Amplified Spontaneous Emission Properties of Solution Processed

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Article

Amplified Spontaneous Emission Properties of Solution Processed CsPbBr Perovskite Thin Films 3

Maria Luisa De Giorgi, Andrea Perulli, Natalia Yantara, Pablo P. Boix, and Marco Anni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00854 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 17, 2017

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The Journal of Physical Chemistry

Amplified Spontaneous Emission Properties of Solution Processed CsPbBr3 Perovskite Thin Films M. Luisa De Giorgia*, Andrea Perullia, Natalia Yantarab, Pablo P. Boix,b,c and Marco Annia. a

Dipartimento di Matematica e Fisica "Ennio De Giorgi", Università of Salento, via Arnesano,

73100 Lecce, Italy b

Energy Research Institute@NTU (ERI@N), Research TechnoPlaza, X-Frontier Block, Level 5,

50 Nanyang Drive, 637553, Singapore c

Instituto de Ciencia Molecular, Universidad de Valencia, c. Catedratico J. Beltran 2, 46980

Paterna, Valencia, Spain

ABSTRACT

Metal halide perovskites are currently emerging as highly promising optoelectronic materials. It has been recently demonstrated that full inorganic solution processed CsPbBr3 perovskite thin films show good electroluminescence properties combined with high thermal stability. In this work, we investigate in details the Amplified Spontaneous Emission properties of CsPbBr3

ACS Paragon Plus Environment

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perovskite thin films, as a function of the temperature and the trap density, modified by changing the CsBr-PbBr2 precursor concentration. ASE is observed in samples from both CsBr-rich solution (low trap density) and equimolar solution (higher trap density), up to about 150 K, with a minimum threshold of 26 µJcm-2and 29 µJcm-2 at 10 K, respectively. However, the different distribution of defect states – mainly above the first exciton level in the former, and below it in the latter–strongly improved optical gain at 10 K and changed the ASE temperature dependence of CsBr-rich films.

INTRODUCTION During the last years, great attention has been devoted to organic-inorganic lead perovskites due to their unique chemical, structural and optical properties. Hybrid lead halide perovskites (such as MAPbX3 where MA is methylammonium and X is Cl, Br or I, or mixed Cl/Br and Br/I systems) have shown great potential as light-absorbing semiconductors. In particular, they demonstrated remarkable photovoltaic properties, showing power conversion efficiencies up to 20%, ascribed to large absorption coefficients, low defect densities and long diffusion lengths1-4. Moreover, as absorber layers, they are appropriate for the realization of highly sensitive photodetectors operating in the visible5, ultraviolet6 and X-ray7 wavelength regions. Rather interestingly organic inorganic perovskites also show excellent emission properties, with photoluminescence

quantum

yield

typically

between

20%3

and

30%8,

promising

electroluminescence performances9 and tunable optical gain in the visible range3. A particularly important limit of organic inorganic perovskites is the lack of photostability, particularly relevant for photovoltaic applications, and of thermal stability due to the presence of the organic cations10-11, limiting applications to light emitting diodes (PeLED). For these reasons,


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fully inorganic perovskites have been recently proposed, being characterized by a direct band gap similar to that of hybrid organic-inorganic perovskites (such as methylammonium lead bromide), but exhibiting a higher thermal stability12. In one of these alternative materials inorganic cations, such as Cs+, substitute methylammonium leading to enhanced thermal stability of the devices as well13,14. As an example CsPbBr3 is demonstrated to be thermally stable up to 250 °C in air, and efficient PeLED have been recently demonstrated exploiting solution processed CsPbBr3 thin films with an optimized relative concentration of the precursors15. It has also been demonstrated that the precursors relative concentration modifies the photoluminescence quantum yield, the photoluminescence relaxation dynamics and electroluminescence properties, without affecting the film morphology. The better properties observed in films deposited from the CsBr-rich solution, were ascribed to a reduction of halogen vacancies (VBr)15. Concerning the optical gain properties, a wide range of results have been reported on CsPbBr3, including the absence of gain in bulk crystals even at 4 K16, the presence of optical gain up to room temperature in microcrystalline films realized by amorphous-crystalline thermal transition17, and high room temperature gain in nanocrystals film18. In this paper we investigate the Amplified Spontaneous Emission (ASE) and the optical gain properties of solution processed CsPbBr3 thin films with two different trap densities obtained by a variation of the precursor solution stoichiometry15. We demonstrate that ASE is present in both films at low temperatures with a similar threshold of 25-30 µJcm-2 at 10K. A maximum optical gain of about 100 cm-1 (60 cm-1) is measured in the sample with low (high) defects density. A different ASE temperature dependence is observed in the two samples, evidencing the presence


