*2.6. Characterization*

A transmission electron microscope (TEM) (JEM 2010F, JEOL Ltd., Tokyo, Japan) was used for morphology and size analysis of CQDs. Fluorescent measurements were performed using a Cary Eclipse Fluorescence Spectrophotometer (Agilent Technologies Inc., Santa Clara, CA, USA). Transmittance measurements were performed using a Lambda 1050 spectrophotometer (Perkin Elmer Inc., Waltham, MA, USA) equipped with an integrating sphere. The *J–V* characteristics of the devices were measured by using a Keithley 2400 sourcemeter (Tektronix Inc., Beaverrton, OR, USA) under an AAA class ORIEL Sol3A solar simulator (Newport Corporation, Irvine, CA, USA) equipped with an Air Mass (AM) 1.5 filter at 100 mW/cm2. The light intensity was calibrated by a Si reference cell from Newport (Newport Corporation, Irvine, CA, USA).

#### **3. Results and Discussion**

The TEM analysis of CQD samples confirmed a narrow size distribution in the range of 3–7 nm (Figure 2a). The down-converting property of the CQD@PMMA films was tested using fluorescence spectroscopy. For this, freshly made CQD@PMMA solution was spin coated on thin microscope glass slides and fluorescence emission spectra of the samples were recorded. The emission spectra of the coatings were tested at the excitation wavelength of 320 nm. As shown in Figure 2b, the emission spectra of a CQD@PMMA film shows a distinct peak centered at 430–440 nm, which is attributed to a typical emission range of CQDs [7]. The emission spectra of a neat PMMA film deposited on a thin microscope glass slide were also recorded for comparison. The absence of an emission peak in the neat PMMA film indicates that the emission peak in the CQD@PMMA film originates from the CQDs embedded in the PMMA matrix.

**Figure 2.** (**a**) TEM image of prepared carbon quantum dots (CQDs), and (**b**) fluorescence emission of neat poly(methyl methacrylate) (PMMA) and CQD@PMMA coatings deposited on thin microscope glass slides.

Figure 3 shows the transmittance spectrum of neat PMMA and CQD@PMMA films deposited on thin microscope glass slides for the measurement range of 300–800 nm. The CQD@PMMA film has lower transmittance in the near-UV region (<350 nm) compared to the neat PMMA film, which suggests absorption of UV light by the CQDs embedded in the PMMA matrix.

**Figure 3.** Transmittance of neat PMMA and CQD@PMMA coatings on thin microscope glass slides.

In the next stage, CQD@PMMA coatings were applied to PSCs in order to evaluate their ability to enhance the light harvesting efficiency of the devices. For this, a PMMA/chlorobenzene mixture with dispersed CQDs was spin-coated (30 s at 3000 rpm) on the illumination side of prefabricated PSCs. The thickness of the CQD@PMMA coating was around 480 nm (Figure 4a). To evaluate the effect of CQD@PMMA coating on device performance, *J–V* characteristics of PSCs before and after their application were measured. Figure 4b shows a comparison of *J–V* curves, both forward and reverse, for devices before and after application of a CQD@PMMA coating. The active area of the device was

0.075 cm2. The *J–V* curves show that the device with the CQD@PMMA coating resulted in a higher photocurrent (*JSC*) compared to the device without the CQD@PMMA coating. The relative increase for *JSC*, which was around 3%, was consistent with the transmittance measurements presented in Figure 3. The *J–V* curves also show that there was a notable enhancement (around a 6% relative increase) in fill factor (*FF*), and some minor improvements (around a 1% relative increase) in open-circuit voltage (*VOC*) after the application of the CQD@PMMA layer on the device (see Table 1).

Control experiments were also performed, in which the effect of neat PMMA coating on the light harvesting ability of PSCs was investigated. For this, we obtained the *J–V* characteristics of PSCs before and after the application of a neat PMMA coating (deposited via spin-coating for 30 s at 3000 rpm). This was done in order to separate and estimate the effect of the PMMA layer alone on the performance of the devices. Comparison of the *J–V* curves for a device before and after application of the neat PMMA coating shows that the PMMA coating alone had a negligible or no effect on the performance of the device (Figure 4c). This indicates that the device performance enhancement observed in the case of CQD@PMMA coating indeed originates from the CQDs embedded in the PMMA matrix—i.e., due to the down-conversion effect of CQDs, effectively converting UV to visible light, affording more photons to pass into the perovskite photo-absorber layer, and, thus, generating more charge carriers.

