PFO polymer composite thin films

PFO polymer composite thin films

Journal Pre-proofs Optical and Structural Properties of CsPbBr3 Perovskite Quantum Dots/PFO polymer composite thin films B.A. Al-Asbahi, Saif M.H. Qai...

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Journal Pre-proofs Optical and Structural Properties of CsPbBr3 Perovskite Quantum Dots/PFO polymer composite thin films B.A. Al-Asbahi, Saif M.H. Qaid, Hamid M. Ghaithan, M.S. AlSalhi, Abdullah S. Al dwayyan PII: DOI: Reference:

S0021-9797(19)31561-9 https://doi.org/10.1016/j.jcis.2019.12.094 YJCIS 25834

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

24 September 2019 16 December 2019 21 December 2019

Please cite this article as: B.A. Al-Asbahi, S.M.H. Qaid, H.M. Ghaithan, M.S. AlSalhi, A.S. Al dwayyan, Optical and Structural Properties of CsPbBr3 Perovskite Quantum Dots/PFO polymer composite thin films, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.12.094

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Optical and Structural Properties of CsPbBr3 Perovskite Quantum Dots/PFO polymer composite thin films

B. A. Al-Asbahia,b,*, Saif M. H. Qaida,c,*, Hamid M. Ghaithana, M. S. AlSalhia,d, Abdullah S. Al dwayyana,e aDepartment of Physics & Astronomy, College of Sciences, King Saud University, Saudi

Arabia bDepartment of Physics, Faculty of Science, Sana'a University, Yemen cDepartment of Physics, Faculty of Science, Ibb University, Ibb, Yemen dResearch Chair on Laser Diagnosis of Cancers, College of Sciences, King Saud University, Saudi Arabia eKing Abdullah Institute for Nanotechnology, King Saud University, Riyadh, Saudi Arabia *Corresponding authors at: [email protected] and [email protected] Abstract: The aim of this study is to investigate the optical and structural properties of polymer/perovskite quantum dots (QDs) composite thin films and estimate the applicability of using these blends as active materials in photonic devices. A solution has been utilized, which is processed based on conjugated polymer and perovskite QDs composite films. The incorporation of CsPbBr3 QDs, with various weight ratios, influences the structure of the thin films, as proven by several techniques. The results of the study showed that the surface of the poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO)/CsPbBr3 thin films improved, when compared to that of the pristine CsPbBr3 thin film. The increase in the steepness parameter and decrease in both the energy gaps and Urbach tail, upon the increment of CsPbBr3 QDs, can be attributed to the decrease in the localized density of electronic states within the forbidden band gap of the hybrids. The overlap between the absorption spectrum of PFO and emission

spectrum of CsPbBr3 QDs, and the enhancement in the emission peak of CsPbBr3 in the blends, confirmed the efficient non-radiative energy transfer between them. Keywords: CsPbBr3; perovskite quantum dots; composite; light-emitting materials; energy transfer; PFO polymer. 1. Introduction: Metal halide perovskite quantum dots (QDs), such as CsPbX3 (X = Br, I, and Cl), have recently emerged as a new class of material with a promising potential for optoelectronic applications, such as photo-detection, lasing, and perovskite light emitting diodes (PeLEDs) [1–4]. The high interest in these materials can be attributed to their advantages, such as short radiative lifetime, broad emission spectra, extremely narrow emission bandwidth, high luminescence quantum yield, and ample options for shape control. These materials have easily adjustable composition versatility, which aids the tuning of their emission wavelength throughout the visible spectrum [2,5–14]. In addition, compared to classical Cd-based chalcogenide QDs and other QDs, perovskite QDs are of interest because of their significant optical properties, such as blinking behavior, non-linear absorption, and stimulated emission [15–19]. Despite the unique advantages of perovskite QDs, a considerable challenge that requires attention is their degradation, which occurs during purification from the synthesis solution [20]. Moreover, efficiency, brightness, and stability of lightemitting devices, based on organic or inorganic perovskite QDs, are low and require improvement [21–24]. The low performance of such light-emitting devices is primarily due to the low binding energy generated in the perovskite layer with

