Enhanced Carrier Lifetimes of Pure Iodide Hybrid Perovskite via Vapor

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Enhanced Carrier Lifetimes of Pure Iodide Hybrid Perovskite via Vapor Equilibrated Re-Growth (VERG) Banu Selin Tosun, and Hugh W. Hillhouse J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00842 • Publication Date (Web): 08 Jun 2015 Downloaded from http://pubs.acs.org on June 9, 2015

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The Journal of Physical Chemistry Letters

Enhanced Carrier Lifetimes of Pure Iodide Hybrid Perovskite via Vapor Equilibrated Re-Growth (VERG) B. Selin Tosun, Hugh W. Hillhouse* Department of Chemical Engineering and Molecular Engineering and Sciences Institute University of Washington, Box 351750, Seattle, WA 98195–1750, USA.

Abstract: Solution deposition of planar films of the hybrid perovskite (HP) methyl ammonium (MA) lead iodochloride (MAPbI3-xClx) often result in very low surface coverage, small grain size, and high density of defects, particularly for the pure iodide HP. This decreases the optoelectronic quality of MAPbI3 (minority lifetimes all less than 10 ns) and creates pinholes that may result in shunt pathways that severely degrade the efficiency of photovoltaic devices. The poor morphology is usually attributed to the formation of large disconnected grains of PbI2 that nucleate first and set the morphology of the final HP layer. As a result, many use PbCl2 as a lead source. The PbCl2 is less soluble, forms smaller grains, and promotes more continuous HP films. Here, we show a highly reproducible deposition method for pure iodide MAPbI3 that yields continuous films with large grain sizes and minority carrier lifetimes greater than 200 ns. The method consists of thermal evaporation of PbI2 and a post-deposition vapor-equilibrated regrowth (VERG) step at 110 ○C in a closed vessel.

Perovskites have stoichiometry of ABX3 and a crystal structure equivalent to calcium titanate (CaTiO3).1 In organic-inorganic hybrid perovskites (HP), the A site is occupied by an organic cation (e.g., CH3NH3+), the B site is a divalent metal cation (e.g., Pb2+, Sn2+), and the X site is occupied by halogen anions (e.g., I-, Cl-, or Br-).2,3 The materials have a sharp photoabsorption onset at the bandgap and show strong absorption above bandgap.4 Importantly, 1 ACS Paragon Plus Environment


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for many compositions, particularly the lead iodochloride HPs, photo-excited electrons and holes may have diffusion lengths comparable to the required absorber film thickness.4,5,6 This enables planar photovoltaic architectures (as opposed to mesostructured architectures).7,8 The power conversion efficiency of both mesostructured and planar hybrid perovskite based solar cells has increased rapidly and dramatically. 4,9-12 Solution deposition of planar films is particular exciting due to its simple device architecture and the potential for low-cost device fabrication. However, the underlying crystallization and film growth mechanisms are still not completely understood. As in the inorganic high efficiency solar cells, deposition methods based on vacuum coevaporation of the precursors, a.k.a. CH3NH3I (MAI) and PbCl2, result in good film coverage and uniformity, which in return result in with good device performances.8 Nonetheless, this method requires high vacuum and could be difficult to implement at high-throughput and large area cost-effective mass production.4 Alternatively, planar MAPbX3 films may be fabricated by spin coating a single solution5,7,12-16 with PbX2 and MAX or by a two-step coating process17,19 (spin coating PbX2 followed by MAX). However, for the pure iodide, simple spin coating typically does not yield a homogenous perovskite layer with uniform coverage over a large area. The resulting pinholes and incomplete surface coverage deteriorate the device performance.13,1926

Wen et. al27 studied the impact of hybrid perovskite film coverage on solar cell performance. They found that discontinuous film morphologies are detrimental for the charge extraction.27 The crystallinity of MAPbI3 films are believed to be dependent on the PbX2 morphology in two-step deposition methods. In particular, films that are deposited using PbCl2 precursor exhibit better MAPbClxI3-x film crystallinity with larger grains,14 and thus far, most published studies attribute the superior properties of MAPbClxI3-x to formation of an sacrificial 2 ACS Paragon Plus Environment

