Hybrid Perovskite Films by a New Variant of Pulsed Excimer Laser

Apr 12, 2015 - ABSTRACT: A new variant of the classic pulsed laser deposition (PLD) process is introduced as a room-temperature dry process for the gr...
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Hybrid Perovskite Films by a New Variant of Pulsed Excimer Laser Deposition: A Room-Temperature Dry Process Umesh Bansode, Rounak Naphade, Onkar Game, Shruti Agarkar, and Satishchandra Ogale* Centre of Excellence in Solar Energy, Physical and Materials Chemistry Division, and Academy of Scientific and Innovative Research, National Chemical Laboratory, (CSIR-NCL), Pune 411008, India S Supporting Information *

ABSTRACT: A new variant of the classic pulsed laser deposition (PLD) process is introduced as a room-temperature dry process for the growth and stoichiometry control of hybrid perovskite films through the use of nonstoichiometric single target ablation and off-axis growth. Mixed halide hybrid perovskite films nominally represented by CH3NH3PbI3−xAx (A = Cl or F) are also grown and are shown to reveal interesting trends in the optical properties and photoresponse. Growth of good quality lead-free CH3NH3SnI3 films is also demonstrated, and the corresponding optical properties are presented. Finally, perovskite solar cells fabricated at room temperature (which makes the process adaptable to flexible substrates) are shown to yield a conversion efficiency of about 7.7%.

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INTRODUCTION Hybrid perovskite systems have been at the focus of intense scientific interest and attention at this time in view of the recent successes in realizing solar photoconversion efficiencies approaching 20% using these materials as an active component in different variants of solar cell architectures, including mesoporous, planar, or multilayer heterostructure configurations.1−19 Thanks to these many interesting and key contributions of several groups to the field, it is poised for further rapid progress and breakthroughs in the years ahead. While the current research is focusing on a few champion materials with which an early success was achieved, it is clear that diverse hybrid materials systems with different inorganic and organic molecular components would be researched soon. A key issue which may propel or hinder further research using these materials is our ability to grow their high quality thin films. At present, solution processing and dual vapor phase growth have been developed with a great degree of success.3,4,14 Many other film growth approaches which have witnessed immense success in the context of inorganic perovskite systems still remain to be explored and applied for hybrid perovskites. A powerful method which has been applied very successfully in the case of a variety of metal oxide perovskite systems such as high temperature superconductors, CMR manganites, ferroelectrics, multiferroics, and the corresponding superlattices is pulsed laser deposition (PLD).20−22 A key strength of this technique is its unique ability to transfer stoichiometry from the bulk to a film via intense pulsed UV laser-induced ablation, rendering a spray of energetic (0.1 to several eV) ions and radicals in the form of a plasma plume heading toward and impinging on a substrate, which could be heated as appropriate. The corresponding growth leads to some of the highest quality crystalline films of functional oxides, thanks to the high energy of the impinging ions. © XXXX American Chemical Society

When the PLD process is attempted in the case of hybrid perovskites, however, some different complications come into play which emanate from the different properties of the easily vaporizable (organic) and not easily vaporizable (inorganic) components of the hybrid materials. The corresponding low and high Z values, respectively, also undergo different vapor phase and surface kinetics. Since the film growth occurs via the kinetics and thermodynamics of formation, dissolution, reactions, and evolution of ion-induced tiny clusters, the thermal stability of the cluster intermediates is also a matter of concern. For example, iodine in the perovskite CH3NH3PbI3 evaporates quickly even at room temperature. Also, the other low Z components in the material have a gas forming tendency. In this paper, we have addressed and resolved these issues via interesting geometric as well as stoichiometry management variants of the classic PLD process, enabling successful deposition of high quality hybrid perovskite films. Moreover, we demonstrate with a few examples the power of this technique to achieve stoichiometry control that is not as easily accomplished by the other currently used methods. We also show that, by our improvised PLD process, we can achieve a conversion efficiency of about 7.7% under fully roomtemperature processing. In our view, the success of the PLD process could also open up possibilities of creating interesting heterostructures involving the new hybrid perovskites and the other functional inorganic materials which can be grown by the same technique. The basic pulsed laser deposition (PLD) arrangement used in this work is depicted in the schematic shown in Figure 1. In this case, ultraviolet laser pulses from an excimer laser (KrF; λ = Received: March 17, 2015 Revised: April 11, 2015

