Multi-Functional Ethylene-vinyl Acetate Copolymer Flexible Composite Film Embedded with Indium Acetate-Passivated Perovskite Quantum Dots

In recent years, all-inorganic cesium lead halide perovskite quantum dots have emerged as promising candidates for various optoelectronic applications, including sensors, light-emitting diodes, and solar cells, owing to their exceptional photoelectric properties. However, their commercial utilization has been limited by stability issues. In this study, we addressed this challenge by passivating the surface defects of CsPbBr3 quantum dots using indium acetate, a metal–organic compound. The resulting CsPbBr3 quantum dots exhibited not only high photoluminescence intensity, but also a remarkably narrow half-peak width of 19 nm. Furthermore, by embedding the CsPbBr3 quantum dots in ethylene-vinyl acetate, we achieved stretchability and significantly enhanced stability while preserving the original luminous intensity. The resulting composite film demonstrated the potential to improve the power conversion efficiency of crystalline silicon solar cells and enabled the creation of excellent white light-emitting diodes with coordinates of (0.33, 0.31). This co-passivation strategy, involving surface passivation and polymer packaging, provides a new idea for the practical application of CsPbBr3 quantum dots.


Preparation of Precursor and EVA Solution
The CsPbBr 3 precursor comprised 0.0426 g CsBr, 0.0734 g PbBr 2 , 0.023 g C 23 H 49 NO 3 S, 4 mL DMF, 1 mL DMSO, 0.5 mL OA, and 0.25 mL OLA.These were added to a 10 mL glass vial and stirred at 70 • C until a clear and homogeneous solution was obtained.
For the EVA solution, 1.2 g EVA and 4 mL toluene were added to a 10 mL glass vial and stirred to obtain a clear homogeneous solution at 70 • C.

Synthesis of CsPbBr 3 -In/EVA Film
We collected 20 µL of the precursor and added it to a mixture of 2 mL toluene and 20 µL methanol.The solution was stirred at room temperature, resulting in the formation of a CsPbBr 3 -In QD solution.Subsequently, we mixed the CsPbBr 3 -In QD solution with the EVA solution and stirred it at room temperature, leading to the formation of the CsPbBr 3 -In/EVA solution.At this point, the mass ratio of QDs to EVA was 0.034%.Finally, the solution was uniformly deposited onto a PET substrate and allowed to dry naturally at room temperature, resulting in the formation of a CsPbBr 3 -In/EVA film as illustrated in Figure 1.

Preparation of Precursor and EVA Solution
The CsPbBr3 precursor comprised 0.0426 g CsBr, 0.0734 g PbBr2, 0.023 g C23H49NO3S, 4 mL DMF, 1 mL DMSO, 0.5 mL OA, and 0.25 mL OLA.These were added to a 10 mL glass vial and stirred at 70 ℃ until a clear and homogeneous solution was obtained.
For the EVA solution, 1.2 g EVA and 4 mL toluene were added to a 10 mL glass vial and stirred to obtain a clear homogeneous solution at 70 °C.

Synthesis of CsPbBr3-In/EVA Film
We collected 20 µL of the precursor and added it to a mixture of 2 mL toluene and 20 µL methanol.The solution was stirred at room temperature, resulting in the formation of a CsPbBr3-In QD solution.Subsequently, we mixed the CsPbBr3-In QD solution with the EVA solution and stirred it at room temperature, leading to the formation of the CsPbBr3-In/EVA solution.At this point, the mass ratio of QDs to EVA was 0.034%.Finally, the solution was uniformly deposited onto a PET substrate and allowed to dry naturally at room temperature, resulting in the formation of a CsPbBr3-In/EVA film as illustrated in Figure 1.

