Letter pubs.acs.org/NanoLett
Observation of Enhanced Hole Extraction in Br Concentration Gradient Perovskite Materials Min-cheol Kim,†,‡ Byeong Jo Kim,§ Dae-Yong Son,∥ Nam-Gyu Park,∥ Hyun Suk Jung,*,§ and Mansoo Choi*,†,‡ †
Global Frontier Center for Multiscale Energy Systems, Seoul 151-742, Republic of Korea Department of Mechanical Engineering, Seoul National University, Seoul 151-742, Republic of Korea § School of Advanced Materials Science & Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Korea ∥ School of Chemical Engineering and Department of Energy Science, Sungkyunkwan University, Suwon, Gyeonggi-do 440-746, Republic of Korea ‡
S Supporting Information *
ABSTRACT: Enhancing hole extraction inside the perovskite layer is the key factor for boosting photovoltaic performance. Realization of halide concentration gradient perovskite materials has been expected to exhibit rapid hole extraction due to the precise bandgap tuning. Moreover, a formation of Br-rich region on the tri-iodide perovskite layer is expected to enhance moisture stability without a loss of current density. However, conventional synthetic techniques of perovskite materials such as the solution process have not achieved the realization of halide concentration gradient perovskite materials. In this report, we demonstrate the fabrication of Br concentration gradient mixed halide perovskite materials using a novel and facile halide conversion method based on vaporized hydrobromic acid. Accelerated hole extraction and enhanced lifetime due to Br gradient was verified by observing photoluminescence properties. Through the combination of secondary ion mass spectroscopy and transmission electron microscopy with energy-dispersive X-ray spectroscopy analysis, the diffusion behavior of Br ions in perovskite materials was investigated. The Br-gradient was found to be eventually converted into a homogeneous mixed halide layer after undergoing an intermixing process. Br-substituted perovskite solar cells exhibited a power conversion efficiency of 18.94% due to an increase in open circuit voltage from 1.08 to 1.11 V and an advance in fill-factor from 0.71 to 0.74. Long-term stability was also dramatically enhanced after the conversion process, i.e., the power conversion efficiency of the post-treated device has remained over 97% of the initial value under high humid conditions (40−90%) without any encapsulation for 4 weeks. KEYWORDS: Perovskite solar cell, halide substitution, halide concentration gradient, photoluminescence, secondary ion mass spectroscopy, diffusion
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being actively researched. In order to obtain such enhanced performance and better stability, chemical composition engineering has been widely undertaken by utilizing a mixture of various suitable materials in the organic and halide sites (organic site: CH3NH3, HC(NH2)2; halide site: I, Br, Cl).7−9,11−14 Doping a certain amount of bromide or chloride into the halide site instead of iodide imparts manifold advantages to the perovskite solar cell. Most importantly, when Br is used as a dopant, the crystalline structure undergoes transition from the structurally metastable tetragonal phase to the stable cubic phase; the long-term stability under high humidity conditions is thus significantly enhanced.15,16 The second positive effect is
ince the inceptive use of organic−inorganic halide perovskite materials as photovoltaic light absorbers, perovskite solar cells have undergone revolutionary improvement in terms of their power conversion efficiency (PCE). Fundamental studies on perovskite materials,1 device architectures,2,3 fabrication processes,4−6 and elemental engineering of materials7−9 have fuelled the rapid development of perovskite solar cells. Consequently, a PCE of up to 22.1% has been achieved, as recently certified by National Renewable Energy Laboratory (NREL).10 Thus, the organic−inorganic halide perovskite solar cell is one of the most promising alternative energy resources with the advantages of high efficiency and low cost. However, despite the tremendous achievements in terms of performance, the long-term stability of these cells toward humidity or photoillumination and current−voltage hysteresis remains controversial. Therefore, higher efficiency and long-endurance perovskite solar cells are © XXXX American Chemical Society
Received: June 16, 2016 Revised: August 5, 2016
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DOI: 10.1021/acs.nanolett.6b02473 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Schematics of bromine doping with fume substitution doping method using HBr solution. Oxidized HBr turns into Br2 gas and consequently substitutes I ions to Br ions at the surface of perovskite layer. The reddish part of perovskite film at the surface is bromine-rich region and the black part of perovskite film at the bottom is iodine-rich region after the substitution. This partially substitution method enables spatial halide doping in pristine perovskite layer with the formation of gradational energy band level.