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of defects below the first exciton state in the sample with higher defect density and above it in the sample with lower defect density.

EXPERIMENTAL SECTION CsPbBr3 lead-halide perovskite films were deposited by spin coating of a mixture of CsBr and PbBr2 with different molar ratios, as precursor solution. Sample (1-1) was prepared by a equimolar ratio of CsBr/PbBr2 (thickness d ∼125 nm), while sample (2-1) was deposited from a CsBr rich solution (2-1 molar ratio of CsBr/PbBr2 and d ∼ 85 nm). In order to investigate the ASE properties, the films were pumped by a nitrogen laser @ 337 nm delivering 3 ns pulses with a repetition rate of 10 Hz and a maximum pulse energy of 155 µJ. The pump beam was focused by a cylindrical lens onto the sample surface in a rectangular stripe (∼5 mm x 100µm). The pumping laser energy density was changed through neutral density filters. The emission, waveguided by the active film, was collected from the sample edge in correspondence of the end of the excitation stripe, by means of an optical fiber coupled to a spectrophotometer (ACTON SpectraPro-750) equipped with a CCD (Andor). The spectral resolution was about 0.5 nm. In order to perform measurements as a function of the temperature, the samples have been placed in a close circuit He cryostat, under vacuum (pressure of about 10-2 mbar). The relative room-temperature PL quantum yield (PLQY) was measured by coupling, through an optical fiber, the ACTON SpectraPro-750 spectrophotometer with an integrating sphere and exciting the samples at a wavelength of 407 nm. The UV-Vis absorption and reflection spectra were acquired using a spectrophotometer equipped with an integrating sphere (Perkin-Elmer UV-Vis Lambda 900) (Figure S1). The sample thickness was measured with a


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stylus-based

surface

profiler

(TencorAlphaStep

200

Profilometer).

Morphological

characterizations were performed through a scanning electron microscope (JEOL JSM-6480LV)).

RESULTS AND DISCUSSION a) Photoluminescence excitation density dependence The excitation density dependence of the photoluminescence (PL) spectra was investigated in the range from about 8µJcm-2 to about 4.8 mJcm-2. The PL spectra of sample (1-1) as a function of the excitation density, at a temperature T=10 K, are reported in Figure 1a, and at the highest excitation density in Figure 1b. At low excitation density the PL is peaked at about 532.5 nm (see also Figure 2a) with a Full Width at Half Maximum (FWHM) of about 6.3 nm (Figure 2b). A linear variation can be observed as the excitation density increases, with the appearance of a narrow band peaked at 534.3 nm, visible for excitation density higher than 50 µJcm-2, progressively dominating the spectra. The PL peak wavelength shows an initial strong (about 2 nm) progressive red-shift (Figure 2a) up to about 0.3 mJcm-2, followed by a weaker red-shift (about 0.5 nm) up to the maximum investigated excitation density of 4.7 mJcm-2.

10

4

10

(a) 10 K

-2

0.009 mJcm 0.30 0.017 0.47 0.026 0.75 0.050 1.4 0.10 2.8 0.15 4.7

3

10

2

10

515 520 525 530 535 540 545 550 555 Wavelength (nm)

(b) 10

5

10

4

10

3

PL Intensity (arb.units)

5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 K 70 K 150 K 230 K 290 K

-2

@4.7 mJcm

510

520

530 540 Wavelength (nm)

550

560


5


Figure 1.PL spectra (a) as a function of the excitation density at T=10 K and (b) at the maximum investigated excitation density for different temperatures (sample (1-1)), showing a clear ASE band up to 150 K, and a broad PL at higher temperatures.

