Selective Stabilization and Photophysical Properties of Metastable

Sep 6, 2017 - International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong Univers...
13 downloads 18 Views 3MB Size
Download PDF
Article pubs.acs.org/cm

Selective Stabilization and Photophysical Properties of Metastable Perovskite Polymorphs of CsPbI3 in Thin Films Yongping Fu,† Morgan T. Rea,† Jie Chen,†,‡ Darien J. Morrow,† Matthew P. Hautzinger,† Yuzhou Zhao,† Dongxu Pan,† Lydia H. Manger,† John C. Wright,† Randall H. Goldsmith,† and Song Jin*,† †

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, P. R. China

‡

S Supporting Information *

ABSTRACT: All-inorganic cesium lead iodide (CsPbI3) perovskite has improved thermal stability over the organic− inorganic hybrid perovskites and a suitable bandgap for optoelectronic and photovoltaic applications, but it is thermodynamically unstable at room temperature and has multiple structural polymorphs. Here, we show that the use of long-chain ammonium additives during thin film deposition as surface capping ligands results in the stabilization of metastable bulk CsPbI3 perovskite phases without alloying mixed cations or anions into the perovskite lattice. Moreover, two different metastable CsPbI3 perovskite polymorphs in the cubic (αCsPbI3) and the much less common orthorhombic (β-CsPbI3) structures can be directly synthesized in a one-step spin coating film deposition by using oleylammonium or phenylethylammonium additives, respectively, and both phases are stable at room temperature for months. Time-resolved photoluminescence and photoluminescence quenching experiments show that the photoexcited species in the stabilized orthorhombic CsPbI3 thin film are mainly free carriers under solar illumination with a carrier lifetime of ∼50 ns and carrier diffusion length on the order of ∼100 nm, which implies efficient carrier transport within the film despite the presence of surface ligands. Our results provide a new chemical strategy to synthesize metastable all-inorganic CsPbI3 perovskites, which, together with the good photophysical properties, will open them up for applications in photovoltaic and other optoelectronic devices.

■

INTRODUCTION Metal halide perovskites with the structural formula AMX3 (where A is methylammonium, formamidinium, or cesium; M is Pb, Sn, or Ge; and X is a Cl, Br, or I) have re-emerged as a class of semiconductor materials with outstanding optical and physical properties for photovoltaic and optoelectronic applications.1−6 Despite exciting and promising device performance achieved in the organic−inorganic hybrid perovskites, their applications have been limited due to instability (i.e., poor thermal stability and moisture sensitivity) attributed to the organic cations.7 To address the stability issue, all-inorganic cesium lead halide perovskites (CsPbX3) have recently attracted more attention8−11 because of their improved stability toward heat and moisture and competitive optoelectronic properties as compared to the organic−inorganic hybrid perovskites.12 The perovskite phase of cesium lead iodide (CsPbI3) has a direct bandgap of ∼1.7 eV and excellent photoluminescence properties and is particularly desirable for a variety of optoelectronic applications.8,13−15 Unfortunately, because the cesium cation is too small to form a stable perovskite crystal structure,16,17 the © 2017 American Chemical Society

perovskite phase of CsPbI3 with a cubic structure (α-CsPbI3) is only stable at high temperature (>320 °C), below which it transforms into a nonperovskite structure of γ-CsPbI3, which is an undesirable semiconductor for optoelectronics due to its unfavorable optical properties (see Figure 1 for the crystal structures of different polymorphs and their phase transitions). Besides the commonly reported cubic perovskite (α-CsPbI3) and nonperovskite (γ-CsPbI3) phases, one might expect that CsPbI3 could experience similar polymorphic phase evolution behaviors to its analogs such as CsSnI3 or CsPbBr3.18,19 Therefore, the high-temperature α-CsPbI3 might also transform into a uncommon orthorhombic perovskite structure (βCsPbI3) via distortion of PbI6 octahedra without breaking the three-dimensional (3D) Pb−I network (Figure 1 right panel) upon cooling to room temperature (RT). However, the exact single-crystal structure of this CsPbI3 perovskite polymorph at Received: July 13, 2017 Revised: August 14, 2017 Published: September 6, 2017 8385

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394

Article

Chemistry of Materials

Figure 1. Crystal structures of different perovskite and nonperovskite polymorphs of CsPbI3 and the phase transitions between them.

°C), the larger density change (0.373 g/cm3 versus −0.005 g/ cm3), and the faster spontaneous phase transformation from the perovskite to nonperovskite polymorphs. Therefore, the stabilization of the perovskite phase of CsPbI3 is a more formidable task based on the thermodynamic equilibria. To the best of our knowledge, there have been no reports of stabilized bulk thin films of CsPbI3 perovskite that can allow for a fundamental study of intrinsic CsPbI3 perovskite properties and practical optoelectronic applications. In this article, we selectively stabilize the pure metastable CsPbI3 perovskite phase in bulk thin films without alloying other cations or anions into the crystal lattice by using longchain alkyl or aromatic ammonium cation ligands during thin film deposition. We found the resultant CsPbI3 perovskite thin films could be selectively stabilized in either the cubic (αCsPbI3) or the much less commonly reported orthorhombic (β-CsPbI3) perovskite phase depending on the choice of ligands, and both perovskite phases were stable over at least 4 months at RT. Using time-resolved photoluminescence quenching experiments, we estimate, for the first time, that the electron and hole diffusion length in the orthorhombic (βCsPbI3) perovskite phase with the presence of surface ligands could be up to 350 and 94 nm, respectively, indicating efficient charge transport within the film. As a proof-of-concept, a planar heterojunction solar cell based on stabilized CsPbI3 perovskite thin film was demonstrated to show a power conversion efficiency of ∼6.5%.