thermal ionization of excitons and the low film quality of perovskite QDs. Therefore, to improve the performance of LEDs based on perovskite QDs, research in this field should aim to limit the large diffusion length of excitons, decrease the possibility of exciton dissociation into carriers, and enhance the uniformity and flatness of the perovskite emitting layer [23]. One of the important factors in understanding LEDs that are based on perovskite, which have high efficiency and full-coverage electroluminescence, is the uniform morphology of a perovskite film. This uniform and continuous morphology of the perovskite film affects the subsequent deposition of functional layers in solution procedures [24,25]. Moreover, there exist two common strategies to improve the stability of such devices. One approach is to conduct a surface modification on perovskite, e.g., incorporating CsPbBr3 QDs into a silica/alumina monolith using a simple sol–gel method [26,27]; the other approach is to prepare a perovskite/polymer composite, such as CsPbBr3–poly(styrene-ethylenebutylene-styrene) [28]. Although many research studies have endeavored to improve the efficiency, the properties of the polymers and perovskite still require further investigation for optical applications. Therefore, in this study, high quality green-emitting CsPbBr3 QDs, which act as an acceptor, were incorporated into the donor conjugated polymer poly(9,9-di-n-octylfluorenyl-2,7-diyl (PFO), using a solution blending method. This composition helps us realize a new class of composites, with high quality and smooth thin films that deposit in one step. The effects of different quantities of CsPbBr3 QDs in PFO/CsPbBr3 QDs composite thin films were investigated, in terms of their structural and optical properties, using different characterization techniques to

improve their optoelectronic applications. Moreover, the interaction mechanism between CsPbBr3 QDs and PFO was suggested. 2. Experimental details 2.1 Materials: PFO (>99.0%) and toluene (anhydrous, >99.9%) were obtained from Sigma–Aldrich, USA. High quality, green-emitting CsPbBr3 QDs (10 mg/mL) with a purity >99.0% in toluene, synthesized by a hot injection method, were obtained from Quantum Solutions LLC, King Abdullah University of Science and Technology (KAUST), Kingdom of Saudi Arabia. All the materials were used as soon as they were received. 2.2 Method: At first, the glass substrates (1 cm × 2 cm) were cleaned using detergent placed in the ultra-sonication bath, followed by de-ionized water, acetone, and isopropanol, for approximately 15 min each. The toluene solutions containing CsPbBr3 QDs (10 mg/ml) and PFO (30 mg/ml) were then prepared as stock solutions. Before spin coating the glass substrates, these two solutions were premixed, using a solution blending method, to form transparent precursor solutions with various weight ratios of PFO/CsPbBr3 (1, 5, 10, 20, 30, 40, and 50 wt%). Subsequently, 50 µL of CsPbBr3 QDs, pure PFO, and PFO/CsPbBr3 QDs blends were dropped onto the clean glass substrates, which were placed in the spin-coating machine (spinning at 3000 rpm for 30 s), to prepare homogenous thin films. A Veeco Dektak 6M Stylus profilometer was employed to measure the film thickness, which was 200 nm for CsPbBr3 QDs. The

thickness reduced in the presence of PFO, in the PFO/CsPbBr3 QDs hybrid thin films, from 150 nm to 60 nm for 50 wt.% and 10 wt.%, respectively. 2.3 Characterizations: The structural properties of the CsPbBr3 QDs were investigated using a JEOL 1400 high-resolution transmission electron microscope (HRTEM) to confirm the morphology and structure of the perovskite QDs. The sample structure was characterized using a Panalytical X’PERT Pro powder X-ray diffractometer (XRD) in the θ–2θ mode. CuKα radiation, at 45 kV and 40 mA, was used in the XRD measurements, and the scanning angle (2θ) was changed between 10° and 80°, with a step size of 0.02° at 0.5° min−1. Vibration spectra were recorded by a Fourier transform infrared (FTIR) spectrometer (RX FTIR System, Perkin Elmer) in the range of 2300 to 3500 cm−1, at a resolution of 1.4 cm−1. The micro-Raman scattering measurements were recorded using a Raman system (JY-Horiba-T64000) in conjunction with a He–Cd Kimmon continuous wave laser, operating at a wavelength of 325 nm, as the excitation source. In addition, the surface atomic structure of the films was characterized using atomic force microscopy (AFM) (Veeco multimode V Scan Probe Microscope). The absorption and photoluminescence spectra were measured by UV–vis spectrophotometers, a JASCO V-670 absorption spectrometer, and a JASCO FP8200 spectrofluorometer, respectively. All characterizations were executed in the ambient atmosphere.