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pure chloride phase (MAPbCl3) which slows down the formation of MAPbClxI3-x and therefore improves the crystallization, coverage, and leads to more complete conversion to perovskite. In addition to having continuous films, the elimination or passivation of defects in the bulk, at grain boundaries, and at surfaces are also crucial for obtaining high quality optoelectronic materials. Like many ionic semiconductor absorbers, CH3NH3PbX3 contains under-coordinated ions at crystal surfaces, grain boundaries, and in bulk of the grains.12 It has been found that hybrid perovskites synthesized via PbCl2 (as opposed to with PbI2) show longer photo-generated carrier lifetimes, which is attributed to the aforementioned better crystallinity and/or lower defect concentration. The effective photogenerated carrier lifetime in the iodochloride (MAPbClxI3-x) is usually measured to be around 200-300 ns while the pure iodide (MAPbI3) is usually measured to be 3-9 ns under low fluences in a monomolecular recombination regime.17,18,28,29 It is unclear if the small amount of residual chloride is important to obtain good optoelectronic quality or whether it is purely a result of the better morphology of the iodochloride. The experiments below indicate that it is primarily a morphology effect. Here, we show 100% substrate coverage with a pure iodide HP (MAPbI3) synthesized by spin coating MAI solution on planar films of thermally evaporated PbI2. We also compare the results to films formed from thermally evaporated PbCl2. We observed more compact and continuous network of MAPbI3 crystals in both PbI2-MAI films and PbCl2-MAI films. Further, we show that re-growing these films with MAI vapor at low temperatures (110 °C) in a closed and equilibrated system improved the film morphology dramatically and resulted in micron size grains with highly compact film coverage. We refer to this as VERG (Vapor Equilibrated ReGrowth). These results are important in two ways. First, thermal evaporation of the lead halide is simple and can be achieved with any thermal evaporator. This is in contrast to co-evaporation of 3 ACS Paragon Plus Environment


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the lead halide and methyl ammonium halide which requires a more complicated instrument and precise control of the evaporation rates from the two sources. Second, the VERG process we report here is much more controlled than other previously reported vapor or solvent assisted sintering methods since in our method the system is closed and the MAI vapor pressure is set by the temperature of the oven and remains constant.4,5 This closed system inhibits any loss of MAI (or methyl amine and hydrogen iodide) from the hybrid perovskite layer during the re-growth process, allowing time for the hybrid perovskite to re-organize. In this paper, we show that VERG treated pure iodide films (MAPbI3) exhibit carrier lifetimes longer than the non-VERG treated films by two orders of magnitude, exceeding the carrier lifetime of films synthesized using PbCl2 precursors. Samples were prepared as described in the Materials and Methods section of the supporting information (SI). Figure 1(a) shows the process flow diagram for deposition of MAPbI3 films for planar heterojunction devices. Briefly, F-doped SnO2 (FTO) coated glass is used as the substrate. Here, the FTO substrates are sputtered with a ~50 nm thick TiO2 compact layer (c-TiO2). This is followed by spin coating PbI2. The formation of MAPbI3 is completed by spin coating MAI solution dissolved in isopropanol (IPA) on the PbI2 layer. The micrographs of the films prepared through this method are shown in Figure 1(c). Figure 1(b) shows the flow diagram for our deposition method that forms MAPbI3 through thermal evaporation of PbI2 followed by spin coating of MAI. The films are shown in Figure 1(d). As seen in the micrographs, the MAPbI3 films deposited through the thermal evaporation of PbI2 followed by spin-coating MAI result in slightly more continuous network compared to the conventional twostep spin-coating/spin-coating process. Since PbI2 acts as a template for the microstructure of the MAPbI3,18 the change in perovskite microstructure is attributed to the fine and continuous grains 4 ACS Paragon Plus Environment

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of the evaporated PbI2 compared to the solution deposited PbI2. Micrographs of the two precursor PbI2 films are given in supporting information Figure SI-1.

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Figure 1: (a) Schematic of the conventional two-step solution based spin coated of MAPbI3 films, (b) Schematic of two-step deposition with thermal evaporation of PbI2 and spin coated of MAI, (c) MAPbI3 films grown via conventional two-step solution based method, (d) MAPbI3 films grown via thermal evaporation of PbI2 and spin coated MAI.