partially converted perovskite films with a halide gradient in accordance with depth. Earlier research suggested postsynthetic halide conversion with halogen gas as a prospective methodological strategy for obtaining partially converted halide perovskite films.25,29 However, this method has relatively fast reaction kinetics; therefore, it is impossible to control the halide diffusion process. In this study, we achieve slow conversion of halide ions in the perovskite layer by utilizing halogen fume produced by vaporization of hydrobromic acid (HBr). This moderate substitution method enables spatial substitution of the upper part of the pristine perovskite film, in contrast with the solution process employing a mixed halide precursor or the fast conversion method that cannot furnish partially substituted films. Using this unique halide substitution technique, Br ions are effectively incorporated into the mixed halide perovskite phase in a gradational manner; enhanced hole extraction and lifetime without surface damage via HBr-treatment is verified by PL measurements. The diffusion behavior of Br ions inside the layer through ion migration intermixing process is also evaluated. This facile halide exchange method also facilitates enhancement of the performance of modified perovskite solar cells. The Br-doped perovskite device exhibited a PCE of 18.94% due to the extreme increase of the Voc from 1.08 to 1.11 V derived from HBr-treatment. Moreover, for the duration of 4 weeks, the PCE of the HBr-treated perovskite device remained as high as 97% with storage under ambient conditions (relative humidity: 40−90%) without any encapsulation, whereas the PCE of the reference perovskite device declined to 83% over the same period. Figure 1 shows the experimental setup and the mechanism of halide substitution via the fume substitution doping method using HBr and the CH3NH3PbI3 (MAPbI3) reference perovskite film. The pristine, smooth, iodine-based reference perovskite film could be easily changed to the bromine-doped CH3NH3Pb(I1−xBrx)3 (MAPb(I1−xBrx)3) mixed halide perov-
the significant enhancement of the carrier transport properties of halide mixed organic−inorganic perovskite films, which leads to enhancement of the device performance.17,18 Br-doped as well as Cl-doped mixed halide perovskite photovoltaic devices exhibit improved carrier mobility and reduced carrier recombination rates.19 The longer carrier diffusion length of such mixed halide devices gives rise to higher fill factor (FF) and PCE values. As an additional advantage, the open circuit voltage (Voc) is also enhanced due to the relatively larger band gap of Br- or Cl-containing perovskite materials.20−23 Band gap engineering is easily possible by controlling the compositional ratio of the dopants, which does not merely lead to large Voc devices.24 Therefore, by exploiting these advantageous features via modification of the composition of organic−inorganic halide perovskite films, enhanced perovskite solar cell performance can be effectively achieved. Especially, a phase separation of I-rich region and Br-rich region, which forms halide concentration gradient and band gap gradient at the perovskite layer, provides several advantages for perovskite devices. The first expectation is that halide gradient brings a driving force for enhanced carrier extraction.25 Band gap tuning via variation of the halide composition dominantly affects the top of the valence band. Therefore, the halide gradient exerts a favorable effect on the drift of holes to the hole transport layer (HTL).26 Hole drift acceleration inside the perovskite layer is especially important for charge carrier balance due to slower mobility of hole than electron inside the layer.27,28 Moreover, a formation of separated Br-rich region on the tri-iodide perovskite layer may enhance long-term stability without the loss of current density because moisture stable cubic-phased Br-rich region protects tri-iodide perovskite, which has higher quantum efficiency.15 Thus far, researchers have blended various lead halide materials (PbI2, PbBr2, and PbCl2) into mixture precursors to obtain mixed halide perovskite films. These methods strictly generate entirely mixed perovskite films and cannot furnish B
DOI: 10.1021/acs.nanolett.6b02473 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters skite film by exposure to vaporized and oxidized HBr vapor for 30 s to 1 min. HBr gas is easily oxidized in air to generate Br2 and H2O gas according to the chemical equation presented in Figure 1, and Br2 gas is then substituted into the iodine-based perovskite film, i.e., HBr does not directly diffuse into the perovskite film.30 This easy conversion reaction is possible due to the difference in the halogen reduction potentials. The standard reduction potentials for I2, Br2, and Cl2 are established as 0.54, 1.07, and 1.36 V vs. SHE, respectively.31 Therefore, I− ions inside the perovskite crystal are prone to undergo oxidation to I2 when Br2 gas is introduced, while the Br2 gas is converted to Br− ions. Through such oxidation and reduction processes, the reference MAPbI3 layer can be converted into the MAPb(I1−xBrx)3 film with generation of Br− ions that