3. Results and discussion 3.1. Structural characteristics Fig. 1 shows the XRD patterns of the CsPbBr3 QDs, pure PFO, and PFO/CsPbBr3 QDs hybrids, respectively. The peaks at (101), (121), and (202) correspond to the CsPbBr3 QDs and indicate that the film was in the polycrystalline phase. The 2θ values of these peaks are in good agreement with the previous reports [29,30]. Broadened X-ray patterns of the PFO confirmed the poor crystallinity of the film, or its amorphous state, as proved in the previous report [31]. The XRD diffractograms of the hybrid thin films displayed the formation of a broad PFO peak, along with numerous narrow peaks representing CsPbBr3 QDs. The (101) and (202) peaks of the PFO/CsPbBr3 film are sharp and highly distinct. This indicates that the crystallization of the PFO/CsPbBr3 thin films was enhanced with the increase in CsPbBr3 content. The incorporation of the CsPbBr3 QDs with various weight ratios had a significant influence on the thin film structure. Crystallite size, dislocation density, and lattice strain were estimated from the XRD at the (202) peak and tabulated in Table 1. The crystallite size (D), dislocation density (δ), and lattice strain (ε) are given by D = 0.9 λ/(βcosθ), δ = 1/D2, and ε = βcosθ/4, respectively, where λ is the X-ray source wavelength, θ is the Bragg angle, and β is the full width at half maximum of the peak [32]. The increase in the crystallite size may be due to the agglomeration of the QDs on the PFO segments. Imperfections, including vacancy and interstitials, may be significant in changing the values of dislocation density and lattice strain, upon the increase of CsPbBr3 QDs in the thin films. These imperfections thus cause a slight shift in the peak positions of the XRD patterns. The large shift of the broadened X-ray

patterns towards the large angle, upon incorporation of CsPbBr3 QDs, can be attributed to the difference in the size of the atoms of CsPbBr3 QDs and PFO, and the change in lattice parameter. A typical HRTEM image of CsPbBr3 QDs is shown in Fig. 2. The CsPbBr3 QDs had an average diameter of 7.3 nm, with a size deviation of ± 1.4 nm. FTIR spectra of CsPbBr3 QDs, pristine PFO, and PFO/CsPbBr3 QDs hybrid thin films are shown in Fig. 3. The existence peaks at 2850 cm-1 and 2920 cm-1 confirm the purity of the CsPbBr3 QDs, as proved in the previous reports [20,33]. The main FTIR spectra of pristine PFO and PFO/CsPbBr3 QDs hybrid thin films are shown at 2850 cm-1, 2920 cm-1, and 2954 cm-1. The peaks at 2850 cm-1 and 2920 cm-1, and at 2954 cm-1 can be attributed to C–H stretching of the alkyl chain and aromatic ring, respectively, which is consistent with the spectra previously published [34,35]. The absence of noticeable variation in FTIR spectra indicates that the chemical interaction between the CsPbBr3 QDs and PFO is insignificant. Fig. 4 shows the Raman spectra of pure PFO and PFO/CsPbBr3 QDs hybrid thin films that were spin-coated onto the glass substrates. A strong Raman active mode was observed in fluorene, at 1380 cm-1. After adding various quantities of CsPbBr3 QDs to the hybrid thin films, only the spectrum of PFO could be observed between 250 and 2800 cm−1. This confirmed that only a trace amount of PFO remained in the PFO/CsPbBr3 thin films and the CsPbBr3 QDs thin film exhibited weak absorption. This was observed in the previous report of the system of PFO/CsPbBr3 QDs thin films [4].

