Iodomethane-Mediated Organometal Halide Perovskite with Record

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Iodomethane-Mediated Organometal Halide Perovskite with Record Photoluminescence Lifetime Weidong Xu, John A McLeod, Yingguo Yang, Yimeng Wang, Zhongwei Wu, Sai Bai, Zhongcheng Yuan, Tao Song, Yusheng Wang, Junjie Si, Rongbin Wang, Xingyu Gao, Xinping Zhang, Lijia Liu, and Baoquan Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05770 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 18, 2016

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ACS Applied Materials & Interfaces

Iodomethane-Mediated Organometal Halide Perovskite with Record Photoluminescence Lifetime Weidong Xu1, John A. McLeod1*, Yingguo Yang2, Yimeng Wang3, Zhongwei Wu1, Sai Bai5, Zhongcheng Yuan5, Tao Song1, Yusheng Wang1, Junjie Si4, Rongbin Wang1, Xingyu Gao2, Xinping Zhang3, Lijia Liu1*, Baoquan Sun1* 1

Jiangsu Key Laboratory of Carbon-based Materials, Institute of Functional Nano and Soft

Materials (FUNSOM), Soochow University, Suzhou, Jiangsu, 215123, China 2

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

Academy of Sciences, Shanghai, 201204, China 3

Institute of Information Photonic Technology and College of Applied Sciences, Beijing

University of Technology, Beijing, 100124, China 4

State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and

Applications, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China 5

Department of Physics, Chemistry and Biology (IFM), Linköping University Campus Valla,

Linköping, SE-58183, Sweden KEYWORDS: iodomethane, perovskite solar cell, photoluminescence lifetime, surface passivation, trap state, transient absorption, microwave detected photoconductivity

ACS Paragon Plus Environment

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ABSTRACT: Organometallic lead halide perovskites are excellent light harvesters for highefficiency photovoltaic devices. However, as the key component in these devices, a perovskite thin film with good morphology and minimal trap states is still difficult to obtain. Herein we show that by incorporating a low boiling point alkyl halide such as iodomethane (CH3I) into the precursor solution, a perovskite (CH3NH3PbI3-xClx) film with improved grain size and orientation can be easily achieved. More importantly, these films exhibit a significantly reduced amount of trap states. Record photoluminescence lifetimes of more than 4 µs are achieved, these lifetimes are significantly longer than that of pristine CH3NH3PbI3-xClx films. Planar heterojunction solar cells incorporating these CH3I-mediated perovskites have demonstrated a dramatically increased power conversion efficiency compared to the ones using pristine CH3NH3PbI3-xClx. Photoluminescence,

transient

absorption

and

microwave

detected

photoconductivity

measurements all provide consistent evidence that CH3I addition increases the number of exciton generated and their diffusion length, both of which assist efficient carrier transport in the photovoltaic device. The simple incorporation of alkyl halide to enhance perovskite surface passivation introduces an important direction for future progress on high efficiency perovskite optoelectronic devices.


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INTRODUCTION Among the organic/inorganic hybrid perovskite family, CH3NH3PbI3 and CH3NH3PbI3-xClx are the two most intensively investigated materials. These two materials are usually made by mixing CH3NH3I and PbI2 (or PbCl2, in the case of CH3NH3PbI3-xClx) as precursor solution and spincoating this solution on the desired substrate. The precursor gradually crystalizes into a perovskite crystal structure during post-annealing at a moderate temperature.1, 2 The last step, however, is difficult to control, since crystallization occurs at a fast rate (usually within less than half an hour). As a result, producing a pin-hole free film with high surface coverage using the solution-based method has always been a challenging task, especially for planar solar cells. Furthermore, CH3NH3PbI3 films usually exhibit a mesoporous or branched structure, due to the incomplete conversion of PbI2.3,

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These rough surfaces contain multiple defects and grain

boundaries that hinder charge carrier transport. These defects and boundaries result in generating numerous carrier trap states. Photocurrent hysteresis has been commonly observed during device operation and attributed to a trap-related origin.5 The film morphology can be improved using a vapor assisted growth, which requires a long growth time,6 or a two-step deposition strategy.7 On the other hand, CH3NH3PbI3-xClx has been more commonly used for planar structured devices, because it is found that addition of Cl increases the crystal domain size with a preferred orientation, hence an increased electron diffusion length.8, 9 Although the initial production of CH3NH3PbI3-xClx still exhibit large pores in the films,10 various techniques has been applied such as thermal annealing, variation of precursor concentrations, or use of different solvents to tune the crystallization dynamics for better film morphology.11, 12, 13 Perovskite films with millimetersale grains achieved using a hot-casting process have also been reported.14


