Synthetic and Post-Synthetic Strategies to Improve Photoluminescence Quantum Yields in Perovskite Quantum Dots

Abstract

Making high-quality raw materials is the key to open the versatile potential of next generation materials. All-inorganic CsPbX₃ (X: Cl⁻, Br⁻, and/or I⁻) perovskite quantum dots (PQDs) have been applied in various optoelectronic devices, such as photocatalysis, hydrogen evolution, solar cells, and light-emitting diodes, due to their outstanding photophysical properties, such as high photoluminescence quantum yield (PLQY), absorption cross-section, efficient charge separation, and so on. Specifically, for further improvement of the PLQY of the PQDs, it is essential to diminish the non-radiative charge recombination processes. In this work, we approached two ways to control the non-radiative charge recombination processes through synthetic and post-synthetic processes. Firstly, we proposed how refinement of the conventional recrystallization process for PbI₂ contributes to higher PLQY of the PQDs. Secondly, after halide exchange from CsPbI₃ PQDs to CsPbBr₃, through an in situ spectroelectrochemical setup, we monitored the positive correlation between bromide deposition of on the surface of the perovskite and photoluminescence improvement of the CsPbBr₃ perovskite film through electrodeposition. These two strategies could provide a way to enhance the photophysical properties of the perovskites for application to various perovskite-based optoelectronic devices.

1. Introduction

The ABX₃ hybrid perovskites (A = MA⁺, FA⁺, and Cs⁺, where MA⁺: methylammonium, FA⁺: formamidinium, B = Pb²⁺ and Sn²⁺, and X = Cl⁻, Br⁻, and I⁻) with various dimensions (0D to 3D) have been widely studied for application to various optoelectronic devices, especially for boosting photoconversion efficiency of the perovskite-based solar cell up to 25.17% [1,2,3]. In the perovskite-based research field, using high-purity precursors is widely accepted among researchers for better photophysical properties such as high photoluminescence quantum yield (PLQY) for perovskite quantum dots (PQDs), high absorption cross-section, efficient charge separation, and advanced device performance [4,5,6]. To enhance the photoconversion efficiency, numerous strategies have been proposed but a major strategy is reducing photogenerated free carrier loss in the device [5,6,7]. The major reasons of current loss could be as follows: (1) free carrier trapping in the perovskite at various internal/surface defects [5,8,9], (2) interfacial charge accumulation between the perovskite/charge transfer materials [10], and/or (3) insufficient charge transfer and regeneration processes [11,12]. To increase the efficient electron/hole separation and photoinduced current generation, researchers have been focused on decreasing surface defects by increasing grain size [8,13,14], surface defect passivation [15], applying recrystallization process of as-synthesized perovskite to reduce impurity to reduce internal defects [4], and so on. In the perovskite, various defects were studied but halide-based defects are easily formed due to their low defect formation energies [16]. Therefore, decreasing halide-induced defects in the perovskite is one of the important strategies for their better photophysical properties and efficient perovskite-based device performance.

In this work, we focused on the early stage, how the conventional recrystallization process for perovskite precursors affects the photophysical performance of the PQDs. Additionally, we figured out the role of the halide compositions to control the non-radiative recombination rate constant (k_nr) to enhance photophysical performances of the PQDs. Through in situ photoluminescence (PL) measurement with and without applied bias, we also observed that the post-synthetic process also provided a positive effect on increasing PLQY. Enriched halide surface passivation through electrodeposition could also be the key to increase PLQY through decreasing k_nr. The findings could also initiate how the halide plays an important role to the perovskites for application to various optoelectronic devices, such as light-emitting diodes (LEDs), solar cells, photocatalysis, hydrogen evolution reactions, and so on.