The AFM images of the pure CsPbBr3 and PFO thin films prepared with 0 wt%, 10 wt%, 20 wt%, and 50 wt% CsPbBr3 content are shown in Fig. 5, and their root-meansquare (RMS) roughness was measured and is tabulated in Table 1. As evident in these AFM images, the surface of the PFO/CsPbBr3 thin films was improved, compared to that of the pristine CsPbBr3 thin film. The RMS roughness of the CsPbBr3 thin film was 27.5 nm and that of the PFO/CsPbBr3 thin films was drastically decreased to 7.86 nm, at 10 wt.% of CsPbBr3. It is expected that the smoother surface will decrease the loss of incident pumping light at the interface of air and thin film, which may be beneficial when it is used as an active layer in optoelectronic devices, or in the creation of amplified spontaneous emission in PFO/CsPbBr3 thin films. Adverse effects, such as a large number of pin-holes and poor surface coverage in the thin films, could result from large-size crystallite agglomeration of CsPbBr3 QDs at high quantities, which is consistent with the XRD results of crystallite size. 3.2. Optical analysis Typical optical normalized absorbance spectra for as-deposited CsPbBr3 QDs, pristine PFO, and PFO/CsPbBr3 QDs hybrid thin films are shown in Fig. 6. The absorbance spectrum of the CsPbBr3 QDs was broad, with a maximum peak at 267 nm and a shoulder at 495 nm. The maximum absorbance peak of pristine PFO was observed at 385 nm, with a shoulder at 255 nm. A unique observation was made upon the increment of the CsPbBr3 QDs content in the hybrid thin films; the peak of PFO at 255 nm was significantly enhanced and the maximum peak gradually red shifted from 385 nm to 395 nm, indicating minor expansions in the conjugation length of the PFO [36].

Moreover, the shoulder peak (495 nm) of CsPbBr3 QDs started to appear when the quantity of the CsPbBr3 QDs exceeded 1.0 wt.%. Investigation of the absorption coefficient (α), using the Tauc relation, was proven to be a reliable approach to anticipate any amendment in the electronic band structure of the polymer materials and to determine the electron transition type. It is possible to determine both the direct and indirect transitions in the band gap of the material by analyzing the optical absorption spectra and using the following formula [37]: αhν = C(hν - Eg)n; α = 2.303 (A/d), where A and d are the absorbance and film thickness, respectively; h is Planck's constant; ν is the incident radiation frequency; C is a constant that is dependent on the transition probability; Eg is the energy band gap; and n = 1/2 and 2 for direct and indirect band gaps, respectively. The analyzing data of the absorption spectra indicated that CsPbBr3 QDs had a direct band gap and pure PFO had both direct and indirect band gaps, as tabulated in Table 2. The existence of both direct and indirect band gap types was confirmed by the linearity in the plot of (αhν)2 and (αhν)1/2 versus photon energy (hν), respectively, which is evident from Fig. 7. As tabulated in Table 2, both direct and indirect band gaps of the PFO significantly reduce with increasing CsPbBr3 QDs content. The direct band gap was reduced from 2.970 to 2.777 eV and the indirect band gap, from 2.846 to 2.673 eV. The existence of both direct and indirect band gaps in PFO/CsPbBr3 QDs hybrid thin films can extend the range of absorption energy to enhance the photoelectric energy conversion [38].

In addition, valuable information regarding the optical band gap can be provided by the study of the fundamental absorption edge. This is determined when the photon excites the electron from the lower to the higher energy states [39]. In thin film semiconductors, defect states are essential in determining the electrical properties of the film. Therefore, it is important to characterize these defect states to assess the suitability of the materials that form thin films to the structure of the devices. The defects-induced local states can be characterized by investigating the spectral dependence of α at a lower photon energy region, so called Urbach tail, which is less than the band gap energy. The Urbach energy can be determined from the inverse of the slope of the plot ln(α) versus photon energy (hν) [40]: α= α0 exp(hν/Eu), where Eu is the Urbach energy and α0 is a temperature-dependent constant. The Urbach energy was interpreted as the width of the exponential absorption edge or the width of the tail of localized states. Fig. 8 shows ln(α) as a function of hν for pristine PFO, CsPbBr3 QDs, and PFO/CsPbBr3 QDs hybrid thin films. It is evident that the absorption edge of the thin films significantly shifted toward the lower photon energy. For further analysis of the absorption spectra (Fig. 6), a minimum value of absorbance was employed to determine the cut-off wavelength (λcut-off), such that Eu can be expressed by the following equation [41]: Eu = 1240/λcut-off .