In order to improve the morphology (increase film continuity and crystal grain size), the hybrid perovskite layers were subjected to an extended sintering step, more appropriately called a re-growth step, in a closed system with additional MAI powder. During the vapor equilibrated 6 ACS Paragon Plus Environment

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re-growth (VERG) step, the vapor phase quickly saturates with MAI (or methyl amine and hydrogen iodide). This inhibits loss of MAI from the hybrid perovskite while allowing for substantial re-structuring and re-growth of the hybrid perovskite film. The films that are subjected to VERG show an increase in the grain size by an order of magnitude; the MAPbI3 film becomes more compact and the surface coverage improves significantly. Figure 2 compares the effect of VERG on the microstructure of the MAPbI3 films. The top-view and cross-sectional SEM micrographs from MAPbI3 films before and after VERG are shown in 2b and 2c, respectively. The grain size increases from ~100 nm to ~1 µm, shown in the top view micrographs. The cross sectional micrographs show that the as deposited MAPbI3 consists of many small grains, while the VERG treated films consist of a single grain though out the film thickness. From the cross-sectional and top-view micrographs, ~10 nm diameter pinholes are distinguishable in the as-deposited films. These pinholes must be avoids since they can cause very low shunt resistance and result in short-circuited devices, especially in planar heterojunction devices as described by Eperon et. al.19 The elimination of pinholes using VERG is expected to improve device performance.



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Figure 2: (a) Simplified schematic of the vapor equilibrated re-growth (VERG) system. A detailed figure is given Figure SI-2. Top and Cross-sectional view micrographs from scanning electron microscope of the MAPbI3 films deposited through a thermal evaporation of PbI2 and spin coating of MAI (b) before VERG and (c) after VERG. The MAI exposure path plays a crucial role in the development of the MAPbI3 film and grain quality. The final film properties depend on the mode of contact (direct solid/liquid contact versus vapor exposure only) between the MAI and the MAPbI3 or PbI2 film. A summary of conditions and their results is given in Table 1 below. MAPbI3 films show grain growth and the most compact film structure when the MAI powder is kept separate in a vial, allowing 8 ACS Paragon Plus Environment

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communication only through the vapor phase, as shown in Figure 2. When the MAI is dissolved in IPA, both PbI2 films and MAPbI3 films disappear completely at the regions in contact with the IPA-MAI solution, while the dry regions remain visually unchanged. When exposed to direct contact with dry MAI powder, the PbI2 films react to form large compact grain MAPbI3 at the regions in direct contact with MAI. However, the regions that are not in contact with the MAI powder remain as deposited PbI2 films. When MAI powder is directly applied directly on the MAPbI3 films, the MAPbI3 film starts to disappear (turn from a dark colored light absorbing film into a semi-opaque white residue) in regions that it is in contact with bulk amounts of pure MAI powder. Afterwards, the entire film slowly degrades in a similar fashion. Currently, the underlying reactions are not yet understood. Table 1: Hybrid perovskite film re-structuring with various MAI exposures

Solid MAI powder MAI dissolved in IPA

Mode of Contact

Starting with PbI2 Film

Starting with MAPbI3 Film

In Direct Contact

MAPbI3 conversion with large grains

MAPbI3 delaminates from the substrate

Not In Contact

Stays as PbI2 film

MAPbI3 grain growth

In Direct Contact

Film disappears

MAPbI3 disappears

Not In Contact

Stays as PbI2 film

MAPbI3 grain growth

Figure 3 compares the x-ray diffraction of the as deposited MAPbI3 films and after the VERG. The diffraction peaks from the FTO substrate are shown in blue vertical lines, and labeled on the diffraction pattern with the respective (hkl) planes. The intensity of the MAPbI3 peak at 14.0 °2Θ doubles, and the FWHM is halved after the VERG. A more thorough Scherrer analysis of the peak width is prevented by the multiple closely spaced peaks of the tetragonal phase (see the peak positions in the bottom trace of Fig. 3). The small peak in the as deposited