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ZnO Nanoparticles Synthesis. Typically, ZnO nanoparticles were synthesized by a reflux and condensation method.25 A 2.95 g portion of Zn(CH3COO)2·2H2O was dissolved in 125 mL of methanol at 65 °C. The KOH solution (1.48 g in 65 mL of methanol) was added dropwise to zinc acetate solution over a period of 10 min, and the mixture was heated at 65 °C for 135 min under constant stirring. The resultant turbid solution was allowed to cool to room temperature, followed by removal of the supernatant solution. The obtained ZnO nanoparticle precipitate was washed twice with methanol and redispersed in 70 mL of n-butanol, 5 mL of methanol, and 5 mL of chloroform. The ZnO nanoparticle dispersion was filtered through the 0.45 μm PVDF syringe filter before use in the device fabrication. Solar Cell Device Fabrication. A part of the ITO-coated glass substrates was first etched using Zn metal powder and diluted HCl, and then subjected to a cleaning protocol which included washing with the soap solution, distilled water, and absolute ethanol and finally boiling in isopropanol. The ZnO nanoparticle dispersion was spin-coated three times on clean ITO at 3000 rpm for 30 s. The perovskite layer was deposited using the PLD process (Lambda Physik KrF Excimer later; λ = 248 nm; pulse width: 20 ns; 5 Hz), wherein the incident energy density on the target was kept fixed to 0.3 J/cm2 for all the cases. The solution of hole transporting material was prepared with 80 mg of spiro-OMeTAD, 28.5 μL of 4-tert-butylpyridine, and 17.5 μL of lithium-bis-(trifluoromethanesulfonyl)imide (LiTFSI) solution (520 mg in 1 mL of acetonitrile) and spincoated on perovskite films at 2000 rpm for 15 s in the glovebox. Then, the films were kept in a drybox (oxygen atmosphere with less than 10% humidity) for the whole night. Later, 80 nm gold was deposited by thermal evaporation using a shadow mask in 10−6 mb vacuum at 2 Å/s. The photovoltaic characteristics and the I−V measurements of the perovskite films were studied using a Newport solar simulator connected to the Keithley 2420 I−V measurement system.

Figure 1. Schematic presentation and photograph of substrate arrangement for “on-axis” and “off-axis” deposition by pulsed laser deposition (PLD).

248 nm; pulse width: 20 ns) are made incident onto a target held in vacuum (or with controlled gas ambient) at a certain pulse repetition frequency (typically 5 Hz) to ablate the target material. In the common PLD process for inorganic solids, the ablated plasma plume is directed onto a substrate heater facing the ablation spot (on-axis case). Another variant of this arrangement called as off-axis deposition is also possible with the substrate mounted parallel to the plume, but is extremely rarely used.23,24 In this case, the ions and radicals diffusing sideways render the film growth and the particulate matter seen in on-axis growth is minimized. In our case, however, we show that the off-axis growth also leads to specific new advantages in terms of stoichiometry control for the hybrid material, which have not been discussed hitherto. We also show that the target stoichiometry needs to be suitably chosen to get the desired film stoichiometry in view of the point regarding the differing kinetics of the component elements discussed earlier.