RT has not been unambiguously determined due to its instability and fast phase transformation into the γ-CsPbI3. Such undesirable phase transitions not only rule out the possible use of these metastable CsPbI3 perovskite phases in practical applications, but also prevents the study of the intrinsic photophysical properties of the various CsPbI3 polymorphs at RT. Many methods have been explored or suggested to access the metastable CsPbI3 perovskite phase in bulk thin films or nanostructures at RT such as anion exchange using stable Csbased perovskite nanostructures as templates,11 kinetic trapping by thermal quenching,17,20 using excess chemical such as hydroiodic acid during the film deposition,8 or solvent engineering.21 However, these kinetically trapped products spontaneously convert into the nonperovskite thermodynamic phase at RT after long-term storage, especially upon exposure to moist air.8,17 Alloying with other cations or anions is an effective strategy to stabilize the perovskite lattice,22−26 but it also changes the electronic band structure of pure CsPbI3 due to the incorporation of extrinsic ions into the lattice. Interestingly, it has been shown that the colloidal nanocrystals of pure CsPbI3 can be stabilized in the α-CsPbI3 perovskite structure at RT.27,28 We suspect that such stabilization could be attributed to surface energy effects, that is, the increasing energy contribution from the surface can outweigh that from the bulk as the size of crystallites decreases.29−31 Although the initially reported α-CsPbI3 nanocrystals were unstable and transform into γ-CsPbI3 after prolonged storage, Swarnkar et al. recently reported a critical purification approach that removed the excess unreacted precursors, which increased the structural stability to several months.14 In addition to the size effect, surface ligand functionalization could also have a profound influence on the phase stability. It was recently suggested that the colloidal CsPbI3 nanocrystals with oleylammonium iodide and/or oleylamine as the predominant surface ligands can have better phase retention, while the introduction of oleic acid destabilizes the surface chemistry and subsequently causes phase instability.32 Very recently, we demonstrated a new strategy to stabilize the metastable perovskite phase of pure formamidinium lead iodide (FAPbI3) via surface functionalization with long-chain alkyl or aromatic ammonium iodide and showed that surface functionalization may alter the phase equilibria due to the reduced surface energy.31 These results suggest the importance of surface chemistry on the structural stability of metastable lead iodide perovskites. We should note that the difference of the Gibbs free energies between the thermodynamically stable and metastable polymorphs of CsPbI3 could be much larger than that of FAPbI3, as implied by the higher phase transition temperate (320 °C versus 150

■

RESULTS AND DISCUSSION The CsPbI3 thin films were deposited by spin-coating a precursor solution that consisted of stoichiometric amounts of CsI and PbI2 in dimethyl sulfoxide (DMSO) solutions with or without the additive of oleylammonium (OA) or phenylethylammonium (PEA) acetate (see Scheme 1 for the chemical structures), followed by annealing at a temperature of 120 °C (lower than the phase transition temperature of 320 °C) for 1 Scheme 1. Chemical Structures of Long Chain Oleylammonium and Phenylethylammonium Cations Used as Stabilizing Ligands in This Work

8386

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394

Article

Chemistry of Materials

Figure 2. Schematic illustration of hypothesized surface ligand functionalization of the CsPbI3 perovskite structures and structural characterizations of the stabilized CsPbI3 thin films. Schematic crystal structures of (a) γ-CsPbI3 without surface ligands, (b) OA-stabilized CsPbI3 perovskite in the cubic phase; and (c) PEA-stabilized CsPbI3 perovskite in the orthorhombic phase. (d) PXRD patterns of the as-deposited γ-phase CsPbI3 thin film without additive, OA-stabilized CsPbI3, and PEA-stabilized CsPbI3 thin films, together with the standard patterns of γ-phase CsPbI3, cubic perovskite phase α-CsPbI3, and the simulated pattern of orthorhombic perovskite phase β-CsPbI3 based on the isostructural CsPbBr3 at RT; SEM images of the (e) γ-phase CsPbI3, (f) OA-stabilized CsPbI3, and (g) PEA-stabilized CsPbI3 thin films; scale bar is 1 μm. Insets are the corresponding photographs of the samples. AFM images of the (h) γ-phase CsPbI3 (scale bar 400 nm), (i) OA-stabilized CsPbI3 (scale bar 200 nm), and (j) PEA-stabilized CsPbI3 (scale bar 200 nm) thin films.

stabilized CsPbI3 film is more uniform and smooth with a small surface roughness of 40 nm). However, these long-chain ammonium ligands in the precursor solution play a critical role to access the metastable phase of CsPbI3 during the film deposition through both thermodynamic and kinetic factors. From the viewpoints of thermodynamics, at the initial nucleation stage, the metastable phase of CsPbI3 seeds at the nanoscale can be thermodynamically selected. The surface functionalization with long-chain ammonium ligands can further reduce the surface energy and make the perovskite phase more favorable to crystallize from the solution. When the nanocrystals reach the critical size, the growth of metastable phase can be still kinetically favored because the nucleation of thermodynamically stable phase would require higher energy barrier. We further suggest that the larger surface energy contribution from smaller grain sizes in the OA-stabilized films helps stabilize the higher-temperature cubic perovskite structure at RT because the OA ligand with longer carbon chains more effectively prevent grain growth and aggregation. This is confirmed by the broader PXRD peak width of the (002) planes for the OA-stabilized film (0.23°) than that of the PEA-stabilized film (0.12°), indicating a smaller crystallite size in the former. The crystallite size can be further estimated by using the Scherrer equation (Dsize = Kλ/Bsize cos θ, see Experimental section for more details), which yields an average crystallite size of 39 and 86 nm for the OA-stabilized and PEAstabilized sample, respectively. Similar size-dependent phase stability has been previously observed in metal oxide nanoparticles.27,29,37 The phase purity and identity of the stabilized CsPbI3 films are further confirmed by optical characterizations that show the deposited films with OA or PEA ligands are the 3D perovskite CsPbI3 structure without any optical features from layered perovskites despite the high content of ammonium cation additives. The absorption spectra of both stabilized CsPbI3 films (Figure 4 solid lines) show the onsets occur at ∼710 nm, corresponding to a bandgap of 1.75 eV. The corresponding PL spectra (Figure 4 dash lines) show a bandedge emission peak centered at 705 nm with a narrow fullwidth-at-half-maximum of ∼34 nm. The peak positions of both stabilized CsPbI3 perovskite phases are nearly identical, and they are consistent with previously reported PL peak position in thermally converted CsPbI3 nanowires20 and significantly red-shifted to that of colloidal CsPbI3 nanocrystals.14,32

Figure 3. Surface analysis and quantification of the ligands presented in the stabilized CsPbI3 films using XPS and TGA characterizations. High-resolution XPS of (a) Pb 4f, (b) I 3d, and (c) Cs 3d regions for the PEA-stabilized CsPbI3 sample. (d) High-resolution XPS of N 1s region for of the PEA-stabilized CsPbI3, the OA-stabilized CsPbI3, and the pure yellow CsPbI3 phase without ligands. (e) TGA curves of the three samples under a O2 atmosphere with a heating rate of 10 °C min−1 from room temperature to ∼750 °C.