coordinate to the Pb2+ centers. As shown in Figure 1, bromide substitution occurs only at the uppermost surface of the films, thereby generating discrete regions; the upper region is the bromine-doped region, and the lower region is the almost pristine iodine-based perovskite region. Therefore, we depict the surface of the perovskite film as reddish and the bottom region as black after substitution. This gradational phase separation may accelerate hole extraction due to gradational configuration of valence band as depicted in Figure 1. First of all, to verify that the bromide ions indeed replace a certain amount of iodide ions in the perovskite film via the present experimental setup, the band gap and crystalline structures of the HBr-treated films were evaluated to provide substantive evidence of bromine doping. To explore such transitions of the band gap and crystalline structures of the HBr-treated perovskite layer, optical absorption spectra and X-ray diffraction patterns were acquired. The bromide ion doping ratio can be determined from these measurements based on the degree of change in the band gap and crystal structure after exposure of the film to HBr. Therefore, measurements were conducted using films HBrtreated for different times (1, 3, and 5 min). Figure 2a shows the change in the optical absorption of the perovskite layers as a function of the HBr-treatment time. The absorption spectrum of the reference MAPbI3 layer is consistent with the results from a previous study.14 For the HBr-treated samples, the onset absorption bands gradually shifted to shorter wavelength with increasing HBr exposure time, corresponding to a gradual increase of the band gap. The variation of the band gap of the samples is represented in Figure S1 based on the Tauc equation.32 As shown in Figure 2b, the magnified X-ray diffraction (XRD) patterns in the specified region (2θ = 28.0− 29.2°) show an apparent transition to the MAPb(I1−xBrx)3 structure; the complete XRD patterns are provided in Figure S2. The diffraction peak position changed from 28.46° for the reference sample to 28.64° for the 5 min treated sample, corresponding to lattice parameters of 6.28 and 6.22 Å,33 respectively. Since the ionic radius of Br is 1.82 Å, which is smaller than that of iodine (2.06 Å), the lattice parameter correspondingly decreased with Br− substitution. The increase in the band gap and the decrease in the lattice parameter as a function of HBr-treatment time definitively indicate that the Iions in the MAPbI3 perovskite layer were substituted with Br− ions. Despite the doping effect demonstrated by the above measurements, evaporated HBr or H2O generated during HBr oxidation may damage the surface morphology of the film. The surface and cross-sectional morphologies of each film were evaluated by acquisition of scanning electron microscopy
Figure 2. (a) Absorption spectra and (b) X-ray diffraction patterns of the reference perovskite film and the perovskite films with HBr treatment for different period. Inset graph in (a) is the magnified region for 700−800 nm wavelength to inspect shift of spectrum, which demonstrates reduction of absorption range. Shift of both absorption range and X-ray diffraction patterns represents the evidence of Br doping inside the CH3NH3PbI3 perovskite layer. SEM images of (c) reference and (d) HBr-treated (1 min) perovskite films on FTO/glass. Insets of SEM images are surface topography images measured by AFM. No damages are observed in HBr-treated perovskite film. All the scale bars in images are 300 nm.
(SEM) images and atomic force microscopy (AFM) images for the reference and HBr-treated perovskite layers. From SEM and AFM images in Figure 2c,d, we confirm that HBr-treatment does not damage the surface of perovskite film observing that HBr-treated film shows the same coverages and morphologies with pristine film. We also find that exposure to HBr rather improves the film morphology through comparing root mean square roughness between HBr-treated and pristine films (Figure S3). Root mean square roughness of treated layer is lower than that of reference layer, which proves surface smoothing effect of HBr-treatment. This smoother perovskite film could enhance the device performance.34 However, exposure for more than 3 min severely affected the film morphology, i.e., voids were generated in the bulk films, thereby increasing the surface roughness (Figure S4). The bromide ion doping effects and the damage on film morphology are trade-off factors for maximizing the photovoltaic performance. Therefore, we examined the photovoltaic performance of films subjected to different HBr-treatment C
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Figure 3. (a) Time-resolved photoluminescence measurements and the fitting curves for reference and HBr treated sample (1 min). Instrumental response function (IRF) for third-order exponential decay deconvolution is also plotted. (b) Steady-state PL spectra of reference and HBr-treated perovskite film 0 h after fabrication or treatment. (c) Steady-state PL spectra of HBr-treated perovskite films after 0, 2, 4, and 6 h of aging. (d) Steady-state PL spectra of reference perovskite films after 0 and 6 h of aging.