2. Results and Discussion

2.1. Impact of Recrystallization of PbI₂ Precursor for Photophysical Properties of the PQDs

To study the impact of the internal defect in the PQDs to their photophysical properties, we firstly focused on the most frequently used PQD, CsPbI₃ PQDs, and even various chemicals which were applied in the hot-injection method to synthesize the CsPbI₃ PQDs (details in Materials and Methods), we focused on one of the most important precursors, PbI₂, which affects 1. stoichiometry and 2. halide-mediated defects in CsPbI₃ PQDs. To specify the role of purification, adapting motivation from our earlier work [6], we used the “Golden Rain” method (see Figure S1, provided by the Royal Society of Chemistry [17]) to obtain precipitation of PbI₂, reaction scheme is as follows:

$$\mathrm{Pb}{\left({\mathrm{CH}}_3\mathrm{COO}\right)}_{2 \left(\mathrm{aq}\right)}+2{\mathrm{KI}}_{\left(\mathrm{aq}\right)}\to { \mathrm{PbI}}_{2 \left(\mathrm{s}\right)}+2{\mathrm{CH}}_3{\mathrm{COOK}}_{ \left(\mathrm{aq}\right)}$$

Additionally, we performed the recrystallization process with varying cooling rates (two different cooling rates, 2.38 h⁻¹ and 0.690 h⁻¹, for Ice Bath and Hot Water PbI₂, respectively) to observe the role of recrystallization on purity and I/Pb stoichiometry. In Figure 1A, X-ray diffraction (XRD) patterns of various PbI₂ are presented. Comparing the patterns to the pattern obtained from commercially available PbI₂ with 99% purity, synthesized and consecutive recrystallized XRD patterns present clear patterns, following hexagonal PbI₂ with P-3 mL space group (PDF# 04-007-2640). Compared to another XRD pattern from commercially available PbI₂ with 99.999% purity, the XRD patterns demonstrated identical PbI₂ XRD patterns without pattern from expected impurities, such as PbO, PbCO₃, and so on (see Figure S2). More details are under investigation to clarify the detailed chemical environment but through X-ray fluorescence (XRF) elemental analysis (Table S1), we were able to clarify that the recrystallized Ice Bath and Hot Water PbI₂ demonstrated the most similar stoichiometry (I/Pb ratio of 2.012 for Ice Bath and 2.000 for Hot Water) compared to As-synthesized and Commercial PbI₂ (2.059 and 2.016, respectively). We speculate that better stoichiometry between I and Pb is essential to avoid excess halide-based defects, such as iodide interstitials in the perovskite, which are easily formed under excess iodide (I-rich) conditions in the perovskite with lower defect formation energy as 0.19 eV over than valence band maximum [16,18,19]. Note that compared to our previous report [6], current I/Pb ratios (2.000–2.059) were shown to contain relatively smaller I/Pb ratios than early reported values (2.15–2.19). Relatively smaller I/Pb ratios could be detrimental for controlling iodide mediated vacancies in the perovskite but in case of relatively smaller I/Pb conditions, the iodide interstitials could be one of the most critical factors to impact the non-radiative recombination process [16,18,19]. To summarize, we were able to obtain better PbI₂ through recrystallization without any impurity XRD patterns with better I/Pb stoichiometry that finally increased the optical performance of the PQDs synthesized with it.

