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Unique Optical Properties of Methylammonium Lead Iodide Nanocrystals Below the Bulk Tetragonal-Orthorhombic Phase Transition Benjamin T. Diroll, Peijun Guo, and Richard D. Schaller Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04099 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Unique Optical Properties of Methylammonium Lead Iodide Nanocrystals Below the Bulk Tetragonal-Orthorhombic Phase Transition Benjamin T. Diroll,1 Peijun Guo,1 and Richard D. Schaller1,2* 1

Center for Nanoscale Materials, Argonne National Laboratory 9700 S. Cass Avenue, Lemont,

IL 60439 2

Department of Chemistry, Northwestern University 2145 Sheridan Rd, Evanston, IL 60208

ABSTRACT. Methylammonium (MA) and formamidinium (FA) lead halides are widely studied for their potential as low-cost, high-performance optoelectronic materials. Here, we present measurements of visible and IR absorption, steady state, and time-resolved photoluminescence from 300 K to cryogenic temperatures. Whereas FAPbI3 nanocrystals (NCs) are found to behave in a very similar manner to reported bulk behavior, colloidal nanocrystals of MAPbI3 show a departure from the low-temperature optical behavior of the bulk material. Using photoluminescence, visible and infrared absorption measurements, we demonstrate that unlike single crystals and polycrystalline films, NCs of MAPbI3 do not undergo optical changes associated with the bulk tetragonal-to-orthorhombic phase transition, which occurs near 160 K. We find no evidence of frozen organic cation rotation to as low as 80 K or altered exciton binding energy to as low as 3 K in MAPbI3 NCs. Similar results are obtained in MAPbI3 NCs

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ranging from 20 to over 100 nm and in morphologies including cubes and plates. Colloidal MAPbI3 NCs therefore offer a window into the properties of the solar-relevant, roomtemperature phase of MAPbI3 at temperatures inaccessible with single crystals or polycrystalline samples. Exploiting this phenomenon, these measurements reveal the existence of an opticallypassive photoexcited state close to the band edge and persistent slow Auger recombination at low temperature.

KEYWORDS. Perovskites, phase transition, exciton binding energy, lifetime, nanocrystals Organolead halide perovskites such as methylammonium (MA) and formamidinium (FA) lead triiodides have rapidly established a presence as candidate materials for high-performance optoelectronic devices including solar cells,1–3 detectors,4–6 and light-emitting media.7–10 The relative ease of synthesis combined with high-defect tolerance,11 large absorption crosssections,12 high mobilities,13 long carrier lifetimes,13,14 and modest carrier mobility15 make such hybrid perovskites promising optoelectronic materials. One complicating feature of studying hybrid perovskites is that these materials may exist in multiple phases with distinct optoelectronic properties.12,16–18 Both the cubic (stable > 327 K) and tetragonal (stable from 327 to 162 K) phases of MAPbI3 display small exciton binding energies,19,20 long photogenerated carrier lifetimes,21 and excellent charge diffusion lengths22 advantageous for high-performance photovoltaics at or near ambient temperatures.16,19,20,23,24 Phase transitions between the cubic and tetragonal phases of MAPbI3 are only weakly apparent, if at all, in optical measurements.25–27 Many groups have reported that the exceptional properties of MAPbI3 arise from the free motion of organic cations that occurs in these phases.19,20,28,29 However, the transition at 162 K from the


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tetragonal phase of MAPbI3, which results in higher exciton binding energy and an abrupt freezing of the cation,24 precludes the study of this phase at lower temperatures. Here, we present the optical properties of FAPbI3 and MAPbI3 nanocrystals (NCs) from 300 K to 3 K. Although nanometer-scale, these materials are only weakly quantum-confined for the size studied and do not show the large characteristic blue-shift and excitonic structure of other quantum-confined perovskites.30 In most respects, as has been reported previously, FAPbI3 NCs show the same temperature-dependent properties of bulk FAPbI3.12,31 However, similarly sized MAPbI3 NCs show distinctive optical properties from the bulk material below 160 K, including no optically-detectable phase transition from tetragonal to orthorhombic phases. Not only is the PL of NCs dominated by the tetragonal phase to low temperature for NCs over a surprisingly wide size range, visible and IR absorption measurements also confirm that MAPbI3 NCs retain the properties of the tetragonal phase to as low as 3 K. MAPbI3 NCs therefore offer an opportunity to study the fundamental properties of MAPbI3 below 160 K which we exploit to find the low exciton binding energy, persistent organic free rotation, photoluminescence from an optically-passive state, and temperature-insensitive Auger recombination. FAPbI3 offers important points of comparison for the understanding of results from MAPbI3 NCs, particularly the persistent free cation rotation to low temperatures. Typical examples of the temperature-dependent properties of 15.6 nm FAPbI3 NCs are shown in Figure 1. In all respects, these results are in agreement with previous measurements on bulk and nanocrystalline FAPbI3.12,31 The PL of a drop-cast film is shown in Figure 1a. The PL red-shifts with cooling, except for a blue-shift between 150 K and 100 K, a trend which is also tracked in the absorption shown in Figure 1b. This region of blue-shifting with cooling is attributed to a phase transition between β and γ orthorhombic phases.31 The band-edge absorption feature,