Material Characterization
An X-ray diffractometer (XRD, D8 Advance, Bruker), field-emission scanning electron microscopy (FESEM, Hitachi New Generation SU8220, Tokyo, Japan) equipped with X-ray energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi, Thermo Scientific, Waltham, MA, USA) with an Al Kɑ source and transmission electron microscopy (TEM, Tecnai G2 F20, FEI, Hillsborough, OR, USA) were used to study the structure, elemental composition, surface morphology, and microscopic morphology of the compound.The optical properties of the QD solution were characterized using a UV-Vis spectrophotometer (UV-Vis, Cary 300, Varian, Hong Kong, China).PL spectra (PL, FS5, Edinburgh, Scotland) were used to investigate the fluorescence characteristics of the perovskite quantum dot solutions and composite films.The carrier lifetime

Material Characterization
An X-ray diffractometer (XRD, D8 Advance, Bruker), field-emission scanning electron microscopy (FESEM, Hitachi New Generation SU8220, Tokyo, Japan) equipped with X-ray energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS, EscaLab 250Xi, Thermo Scientific, Waltham, MA, USA) with an Al Kα source and transmission electron microscopy (TEM, Tecnai G2 F20, FEI, Hillsborough, OR, USA) were used to study the structure, elemental composition, surface morphology, and microscopic morphology of the compound.The optical properties of the QD solution were characterized using a UV-Vis spectrophotometer (UV-Vis, Cary 300, Varian, Hong Kong, China).PL spectra (PL, FS5, Edinburgh, Scotland) were used to investigate the fluorescence characteristics of the perovskite quantum dot solutions and composite films.The carrier lifetime of the perovskite quantum dots was recorded by transient time-resolved photoluminescence (TRPL, FLS 980, Edinburgh, Scotland).The J-V curve of the electrochemical workstation (Keithley 2420 Source Meter, Hong Kong, China) was used to measure the photovoltaic performance of the silicon solar cell under a standard AM 1.5 simulated sun (100 mW•cm −2 , Oriel Sol 3A, Newport, Irvine, CA, USA).