Figure 2A presents UV-Vis. absorption spectrum of the CsPbI₃ PQDs with an absorption band edge at 665 nm and corresponds to the PL spectrum with a peak position at 685 nm. Around a 20 nm stoke shift was observed due to the relaxation of the vibration process to the valence band maximum in the PQD. The shift is reproducible compared to previously reported values [1]. With dependence on crystalline core size of the CsPbI₃ PQDs, due to the quantum confinement effect, the absorption band edge and emission peak shift could occur [1], but in this report, PQDs made using the four PbI₂ (Commercial, As-synthesized, Ice Bath, and Hot Water PbI₂) demonstrated negligible absorption band edge/emission peak position differences (see Figure S3). Furthermore, in Figure 2B, crystallized CsPbI₃ PQDs with around 7 to 9 nm cubic shapes were observed. In Figure S4, the size similarity can be observed by comparing PQDs made using the four PbI₂, within their size distributions. Due to their similar sizes of PQDs, even using different PbI₂ for the PQD synthesis, we were able to obtain similar absorption cross-sections and molar extinction coefficients [20], which implies similar intensity of light absorbed (I_a), and so negligible differences regarding the number of excited electrons in the various PQDs was estimated. This similar I_a is one of the important factors for comparing radiative and non-radiative recombination kinetics for the various PQDs using different PbI₂. In the perovskite, if the I_a and/or number of excited electrons would be different, major recombination-trap assisted recombination, bimolecular recombination of exciton/free carriers, or Auger recombination processes could be changed [21]. Furthermore, the similar sizes of PQDs (7–9 nm) demonstrate similar surface-to-volume ratios, which is one of the key factors that affects the number of surface defects in the PQDs. The surface defect density and surface defect induced non-radiative recombination kinetics could be changed by ligand, surface density [22], type of ligands [23,24], formation of core or core-shell structures [25,26], varying surface-to-volume ratios or sizes [27], and so on. However, in this study, we used the same QD structure, ligands, and their amounts for all synthetic procedures of the PQDs. Therefore, the effect of ligands on PQD surface to non-radiative recombination kinetics could be experimentally controlled for all PQDs made using the four PbI₂. Consequently, the observed PLQY and k_nr can be associated with defects associated with different characters of PbI₂ precursors with different I/Pb elemental ratio.

To monitor the effect of the PbI₂ precursor purification on photophysical properties of CsPbI₃ PQDs, the four types of CsPbI₃ PQDs were synthesized using the four different PbI₂ (Commercial, As-synthesized, Ice Bath, and Hot Water PbI₂) and their PL spectra are shown in Figure 3A. Note that the emission spectra were taken from the PQD dispersed solutions by maintaining their absorbances at their absorption band edge around 0.1 ± 0.005 to obtain PLQY, as shown in Figure 3B. To obtain PLQY without dependency of degradation of the PQDs, the PLQY values were taken right after PQD synthesis in every time with comparisons of reference dye (riboflavin, see details in the Materials and Methods section) with the same absorbance at the band edge, through the following equation:

$${\mathsf{\varphi }}_{\mathrm{PQD}}={\mathsf{\varphi }}_{\mathrm{standard}}\times \frac{{\mathrm{Area}}_{\mathrm{PQD}}}{{\mathrm{Area}}_{\mathrm{standard}}}\times \frac{{\mathsf{\eta }}_{\mathrm{PQD}}^2}{{\mathsf{\eta }}_{\mathrm{standard}}^2}$$

(1)

In Equation (1), ${\mathsf{\varphi }}_{\mathrm{PQD}}$,${\mathsf{\varphi }}_{\mathrm{standard}}$,${ \mathrm{Area}}_{\mathrm{PQD}}$,${ \mathrm{Area}}_{\mathrm{standard}}$,${ \mathsf{\eta }}_{\mathrm{PQD}}$, and ${\mathsf{\eta }}_{\mathrm{standard}}$ represent PLQY of PQD, PLQY of standard dye (riboflavin), PL peak area of PQD, PL peak area of standard dye, refractive index of PQD dispersed solution (hexane), and refractive index of PQD dispersed solution (ethanol), respectively. PLQYs of PQDs using Commercial, As-synthesized, Ice Bath, and Hot Water PbI₂ are 52.3 ± 5.2%, 61.8 ± 5.6%, 81.3 ± 7.7%, and 80.3 ± 8.0%, respectively. In general, PLQY were assigned as follows:

$$\mathrm{PLQY}=\frac{{\mathrm{k}}_{\mathrm{r}}}{{\mathrm{k}}_{\mathrm{r}}+{\mathrm{k}}_{\mathrm{nr}}}$$