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although it presents a greater slope at low temperature, does not include a distinct excitonic absorption down to 3 K, suggesting that the exciton binding energy remains quite small, as found in earlier work.32 Accompanying the absence of a strong excitonic absorption, IR absorptions shift slightly (~1 cm-1) to lower energy when cooling from 300 K to 80 K with an increase in intensity of approximately 60%, most likely reflecting reduced thermal sampling of different environments in the crystal. The PL excited-state lifetime decreases upon cooling from 300 to 40 K, whereupon a long-lived tail of the emission from thermal trapping of photoexcited carriers, consistent with a dark state, appears and raises the average lifetime slightly, while the fast component of emission continues to become faster to 3 K. Spectrally-resolved measurements at high temporal resolution (Supporting Information Figure S2) confirm previous reports that FAPbI3 NCs display a sub-nanosecond red-shift of PL from a singlet emissive state to an optically passive state with longer lifetime.12

Figure 1. (a) Temperature-dependent PL of FAPbI3 NCs with TEM image of the sample shown inset. (b) Integrated PL as a function of temperature, normalized to the maximum observed intensity. (c) Temperature-dependent visible absorption spectra of FAPbI3 NC sample. (d) Plot of


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the full-width at half maximum and PL center energy of the FAPbI3 NCs as a function of temperature in solid circles. Separately, the change in PL and absorption band gap energies (referenced to 3 K) are plotted as open circles and closed triangles, respectively. (e) Temperature-dependent infrared absorption of FAPbI3 NCs and (f) plot of the absorption strength of mid-IR transitions associated with the formamidinium cation. Inset shows the IR absorption spectrum with the vibrations labeled. (g) Temperature-dependent PL dynamics of FAPbI3 NCs and (h) plot of the average PL lifetime versus temperature.


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Figure 2 compares the low-fluence (400 pJ/cm2) PL properties of a sample of 15.1 nm MAPbI3 NCs shown in Figure 2a with a polycrystalline film of the same composition shown in Figure 2b. The PL of the samples, shown in Figures 2c and 2d, differs dramatically below 160 K. As shown for polycrystalline films and single crystals, the phase transition from tetragonal to orthorhombic MAPbI3 results in an initial blue-shift of the PL peak followed below 120 K by substantially more complex behavior, including a significant broadening of PL.21,25,33,34 The NC sample, by contrast, shows a continuous red-shift with decreasing temperature and only a single PL feature at all temperatures (See also Supporting Figure S3). Temperature-dependent PL measurements repeated for several samples of different size and shape, including cubes and plates, show similar changes in band gap with temperature (See Supporting Information Figure S4). As reported previously, NCs smaller than 12 nm showed a more complexly structured PL, but at larger sizes with weaker quantum confinement, PL was dominated by single peak features. Surprisingly, even ensembles containing plates as large as 150 nm in edge length showed no evidence of the tetragonal-to-orthorhombic phase transition.