This analysis revealed that the Eu values of CsPbBr3 QDs and PFO were 2.338 eV and 2.928 eV, respectively. As tabulated in Table 2, the Eu value gradually decreased with the increase of CsPbBr3 QDs content. This decrease may be attributed to the variation in defect density strain and crystallite size, as described in the XRD analysis. A strong correlation between Eu and Eg was observed from the shift trend upon the incorporation of CsPbBr3 QDs. This correlation indicated that the transitions responsible for the Urbach edge and the transitions across the band gap are both electronic transitions. Thus, these electronic transitions were not featured, except for the variations in the relevant states densities. This correlation between Eu and Eg has been recently presented in several other studies [32,42]. The steepness parameter (σ) can be calculated using the following formula [40]: σ = kβT/Eu , where T is the absolute temperature and kβ is the Boltzmann constant. This formula describes the shrinkage/broadening of the optical absorption edge, due to the exciton–phonon or electron–phonon interactions. As shown in Table 2, the decrease in Eu or increase in σ, upon the increment of CsPbBr3 QDs, can be attributed to the decrease in the localized density of electronic states within the forbidden band gap of the hybrids [32,43]. Normalized fluorescence spectra of CsPbBr3 QDs, pristine PFO, and PFO/CsPbBr3 QDs hybrid thin films are shown in Fig. 9. One peak was observed for CsPbBr3 QDs at 507 nm, whereas three peaks and a shoulder were observed for pristine PFO at 419, 438, 465 and 497 nm, respectively. The three peaks and shoulder were attributed to 0-0,

0-1, 0-2, and 0-3 vibronic transitions, respectively. The intensity of the 0-0 transition was inhibited for quantities of QDs more than 5 wt.%, whereas the shoulder intensity increased with the increase of the QDs and significantly red shifted to 505 nm. This observation was attributed to the increase in the conjugation length of PFO [44]. Moreover, this observation confirmed that the non-radiative energy transfer occurred from PFO to CsPbBr3 QDs. This was also confirmed by the significant overlap between the absorption spectrum of CsPbBr3 QDs, and the fluorescence spectrum of PFO, as shown in Fig. 10. The energy levels of CsPbBr3 existed between that of PFO [45, 46], which provided more evidence for the possibility of a Förster energy transfer mechanism in the system of the PFO/CsPbBr3 hybrid. In this mechanism, the PFO (donor) irradiates with a light energy of hν and its oscillating dipole then produces, and resonates with, the oscillating dipole of CsPbBr3 (acceptor). Thus, the energy of the excited state can be transferred from the PFO to CsPbBr3 through space, without the exchange of electrons (dipole-dipole interaction). When the PFO returns to the ground state, a non-radiative energy transfer occurs and the CsPbBr3 is brought to the excited state. A scheme of the Förster mechanism from the PFO to CsPbBr3 is described in Fig. 11. The efficiency of this mechanism can be enhanced by controlling the acceptor quantity in the blend, as proven in previous studies [47-49]. Consequently, the optical properties of the composite thin film can be adjusted by varying the CsPbBr3 content in the hybrids, as shown in this work. In future, the use of such hybrids, which have a smooth surface and significant Förster resonance energy transfer, may be significant in the improvement of the performance of optoelectronic devices. Conclusion:

In this work, the influence of the incorporation of CsPbBr3 QDs on the structural and optical properties of the PFO/CsPbBr3 QDs thin films was investigated using several techniques. Insignificant chemical interaction was observed between the CsPbBr3 QDs and PFO, as there were no noticeable variations in XRD diffractograms, FTIR, and Raman spectra. Subsequently, it was observed that the surface of the PFO/CsPbBr3 thin films was improved in comparison to that of the pristine CsPbBr3 thin film. In addition, both direct and indirect band gaps of the PFO significantly reduced from 2.970 to 2.777 eV and from 2.846 to 2.809 eV, respectively, with an increase in CsPbBr3 QDs content. Further, the decrease in Eu and increase in σ, upon the increase in CsPbBr3 QDs, can be attributed to the decrease in the localized density of electronic states within the forbidden band gap of the blends. Moreover, the blending of PFO with CsPbBr3 QDs, not only improved the thin film morphology and tuned the optical properties of thin film, but also facilitated the non-radiative energy transfer (Förster type) from PFO to CsPbBr3 QDs. Förster energy transfer is an important factor, responsible for enhanced electron–hole recombination and, subsequently, the improvement in the performance of optoelectronic devices. Therefore, the detailed study of the non-radiative energy transfer mechanism for this hybrid, and the employment of its optimized ratio as an emissive layer in optoelectronic devices, such as PeLEDs, will be a unique study in the future.