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Like other inorganic crystals, ion vacancies are the major type of defect in these perovskites. Theoretical studies have shown that there are under-coordinated Pb ions within the perovskite crystals, in other words, iodide vacancies.15, 16 These defect centers could act as charge traps, reducing the amount of free charge carriers. Because exciton separation and carrier transportation are largely influenced by defects within the active layer, perovskite layers with poor crystallinity result in high nonradiative recombination rates. Exciton decay lifetime hence becomes one important parameter to evaluate perovskite thin film quality, and this can be measured using time-resolved photoluminescence (PL) spectroscopy. The commonly observed PL lifetime in thin film CH3NH3PbI3 and CH3NH3PbI3-xClx is several tens of ns, and by optimizing preparation techniques, the value can be brought up to several hundreds of ns.9, 17, 18 On the other hand, single crystal perovskite CH3NH3PbI3 has demonstrated an extra-long diffusion length of >10 µm with a PL decay lifetime up to 1 µs.19 Aside from controlling the growth conditions for perovskite layers, additives have been used to reduce the formation of defect and improve crystallization dynamics. There have been several successful examples for defect passivation by introducing an extra iodide source or electron-rich species during synthesis or as post-treatment. For example, adding the proper amount of extra PbI2 in the precursor for a CH3NH3PbI3 film leads to reduced recombination in the grain boundaries, and the resulting film has a PL lifetime of 100 ns.20 Treating the film with iodopentafluorobenzene (IPFB) and Lewis bases such as pyridine and thiophene has been reported as an effective way to passivate surface defects, and this treatment produces films with a PL lifetime up to 2 µs.21, 22 On the other hand, it has been reported that introducing long chain diiodo-alkane such as diiodooctane during the perovskite formation leads to better


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crystallization.23, 24 However, a decrease in PL lifetime has been observed from films prepared with these additives.23 IPFB or Lewis base treatment only helps reducing the surface defect due to the large molecule size. Since perovskite crystal structure (i.e. lattice size) is so sensitive to the size of the organic cations,25, 26 we need to seek other electron-rich species that are small enough to infiltrate the PbI2 or PbCl2 lattice without jeopardizing the formation of corner-sharing PbI6 octahedra. Previously, we have observed that during the formation of perovskite by thermal evaporation, there are two carbon species in the X-ray photoelectron spectroscopy (XPS) spectra. We have attributed them to CH3NH3+ and a CH3I-related species, respectively.27 This work has inspired us to deliberately add CH3I into the precursor solution. CH3I is a suitable size for intercalation into the PbI6 lattice. Therefore, rather than solely providing surface passivation, it is more like a meditator, which screens the overall lattice and patches the I vacancies. CH3I also exhibits a low boiling point, which can easily escape from the system without leaving unwanted organic residue, making room for the CH3NH3+ cation to build the perovskite structure. In this paper, we demonstrate that by incorporating CH3I as a mediating agent, thin film CH3NH3PbI3-xClx can be made with good crystallinity as confirmed by 2-dimensional grazing incident X-ray diffraction (2D GIXRD). Ultraviolet-visible (UV-vis) absorption spectra, transient absorption (TA) and microwave detected photoconductivity (MDP) results show the mediated film contains more free excitons upon light absorption, and significantly reduced trap states. These films exhibit a PL lifetime up to 4 µs, which is much longer than any perovskite films reported so far. Planar heterojunction solar cells incorporating these films demonstrate an increased power conversion efficiency (PCE) compared to those using regular CH3NH3PbI3-xClx films.


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EXPERIMENTAL SECTION Materials and synthesis. N,N-dimethylformamide (DMF) (anhydrous, amine free; 99.9%), hydroiodic acid (57% in water), PbCl2 (99.999%), and tetramethylammonium hydroxide (TMAH) (98%) were purchased from Alfa-Aesar. Poly(methylmethacrylate) (PMMA), chlorobenzene (anhydrous; 99.9%), zinc acetate dihydrate (> 98%) and methylamine (33% in absolute ethanol) were purchased from Sigma-Aldrich. Ethanol (HPLC grade), ethyl acetate, and dimethyl sulphoxide (DMSO) were purchased from J & K Chemicals. Phenyl-C61-butyric acid methyl

ester

(PCBM)

and

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS) (CLEVIOS Al 4083) were purchased from Merck, Solenne and Heraeus respectively. All chemicals were used upon receiving without further purification. CH3NH3I was synthesized according to standard procedure described in literature.2 Pristine CH3NH3PbI3-xClx precursor solution was prepared by dissolving CH3NH3I and PbCl2 in DMF with a molar ratio of 3:1. The mixture was heated and stirred at 60 °C overnight, and filtered through polytetrafluoroethylene (PTFE) filters (0.45 µm). The modified precursor solution was prepared following the same process, which CH3I partly substituting CH3NH3I in the mixture. Based on device performance test, the molar ratio of the three components had been optimized to 2.1:0.9:1 (CH3NH3I : CH3I : PbCl2). All procedures were done in glove box. Colloidal ZnO nanocrystals were synthesized by a solution-precipitation process following literature procedures.28 In brief, 15 mL of 0.276 M TMAH in ethanol was added dropwise to a solution of 0.051 M zinc acetate in 90 mL DMSO under constant stirring. Afterwards, ethyl acetate was added to precipitate ZnO nanocrystals from the liquid suspension. ZnO particles were collected by centrifuge, and redissolved in ethanol.