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films (labeled as No VERG) at 12.7° could be interpreted as a residual PbI2 phase; however the lack of most intense PbI2 peak at 25.9° makes this unlikely. We believe that this peak is due to the existence of orthorhombic MAPbI3 (12.5°). The disappearance of peak at 12.7°, along with the tetragonal phase peak at 23.5°, after the VERG is attributed to the textured grain growth along (110) direction of the tetragonal structure. The lack of peak at 14.0° after the VERG, in agreement with the SEM micrographs seen in Figure 2, shows that the MAPbI3 films subjected to VERG do not contain any detectable PbI2 phase. MAPbI3 can decompose into its precursors MAI and PbI2 by the loss of MAI to the vapor phase. When this occurs, the initial hybrid perovskite film turns yellow as it is fully converted to PbI2. Chen et. al report that the MAI loss mainly takes place at the grain boundaries.4 Another post-annealing method, the Vapor Assisted Sintering Process (VASP)30, consists of a hot plate annealing step after adding MAI powder directly in contact with the film surface and partially enclosing the film and MAI using a petri dish. There are a couple of complicating issues with this method. First, the system is not closed. MAI may escape as it is volatilized. The process is dynamic and can be difficult to reproduce. When successful, it also results in large grains with compact films. However, the VASP method often results in formation of too much PbI2 due to the loss of MAI or disordering of the film due to too much solid MAI. In the VERG treatment reported here, the re-growth process is conducted in a closed system. MAI (or methyl amine and hydrogen iodide) saturates the atmosphere. Thus, the vapors are in equilibrium with the MAI solid and the hybrid perovskite film. As a result, the perovskite film does not decompose. Moreover, the possible remaining PbI2 sites are also reacted with MAI during the growth and passivated via excess MAI vapor.


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Figure 3: X-ray diffraction of the MAPbI3 films deposited in TiO2 coated FTO glass substrates, as deposited (shown as “No VERG”) and VERG treated (shown as “VERG”). The subscript “S” indicates the peak aligns with the SnO2 phase powder diffraction, “P” indicates the peak aligns with the PbI2 phase powder diffraction, and “T” indicates the peak is aligns with MAPbI3-T (tetragonal) phase powder diffraction. The optoelectronic quality of the MAPbClxI3-x and MAPbI3 films (both with and without VERG) are probed with time-resolved photoluminescence (TRPL) using the time-correlated single photon counting (TCSPC) method. VB1 to CB transitions31 are excited using a laser centered at 481 nm with 250 MHz repetition rate, and 1.2 nJ/cm2/pulse fluence. The decays are primarily exponential as seen in Figure 4. Over a range of excitation intensities, the recombination kinetics are modeled by the simple rate equation (eq. 1), where n is the photoexcited excess carrier density and t is time.32 The physical interpretations of the three terms are: (i) first-order decay rate due to either trap-mediated (Shockley-Read-Hall) recombination or band-to-band recombination under low injection conditions, both are effectively monomolecular;



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(ii) second-order decay rate due to band-to-band recombination under high injection which is bimolecular; (iii) third-order decay rate due to Auger recombination which is trimolecular. The data are collected under low-injection and thus strong second-order and third-order terms from equation 1 are not observed. Reasonable fits are obtained using only the first order term. However, in order to capture the deviation from a single exponential decay, a superposition of two exponentials are used to fit the intensity versus time data (eq. 2). This is the so-called "biexponential" fit. Time constants for both exponentials as well as their weights and a calculated average lifetime (eq. 3) are shown in Table 2.

−

= + +

(1)

() = + / + / =

+

(2) (3)


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Figure 4: Intensity normalized Time-Resolved-Photoluminescence decay of the (a) as deposited and VERG treated MAPbI3 films on quartz substrates, (b) as deposited and VERG treated MAPbClxI3-x films on quartz substrates.

Table 2: The bi-exponential model fitting and average lifetime of the carriers. τ1

τ2

τave

MAPbI3 (No VERG)

1.2 ns

12.8 ns

1.5 ns

MAPbI3 (VERG)

86.4 ns

495.0 ns

208.0 ns

MAPbClxI3-x (No VERG)

2.2 ns

32.8 ns

3.2 ns

MAPbClxI3-x (VERG)

68.6 ns

308.8 ns

113.9 ns

MAPbClxI3-x perovskites have been observed to have long effective minority carrier lifetimes. This has been attributed to the low density of defects as a result of better film crystallinity (compared to the pure iodide).28 Further, the passivation of surface defects in perovskites has been studied by Noel et. al. It was shown that when MAPbClxI3-x is treated with



