Efficient Perovskite Solar Cells with Reduced

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Article Cite This: ACS Omega 2018, 3, 7069−7076

Efficient Perovskite Solar Cells with Reduced Photocurrent Hysteresis through Tuned Crystallinity of Hybrid Perovskite Thin Films Jun Qi,†,§ Xiang Yao,†,§ Wenzhan Xu,† Xiao Wu,† Xiaofang Jiang,† Xiong Gong,*,†,‡ and Yong Cao† †

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Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, P. R. China ‡ Department of Polymer Engineering, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States ABSTRACT: Hybrid perovskite materials used for realization of efficient perovskite solar cells have drawn great attention in both academic and industrial sectors. It was reported that the crystallinity of hybrid thin-film perovskite materials plays an important role in device performance. In this study, we report a novel and simple method to tune the crystallinity of CH3NH3PbI3 thin film for device performance of perovskite solar cells. By employing tetraphenylphosphonium chloride on the top of PbI2 thin layer in the two-step perovskite deposition processes, the crystallinity of the resultant CH3NH3PbI3 thin film was tuned. As a result, perovskite solar cells by the CH3NH3PbI3 thin film with tuned crystallinity exhibit an enlarged open-circuit voltage and enhanced short-circuit current, thus boosted efficiency as well as reduced photocurrent hysteresis compared to pristine CH3NH3PbI3 thin film. These results indicate that our study provides a new simple way to boost device performance of perovskite solar cells through tuning the crystallinity of CH3NH3PbI3 thin film.

1. INTRODUCTION In the past years, hybrid perovskite materials used for realization of efficient perovskite solar cells (PSCs) have drawn great attention in both academic and industrial sectors due to their advanced features, such as large absorption coefficient,1,2 high charge-carrier mobility,3,4 and long chargecarrier diffusion lengths.5,6 Power conversion efficiencies (PCEs) of more than 22% have been reported from PSCs fabricated by CH3NH3PbI3 with large crystal and generic interfacial engineering in PSCs device structure.7−10 However, there is still a gap to realize 31% PCE, a theoretical value, from CH3NH3PbI3-based PSCs.11 Toward the end, device performance parameters, short-circuit current (JSC), open-circuit voltage (VOC), and fill factors (FFs) are required to be enhanced for boosting PCE. In the case of PSCs fabricated by CH3NH3PbI3 thin film, VOC is estimated to be 1.3 V.11 But the reported VOC always exhibited unavoidable optical and electrical losses,12,13 which were induced by interfacial states.14−16 Toward the end, passivating the charge defects and improving the energy disorder at the electron extraction layer have been utilized to realize large VOC.16−22 On the other hand, many effects have been also devoted to enhance JSC PSCs.11,23−25 Han et al. reported a solvent-annealing process to control the crystal orientation transformation to improve charge-carrier collection and prolong charge-carrier lifetime, thus boosting JSC.26 Recently, Leblebici et al. have found that © 2018 American Chemical Society

different crystal facets of individual grains have a direct impact on both VOC and JSC.27 Thus, optimization of microscopic facet orientation of CH3NH3PbI3 crystals in polycrystalline perovskite thin film could boost both VOC and JSC.26−29 In this study, we report a novel and simple method to tune the crystallinity of CH3NH3PbI3 thin film for boosting PSCs device performance. By employing tetraphenylphosphonium chloride (TPPCl) on the top of PbI2 thin layer in the two-step perovskite deposition processes, the crystallinity of the resultant CH3NH3PbI3 thin film was tuned. As a result, the PSCs fabricated by the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer exhibit a VOC of 1.12 V, a JSC of 21.71 mA/cm2, an FF of 74.43, and a corresponding PCE of 18.04%, with dramatically reduced hysteresis, compared to the PSCs fabricated by pristine CH3NH3PbI3 thin film, which exhibit a VOC of 1.04 V, a JSC of 21.08 mA/cm2, an FF of 74.41, and a corresponding PCE of 16.34%, with serious hysteresis.