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EXPERIMENTAL METHODS Pulsed Laser Deposition. For pulsed laser deposition, the targets were made by homogeneously mixing PbA2 (A: I, Cl, or F) and MAI in different compositions in the glovebox ambient and pressing them at 10 Tons pressure. Pulses of an excimer laser (λ = 248 nm; energy density: 0.3 J/cm2; pulse rep rate: 5 Hz) were made incident on the target held in a vacuum chamber held at 10−6 Torr. In one experiment, two separate targets were also made by homogeneously mixing PbCl2 and MACl (no iodine case); and PbCl2, MACl, and MAI (1:2:2) in the glovebox ambient and pressing them at 10 Tons pressure. Other parameters of compositions, deposition geometry, and temperature are described in the text for the respective cases. For the cation replacement case, SnI2 was used instead of PbI2. After optimizing the PLD process, we used the protocol reported by Liu and Kelly13 to make perovskite-based solar cells, except that the deposition of perovskite was carried out by the PLD technique optimized in this study. Although ZnO is relatively unstable as a compact layer as compared to other oxides, Liu and Kelly13 emphasized that ZnO has the advantage that it can be deposited rather easily by spin-coating and does not require heating or sintering step. Since, in our pulsed laser deposition work, we intended to grow the films and full devices at room temperature only, we used this ZnO compact layer protocol in this first study on PLD films. Further work will definitely involve use of other metal oxide options and the corresponding property optimization.

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RESULTS AND DISCUSSION Initially, as a starting point for the process of optimization, we used a stoichiometric (1:1) compressed uniform physical mixture of PbI2 and MAI as the target. A low energy density of 0.3 J/cm2 was used at a pulse repetition rate of 5 Hz so that the target does not break and crumble. The X-ray diffraction XRD pattern for this target for the on-axis deposition case (film grown on a substrate directly facing the plasma plume, as is normally done in the PLD of inorganic oxides and other inorganic compounds) is shown in Figure S1a (Supporting Information). It shows that the right (desired) perovskite phase is not formed by this conventional on-axis PLD approach using a stoichiometric target. Indeed, the inset shows the yellow color of the film, which clearly indicates the dominant presence of PbI2 therein. Clearly, it is deficient in iodine as well as the organic components. This was also borne out by the field emission scanning electron microscopy−energy dispersive Xray (FESEM−EDAX) analysis data shown in Figure S1b. The problem with molecular iodine is that it sublimates at room temperature, while the organic components have a tendency toward gasification unless they are chemically incorporated efficiently into the growing film surface. Moreover, the PLD generated plume has high energy ions and radicals (0.1 to a few eV) as against the case of simple vapor deposition (hundreds of meV). The high energy of ions and radicals in the PLD process B

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Figure 2. (a) XRD of all cases deposited using mixed targets with varying concentrations of organic component (MAI). The target compositions are specified as PbI2:MAI ratio. (b) XRD of MAPbI3 with 1:4 (PbI2:MAI). (c) FESEM−EDAX top view and (d) FESEM cross section of perovskite film deposited with 1:4 (PbI2:MAI) target. The elements C, N, and Si were identified with K shell X-rays and I, Pb with L shell X-rays.

content, and checked the film stoichiometry for the off-axis grown films. The XRD data for these cases are summarized in Figure 2a. It is clear that, for a ratio of PbI2:MAI = 1:4 mol in the target, the off-axis grown film gives near-perfect stoichiometry of the perovskite phase, as shown in Figure 2b. Importantly, the line widths of XRD peaks are also narrow (radians ∼ 0.0019), which correspond to a grain size of about 76 nm, as estimated by the Scherrer formula. Growth of such fairly large grains even for deposition at room temperature implies that the energy of impinging ions and radicals from the PLD generated plume facilitates the local growth kinetics. It would be possible to control the crystal size further by changing the substrate temperature, laser energy density, and pulse repetition rate. This work will be done in due course of time. Figure 2c shows the FESEM−EDAX data for this case, which further reinforces this conclusion. Figure 2c,d shows the top view and crosssectional view of the film. The top view shows a laterally homogeneous microstructure, while the side view shows the oriented nature of grains and the thickness, suggesting the growth rate to be about 1.9 Å/s. Encouraged by the above results, we also re-examined the possibility of realizing a stoichiometric film even for the case of direct on-axis growth, but this time by using higher organic (MAI) content in the target, some of which is dynamically lost from the growth front due to impinging high energy radicals, as discussed earlier. Toward this end, we carried out on-axis deposition using a target composition of PbI2:MAI (1:18). Possibly because we added the organic component (MAI) a bit too much, the room-temperature deposition for this case did