537 536 535 534 533 532 531 530 529 528 527

20 10K 70K 150K 230K 290K

(a)

18

(b)

16 14

FWHM (nm)

λ max (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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10K 70K 150K 230K 290K

12 10 8 6 4 2

0.01

0.1 1 Excitation Density (mJ cm-2)

0.01

0.1

1

Excitation Density (mJ cm-2)

Figure2.Plot of (a) λmax and (b) FWHM as a function of excitation density at different temperatures for sample (1-1). Concerning the FWHM (Figure 2b) an almost constant value of about 6.3 nm is observed up to 26 µJcm-2 followed by a clear line narrowing, down to about 2.3 nm, up to about 0.3 mJcm-2, and finally a broadening as the excitation density further increases. Finally, the total PL intensity shows a linear increase with the excitation density, up to 26 µJcm-2, followed by a slope increase at higher excitation density (Figure S2). Overall, these features are the typical signature of ASE. The first strong red-shift and the line narrowing take place in the same excitation density range, and are ascribed to the appearance of the ASE band. The weaker red-shift and the ASE broadening at excitation densities above 0.3 mJcm-2, are likely due to exciton-exciton scattering17. An ASE threshold of about 29 µJcm-2 has been estimated from the analysis of the FWHM excitation density dependence, by fitting the low excitation density data point with a


6

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constant, and the decreasing part of the data with a decreasing straight line, considering as threshold the crossing point between the two fitting lines. The excitation density dependence has been also measured at four higher temperatures, namely 70 K, 150 K, 230 K and 290 K (Figure S3). At 70 K and 150 K, features very similar to spectra at 10 K are observed, with the appearance of a clear ASE band with a PL red-shift and narrowing. However, the ASE threshold increases with the temperature up to 66 µJcm-2and 200 µJcm-2, at T = 70 K and T = 150 K, respectively. On the contrary, no clear line shape variation, strong red-shift and line narrowing are observed at 230 K and 290 K, evidencing the lack of ASE in the investigated excitation density range, as also indicated by the broad PL spectrum at the highest excitation density (Figure 1b). A weak red-shift and broadening are observed in both cases, evidencing the presence of exciton-exciton scattering. The excitation density dependence of the PL spectra of sample (2-1) are qualitatively similar (Figures 3, 4, S2 and S4). A clear ASE band is observed for temperatures up to 150 K, while no ASE is present at higher temperatures. The ASE threshold at 10 K and 70 K are about 26 µJcm-2 and 63 µJcm-2, respectively, comparable with the values of sample (1-1). On the contrary, a much higher ASE threshold of about 1.7 mJcm-2 is observed at 150 K. For the sake of comparison, reports have shown microcrystalline CsPbBr3 films deposited from amorphous-crystalline transition17 with an ASE threshold of about 50 kW/cm-2, in backscattering collection geometry, with an estimate of about 10 kW/cm-2 (not measured) in waveguide geometry. In our samples the 70 K average power density threshold is about 21-22 kW/cm-2, that is two times better than the measured value, and 2 times larger than the best


7


estimate one. Overall, the low temperature ASE properties of our solution processed films are comparable with the microcrystalline ones.


10

0.008 0.015 0.023 0.045 0.090

10 K

4

10

-2

mJcm 0.13 0.27 0.43 0.68 1.3 2.6 4.3


(a) 5

3

10

2

10

(b) 5 10 @4.3 mJcm-2

10

4

10

3

510

515 520 525 530 535 540 545 550 555

520

Wavelenght (nm)

530 540 Wavelength (nm)

10 K 70 K 150 K 230 K 290 K

550

560

Figure 3: PL spectra (a) as a function of the excitation density at 10 K temperatures and (b) at the maximum investigated excitation density for different temperatures (sample (2-1)). A weak ASE band is observed at 150 K, while only spontaneous emission is present at higher temperatures.