2. RESULTS AND DISCUSSION TPPCl is selected to tune the crystallinity of CH3NH3PbI3 thin film through modification of PbI2 thin-layer surface originated Received: May 19, 2018 Accepted: June 19, 2018 Published: June 28, 2018 7069

DOI: 10.1021/acsomega.8b01061 ACS Omega 2018, 3, 7069−7076

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ACS Omega Scheme 1. (a) Molecular Structure of TPPCl and (b) the Device Structure of Perovskite Solar Cells

Figure 1. Top view of SEM images of pristine PbI2 thin film and PbI2 thin film treated with the TPPCl ultrathin layers cast from TPPCl solutions with concentrations of (a) 0 mg/mL, (b) 0.25 mg/mL, (c) 0.5 mg/mL, (d) 1 mg/mL, and (e) 1.5 mg/mL. Top view of SEM images of pristine CH3NH3PbI3 thin film and CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layers cast from TPPCl solutions with concentrations of (f) 0 mg/mL, (g) 0.25 mg/mL, (h) 0.50 mg/mL, (i) 1 mg/mL, and (j) 1.5 mg/mL.

from the Cl− anion, which probably chelated with the Pb2+ cations and altered the kinetics of thin-film formation.29 The molecular structure of TPPCl is shown in Scheme 1a. Figure 1a−e presents the scanning electron microscopy (SEM) images of pristine PbI2 thin film and PbI2 thin films treated with the TPPCl ultrathin layers. It is found that pristine PbI2 thin film possesses layered crystals with sizes of tens of nanometers and many voids. Such observation is in good agreement with the morphologies of PbI2 thin film reported by others.30,31 After the PbI2 thin films are treated with different TPPCl ultrathin layers cast from different concentrations of TPPCl solutions, PbI2 thin films still possess layered crystals with sizes of tens of nanometers and many voids, but a mass of interlaced particles like strip is adhered to the surfaces of PbI2 thin films. As the TPPCl solution is at 1 mg/mL, several welldistributed large TPPCl particles are formed on the surface of PbI2 thin film. As the TPPCl solution is further increased to 1.5 mg/mL, large TPPCl nanoparticles are aggregated on the surface of PbI2 thin film. Figure 1f−j presents the SEM images of pristine CH3NH3PbI3 thin film and CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers. Compared to pristine CH3NH3PbI3 thin film, no significant change in the film morphology is observed in the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from TPPCl solution with a concentration of 0.25 mg/mL. However, the

CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from TPPCl solution with a concentration of 0.5 mg/mL, displays larger and more inerratic grains. The CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from TPPCl solution with a concentration of 1.0 mg/mL, shows larger gains and higher gains continuity with distinctly less grain boundaries. The CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from TPPCl solution with a concentration of 1.5 mg/mL, possesses very fine grain with flatness feature of grain boundary. Thus, these results demonstrate that the TPPCl ultrathin layers affect the qualities of resultant CH3NH3PbI3 thin films. Figure 2 presents the X-ray diffraction (XRD) patterns of pristine CH3NH3PbI3 thin film and CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers. No XRD patterns assigned to TPPCl ultrathin layer are found in all CH3NH3PbI3 thin films, indicating that TPPCl is evaporated during the thermal annealing for the formation of CH3NH3PbI3 thin films. All CH3NH3PbI3 thin films exhibit the same diffraction peaks located at 14.1, 28.5, and 31.9°, corresponding to the (110), (220), and (114) crystal planes. No XRD patterns assigned to TPPCl ultrathin layer are found in all. These results indicate that all CH3NH3PbI3 thin films possess the tetragonal crystal phase.29,32 However, the peak intensities are different. The highest peak intensity (the (110) plane) is observed from the CH3NH3PbI3 film pretreated with 7070


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film. However, the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layers possesses gradually enhanced absorbance ranging from 380 to 770 nm along with the TPPCl ultrathin layers cast from increased concentrations of TPPCl solutions. The CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from a 1.0 mg/mL concentration of TPPCl solution possesses the highest absorbance. Such high absorbance indicates that more light will be absorbed by the CH3NH3PbI3 thin film, implying that more photocurrent could be generated. The photovoltaic properties of pristine CH3NH3PbI3 thin film and CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers are evaluated through investigation of device performance of PSCs with a device structure of ITO/PTAA/ CH3NH3PbI3/PC61BM/BCP/Ag, as shown in Scheme 1b, where ITO is indium tin oxide, which acts as the anode; PTAA is poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], which acts as the hole extraction layer; PC61BM is 6,6-phenyl-C61butyric acid methyl ester, which acts as the electron extraction layer; BCP is bathocuproine, which acts as the hole blocking layer; and Ag is sliver, which acts as the cathode. The current density versus voltage (J−V) characteristics of PSCs are shown in Figure 4a. The device performance parameters are summarized in Table 1. The PSCs fabricated by the pristine CH3NH3PbI3 thin film exhibit a VOC of 1.04 V, a JSC of 21.08

