporting Information). It has been reported that the hysteresis between the reverse and forward scan is mainly due to the existence of enormous traps on TiO2 surface and within perovskite layer as well as ion migration.8,54,55 Hence, the dramatic suppression of the hysteresis of J-V curves after HAVA post-treatment is expected to result from the reduced trap states and inhibited ion migration from grain boundaries. To probe the effect of HAVA post-treatment on the trap states of the MAPbI3 perovskite layer,

we

carried

out

steady-state

and

time-resolved

photoluminescence

(PL)

characterizations, which provide valuable information on the charge recombination dynamics within the perovskite absorber.8,19,57 Steady-state PL spectra of the MAPbI3 perovskite film 18 Environment ACS Paragon Plus

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before and after HAVA post-treatment measured under an excitation wavelength of 460 nm are compared in Figure 5A. For the pristine MAPbI3 film, the PL peak at 764 nm is assigned to its characteristic emission.8,19,57 After 2 min HAVA post-treatment, such a PL peak is obviously enhanced by around 3 times, suggesting the dramatic suppression of the nonradiative decay.35,46,56 In particular, the detected PL peak of MAPbI3 at 764 nm blue-shifts to 758 nm, and this can be interpreted by synergistic effect of the enlarged bandgap due to the generation of phase-pure MAPbCl3 and reduced trap density after HAVA post-treatment.8,57,58 Besides, as shown in time-resolved photoluminescence (TRPL) spectra (Figure 5B), the pristine MAPbI3 perovskite film shows a lifetime of τ = 75.26 ns, which increases to 157.0 ns after 2 min HAVA post-treatment (see Supporting Information S12 for detailed analyses), suggesting the decrease of the trap states within MAPbI3.43

Figure 5. Steady-state (A) and time-resolved (B) photoluminescence (PL) spectra of the MAPbI3 perovskite films on glass substrate before and after 2 min HAVA post-treatment. (c) Dark current-voltage responses of the electron-only devices (shown in the inset), from which the VTFL is determined as the kink point. (D) Nyquist plots of the control and HAVA post-treated devices measured in the dark under a reverse potential of 1.0 V. The fitted curves are shown as solid lines, and the experimental data are shown as corresponding points. Inset: the equivalent circuit model employed for the fitting of the impedance spectra.



The definitive evidence on the effect of HAVA post-treatment on the trap states within the MAPbI3 perovskite layer is obtained by measuring the electron trap state density within the perovskite layer using space charge limited current (SCLC) method. An electron-only device with a structure of FTO/TiO2/perovskite/PCBM/Ag was fabricated, and the corresponding I-V curves for the devices based on MAPbI3 perovskite before and after HAVA post-treatment is presented in Figure 5C. The measured trap-state density (nt) of MAPbI3 perovskite films before and after HAVA post-treatment are 5.52 × 1015 cm-3 and 2.37 × 1015 cm-3, respectively (see Supporting Information S13 for detailed analyses). Such a decrease of the nt value by around 60% after HAVA post-treatment indicates the reduced trap states, consistent with the inference from the steady-state and time-resolved PL results discussed above. We further investigated the effect of HAVA post-treatment on the carrier mobility by using SCLC method based on different electron-only and hole-only devices, and found that the electron mobility of perovskite films increased from 9.21×10-6 cm2•V-1•s-1 to 3.88×10-5 cm2•V-1•s-1 after 2 min HAVA post-treatment, while the hole mobility improves as well from 1.75×10-5 to 3.48×10-5 cm2•V-1•s-1 (see Supporting Information S14 for detailed analyses). Interestingly, the HAVA post-treatment can not only improve both electron and hole mobilities, but also lead to a more balanced charge transport rate (µe/µh), which is crucial for the charge injection/transfer efficiency that greatly affect the photocurrent density.63 Such an obvious improvement of the carrier mobility primarily origins from the generation of MAPbCl3 which has a higher carrier mobility (179 cm2 • V-1 • s-1) than MAPbI3 (34 cm2 • V-1 • s-1).64 Higher carrier mobility and more balanced charge transport render better charge transport and longer diffusion length, contributing to the increase of Jsc. In addition to the influence on the MAPbI3 perovskite layer, it is of importance to investigate whether HAVA post-treatment affects the interfacial charge transport behavior of the PSC devices. We carried out the electrochemical impedance spectroscopy (EIS)


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measurements in dark under a reverse potential of 1.0 V which is near the open circuit potential. The Nyquist plots of the PSC devices before and after HAVA post-treatment are plotted in Figure 5D, which also includes the corresponding fitted curves as well as the equivalent circuit model employed for fitting.30,34,59,65,66 It is known that the lower Rco (contact resistance) value implies more favorable electron transport while the larger Rrec (recombination resistance) suggests the lower recombination rate.34,65,66 For the PSC device after HAVA post-treatment, the smaller Rco along with the large Rrec value indicates that HAVA post-treatment can effectively inhibit electron-hole recombination at perovskite/HTL interface (see Supporting Information S15 for detailed analyses). One reason for such an effect of HAVA post-treatment is the improved interfacial contact between perovskite and HTL layers due to the smoother MAPbI3 perovskite surface with lower RMS roughness resulted from the improved film uniformity as discussed above. Besides, the reduced trap states within the perovskite layer may contribute to the suppressed recombination and consequently benefit the charge transport. We further studied the mechanism of PCE enhancement after HAVA post-treatment from the viewpoint of energy level alignment. Ultraviolet photoelectron spectroscopy (UPS) was used to determine the changes of the work function (WF) and band energy levels of MAPbI3 perovskite before and after HAVA post-treatment, and the energy level diagram of the different layers is constructed (see Supporting Information S16 for detailed analyses). Based on the energy level alignments of perovskite and TiO2, it is clear that HAVA (2min)-treated perovskite (with an EV = -5.71 eV and EC = -4.10 eV) seems to be most favorable for electron extration at perovskite/TiO2 interface due to the minimum energy level offset at the interface67 Such a facilitated electron extration may result in increases of Jsc and Voc and consequently improved device perfomance. Effect of HAVA Post-treatment on the Stability of the PSC Device. The effect of HAVA post-treatment on the stability of the PSC device is checked finally. The PCE of the 21 Environment ACS Paragon Plus