536 534

10K 70K 150k 230K 290K

(a)

FWHM (nm)

538

λmax (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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532 530 528 526

0.01

0.1

1 -2

Excitation Density (mJ cm )

22 20 18 16 14 12 10 8 6 4 2

10K 70K 150K 230K 290K

(b)

0.01 0.1 1 -2 Excitation Density (mJ cm )

Figure 4: Plot of (a) λmax and (b) FWHM as a function of excitation density at different temperatures for sample (2-1).


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b) Optical gain at T =10 K We determined the net gain g’ = g – α, where g is the sample gain and α are the waveguide propagation losses, by using the variable stripe length (VSL) method, by measuring the PL spectra at fixed excitation density of about 25 times the ASE threshold, as a function of the pump stripe length (Figure S5). The PL intensity dependence on the stripe length at the ASE peak wavelength shows an initial exponential increase, followed by a progressive saturation (Figure 5).The gain values at different wavelength were obtained by fitting these experimental data to two different equations for the intensity dependence on the stripe length.

4

10

(2-1)

10


(1-1) PL Intensity (arb.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

@ 535 nm 10

2

0,1

0,2 0,4 0,6 0,8 1 Stripe Length (mm)

3

10

@ 535 nm 2

10

2

0,1

0,2

0,4

0,6 0,8 1

2

Stripe Length (mm)

Figure 5.Plot of intensity as a function of the excitation stripe length at 535 nm and data fit with equation (1) (red line) and equation (2) (blue line).

The first equation is typically used in semiconductor films and is valid far from saturation19:


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I (z ) =

Is A [exp(gz ) − 1] g

Page 10 of 26

(1)

Where I(z), g, and z are the detected light intensity, gain coefficient and excitation stripe length, respectively; Is is the spontaneous emission rate per unit volume and A is the crosssectional area of the excited volume. The second equation20 is empiric and proposed to quantify the gain of II-VI nanocrystals films, including saturation:

   − ( z − z 0 )     I ( z ) = exp  glα 1 − exp  l α     

(2)

where I(z), g, and z are the ASE intensity, gain coefficient and excitation stripe length, respectively; while z0 accommodates for possible pump beam inhomogeneity and delayed ASE onset. lαis a parameter that accounts for the saturation in ASE intensity which is defined as the gain lifetime multiplied by the speed of light within the gain medium. The best fit values are reported in Figure 6 for both samples. We observe that the net gain values obtained from the fit with equation (2) are about 5 times larger than those obtained from the fit with equation (1) (as also reported in literature3), reaching extremely high values (about 400 cm-1 for sample (2-1)). Nevertheless, it is important to observe that the fit with equation (1) correctly reproduces (see Figure 5) the low length range in which, far from saturation, the experimental trend are determined by the unsaturated gain value. On the contrary, the best fit with equation (2) correctly reproduces the saturation, but fails to reproduce the low length range. Moreover the best fit values obtained with equation (2) indicate very large


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gain values also out from the ASE spectral range, that are not physically meaningful, while the values obtained from equation (1) correctly go to 0 out from the ASE spectral range. For these reasons, we believe that the values obtained with equation (1) are a more reliable estimate of the real gain value in the two samples. In particular, remarkable peak gain values of about 60 cm-1 and of 100 cm-1 are obtained for sample (1-1) and (2-1) respectively, as shown in Figure 6.

250

(1-1)

-1

-1

Gain (cm )

200 Gain (cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150 100 50 0 520

525

530 535 540 Wavelenght (nm)

545

550

450 (2-1) 400 350 300 250 200 150 100 50 0 520 525

530 535 540 Wavelenght (nm)

545

550

Figure 6.Plot of optical gain as a function of wavelength as deduced from fitting with equation (1) (open squares) and equation (2) (solid squares).

