Self-Assembled Lead Halide Perovskite Nanocrystals in a Perovskite

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Self-assembled Lead Halide Perovskite Nanocrystals in a Perovskite Matrix. Daniela Marongiu, Xueqing Chang, Valerio Sarritzu, Nicola Sestu, Riccardo Pau, Alessandra Geddo Lehmann, Alessandro Mattoni, Francesco Quochi, Michele Saba, Andrea Mura, and Giovanni Bongiovanni ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00046 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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ACS Energy Letters

Self-Assembled Lead Halide Perovskite Nanocrystals in a Perovskite Matrix. Daniela Marongiu†, Xueqing Chang†, Valerio Sarritzu†, Nicola Sestu†, Riccardo Pau†, Alessandra Geddo Lehmann†, Alessandro Mattoni‡, Francesco Quochi†, Michele Saba†*, Andrea Mura†, Giovanni Bongiovanni†. †

Dipartimento di Fisica, Università degli Studi di Cagliari, I-09042 Monserrato, Italy.

‡

Istituto Officina dei Materiali, CNR-IOM Cagliari SLACS, Cittadella Universitaria, I-09042

Monserrato (CA), Italy.

AUTHOR INFORMATION Corresponding Author *[email protected]

ABSTRACT. Hybrid metal halide perovskite materials are produced with facile routes, but their morphology is sensitive to water, oxygen, temperature and exposure to light. While phase separation and self-assembly of perovskite nanostructures have been demonstrated, the realization of controlled perovskite-perovskite heterostructures has been limited up to now. We demonstrate here the growth of stable CH3NH3PbI3-xBrx nanocrystals in a CH3NH3PbBr3 matrix. Optical emission from the nanocrystals can be reversibly activated upon illumination through a

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photobrightening process. Optical microscopy images show that nanocrystals are stable in time, through several illumination cycles. Ultrafast photoluminescence measurements imply that optical excitations are funneled from the matrix into the lower bandgap nanocrystals. Since the nanocrystals represent less than 2% of the materials volume, the local carrier concentration is higher in the nanocrystals than in the matrix, leading to an increase in the photoluminescence quantum yield, highlighting the promise of such self-assembles heterostructures for efficient light-emitting devices.

TOC GRAPHICS

Hybrid lead halide perovskites with organic anions are very popular solution process semiconductor materials in solar cell and optoelectronics research.1,2 Thanks to the substitution of the halide, materials can be produced with almost any bandgap within the visible spectrum. As an example, prototypical CH3NH3PbI3, the standard choice for solar cells, has a ~1.6 bandgap, while in CH3NH3PbBr3 the bandgap is above 2.3 and further substitution of the halide with Cl brings the gap to ~3 . A major technological advance for perovskite materials


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would then be to produce perovskite-perovskite heterostructures, useful for directional charge separation in photoconversion, or for carrier confinement and efficient photoluminescence in light-emitting devices.3,4 It is however very difficult to control the morphology of perovskite materials with non-homogeneous composition. Both solid solutions and phase separation have been obtained in mixed halide materials, with reports of associated large instabilities in the optical emission spectrum under illumination, which have been linked to halide ion migrations and trap saturation.5-28 At present, it is not clear whether or not phase separation in mixed halide perovskites could be used to create stable heterostructures, particularly in view of efficient light emission. Here we demonstrate the creation of self assembled CH3NH3PbI3-xBrx nanocrystals in a CH3NH3PbBr3 matrix. We show that the heterostructures are stable in time, not dynamically created by halide ion migrations, and optical instabilities are related to the neutralization of traps during illumination. The CH3NH3PbI3-xBrx nanocrystals funnel optical excitations from the CH3NH3PbBr3 matrix, concentrating them in a small fraction of the materials volume and giving rise to optical emission that is efficient even at low excitation intensities, a very promising feature for the realization of perovskite LEDs.29-34


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Figure 1. Characterization of mixed halide perovskite films. (a) UV-Vis absorption spectrum. In the inset is shown the XRD spectrum stand-alone intrinsic layer. (b) Photoluminescence spectra acquired from a mixed halide perovskite film during sample illumination.

Perovskite films were prepared with PbAc2*3H2O, CH3NH3I and CH3NH3Br dissolved in DMF. CH3NH3Br was prepared by reacting methylamine, 33 wt % in ethanol, with hydrobromic acid (HBr), 48 wt % in water as described elsewhere.35 CH3NH3I was prepared in a similar way by reacting methylamine, 33 wt % in ethanol, with hydroiodic acid (HI), 57 wt % in water. Both CH3NH3Br and CH3NH3I salts were crystallized at 60 °C using a rotary evaporator until the powders reached a white and white-brown color, respectively. The salts were washed and filtered three times with diethyl ether, then dried in vacuum overnight to finally obtain a white powder.