(2)

k_r represents the radiative recombination rate constant. In Figure 3A, emission intensities are higher for PQDs using recrystallized PbI₂ precursors. Additionally, in Figure 3B, in the case of the PQDs using recrystallized PbI₂, PLQYs were around 20% higher than PQDs using As-synthesized PbI₂. The PLQY improvements could be concluded as decreasing k_nr due to following reasons: (1) similar intensity of light absorbed, I_a, using the same laser power excitation, which expects a similar number of excited electrons in the PQDs, and (2) removal of impurity in the PQDs, monitored through XRD patterns (see Figure S5). Especially, we speculate that the recrystallization of the PbI₂ precursor gives a positive effect to decrease iodide interstitial defects by giving better stoichiometry between I/Pb as 2.01–2.00 in PQDs [16,18,19]. Note that detailed PLQY were obtained through Equation (1) by comparing emissions of riboflavin dye by matching fine absorption match and independent measurements right after QD synthesis to exclude the degradation effect, so it is hard to differentiate PLQY difference among 81.3 ± 7.7%, and 80.3 ± 8.0%. Furthermore, overall PLQYs are below 90%, by comparing early reported high PLQY values (>90%) [28] to the PLQY, there is still a significant impact from the non-radiative recombination process from charge trapping at the surface defect. More details could be investigated further regarding optimizing experimental QD synthetic conditions to increase overall PLQY. In short, the recrystallization process of precursor for PQDs could contribute to providing qualified crystalline quality of the PQDs and more emissive properties with higher PLQY.

To observe how recrystallization of the precursor affects the non-radiative recombination process in the PQDs, time-correlated single photon counting (TCSPC) was performed to monitor time-resolved photoluminescence (TRPL) kinetic trace for the four CsPbI₃ PQDs made using Commercial, As-synthesized, Ice Bath, and Hot Water PbI₂ (Figure 4 and

Table 1. Averaged photoluminescence decay lifetime (τ_avg), PLQY, k_r, and k_nr for the four CsPbI₃ PQDs made using the Commercial, As-synthesized, Ice Bath, and Hot Water PbI₂.

PbI2	${\mathsf{\tau }}_{\mathbf{a}\mathbf{v}\mathbf{g}} \left(\mathbf{ns}\right)$	PLQY (%)	kr (s−1)	knr (s−1)
Commercial	72.3 ± 1.4	52.3 ± 5.2	7.23 ± 0.14 (×106)	6.61 ± 0.13 (×106)
As-synthesized	50.0 ± 1.0	61.8 ± 5.6	1.24 ± 0.03 (×107)	7.64 ± 0.16 (×106)
Ice Bath	69.5 ± 0.4	81.3 ± 7.7	1.17 ± 0.07 (×107)	2.70 ± 0.02 (×106)
Hot Water	85.5 ± 0.6	80.3 ± 8.0	9.39 ± 0.07 (×106)	2.31 ± 0.02 (×106)

). The averaged photoluminescence decay lifetimes (τ_avg) were obtained through a bi-exponential function fitting to the TRPL, as following Equation (3) [29]:

$$\mathrm{y}={\mathrm{y}}_0+{\mathrm{A}}_1{\mathrm{e}}^{-\mathrm{x}/{\mathsf{\tau }}_1}+{\mathrm{A}}_2{\mathrm{e}}^{-\mathrm{x}/{\mathsf{\tau }}_2}$$

(3)

In Figure 4A, the emission intensities were monitored with time delay. A_i and τ_i demonstrate weighted coefficients for each component and their time constants, respectively. The multi-exponential decay behavior of the PQDs was attributed to various fluctuations in the non-radiative recombination process, which include quenching of the excited electrons to trap sites [29,30]. In Table S1, the parameters through the fitting were demonstrated. The τ_avg were obtained through the following Equation (4) [29]:

$${\mathsf{\tau }}_{\mathrm{avg}}=\frac{{\mathrm{A}}_1{\mathsf{\tau }}_1^2+{\mathrm{A}}_2{\mathsf{\tau }}_2^2}{{\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2}$$

(4)

Overall, τ_avg for PQDs using recrystallized PbI₂ presented longer than τ_avg for PQDs using As-synthesized PbI₂ due to the bigger contribution of amplitude A₂ for longer time constants. The elongation of τ_avg is a result of decreasing k_r and/or k_nr, as follows [6]:

$${\mathsf{\tau }}_{\mathrm{avg}}=\frac{1}{{\mathrm{k}}_{\mathrm{r}}+{\mathrm{k}}_{\mathrm{nr}}}$$