Figure 2. XRD patterns of pristine CH3NH3PbI 3 film and CH3NH3PbI3 thin films pretreated with the TPPCl ultralayers cast from TPPCl solutions with concentrations of 0, 0.25, 0.50, 1, and 1.5 mg/mL.

the TPPCl ultrathin layer cast from a 1.0 mg/mL concentration of TPPCl solution, indicating that the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer possesses the best crystallinity, which is in good agreement with the SEM results. All of these results imply that the CH3NH3PbI3 film pretreated with the TPPCl ultrathin layer cast from a 1.0 mg/mL concentration of TPPCl solution, possesses optimal photovoltaic properties. Figure 3 depicts the UV−vis absorption spectra of pristine CH3NH3PbI3 thin film and CH3NH3PbI3 film pretreated with the TPPCl ultrathin layers. All CH3NH3PbI3 thin films have identical absorption spectrum with the same onset of absorption, indicating that the TPPCl ultrathin layers do not have any influence on the band gap of the CH3NH3PbI3 thin

Figure 4. (a) J−V characteristics and (b) external quantum efficiency (EQE) spectra of PSCs fabricated by pristine CH3NH3PbI3 film and CH3NH3PbI3 films pretreated with the TPPCl ultrathin layers cast from TPPCl solutions with concentrations of 0, 0.25, 0.50, 1, and 1.5 mg/mL.

Figure 3. UV−vis spectra of pristine CH3NH3PbI3 film and CH3NH3PbI3 films pretreated with the TPPCl ultrathin layers cast from TPPCl solutions with concentrations of 0, 0.25, 0.50, 1, and 1.5 mg/mL. 7071


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measurements are conducted to assess the intrinsic properties of charge-carrier extraction and charge-carriers recombination in PSCs.35,36 Figure 5a presents the normalized TPC of the PSCs fabricated by either pristine CH3NH3PbI3 thin film or CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers. The PSCs fabricated by CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers possess slightly shorter charge extraction lifetimes (0.17, 0.24, and 0.23 μs) compared to that (0.29 μs) by the PSCs fabricated the pristine CH3NH3PbI3 thin film, indicating that reduced trap-assisted charge recombination and better charge extraction efficiencies have taken place in these PSCs. However, the longest charge extraction lifetime (0.43 μs) is observed in the PSCs fabricated by CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layers cast from a 1.5 mg/mL concentration of the TPPCl solution, indicating that PSCs fabricated by such thin film exhibit the poorest device performance. Figure 5b shows the normalized TPV of the PSCs fabricated by either pristine CH3NH3PbI3 thin film or CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers. The PSCs fabricated by CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers possess longer charge recombination lifetimes to reach the same TPV compared to those fabricated by pristine CH3NH3PbI3 thin film. The longest charge recombination lifetime is observed in the PSCs fabricated by CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from a 1 mg/mL concentration of the TPPCl solution. The longer charge recombination lifetime demonstrates the more suppressed charge recombination in PSCs. Thus, the PSCs fabricated by CH3NH3PbI3 thin film pretreated with TPPCl ultrathin layers possess larger JSC values compared to those fabricated by pristine CH3NH3PbI3 thin film. The light intensity dependence on V OC is further investigated to illustrate the charge-carrier recombination in PSCs. Figure 6a displays the light intensity dependence on VOC for the PSCs fabricated by either pristine CH3NH3PbI3 thin film or CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers. According to VOC ∝ S In(I)37 (where S is the slope and I is the light intensity), the smallest S (0.038) is observed in the PSCs fabricated by the CH3NH3PbI3 thin film

Table 1. Device Performance Parameters of PSCs solution concentrations for deposition of the TPPCl ultrathin layer (mg/mL)

VOC (V)