is normally considered as an advantage in inorganic (e.g., oxide) PLD because that energy is translated into high surface mobility, leading to enhanced growth quality as well as densification of the film. These advantages would persist in the PLD process for hybrid perovskite case as well; however, low energy ion sputtering and/or evaporation of low Z elements and iodine loss by early formation and evaporation of its tiny clusters are an issue. We used two approaches to address and resolve this problem: (a) We explored whether of faxis deposition (substrate held parallel to the plume direction) can hold better stoichiometry because the energies of ions/ radicals are lower for the ions drifting sideways in the scattering processes. (b) We studied the use of a nonstoichiometric mixture in the target with higher organic component therein so that its loss is compensated for. We first present the results for the off-axis deposition case. Figure S1c shows the XRD for the off-axis deposited film under the same conditions as those for the case of direct deposition shown in Figure S1a. As conjectured, it immediately becomes clear that the off-axis grown film holds the stoichiometry much better, including the iodine component, as compared to the direct or on-axis deposited film. This is also clearly established by the FESEM−EDAX results shown in Figure S1d. The inset of Figure S1c shows the brownish color of the film. However, it is clear that, even for the off-axis case, the stoichiometry is not perfect and necessitates control of the starting target constitution, as is required also in some examples of sputtering of inorganic materials. We made a series of targets with different ratios of PbI2 and methyl ammonium iodide (MAI), with increasing the latter’s C

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Figure 3. (a) XRD of PLD grown films of CH3NH3SnI3. (b) Transformed Kubelka−Munk spectrum and photoluminescence (red) spectra of PLD grown films of CH3NH3SnI3.

Figure 4. (a) XRD of MAPbI3, Cl−, and F− mixed halide perovskite films. (b) FESEM−EDAX of Cl− mixed halide perovskite. (c) FESEM−EDAX of F− mixed halide perovskite. The elements C, N, F, Cl, and Si were identified with K shell X-rays and I, Pb with L shell X-rays. FESEM cross section of (d) Cl− and (e) F− mixed halide perovskite films.

It is interesting to point out that achieving full cation replacement or its partial substitution is entirely easy with the PLD process by simple target content control. Importantly, the phase formation in the target is not a necessary condition for the success of the PLD process, since the laser ablation anyway converts the target constituents into a spray of ions and radicals in the same proportion as present in the target. We, therefore, first examined and demonstrated the use of the PLD process to deposit lead-free tin-based perovskite (CH3NH3SnI3). The target constitution and growth parameters for Sn-based perovskite case were similar to the lead-based perovskite case.

show some organic precursor impurities along with the desired perovskite phase. In the next deposition, therefore, we heated the substrate to 90 °C during deposition so as to dynamically remove the extra unincorporated organic components from the growth front. Very interestingly, this resulted in a highly (110) oriented growth of the desired perovskite phase, as shown in Figure S2 (Supporting Information). Such highly oriented growth of the perovskite phase on an amorphous substrate (glass) is particularly impressive and will be the subject of future investigation. D

