2.1.

Perovskite Quantum Dot Synthesis

${\mathrm{CsPbX}}_3$ (X = Br and I) PQDs were synthesized using the methods reported in Refs. 25 and 26. The same process reported in Ref. 27 was used to prepare the cesium (Cs)-oleate precursor solution and for synthesis in the reaction flask (RF). The synthesis temperature was set at 170°C to produce $\sim 8\text{-}\mathrm{nm}$²⁵ to 9.5-nm²⁶ PQD nanocrystals. The synthesized PQDs were diluted in hexane to achieve a $5\text{-}\mathrm{mg}/\mathrm{mL}$ ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD solution. The detailed procedure of the ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD solution preparation can be found in Appendix A.

2.2.

Perovskite Quantum Dot-Cyclic Olefin Copolymer Solution

Perovskite materials, by themselves, are known to be easily degraded by moisture; hence, their performance as photonic materials easily decline over a short period of time, from a few weeks to a few minutes under typical atmospheric conditions.^30–38

To protect the PQDs from humidity, polymer matrices can be used. Polymer matrices can be made from one or a combination of materials from the group consisting of polystyrene (PS), polycarbonate (PC), cellulose acetate (CA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), polyurethane (PU), polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polylactic acid (PLA), acrylonitrile butadiene styrene (ABS), polymaleic anhydride-alt-1-octadecene (PMA), and polynitriles.^39–44 Aside from these, amorphous thermoplastics such as cyclic olefin copolymers (COCs) are known to be moisture-resistant^45,46 and are transparent to wavelengths beyond 300 nm^47,48 for about 88%, as shown in Fig. 1(a). Figure 1(a) shows the light transmission spectrum of the COC film as found in the manufacturer’s datasheet.⁴⁸ Experimentally, COC was found to be transparent to wavelengths between 200 and 1000 nm by 91%, as shown in Fig. 1(b). This was derived from the reflectance measurements of COC on a quartz substrate. The reflectance measurements were obtained using a commercial ultraviolet (UV)–visible light spectrophotometer. It also has a refractive index of 1.5, making it optically compatible with III-V optoelectronic materials used in solar cell fabrication. Solar cell devices are designed to absorb UV, visible, and infrared (IR) wavelengths from the sun and are made of III-V semiconductors typically with refractive indices of more than 2.0 for III-V materials and more than 1.5 for II-VI materials.⁴⁹ Because of its suitable optical properties and its capability to protect perovskite materials from moisture, this amorphous thermoplastic film was used as the ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD encapsulant by mixing.

Fig. 1

The transmission spectrum of TOPAS^® COC, which was used in the PQD-COC mixture (a) from the manufacturer’s specification sheet⁴⁸ and (b) from reflectance measurement.

COC pellets dissolved in cyclohexane (${\mathrm{C}}_6{\mathrm{H}}_{12}$) were added to the synthesized ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD solution before substrate dip coating. In this work, about $50\text{-}\mathrm{mg}/\mathrm{mL}$ COC dissolved in ${\mathrm{C}}_6{\mathrm{H}}_{12}$ by ultrasonic bath were mixed with at least $10\text{-}\mathrm{mg}/\mathrm{mL}$ ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQDs dispersed in hexane (${\mathrm{C}}_6{\mathrm{H}}_{14}$) to form moisture-robust PQD films on a target substrate. Considering ${\mathrm{C}}_6{\mathrm{H}}_{12}$ and ${\mathrm{C}}_6{\mathrm{H}}_{14}$ volatilities, only a rough estimate of the solution concentration at the time of preparation can be made, based on the measured masses of the pure COCs and ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQDs and the injected volumes of ${\mathrm{C}}_6{\mathrm{H}}_{12}$ and ${\mathrm{C}}_6{\mathrm{H}}_{14}$, respectively. The COC in ${\mathrm{C}}_6{\mathrm{H}}_{12}$ and PQD in ${\mathrm{C}}_6{\mathrm{H}}_{14}$ solutions were mixed at a 1:1 proportion before dip coating the III-V MJSC samples. Hence, the approximate concentrations of each solution in the mixture were effectively halved. Concentrations of COC in ${\mathrm{C}}_6{\mathrm{H}}_{12}$ and PQD in ${\mathrm{C}}_6{\mathrm{H}}_{14}$ became 25 and $5\mathrm{mg}/\mathrm{mL}$, respectively. The MJSCs dipped into the PQD-COC solution were commercial, third-generation, terrestrial grade $0.31{\mathrm{cm}}^2$ InGaP/GaAs/Ge 3JSCs samples. These samples were mounted on copper plate and were partially encapsulated. Each of the samples is connected to a bypass diode.