0 0.25 0.5 1 1.5

1.04 1.06 1.08 1.12 1.08

JSC (mA/cm2) FF (%) 21.08 21.32 21.77 21.71 21.38

74.41 75.17 74.84 74.43 65.75

PCE (%)

HI (%)

16.34 17.04 17.51 18.04 15.26

3.74 1.73 1.11 1.12 −3.11

mA/cm2, an FF of 74.41, and a corresponding PCE of 16.34%. These device performance parameters are consistent with reported values from PSCs with the same device structure.7−9 The PSCs fabricated by the CH3NH3PbI3 thin film pretreated with TPPCl ultrathin layers exhibit enlarged VOC, enhanced JSC, and thus boosted PCEs. The best device performance (VOC of 1.12 V, JSC of 21.71 mA/cm2, FF of 74.43, and the corresponding PCE of 18.04%), is observed from the PSCs fabricated by the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from a 1.0 mg/mL concentration of TPPCl solution. Figure 4b presents the external quantum efficiencies (EQE) of PSCs fabricated by either pristine CH3NH3PbI3 thin film or CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers. Enhanced EQE values are observed from the PSCs fabricated by the CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers compared to those by pristine CH3NH3PbI3 thin film. The integrated JSC values are 20.16 mA/cm2 for the PSCs fabricated by pristine CH3NH3PbI3 thin film, and 20.42, 20.83, 20.80, and 20.46 mA/cm2 for the PSCs fabricated by CH3NH3PbI3 thin films pretreated with the TPPCl ultrathin layers cast from the TPPCl solutions with concentrations of 0.25, 0.5, 1.0, and 1.5 mg/mL, respectively. These JSC values are consistent with the JSC extracted from J−V characteristics, as shown in Figure 4a. The increased JSC is attributed to the enlarged grain size (Figure 2) and stronger light-harvesting ability of CH3NH3PbI3 thin films pretreated with TPPCl ultrathin layers (Figure 3).33,34 To further understand enhanced JSC, transient photocurrent (TPC) and transient photovoltage (TPV)

Figure 5. (a) Transient photocurrent and (b) transient photovoltage decays of PSCs fabricated by pristine CH3NH3PbI3 thin film and CH3NH3PbI3 thin films pretreated with the TPPCl ultrathin layers cast from TPPCl solutions with concentrations of 0, 0.25, 0.50, 1, and 1.5 mg/ mL. 7072


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Figure 6. (a) VOC dependence on different light intensities and (b) J−V characteristics, in the dark, of the PSCs fabricated by pristine CH3NH3PbI3 thin film and CH3NH3PbI3 thin films pretreated with the TPPCl ultrathin layers cast from TPPCl solutions with concentrations of 0, 0.25, 0.50, 1, and 1.5 mg/mL.

17.66% under forward and reversed scan directions, respectively, which reveal an HI value of 3.74%. A stable PCE and the smallest HI are observed from the PSCs fabricated by the CH3NH3PbI3 thin film pretreated with TPPCl ultrathin layer cast from a 1 mg/mL concentration of the TPPCl solution. Thus, the PSCs fabricated by the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layers possess reduced photocurrent hysteresis. Such reduced photocurrent hysteresis behavior probably originated from suppressed charge recombination due to the improved crystallinity of CH3NH3PbI3 thin films.

pretreated with the TPPCl ultrathin layer cast from a 1 mg/mL concentration of the TPPCl solution, indicating that suppressed trap-assisted charge recombination occurred in PSCs.38 Therefore, the PSCs fabricated by the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from a 1 mg/mL concentration of the TPPCl solution exhibit the highest JSC value among all PSCs. To understand the underlying physics of enlarged VOC, the J−V characteristics of PSCs are measured in the dark, and the results are shown in Figure 6b. According to the Shockley− Queisser model,39 the relationship between J and VOC is described as VOC

nkT ijj JSC yzz = lnjjj zzz j J0 z q k {

3. CONCLUSIONS In summary, we reported a simple method to tune the crystallinity of CH3NH3PbI3 thin film in a two-step process. Compared to pristine CH3NH3PbI3 thin film, large and inerratic grains have been observed in the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer. As a result, the PSCs fabricated by CH3NH3PbI3 thin film pretreated with TPPCl ultrathin layer exhibit a VOC of 1.12 V, a JSC of 21.71 mA/cm2, an FF of 74.43, and a corresponding PCE of 18.04%, with dramatically reduced hysteresis compared to the PSCs fabricated by pristine CH3NH3PbI3 thin film, which exhibit a VOC of 1.04 V, a JSC of 21.08 mA/cm2, an FF of 74.41, and a corresponding PCE of 16.34%, with serious hysteresis. Our results provide a new strategy to boost device performance through tuning the crystallinity of perovskite thin films.