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using PbCl 2 and PbF 2 , respectively, as the inorganic components in the target mixture, but the organic component was retained as MAI only. Since the mixing ratio for getting the desired stoichiometric phase in the film was 1 (inorganic):4 (organic) component, the percentages of Cl− and F− in the final PLD films were indeed low. The presence of Cl− and F− is, however, clearly established by FESEM−EDAX as shown in Figure 4b,c, respectively. Figure 4d,e shows the cross sections of Cl− and F− mixed halide perovskite films, which clearly show the uniform deposition over the scale of several micrometers. We employed the FESEM elemental mapping technique to examine whether the presence of the halide ion (not necessarily implying substitution) in the PLD films is uniform across the area. These images and the corresponding remarks are presented in the Supporting Information (Figures S3 and S4). It is clear that the distribution is indeed fairly uniform. We also performed X-ray photoelectron spectroscopy to independently verify the presence of Cl− in the PLD film. The corresponding data shown in Figure S5 in the Supporting Information do establish the presence of Cl− in the film through the clear signal at about 198 eV corresponding to the Cl 2p3/2 contribution, as seen and reported by others as well.33−35 We also studied the optical properties of the iodide perovskite films with Cl− and F− incorporation. Figure S6 (Supporting Information) shows the absorbance curves for the different perovskite cases deposited on glass using PLD. The CH3NH3PbI3 perovskite shows an absorption onset at 782 nm, corresponding to an energy gap of 1.58 eV, which is in accordance with the literature values reported for the band gap of CH3NH3PbI3.36 It can be observed from the inset of Figure S6 that the chlorine and fluorine mixed halide perovskites films (only nominally represented as CH3NH3PbI3−xClx and CH3NH3PbI3−xClx) deposited by using PLD show slight blue shifts in the absorption onset. This points toward the increase in the energy gaps as compared to the undoped perovskite. Such a change in the energy gap of MAPbI3 perovskite with Cl− or Br− is in accordance with the reported experimental observations and theoretical calculations.32,36 According to these, the addition of Cl− ions introduces the chlorine 3p states, which are placed energetically lower than the valence band maximum formed by the iodine 5p states, which, along with the reduction in lattice parameter, leads to an increase in the energy gap of the resultant perovskite.32 Another possibility which could cause blue shifts would be the smaller size of the perovskite crystallites, as pointed out by D’Innocenzo et al.37 Indeed, in the chlorine doped case, the crystallite size as estimated by the Scherrer formula was found to be smaller (by about 40%) than that in the MAPbI3 case, whereas the F− doped case did not show much change in the crystallite size. It is now useful to mention that, if one desired to introduce higher concentrations of the substituted halide components, one could easily increase their contribution in the methyl ammonium component as well. We performed additional experiments to demonstrate this. One deposition was performed with a mixture of methyl ammonium chloride and PbCl2 wherein no iodine was involved. This gave us a fully chorine-based perovskite phase, as seen from Figure 5a, which matches with the XRD pattern reported by Kitazawa et al.38 In order to further ascertain the nature of this phase, we performed optical absorption and photoluminescence (PL) measurements. As shown in Figure S7a (Supporting Information), this fully chlorine-based perovskite showed a