Results and Discussion
The structure of CsPbBr 3 -In is depicted in Figure 2a.Initially, an XRD analysis was performed to characterize the synthesized sample, as depicted in Figure 2b.A comparison with the PDF standard card (54-0752) [41] revealed that CsPbBr 3 and CsPbBr 3 -In shared the same crystal structure (space group Pm3m, lattice constant a = b = c = 5.83 Å, α = γ = β = 90 • ).Notably, two prominent diffraction peaks at 15.18 • and 30.64 • corresponded with the (100) and (200) crystal faces of the cubic perovskite structure.The results showed that the QDs had a cubic perovskite structure and a preferred orientation of (100) [42].Importantly, the addition of In(C 2 H 3 O 2 ) 3 did not lead to a shift in the diffraction peaks or alter the crystal structure of CsPbBr 3 , indicating that In 3+ was not incorporated into the crystal structure of CsPbBr 3 -In.The SEM and EDS images are presented in Figure 2c,d, respectively.The observed sample surface appeared smooth and flat, with an even distribution of Cs, Pb, Br, and In elements.Further evidence of the uniform distribution of these elements (such as In) was obtained through the element mapping of Cs, Pb, Br, and In in single CsPbBr 3 -In QDs, as shown in Figure 2g-j, illustrating the effective coverage of indium acetate ligands on the perovskite QDs.Subsequently, TEM and HRTEM were employed for the further characterization of the obtained samples.As shown in Figure 1e, the TEM image revealed a significant number of uniformly dispersed CsPbBr 3 -In rectangular squares.A size distribution analysis of the CsPbBr 3 -In QDs indicated an average size of 13.45 nm, as shown in the illustration in Figure 2e.In Figure 2f, the measured distances between the crystal faces of CsPbBr 3 -In were found to be 0.41 nm and 0.29 nm, corresponding with the (110) and (200) crystal faces, respectively [43,44].This further validated the XRD results, supporting the conclusion that the addition of In(C 2 H 3 O 2 ) 3 did not affect the crystal structure.QD solutions with varying concentrations of In(C2H3O2)3 are shown in Figure 4 upper image was taken in an indoor environment, while the lower image was cap under UV light.Figure 4b displays the PL spectra corresponding with the differen tents of In(C2H3O2)3 shown in Figure 4a.It was obvious that the addition of In(C2H had an impact on both the luminous intensity and peak position of CsPbBr3.Specifi when 0.02 mmol In(C2H3O2)3 was added, the luminous intensity increased by a fac 1.56.Therefore, in the follow-up test, we chose a precursor solution with a content o mmol In(C2H3O2)3.However, excessive amounts of In(C2H3O2)3 led to a decrease luminescence intensity of CsPbBr3, accompanied by a blue shift in the peak position observation aligned with the UV-Vis absorption spectrum displayed in Figure 4c.TRPL measurements were conducted on samples with varying concentratio In(C2H3O2)3, as depicted in Figure 4d.The photoluminescence decay curves for both bBr3 and CsPbBr3-In followed a double exponential pattern.The decay time, decay ponent ratio, and average decay lifetime are shown in Table 1.The longer-lived co nent (τ2) of CsPbBr3 could be attributed to typical radiative recombination, where  In order to further passivate the defects and improve the stability of the QD encapsulated the CsPbBr3 and CsPbBr3-In QDs in EVA and investigated the stability o CsPbBr3/EVA and CsPbBr3-In/EVA films under extreme conditions, including exposu water and high temperatures.The results of the stability tests at 90 °C are present Figure 5a-c.It was evident that the addition of In(C2H3O2)3 significantly enhance thermal stability.Subsequently, we immersed the films in water and conducted tes shown in Figure 5d-g.It was observed that both the CsPbBr3/EVA and CsPbBr3-In films experienced substantial degradation on the first day.However, after 7 days, the bBr3-In/EVA films maintained a good luminous performance, while the CsPbBr3 films exhibited more significant deterioration.This improvement in stability may b  In order to further passivate the defects and improve the stability of the QDs, we encapsulated the CsPbBr 3 and CsPbBr 3 -In QDs in EVA and investigated the stability of the CsPbBr 3 /EVA and CsPbBr 3 -In/EVA films under extreme conditions, including exposure to water and high temperatures.The results of the stability tests at 90 • C are presented in Figure 5a-c.It was evident that the addition of In(C 2 H 3 O 2 ) 3 significantly enhanced the thermal stability.Subsequently, we immersed the films in water and conducted tests, as shown in Figure 5d-g.It was observed that both the CsPbBr 3 /EVA and CsPbBr 3 -In/EVA films experienced substantial degradation on the first day.However, after 7 days, the CsPbBr 3 -In/EVA films maintained a good luminous performance, while the CsPbBr 3 /EVA films exhibited more significant deterioration.This improvement in stability may be attributed to the passivation of QD defects and enhanced stability resulting from the presence of In(C 2 H 3 O 2 ) 3 .Furthermore, we performed a contact angle test (Figure 5h,i), which revealed that the contact angle increased by 7.5 • upon the addition of indium.Consequently, the perovskite film exhibited enhanced hydrophobicity.Therefore, the inclusion of In(C 2 H 3 O 2 ) 3 has the potential to improve the stability of perovskite films in water and high-temperature environments.This enhancement in stability further expands the application range of quantum dots (QDs) in various fields.With the development of flexible solar cells and displays, the flexibility of QD fi has attracted more and more attention.Therefore, the flexibility of the film is then furt tested [48].As shown in Figure 6a, the flexibility of CsPbBr3-In/EVA films was dem strated when bent at different angles.The film exhibited excellent flexibility, allowin to be easily shaped into various forms.To assess its performance under stretching, the strength measurement was obtained before and after stretching (see Figure 6b,c), and With the development of flexible solar cells and displays, the flexibility of QD films has attracted more and more attention.Therefore, the flexibility of the film is then further tested [48].As shown in Figure 6a, the flexibility of CsPbBr 3 -In/EVA films was demonstrated when bent at different angles.The film exhibited excellent flexibility, allowing it to be easily shaped into various forms.To assess its performance under stretching, the PL strength measurement was obtained before and after stretching (see Figure 6b,c), and the following stretching equation was adopted to calibrate its performance: where L 0 and L h represent the initial and final lengths of the film, respectively.During the tensile test, the CsPbBr 3 -In/EVA film was initially 2 cm in length and did not break, even when subjected to a stretching rate ranging from 0% to 200%.This exceptional performance was primarily attributed to the stretchability of the EVA polymer.By assessing the PL at different stretching rates, there existed only a slight decrease in the luminescence intensity of the CsPbBr 3 -In/EVA film before and after stretching, which indicated that the use of EVA packaging enabled the QDs to exhibit flexible stretchability, thus expanding the potential applications of QD films in various fields.In addition, we also investigated the effects of different amounts of EVA polymers on the bending and stretching properties, as shown in Figures S1 and S2.When the mass ratio of CsPbBr 3 -In and EVA was 0.023%, although the film had good tensile and bending properties, the flatness of the film was poor and the quantum dots aggregated.When the mass ratio of CsPbBr 3 -In to EVA was 0.045%, the surface of the sample was relatively flat and there was no obvious aggregation phenomenon of quantum dots, but the bending and tensile properties were poor.We theorized that the reason for this phenomenon was when there was less EVA, there was not a good link between the polymer chains, resulting in poor film formation.When there was more EVA, the force between the molecular chains increased and the stretching effect became worse.