The EDS mapping of the sample shows uniform distribution of the ligand in the sample (Figure S1). In addition, the quantification of the XPS spectrum of the PEA-stabilized sample yields a molar ratio of ligand (or N) to Pb of ∼0.42 and ∼0.30 for the OA-stabilized and PEA-stabilized sample, respectively. However, note that XPS is a surface sensitive technique and might not sample all of the Pb atoms. Therefore, thermogravimetric analysis (TGA) of the samples in a O2 atmosphere was carried out to quantify the final amounts of the ligands presented in the stabilized films (Figure 4e). If we assume all the organic species can be completely removed before the decomposition of CsPbI3 (which is at 325 °C), the weight percent of the organic species were estimated to be 4.0% and 5.9% for the PEA-stabilized and OA-stabilized sample, respectively. We then quantified the molar ratios of ligands to CsPbI3 to be 0.17 and 0.14, respectively, by simply assuming

Figure 4. Absorption and PL spectra of the stabilized CsPbI3 thin films. 8388

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394

Article

Chemistry of Materials

photoexcited carriers are mainly free carriers and the emission is dominated by free carrier bimolecular recombination at low N0.46 As N0 increases (>1018 cm−3), the TRPL exhibits accelerated decay dynamics due to the existence of a higherorder recombination channel (i.e., Auger recombination). We also plot the relative quantum yield under different excitation power densities (Figure S3), which reveal the relative contribution of the radiative and nonradiative recombination. The relative quantum yield was obtained by integrating the total signal from the TRPL curves, which were then divided by the excitation power. At the low carrier density region (n0 < 1018 cm−3), the quantum yield increased with the injected carrier density. This can be attributed to the increasing contribution of the electron−hole radiative recombination relative to the trap-assisted nonradiative recombination. After reaching a maximum, the quantum yield then decreases due to the occurrence of the Auger recombination at a high carrier density. The PL lifetime (τperovskite) at low carrier density N0 = 2.8 × 1015 cm−3 (similar to solar illumination) is estimated to be ∼50 ns using a stretched exponential fit (PL ∝ exp[−(kt)β], the fitting values of k and β are 0.020 ns−1 and 0.66). This is longer than the lifetime reported for CsPbI3 quantum dots in either solution or film,14,27 suggesting effective defect passivation via surface functionalization in this stabilized CsPbI3 film. The carrier kinetics behave similarly to that of hybrid lead iodide perovskites46,47 and bulk CsPbBr3 perovskite43 and is consistent with the recently determined small exciton binding energy of metastable CsPbI3 thin film.48 This suggests that CsPbI3, if it can be stabilized in the perovskite phase, can be an alternative excellent and robust perovskite material for light emitting and photovoltaic applications. We further used PL quenching experiments to study the charge transport properties of the stabilized CsPbI3 thin film by following previous reports on hybrid perovskites.49,50 Briefly, we prepared the quenched samples by spin-coating either an electron extraction layer of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) or a hole extraction layer of Spiro-OMeTAD on the top of a PEA-stabilized CsPbI3 thin film with a thickness of 75 nm (see Experimental Section for the details of the sample preparation and see details about this CsPbI3 thin film sample in Figure S4). Through comparing the TRPL spectra of these three samples: neat CsPbI3, CsPbI3/PCBM, and CsPbI3/spiroOMeTAD, the carrier lifetimes, diffusion constants, and thus carrier diffusion lengths can be extracted. Figure 5c shows that the steady-state PL intensities of the CsPbI3 films coated with charge extraction layers are significantly quenched compared to that of the neat CsPbI3 film under the same excitation density, indicating the carriers efficiently transport to the interface. Moreover, when the film is coated with a charge extraction layer, the PL decay is substantially faster with a much shorter lifetime (τheterojuction) fitted by a single exponential function: 7.2 ns for CsPbI3/spiro-OMeTAD and 0.8 ns for CsPbI3/PCBM (Figure 5d). We estimated the hole and electron transfer efficiency (Φ) are 86% and 99%, respectively, using the following equations:50

Moreover, there are no other additional excitonic absorption peaks, which would be expected if 2D layered perovskite structures were formed.38 While the photophysical properties of the organic−inorganic hybrid perovskites have been well studied,39−43 the investigation of fundamental photophysical properties of bulk CsPbI3 perovskite, such as the nature of photogenerated species and carrier diffusion lengths, remain largely unknown due to the phase instability issues. As mentioned previously, (Cl-doped) CsPbI3 nanocrystals in the cubic perovskite phase were reported;14,25,27,28,32 therefore, previous reports on the photophysics have mainly focused on the colloidal nanocrystals of CsPbI3 that exhibit quantum confinement,44,45 with little attention to the bulk CsPbI3 perovskite. Therefore, we used time-resolved photoluminescence (TRPL) to probe the carrier dynamics of the PEA-stabilized CsPbI3 thin film (see Experimental Section for details). Note that this is the orthorhombic β-CsPbI3 perovskite that has been less commonly reported and rarely (if at all) investigated by timeresolved spectroscopy. Figure 5a shows normalized TRPL kinetics of a PEAstabilized CsPbI3 film under different injected carrier densities (N0) ranging from 1015−1019 cm−3. The initial PL intensity (PL0) just after excitation as a function of carrier density (Figure 5b) shows PL0 increases quadratically from 2.8 × 1015 to 3.9 × 1017 cm−3 and then approaches saturation at higher carrier densities. This quadratic dependence establishes that the

Figure 5. (a) TRPL decay kinetics of a PEA-stabilized orthorhombic CsPbI3 perovskite thin film at different injected carrier densities; (b) initial PL intensity after laser excitation as a function of injected carrier density shows the quadratic dependence at lower injected carrier density in the range of ∼1015−1017 cm−3 (solid red line); (c) steadystate PL of the neat CsPbI3, CsPbI3/PCBM, and CsPbI3/spiroOMeTAD films excited at the same excitation fluence; (d) TRPL spectra of the neat CsPbI3, CsPbI3/PCBM, and CsPbI3/spiroOMeTAD films under the same injected carrier density of 2.9 × 1015 cm−3, along with the stretched exponential fit for the neat CsPbI3 film and the fits to one-dimensional diffusion model for CsPbI3/ PCBM and CsPbI3/spiro-OMeTAD films.

k heterjunction = k perovskite + k CT

Φ = k CT/k heterjunction

where kCT is the rate constant for the charger transfer process, kperovskite is the recombination rate constant of the neat CsPbI3 film (which is equal to 1/τperovskite), and kheterjunction is recombination rate constant of the quenched samples (which 8389