the time-resolved PL decays, which is convoluted using thirdorder exponential decay functions, the average PL liftetime (τav) of HBr-treated sample (22.85 ns) is much shorter than that of the reference sample (177.98 ns), as listed in Table S1. This means that the charge injection of HBr-treated perovskite to hole transport layer is more efficient than reference perovskite layer.36 Moreover, despite the fast component (τ1) of HBr-treated sample is shorter, both samples show similar value of slow component (τ3). As slow component is related to the recombination, HBr-treated layer has similar defect sites with untreated reference sample.37 In order to examine the charge injection characteristics using PL quenching mechanisms, steady-state PL is measured. Figure 3b−d shows steadystate PL spectra for each perovskite sample with different aging periods. In Figure 3b, PL peak position of reference film (black dashed line: 772 nm) is shifted to short wavelength with HBrtreatment (red dashed line: 762 nm), which supports the evidence of Br doping.38 HBr-treated perovskite film also presents the significant quenching on PL intensity. This result indicates the hole extraction enhancement, possibly originated from halide concentration gradient. The Br gradient inside the perovskite layer induces valence band energy gradient which accelerates the hole drift.25,26 However, the PL intensity of HBr-treated film has increased gradually for 6 h in Figure 3c, while that of reference film has remained identically for 6 h as
times (1, 3, and 5 min) as plotted in Figure S5. For 1 and 3 min of treatment, the open circuit voltage (Voc) gradually increased from 1.09 to 1.11 V and the short circuit current density (Jsc) decreased from 22.66 to 21.92 mA cm−2 with increasing HBrtreatment time. This performance change is ascribed to the increased band gap resulting from Br− ion substitution. In comparison, the device fabricated with the film treated for 1 min exhibits the highest fill factor (FF) of 0.73, and further HBr-treatment led to a decrease of the FF. In the case of the 5 min treatment, due to deterioration of the film morphology, i.e., formation of voids as well as increased film roughness, all the photovoltaic performance factors are degraded. The optimized HBr-treatment time was found to be 1 min under the present experimental conditions. In order to investigate the halide gradient effect, charge carrier extraction measurement is performed. The most established method to examine the carrier extraction rate is photoluminescence (PL) measurement. We carry out timeresolved PL (TRPL) decay and steady-state PL measurement for prepared samples with configuration of glass/NiOx/ reference or glass/NiOx/HBr-treated perovskite. NiOx is a well-known hole extraction layer for stable perovskite device.35 Figure 3a shows PL decay profiles of reference and HBr treated samples. Clear difference is observed in the TRPL response between reference and HBr-treated sample. From comparing D
DOI: 10.1021/acs.nanolett.6b02473 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. Time of flight secondary ion mass spectroscopy (ToF-SIMS) depth profiles of the identical 1 min of HBr treated sample (a) after 0 h and (b) after 6 h of aging. The molecular fragments from the perovskite (Br−, I2−, PbI3−), and the FTO (Sn−) are shown. Putative layers from the intensity of ToF-SIMS are marked as colored region. (c) Cross-sectional view of HBr treated perovskite after focused ion beam (FIB) milling is shown as transmission electron microscopy (TEM) images. Magnified perovskite layer and the area in the white box is selected for energy-dispersive X-ray spectroscopy (EDX) mapping. (d) Pb, (e) Br, and (f) I atoms are well distributed in the entire perovskite film. (g) Device structures of perovskite device immediately after HBr treatment with halide gradient and entirely mixed HBr treated device with schematics of ion migration mixing process.