Since the Sn-based perovskite is unstable in air due to oxidation of tin, we in situ deposited a capping gold layer on the surface of CH3NH3SnI3 perovskite film in order to retard its rate of oxidation, allowing us to do at least the XRD measurements in air. The XRD in Figure 3a clearly shows successful deposition of CH3NH3SnI3 perovskite phase in of f-axis deposition geometry.26,27 We also studied the optical properties of PLD grown CH3NH3SnI3 films by diffused reflectance spectroscopy (DRS) with JASCO V-550 (integrating sphere geometry) and photoluminescence measurements with FLS 980 Edinburgh instruments. To carry out these optical measurements, we deposited the CH3NH3SnI3 film by PLD on a quartz plate substrate and immediately transferred it into a glovebox for sealing with Surlyn and glass coverslip to avoid significant oxidation of CH3NH3SnI3. We found that, after such sealing, the CH3NH3SnI3 film is quite stable for the intended measurements. The optical band gap was calculated using the Kubelka−Munk equation [F(R) = (1 − R)2/2R], where α and S are absorption and scattering coefficients, respectively, and R is the percentage of the diffuse reflected light.28 From Figure 3b, which shows the transformed Kubelka−Munk spectrum [F(R)hν]2,10 it can be seen that the PLD grown CH3NH3SnI3 film has a direct band gap of 1.3 eV.26,29,30 The photoluminescence (PL) of the same PLD grown CH3NH3SnI3 film is shown in Figure 3b (red curve) over the near IR region (from 800 to 1100) with photoexcitation at 532 nm. These results clearly establish the power of the PLD process to deposit organo-metal halide perovskites with different compositions including lead-free systems. In order to make devices based on such CH3NH3SnI3 films, however, we need to connect our PLD chamber to a glovebox with a sample transfer facility. This will be done in due course of time. Another issue to address is the possibility of introducing dopants in the growing films. There are several reports on the inclusion (not necessarily lattice incorporation, currently a controversial matter) of Cl− in the perovskite matrix using chemical or vapor deposition techniques.2,12,14 However, interestingly, there are no reports on F− inclusion in perovskite, possibly due to limitations in solution processability of fluorinecontaining precursors. Herein, we show that, with the PLD process demonstrated herein, one can introduce Cl− as well as F− in the growing hybrid perovskite film. Figure 4a shows the XRD of Cl−- and F−-based mixed halide hybrid perovskite films grown by PLD along with the pattern for the PLD grown MAPbI3 case. The use of the term mixed halide cautions that it need not necessarily mean lattice incorporation of one halide ion in the matrix of the other. This follows the terminology by Zhang et al.31 The inset of Figure 4a shows the region of the (110) and (002) peaks on the expanded scale. It can be clearly seen that, in the fluorine case, there is a tiny, but definitive, shift of the (110) and (002) peaks toward larger 2θ values, as compared to the pure MAPbI3 case. Since the ionic size of F− is smaller than that of I−, the lattice parameter can be expected to shrink, causing a peak shift toward higher 2θ. This suggests F− incorporation at the I− site in the matrix. The Cl− case, however, exhibits a change in the relative intensities of the two contributions (as also reported by others);32,33 hence, the shift, if any, could possibly be ascertained only by using a fitting procedure, which is not desirable for such tiny shifts. It may be pointed out that, in the doping results reported above, the targets for Cl− and F− doping were prepared by E

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given small difference between the lattice parameters between two lattices (for example, due to dopant incorporation). This can be seen from the differentiation of Bragg’s equation, which gives Δθ = −Δd /d ·tan θ

(1)

Here, θ is the angle of diffraction, d is the corresponding lattice spacing for a specific set of planes, and Δ θ is the angular separation for a change in lattice parameter of Δd. Since tan θ is an increasing function of θ, the angular separation Δθ between peaks increases with θ. From the inset of Figure 5b, it is clearly seen that peak(s) for the (1:2:2) chlorine doped compound are slightly right shifted as compared to the MAI case. Thus, we believe that a small concentration of Cl− does get incorporated in the iodide perovskite and the rest forms a separate chloride perovskite phase. Interestingly, the presence of PbI2 is also seen for the case of this dual phase separated perovskite film. It means that the kinetics of phase separation does not allow all PbI2 to react fully during the film growth. Further work is needed to see if this can be suppressed by other controls. The optoelectronic properties of PLD perovskite films were further studied using the in-plane I−V measurements under dark and under illumination. The MAPbI3 perovskite films without and with Cl− or F− (targets used as 1:4 PA2:MAI (A = Cl, F) were deposited on a 2 cm × 2 cm glass substrate using PLD. Two gold electrical contacts (2 mm × 8 mm × 120 nm each) with a spacing of 2 mm were deposited on the top of perovskite films using thermal evaporation. Figure 6a shows the current vs time profiles (photoresponse) of MAPbI3 and Cl−/F− mixed halide perovskite films under light ON and OFF cycles, where a potential difference of 20 V was applied between the two electrodes. It can be clearly seen that all the perovskite films show considerable photocurrent under illumination. As mentioned earlier, in this on/off photoresponse study, the perovskite films were tested only for their in-plane photoconductivity and no full out-of-plane solar cell device configuration. Since the in-plane distances are large and no specific architecture is provided for electron hole separation as in a solar cell configuration, the current values are low, as expected. Importantly, the magnitude of photoresponse (Ilight/Idark) is the maximum in F− mixed halide perovskite film, followed by Cl− mixed halide case and then MAPbI3 film. Du40 has predicted using density functional theory (DFT) and Perdew− Burke−Ernzerhof (PBE) functionals that the chloride inclusion within perovskite matrix leads to a shallow acceptor level due to chlorine (or fluorine) interstitials which reduce the carrier trapping and nonradiative recombination. Therefore, this may explain the high photoresponse (Ilight/Idark) in F− (∼750) and Cl− mixed halide (∼200) hybrid perovskites as compared to MAPbI3 (∼90) perovskite. Importantly, a systematic difference can be observed in the case of the saturation times of photocurrents of MAPbI3 and Cl−/F− incorporated hybrid perovskites (Figure 6b). The photocurrent of MAPbI3 takes nearly 10 s to saturate, which is also observed previously by Zaban et al.7 The slow photoresponse in the case of MAPbI3 is attributed to two effects: (1) the photoinduced reduction in the binding energy of methyl ammonium ion (MA+), which results in its higher rotational freedom and realignment in the direction of applied electric field, and (2) the slow adjustment of the inorganic scaffold to the realignment of MA+ under illumination, which leads to a slow photoresponse. The structural origins of the above-