(5)

Through experiments, PLQY and τ_avg were obtained. Applying the PLQY and τ_avg values to Equations (2) and (5), k_r and k_nr were obtained and they are listed in Table 1. Interestingly, using recrystallized PbI₂ for the CsPbI₃ PQD synthesis decreases k_nr about 1/4 times slowly than the k_nr of PQDs using As-synthesized PbI₂ as precursor. Even though k_r also decreases 5.64% in the case of Ice Bath PbI₂ and 24.3% for the case of Hot Water PbI₂ (as compared to k_r for the case of using As-synthesized PbI₂), usage of recrystallized PbI₂ enabled us to retard k_nr (65% slower rate for Ice Bath and 70% slower rate for Hot Water, compared to the k_nr for As-synthesized) in the CsPbI₃ PQDs. It means that there is a positive correlation between recrystallization of PbI₂ as a precursor and retarding k_nr for favorable emission and reduction of the excited electron quenching at defect sites. We speculate that reduction of interstitial iodide in the perovskite could contribute to reducing the easily favored non-radiative recombination sites so that the relatively unfavored non-radiative recombination process retards the k_nr [16,18,19].

2.2. Halide Vacancy Mediated Surface Defect Passivation through Electrodeposition

To monitor reduction of excited electron trapping at surface defects and to occur favorable radiative recombination process, we also performed in situ PL spectroscopy while undergoing electrodeposition of CsPbBr₃/ITO film with positive voltage to cover the surface defects to bromide (Figure 5 and see details in Materials and Methods). In short, dispersed CsPbI₃ PQDs interacted with tetraoctylammonium bromide in the same solution phase. Under the ambient condition, CsPbBr₃ PQDs have better stability and form bulk CsPbBr₃ film following through annealing process to firmly attach on to the ITO substrate. In the films with positive applied bias (+0.6 V), the positive electric field attracted to bromide in the octylammonium bromide solution and deposited the bromide on the surface of the CsPbBr₃ perovskite layer. The +0.6 V corresponds to the relative higher energy level to valence band maximum of the perovskite and the energy level similarly matches to the halide vacancy and its hole mediated oxidation [18]. Note that the CsPbBr₃ perovskite film was surrounded by an octylammonium bromide solution so the electrical field in between the two electrodes made bromide move to the anode and octylammonium cation move to the cathode, instead of the hole-mediate oxidation under the electrical field. The quantitative movements of ion in between the electrodes and the accumulated number of electrons calculated through current are presented in Figure 5A. Under the constant applied bias, the current decreased while the electrodeposition occurred. We speculate that constant ion movements to the two electrodes (bromide to surface of the CsPbBr₃ perovskite and octylammonium cation to counter electrode) continuously decreased the overall concentration of the octylammonium bromide. However, the accumulated electron flow indicated that the successful electrodeposition of ions occurred and the successive bromide surface passivation on the CsPbBr₃ perovskite film surface occurred as well. Figure 5B demonstrates the emission enhancement during the electrodeposition process because of bromide passivation on the perovskite surface. Figure 5C presents electrodeposition-time based relative PLQY enhancement through the following equation, derived from Equation (1):

$$\frac{{\mathsf{\varphi }}_{\mathrm{t}}}{{\mathsf{\varphi }}_0}=\frac{{\mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_0}\times \frac{{\mathsf{\eta }}_{\mathrm{t}}^2}{{\mathsf{\eta }}_0^2}$$

(6)