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394

Article

Chemistry of Materials is equal to 1/τheterojuction). The difference confirms that highly efficient charge transfer occurs at the interface. We then quantitatively calculated the carrier diffusion coefficient based on the one-dimensional diffusion equation: ∂ 2n(z , t ) ∂n(z , t ) =D − k(t )n(z , t ) ∂t ∂z 2

where D is the diffusion coefficient, k(t) is the carrier recombination rate of neat CsPbI3 without the charge extraction layer, and n(z,t) is the spatial charge carrier density. The initial distribution of the photoexcited carrier density within the film can be calculated using (z,0) = n0 exp(−αz), where α is the absorption coefficient (α = 2.0 × 104 cm−1 at 639 nm, as shown in Figure S4b), n0 is the carrier density on the surface, which is estimated by the photon fluence times absorption coefficient. In the quenching experiments, it is assumed that all the photoexcited carriers that reach the interface will be quenched, which yields n(L,t) = 0, where L is the thickness of the CsPbI3 film (75 nm). Through fitting the PL dynamics (Figure 5d), we estimated the diffusion coefficients for holes and electrons to be 0.0018 ± 0.0001 and 0.025 ± 0.001 cm2 s−1, respectively (see Experimental Section and Figure S5 for more details). The charge diffusion length LD is given by L D = D/k neat , where kneat is the recombination rate of neat CsPbI3 film. Despite the large amount of PEA ligands present in this stabilized CsPbI3 film, we found the electron and hole diffusion lengths are up to 350 and 94 nm, respectively, which are close to or slightly shorter than previously reported values for the polycrystalline MAPbI3 thin film measured by the same spectroscopic methods.49−51 It is suspected that the CsPbI3 crystallites may be electronically connected rather than isolated within the film. We note that the PL quenching experiments were carried out on different spots on these set of samples and the statistics of the fitting results and the average values are presented in Figure S6 and Table S1 of the Supporting Information. The unbalanced electron and hole transport properties may be due to the different electron and hole mobility, which requires further investigation. Encouraged by the good photophysical properties, we fabricated solar cells using PEA-stabilized CsPbI3 films as a proof-of-concept. To ensure the efficient hole collection with the shorter diffusion length in the stabilized material, we fabricated the devices using the architecture of ITO/ PEDOT:PSS/stabilized CsPbI3/PCBM/Al (Figure 6a, see details of device fabrication in Experimental Section), in which most carriers are generated in the region close to the PEDOT:PSS hole extraction layer on the side of illumination, thus reducing the potential impact of the small hole diffusion length. The device has a short-circuit current density (Jsc) of 15.0 mA/cm2, an open-circuit voltage (Voc) of 1.06 V, a fill factor (FF) of 0.41, and a power conversion efficiency (PCE) of 6.5% (Figure 6b). While the PCE is lower than the state-of-art CsPbI3 solar cells made of quantum dots or vapor-phase deposited films (∼10%),13,14 it is comparable to those of other solution-processed metastable CsPbI3 thin films.8,21,38,52 Further optimization of the device engineering, including improving electron or hole extraction layers, thickness, and interfaces, could further improve the solar cell efficiency. More importantly, we tested the phase stability of these CsPbI3 films and found them to be stable over several months at RT, as demonstrated by the PXRD and absorption spectra remaining largely unchanged even after 4 months of storage in

Figure 6. (a) Schematic of the CsPbI3 thin film solar cell device architecture; (b) current−voltage curves for a PEA-stabilized CsPbI3 solar cell; (c) PXRD patterns of the initial stabilized CsPbI3 thin films in comparison with the patterns taken after 4 months; (d) absorption spectra of the initial stabilized CsPbI3 thin films in comparison with those taken after 4 months. Insets are the photographs of the samples taken after 4 months.

the air (Figure 6c,d). Such excellent perovskite structural stability enabled by the surface functionalization31 is already superior to those previously reported metastable CsPbI3 films or nanostructures accessed via kinetic pathways, even though our stabilized films may still be metastable. For example, we have also attempted to make the black perovskite using HI as the additive by following the previous procedure but in the ambient air. While the black perovskite phase can be formed, we found that it quickly converted into the nonperovskite polymorphs within a few mins (the humidity in our lab was ∼30%), which is much faster than our stabilized films made under the same environmental conditions. We attributed such phase stability improvement seen in our samples to the more stable perovskite surface, where the surface ligands not only reduce the surface energy, but also prevent the interaction with moisture. Both are beneficial to the phase retention. Although the processed colloidal CsPbI3 quantum dots could be stable for at least 2 months,14 our strategy represents a simple method to synthesize pure CsPbI3 perovskite thin films that are ready for device fabrication.

■

CONCLUSIONS In summary, we demonstrate a new chemical approach to selectively stabilize the metastable CsPbI3 perovskite structures in the bulk form during thin film deposition by using long chain ammonium additives as surface capping ligands during a onestep spin coating thin film deposition. Moreover, different ammonium ligands enable the selective stabilization of two different perovskite polymorphs: the cubic α-CsPbI3 perovskite commonly observed in CsPbI3 nanocrystals and the much less common orthorhombic β-CsPbI3 phase with distorted perovskite structure, and both perovskite thin films are stable in the 8390