Figure 5. (a) XRD of MAPbCl3 film grown by PLD. (b) XRD of 1:2:2 mol (PbCl2:MAI:MACl), MAPbCl3, and MAPbI3 films grown by PLD. Inset shows the (220) peak contributions on an expanded scale.

band gap of ∼3 eV (as reported by others as well), as against that of the MAPbI3 phase, which is 1.58 eV. Interestingly, no discernible PL signal was obtained for the chlorine compound, which is consistent with the report by Dimesso et al., who suggested that the full chlorine-based perovskite is PL inactive.39 The MAPbI3 system did show the expected PL, as seen from the results shown in Figure S7b. In another experiment, we used PbCl2, MAI, and MACl in a 1:2:2 proportion in the target. Interestingly, in this case, we observe from the XRD data of Figure 5b that one gets a clear mixed phase (or phase separated) system of the iodide perovskite phase and chloride perovskite phase. In order to understand whether some small amount of chlorine is incorporated in the iodide perovskite, one has to look for XRD line shifts in the iodide phase component. As stated earlier and also reported by others, in the full Cl− mixed halide perovskite case, the relative intensities of the (110) and (002) peaks are different as compared to those in the MAPbI3 case; hence, it is hard to ascertain tiny shifts, if any, from this region of the pattern. On the other hand, as per the Bragg’s law (nλ = 2d sin θ), at larger diffraction angles, the peaks are more separated for a F

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Figure 6. (a) Current (I) vs time (t) profiles of perovskite films under light ON and OFF cycles at an applied bias of 20 V. Graph (a) plotted on the semilog scale for clarity in photoresponse (Ilight/Idark) differences. (b) The I vs t graph for a single light ON and OFF cycle plotted on the linear scale (saturation times mentioned in the respective graphs). (c) J−V curve with a scan rate of 0.04 V/s for Cl− mixed halide perovskite solar cell with forward and reverse scans (inset shows the device and the table of the solar cell parameters). (d) FESEM cross section of the complete solar cell device.