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Device fabrication. Patterned indium tin oxide (ITO) coated glass substrates (20Ω·sq–1) were ultrasonicated with acetone, ethanol, and deionized water for 20 min each, and treated with ultraviolet ozone plasma for 15 min. PEDOT:PSS was then spin-coated at 4000 rpm for 30 s and baked at 140 °C for 20 min. Both kinds of perovskite films were deposited by spin-coating precursor solutions on the as-prepared glass/ITO/PEDOT:PSS substrates at 3,000 rpm for 40 s in a nitrogen-filled glovebox. After lying in a Petri dish at room temperature for 10 min, the perovskite films were baked on a hot plate at 95 °C for 30 min, allowing the color of films turn into dark brown. Then a solution of PCBM with 30 mg/mL in chlorobenzene was spin-coated first at 1000 rpm for 6 s and then 2000 rpm for 30 s, followed by a solution of colloidal ZnO nanocrystals with 8 mg/mL in ethanol at 3000 rpm for 30 s. Finally 120 nm Al electrode was deposited through a shadow mask under 10-6 Torr using a thermal evaporation system (Mini SPECTRO, Kurt J. Lesker Co.). The device area is 7.25 mm2 determined by the top aluminum electrode coverage. Characterization. J–V curves of devices were measured under a Newport 94023A solar simulator with a 300 W Xenon lamp in a glovebox, while an air mass filter was used to generate a simulated AM 1.5G solar spectrum irradiation source. Calibrated by a Newport standard silicon solar cell 91150, the irradiation intensity was 100mW cm2. External quantum efficiency (EQE) was measured by a Newport monochromator 74125 and power meter 1918 with a silicon detector 918D. A Keithley 2612 was used to record all the electrical data. An Edinburgh Instruments FLS920 fluorescence spectrometer was used to acquire the time-resolved and steady-state PL spectra with the excitation density of 4 nJ cm-2. A field emission scanning electron microscope (FEI Quanta 200) was used to conduct scanning electron microscopy (SEM) images. A UV-vis spectrophotometer (PerkinElmer Lambda 750) was used to obtain optical


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characterization. XPS measurement was carried out in a SPECSTM ultrahigh vacuum angular resolved photoelectron spectroscopy system which includes an analysis chamber and an evaporation chamber. The base pressures in two chambers preceded 2 × 10−10 and 4 × 10−10 mbar, respectively. XPS spectra were measured by using a monochromatized Al Kα source (1486.7 eV) and calibrated by Au. The regular XRD patterns were obtained by PANalytical (Empyrean) equipment while the GIXRD measurements were performed at the BL14B1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF). Two-dimensional GIXRD patterns were acquired by a MarCCD mounted vertically at a distance 223 mm from the sample, the grazing incidence angle of X-ray was 0.2 degree with respect to the surface plane and exposure time was 20 s. The 2D GIXRD patterns were analyzed using the FIT2D software and displayed in scattering vector q coordinates, where q = 4πsinθ/λ, θ is half of the diffraction angle, and λ is the X-ray wavelength. The transient absorption was obtained by a laser system consists of a commercial Coherent Mira 900 femtosecond laser pumped by a Verdi V6 laser with 6 W @ 532 nm laser. As a result, 200 fs pulses with a repetition rate of 76 MHz at wavelengths between 700 and 980 nm were generated. The output of the femtosecond laser was amplified by a regenerative amplifier (Legend Elite Duo) at 1 KHz. The amplified pulses with energy of about 12 mJ and 130 fs pulse duration with the wavelength of 800 nm were obtained. The output of the laser was split into two beams. The one pass through the crystal of BBO to generate the femtosecond laser at 400 nm as the pump pulse, and the residual was focused in a 1cm cell containing a heavy water to generate whitelight continuum (425-1200 nm). The continuum was used as the probe pulse. The pump pulse was passed through several reflectors and a delay line (up to 3.5 fs). Then the probe beam was overlapped with the pump beam in the sample. The probe beam passed through the concave


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mirror with the focus distance of 200 mm, and detected by the Avantes spectrum. The steadystate photoconductivity curves and its decay with a resolution of 0.5µs were obtained by the MDP map-laboratory equipment from Freiberg Instruments in Germany. The instrument provides a 785 nm laser with energy varies from 20 to 65 mW. Conditions with pulse of 300 µs, decay of 300 µs, summation of 1000, and spot diameter of 0.2 mm and a laser intensity of 65 mW were chosen. For the characterization of the perovskite layers, glass/ITO/PEDOT:PSS/Perovskite structures were prepared under the identical condition as used in device fabrication. For regular XRD, 2D GIXRD, PL, TA and UV-vis light absorption measurements, samples were sealed by a spincoated top layer of PMMA (10 mg/mL in chlorobenzene) to prevent decomposition. For XPS and SEM measurement, fresh samples were prepared and transferred from glovebox into the load lock chambers within 5 minutes to minimize the air exposure. RESULTS AND DISCUSSION The intrinsic solubility of PbCl2 in DMF is lower than 30 mg/mL.29 The addition of CH3I drastically improves the solubility of PbCl2 in the precursor solution up to more than 300 mg/mL. It is interestingly found that not only CH3I but other small alkyl iodides, such as CH3CH2I and CH2I2, also increase the solubility of PbCl2 in DMF. The improved solubility of PbCl2 in DMF is shown visually in Figure S1, which is because CH3I can act as a chelating agent to break Pb-Cl bond and ease the production of Pb2+. The detailed mechanism of the chelation is similar to Chueh’s work.24 Photovoltaic devices using CH3NH3PbI3-xClx active layers formed from precursor containing CH3I at a ratio of 2.1:0.9:1 (CH3NH3I : CH3I : PbCl2) yields the best PCE of all alkyl halide species and concentrations we investigated. The device PCEs of various ratios of CH3NH3I : CH3I : PbCl2 are detailed summarized in Table S1. Hence our discussion