where J0 is the reverse dark current density, q is the electron charge, n is the diode ideality factor, k is the Boltzmann constant, and T is the temperature. Thus, it concludes that a large VOC is anticipated from the PSCs with a low value of J0. As shown in Figure 6b, the PSCs fabricated by the CH3NH3PbI3 thin film pretreated with TPPCl ultrathin layers possess low J0 values compared to those fabricated by the pristine CH3NH3PbI3 thin film. In particular, the PSCs fabricated by the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from a 1 mg/mL concentration of the TPPCl solution possess the lowest J0. Thus, the PSCs fabricated by the CH3NH3PbI3 thin film pretreated with the TPPCl ultrathin layer cast from a 1 mg/mL concentration of the TPPCl solution exhibit the largest VOC value. The hysteresis behaviors of PSCs are further investigated. Figure 7a−e presents the J−V characteristics of the PSCs fabricated by either pristine CH3NH3PbI3 thin film or the CH3NH3PbI3 thin film pretreated with TPPCl ultrathin layers under either forward scan direction (from the negative voltage to the positive voltage) or reserved scan direction (from the positive voltage to the negative voltage). The hysteresis index PCEreverse − PCEforward 40 (HI) is defined as HI = . Table 1 PCE

4. EXPERIMENTAL SECTION 4.1. Materials. Bathocuproine (BCP), tetraphenylphosphonium chloride (TPPCl), n-butyl alcohol, anhydrous N,Ndimethylformamide (DMF), and ethanol were purchased from Sigma-Aldrich. Lead(II) iodide (PbI2) was purchased from Alfa Aesar. Phenyl-C61-butyric acid methyl ester (PC61BM) was purchased from 1-Material Inc. Poly[bis(4-phenyl)(2,4,6trimethylphenyl)amine] (PTAA) and methylammonium iodide (MAI) were purchased from Xi’an Polymer Light Technology Corp. All materials were used as received without further purification. 4.2. Solution Preparation. PbI2 was first dissolved in DMF at a concentration of 400 mg/mL and then stirred at 60

reverse

summarizes PCEs and the corresponding HI values of PSCs under different scan directions. Clearly, the PSCs fabricated by pristine CH3NH3PbI3 thin film exhibit PCEs of 17.00 and 7073


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Figure 7. J−V characteristics of the PSCs fabricated by pristine CH3NH3PbI3 thin film and the CH3NH3PbI3 thin films pretreated with the TPPCl ultrathin layers cast from TPPCl solutions with concentrations of 0, 0.25, 0.50, 1, and 1.5 mg/mL and measured under forward and reverse scan directions.

°C for 12 h to form a PbI2 solution. Afterward, the PbI2 solution was filtered using a filter of size 0.45 mm to obtain a transparent yellow solution. The MAI was dissolved in ethanol to form an MAI solution with a concentration of 35 mg/mL. TPPCl was dissolved in n-butyl alcohol with concentrations of 0.25, 0.5, 1, and 1.5 mg/mL. 4.3. Preparation of CH3NH3PbI3 Thin Film. The CH3NH3PbI3 thin films were prepared by a two-step deposition method. PbI2 solution was first spin-cast on the top of the PTAA layer, which was cast from the corresponding solution, and thermally annealed at 80 °C for 5 min. After PbI2 thin film was cooled to room temperature, the TPPCl ultrathin

layer was coated on the top of PbI2 thin film from TPPCl solution at 4000 rpm for 30 s. Afterward, the MAI thin layer was spin-coated on the top of either pristine PbI2 thin film or the PbI2/TPPCl thin film and thermally annealed at 100 °C for 2 h for converting PbI2 and CH3NH3I into CH3NH3PbI3 thin film. 4.4. Characterization of CH3NH3PbI3 Thin Film. The surface profilometer (Tencor, Alpha-500) was used to measure the film thickness. UV−vis absorption spectra of CH3NH3PbI3 thin films were recorded on an HP 8453 spectrophotometer. Top-view images of CH3NH3PbI3 thin films were obtained 7074