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All the reactants were purchased from Sigma-Aldrich. The perovskite films were prepared by spin-coating of few drops of a 30 wt% perovskite fresh solution at 60°C containing PbAc2*3H2O and CH3NH3I with CH3NH3Br on a sodalime glass substrate at 7000 rpm for 30 s in a nitrogenfilled glovebox. The molar ratio between PbAc2*3H2O and the methylammonium salts was 1:3 while the molar ratio between CH3NH3I and CH3NH3Br was 1:10. The choice of precursors, particularly halide-free PbAc2*3H2O instead of the more common PbI2 and PbBr2 as lead sources, created a peculiar synthesis environment, where halides and lead were mixed in stoichiometric ratios, while CH3NH3 was in excess. After spin-coating, the films were annealed at 100°C for 1 h.36 It should be pointed out that the CH3NH3I/CH3NH3Br ratio employed in sample preparation does not reflect the final composition of the films: it has been recently demonstrated26 that Br ions have much stronger affinity than I ones for complexation with Pb ions, making them the dominant halide in mixed perovskites and, under the some condition (like halide excess) even producing pure bromide perovskites. To sum it up, with respect to other studies in literature on mixed halide perovskite films,5,9,15,18,20,37 our preparation route involves significantly lower amounts of I and different precursors, particularly PbAc2*3H2O, leading to the peculiar perovskite composition outcome that will be described in the following. The UV-Vis absorption spectrum was measured with a Hamamatsu Quantaurus system employing an integrating sphere and therefore automatically excluding the Rayleigh diffusion background. Figure 1a shows in mixed halide perovskite films the onset of interband absorption typical of CH3NH3PbBr3 around 530 in wavelength, corresponding to ~2.3 in bandgap, together with a well-defined excitonic peak.38 No absorption step is however present corresponding to the CH3NH3PbI3 bandgap (~1.6 or 780 ), signaling that the amount of I in the films is too low to be detected. If phase separated islands of CH3NH3PbI3 are present, their


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fraction is too small to be detected in absorption or, if a solid solution is created, the iodine component is too small to cause a measurable shift in bandgap. The XRD spectrum in the inset of Fig. 1a is compatible with what expected for pure CH3NH3PbBr3, confirming that the I/Br fraction is below our detection limit. The photoluminescence spectra in Figure 1b however tell a different story: mixed halide perovskite films initially exposed to illumination emit green light (~530 in wavelength), as expected for pure CH3NH3PbBr3 films, while keeping the film exposed to illumination produces important modifications in the emission spectrum, as a red peak appears (~710 in wavelength) and rises with time, growing to over 10 times more intense than the green peak before saturating. The photoluminescence quantum yield correspondingly jumps from ~5 10 to ~5 10 under 5 / illumination intensity (as we will se later, the quantum yield does not stay constant while varying the illumination level). The illumination was provided by a frequency-doubled mode-locked Ti:sapphire laser (80 repetition rate, 780 in wavelength, 100 long pulses), which also excited photoluminescence (similar results were obtained illuminating the films with a continuous wave blue laser diode, 405 in wavelength); the photoluminescence was then dispersed with a grating spectrometer and detected with a CCD camera. While pure CH3NH3PbI3 films would emit light ~780 in wavelength, the red peak is clearly related to the presence of iodine rich domains. Assuming that mixed halide domains are present, the blueshift of photoluminescence should be linearly correlated to the I/Br fraction through Vegard’s law, and corresponds to CH3NH3PbI2.6Br0.4, i.e. domains containing approximately 20% Br and 80% I. Such estimate clearly neglects any possible contribution of spatial confinement to the bandgap energy shift.


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After about 5 minutes in the dark, the mixed halide perovskite film resumed the initial emission spectrum measured before illumination. On the other hand, the UV-Vis absorption spectrum measured right after illuminating the samples did not show any modification with respect to the initial absorption spectrum obtained before illumination. This observation may hint to the fact that chemical photoreactions do not play a major role here.