${\mathsf{\varphi }}_{\mathrm{t}}$,${\mathsf{\varphi }}_0$,${ \mathrm{A}}_{\mathrm{t}}$,${ \mathrm{A}}_0$,${ \mathsf{\eta }}_{\mathrm{t}}$, and ${\mathsf{\eta }}_0$ represent PLQY of the CsPbBr₃ perovskite film at a certain time, PLQY of the CsPbBr₃ perovskite film at time zero, PL peak area of the CsPbBr₃ perovskite film at a certain time, PL peak area of the CsPbBr₃ perovskite film at time zero, the refractive index of the octylammonium bromide solution at a certain time, and the refractive index of the octylammonium bromide solution at time zero, respectively. Note that the change of refractive index of the octylammonium bromide solution under the electrodeposition process is negligible so $\frac{{\mathsf{\eta }}_{\mathrm{t}}^2}{{\mathsf{\eta }}_0^2}$ in Equation (6) could be disregarded. Equation (6) could be simplified to Equation (7), as follows:

$$\frac{{\mathsf{\varphi }}_{\mathrm{t}}}{{\mathsf{\varphi }}_0}=\frac{{\mathrm{A}}_{\mathrm{t}}}{{\mathrm{A}}_0}$$

(7)

Therefore, through calculating the ratio between PL emission peak area at certain time and emission peak area at time zero, we present the relative PLQY change with the electrodeposition reaction time in Figure 5C. Note that the overall PLQY of CsPbBr₃ perovskite film is below 1% because efficient exciton separation occurs to form free carriers at room temperature due to similar energy values between exciton dissociation energy and thermal energy k_BT ≈ 25 meV [11,23]. Additionally, dominant defect-mediated excited electron trapping induces the favorable non-radiative recombination process and so the favorable non-radiative recombination process also diminishes the overall PLQY of the perovskite film [11,23]. Furthermore, we also observed that there is a positive correlation between increasing applied electric field and emission enhancement (Figure S6). It means that the increased potential facilitates ion movement in the solution for the electrodeposition and so that the passivation of bromide on to the anode, perovskite film. Note that a similar electrodeposition technique could be performed for the PQD deposited film but by being placed in the electrolyte, both ion passivation and loss of PQD from film to electrolyte could be performed so that detailed experimental control could be issued. Through electrodeposition of bromide to perform bromide surface passivation on the CsPbBr₃ perovskite film, it could be possible to conclude that bromide surface passivation is one strategy to decrease non-radiative recombination processes at surface defects.

2.3. The Two Different Synthetic and Post-synthetic Strategies to Improve Photoluminescence Quantum Yield of the PQDs

The two experimental strategies to control the non-radiative process at defects in the perovskite in this study are proposed in Scheme 1. First, recrystallization of the PbI₂ precursor positively affected the enhancement of the PLQY of CsPbI₃ PQDs due to removal of impurities in the PQD crystalline structure and retarding k_nr to increase the PLQY, following Equation (2). The purification of the precursor could positively affect the formation of PQDs with a lower number of defects so that the overall PLQY could be enhanced from 61.8% to 80.3%. The reason could be reduction of the interstitial iodide to induce favorable radiative recombination. Additionally, the post-synthetic process, electrodeposition, provided effective bromide surface passivation on the CsPbBr₃ perovskite film so that PLQY increased due to decreased excited electron quenching at defects, estimated by bromide vacancy. The bromide vacancy on the surface could also play a role in inducing a trapping site to the photoinduced excited electrons and further induce the non-radiative recombination process, which was detrimental for the PLQY.

3. Materials and Methods

3.1. Materials

Lead acetate (Pb(CH₃CO₂)₂, 6080-56-4, 99.7%, Fisher Chemical, Waltham, Massachusetts, US), Potassium iodide (KI, 7681-11-0, 99.5%, Daejung Chemicals, Siheung-si, Korea), Hydrochloric acid (HCl, 7647-01-0, 35.0%, Daejung Chemicals, Siheung-si, Korea), Ethyl acetate (141-78-6, Duksan Chemicals, Ansan-si, Korea), n-octane (111-65-9, 97.0%, Daejung Chemicals, Siheung-si, Korea), Tetraoctylammonium bromide (14866-33-2, 98%, Sigma-Aldrich, Saint Louis, Missouri, US), Cs₂CO₃, (Cs₂CO₃, 534-17-8, 99.5%, Samchun Chemicals, Seoul, Korea), oleic acid (OA, 112-80-1, 90%, Alfa Aesar, Haverhill, US, ), 1-octadecene (112-88-9, 90%, Sigma-Aldrich, Saint Louis, Missouri, US), PbI₂ (10101-63-0, 99%, Sigma-Aldrich, Saint Louis, Missouri, US), PbI₂ with higher purity (10101-63-0, 99.999%, TCI, Paris, France), and oleylamine (112-90-3, 50%, OLA, TCI, Paris, France)