device after HAVA post-treatment can be stabilized at 17.98% at the maximum power point (Figure S13 and Table S12, Supporting Information). Furthermore, we evaluated the ambient stability of the PSC device by storing the device without any encapsulation in ambient condition (temperature: 20 ºC, relative humidity: 35%) for 500 h. The control device is obviously vulnerable to the ambient environment, and a PCE drop of ca. 53% after 500 h storage is observed. This phenomenon is similar to those reported extensively in literatures, mainly due to the decomposition of MAPbI3.10,68 After 2 min HAVA post-treatment, a ca. 85% retaining of the PCE is achieved (Figure S14(a), Supporting Information), indicating the dramatic improvement on ambient stability. It is then intriguing to unravel that, within MAPbI3•MAPbCl3 mixed phase, which one is more prone to degrade in ambient condition. MAPbI3 films before and after HAVA posttreatment (2 min) were prepared on FTO/TiO2 substrate and stored in the same ambient condition for 30 days, and their compositional changes were monitored by XRD measurements. While a severe degradation after ambient storage for 30 days is observed for the pristine MAPbI3 film, the MAPbI3•MAPbCl3 mixed phase film upon 2 min HAVA posttreatment exhibits little change on the XRD pattern specifically for the (110) peak of MAPbI3 and (100) peak of MAPbCl3 (15.5°) after ambient storage for 30 days. Therefore, we conclude that the generation of MAPbCl3 as the secondary phase, which is more stable in ambient condition than MAPbI3, may protect MAPbI3 from decomposition. A plausible explanation is that the stronger coordination between Pb2+ and Cl- than I- may strengthen the Pb-halide bonding, fulfilling improved stability of the mixed phase perovskite. Conclusions In summary, by developing a low-cost and facile HAVA post-treatment method, we realize a rapid conversion of MAPbI3 to phase-pure MAPbCl3, demonstrating a new concept of phase engineering of perovskite materials toward efficiency enhancement of PSCs for the first time. Under the optimized HAVA post-treatment time (2 min), we achieved a significant 22 Environment ACS Paragon Plus

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enhancement of PCE of MAPbI3-based PHJ-PSC device from 14.02% to 17.40% (the highest PCE reaches 18.45%) with greatly suppressed hysteresis of current-voltage response. The advantage of our method is that, in addition to the low temperature (below 50 ºC) process conducted at ambient atmosphere, the stiochiometric control of the Cl-doping ratio for the mixed-halide form MAPbI3-xClx commonly used for previous methods can be avoided. The effects of HAVA post-treatment on the grain size, phase structure, crystallinity, bandgap and energy level of the perovskite material and film are investigated systematically, revealing that HAVA post-treatment is beneficial for increasing the average grain size of perovskite film with reduced defect states and enhanced crystallinity. More importantly, the generation of MAPbCl3 secondary phase via phase engineering is beneficial for improving the carrier mobility with a more balanced carrier transport rate and enlarging the bandgap of perovskite film along with optimized energy level alignment at perovskite/TiO2 interface. Furthermore, the ambient stability of the MAPbI3-based PSC device is greatly improved after HAVA posttreatment, and the generation of MAPbCl3 as the secondary phase may protect MAPbI3 from decomposition due to the strengthened Pb-halide bonding resulted from the stronger coordination between Pb2+ and Cl- than I-. HAVA post-treatment developed in this study is facile, effective, and can be extended to other types of haloid acids and perovskite absorbers, thus phase engineering of perovskite materials is quite promising for achieving highefficiency perovskite solar cells. Supporting Information Available: Optimization of the HAVA post-treatment time, EDS study, UV-vis absorption spectra, Estimation of the SCLC carrier mobility of the MAPbI3 perovskite film after HAVA posttreatment, Estimation of the percentage of MAPbCl3 within MAPbI3•MAPbCl3 mixed phase, Histograms and box plots of photovoltaic parameters, hysteresis, stabilized photocurrent density and power output of the device, fitting parameters for EIS data, TRPL data, trap-state



density measurements, UPS data, stability characterization, etc. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements This work was partially supported by the National Key Research and Development Program of China (2017YFA0402800, 2017YFA0205004), National Natural Science Foundation of China (21371164, 51572254, 11674295), the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (No. 2016FXZY003), and the Fundamental Research Funds for the Central Universities (WK3430000002, WK2060140023).

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Phase Engineering of Perovskite Materials for High-Efficiency Solar

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