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below focuses on the differences between pristine CH3NH3PbI3-xClx and that formed from CH3I at a mole ratio of 2.1:0.9:1 (CH3NH3I : CH3I : PbCl2). Figure 1a and Figure 1b compare the SEM images of perovskite films prepared with and without the introduction of CH3I in the precursor (hereafter referred to as pristine and CH3I-mediated, respectively). The two films exhibit obviously different morphologies. The pristine film displays a surface morphology with random grains overlapping each other with sizes of 0.5~1 µm, which is similar to films commonly observed in the literature,30, 31 while the grain sizes in the CH3Imediated film are significantly two times larger of 2~3 µm. The crystal structures of these two films are characterized by regular XRD and 2D GIXRD, respectively. The regular XRD patterns are shown in Figure S2 with similar strong-diffraction peaks while the 2D GIXRD patterns are shown in Figure 1c and Figure 1d, and the azimuthally integrated scattering intensities are compared in Figure 1e. The two films exhibit similar line profiles, confirming the perovskite structure is retained with CH3I addition. The intense peak at q = 10 nm-1 is assigned to the (110) plane of CH3NH3PbI3-xClx perovskite structure. In the 2D images, it is clear that the CH3Imediated perovskite exhibits sharper diffraction spots, and weaker diffraction rings than the pristine perovskite, and the CH3I-mediated film has a smaller full width at half maximum (FWHM) (Figure 1f) at the azimuth angle of 90°. These observations indicate that CH3Imediated perovskite has a preferred orientation parallel to substrate and a higher crystal quality than our pristine perovskite. To verify the chemical composition of the perovskite films, especially the carbon species, XPS measurements were conducted. Figure 2 shows the core level spectra of all elements in the perovskite films. It can be seen that the two films contains nearly identical peak profiles at the N 1s, Pb 4f, I 3d, and Cl 2p, but display a noticeable difference at the C 1s. The pristine film


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contains two carbon peaks at 285.5 eV and 286.6 eV, respectively, and both are of relatively equal intensities. This is consistent with our previous results on thermal evaporated perovskite film,27 and is also in agreement with the results commonly observed in other literature.32, 33 In our previous work, we pointed out that the peak at the lower energy side was related to another carbon species, and based on its binding energy, we attributed it to C element in a CH3I environment. Here, this peak is enhanced in the CH3I-mediated perovskite, which supports our assumption that a CH3I-type species is present in the perovskite, and the amount can be enhanced by deliberately adding CH3I during the fabrication process. Here it should be noted that we are not saying that there necessarily is free CH3I in the final perovskite, as there could also be C atoms chemically bonded to I, or present in a chemical environment that resembles the one in CH3I. Since the XPS is mostly surface detected, combining with the unshifted XRD diffraction peaks, we believe CH3I exists on the surfaces and boundaries of the perovskite crystal grains to passivate the I vacancies. However we should note that angle-resolved XPS (in which grazing incidence can enhance the signal from surface monolayers) produces essentially identical spectra suggesting that CH3I is more-or-less homogeneously distributed to a depth of a few nanometers. Planar

heterojunction

solar

cells

are

fabricated

with

a

configuration

of

ITO/PEDOT:PSS/perovskite/PCBM/ZnO/Al. Figure 3a shows a representative cross section SEM image of the device. The pristine and CH3I-mediated perovskite films both have similar thicknesses of ~300 nm. The devices with CH3I-mediated perovskite exhibit an average PCE of 15.06%, with the best being 16.18%, which represents a significant improvement over the ones with pristine perovskite (average 13.07%, highest 14.02%). The current density–voltage (J-V) curves of the best device are shown in Figure 3c and the full cell parameters are shown in Table 1. The histograms of PCEs measured from 30 devices can be found in Figure S3. It can be seen