3.2. Methods

3.2.1. Synthesis of the PbI₂ Perovskite Precursors and Recrystallization Process

First, 0.339 g lead acetate and 0.238 g potassium iodide were each dissolved in 100 mL of distilled water [6]. A total of 0.3 mL of 1 M Hydrochloric acid was added to the lead acetate solution to prevent hydrolysis. We simply mixed the two solutions with a few drops at an early moment. Soon, bright yellow lead iodide was produced. After the precipitate was formed, heat was applied to the solution to ~80 °C in 1 h by putting the solution in a water bath to induce complete dissolution of PbI₂. Then cooling applied either (1) or (2), as follows:

(1)

Recrystallization process with slow cooling (called “Hot Water”)

The heating was turned off and the solution was placed in the water bath. Gradual cooling down to room temperature occurred on its own (~ 2 h). After watching the crystals for around 2 h, the flask was kept in the lab fridge for 3 h to obtain as many crystals as possible, then the precipitation was filtered to isolate the lead iodide. The filtered lead iodide was dried under a vacuum overnight.

(2)

Recrystallization process with fast cooling (called “Ice”)

The heated solution was placed in a flask inside the ice bath in the lab fridge for 3 h to obtain as many crystals as possible. Then, the crystallized lead iodide was filtered, and dried under vacuum overnight.

3.2.2. Synthesis of CsPbI₃ Perovskite Quantum Dots

The synthetic procedure for CsPbI₃ perovskite quantum dots was from our previous work [3,6,31]. In brief, a mixture composed of 0.407 g of Cs₂CO₃, 1.25 mL of oleic acid, and 20 mL of 1-octadecene was degasified in a 50 mL three neck flask under vacuum for 30 min at 80 °C, under vacuum for 30 min at 120 °C, and then heated at 140 °C under N₂ conditions until Cs₂CO₃ reacted completely with OA. A translucid clear Cs-oleate solution was achieved. Before injection, the solution was preheated at 115 °C, keeping the N₂-purging, to avoid the precipitation of Cs-oleate. Note that the appearance of a dark orange color in the Cs-oleate solution is indicative of Cs-oleate oxidation to form Cs₂O.

To synthetize CsPbI₃ quantum dots, 0.5 g of PbI₂ was mixed with 50 mL of 1-ODE in a 100 mL three neck flask. The mixture was dried under vacuum at 120 °C for 30 min. Then, preheated OA and OAm (2.5 mL of each) were loaded in the solution at 130 °C, and briefly it was degasified until the complete dissolution of precursors.

Under N₂-purging, the temperature of reaction was raised to 170 °C, and 2 mL of Cs-oleate solution was quickly injected. Lastly, the reaction was quenched after adding the reaction flask in an ice bath until a fall in temperature in the range of 60–70 °C.

3.2.3. Halide Exchange Process and Deposition of Perovskite Quantum Dots and on ITO to form Stable CsPbBr₃ Perovskite/ITO Films in Ambient Condition at Room Temperature

First, halide exchange was performed by reacting 2 mL CsPbI₃ PQDs with 0.2 g tetraoctylammonium bromide under vigorous stirring. We obtained the supernatant, CsPbBr₃ PQD. A total of 3 mL ethyl acetate was added to 1.5 mL halide exchange QDs solution and then centrifuged at 6000 rpm for 20 min. The supernatant was discarded, and the QDs was dispersed again in 1 mL hexane and then 300 rpm 3 min. Dispersed solutions were taken and N₂ blow applied for 10 min to remove hexane solvent. The remained PQDs were dispersed in 2 mL n-octane. The PQD dispersed octane solution was drop-cast on ITO and dried under N₂ gas flow. To form CsPbBr₃ bulk film, the drop-cast PQD/ITO film was annealed at 225 °C for 3 min, under ambient condition, as reported in earlier literature [31].