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Figure 2. (a) TEM of MAPbI3 NCs. (b) SEM of MAPbI3 thin film. (c, d) Normalized temperature-dependent emission of (c) NC sample and (d) polycrystalline film collected at 400 pJ/cm2 excitation fluence. (e) Plot of the integrated PL intensity for each of the samples as a function of temperature. (f) Plot of the band gap determined from fitting the PL of each sample. This result is consistent with one recent on work on quantum confined MAPbI3 that has demonstrated the dominance of tetragonal phase PL in ensembles of MAPbI3 NCs at temperatures below 160 K, despite the presence of an orthorhombic phase in the X-ray diffraction pattern.35 Figure 3 shows temperature-dependent absorption data, which are perhaps even more revealing because unlike PL measurements, they are not affected by small inclusions or optically-passive states.36–38 PL studies may be dominated by a minority component of the sample; absorption measurements confirm that the unique properties of NCs reflect the ensemble behavior of the samples and extend not only to changes in the band gap, but also the putativelyimportant vibrational motion of the organic sublattice. In visible absorption measurements, plotted in Figure 3a, the NCs are shown to follow a continuous red-shift of the band gap as the


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temperature is lowered, exactly mirroring the change in the PL. The polycrystalline bulk film, on the other hand, shows a blue-shift and the rise of a strong excitonic absorption feature between 165 K and 155 K due to the phase transition from the tetragonal to orthorhombic phases.18,20 This strong excitonic feature is characteristic of orthorhombic MAPbI3.18,20 The absence of a phase transition apparent in optical data suggests that NCs of MAPbI3 can be exploited to find the exciton binding energy of the tetragonal phase. Estimates of the exciton binding energy of the tetragonal phase of MAPbI3 vary from different optical fittings ranging from 2 meV39 or 5 meV19, to higher values such as 1240,41 to 55 meV.20 Values for the orthorhombic phase, with a clear excitonic absorption feature, are also reported over a wide range of values as 12 meV,32 15 meV,19 37 meV,42 or 45 meV.18 In our data for NCs of MAPbI3, at no recorded temperature is an excitonic feature observed which can be distinguished from continuum absorption. It is possible that strain variation or size polydispersity of the weakly-confined NCs washes out an excitonic feature. Nonetheless, inhomogeneous broadening is unlikely to wash out strong excitonic absorption. Based upon the absence of excitonic absorption from 3 K to 300 K, we conclude that the exciton binding energy of the MAPbI3 NCs are equal to or smaller than the smallest reported values for polycrystalline bulk films (≤ 5 meV). Persistent low exciton binding energy may enable new uses for MAPbI3 which were previously constrained by excitons at low temperature.43 The contrast between bulk polycrystalline films and NCs of MAPbI3 in infrared absorption, shown in Figures 3d and 3e, is also clear. For the bulk, free rotation of the methylammonium cation in the cubic and tetragonal phases produces fairly broad absorption from asymmetric N-H stretches. However at 155 K and below, a significant sharpening of the absorption lines at the N-H υ1 and υ2 stretches (respectively at 3125 cm-1 and 3172 cm-1, varying


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somewhat with temperature) appears upon freezing out of such free rotation.44–47 The IR absorption properties of the MAPbI3 NCs in Figure 3d resemble those of FAPbI3.. The υ1 stretch near 3125 cm-1 increases by a factor of nearly 5 between 300 K and 80 K and the υ2 saturates below 160 K for the bulk film. In contrast, both resonances increase by a factor of approximately 2 in the NC sample, with a smaller frequency shift (Supporting Information Figure S6). These findings show that unlike in bulk MAPbI3, free rotation of the methylammonium ion is not restricted strongly by hydrogen bonding in the MAPbI3 NCs and is instead only more gradually restricted by available thermal energy, similar to FAPbI3. As the free rotation of methylammonium is incompatible with the orthorhombic phase of MAPbI3, this is additional evidence that the samples exist in the tetragonal phase.48 The free rotation of the methylammonium is also consistent with a higher dielectric environment24,27,49 and lower exciton binding energy, which helps to explain the absence of excitonic absorption in the low temperature visible absorption of MAPbI3 NCs.


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Figure 3. Visible energy absorption spectra of (a) FAPbI3 NCs and (b) bulk polycrystalline FAPbI3 recorded from 300 K to 3 K. (c) A plot of the bad gap for the two samples estimated based upon linear extrapolation in a Tauc plot. Infrared absorption of the (d) MAPbI3 NCs and (e) polycrystalline film as a function of temperature from 300 K to 80 K. (f) Normalized absorption (to 300 K) of the infrared absorption resonances υ1 at 3124 cm-1 and υ2 at 3172 cm-1 for the NC and polycrystalline film. Collectively, the findings presented thus far underline that the NCs do not obey the bulk phase behavior of MAPbI3. Though not always well-understood, colloidal NCs are often observed to exhibit distinctive thermodynamic phase stability from bulk materials.50 For example, CsPbX3 NCs are also observed to have distinct behavior with temperature than the bulk material.51–53 Unlike previous works which have demonstrated kinetic trapping of FAPbI3,54 NC samples show only small hysteresis (Supporting Information Figure S8), indicating that the tetragonal phase is substantially more stable in the NC form. Based upon earlier reports of ligand-induced stabilization of micrometer-scale cubic FAPbI3 and MAPbBr3 NCs,55,56 it is