In the past, it was found that the optimal withdrawal speed of GaAs, a III-V material, to produce optically smooth and emissive films was $5\mathrm{mm}/\mathrm{s}$, where the values tested were between 1 and $20\mathrm{mm}/\mathrm{s}$.²⁷ In dip coating, the withdrawal speed is the most critical parameter that affects both the film thickness and its quality.^50,51 On the other hand, the number of dipping cycles was fixed at 5 times.

2.3.

Electro-optical Characterization

The dip-coated MJSC samples were evaluated through a laser beam induced current (LBIC) mapping method at room temperature. Here each subcell was made current limiting and subjected to either LC effect excitation or direct laser irradiation.⁸ These measurements were validated through external quantum efficiency (EQE) and current–voltage ($J-V$) characteristics measurements.

2.4.

Data Analysis

3.1.

Preliminary PQD-COC Deposition on a III-V Substrate

Before application on full III-V MJSC devices, PQD-COC was first deposited on semi-insulated, undoped GaAs substrates. From the single-layer samples, the following were obtained: (1) the PL spectrum, from which the peak emission wavelength was acquired, (2) the qualitative PL under 365-nm UV light to augment the PL spectroscopy results, (3) the surface roughness, and (4) the thickness of PQD-COC films.

The PL spectrum of the ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD without and with COC are shown in Figs. 7(a) and 7(b) (Appendix B), respectively. As shown in these images, the peak emission wavelength of the PQD on GaAs is about 680 nm. In a span of 2 months, the PQD-COC on the GaAs sample exhibited less PL intensity degradation. This suggests that the COC preserved the PL properties of the ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQDs under ambient air conditions. Moreover, the UV PL from the PQD-COC film on GaAs was observed to be more emissive than the sample without COC. The PQD films without (left) and with COC (right) under white room light and under 365-nm UV PL are shown in Figs. 9(a) and 9(b) (Appendix B), respectively.

The surface roughness of the ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD on GaAs without and with COC was obtained through atomic force microscopy (AFM) imaging, as shown in Fig. 10. The measurements of the surface roughness of the samples without and with COC were 9.79 and 1.33 nm, respectively. Meanwhile, the average thicknesses of the PQD film on GaAs without and with COC were found to be 1.021 and $1.089\mu \mathrm{m}$, respectively. These values were obtained through the stylus profilometer measurements shown in Fig. 11 (Appendix B).

More details on the optical and surface characterization of PQD-COC on GaAs can be found in Appendix B.

3.2.

Current Collection and Homogeneity

To determine the effect of adding PQD-COC on InGaP/GaAs/Ge 3JSCs, the LBIC maps were acquired from each subcell before and after dip coating 5 times, with GaAs being the limiting subcell in some cases.^53,54 Figure 2 shows the simplified illustration of the 3J sample with PQD-COC film on the front surface. The series of LBIC maps before and after PQD-COC DC are shown in Fig. 3. Also the $J_{x,\mathrm{ave}}$ and ${\sigma }_x$ values together with their percent differences before and after PQD-COC DC are summarized in Tables 3 and 4, respectively.

Fig. 2

InGaP/GaAs/Ge 3JSC with PQD-COC film on the front surface formed after 5 times of dip coating.

Fig. 3

LBIC measurements acquired before and after PQD-COC dip coating by (a), (b) direct InGaP excitation using a 450-nm laser; (c), (d) GaAs LC effect excitation using 450-nm laser; (e), (f) direct GaAs excitation using a 785-nm laser; (g), (h) Ge LC effect excitation using a 785-nm laser; and (i), (j) direct Ge excitation using a 1064-nm laser, respectively.