Acknowledgements: The authors thank the Deanship of Scientific Research at King Saud University for funding this work through Research Group No. RG-1440-37.

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resonance

energy

transfer

(FRET)

in

blue-emitting

poly

vinylpyrrolidone (PVP)-passivated zinc oxide (ZnO) nanoparticles. J. colloid interface sci., 488 (2017)348-355.

1400

(101)

Intensity (a. u.)

1200

PFO CsPbBr 3 QDs

(121) (202)

10%

1000

20% 50%

800 600 400 200 0

10

20

30

40

50

60

70

2  Fig. 1 XRD diffractograms of pristine PFO, CsPbBr3 QDs, and PFO/ CsPbBr3 QDs hybrid thin films

80

25

Particle size=7.3 nm

Frequency

20 15 10 5 0

2

4

6

8

10

Particle size, nm

Fig. 2 TEM image of CsPbBr3 QDs with particle size distribution.

12

14

2400

2600

2800

3000

3200

3400

3000

3200

3400

150 140

Transmission (a.u.)

130

50%

120

20%

110

10% 5%

100 90 80

1%

70

PFO

60

CsPbBr 3 QDs

50 40 30

2400

2600

2800

Wavenumber (cm -1) Fig. 3 FTIR spectra of pristine PFO, CsPbBr3 QDs, and PFO/ CsPbBr3 QDs hybrid thin films.

Raman Intensity (a. u.)

50 wt.% 20 wt.% 10 wt.% 5 wt. % PFO 500

1000

1500

2000

2500

Wavenumber (cm -1) Fig. 4 Raman spectra of pristine PFO and PFO/ CsPbBr3 QDs hybrid thin films.

(a) CsPbBr3 QDs

(b)PFO

(c)10wt.% CsPbBr3 QDs

(d) 20wt.% CsPbBr3 QDs

(e) 50wt.% CsPbBr3 QDs

Fig. 5

2D and 3D AFM images of pristine PFO, CsPbBr3 QDs, and PFO/ CsPbBr3 QDs hybrid thin films.

0.7

0.20 CsPbBr3 QDs 10 mg/ml

Absorbance

0.15

0.6

0.10

0.05

0.5

0.00

Absorbanc

300

400

500

600

700

 nm)

0.4

PFO 10 mg/ml 1.0 wt.% QDs 5.0 wt.% 10 wt.% 20 wt.% 30 wt.% 40 wt.% 50 wt.%

0.3 0.2 0.1 0.0 300

400

500

600

700

 (nm) Fig. 6 Absorbance spectra of pristine PFO and PFO/CsPbBr3 QDs hybrid thin films. The inset is for pristine CsPbBr3 QDs.

(a)

40

3000

CsPbBr 3 QD 10 mg/ml

35

(h)2 (cm-1.eV)2

30

(h )2 (cm -1.eV)2

2500 2000

25 20 15 10 5 0 1.5

1500 1000 500 0 2.6

Pure PFO 10 mg/ml 1.0 wt.% QDs 5.0 wt.% 10 wt.% 20 wt.% 30 wt.% 40 wt.% 50 wt.% 2.7

2.8

3.0

2.5

2.0

h  (eV)

2.9

h  (eV)

3.0

3.1

3.2

8

6 5 4

h )

1/2

-1

(cm . eV)

1/2

7

(b)

Pure PFO 1.0 wt. % QDs 5.0 wt. % 10 wt. % 20 wt. % 30 wt. % 40 wt. % 50 wt. %

3 2 1 2.4

2.6

2.8

3.0

3.2

h  (eV) Fig. 7 a) Direct band gap and b) Indirect band gap for pristine PFO and PFO/CsPbBr3 QDs hybrid thin films. The inset is for direct band gap of pristine CsPbBr3 QDs.

1.4

3

CsPbBr 3 QDs

1.2

ln

1.0 0.8 0.6

2

0.4 0.2 0.0

ln 

2.2

2.4

2.6

2.8

3.0

3.2

3.4

A

Pure PFO 1.0 wt. % QDs 5.0 wt. % 10 wt. % 20 wt. % 30 wt. % 40 wt. % 50 wt. %

1

0

2.6

2.8

3.0

3.2

h  (eV) Fig. 8 Urbach energy for pristine PFO, CsPbBr3 QDs and PFO/CsPbBr3 QDs hybrid thin films.