In order to correlate the ASE threshold of the two samples to the measured gain values we observe that the ASE threshold is the excitation density at which the gain, g, compensates the propagation losses, α, along the waveguide, and thus the net gain g’ = g-α = 0. The gain coefficient depends on the population inversion density N in the sample through the relation g=σN, where σ is the gain cross section. Moreover, far from saturation and in steady state, the population inversion density can be written as N=FabsDτ/(dhν), where Fabs is the fraction of


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absorbed pump photons, D is the pump excitation density, d the sample thickness, hν the pump photons energy and τ the emitting state lifetime. Thus the gain value is directly proportional to the product of the gain cross section by the emitting state lifetime (g ∝ στ), and the net gain linearly increases with D, with a slope proportional to στ. As the two samples show almost identical ASE thresholds, at a common excitation density the net gain is 0 in both samples. However at a common, higher, excitation density sample (2-1) shows a net gain which is about 1.7 times larger than sample (1-1) evidencing that the στ value is 1.7 times larger in sample (2-1) than in (1-1). Considering that the excited state lifetime in sample (2-1) is about 2 times larger than the (1-1) sample15 , this result indicates that the gain improvement in sample (2-1) mainly comes from the slower photoluminescence relaxation dynamics, and not by a higher gain cross section. Moreover the identity of the ASE threshold values, despite the large gain value in sample (2-1) than in sample (1-1) at a given excitation density evidences that the propagation losses are larger in sample (2-1) than in sample (1-1).

c) ASE temperature dependence In order to further investigate the role of the different composition on the ASE properties of the two samples we measured the temperature dependence of the PL spectra, at a fixed excitation density, 25 times higher than the 10 K ASE threshold. The temperature dependence of the PL spectra of sample (1-1) at an excitation density of 0.75 mJ/cm2 is reported in Figure7. As the temperature increases the following features are observed: 1) the peak wavelength progressively shifts to lower values (blue-shift) (see also Figure S6a);


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2) a clear ASE band is observed up to a temperature of 150 K, while broad spontaneous emission spectra are observed at higher temperatures; 3) a small temperature-independent FWHM is observed up to 130 K, followed by a rapid broadening up to 170 K, and by a progressive slower broadening at higher temperatures (Figure S6b); 4) the total intensity shows an initial increase, up to about 70 K, followed by a decrease at higher temperatures (Figure 8). The spectra of sample (2-1) (Figure 7) show similar features, with a progressive spectra blueshift, a temperature dependent broadening (Figure S7) and an ASE band up to 130 K, while the total intensity continuously decrease as the temperature increases, with a first small decrease up to about 90 K and a stronger exponential decrease at higher temperatures (Figure 8).

10K 30K 50K 70K 90K 110K 130K

(1-1) -2 @ 0.75 mJcm

4

10

150K 170K 190K 210K 230K 250K 270K 290K

3

10

2

10

500

510

520

530

540

550

560

5

10 PL Intensity (arb.units)

5

10 PL Intensity (arb.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2-1) @ 0.68 mJ/cm^2

4

10

10K 30K 50K 70K 90K 110K 130K

150K 170K 190K 210K 230K 250K 270K 290K

3

10

2

10

500

510

Wavelength (nm)

520 530 540 Wavelength (nm)

550

560

Figure 7.Temperature dependence of PL spectra at 0.75 mJ/cm2 (sample (1-1)) and at 0.68 mJ/cm2 (sample (2-1)) (about 25 times higher than the threshold value at a 10 K).


13


5 x 1-1 Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2-1

6

10

5

10

0

200

400

600

800

1000 1200

-1

1/kBT (eV )

Figure 8.Temperature-dependent integrated photoluminescence intensities. The full black and red symbols evidence the temperature range in which the emission is dominated by ASE, the gray symbols the transition regime from ASE to spontaneous emission, and the empty symbols the temperature range in which only spontaneous emission is present. The lines are the best fit curves in the ASE (dotted) and spontaneous emission (continuous) regimes. The data of the (1-1) sample have been vertically scaled of a factor 5 for clarity.