architecture was ITO/ZnO/Cl− mixed halide perovskite/spiroOMeTAD/gold. The solar cell measurement was carried out over an active area of 0.09 cm2 under the illumination of 0.95 sun light, and the corresponding J−V curves are included in Figure 6c. As seen, a power conversion efficiency of 7.7% is realized using a fully room-temperature process. The hysteresis observed when the device was scanned from forward bias to short circuit (FB-SC) and reverse scan from short circuit to forward bias (SC-FB) is shown in Figure 6c. This hysteresis in the planar heterojunction perovskite solar cell is observed due to excess ion migration,30,41−45 ferroelectric properties of hybrid perovskite,43,46,47 or the large defect states present in the perovskite material surface acting as trap states for the electron and hole.43 With further optimization and device skill development, higher efficiency values are certainly possible. Figure 6d shows the cross-sectional FESEM image of the complete solar cell device, which clearly shows the compact and uniform deposition of perovskite on the ZnO compact layer using the PLD process.

mentioned effects lie in the photoinduced I → Pb charge transfer and the weak hydrogen bonding between the inorganic cage and MA+. The partial replacements of I− with Cl−/F− are likely affecting both the charge transfer mechanism as well as hydrogen bonding, making it stronger due to higher electronegativities of Cl− and F− than I−. Therefore, a faster photosaturation can be expected in the case of Cl−/F− mixed halide perovskites as compared to MAPbI3 perovskite due to strong hydrogen bonding in the case of Cl−/F−, which helps the faster restructuring of the inorganic cage than in the case of MAPbI3 perovskite. Photoinduced ion migration in perovskites is another intriguing possibility which might be responsible for the transient optoelectronic effects in the perovskites.30,41−45According to this phenomenon, the photoexcitation of perovskite causes halide ion vacancies, which facilitate the migration of halide ions (lattice or interstitial) under an applied external electric field, which may assist or oppose the charge carrier extraction. The size of the halide ion can also play an important role in the time dynamics of the ion migration process and hence the optoelectronic response times. Therefore, the faster rise time in the case of F− incorporated perovskite as compared to MAPbI3 perovskites can be attributed to the efficient photoinduced ion migration process due to the smaller ionic radius of the F− ion as compared to I− or Cl−. Figure 6b clearly shows that the Cl−/F− mixed halide perovskite-based photoconductive devices indeed show a faster photosaturation within ∼3−4 s as compared to the case of MAPbI3 perovskite. This can be regarded as an indirect indication of successful incorporation of at least a small quantity of Cl−/F− using PLD to yield hybrid perovskites such as CH3NH3PbI3−xClx or CH3NH3PbI3−xFx. Finally, we fabricated planar heterojunction perovskite solar cells with perovskite film deposited using PLD. The device

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CONCLUSIONS

In conclusion, we have demonstrated the use, power, and versatility of a new variant of the pulsed laser deposition (PLD) technique for achieving room-temperature growth of high quality thin films of hybrid perovskites. The inclusion of Cl− and F− ions in hybrid perovskite is successfully carried out, and interesting optical properties and photoresponse are revealed. We have further optimized the PLD deposition parameters for lead-free hybrid perovskite (CH3NH3SnI3). Finally, we have implemented this technique to fabricate perovskite-based solar cells at room temperature exhibiting a power conversion efficiency of about 7.7%, which allows its use for flexible substrates. G

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PLD is a fully dry process which is widely used for inorganic oxides, sulfides, and nitrides as well as some polymeric systems. Our demonstration of the success of PLD for perovskite growth opens doors for in situ integration of the new hybrid perovskites with other inorganic material systems in heterostructures or modulated structure configurations, promising new science and applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information contains XRD and FESEM− EDAX analysis for 1:1 and 1:18 (PbI2:MAI) PLD process hybrid perovskite films, elemental mapping for Cl− and F− mixed halide perovskite, XPS of Cl 2p, and UV−vis spectra of anion (Cl−, F−) mixed halide perovskite. Optical properties of MAPbCl3 are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS All authors wish to thank UK-India APEX program in solar energy (DST), TAPSUN (CSIR) program, and MNRE for funding support. U.B. and R.N. would like to thank CSIR for a fellowship and AcSIR.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b02561 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b02561 J. Phys. Chem. C XXXX, XXX, XXX−XXX