Table 3

Summary of the average current densities Jx,ave, before and after PQD-COC dip coating and their percent differences %ΔJx,ave. Here positive %ΔJx,ave indicates current collection improvement.

Table 4

Summary of the standard deviations of the current collection σ before and after dip coating and their percent differences %Δσ. Here negative %Δσ indicates current homogeneity improvement after PQD-COC dip coating.

Comparing the LBIC maps acquired through direct InGaP excitation before [Fig. 3(a)] and after [Fig. 3(b)] dip coating the 3J sample into the PQD-COC solution, the current collection in InGaP decreased after dip coating by 13.5%, based on the $J_{\mathrm{InGaP},\mathrm{ave}}$ values calculated from LBIC maps acquired. Furthermore, ${\sigma }_{\mathrm{InGaP}}$ increased after dip coating by 11.4%, indicating spatial current uniformity degradation. This suggests that the PQD-COC film was able to absorb some portion of the incident light before it reached the top cell.

As for the GaAs LC and direct 785-nm laser excitation, 12.3% and 7.5% increases in the current collection were observed after dip coating [Figs. 3(d) and 3(f)], as compared with the set of LBIC maps before dip coating [Figs. 3(c) and 3(e), respectively]. These results show that the PQD with COC was able to re-emit light toward GaAs. The PQD-COC deposition also improved the homogeneity of GaAs current collection by 19.2% [LC effect] and 21.0% [direct 785 nm]. Meanwhile, 6.6% and 20.2% current collection increases and 5.54% and 5.66% current homogeneity improvements were observed from Ge LC and direct 1064-nm laser excitation, respectively. Because the light absorption in the GaAs middle cell was enhanced by the light emitted from the deposited PQD-COC film, the LC effect between GaAs and Ge subcells became stronger. This was reported in past literature that specifically tackled the LC effect between these subcells in InGaP/GaAs/Ge 3JSC devices.^7–9,14,15

Collectively, the results from the LBIC measurements suggest that dip coating PQD on full III-V MJSCs is a potential solution for both current matching and homogenizing subcell current collection.

3.3.

Subcell EQE Measurements

The trend observed from the LBIC measurements shown in Fig. 3 was investigated further by comparing the subcell EQE measurements before and after PQD-COC dip coating. These measurements are shown in Fig. 4. Although the InGaP EQE decreased, some increase in the longer wavelength region of GaAs and an increase in Ge EQE were observed. The decrease of the InGaP EQE is inferred as the decrease in InGaP current density. On the other hand, the increases in both GaAs and Ge EQE measurements after PQD-COC dip coating indicate an increase in GaAs and Ge current densities. These were numerically confirmed by the calculations of $J_{\mathrm{AM}1.5\mathrm{G},i}$ from the subcell EQE measurements using Eq. (1). These are summarized in Table 5. Before dip coating, the current-limiting cell was found to be the GaAs middle cell, having the lowest current density value of $10.3\mathrm{mA}/{\mathrm{cm}}^2$ among the subcells. After dip coating the InGaP/GaAs/Ge 3JSCs into the 680-nm PQD-COC solution, EQE measurement and $J_{\mathrm{AM}1.5\mathrm{G},i}$ calculations revealed that photon re-emission from the PQD-COC film toward the middle cell occurred. These results then confirm that controlling current matching among subcells is possible through PQD-COC film deposition on full III-V MJSCs. On the other hand, the EQE increase in the Ge bottom cell further suggests that the increased absorption in the GaAs middle cell has led to a stronger LC effect between GaAs and Ge subcells.

Fig. 4

Subcell EQE measurements from InGaP/GaAs/Ge 3JSC before (dotted lines) and after PQD-COC deposition (solid lines) by dip coating (DC) 5 times.

Table 5

Integrated current density JAM1.5G,i values calculated from the subcell EQE measurements.

3.4.