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thiophene or pyridine the lifetime increases dramatically.12 It was proposed that the surface of the crystals suffers from the loss of iodine, resulting in a positively charged trap site that captures the photogenerated electrons. The Lewis base sites on the thiophene and pyridine then donate negative charge that passivates the trap. As a result, the photoluminescence quantum yield and the effective carrier lifetime increase an order of magnitude. During the VERG, MAI should passivate the grain boundaries, but we also believe that it might passivate the bulk defects in the MAPbI3 films through the diffusion of MAI. Hence, the VERG treated MAPbI3 films appear to be highly compact films with large crystal domains spanning the thickness and appear to be textured as observed from the electron micrographs and x-ray diffraction, respectively. PbI2 is known to undergo intercalation reactions due to its layered structure, especially with the Lewis bases as pyridine and methylamine (MA).33,34 Liang et. al35 has previously reported that vacuum deposited PbI2 may require more than 1 hour of MAI contact to fully convert to MAPbI3. In our method, 30 minutes of contact between MAI and PbI2 or PbCl2 is applied at least three times. This converts most of the PbI2 into MAPbI3, however we believe that the intercalation into PbI2 layers is not well coordinated. Thus, in the as deposited films the crystals remain small, and the films have a high density of defects. In MAPbClxI3-x, even though the reaction path involves formation of intermediate species before the Cl- and I- ion exchange, we believe that the intercalation of PbCl2 in the path of MAPbClxI3-x is not significantly different than the intercalation and formation of pure MAPbI3. Thus, both MAPbI3 and MAPbClxI3-x are predicted to have similar intercalation defects, and therefore both show low carrier lifetimes before the VERG treatment. The MAI decomposes into methyl ammine (CH3NH2, -6 °C boiling point) and hydrogen iodide (HI) at around 280 °C.36,37 While the PbI2 and MAI react immediately and form MAPbI3, it has been proposed that the PbCl2 and MAI 14 ACS Paragon Plus Environment

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reaction goes through a MAPbCl3 intermediate phase.19 As Williams et. al13 discussed recently, this intermediate phase acts a template for the developing MAPbI3. Thus, in general MAPbClxI3-x films show better crystallinity along with better electronic properties due to its slower formation process.38 We attribute the relatively slower photoluminescence decay in the non-sintered MAPbClxI3-x film compared to non-sintered MAPbI3 film to the possible lower defect density. In addition, we believe that a passivation of the MAPbI3 crystals similar to Noel et. al’s is taking place, and there are iodine vacancies on the initial small grain surfaces which are passivated by HI, as the iodine ion bonds to Pb surface during the VERG treatment. This agrees with the photoluminescence dynamics; the as deposited films exhibit an almost mono-molecular decay due to the high density of defect states, where as the photoluminescence decay after VERG becomes bi-exponential. The excess MAI vapor at 110 °C in the VERG treatment is believed to coordinate the MA molecules in the perovskite films both in grain boundaries and bulk of deposited films that results in with grain growth and film texturing while the defects are passivated. In conclusion, we have shown a deposition method, which leads to pinhole free MAPbI3 perovskite films with 100% substrate coverage for planar heterojunction solar cell architectures. Further, and more importantly, we have demonstrated an equilibrium post-deposition treatment (Vapor Equilibrated Re-Growth, VERG) that results in micron size MAPbI3 grains. The VERG treatment results in MAPbI3 grains textured along (110) direction, and the electron micrographs are consisted with single grains along the film thickness. The improvement in the photoluminescence lifetime of the photogenerated carriers with the VERG is likely due to decreasing ratio of grain boundary to bulk recombination through the growing larger crystals, but there may also be effects of passivation at the grain boundaries and the bulk of the films. We 15 ACS Paragon Plus Environment


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hypothesize that the enhanced carrier lifetime might also be a result of the textured MAPbI3 grains as the carrier diffusion is favored along the film thickness.13,39 As a result, the VERG treatment of the MAPbI3 may lead to higher short circuit currents and higher shunt resistances in planar heterojunction solar cells.

ASSOCIATED CONTENT (S) Supporting Information The experimental methods of the fabrication, post-fabrication treatments, and measurements of the corresponding samples fabricated in this work. SEM of the MAPbClxI3-x of VERG treated and non-treaded films fabricated in this work. This material is available free of charge via the Internet website http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy SunShot Initiative as part of the Next Generation Photovoltaics 3 program (DE-EE0006710) and by the University of Washington Clean Energy Institute.