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394

Article

Chemistry of Materials

Bsize2 = Bobserved2 − Binstrument2. For all raw PXRD patterns, the Cu Kα2 peaks were first subtracted using Jade program. The instrumental broadening width was estimated by measuring the peak width of a standard crystalline Al2O3 sample with very large crystallite domain size under the same data acquisition conditions, which is determined to be 0.06° at 2θ ≈ 30°. The peak width of PEA-stabilized and OAstabilized film is 0.12° at 2θ = 28.54° and 0.23° at 2θ = 28.90°, which yields a crystallite size of 39 and 86 nm, respectively. Samples for TRPL and PL Quenching Experiments. Before thin film deposition, the glass cover slides were ultrasonically cleaned in IPA and then cleaned with oxygen plasma (150 W RF, 1 sccm O2, < 200 mTorr, 20 min) to improve the wetting property with precursor solution. The PEA-stabilized CsPbI3 films were deposited on cover slides by spin coating a precursor solution as described above, but further diluted with DMSO at a volume ratio of 1 to1, at 4000 rpm for 30 s, and then annealed at 120 °C for 20 s. The hole-quenched sample of CsPbI3/Spiro-OMeTAD was obtained by spin coating a solution containing 70 mg spiro-MeOTAD in 1 mL chlorobenzene on the above stabilized CsPbI3 film at 2000 rpm for 1 min. The electronquenched sample of CsPbI3/PCBM was obtained by spin coating a solution containing 20 mg PCBM in 1 mL of chlorobenzene on the above stabilized CsPbI3 film at 2000 rpm for 1 min. All samples were sealed between two pieces of cover slides using parafilm as a spacer on a hot plate (100 °C) in the ambient air before spectroscopy experiments. Calculations of Relative Quantum Yield and Charge Transfer Efficiency. For the neat PEA-stabilized sample, the total signal was integrated from the TRPL curves with different excitation power densities, and then divided by the excitation power to obtain the relative quantum yield. For the quenched samples, the charge transfer efficiency was followed the previously reported method.50 The total recombination rate of the quenched sample (kheterjunction) is given by the sum of the recombination rate in the perovskite layer (kperovskite) and the charge transfer rate of the carriers from perovskite layer to the electron or hole transport layer (kCT), which can be expressed as kheterjunction = kperovskite + kCT. The electron or hole transfer efficiency (Φ) can be estimated using the equation of Φ = kCT/kheterjunction. The TRPL kinetics of the perovskite sample and the two quenched samples were then fitted to extract the corresponding rate constants. Please note that we used a stretched exponential function to fit the TRPL of perovskite sample (PL ∝ exp[−(kt)β], the fitting values of kperovskite and β are 0.020 ns−1 and 0.66), and a single exponential function to fit the TRPL of the quenched samples [PL ∝ exp(−kt), the fitting values of kheterjunction are 7.2 ns−1 and 0.8 ns−1 for the hole-quenched sample and the electron-quenched sample]. The lifetime (τ) is defined by the time of PL intensity decays to 1/e of initial intensity; thus, the rate constant is equal to 1/ τ. TRPL Experiments. The TRPL experiments on various thin films samples were performed on a home-built confocal fluorescence microscope with excitation provided by a 639 nm picosecond pulsed diode laser (LDH-D-C-640, PicoQuant, Berlin, Germany) with a repetition rate of 125 kHz.47 The excitation was focused to a spot ∼0.53 μm2 through a CFI Plan Fluor 40× air objective (NA 0.75) (Nikon, Melville, NY) mounted on a Nikon Eclipse Ti−U inverted microscope, and the emission was collected through the same objective. The emission passed through a 635 nm dichroic beamsplitter (Semrock, Rochester, NY), a 647 nm long-pass filter (Semrock), and a 200 μm pinhole (Newport Corporation, Irvine, CA). The instrument response function was 580 ps fwhm. The filtered signal was then attenuated using absorptive neutral density filters (Thorlabs, Newton, NJ) to ensure ≤ 1 photon was detected for each excitation pulse. Emission was detected on a τ-SPAD single photon counting module (PicoQuant) and recorded using a PicoHarp 300 time-correlated single photon counting module (Pico-Quant) with 64 ps resolution, except for the IRFs, which were taken with a 16 ps resolution. The injected carrier density, N0, was estimated by N0 = jα, where j is the pump fluence (photons/cm2) and α is the absorption coefficient of the stabilized CsPbI3 film, which is 2.0 × 104 cm−1 at 639 nm (Figure S2b).

air for months at room temperature. Furthermore, by performing time-resolved PL and PL quenching experiments, we found the photoexcited species in the stabilized orthorhombic CsPbI3 thin film are mainly free carriers with a carrier lifetime of ∼50 ns under solar light illumination conditions and the carrier diffusion length is on the order of 100 nm, which can allow efficient carrier collection. A proof-ofconcept solar cell based on the stabilized CsPbI3 perovskite thin film demonstrated a power conversion efficiency of ∼6.5%. The excellent room temperature phase stability of the CsPbI3 perovskite thin films enabled by surface functionalization, together with their good photophysical properties revealed herein, could open up new opportunities for high performance optoelectronic devices based on stabilized CsPbI3 perovskites.

■

EXPERIMENTAL SECTION

All chemicals and regents were purchased from Sigma-Aldrich and used as received unless noted otherwise. CsPbI3 Thin Film Deposition. The yellow γ-phase CsPbI3 film was deposited on a glass slide by spin coating a precursor solution of CsI (260 mg, 1 mmol) and PbI2 (461 mg, 1 mmol) in 1 mL of DMSO solution at 4000 rpm for 30 s, and subsequently annealed at 120 °C for ∼1 min. Note that upon heating to remove the residual solvent, the film first appeared black, then quickly converted into yellow. The phenylethylammonium (PEA)-stabilized CsPbI3 film was deposited on a glass slide by spin coating mixed precursors of CsI (260 mg, 1 mmol), PbAc2·3H20 (379 mg, 1 mmol), and PEAI (498 mg, 2 mmol) in 1 mL of DMSO solution at 4000 rpm for 30 s, and subsequently annealed at 120 °C for ∼1 min. The oleylammonium (OA)-stabilized CsPbI3 film was deposited on a glass slide by spin coating mixed precursors of CsI (260 mg, 1 mmol), PbI2 (461 mg, 1 mmol), oleyamine (70% grade, 0.47 mL, ∼1 mmol), and acetic acid (0.12 mL, ∼2 mmol) in 1 mL of DMSO solution at 4000 rpm for 30 s, and subsequently annealed at 120 °C for ∼1 min. Acetic acid was added to ensure the protonation of oleylamine into oleylammonium cation. Before deposition, all the precursor solutions and glass slides were preheated on the hot plate at 120 °C. After deposition, the edge and corner of the film (that appeared less stable) were scratched off. The samples were sealed between two pieces of glass slides using parafilm as a spacer on a hot plate (100 °C) in the ambient air, and then kept in a desiccator in air for storage in the stability test. Structural Characterizations of the (Stabilized) CsPbI3 Films. The PXRD patterns were collected on as-deposited samples on glass substrates using a Bruker D8 Advance Powder X-ray Diffractometer with Cu Kα radiation. The samples were examined on a LEO SUPRA 55 VP field-emission scanning electron microscope (SEM) operated at 3 kV. The atomic force microscopy (AFM) images were collected using an Agilent 5500 AFM. The UV−vis absorption of as-grown stabilized thin films were collected using a JASCO V-550 spectrometer. The photoluminescence (PL) of the samples was collected with an Aramis Confocal Raman Microscope using a 442 nm laser source. Thermogravimetric analyses of the as-prepared samples were performed using a TA Instruments Q500 Thermogravimetric Analyzer with a ramping rate of 10 °C min−1 from room temperature up to ∼750 °C under an oxygen environment with a flow rate of 50.0 mL/min. The samples were scratched from the glass slides. Estimation of Crystallite Size. The crystallite size can be estimated by the Scherrer equation,53,54 which is given by Dsize = Kλ/ Bsize cos θ, where Dsize is the Scherrer crystallite size, K is a unit cell geometry dependent constant (we took K = 0.94), λ is the X-ray wavelength (λ= 1.5418 Å), Bsize is the peak width broadening due to the size effect, and θ is the Bragg angle. The peak broadening is a cumulate event, which comes from two sources: instrumental contribution and sample contribution. If both instrumental broadening profile and diffraction peak are Gaussian function, the observed peak width of the sample follows a simple square law, which can be expressed as Bobserved2 = Bsample2 + Binstrument2. We only consider the broadening contribution of crystallite size in the sample, therefore 8391