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ACS Omega using a field-emission scanning electron microscope (Zeiss Merlin). 4.5. Fabrication of Perovskite Solar Cells. Precleaned indium tin oxide (ITO) glass substrates were first treated by oxygen plasma for 3 min. Afterward, ∼10 nm of the PTAA film was spin-cast on the top of the ITO substrates from PTAA solution and then thermally annealed at 100 °C for 10 min. The CH3NH3PbI3 thin films were prepared by the two-step deposition method described above. Then, an ∼40 nm thick PC61BM layer was spin-cast on the top of the CH3NH3PbI3 thin film. Afterward, an ∼10 nm BCP thin film was spin-cast on the top of the PC61BM layer. Finally, an ∼100 nm silver (Ag) electrode was thermally evaporated on the top of the BCP layer in vacuum with a base pressure of 2 × 10−6 mbar. The device area was measured to be 5.7 mm2. 4.6. Characterization of Perovskite Solar Cells. The current density−voltage (J−V) characteristics of PSCs were measured under 1 sun from an AM 1.5 G solar simulator (Japan, SAN-EI, XES-40S1), where light intensity was calibrated using a standard silicon solar cell with a KG5 visible filter. The photocurrent hysteresis properties of PSCs were assessed by testing the PSCs at both forward (from −1.2 to 0.2 V) and reverse (from 0.2 to −1.2 V) directions at a scan rate of 3 V/s. The external quantum efficiency (EQE) spectra of PSCs were recorded on a DSR100UV-B spectrometer with an SR830 lock-in amplifier, a bromine tungsten light source, and a calibrated Si detector. 4.6.1. Transient Photovoltage and Transient Photocurrent Measurements. A digital oscilloscope (Tektronix TDS 3052C) with input impedances of 1 MΩ and 50 Ω was used to monitor the charge density decay and charge extraction time of PSCs. The transient photovoltage of PSCs was measured under 0.03 sun illumination with a small perturbation by an attenuated laser pulse (500 nm). The laser-pulse-induced photovoltage variation (ΔV) was smaller than 5% of the VOC produced by the background illumination. The transient photocurrent of PSCs was measured by applying laser pulses to the short-circuited devices in the dark with an excitation wavelength of 500 nm, a pulse width of 120 fs, and a repetition rate of 1 kHz.