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(13) Williams, S.T.; Zuo, F.; Chueh, C.-C.; Liao, C.-Y.; Liang, P.-W.; Jen, A. K.-Y. Role Of Chloride In The Morphological Evolution Of Organo-Lead Halide Perovskite Thin Films. ACS Nano 2014, 8, 10640-10654. (14) 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. (15) Yu, H.; Wang, F.; Xie, F.; Li, W.; Chen, J.; Zhao, N. The Role Of Chlorine In The Formation Process Of “CH3NH3PbI3-xClx” Perovskite. Adv. Funct. Mater. 2014, 24, 7102-7108. (16) Wehrenfennig, C.; Eperon, G.E.; Johnston, M.B.; Snaith, H.J.; Herz, L.M. High Charge Carrier Monilities And Lifetimes In Organolead Trihalide Perovskites. Adv. Mater. 2014, 26, 1584-1589. (17) Docampo, P.; Hanusch, F.; Stranks, S.D.; Doblinger, M.; Feckl, J.M.; Ehrensperger, M.; Minar, N.K.; Johnston, M.B.; Snaith, H.J.; Bein, T. Solution Deposition-Conversion For Planar Heterojunction Mixed Halide Perovskite Solar Cells. Adv. Energy Mater. 2014, 4, 1400355. (18) Burschka, J.; Pellet, N.; Moon, S-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M.K.; Gratzel, M. Sequential Deposition As A Route To High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. (19) Eperon, G.E.; Burlakov, V.M.; Docampo, P.; Goriely, A.; Snaith, H. Morphological Control For High Performance, Solution-Processed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater. 2013, 24, 151-157. (20) 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. (21) Liang, P.-W.; Liao, C.-Y.; Chueh, C.-C.; Zuo, F.; Williams, S.T.; Xin, X.-K.; Lin, J.; Jen, A.K.-Y. Additive Crystallization Of Solution-Processed Perovskite For Highly Efficient Planar Hetero-Junction Solar Cells, Adv. Mater. 2014, 26, 3748-3754. (22) Ball, J.M.; Lee, M.M.; Hey, A.; Snaith, H.J. Low Temperature Processed MesoSuperstructured To Thin-Film Perovskite Solar Cells. Energy Environ. Sci. 2013, 6, 1739-1743. (23) Carnie, M.J.; Charbonneau, C.; Davies, M.L.; Throughton, J.; Watson, T.M.; Wojciechowski, K.; Snaith, H.J.; Worsley, D.A. One Step Low Temperature Processing Route For Organolead Halide Perovskite Solar Cells, Chem. Commun. 2013, 49, 7893-7895. (24) Tan, K.W.; Moore, D.T.; Saliba, M.; Sai, H.; Estroff, L.A.; Hanrath, T.; Snaith, H.J.; Wiesner, U. Thermally Induced Structural Evolution And Performance Of Mesoporous Block Coploymer-Directed Alumina Perovskite Solar Cells. ACS Nano 2014, 8, 4730-4739. (25) Burlakov, V.M.; Eperon, G.E.; Snaith, H.J.; Chapman, S.J.; Goriely, A. Controlling Coverage Of Solution Cast Materials With Unfavorable Surface Interactions. Appl. Phys. Lett. 2014, 104, 091602.


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53x52mm (96 x 96 DPI)

ACS Paragon Plus Environment


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Figure 1: (a) Schematic of the conventional two-step solution based spin coated of MAPbI3< films, (b) Schematic of two-step deposition with thermal evaporation of PbI2 and spin coated of MAI, (c) MAPbI3< films grown via conventional two-step solution based method, (d) MAPbI3< films grown via thermal evaporation of PbI2< and spin coated MAI 175x184mm (96 x 96 DPI)


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Figure 2: (a) Simplified schematic of the vapor equilibrated re-growth (VERG) system. A detailed figure is given Figure SI-2. Top and Cross-sectional view micrographs from scanning electron microscope of the MAPbI3 films deposited through a thermal evaporation of PbI2 and spin coating of MAI (b) before VERG and (c) after VERG. 155x182mm (96 x 96 DPI)



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Figure 3: X-ray diffraction of the MAPbI3 films deposited in TiO2 coated FTO glass substrates, as deposited (shown as “No VERG”) and VERG treated (shown as “VERG”). The subscript “S” indicates the peak aligns with the SnO2 phase powder diffraction, “P” indicates the peak aligns with the PbI2 phase powder diffraction, and “T” indicates the peak is aligns with MAPbI3-T (tetragonal) phase powder diffraction. 273x209mm (300 x 300 DPI)


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Figure 4: Intensity normalized Time-Resolved-Photoluminescence decay of the (a) as deposited and VERG treated MAPbI3 films on quartz substrates, (b) as deposited and VERG treated MAPbClxI3-x films on quartz substrates. 271x207mm (300 x 300 DPI)


Enhanced Carrier Lifetimes of Pure Iodide Hybrid Perovskite via Vapor

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