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394

Article

Chemistry of Materials Description of Fitting Procedure for Extracting Carrier Diffusion Length. The spectrometer’s instrument response function (IRF) was measured by exciting a glass coverslip with the 200 μm pinhole in place but with no long-pass filter. We convolved this response function with our model’s material response to fit the measured TRPL transient. The neat CsPbI3 film is modeled as having a decay described by a stretched exponential  the TRPL community uses stretched exponentials when samples are heterogeneous and have a distribution of lifetimes associated with a decay pathway:49

60623). The I−V curve measurements were recorded in a twoelectrode configuration using a Bio-Logic SP-200 potentiostat with a scan rate of 100 mV/s and reverse scanning. The area of the device was defined by the overlap of Al contact and ITO electrode, which was around 0.06 cm2.

■

* Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02948. Additional PXRD patterns; SEM images and absorption coefficient of stabilized CsPbI3 film; more details on TRPL data fitting (PDF)

signal(t ; a , k , β) = a exp[− (kt )β ] The samples coated with extraction layers are modeled with a modified form of the one-dimensional diffusion model equation used by Xing et al.:50

⎡ ⎛π m + ∞ ⎢ 2N0L ⎜ N (t ) = exp[− kt ] ∑ exp⎢ −Dt ⎜ π L m=0 ⎢ ⎝ ⎣

(

1 2

2⎤

■

) ⎞⎟ ⎥ ⎟⎥ ⎠ ⎥⎦

*E-mail: [email protected].

( 12 ) + (−1)m αL 2 1 2 1 2 2 α + π + ( L ) m m + 2) ( ) ( 2 ) (

ORCID

Yongping Fu: 0000-0003-3362-2474 Jie Chen: 0000-0002-2007-0896 Darien J. Morrow: 0000-0002-8922-8049 Randall H. Goldsmith: 0000-0001-9083-8592 Song Jin: 0000-0001-8693-7010

in which L is the thickness of the sample, D is the diffusion coefficient, and α is the absorption coefficient at the TRPL pump wavelength. We modified this equation to account for our stretched exponential as well as the fact that we found any term more than m = 0 to not increase the accuracy of our fit. Note: when only m = 0 is used, much of the above equation becomes merely a proportionality constant. Therefore, the equation we fit becomes

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Y.F., D.J.M., M.P.H., Y.Z., X.P., J.C.W., and S.J. acknowledge support by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award No. DE-FG02-09ER46664 for materials synthesis, structural characterization, device fabrication, and spectroscopic data analysis. J.C. acknowledges support from the China Scholarship Council. M.T.R., L.H.M., and R.H.G. acknowledge support by National Science Foundation (DMR-161034) that allowed the collection of the time-resolved spectroscopy results.

⎡ ⎛ π ⎞2 ⎤ N (t ) = b exp[− (kt )β ] exp⎢Dt ⎜ ⎟ ⎥ ⎣ ⎝ 2L ⎠ ⎦ We then extracted the diffusion constant from our data using the following algorithm. (i) Fit stretched exponential convolved with measured IRF to the response of neat CsPbI3 thin film to extract the decay and stretching parameter which ought to be characteristic of all studied iterations. (ii) Fit the diffusion model convolved with Gaussian IRF for the TRPL transients of the Spiro-OMeTAD and PCBM coated CsPbI3 thin films. The free parameters of the fit are the diffusion coefficient and prefactor. (iii) The diffusion length of the carriers is then calculated by L D =

D k neat

AUTHOR INFORMATION

Corresponding Author

exp[− αL]π m +

×

ASSOCIATED CONTENT

S

■

. We examined ∼6 spots for

each type of sample. Representative fitting curves are shown in Figure S5, and the histograms of the fitting results are shown in Figure S6 and the fitting results summarized in Table S1. Fabrication and Characterization of Perovskite Solar Cells. The ITO substrates were first patterned by etching with a 2 M HCl solution and Zn powders around a mask formed by strips of adhesive tape (3M, Scotch Magic Tape). Before various active layers were deposited, the ITO substrates were ultrasonically cleaned in acetone and then IPA for 5 min, and then cleaned with oxygen plasma for 10 min to remove organic residues. A layer of PEDOT:PSS was introduced on the prepatterned FTO substrate by spin-coating a PEDOT:PSS solution (Ossila, Al 4083) at 3000 rpm for 30 s. The substrate was then annealed on a hot plate at 120 °C. A PEA-stabilized CsPbI3 film was subsequently deposited following the deposition procedure using diluted DMSO solution described above for making samples for TRPL. After that, an electron transport layer was deposited on the stabilized CsPbI3 film by spin-coating a solution containing 20 mg of PCBM in 1 mL of chlorobenzene at 3000 rpm for 1 min. Finally, Al electrode (100 nm) was deposited on the PCBM film by e-beam evaporating Al (Kurt J. Lesker Co., 99.99%) at 1 Å/s. For solar cell measurements, a 1 kW Xe arc lamp solar simulator (Newport Corp., Model 91191) with a AM1.5G filter was used to illuminate the devices at an intensity of 100 mW/cm2, which was calibrated by using an NREL-calibrated and NIST-traceable monocrystalline Si reference solar cell (Photo Emission; model no.