Br− ions (red line) was the highest on the upper surface of the perovskite film and gradually decreased with depth, verifying the formation of a halide gradient. This halide gradient is defined by a remarkable difference between the top and bottom region in that the intensity of the Br− signal at the surface is almost ten times higher than that at the bottom of the film. However, as supposed from PL measurement, the concentration of Br− ions of HBr-treated film becoming almost consistent across the entire perovskite film after 6 h of aging time (Figure 4b), which indicates that the Br− ions diffused and were homogeneously distributed throughout the entire perovskite film without any thermal treatment. As a result, the halide gradient inside the perovskite layer is formed at the initial conditions but cannot be persisted permanently. The intermixed phase was also observed via energy dispersive Xray spectroscopy (EDX) measurement with the aid of focused
shown in Figure 3d. This implies that valence band energy gradient accelerating the hole extraction has diminished with aging time. Moreover, PL peak position of perovskite film, which was immediately measured after HBr-treatment (0 h) (red dashed line: 762 nm), is shifted to shorter wavelength with aging time of 2−6 h (green dashed line: 758 nm), which demonstrates the change of halide gradient. To ensure the halide gradient formation and vanishment in terms of atomic perspective, changes in the spatial distribution of Br were monitored by measuring the depth profile of the perovskite film obtained by time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The change in the depth profiles of the Br−, I2−, and PbI3− ions associated with the mixed halide perovskite film was monitored to evaluate the compositional changes in the mixed halide perovskite film.39 Immediately after exposure to HBr gas (Figure 4a), the intensity of the signal of E
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Figure 5. (a) Current density−voltage curves of the optimized best devices under one sun conditions (100 mW cm−2, AM 1.5G) for the reference perovskite film and the HBr-treated perovskite film. (b) Stabilized current density of the HBr-treated perovskite device measured with 0.914 V biased voltage, which is the voltage of maximum power point. (c) Normalized device performances (short-circuit current density, open-circuit voltage, fillfactor, power conversion efficiency) of reference perovskite device and HBr-treated perovskite device as a function of time (day) and ambient environment (20−25 °C, 40−90% humidity).
Furthermore, the increased band gap resulting from Br− doping and the increased recombination resistance (Rrec) of the mixed perovskite materials retards charge recombination, concurrently enhancing the Voc and FF.19 Even after ion migration process, outstanding hole selectivity characteristics of HBr-treated perovskite device measured from steady-state PL intensity in Figure S7 also affects the better photovoltaic performance. High standard of Jsc for both reference and HBrtreated devices are approved by measuring external quantum efficiency (EQE) (Figure S6). Photon to current conversion range transits to shorter wavelength with 1 min of HBrtreatment, which shows the equivalent trend to absorption range. Integrated Jsc calculated from EQE spectra corresponds with the value and trend of J−V curves. Superior reproducibility was verified by plotting histograms of the respective device performance characteristics (Jsc, Voc, FF, and PCE) for 20 individually fabricated devices (Figure S8). In order to examine the hysteresis characteristics of each device, reverse (from Voc to Jsc) and forward (from Jsc to Voc) scan curves are plotted together in Figure S9. The steady-state current density is also presented to prove that the device is stabilized within a few seconds (Figure 5b). Another outstanding effect of HBr-treatment is the transition of the crystal morphology from the tetragonal structure (MAPbI3), which is relatively vulnerable to humidity, to the stable cubic structure (MAPb(I1−xBrx)3). In the present case, the perovskite film subjected to 1 min treatment had an XRD peak at 28.52°, corresponding to the (200) cubic phase, and the bromide doping ratio was calculated as x = 0.2.15 Therefore, enhanced device stability was achieved with the film subjected to HBr-treatment. The average photovoltaic characteristics (Jsc, Voc, FF, and PCE) for eight devices based on HBr-treated and