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that the solar cells using CH3I-mediated perovskite exhibit a higher open-circuit voltage (VOC), short-circuit current (JSC), and fill factor (FF) than those using pristine perovskite. The integrated EQE spectra in Figure S4 are in good agreement with the measured JSC, and the CH3I-mediated devices exhibit EQE enhancement over the entire absorption range. The above observation suggests that the large, homogeneous crystal grains in perovskite film formed due to CH3I mediation can offer improved percolation path for excitons and carriers to transport and diffuse. We will further illustrate this point in detail. The improvement in the FF of CH3I-mediated cells can be attributed to the better perovskite crystal formation as has been observed in 2D GXRD. Large crystal grains provide increased contact area for charge transport. To rationalize the increase in VOC and JSC, we attempt to fit the J-V curves using a realistic diode model,34 following equation below: ܸ݁ + ܴௌ ‫ܬ‬ ܸ + ܴ௦ ‫ܬ‬ ‫ܬ = ܬ‬଴ ൤1 − ݁‫ ݌ݔ‬൬ ൰൨ + ‫ܬ‬௣௛ − ݊݇஻ ܶ ܴ௦௛ The schematic layout of the diode model is also illustrated in (Figure 3b) according to the equation. The series resistance Rs is a measure of contact resistance, the shunt resistance Rsh is a measure of the unwanted conductive channels in the photovoltaic layer, the photocurrent Jph is a measure of the current generated by light illumination, the leakage current J0 is a measure of the diode current under reverse bias, and the ideality factor n is a measure of the quality of the diode. The fitted results are listed in Table S2. Note that an ideal photovoltaic should have a large Jph, a small (but non-zero) J0, a zero Rs, an infinite Rsh, and n = 1. The fitted Jph and Rs are very similar for both forward and backward scans from both devices, as shown in Table S2. This indicates that there are the analogous contacts between the layers of the devices. This is in agreement with the XRD data, that the two films exhibit identical diffraction peaks. J0 however, is substantially


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lower, and Rsh is substantially higher for the CH3I-mediated devices than that of the pristine devices. These differences are the main origin of the higher JSC and VOC in the CH3I-mediated devices. The ideality factor is also lower for the CH3I-mediated devices. A high ideality factor is usually associated with a large quantity of charge traps,35 so here a reduced n from 1.98 ± 0.07 to 1.60 ± 0.1 by average indicates that the introduction of CH3I during the perovskite film fabrication can reduce the number of charge traps. Another noteworthy property that contributes to the improved PCE is the huge enhancement in the UV-vis absorption in the CH3I-mediated perovskite, as shown in Figure 4a. To examine the spectra quantitatively, we use an Elliot-Wannier exciton model to obtain the values of band gap,36 and the results are listed in Table S3. It can be seen that the model matches the experimental very well, even below the band edge where the CH3I-mediated perovskite exhibits more absorption than the pristine perovskite (also see Figure S5 for the UV-vis absorption extending over a larger range in wavelength). The CH3I-mediated perovskite shows a much more prominent excitonic peak in comparison with that of reference, as well as an overall increase in absorbance, compared to the pristine perovskite. Excitons in CH3NH3PbI3 are thought to be relatively freely-moving,36 and “self-trapped” excitons are not thought to have a significant effect in 3-dimensional halide perovskites.37, 38 Furthermore, since the exciton binding energy is close to room temperature, excitons can easily thermalize into free carriers. Here the larger concentration of excitons in CH3I-mediated films with respect to the pristine films implies an increase in the number charge carriers over the time scales relevant to photovoltaic device operation. Another amazing property of the CH3I-mediated perovskite is its incredibly long photoluminescence lifetime. As shown in Figure 4b, the time-resolved PL spectrum of the CH3I-


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mediated perovskite exhibits a much slower decay than that of the pristine perovskite, suggesting the excitons have a longer lifetime compared to the pristine film. It has been found that perovskite PL usually contains two components: the fast decay component, τ1, might come from bimolecular recombination. The long decay component τ2 could be attributed to recombination of free carriers in the radiative channel.20, 39 Thus a bi-exponential model is used to fit the two spectra according to the following equation: ܲ‫ܮ‬ሺ‫ݐ‬ሻ = ‫ܣ‬ଵ exp ൬−

‫ݐ‬ ‫ݐ‬ ൰ + ‫ܣ‬ଶ exp ൬− ൰ ߬ଵ ߬ଶ

The detailed fitting parameters can be found in Table 2. The result reveals that the CH3Imediated film contains two decay lifetimes, 4210 ns and 1210 ns, while the pristine films are 277 ns and 84 ns, respectively. Here even the fast component in the CH3I-mediated film is still much longer than the slow components in the pristine film in our study as well as other reported values.19 This remarkable PL lifetime enhancement reveals an overall reduction in nonradiative recombination within the material, giving the strong evidence of a reduced number of charge traps upon CH3I mediation. Regarding to the steady-state PL spectra in the inset of Figure 5b, both films emit light at the same wavelength, but the intensity of CH3I-mediated film shows one order of magnitude higher than that of the pristine one. The high PL intensity indicates a large number of electron-hole pair recombination, indicating that more excitons are generated in the CH3I-mediated film. This result is also in agreement with the enhanced absorption and increased exciton binding energy extracted from UV-vis spectroscopy for CH3I-mediated film, as has been shown in Figure 4a, which supports the higher JSC value obtained from device performance test. TA spectroscopy is conducted to further examine the interplay between free carriers and excitons. Figure 5a and Figure 5b compare the dynamics of pristine and CH3I-mediated perovskite films probed at three wavelengths selected based on the observed PL emission. 781