REFERENCES

(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643. (3) 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. (4) Tan, Z. K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright lightemitting diodes based on organometal halide perovskite. Nat. Nanotechnol. 2014, 9, 687. (5) Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X. Lead halide perovskite nanowire lasers with low lasing thresholds and high quality factors. Nat. Mater. 2015, 14, 636. (6) Stoumpos, C. C.; Kanatzidis, M. G. The Renaissance of Halide Perovskites and Their Evolution as Emerging Semiconductors. Acc. Chem. Res. 2015, 48, 2791. (7) Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.; D’Haen, J.; D’Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.; Mosconi, E.; Angelis, F. D.; Boyen, H.-G. Intrinsic Thermal Instability 8392

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394

Article

Chemistry of Materials of Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater. 2015, 5, 1500477. (8) Eperon, G. E.; Paterno, G. M.; Sutton, R. J.; Zampetti, A.; Haghighirad, A. A.; Cacialli, F.; Snaith, H. J. Inorganic caesium lead iodide perovskite solar cells. J. Mater. Chem. A 2015, 3, 19688. (9) Beal, R. E.; Slotcavage, D. J.; Leijtens, T.; Bowring, A. R.; Belisle, R. A.; Nguyen, W. H.; Burkhard, G. F.; Hoke, E. T.; McGehee, M. D. Cesium Lead Halide Perovskites with Improved Stability for Tandem Solar Cells. J. Phys. Chem. Lett. 2016, 7, 746. (10) Kulbak, M.; Gupta, S.; Kedem, N.; Levine, I.; Bendikov, T.; Hodes, G.; Cahen, D. Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells. J. Phys. Chem. Lett. 2016, 7, 167. (11) Fu, Y.; Zhu, H.; Stoumpos, C. C.; Ding, Q.; Wang, J.; Kanatzidis, M. G.; Zhu, X.; Jin, S. Broad Wavelength Tunable Robust Lasing from Single-Crystal Nanowires of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I). ACS Nano 2016, 10, 7963. (12) He, X.; Qiu, Y.; Yang, S. Fully-Inorganic Trihalide Perovskite Nanocrystals: A New Research Frontier of Optoelectronic Materials. Adv. Mater. 2017, 29, 1700775. (13) Frolova, L. A.; Anokhin, D. V.; Piryazev, A. A.; Luchkin, S. Y.; Dremova, N. N.; Stevenson, K. J.; Troshin, P. A. Highly Efficient AllInorganic Planar Heterojunction Perovskite Solar Cells Produced by Thermal Coevaporation of CsI and PbI2. J. Phys. Chem. Lett. 2017, 8, 67. (14) Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum dot−induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics. Science 2016, 354, 92. (15) Ahmad, W.; Khan, J.; Niu, G.; Tang, J. Inorganic CsPbI3 Perovskite-Based Solar Cells: A Choice for a Tandem Device. Solar RPL 2017, 1, e201700048. (16) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties. Inorg. Chem. 2013, 52, 9019. (17) Dastidar, S.; Hawley, C. J.; Dillon, A. D.; Gutierrez-Perez, A. D.; Spanier, J. E.; Fafarman, A. T. Quantitative Phase-Change Thermodynamics and Metastability of Perovskite-Phase Cesium Lead Iodide. J. Phys. Chem. Lett. 2017, 8, 1278. (18) Chung, I.; Song, J.-H.; Im, J.; Androulakis, J.; Malliakas, C. D.; Li, H.; Freeman, A. J.; Kenney, J. T.; Kanatzidis, M. G. CsSnI3: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions. J. Am. Chem. Soc. 2012, 134, 8579. (19) Hirotsu, S.; Harada, J.; Iizumi, M.; Gesi, K. Structural Phase Transitions in CsPbBr3. J. Phys. Soc. Jpn. 1974, 37, 1393. (20) Lai, M.; Kong, Q.; Bischak, C. G.; Yu, Y.; Dou, L.; Eaton, S. W.; Ginsberg, N. S.; Yang, P. Structural, optical, and electrical properties of phase-controlled cesium lead iodide nanowires. Nano Res. 2017, 10, 1107. (21) Luo, P.; Xia, W.; Zhou, S.; Sun, L.; Cheng, J.; Xu, C.; Lu, Y. Solvent Engineering for Ambient-Air-Processed, Phase-Stable CsPbI3 in Perovskite Solar Cells. J. Phys. Chem. Lett. 2016, 7, 3603. (22) Li, Z.; Yang, M.; Park, J.-S.; Wei, S.-H.; Berry, J. J.; Zhu, K. Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys. Chem. Mater. 2016, 28, 284. (23) Sutton, R. J.; Eperon, G. E.; Miranda, L.; Parrott, E. S.; Kamino, B. A.; Patel, J. B.; Hörantner, M. T.; Johnston, M. B.; Haghighirad, A. A.; Moore, D. T.; Snaith, H. J. Bandgap-Tunable Cesium Lead Halide Perovskites with High Thermal Stability for Efficient Solar Cells. Adv. Energy Mater. 2016, 6, 1502458. (24) Cottingham, P.; Brutchey, R. L. Compositionally Dependent Phase Identity of Colloidal CsPbBr3−xIx Quantum Dots. Chem. Mater. 2016, 28, 7574. (25) Dastidar, S.; Egger, D. A.; Tan, L. Z.; Cromer, S. B.; Dillon, A. D.; Liu, S.; Kronik, L.; Rappe, A. M.; Fafarman, A. T. High Chloride