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(3) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. (4) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths >175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967−970. (5) Slavney, A. H.; Hu, T.; Lindenberg, A. M.; Karunadasa, H. I. A Bismuth-Halide Double Perovskite with Long Carrier Recombination Lifetime for Photovoltaic Applications. J. Am. Chem. Soc. 2016, 138, 2138−2141. (6) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites. Adv. Mater. 2014, 26, 1584−1589. (7) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (8) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234−1237. (9) Bi, D.; Yi, C. Y.; Luo, J. S.; Decoppet, J. D.; Zhang, F.; Zakeeruddin, S. M.; et al. Polymer-templated nucleation and crystal growth of perovskite thin film for solar cells with efficiency greater than 21%. Nat. Energy 2016, 1, 16142−16146. (10) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells. Science 2017, 356, 1376−1379. (11) Sha, W.; Ren, X. G.; Chen, L. Z.; Choy, W. C. H. The efficiency limit of CH3NH3PbI3 perovskite solar cells. Appl. Phys. Lett. 2015, 106, No. 221104. (12) Leong, W. L.; Ooi, Z. E.; Sabba, D.; Yi, C. Y.; Zakeeruddin, S. M.; Graetzel, M.; Gordon, J. M.; Katz, E. A.; Mathews, N. Identifying Fundamental Limitations in Halide Perovskite Solar Cells. Adv. Mater. 2016, 28, 2439−2445. (13) Kim, H. D.; Ohkita, H.; Benten, H.; Ito, S. Photovoltaic Performance of Perovskite Solar Cells with Different Grain Sizes. Adv. Mater. 2016, 28, 917−922. (14) Heumueller, T.; Burke, T. M.; Mateker, W. R.; Sachs-Quintana, I. T.; Vandewal, K.; Brabec, C. J.; McGehee, M. D. Disorder-Induced Open-Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn-In. Adv. Energy Mater. 2015, 5, No. 1500111. (15) Blakesley, J. C.; Neher, D. Relationship between energetic disorder and open-circuit voltage in bulk heterojunction organic solar cells. Phys. Rev. B 2011, 84, No. 075210. (16) Shao, Y. C.; Yuan, Y. B.; Huang, J. S. Correlation of energy disorder and open-circuit voltage in hybrid perovskite solar cells. Nat. Energy 2016, 1, 15001−15006. (17) Shao, Y.; Xiao, Z. G.; Bi, C.; Yuan, Y. B.; Huang, J. S. Origin and elimination of photocurrent hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar cells. Nat. Commun. 2014, 5, No. 5784. (18) Sun, C.; Wu, Z. H.; Yip, H. L.; Zhang, H.; Jiang, X. F.; Xue, Q. F.; Hu, Z. C.; Hu, Z. H.; Shen, Y.; Wang, M. K.; Huang, F.; Cao, Y. Amino-Functionalized Conjugated Polymer as an Efficient Electron Transport Layer for High-Performance Planar-Heterojunction Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, No. 1501534. (19) Park, S. J.; Jeon, S.; Lee, I. K.; Zhang, J.; Jeong, H.; Park, J. Y.; Bang, J.; Ahn, T. K.; Shin, H. W.; Kim, B. G.; Park, H. J. Inverted planar perovskite solar cells with dopant free hole transporting material: Lewis base-assisted passivation and reduced charge recombination. J. Mater. Chem. A 2017, 5, 13220−13227. (20) Zheng, X. P.; Chen, B.; Dai, J.; Fang, Y. J.; Bai, Y.; Lin, Y. Z.; Wei, H. T.; Zeng, X. C.; Huang, J. S. Defect passivation in hybrid perovskite solar cells using quaternary ammonium halide anions and cations. Nat. Energy 2017, 2, 17102−17110. (21) Wang, N.; Zhao, K. X.; Ding, T.; Liu, W. B.; Ahmed, A. S.; Wang, Z. R.; Tian, M. M.; Sun, X. W.; Zhang, Q. C. Improving

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (330) 972 3406. ORCID

Xiong Gong: 0000-0001-6525-3824 Author Contributions §

J.Q. and X.Y. contributed to this work equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by NSFC (51329301). REFERENCES

(1) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties. Inorg. Chem. 2013, 52, 9019−9038. (2) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat. Photonics 2014, 8, 506−514. 7075


Article

ACS Omega

polymer/nanocrystal solar cells. Phys. Chem. Chem. Phys. 2014, 16, No. 25684. (38) Wang, K.; Liu, C.; Yi, C.; Chen, L.; Zhu, J. H.; Weiss, R. A.; Gong, X. Efficient Perovskite Hybrid Solar Cells via Ionomer Interfacial Engineering. Adv. Funct. Mater. 2015, 25, 6875−6884. (39) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p-n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510− 519. (40) Lee, J. W.; Kim, S. G.; Bae, S. H.; Lee, D. K.; Lin, O.; Yang, Y.; Park, N. G. The Interplay between Trap Density and Hysteresis in Planar Heterojunction Perovskite Solar Cells. Nano Lett. 2017, 17, 4270−4276.