ion beam transmission electron microscopy (FIB-TEM). Compositional analysis of the mixed halide perovskite film was performed by monitoring the Br (yellow dot), I (red dot), and Pb (green dot) distributions (as indicated in Figure 4c), and the data are displayed in Figure 4d−f, respectively. The Br atoms in the perovskite layer with over 6 h of aging time were well distributed over the entire film, which is in good agreement with the SIMS depth profile data. This phenomenon is mainly due to relatively free halide ion migration in the perovskite film. Since the activation energy for migration of iodide ions is the lowest among the components of the organic−inorganic perovskite material (I−, Pb2+, and CH3NH3+), transport and diffusion of iodide ions in the perovskite layer is the fastest.40,41 Similarly, the bromide ions can easily diffuse into iodide vacancies with consequent formation of the homogeneously mixed halide perovskite layer via intermixing of the iodide and bromide ions. The fume substitution doping method assists in improving the photovoltaic performance, as well as in development of the halide gradient in the perovskite films. The J−V characteristics of the optimized HBr-treated perovskite solar devices and the reference devices are presented in Figure 5a. The optimized device with the film subjected to HBr-treatment exhibited a power conversion efficiency (PCE) of 18.94% with Jsc, Voc, and FF values of 22.92 mA cm−2, 1.11 V, and 0.74, respectively. The reference device exhibited corresponding respective values of 18.12%, 23.53 mA cm−2, 1.08 V, and 0.71. Optimized devices performance value and average value of reference and HBrtreated perovskite solar cells are summarized in Table S2. Although the Jsc of the optimized device is slightly lower than that of the reference device, the Voc and FF of the former were notably higher, thereby yielding a higher overall PCE. F
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500 °C for 1 h. The prepared solution of MAPbI3, filtered using syringe filter having 0.45 μm pore size, was spin coated onto mesoporous TiO2 at 4000 rpm for 20 s. During spin coating, 0.5 mL of diethyl ether was dropped on the MAPbI3 precursor. The MAPbI3 layer was crystallized on the hot plate at 130 °C for 20 min.42 Subsequent to the heating, 20 μL of holetransport layer solution was coated by dropping on the spinning device. The Au electrode was deposited on the prepared samples using thermal evaporation system under 10−6 Torr. The size of fabricated perovskite device and active area were 2 × 2 cm2 and 0.14 cm2, respectively. Halide Substitution Process. For substitution of halide ion in the perovskite layer using a halogen fume treatment, a HBr solution (33 wt % in acetic acid) was used. The HBr solution was dropped at the center of bottom side in a 70 mL cylindrical vial. Prepared samples, which are MAPbI3/mesoporous TiO2/blockingTiO2/FTO/glass, are adhered using adhesive onto the cylinder cover. The Br2 gas, which was generated by oxidation of evaporated HBr gas, reacts with upper layer of perovskite materials. After halogen fume substitution process, hole transport layer and metal electrode were coated like the foregoing discussion. Characterization. All devices are stored in ambient condition before and after the measurements. The timeresolved and steady-state photoluminescence measurement was observed by a fluorescence lifetime spectrometer (QuantaurusTau C11367-12, HAMAMATSU). The films were photoexcited with a 464 nm laser (PLP-10, HAMAMATSU) pulsed at a frequency of 10 MHz. The PL was detected by high sensitivity photon counting near IR detector. The depth profiles of the halogen fume treated perovskite layer on substrate were analyzed via time-of-flight secondary ion mass spectroscopy (ToF-SIMS-5, ION-TOF). Samples for TEM were prepared via FIB equipment (JIB-4601F, JEOL); consequently, TEM/EDX (JEM ARM 200F, JEOL) was carried out. EDX maps were acquired over an area of around 0.4 μm2. The cross-sectional and surface high-resolution images were examined using FESEM (JSM-7600F, JEOL). The transmittance spectra and the crystalline diffraction patterns were determined using UV−vis/NIR spectrometry (Cary 5000, Agilent Technologies) and X-ray diffractometer (New D8 Advanced, Bruker), respectively. The performance of perovskite devices was measured under 1 sun condition with AAA solar simulator (Newport Oriel Solar 3A Class AAA, 64023A), and the light intensity was calibrated using a standard silicon solar cell (Oriel, VLSI standards). In order to measure current density of perovskite device, a bias was applied with potentiostat (CHI 600D, CH Instruments), and current density was collected with scan rate of 0.05 V/s. IPCE measurements were conducted by Newport IQE200 system equipped with a 300 W xenon light source for generating monochromatic beam and a lock-in amplifier under AC mode.