Doping Levels Stabilize the Perovskite Phase of Cesium Lead Iodide. Nano Lett. 2016, 16, 3563. (26) Hoffman, J. B.; Schleper, A. L.; Kamat, P. V. Transformation of Sintered CsPbBr3 Nanocrystals to Cubic CsPbI3 and Gradient CsPbBrxI3−x through Halide Exchange. J. Am. Chem. Soc. 2016, 138, 8603. (27) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692. (28) Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Grotevent, M. J.; Kovalenko, M. V. Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I). Nano Lett. 2015, 15, 5635. (29) Navrotsky, A.; Mazeina, L.; Majzlan, J. Size-Driven Structural and Thermodynamic Complexity in Iron Oxides. Science 2008, 319, 1635. (30) McHale, J. M.; Auroux, A.; Perrotta, A. J.; Navrotsky, A. Surface Energies and Thermodynamic Phase Stability in Nanocrystalline Aluminas. Science 1997, 277, 788. (31) Fu, Y.; Wu, T.; Wang, J.; Zhai, J.; Shearer, M. J.; Zhao, Y.; Hamers, R. J.; Kan, E.; Deng, K.; Zhu, X. Y.; Jin, S. Stabilization of the Metastable Lead Iodide Perovskite Phase via Surface Functionalization. Nano Lett. 2017, 17, 4405. (32) Wang, C.; Chesman, A. S. R.; Jasieniak, J. J. Stabilizing the cubic perovskite phase of CsPbI3 nanocrystals by using an alkyl phosphinic acid. Chem. Commun. 2017, 53, 232. (33) Stoumpos, C. C.; Malliakas, C. D.; Peters, J. A.; Liu, Z.; Sebastian, M.; Im, J.; Chasapis, T. C.; Wibowo, A. C.; Chung, D. Y.; Freeman, A. J.; Wessels, B. W.; Kanatzidis, M. G. Crystal Growth of the Perovskite Semiconductor CsPbBr3: A New Material for HighEnergy Radiation Detection. Cryst. Growth Des. 2013, 13, 2722. (34) Peng, W.; Miao, X.; Adinolfi, V.; Alarousu, E.; El Tall, O.; Emwas, A.-H.; Zhao, C.; Walters, G.; Liu, J.; Ouellette, O.; Pan, J.; Murali, B.; Sargent, E. H.; Mohammed, O. F.; Bakr, O. M. Engineering of CH3NH3PbI3 Perovskite Crystals by Alloying Large Organic Cations for Enhanced Thermal Stability and Transport Properties. Angew. Chem., Int. Ed. 2016, 55, 10686. (35) Wang, Z.; Wen, X.-D.; Hoffmann, R.; Son, J. S.; Li, R.; Fang, C.C.; Smilgies, D.-M.; Hyeon, T. Reconstructing a solid-solid phase transformation pathway in CdSe nanosheets with associated soft ligands. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 17119. (36) Ma, D.; Fu, Y.; Dang, L.; Zhai, J.; Guzei, I. A.; Jin, S. Singlecrystal microplates of two-dimensional organic−inorganic lead halide layered perovskites for optoelectronics. Nano Res. 2017, 10, 2117. (37) Garvie, R. C. The Occurrence of Metastable Tetragonal Zirconia as a Crystallite Size Effect. J. Phys. Chem. 1965, 69, 1238. (38) Liao, J.-F.; Rao, H.-S.; Chen, B.-X.; Kuang, D.-B.; Su, C.-Y. Dimension engineering on cesium lead iodide for efficient and stable perovskite solar cells. J. Mater. Chem. A 2017, 5, 2066. (39) Herz, L. M. Charge-carrier dynamics in organic-inorganic metal halide perovskites. Annu. Rev. Phys. Chem. 2016, 67, 65. (40) D’Innocenzo, V.; Grancini, G.; Alcocer, M. J. P.; Kandada, A. R. S.; Stranks, S. D.; Lee, M. M.; Lanzani, G.; Snaith, H. J.; Petrozza, A. Excitons versus free charges in organo-lead tri-halide perovskites. Nat. Commun. 2014, 5, 3586. (41) Johnston, M. B.; Herz, L. M. Hybrid Perovskites for Photovoltaics: Charge-Carrier Recombination, Diffusion, and Radiative Efficiencies. Acc. Chem. Res. 2016, 49, 146. (42) Zhu, H.; Miyata, K.; Fu, Y.; Wang, J.; Joshi, P. P.; Niesner, D.; Williams, K. W.; Jin, S.; Zhu, X.-Y. Screening in crystalline liquids protects energetic carriers in hybrid perovskites. Science 2016, 353, 1409. (43) Zhu, H.; Trinh, M. T.; Wang, J.; Fu, Y.; Joshi, P. P.; Miyata, K.; Jin, S.; Zhu, X. Y. Organic Cations Might Not Be Essential to the Remarkable Properties of Band Edge Carriers in Lead Halide Perovskites. Adv. Mater. 2017, 29, 1603072. 8393

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394

Article

Chemistry of Materials (44) Liu, Q.; Wang, Y.; Sui, N.; Wang, Y.; Chi, X.; Wang, Q.; Chen, Y.; Ji, W.; Zou, L.; Zhang, H. Exciton Relaxation Dynamics in PhotoExcited CsPbI3 Perovskite Nanocrystals. Sci. Rep. 2016, 6, 29442. (45) Zhang, Y.-X.; Wang, H.-Y.; Zhang, Z.-Y.; Zhang, Y.; Sun, C.; Yue, Y.-Y.; Wang, L.; Chen, Q.-D.; Sun, H.-B. Photoluminescence quenching of inorganic cesium lead halides perovskite quantum dots (CsPbX3) by electron/hole acceptor. Phys. Chem. Chem. Phys. 2017, 19, 1920. (46) Saba, M.; Cadelano, M.; Marongiu, D.; Chen, F.; Sarritzu, V.; Sestu, N.; Figus, C.; Aresti, M.; Piras, R.; Geddo Lehmann, A.; Cannas, C.; Musinu, A.; Quochi, F.; Mura, A.; Bongiovanni, G. Correlated electron−hole plasma in organometal perovskites. Nat. Commun. 2014, 5, 5049. (47) Manger, L. H.; Rowley, M. B.; Fu, Y.; Foote, A. K.; Rea, M. T.; Wood, S. L.; Jin, S.; Wright, J. C.; Goldsmith, R. H. Global Analysis of Perovskite Photophysics Reveals Importance of Geminate Pathways. J. Phys. Chem. C 2017, 121, 1062. (48) Yang, Z.; Surrente, A.; Galkowski, K.; Miyata, A.; Portugall, O.; Sutton, R. J.; Haghighirad, A. A.; Snaith, H. J.; Maude, D. K.; Plochocka, P.; Nicholas, R. J. Impact of the Halide Cage on the Electronic Properties of Fully Inorganic Cesium Lead Halide Perovskites. ACS Energy Lett. 2017, 2, 1621. (49) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341. (50) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344. (51) Li, Y.; Yan, W.; Li, Y.; Wang, S.; Wang, W.; Bian, Z.; Xiao, L.; Gong, Q. Direct Observation of Long Electron-Hole Diffusion Distance in CH3NH3PbI3 Perovskite Thin Film. Sci. Rep. 2015, 5, 14485. (52) Kim, Y. G.; Kim, T.-Y.; Oh, J. H.; Choi, K. S.; Kim, Y.-J.; Kim, S. Y. Cesium lead iodide solar cells controlled by annealing temperature. Phys. Chem. Chem. Phys. 2017, 19, 6257. (53) Langford, J. I.; Wilson, A. Scherrer after sixty years: a survey and some new results in the determination of crystallite size. J. Appl. Crystallogr. 1978, 11, 102. (54) Monshi, A.; Foroughi, M. R.; Monshi, M. R. Modified Scherrer equation to estimate more accurately nano-crystallite size using XRD. World J. Nano Sci. Eng. 2012, 2, 154.

8394

DOI: 10.1021/acs.chemmater.7b02948 Chem. Mater. 2017, 29, 8385−8394