Interfacial Charge Recombination in Planar Heterojunction Perovskite Photovoltaics with Small Molecule as Electron Transport Layer. Adv. Energy Mater. 2017, 7, No. 1700522. (22) Gu, P. Y.; Wang, N.; Wang, C. Y.; Zhou, Y. C.; Long, G. K.; Tian, M. M.; Chen, W. Q.; Sun, X. W.; Kanatzidis, M. G.; Zhang, Q. C. Pushing up the efficiency of planar perovskite solar cells to 18.2% with organic small molecules as the electron transport layer. J. Mater. Chem. A 2017, 5, 7339−7344. (23) Lee, Y. H.; Luo, J. S.; Son, M. K.; Gao, P.; Cho, K. T.; Seo, J.; Zakeeruddin, S. M.; Gratzel, M.; Nazeeruddin, M. K. Enhanced Charge Collection with Passivation Layers in Perovskite Solar Cells. Adv. Mater. 2016, 28, 3966−3972. (24) Huang, X.; Wang, K.; Yi, C.; Meng, T. Y.; Gong, X. Efficient Perovskite Hybrid Solar Cells by Highly Electrical Conductive PEDOT:PSS Hole Transport Layer. Adv. Energy Mater. 2016, 6, No. 1501773. (25) Chang, J. J.; Lin, Z. H.; Zhu, H.; Isikgor, F. H.; Xu, Q. H.; Zhang, C. F.; Hao, Y.; Ouyang, J. Y. Enhancing the photovoltaic performance of planar heterojunction perovskite solar cells by doping the perovskite layer with alkali metal ions. J. Mater. Chem. A 2016, 4, 16546−16552. (26) Liang, Q. J.; Liu, J. G.; Cheng, Z. K.; Li, Y.; Chen, L.; Zhang, R.; Zhang, J. D.; Han, Y. C. Enhancing the crystallization and optimizing the orientation of perovskite thin film via controlling nucleation dynamics. J. Mater. Chem. A 2016, 4, 223−232. (27) Leblebici, S. Y.; Leppert, L.; Li, Y.; Reyes-Lillo, S. E.; Wickenburg, S.; Wong, E.; Lee, J.; Melli, M.; Ziegler, D.; Angell, D. K.; Ogletree, D. F.; Ashby, P. D.; Toma, F. M.; Neaton, J. B.; Sharp, I. D.; Weber-Bargioni, A. Facet-dependent photovoltaic efficiency variations in single grains of hybrid halide perovskite. Nat. Energy 2016, 1, 16093−16099. (28) Maitani, M. M.; Satou, H.; Ohmura, A.; Tsubaki, S.; Wada, Y. Crystalline orientation control using self-assembled TiO2 nanosheet scaffold to improve CH3NH3PbI3 perovskite solar cells. Jpn. J. Appl. Phys. 2017, 56, No. 08MC17. (29) Sun, C.; Xue, Q. F.; Hu, Z. C.; Chen, Z. M.; Huang, F.; Yip, H. L.; Cao, Y. Phosphonium Halides as Both Processing Additives and Interfacial Modifiers for High Performance Planar-Heterojunction Perovskite Solar Cells. Small 2015, 11, 3344−3350. (30) Wu, C. G.; Chiang, C. H.; Tseng, Z. L.; Nazeeruddin, M. K.; Hagfeldt, A.; Gratzel, M. High efficiency stable inverted perovskite solar cells without current hysteresis. Energy Environ. Sci. 2015, 8, 2725−2733. (31) Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J. Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Nat. Commun. 2013, 4, No. 2761. (32) Wang, K.; Liu, C.; Du, P. C.; Zheng, J.; Gong, X. Bulk heterojunction perovskite hybrid solar cells with large fill factor. Energy Environ. Sci. 2015, 8, 1245−1255. (33) Im, J. H.; Jang, I. H.; Pellet, N.; Gratzel, M.; Park, N. G. Growth of CH3NH3PbI3 cuboids with controlled size for highefficiency perovskite solar cells. Nat. Nanotechnol. 2014, 9, 927−932. (34) Nie, W.; Tsai, H. H.; Asadpour, R.; Blancon, J. C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H. L.; Mohite, A. D. High-efficiency solution-processed perovskite solar cells with millimeter-scale grains. Science 2015, 347, 522−525. (35) Xu, W.; Yao, X.; Meng, T. Y.; Wang, K.; Huang, F.; Gong, X.; Cao, Y. Perovskite hybrid solar cells with a fullerene derivative electron extraction layer. J. Mater. Chem. C 2017, 5, 4190−4197. (36) Nian, L.; Gao, K.; Liu, F.; Kan, Y. Y.; Jiang, X. F.; Liu, L. L.; Xie, Z. Q.; Peng, X. B.; Russell, T. P.; Ma, Y. G. 11% Efficient Ternary Organic Solar Cells with High Composition Tolerance via Integrated Near-IR Sensitization and Interface Engineering. Adv. Mater. 2016, 28, 8184−8190. (37) Li, Z.; Wang, W. Y.; Greenham, N. C.; McNeill, C. R. Influence of nanoparticle shape on charge transport and recombination in 7076