Impact on the 1-sun J–V Characteristics

To determine the impact of adding ${\mathrm{CsPbX}}_3$ PQDs with the moisture encapsulant on the net performance of the MJSCs, $J\text{–}V$ characteristics were obtained across the sample terminals before and after PQD-COC deposition on full InGaP/GaAs/Ge 3JSC devices at 1 sun. The $J\text{–}V$ characteristics measurement at AM 1.5G, 1 sun condition, showed a decrease [Fig. 5], confirming further that InGaP became the current limiting cell. This decrease, which corresponds to its EQE and $J_{\mathrm{AM}1.5\mathrm{G},\mathrm{InGaP}}$ decreases, implies that the thickness of the PQD film is not yet optimum. Hence, optimizing the PQD-COC film thickness to be applied on current-mismatched MJSC devices could be a step closer to improved MJSC conversion efficiency in the bigger picture.

Fig. 5

One sun, AM 1.5G light $J\text{–}V$ characteristic curves before (dotted lines) and after PQD-COC deposition (solid lines) by dip coating (DC) 5 times.

3.5.

PQD-COC Deposition as an Additional Step in III-V MJSC Device Fabrication

Reflectance measurements of PQD on GaAs samples with and without COC solution added to the ${\mathrm{CsPbX}}_3$ PQD solution were obtained. This was done to determine if the refractive indices $n$ of the films are larger or smaller than those of the III-V materials typically used as solar cell absorbers. Details on how to qualitatively determine $n$ can be found in Appendix C.

Figure 6 shows the reflectivity spectra of a GaAs substrate dipped 5 times on a PQD-COC solution, together with those of HCl-treated, bare GaAs substrate and those from the published spectrally resolved $n$ and extinction coefficient $k$ values of InGaP⁵⁵ and GaAs.⁵⁶ The spectra were calculated using Eq. (6). The PQD-COC film had a larger $R$, and therefore a larger $n$, than silicon nitride (${\mathrm{SiN}}_x$). ${\mathrm{SiN}}_x$ is typically used as an antireflection coating (ARC) for solar cells, with $n$ value being less than those of the III-V materials. Considering light refraction, the active layers of an MJSC should be stacked from the lowest refractive index to the highest, following the direction of the incident light. Hence, it can be inferred from Fig. 6 that the PQD-COC film can be deposited after growing the III-V stack but before the deposition of an ARC such as ${\mathrm{SiN}}_x$.⁵⁷

Fig. 6

Reflectivity spectra derived from GaAs substrates dip coated 5 times on PQD solutions with and without COC added. These are plotted together with the reflectivity of InGaP,⁵⁵ GaAs,⁵⁶ and ${\mathrm{SiN}}_x$,⁵⁷ derived from optical constants, $n$ and $k$. Plots with * on their labels were obtained from previous literature, while plots with ** were acquired experimentally.

Alternately, if the MJSC is already fabricated, the limiting subcell current should be determined first. Then an optimum PQD-COC film thickness should be fabricated, as recommended earlier.

5.1.

Part 1: 100-mL Cs-oleate Precursor Flask

5.2.

Part 2: CsPbI_2.5Br_0.5 PQD Synthesis

5.3.

Part 3: Synthesized CsPbI_2.5Br_0.5 PQD Wash

5.4.

Part 4: Setting the PQD Solution Concentration

6.1.

Photoluminescence Spectroscopy of PQD-COC on GaAs Substrate

PL spectroscopy measurements were obtained for ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQDs on GaAs substrates between 500 and 950 nm. In the PL measurement setup, a continuous 532-nm laser was used to excite the PQDs on GaAs samples, having it pass through an IR-absorbing filter, a transmission filter, and a series of mirrors and a beam splitter. The output power of the laser before it passes through the mentioned optical elements ranges from 60 to 70 mW. The spot size of the 532-nm laser is about $100\mu \mathrm{m}$. The transmissivity of the filter used was 10% and the exposure time of the charge-coupled device camera detector was fixed at 20 ms. These settings were used such that PL intensities of the samples would not saturate the detector. Also the grating and the slit width of the photodetector were fixed at $150\text{gratings}/500\mathrm{nm}$ and $50\mu \mathrm{m}$, respectively. To compare the PL emission spectra appropriately, the samples were mounted alongside each other such that the mount was only moved along one axis. Then each sample was excited approximately at its center.