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nm corresponds to the emission maximum, while 758 nm and 804 nm are the position of FWHM at the higher and lower energy side, respectively. A jump change (∆m) of optical density (OD) appears with a ~100 fs delay after the pump probe, which can be interpreted as exciton generation. The intensity decay in the ∆m OD afterward represents the separation of electron and hole pairs. The intensity decay at 758 nm, 781 nm, and 804 nm can be fit with a bi-exponential function (of the same form as was used for PL fitting). The results are shown in Table S4. The shorter time constant is on the order of 1 ps, and these results are similar to those previously obtained for perovskite films.40 The longer time constant is on the order of 1 ns for both our films and is somewhat longer than is typically observed. However this was also the case with our PL time constants so these two measurements are consistent. It can also be seen that the CH3Imediated film shows a slower decay than pristine film, suggesting a longer electron-hole pair separation time. Comparing the TA spectra in Figure 5c and Figure 5d, the TA from pristine perovskite shows significant photo-bleaching at the exciton level, i.e. 1.648 eV, as indicated by the dashed line marked as “Exc”, while the CH3I-mediated perovskite does not, this is also evident from the fitted ∆m OD amplitude at 758 nm which are larger in the pristine film by roughly a factor of 4. On the other hand, the UV-vis absorption is more intense in the CH3Imediated film, as noted above. This means that exciton in the CH3I-mediated perovskite are more mobile, in other words, there is more efficient charge separation. The time constant for the decay of the 758 nm TA is too long in the CH3I-mediated film to be accurately fit in our model, while the pristine film has a time constant of around 1 ns. This supports the observations from the PL decay for reduced recombination and trap density in the CH3I-mediated film compared to the pristine film. In fact, all of the TA time constants are significantly longer in the CH3Imediated film than in the pristine film. Note however that the time window for the TA


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measurements (1 ns) is too short to determine if there are TA kinetics that match the time constants found from the PL decay (which were at least 10s of ns). MDP is utilized to explore all possible transitions between defect levels and bands in the forbidden gap of the perovskite films.41 In this technique, the photo-induced conductivity of the perovskite film

can

be obtained

through

a contactless

microwave detector.

The

photoconductivity as a function of time for both perovskite films is shown in Figure 6. The photoconductivity instantly increases with the initial laser illumination, as the laser generates free

carriers.

The

photoconductivity

then

gradually

increases

to

saturation.

The

photoconductivity increase is interpreted as the trap state filling in perovskite film until saturation is reached. This process can be correlated with the device photovoltaic soaking process. Photoconductivity decay is observed after the laser switches off. The decay generally shows two portions: a very fast portion from exciton related direct band-to-band recombination, and a slower portion trap state related from thermal re-emission.41 In our attempt to fit the MDP curves, the exciton related band-to-band recombination region (denoted as region A in Figure 6a) can be adequately fit with an exponential decay using the longer of the two time constants obtained from PL decay fitting, as shown in Table S5. In addition, the exciton life constant of CH3I-mediated film is 10.9 ± 0.7 µs extracted from MDP, which is longer than that of pristine (8.1 ± 0.2 µs). The increased exciton life time constant extracted from MDP of CH3I-mediated film is in line with exciton lifetime of PL. To fit the thermal re-emission of trap states (denoted as regions B and C in Figure 6a), we had to fit two additional exponential decays to obtain satisfactory results. The detailed fitting parameters can be found in Table S6. In fact, if we follow the relationship between energy of the traps (Etrap) and the time constant t for thermal emission by


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‫ܧ‬௧௥௔௣ = ݇஻ ݈ܶ݊ሺߙ‫ݐ‬ሻ assuming material-specific constant α equals to 1, we obtain the energies for the trap states, shown in Table S5. CH3I-mediated film shows slightly shallower photobleaching states of 51.1 ± 0.5 meV that that of pristine film (55.4 ± 0.6 meV). However, the CH3I-mediated film displays much shorter photobleaching state time constant of 111 ± 2 µs than that of pristine one (164 ± 4 µs ), which indicates that there are less trap states once CH3I is incorporated in perovskite. The trap sites analysis is in line with TA measurement. CONCLUSIONS In summary, we demonstrate that incorporating iodomethane CH3I in the precursor solution results in dramatically enhancing CH3NH3PbI3-xClx film quality with less trap states. This synthesis technique is a simple, highly reproducible, low cost method, yet it increases the solubility of PbCl2 in DMF, facilitates the production of thin films with increased grain size and crystallinity, and substantially improves the optical properties of the CH3NH3PbI3-xClx thin film. We confirm that the CH3I-mediation significant reduces the quantity of trap states in the perovskite, and increases the mobility of the excitons. The PL lifetime of CH3I-mediated perovskite is more than 4 µs, which is longer than any PL lifetime in CH3NH3PbI3-xClx reported so far. CH3I-mediated perovskites exhibit increased UV-vis absorbance, a stronger exciton resonance, and a larger photoconductivity than pristine perovskites. Combined with the ultralong PL lifetime in the CH3I-mediate perovskites, these findings all suggest that CH3I mediation helps increase the generation and diffusion length of excitons in CH3NH3PbI3-xClx. As excitons that reach the boundary of the perovskite layer can dissociate into charge carriers, this explains why CH3I mediation improves photovoltaic device performance. Planar solar cells incorporating such perovskite films exhibit improved performance compared to regular ones. These results