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suspected that the phase stability of MAPbI3 NCs observed in this work has an origin arising from ligand-induced changes in surface energy. The stability of tetragonal MAPbI3 optical properties below 160 K enables the examination of several important questions regarding the photophysics. One of these questions, denoted above, regards the exciton binding energy of tetragonal MAPbI3. Another regards the temperature dependence of PL lifetime and the origin of long lifetimes in these materials, which are atypical for direct gap materials. Several theories exist to explain the long PL lifetime in MAPbI3 including high dielectric effective charge screening from organic cation rotation leading to exciton fission,19,20,24,27,49,57,58 introduction of polar ferroelectric domains which enhance charge separation,59–62 or the proposal that MAPbI3 behaves as an indirect bandgap semiconductor either statically through Rashba splittings63–65 or dynamically due to methylammonium rotations which lead to fluctuations of the band edges.28,66 To address the origin of long lifetimes in MAPbI3, temperature-dependent measurements of PL lifetime were performed at low fluence (400 pJ/cm2). Time-resolved PL measurement of NCs also offer a window into the properties of a material in which diffusion of carriers is largely eliminated. Unlike bulk MAPbI3, the time-resolved properties at low fluence are not clearly related to trapping.38 For example, the small, nearly constant energy difference in the band gap determined by absorption and PL—compared to the highly-variable difference in the bulk material—is a strong indication of bright, band-edge emitting states (See Supporting Information Figure S9). The time-resolved PL data shown in Figure 4a reflect a sample which is not dramatically differing quantum yield, and therefore the lifetimes are not as strongly reflective of radiative and non-radiative decay pathways. The average lifetime of PL is plotted in Figure 4b (see Supporting Information for more details). From 300 K to 145 K the NC PL is dominated by


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a single exponential decay process of decreasing lifetime. For 135 K and lower, a fast component of the PL continues to decrease in lifetime, reaching 460 ps at 3 K, while an increasing fraction of the total PL is emitted as a long-lived tail with a lifetime reaching >200 ns. High timeresolution PL in Figures 4c and 4d show that at 3 K, the fast emission is blue-shifted by 10 meV from the slow emission component, which is likely also responsible for the increased PL linewidth at low temperature (Supporting Information Figure S10). Such red-shifts in NC ensembles could arise from energy migration via Forster resonant energy transfer, but for NCs with long ligands, this process is typically much slower (>1 ns).67 Other works on bulk perovskites have claimed such fast PL changes arise due to hot carriers.29,68 Here, the appearance of long-lived emission below 145 K (where KT = 12.5 meV) and the 10 meV blue-shift observed in transient PL implie activation from a lower, optically-passive state. The low temperature PL data showing a sub-nanosecond fast emission, long-lived slow emission, and a red-shifted energy between these two features is proposed to follow the model in Figure 4f, with a higher-energy bright state and lower-energy dark state, similar to FAPbI3.12


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Figure 4. (a) Time-resolved PL of MAPbI3 NCs collected from 300 K to 3 K. (b) Average lifetime (closed circles) from fitting of PL decays. (c) Time- and energy-resolved PL of MAPbI3 NCs at 3 K. (d) Spectra from different time-ranges of PL at 3 K with gaussian fit lines. (e) Normalized time-resolved PL of MAPbI3 NCs shown at early and late times with red fit lines. (f) Model of the PL at low temperature, reflecting fast bright-state emission and slow opticallypassive emission. Although the slow component of MAPbI3 NC emission, which is increasingly prominent at lower temperatures, may be explained by a previously unobserved optically-passive state, the fast component of emission at 3 K observed in the tetragonal phase presents a significant challenge to explanations of long lifetime at or near room temperature. Our measurements of visible and IR absorption allow additional insight into those mechanisms of enhanced carrier lifetime when combined with time-resolved PL. First, multiple reports have shown that the real component of the low frequency (

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