Norm. PL Intensity

a)

530nm 710nm

b)

Norm. PL Intensity

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0.28 W/cm 2 2.4 W/cm 2 8 W/cm 0

200

400

2

600

Time (s)

Figure 2. Photostability of mixed halide perovskite films. a, The integrated emission from the red and green peaks as a function of time are shown for an excitation fluence of 0.28 / . Values are normalized to the maximum of each curve for clarity of display. b, The rise with time of the intensity of red photoluminescence is measured for various excitation intensities, showing a much faster rise with larger excitations. The gray lines represent fits to a model described in the discussion section. Figure 2a shows the time-scales for the photobrightening effect. Within few tens of seconds after initial exposure to illumination, the green photoluminescence peak decreases and tends to


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stabilize to around half its initial values; on the other hand the red emission starts from zero and keeps growing for several minutes before finally saturating. Figure 2b highlights that the photobrightening time-scale depends on the illumination intensity, particularly it becomes faster with larger illumination intensities. a) Log PL intensity

Norm PL Intensity

0.4

b)

0.8 Time (ns) 2

3.5 nJ/cm 2 10 nJ/cm 2 30 nJ/cm 2 100 nJ/cm

1.2 Log PL

0.0

Log PL intensity

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x

1

x

1.5

Log Φ

500 550 600 650 700 750 800 Wavelength (nm)

Figure 3. Time-resolved photoluminescence. a, Time-resolved photoluminescence trace for the 530 (green) and 710 (red) emissions. b, Time-integrated photoluminescence spectra for different values of laser fluence. Inset: power dependence of the integrated emission on laser fluence (Φ) shown in log-log scale. In order to understand the microscopic origin of photobrightening, we investigated the ultrafast dynamics of light emission from the mixed halide perovskites. Photoluminescence was excited with the same Ti:sapphire laser described before and detected with a streak camera coupled to a


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grating spectrometer. Up to right before shining the ultrafast pulses, the sample was kept under continuous background illumination in order to activate the red emission through photobrightening. Results are displayed in Figure 3: the green emission shows a sub-nanosecond decay after the excitation pulse with at least two characteristic times, while the red emission clearly has a rise time, which we attribute to a ’charging’ process, lasting several hundreds of picoseconds. The red luminescence intensity right after the excitation pulse is virtually zero, signaling that there is negligible direct excitation of iodine-rich nanocrystals with the laser light. Such dynamics are typical of a sensitization phenomenon with charge or energy transfer, meaning that optical excitations have to spatially migrate within the films before finally being loaded into the red-emitting nanocrystals. It is very interesting to also observe the time-integrated photoluminescence spectra as a function of the laser fluence (detection with a CCD camera ensured light emitted at all times after laser pulse excitation was collected): while the green emission does not shift with the excitation level, the red one clearly blueshifts and broadens for increasing excitation fluence. In addition, the emission intensity on the low energy side of the peak saturates. Such observation provides evidence that increasing the laser fluence, the efficient funneling of optical excitations in the nanocrystals leads to filling of the lowest-energy excited states, followed by a gradual occupation of the higher electronic levels according to the Pauli’s exclusion principle. The spectrally integrated intensity of the two peaks behaves also differently as a function of fluence: the green peak grows with a 3/2 power law, the typical dependence in pure lead halide perovskites at room temperature, which has been attributed to the presence of free carriers and unsaturated traps;39,40 conversely, the red peak scales linearly with the laser fluence, which is expected from a monomolecular recombination process.


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Figure 4. Optical microscopy. Photographs of the photoluminescence of a mixed halide film kept under illumination. A key point to understand is if illumination induces morphological modification in the mixed halide perovskite film, causing the nucleation of iodine-rich nanocrystals or if light-induced effects are limited to the electronic states. To this goal, we have monitored the photoluminescence from the films with an optical microscope equipped with a color camera (Figure 4). The first observation is that green and red emissions come from different areas in the sample. The second main observation is that after cycling the sample through a dark/illumination sequence, the red spots always reappear at the same places, meaning that the iodine-rich spots


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are always in the same place and illumination does not induce iodine migration over the length scales observed here.

Figure 5. Microanalysis. SEM images (left) of the mixed perovskite film surface with two corresponding EDX elemental profiles (right), the first one (1) showing almost pure CH3NH3PbBr3, the second one (2) an iodine-rich region. The microanalysis performed with a scanning electron microscope (Figure 5) on a pristine mixed halide perovskite film confirms that two phases are already present before illumination, with very different relative abundance of Br and I, particularly a majority phase of almost pure CH3NH3PbBr3 and sub-micrometer spots rich in I.