Polymers 2023, 15, x FOR PEER REVIEW intensity of the CsPbBr3-In/EVA film before and after stretching, which indicated t use of EVA packaging enabled the QDs to exhibit flexible stretchability, thus expa the potential applications of QD films in various fields.In addition, we also invest the effects of different amounts of EVA polymers on the bending and stretching p ties, as shown in Figures S1 and S2.When the mass ratio of CsPbBr3-In and EV 0.023%, although the film had good tensile and bending properties, the flatness of t was poor and the quantum dots aggregated.When the mass ratio of CsPbBr3-In t was 0.045%, the surface of the sample was relatively flat and there was no obvious gation phenomenon of quantum dots, but the bending and tensile properties were We theorized that the reason for this phenomenon was when there was less EVA was not a good link between the polymer chains, resulting in poor film formation.there was more EVA, the force between the molecular chains increased and the stre effect became worse.In order to verify the photovoltaic performance after the film coverage, we tes UV-visible absorption spectra of the EVA, CsPbBr3/EVA, and CsPbBr3-In/EVA film ure S3a).After adding indium acetate, the absorption performance improved to a extent.Subsequently, the EVA, CsPbBr3/EVA, and CsPbBr3-In/EVA films were dep onto the surface of a silicon solar cell, as depicted in Figure 7a-c.Upon exposure light, both the CsPbBr3/EVA and CsPbBr3-In/EVA films exhibited intense green lu cence.A rigorous comparison of the silicon solar cell's performance was performed and after applying the respective polymer films.The corresponding J-V curve of t In order to verify the photovoltaic performance after the film coverage, we tested the UV-visible absorption spectra of the EVA, CsPbBr 3 /EVA, and CsPbBr 3 -In/EVA films (Figure S3a).After adding indium acetate, the absorption performance improved to a certain extent.Subsequently, the EVA, CsPbBr 3 /EVA, and CsPbBr 3 -In/EVA films were deposited onto the surface of a silicon solar cell, as depicted in Figure 7a-c.Upon exposure to UV light, both the CsPbBr 3 /EVA and CsPbBr 3 -In/EVA films exhibited intense green luminescence.A rigorous comparison of the silicon solar cell's performance was performed before and after applying the respective polymer films.The corresponding J-V curve of the silicon solar cell is shown in Figure 7a-c, and Table 2 provides the photovoltaic parameters.The results showed that the power conversion efficiency (PCE) improvements were 0.10%, 0.14%, and 0.31% for the devices incorporating EVA, CsPbBr 3 /EVA, and CsPbBr 3 -In/EVA, respectively.In addition, in order to further verify the impact of CsPbBr 3 -In/EVA on the efficiency of the battery, we also tested the J-V curve under ultraviolet light, as shown in Figure S3b-d and Table S1.Due to the weak ultraviolet light, the battery efficiency was low.Therefore, we used the following equation to evaluate the relative improvement in battery efficiency, E R : where E 0 and E 1 represent the initial cell efficiency and the packaged battery efficiency, respectively.Through the calculation, we found that after EVA, CsPbBr 3 /EVA, and CsPbBr 3 -In/EVA packaging, the battery efficiency relatively increased by 10%, 22%, and 37%, respectively.The underlying mechanisms responsible for the enhanced device performance of EVA, CsPbBr 3 /EVA, and CsPbBr 3 -In/EVA were distinct; the improvement in EVA mainly stemmed from its ability to mitigate light reflection on the silicon solar cell's surface, leading to an increase in the short-circuit current (J SC ) [49].In the case of the CsPbBr 3 /EVA and CsPbBr 3 -In/EVA composite films, the J SC enhancement could be attributed to two factors.S1.Due to the weak ultraviolet light, the battery efficiency wa low.Therefore, we used the following equation to evaluate the relative improvement in battery efficiency, ER: where E0 and E1 represent the initial cell efficiency and the packaged battery efficiency respectively.Through the calculation, we found that after EVA, CsPbBr3/EVA, and CsP bBr3-In/EVA packaging, the battery efficiency relatively increased by 10%, 22%, and 37% respectively.The underlying mechanisms responsible for the enhanced device perfor mance of EVA, CsPbBr3/EVA, and CsPbBr3-In/EVA were distinct; the improvement in EVA mainly stemmed from its ability to mitigate light reflection on the silicon solar cell's sur face, leading to an increase in the short-circuit current (JSC) [49].In the case of the CsP bBr3/EVA and CsPbBr3-In/EVA composite films, the JSC enhancement could be attributed to two factors.Firstly, similar to the EVA film, the composite films exhibited an anti-re flection effect, facilitating greater light transmission into the silicon solar cell.Secondly the incorporation of CsPbBr3 quantum dots (QDs) into these films allowed for the conver sion of low-efficiency, high-energy photons into more efficient, lower-energy photon [13].Next, a white light-emitting diode (WLED) was developed by integrating a green light-emitting CsPbBr3-In/EVA film, a KSF red phosphor, and an InGaN blue chip (Figur 8a), aiming to demonstrate its potential applications in the displays.As shown in Figur 8b, the PL spectrum exhibited clear separation into three distinct regions-blue, green  Next, a white light-emitting diode (WLED) was developed by integrating a green lightemitting CsPbBr 3 -In/EVA film, a KSF red phosphor, and an InGaN blue chip (Figure 8a), aiming to demonstrate its potential applications in the displays.As shown in Figure 8b, the PL spectrum exhibited clear separation into three distinct regions-blue, green, and red emissions-corresponding with the blue LED chip, CsPbBr 3 -In/EVA film, and KSF phosphor, respectively.Without optimizing the device structure, the resulting WLED achieved color coordinates of (0.33, 0.31), which corresponded with 131.34%NTSC and 98.07%Rec.2020 color gamut coverage, as depicted in the CIE coordination in Figure 8c.This suggests that the CsPbBr 3 -In/EVA film holds significant potential as a green light source for LED display devices.