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provide insight in the synthesis of high quality CH3NH3PbI3-xClx films for better device performance. We anticipate that including CH3I in the precursor solution as well as using one of the more sophisticated synthesis techniques shown to improve CH3NH3PbI3-xClx optoelectronic performance (such as solvent annealing, hot casting, etc.) may yield even further improvements to solar cell PCEs. Besides, our concept is a general method to organic-inorganic metal halide perovskite system, for example formamidinium (FA)-, Cs-, Br-, Sn- or Ge- based perovskite. Hence it has a great potential in high quality and none-toxic metal perovskite in the future study. Furthermore, the ultra-long PL lifetime of CH3I-mediated CH3NH3PbI3-xClx may also provide a promising strategy for increasing the performance of hybrid perovskite-based LEDs and lasers. ASSOCIATED CONTENT Supporting Information Solution Images of different alkyl iodides enhance PbCl2 solubility in DMF; Histogram of PCEs from perovskite solar cells; EQE spectra of devices; TA spectra from perovskite films; Tables with parameters from J-V curves fitting, summary of PCEs, UV-vis and TA absorption spectra estimating, MDP fitting. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected], [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEGMENT This work is funded by the National Basic Research Program of China (973 Program) project number 2012CB932402, and the National Natural Science Foundation of China (NSFC) project number U1432106. The authors acknowledge support from Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO-CIC), Soochow University. REFERENCES (1) Im, 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, 4088-4093. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid Solar Cells Based on Meso-superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (3) Tidhar, Y.; Edri, E.; Weissman, H.; Zohar, D.; Hodes, G.; Cahen, D.; Rybtchinski, B.; Kirmayer, S., Crystallization of Methyl Ammonium Lead Halide Perovskites: Implications for Photovoltaic Applications. J. Am. Chem. Soc. 2014, 136, 13249-13256. (4) Wu, Y.; Islam, A.; Yang, X.; Qin, C.; Liu, J.; Zhang, K.; Peng, W.; Han, L., Retarding the Crystallization of PbI2 for Highly Reproducible Planar-Structured Perovskite Solar Cells via Sequential Deposition. Energy & Environ. Sci.2014, 7, 2934-2938. (5) Shao, Y.; Xiao, Z.; Bi, C.; Yuan, Y.; Huang, J., Origin and Elimination of Photocurrent Hysteresis by Fullerene Passivation in CH3NH3PbI3 Planar Heterojunction Solar Cells. Nat. Commun. 2014, 5.5784. (6) Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y.; Li, G.; Yang, Y., Planar Heterojunction Perovskite Solar Cells via Vapor-assisted Solution Process. J. Am. Chem. Soc. 2013, 136, 622-625. (7) Zhao, Y.; Zhu, K., Solution Chemistry Engineering Toward High-efficiency Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 4175-4186.


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(8) Zhao, Y.; Zhu, K., CH3NH3Cl-assisted One-step Solution Growth of CH3NH3PbI3: Structure, Charge-carrier Dynamics, and Photovoltaic Properties of Perovskite Solar Cells. J. Phys. Chem. C 2014, 118, 9412-9418. (9) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; 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. (10) Liu, M.; Johnston, M. B.; Snaith, H. J., Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. (11) Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J., Morphological Control for High Performance, Solution‐Processed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater. 2014, 24, 151-157. (12) Conings, B.; Baeten, L.; De Dobbelaere, C.; D'Haen, J.; Manca, J.; Boyen, H. G., Perovskite‐based Hybrid Solar Cells Exceeding 10% Efficiency with High Reproducibility Using a Thin Film Sandwich Approach. Adv. Mater. 2014, 26, 2041-2046. (13) Dualeh, A.; Tétreault, N.; Moehl, T.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M., Effect of Annealing Temperature on Film Morphology of Organic–Inorganic Hybrid Pervoskite Solid‐ State Solar Cells. Adv. Funct. Mater. 2014, 24, 3250-3258. (14) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A., High-efficiency Solution-processed Perovskite Solar Cells with Millimeter-scale Grains. Science 2015, 347, 522-525. (15) Yin, W.-J.; Shi, T.; Yan, Y., Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104, 063903. (16) Kim, J.; Lee, S.-H.; Lee, J. H.; Hong, K.-H., The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite. J. Phys. Chem. Lett. 2014, 5, 1312-1317. (17) Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X., Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636-642. (18) De Quilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S., Impact of Microstructure on Local Carrier Lifetime in Perovskite Solar Cells. Science 2015, 348, 683-686.