PL emission spectra were obtained at different times after dip coating GaAs substrates five times into ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD solutions without [Fig. 7(a)] and with COC [Fig. 7(b)], simultaneously. It was observed that the PL emission intensities were larger in samples with no COC than those from substrates with COC within 48 h after dip coating. In this instance, the samples had experienced an average of 60% humidity. The larger PL from samples without COC may be attributed to the lower volatility of the PQD-COC solution than the PQD solution without COC, that is, the latter’s solvent evaporated faster than that of the PQD-COC. As a result, the PQD solution without COC may have had a larger effective concentration than the PQD-COC solution, which then could have allowed a larger number of PQDs to be deposited on the GaAs substrates. Hence, a higher PL was observed within the spot area of the 532-nm excitation laser of the PQD on the GaAs sample. However, after a week to 2 months of experiencing average humidity between 56% and 57%, the PQD-COC on the GaAs sample exhibited less variation in PL emission, whereas the PQD on the GaAs sample showed a decrease in PL emission over time. From these results, it may be inferred that the COC was able to protect the PQDs from ambient moisture, and without COC, the PQD film oxidized overtime under ambient humidity. Hence, COC, a hydrophobic material, can act as a moisture barrier that may preserve the desired optical properties of the ${\mathrm{CsPbX}}_3$ PQDs.

Fig. 7

PL emission spectra at 532-nm laser excitation upon (a) ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD on GaAs and (b) ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD-COC on GaAs after dip coating 5 times and being kept under average (ambient) humidity of 56% to 60% for different lengths of time. Emission peaks labeled with numbers 1, 2, and 3 correspond to those from PQDs having about 10-nm diameter, 20-nm diameter, and from GaAs substrates, respectively.

The addition of the COC solution to the PQD solution before substrate dip coating did not alter the peak emission wavelength ${\lambda }_{\text{peak},1}$, which was around 680 nm [Table 6]; hence, the COC did not cause regressive effects on the ${\lambda }_{\text{peak}}$ of the PQDs. On the other hand, a blue shift of about 10 nm was observed from ${\lambda }_{\text{peak},1}$ of the PQD sample without COC after 2 months. This could be due to the oxidation of the film, leading to film volume reduction as observed in quantum dots intended for live cells.⁵⁸ The expected PQD sizes $D_{\mathrm{PQD}}$ were calculated using an empirical formula⁵⁹

Table 6

Summary of peak PL intensities λpeak obtained from CsPbI2.5Br0.5 PQD on GaAs samples without and with COC.

Table 7

Summary of estimated PQD diameters DPQD based on the measured λpeak of CsPbI2.5Br0.5 PQD on GaAs samples without and with COC.

Although it was not expected, a weak secondary peak emission, ${\lambda }_{\text{peak},2}$, appeared between 760 and 840 nm wavelengths in all logarithmic PL spectra plots measured from both samples, as shown in Fig. 7. This could be attributed to the PQDs that formed aggregates during dip coating, making the effective dot size in some areas of the substrate be about 20 nm. The sizes of the PQD aggregates based on the measured ${\lambda }_{\text{peak},2}$ ($D_{\mathrm{PQD},2}$) are summarized in Table 7. These were confirmed by scanning electron microscopy (SEM) imaging of ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQDs on the GaAs substrate shown in Fig. 8. During the counting, the SEM image was partitioned into $4\times 6$ parts or a total of $400\times 600{\mathrm{nm}}^2$. A box plot of the number of dots per area was provided [Fig. 8(c)]. Here the dots were clustered as either having a diameter of $\sim 10$ or 20 nm.