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Figure 6. Scheme of optical emission in mixed halide perovskites. Bands are aligned according to recent reports.41 Charge transfer from the matrix produces a much larger excitation density in the iodine rich nanocrystal than in the matrix. Recent reports of photobrightening on mixed halide (CH3NH3PbIxBr3-x) perovskites with a much larger I fraction than our case have attributed an important role in the phenomenon to spatial migration and subsequent segregation of halides under illumination.5,9,15,18,20,37 Our measurements show instead that for an I/Br ratio of less than 2% the segregation of iodine-rich nanocrystals inside the CH3NH3PbBr3 matrix is stable in the films and does not nucleate randomly during illumination. Illumination however is still responsible for the photobrightening effect. The possible mechanisms have been discussed in literature and involve the fact that under illumination the films are flooded with a gas of highly mobile photoexcited free carriers that fill or neutralize the traps responsible for non-radiative recombination.5,9,37,42,43 The fact that only the red emission brightens under illumination while the green one does not may suggest that traps involved in the photobrightening process are mostly linked to the iodine rich nanocrystals or their interface with the matrix; however the evidence is not strong enough to exclude contributions from traps in the CH3NH3PbBr3 matrix. To support our interpretation that the nanocrystals are not nucleated under illumination, but simply their optical emission is activated by light, we provide an analysis of photobrightening within the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model framework, which is employed in literature to identify nucleation in the kinetics of crystallization and changes of phase in materials.44,45 Particularly, we fit the rise of red photoluminescence with time under illumination (

!"#

in Figure 2b) using an adapted version of the Avrami equation, namely

!"#

= 1−

*

&'() , where + is the growth rate and , is the growth exponent, while we assume that

!"#


12

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is proportional to the fractional volume of the red-emitting nanocrystals that have been activated. The strength and wide applicability of such analysis is that the growth exponent is largely independent on the specific material and transformation involved, and only sensitive to the mechanism of nucleation (e.g. site saturated or continuous) and growth (e.g. interface- or diffusion-limited). For the photobrightening kinetics the characteristic growth rate increases with the illumination level (0.0043 - , 0.017 - and 0.04 - respectively for increasing laser fluence), but the best fit value for the growth exponent , is always 1.6 ± 0.1. Even though the observed time scales of photobrightening are compatible with the fast mobility of ions in hybrid perovskites films,46 the nucleation of the iodine-rich nanocrystals under illumination is unlikely as it would require a growth exponent close to 5/2.44 The dependence of the photoluminescence on laser intensity (inset in Figure 3b) provides important information on the photophysical processes involved in photobrightening. The repetition time between subsequent pulses of the 80 laser employed proved to be long with respect to the ultrafast charge transfer dynamics, yet short with respect to the lifetime of trapped carriers and photobrightening processes. As a consequence,

we can analyze the

photoluminescence emission intensity as a cw quantity. Emission from the CH3NH3PbBr3 matrix is superlinear in the laser fluence and it has been demonstrated that the dependence stems from an unbalanced concentrations of electrons and holes, with one of the two species being predominantly trapped. A rate equation model for the populations per unit volume of the electrons in conduction band ( / ), holes in valence band ( 0 ) and traps ( ( ) clearly illustrates the phenomenon, assuming that electrons can be trapped efficiently, while holes recombine predominantly with trapped electrons through a bimolecular process: 1 / = −3(,/ / − 5 / 0 + 7 ≅ −3(,/ / + 7 12


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1 0 = −3(,0 0 − 5 / 0 − 5′ 0 ( + 7 ≅ −5′ 0 ( + 7 12 1 ( = −5′ 0 ( + 3(,/ / 12 where 7 is the electron-hole pair generation rate (per unit volume) due to laser illumination, 3(,/ and 3(,0 the trapping rates for electrons and holes respectively, 5 and 5′ are the bimolecular electron-hole (which is the process originating photoluminescence) and trapped electron-hole recombination rates respectively. Under the assumptions that electrons are trapped more efficiently than holes, 3(,/ ≫ 3(,0 and 5′ ( ≫ 5 / ≫ 3(,0 , in steady state, with all the population rates equal to zero, the population of electrons is proportional to the generation rate, / ∝ 7,

∝ / 0 ∝ 7 = .39,40 Under such conditions, the steady state populations of electrons and holes are unbalanced, with / ≪ 0 because the electron population is depleted by traps. The presence of iodine rich nanocrystals embedded in the CH3NH3PbBr3 matrix leads to a one order of magnitude enhancement of the photoluminescence quantum yield. We explain this result as due to the higher carrier concentration accumulating in the iodine nanocrystals of the film, the photoluminescence intensity being proportional the product of electron and hole concentrations. In these regions, the generation rates for electrons and holes are provided by the corresponding populations in the matrix through a charge transfer process. We can rewrite the rate equations for the nanocrystals, calling the electron, hole and trap concentrations in the nanocrystals @/ , @0 and @( , while 3A( is the charge transfer rate (symbols marked with a tilde have the same meaning as the corresponding ones in the CH3NH3PbBr3 rate equations):


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1@/ = −3B(,/ @/ − 5C @/ @0 + 3A( / 12 1@0 = −3B(,0 @0 − 5C @/ @0 − 5C ′@0 @( + 3A( 0 12 According to the previous analysis of the kinetic rate equations, we find that @/ ∝ / ∝ 7,

Self-Assembled Lead Halide Perovskite Nanocrystals in a Perovskite

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