Conclusions
In order to improve the stability of perovskite quantum dots and expand their application range, this work presents a straightforward synthesis method for CsPbBr3 QDs that effectively passivated QD defects by incorporating In(C2H3O2)3.When the content of In(C2H3O2)3 in the precursor was 0.02 mmol, the PL intensity was the highest.Subsequently, a co-passivation strategy was further adopted and the CsPbBr3-In QDs were encapsulated with the polymer EVA.When the mass ratio of QDs to EVA polymer was 0.034%, the CsPbBr3-In/EVA had excellent tensile and bending properties, thereby improving the stability of perovskite films in challenging environments and broadening their potential applications.To further enhance the performance of silicon solar cells, the CsPbBr3-In/EVA film was applied as a cover, effectively boosting the PCE of the cell.Lastly, the development of a WLED device that combined a blue chip, KSF red phosphor, and CsPbBr3-In/EVA film resulted in a WLED device with a wide color gamut for highquality display applications, also exhibiting significant potential.

Figure 2 .
Figure 2. Structure characterization of CsPbBr3-In.(a) Diagram of CsPbBr3-In QDs.(b) XRD patterns of CsPbBr3-In QDs (red) and CsPbBr3 (black).The bottom data are the standard XRD cards of CsP-bBr3.(c) SEM images of CsPbBr3-In/EVA film.(d) Elemental mapping images corresponding with (c).(e) TEM and (f) HRTEM images of CsPbBr3-In QDs, respectively.(g-j) Elemental mappings of Cs, Pb, Br, and In elements in the CsPbBr3-In/EVA film of Figure 2d.