Fig. 8

SEM images of ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD film on undoped, semi-insulated, GaAs substrate at (a) $\times 200\mathrm{k}$ magnification and (b) $\times 500\mathrm{k}$ magnification of the region enclosed in the white box in (a). The sample was withdrawn at a speed of $5\mathrm{mm}/\mathrm{s}$ from a tank containing a ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD solution for 5 dip cycles. (c) Box plot of the number of PQDs having different sizes for every $100\times 100{\mathrm{nm}}^2$ area observed.

PL peak emission from the GaAs regime of all samples ${\lambda }_{\text{peak},3}$ was observed between 800 and 900 nm. This was weak compared with the ${\lambda }_{\text{peak},1}$ because there was no carrier confinement structure⁶⁰ or a passivation layer^61–70 that may assist absorption or emission grown on the GaAs substrate. Hence, improving the interface between the PQD film and the GaAs substrate⁷¹ may increase the PL intensity in the latter.

To further investigate the PL observations, the samples were subjected to 365-nm UV irradiation in a dark room within 48 h after DC. Figures 9(a) and 9(b) show the PQD films without and with COC under white room light and under a 365-nm UV lamp, respectively. Upon exposing the samples under the 365-nm UV lamp at room temperature with 49% humidity on the day of dip coating (December 6, 2019), the PL emission from the PQD-COC films were observed to be much stronger as compared with samples having PQD films only. Comparing with the PL spectra obtained through 532-nm laser excitation within 48 h after dip coating, the UV-PL images suggest that the PL emission strength of PQD films have an excitation wavelength dependence. Nevertheless, the UV-PL images confirm the effectiveness of COC in protecting the optical performance of ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQDs under an environment with high-humidity and high-light energy exposure.

Fig. 9

(a) ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD/GaAs and ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD-COC/GaAs samples under white room light. (b) PL emission of ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD/GaAs and ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD-COC/GaAs samples under a 365-nm UV lamp, within 48 h after deposition.

6.2.

Surface Morphology

The surface morphology of ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQDs deposited on GaAs substrates was evaluated through AFM imaging. During imaging, a $3\mu \mathrm{m}\times 3\mu \mathrm{m}$ surface approximately within the center of the sample was observed. From these measurements, the root-mean-square values of the PQD film surface roughness $S_q$ were obtained. This parameter was interpreted depending on the expected PQD nanocrystal size $D_{\mathrm{PQD}}$, which can be predicted using Eq. (4). If the $S_q$ value acquired was less than or equal to $D_{\mathrm{PQD}}$, the deposited film was considered optically smooth. Also a lower $S_q$ value indicates better PQD deposition uniformity.

To compare the surface morphologies of the samples with and without the COC solution added to the PQD solution, AFM images were obtained from PQD films without and with COC on GaAs. These images are shown in Figs. 10(a) and 10(b), respectively. The one with COC garnered a lower $S_q$ of 1.33 nm; hence, it is optically smoother than the one without COC ($S_q=9.79\mathrm{nm}$). In this aspect, PQD films with COC are more desirable than those without COC.

Fig. 10

AFM images of (a) ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD on GaAs and (b) ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD-COC on GaAs after dip coating 5 times and kept under an average (ambient) humidity of 56% to 60%. These images were obtained within 48 h after 5 times of dip coating.

6.3.

Film Thickness Measurement

The thicknesses of ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD films on GaAs substrates were measured using a stylus profilometer. These were measured at a scanning resolution of $0.167\mu \mathrm{m}$ using a $12.5\text{-}\mu \mathrm{m}$-radius stylus set to 2-mg scanning force. The average thicknesses of the PQD film on GaAs without and with COC were 1.021 and $1.089\mu \mathrm{m}$, respectively. The average values were acquired from the height profile shown in Fig. 11 after a leveling correction was applied. These measurements were obtained 3 days after dip coating.

Fig. 11

Height profiles of ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD on GaAs (dotted line) and ${\mathrm{CsPbI}}_{2.5}{\mathrm{Br}}_{0.5}$ PQD-COC on GaAs (solid line) after DC 5 times and being kept under an average (ambient) humidity of 56% to 60% acquired through stylus profilometry. These profiles were obtained 3 days after DC.