3.2.4. Characterizations

The steady state absorption spectra of colloidal CsPbI₃ quantum dot solutions and films were achieved by using a UV/Vis absorption and photoluminescence spectrophotometer (Lab junior, K-MAC) by using a 450 nm LED light source (power density ~1 mW/cm²). The in situ changes of the absorption/emission features of the CsPbBr₃ perovskite films were monitored with a CCD camera with and without applied bias (WPG100e potentiostat/galvanostat, Wonatech). In addition, combining incident photon-to-electron conversion efficiency (IPCE) setup, charge-coupled devices (CCD) detector, and additional optic tools, we prepared a homemade setup (as IPCE-CCD) for in situ absorption/emission measurement with and without applied bias during the electrodeposition process. The CsPbBr₃ perovskite/ITO films were connected to the anode and dipped in an octylammonium bromide isopropanol solution (100 mg/mL). Another ITO film was used as a counter electrode. Various neutral density filters were applied in front of the detector to control overall emission intensity incoming to the detector.

Lead iodide crystal structure was measured using X-ray diffraction (XRD) analysis (XRD, Mini Flex 600, Rigaku). The perovskite quantum dot crystal structure was measured using a field emission transmission electron microscope (FE-TEM, Tecnai G2 F20 S-TWIN, FEI Korea). The relative PLQY of the quantum dots were obtained by comparing emission from riboflavin dye [6]. TRPL were measured through TCSPC (Fluotime 300, PicoQuant, Berlin, Germany) with 520 nm pulse laser excitation (pulse duration < 100 ps). Elemental analysis of PbI₂ was characterized through an energy-dispersive X-ray fluorescence spectrometer (PanalyticalEpsilon, 3-XLE, Malvern, UK) at the Core Research Support Center for Natural Products and Medical Materials at Yeungnam University.

4. Conclusions

We presented two strategies for higher PLQY of the perovskites by using i) higher quality PbI₂ with better I/Pb stoichiometric ratios to decrease iodide interstitials in the PQDs and ii) using the electrodeposition technique to passivate halide surface vacancies. The recrystallized PbI₂ precursors for perovskite improved the quality of the PQDs for two reasons. First, the XRD patterns of the PbI₂ precursors provided a positive role in the recrystallization process to remove impurities. Second, XRF confirmed enhanced I/Pb stoichiometry in the PbI₂ from applying the recrystallization process. Using recrystallized PbI₂ contributed to enhancing the PLQY of PQD from 62% to over 80% due to enriched iodine synthetic atmosphere, which implies decreased iodide-based interstitial defects. Additionally, the recrystallized PbI₂ improved PQD synthetic quality with diminished byproducts, monitored XRD patterns of PQDs made with and without using the recrystallized PbI₂. TCSPC results supports selectively retarded k_nr with a four times slower rate by applying the additional recrystallization process. Furthermore, to study the role of halide vacancy mediated defects, we prepared the CsPbBr₃ film on ITO using halide exchange (bromide to iodide) and subsequent annealing process to form agglomerated bulk morphology on ITO [32]. Under the applied bias, we observed the positive correspondence between current as results of halide movements in the solution and deposition on the anode (CsPbBr₃ film), and enhancement of PL. The correspondence implies that the surface passivation of halide-mediated defects suppresses the halide vacancy mediated non-radiative recombination sites. Therefore, these two different strategies to control defects could contribute to how conventional chemistry techniques applied to the modern renewable energy research field can enhance the photophysical properties of the key materials for the next generation, such as perovskite. This work could open both fundamentals of halide defect chemistry for perovskites and practical challenges to improve the performance of optoelectronic devices.