We observe in both samples that the spectra FWHM is low and weakly temperature dependent for all the temperatures at which ASE is evident. We thus ascribe the sudden spectra broadening between 130 and 170K for sample (1-1) and between 130 K and 150 K for sample (2-1) to the transition from ASE to spontaneous emission. As the FWHM is mainly determined by the most important process between ASE and spontaneous emission we conclude that PL spectra are mainly dominated by ASE up to 110 K in both samples (highest temperature before the


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beginning of the spectra broadening), while spontaneous emission dominates above 170 K in sample (1-1) and above 150 K in sample (2-1). In order to analyze the origin of the observed results, we start by observing that the fast disappearance of the ASE band cannot be related to crystalline phase transition, observed in the range from 140 to 155 K in hybrid organic–inorganic methylammonium lead iodide perovskite (CH3NH3PbI3)21. Indeed, CsPbBr3 shows phase transitions around 361 K and around 403 K which are above room temperature22. We also investigated the sample morphologies by SEM measurements (Figure S8) observing a similar morphology and grain sizes of the two samples, as also previously reported in thicker samples15. This result indicates that the differences in optical properties are not due to morphology differences between the samples, and confirm that the CsBr content of the solution does not affect the surface characteristics of the spin coated films15. The eventual role of the surface/volume differences between the samples, related to their thickness difference, has been investigated by measuring the relative room-temperature PL quantum yield (Figure S9). We find that , the sample (2-1) results about 4 times more efficient than (1-1), despite its lower thickness, and thus its higher surface/volume ratio. This clearly indicate that the defects affecting the emission properties are not surface defects, but bulk defects related to the different chemical composition of the precursors, and thus not depending on the surface/volume ratio. As the PL intensity directly depends on the sample emission efficiency, and thus on the interplay between radiative and non-radiative relaxation processes, we focused our attention on the temperature dependence of the intensity. The integrated PL intensity (Figure 8) shows a gradual increase for the sample (1-1) as the temperature increases from 10 K to about 70 K, while


15


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for the sample (2-1) a slow decrease is observed in the same range of temperatures. At higher temperatures both samples behave similarly, with a significant decrease of PL intensities. The presence of a thermal dependence of the emission intensity and, in particular, the exponential decrease observed above 90 K, are the typical signature of thermally activated processes. Moreover the presence of a clear exciton peak in the absorption spectra at room temperature (Figure S1) suggests that the dominant excited species in both samples are excitons up to room temperature. In order to understand quantitatively the experimental results, we thus modeled the time dependence of the emitting state population with the following rate equation:

 ∆E1   ∆E 2 dn n n n  ± = R (t ) − − exp  − exp  − dt τ r τ nr1  k BT  τ nr 2  k BT

  

(3)

where R(t) is the pump rate, τr the ASE or spontaneous emission radiative lifetime, ∆E1 is the activation energy of a non-radiative process, depleting the emitting state, whose non-radiative lifetime is related toτnr1 and ∆E2 is the activation energy of a non-radiative process, depleting the emitting state (with minus sign) or filling the emitting state (with plus sign) characterized by τnr2. The steady state solution for the PL intensity is:

I (T ) =

I0  ∆E   ∆E  1 + A1 exp − 1  + A2 exp − 2   k BT   k BT 

(4)