Figure 3 .Figure 4 .
Figure 3. XPS characterization.(a) XPS spectra of CsPbBr 3 before and after In(C 2 H 3 O 2 ) 3 treatment.XPS map of (b) Cs 3d, (c) Pb 4f, (d) Br 3d, (e) O 1s, and (f) In 3d.The bottom images in (b-e) are CsPbBr 3 QDs, and the upper of (b-e) are XPS spectra in CsPbBr 3 -In QDs.QD solutions with varying concentrations of In(C 2 H 3 O 2 ) 3 are shown in Figure 4a.The upper image was taken in an indoor environment, while the lower image was captured under UV light.Figure 4b displays the PL spectra corresponding with the different contents of In(C 2 H 3 O 2 ) 3 shown in Figure 4a.It was obvious that the addition of In(C 2 H 3 O 2 ) 3 had an impact on both the luminous intensity and peak position of CsPbBr 3 .Specifically, when 0.02 mmol In(C 2 H 3 O 2 ) 3 was added, the luminous intensity increased by a factor of 1.56.Therefore, in the follow-up test, we chose a precursor solution with a content of 0.02 mmol In(C 2 H 3 O 2 ) 3 .However, excessive amounts of In(C 2 H 3 O 2 ) 3 led to a decrease in the luminescence intensity of CsPbBr 3 , accompanied by a blue shift in the peak position.This observation aligned with the UV-Vis absorption spectrum displayed in Figure 4c.Then, TRPL measurements were conducted on samples with varying concentrations of In(C 2 H 3 O 2 ) 3 , as depicted in Figure 4d.The photoluminescence decay curves for both CsPbBr 3 and CsPbBr 3 -In followed a double exponential pattern.The decay time, decay component ratio, and average decay lifetime are shown in Table 1.The longer-lived component (τ 2 ) of CsPbBr 3 could be attributed to typical radiative recombination, whereas

Figure 4 .
Figure 4. (a) Photos of perovskite quantum solutions with different contents of In(C 2 H 3 O 2 ) 3 under natural light and UV light.(b) PL, (c) UV-Vis, and (d) TRPL spectra of CsPbBr 3 QDs with different contents of In(C 2 H 3 O 2 ) 3 .
mers 2023, 15, x FOR PEER REVIEW 8 o of In(C2H3O2)3 has the potential to improve the stability of perovskite films in water a high-temperature environments.This enhancement in stability further expands the ap cation range of quantum dots (QDs) in various fields.

Figure 5 .
Figure 5. (a-c) Heat stability tests of CsPbBr3/EVA and CsPbBr3-In/EVA films.(d) Photos of per skite quantum dot films under natural and UV light before and after immersion in water.(e-g) ter stability tests of CsPbBr3/EVA and CsPbBr3-In/EVA films.Contact angle tests of (h) CsPbBr3/E and (i) CsPbBr3-In/EVA films.

Figure 5 .
Figure 5. (a-c) Heat stability tests of CsPbBr 3 /EVA and CsPbBr 3 -In/EVA films.(d) Photos of perovskite quantum dot films under natural and UV light before and after immersion in water.(e-g) Water stability tests of CsPbBr 3 /EVA and CsPbBr 3 -In/EVA films.Contact angle tests of (h) CsPbBr 3 /EVA and (i) CsPbBr 3 -In/EVA films.

Figure
Figure (a) CsPbBr 3 -In/EVA film with different bending angles under UV lamp.(b,c) Stretch stability tests of CsPbBr 3 -In/EVA film.

1 Figure
Figure S3b-d and TableS1.Due to the weak ultraviolet light, the battery efficiency wa low.Therefore, we used the following equation to evaluate the relative improvement in battery efficiency, ER:

Table 1 .
The fitted results of TRPL of CsPbBr3 with different In(C2H3O2)3 contents.

Table 1 .
The fitted results of TRPL of CsPbBr 3 with different In(C 2 H 3 O 2 ) 3 contents.

Table 2 .
Photovoltaic parameters of devices with EVA, CsPbBr3/EVA, and CsPbBr3-In/EVA compo site films.