reference films measured on a day-by-day basis for 28 days are presented in Figure 5. Both devices were stored under dark ambient relative humidity of 40−90% without any encapsulation; the specific values of the relative humidity are indicated as a blue dashed line. The current densities of both devices showed similar tendencies, with retention of almost over 95% of the initial values. However, the Voc and FF of the reference cells decreased gradually after 2 days. Consequently, the efficiency of the reference devices was reduced to less than 80% of the initial value after 4 weeks due to the decreased Voc and FF. However, the HBr-treated devices retained 97% of the initial PCE by virtue of the slightly increased Voc and FF. Therefore, this simple HBr-treatment method for perovskite solar cells is a promising way to improve the long-term stability as well as energy conversion efficiency of the cells. In order to achieve gradational distribution of halide ions to enhance the hole extraction efficiency of perovskite films, we designed a novel and facile fume substitution doping method using HBr that enables control of the doping profile along the film depth. By utilizing this method, we obtained a halide gradient in the spatially substituted perovskite layer, starting from the pristine iodine-based layer. The enhancement in hole extraction was observed in the halide concentration gradient perovskite materials using PL measurement. Halide concentration gradient was also verified by depth profiling and compositional analysis. These results also revealed diffusion behavior of bromide ions in the mixed halide perovskite layer. The halide gradient is eventually converted into a homogeneous mixed halide layer after undergoing an ion migration based intermixing process. This facile route also provides a convenient method for enhancing the performance of perovskite solar cells, including the PCE and long-term stability, via halide doping. In future research, a permanent halide gradient may be realized by suppressing ion migration through lowering the temperature or exchanging with immobile ions. Moreover, with the aid of interlayers that block ion migration, we can achieve a persistent halide gradient. Materials and Methods. Materials Preparation. All chemical solutions were purchased from Sigma-Aldrich and used as received. Methylammonium iodide (CH3NH3I) was prepared using a previously reported method.3 A solution of 57 wt % hydriodic acid, in the distilled H2O was added to a mixture solution of 33 wt % methylamine (CH3NH2), in the absolute ethanol, and 100 mL of ethanol using drop by drop at 0 °C. The white powder of CH3NH3I was obtained using a rotary evaporator. The synthesized CH3NH3I was washed and recrystallized with diethyl ether and ethanol, followed by drying in a vacuum oven for 24 h. A precursor solution of MAPbI3 was prepared with 1:1:1 ratio of CH3NH3I, PbI2, and DMSO in DMF to obtain 50 wt % solution at the room temperature. A hole transport layer solution was made with 72 mg of spiroMeOTAD (Lumtec), 28.8 μL of 4-tert-butylpyridine, and 17.6 μL of Li-TFSI solution (720 mg of Li-TFSI in acetonitrile), in 1 mL of chlorobenzene. Device Fabrication. Etched FTO glass substrates (TEC 8, Pilkington) were cleaned with acetone, ethanol, and distilled H2O for 10 min using ultrasonic bath, respectively. A holeblocking layer solution, 0.15 M titanium diisopropoxide bis(acetylactonate) in 1-butanol, was spin coated on FTO glass at 3000 rpm 20 s and then this layer was soft baked at 125 °C for 5 min. A mesoporous TiO2 layer was formed onto the blocking layer using diluted TiO2 paste (5.5:1 ratio, anhydrous ethanol/Dyesol 18 NRT TiO2 paste), followed by sintering at
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b02473. Absorption spectra and correlated band gap data, XRD patterns, AFM images, SEM images, and photovoltaic performance data for reference and HBr-treated perovskite films. Time-resolved PL lifetime parameters. G
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Summary of photovoltaic performance for average and optimized devices. External quantum efficiency for reference and HBr-treated perovskite devices. Steadystate PL spectra of reference and HBr-treated perovskite on glass/HTL(NiOx) substrates after 6 h of aging. Histogram of photovoltaic characteristics of HBr-treated perovskite solar cells. Reverse and forward scan current density−voltage curves for HBr-treated perovskite solar cell (PDF)
AUTHOR INFORMATION
Corresponding Authors
*(H.S.J.) E-mail:
[email protected]. *(M.C.) E-mail:
[email protected]. Author Contributions
M.K. and B.J.K. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea (under contact No. 2011-0031561 and 2012M3A6A7054855). This work was also supported from the Technology Development Program to Solve Climate Changes (2015M1A2A2056827).
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DOI: 10.1021/acs.nanolett.6b02473 Nano Lett. XXXX, XXX, XXX−XXX