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where I(T) represents the integrated PL intensity at temperature T. I0 is the intensity when the temperature approaches 0 K, A1 is given by τr/τnr1and A2 by ±τr/τnr2. The detailed description of the excitation/relaxation processes involving exciton and trap states and the energy level scheme (Figure S10) with the characteristic relaxation time is reported in the Supporting information. The obtained expression is the equivalent of the fitting function used in literature for the analysis of the temperature dependence of the PL intensity in perovskites23-24, including two thermally activated processes instead that one. The experimental data have been fitted to equation (4), by using the minimum necessary number of fitting parameter, and separating the temperature range in which ASE or spontaneous emission dominate. When ASE dominates the emission (see Figure 8) the PL intensity decrease of sample (2-1) can be well reproduced by the fit with equation (4), in which only one thermally activated quenching process is included, with an activation energy of 3.6±0.6 meV. In (1-1) sample the experimental results can be reproduced by including two thermally activated processes, one enhancing the ASE with an activation energy of 17.1±1.5 meV, and one quenching the ASE with an activation energy of 22±2 meV. The observed differences between the samples suggest the presence of a thermally induced exciton trapping in sample (1-1), and of an initially dominating thermally activated exciton detrapping in sample (2-1). This suggests a different distribution of defects states in the two samples, leading to the tentative energy levels scheme reported in Figure 9.


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Figure 9.Proposed energy level scheme of the samples. The continuous lines represent the intrinsic energy levels, the dotted lines the non-radiative defects energy levels, the continuous arrows the radiative transition and the dotted arrow the thermally activated non-radiative transitions. The bold line represents the level that is predominantly populated at low temperature.

The sample (2-1) shows non-radiative defects about 3.6 meV above the exciton ground state. These levels are almost empty at low temperature. As the temperature increases part of the photoexcited electrons are thermally trapped, thus resulting in a thermally activated PL quenching. The sample (1-1) shows intead non-radiative defect levels about 17 meV below the exciton ground state. At low temperature, after photoexcitation, the defect states are mainly populated. As the temperature increases the defect states are thermally depleted, and the ASE intensity increases. The presence of defect states below the exciton state in (1-1) sample is consistent with the attribution of its reduced emission efficiency to a higher density of halogen vacancies15.


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In the high temperature range a clear exponential intensity decrease is observed in both samples, allowing to fit the experimental data with only one non radiative process and approximating the fitting function with I = B exp(∆E1 / kBT ) . An activation energy of 43.0±0.8 meV and of 55±4 meV was obtained for sample (2-1) and (1-1), respectively. This process could be associated to an exciton thermal dissociation, and evidences a high exciton binding energy. A further possible attribution is relative to thermally activated trapping, as proposed for (C6H5C2H4-NH3)2-PbI4 perovskite25 that would indicate even higher exciton binding energy. Our results, thus, indicate that the exciton binding energy in the films is at least 43 meV. This is higher than the reported values for both bulk (35 meV) and nanocrystals (40 meV) of CsPbBr3 and much higher than the thermal energy at room temperature (25 meV), consistently with the well visible exciton absorption at room temperature. The differences between the two values of the high temperature activation energy could be due both to a variation of traps density and distribution in the two samples, or could indicate an exciton binding energy dependent on the sample preparation procedure, as suggested24 for CH3NH3PbI3.

CONCLUSIONS We demonstrated that solution processed CsPbBr3 thin films show Amplified Spontaneous Emission and PL properties affected by the defects density. Optical gain up to 100 cm-1 at 10 K is found in the film (1-2) realized from CsBr-rich solutions, while lower gain (about 60 cm-1) is found in the sample (1-1) realized from equimolar CsBr-PbBr2 solutions. As the temperature increases, in the sample (2-1), showing the best gain


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properties at low temperatures, ASE disappears, rather counterintuitively, at lower temperatures with respect to the sample (1-1). This is ascribed to the presence of thermally induced exciton trapping between 10 and 90 K, while thermally induced detrapping takes place in the (1-1) sample. Our results also suggests that the PL in the films is due to exciton recombination up to room temperature, with an exciton binding energy of at least 43 meV.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Absorbance for both samples; PL spectra as a function of the excitation density at different temperatures; dependence of integrated emission intensity upon the excitation density at different temperatures; PL spectra at increasing values of the excitation stripe length; λmax and FWHM temperature dependence, SEM characterizations, relative room-temperature PL quantum yield measurements, detailed description of the excitation/relaxation processes involving exciton and trap states.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Dr Mauro Lomascolo is kindly acknowledged for useful discussions.


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TOC Graphic


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Amplified Spontaneous Emission Properties of Solution Processed

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