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(19) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K., Low Trap-state Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-522. (20) Chen, Q.; Zhou, H.; Song, T.-B.; Luo, S.; Hong, Z.; Duan, H.-S.; Dou, L.; Liu, Y.; Yang, Y., Controllable Self-induced Passivation of Hybrid Lead Iodide Perovskites Toward High Performance Solar Cells. Nano Lett. 2014, 14, 4158-4163. (21) Abate, A.; Saliba, M.; Hollman, D. J.; Stranks, S. D.; Wojciechowski, K.; Avolio, R.; Grancini, G.; Petrozza, A.; Snaith, H. J., Supramolecular Halogen Bond Passivation of Organic– inorganic Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 3247-3254. (22) Noel, N. K.; Abate, A.; Stranks, S. D.; Parrott, E. S.; Burlakov, V. M.; Goriely, A.; Snaith, H. J., Enhanced Photoluminescence and Solar Cell Performance via Lewis Base Passivation of Organic–inorganic Lead Halide Perovskites. ACS Nano 2014, 8, 9815-9821. (23) Liang, 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, 3748-3754. (24) Chueh, C.-C.; Liao, C.-Y.; Zuo, F.; Williams, S. T.; Liang, P.-W.; Jen, A. K.-Y., The Roles of Alkyl Halide Additives in Enhancing Perovskite Solar Cell Performance. J. Mater. Chem. A. 2015, 3, 9058-9062. (25) 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. (26) Mitzi, D. B., Synthesis, Structure, and Properties of Organic‐inorganic Perovskites and Related Materials. Prog. Inorg. Chem. 2007, 48, 1-121. (27) Liu, L.; McLeod, J. A.; Wang, R.; Shen, P.; Duhm, S., Tracking the Formation of Methylammonium Lead Triiodide Perovskite. Appl. Phys. Lett. 2015, 107, 061904. (28) Bai, S.; Wu, Z.; Wu, X.; Jin, Y.; Zhao, N.; Chen, Z.; Mei, Q.; Wang, X.; Ye, Z.; Song, T.; Liu, R.; Lee, S.-t.; Sun, B., High-performance Planar Heterojunction Perovskite Solar Cells: Preserving Long Charge Carrier Diffusion Lengths and Interfacial Engineering. Nano Res. 2014, 7, 1749-1758. (29) Synnott, J. C.; Butler, J. N., Chloride Reversible Electrodes for Use in Aprotic Organic Solvents. Anal. Chem. 1969, 41, 1890-1894.


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(40) Wang, L.; McCleese, C.; Kovalsky, A.; Zhao, Y.; Burda, C., Femtosecond Time-resolved Ttransient Absorption Spectroscopy of CH3NH3PbI3 Perovskite Films: Evidence for Passivation Effect of PbI2. J. Am. Chem. Soc. 2014, 136, 12205-12208. (41) Berger, B.; Schüler, N.; Anger, S.; Gründig‐Wendrock, B.; Niklas, J. R.; Dornich, K., Contactless Electrical Defect Characterization in Semiconductors by Microwave Detected Photo Induced Current Transient Spectroscopy (MD ‐ PICTS) and Microwave Detected Photoconductivity (MDP). Phys. Status Solidi A. 2011, 208, 769-776.


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FIGURES AND TABLES

Figure 1. SEM images of (a) pristine and (b) CH3I-mediated CH3NH3PbI3-xClx films; 2D GIXRD images of (c) pristine and (d) CH3I-mediated CH3NH3PbI3-xClx films; (e) azimuthally integrated intensity plots and (f) radially integrated intensity plots along the ring at q = 10 nm-1 of the corresponding GIXRD patterns of the two films.


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Figure 2. XPS spectra of (a) C 1s, (b) N 1s, (c) Pb 4f, (d) I 3d, (e) Cl 2p. The colours are consistent for all subplots.


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Figure 3. (a) Cross section SEM of a representative device, (b) Diode model used for J-V curve simulation, and (c) J-V curves with forward and backward scans of pristine and CH3I-mediated solar cells, with fitted curves based on the diode model.


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Figure 4. (a) UV-vis absorption spectra of pristine and CH3I-mediated perovskite films. The spectra are fit with the Elliot-Wannier exciton model, shown by the dashed lines. (b) Timeresolved PL spectra of pristine and CH3I-mediated perovskite films. The dashed lines are the fit using bi-exponential function. The steady-state PL of the two films are shown in the inset.


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Figure 5. Dynamics of (a) pristine and (b) CH3I-mediated perovskite films probed at 758 nm, 781 nm, and 804 nm. Inset in each figures: the expanded area at delay time before 20 ps. TA spectra between 0 ps and 50 ps of (c) pristine and (d) CH3I-mediated perovskite films. Their corresponding UV-vis and PL spectra are included for reference.


28

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(a)

(b)

Figure 6. Photoconductivity curves of pristine and CH3I-mediated perovskite films. The full spectra are shown in the inset, while the main figure shows the enlarged region after the light is switched off.


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Table 1. Electrical output characteristics of the devices with perovskite films with and without CH3I under stimulated AM 1.5 solar illumination at 100 mW cm-2 based on 30 devices each. Type

VOC

JSC

FF

PCE

[V]

[mA/cm2]

Pristine

0.93 ± 0.02

19.38 ± 0.84

0.72 ± 0.03

13.07 ± 0.64

CH3I-mediated

0.96 ± 0.02

20.60 ± 0.60

0.76 ± 0.03

15.06 ± 0.48

[%]

Table 2. Photoluminescence lifetime (PL) fitting parameters for perovskite films with and without CH3I. Sample

A1

τ1

A2

τ2

[a.u.]

[ns]

[a.u.]

[ns]

Pristine

0.46 ± 0.01

277 ± 4

0.50 ± 0.01

84 ± 2

CH3I-mediated

0.68 ± 0.02

4210 ± 60

0.29 ± 0.01

1210 ± 60


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ToC Figure


31

Iodomethane-Mediated Organometal Halide Perovskite with Record

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