1.0

Normlized Int. (a. u.)

1.0

0.8

0.6

Normalized int. (a. u.)

Normalized Int. (a. u.)

0.20

0.15

CsPbBr3 QDs 10 mg/ml

0.8

0.6

0.4

0.2

0.10 0.0 400

450

550

600

PFO 10 mg/ml 1.0 wt.% QDs 5.0 wt.% 10 wt.% 20 wt.% 30 wt.% 40 wt.% 50 wt.%

0.05 490

500

510

520

530

 (nm)

0.4

0.2

0.0 400

500

 (nm)

ex at 355 nm

450

500

550

600

 (nm) Fig. 9 PL spectra of pristine PFO, CsPbBr3 QDs, and PFO/ CsPbBr3 QDs hybrid thin films.

Normalized Int. (a. u.)

1.0

Normalized Absorbance of QDs Normalized fluorescence of PFO

0.8

0.6

0.4

0.2

0.0 300

400

500

600

700

800

 (nm) Fig. 10 PL and Absorption Normalized spectra of pristine PFO and CsPbBr3 QDs thin films.

S1 hν

S0 PFO

hν'

CsPbBr3 CsPbBr3

(PFO)*

CsPbBr3

PFO PFO

(CsPbBr3)** (CsPbBr3)

PFO PFO

Fig. 11 Scheme of Förster energy transfer between PFO (donor) and CsPbBr3 (acceptor).

CsPbBr CsPbBr33

Table 1: Structural properties of pristine PFO, CsPbBr3 QDs, and PFO/ CsPbBr3 QDs

hybrid thin films Sample

FWHM (deg.)

D (nm)

lattice strain ε × 10-3

Dislocation density δ × 10-3 (nm)-2

RMS Roughness (nm)

Pure QDs

0.716

11.501

1.246

7.559

27.5

Pure PFO

-

-

-

-

6.75

0.360

22.890

6.266

1.909

7.86

20 wt.%

0.2952

27.911

5.138

1.284

19.7

50 wt.%

0.2952

27.906

5.138

1.284

24.3

10 wt.%

Table 2: Optical properties of pristine PFO, CsPbBr3 QDs, and PFO/ CsPbBr3 QDs

hybrid thin films Sample

Egd (eV)

Egi (eV)

Wavelength (cut-off)

Eu (eV)

σ=kβT/Eu × 10-3

Pure QD

2.282

-

530.3

2.338

11.12

Pure PFO

2.970

2.846

423.4

2.928

8.877

1 wt.%

2.958

2.809

426.2

2.909

8.934

5 wt.%

2.805

2.725

427.5

2.900

8.936

10 wt.%

2.790

2.696

429.7

2.886

8.963

20 wt.%

2.791

2.695

429.2

2.889

8.999

30 wt.%

2.785

2.688

429.8

2.885

9.012

40 wt.%

2.793

2.709

429.2

2.889

8.999

50 wt.%

2.777

2.673

432.9

2.864

9.077

Graphical abstract

1.0

Normlized Int. (a. u.)

1.0

0.8

0.6

Normalized int. (a. u.)

Normalized Int. (a. u.)

0.20

0.15

CsPbBr3 QDs 10 mg/ml

0.8

0.6

0.4

0.2

0.10 0.0 400

450

550

600

PFO 10 mg/ml 1.0 wt.% QDs 5.0 wt.% 10 wt.% 20 wt.% 30 wt.% 40 wt.% 50 wt.%

0.05 490

500

510

520

530

 (nm)

0.4

0.2

0.0 400

500

 (nm)

ex at 355 nm

450

500

 (nm)

550

600

Highlights -

High quality of the PFO/CsPbBr3 QDs thin films were synthesized

-

Insignificant chemical interaction was observed between the CsPbBr3 QDs and PFO

-

Direct and indirect band gap of the PFO reduced with increasing CsPbBr3 QDs content

-

Non-radiative energy transfer from PFO to CsPbBr3 QDs was detected

Author Contributions: B.A. Al-Asbahi: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing, Visualization. S.M. Qaid: Conceptualization, Methodology, Validation, Investigation. H. M. Ghaithan: Investigation. M. S. AlSalhi: Supervision. A.S. Aldwayyan: Resources, Supervision.