The external quantum efficiency (EQE) measurements were also performed on PSCs to obtain their optical spectral response before and after application of CQD@PMMA coating. Figure 4c shows EQE spectra of a device measured before and after application of a CQD@PMMA coating. The comparison of EQE spectra indicates that after deposition of the CQD@PMMA coating, the EQE response of the device in the blue region has improved [8–10]. The calculated *JSC* from EQE spectra obtained before and after deposition of the CQD@PMMA coating results in values of 16.31 mA cm<sup>−</sup><sup>2</sup> and 17.14 mA cm<sup>−</sup>2, respectively, which exhibits the consistent improvement in *JSC* for the device after deposition of the CQD@PMMA coating. It is noteworthy that an ~26%–28% discrepancy was observed between the calculated *JSC* from the EQE spectra and the measured *JSC* from the solar simulator. The underestimation of EQE commonly happens in PSCs. This can be due to the barrier for the photocurrent of PSC becoming dominant under the monochromatic illumination with low light intensity during the EQE measurement, while the barrier will be alleviated by photodoping of the perovskite under the strong illumination. Nevertheless, the enchantment trend, as determined from EQE, is consistent with the trend of the *J–V* characteristics measured under 1 sun illumination.

Finally, we investigated the dependence of the PSC performance on the thickness of the CQD@PMMA coating. For this, we deposited a CQD@PMMA coating on the glass side of PSCs at various spinning speeds. The performance of the devices was measured before and after application of the CQD@PMMA coating, and relative changes in *JSC*, *FF*, and *VOC* and the efficiency of the devices were investigated. The relative change for photovoltaic parameters is given as a percentage; i.e., it indicates to what extent a certain photovoltaic parameter is changed (improved or worsened) after the deposition of CQD@PMMA coating.

Figure 5 shows the dependence of relative change in the photovoltaic parameters on the spinning speed used to deposit CQD@PMMA coating on PSCs. In order to verify the reproducibility of the PSCs, at least 15 devices for each spinning speed condition were examined. Similar ranges of deviations in photovoltaic parameters of the devices were observed among all spinning conditions, which were predicted due to the high sensitivity of the device performance to the fabrication ambient. Nevertheless, the trend of the device performance with varying thicknesses of the CQD@PMMA coating can clearly be observed. Figure 5a,b show the device performance parameters extracted from *J–V* curves measured in forward and reverse scan directions, respectively. The curves for device efficiency for forward and reverse scan directions show that there is a gradual increase from a lower spinning speed up to 3000 rpm, and then it drops. This trend, especially for forward scan directions (see Figure 5a), originates from a similar behavior in relative changes of *JSC* and *FF*. The initial increase up to 3000 rpm and the following decrease in the relative change of *JSC* in Figure 5a,b can be attributed to the thickness of the CQD@PMMA coating. Thicker CQD@PMMA coating has many CQD particles, which leads to

transmission losses, while a CQD@PMMA coating that is too thin is not effective in converting UV into visible light. Based on this assumption, we can cautiously sugges<sup>t</sup> that 3000 rpm for deposition of CQD@PMMA coating, yielding a thickness of around 480 nm, is the optimal value for spinning speed. Meanwhile, a similar dependence of *FF* on spinning speed—i.e., an initial increase up to 3000 rpm and a subsequent decrease at a higher spinning speed—for both scan directions is not as intuitive to explain. However, a similar trend was reported by Jin et al. [10] in related studies. A possible explanation for this phenomenon could be the UV-filtering effect of CQDs, which prevents the perovskite layer from photodegradation. However, a more detailed investigation of this phenomenon is required and will be the subject of future studies. The relative changes in *VOC* for both scanning directions, however, show very subtle improvements after deposition of CQD@PMMA coating, and slowly decrease with a higher spinning speed, indicating a negligible relative change in *VOC* for thinner CQD@PMMA layers.

**Figure 4.** (**a**) Cross-sectional SEM image of a CQD@PMMA coating deposited onto a fluorine-doped tin oxide (FTO) substrate at 3000 rpm. (**b**) *J–V* characteristics (scan rate 0.5 V s<sup>−</sup>1) of a PSC device before and after application of a CQD@PMMA coating tested under 1 sun illumination and in the dark (blue solid line). (**c**) *J–V* curves of a PSC device before and after application of a neat PMMA layer. (**d**) External quantum efficiency (EQE) spectra of a PSC before and after application of a CQD@PMMA layer.


**Table 1.** Photovoltaic parameters for a PSC device before and after application of a CQD@PMMA layer tested under 1 sun illumination.

**Figure 5.** Dependence of relative change in the photovoltaic parameters on the spinning speed used to deposit a CQD@PMMA coating on PSCs. Key photovoltaic parameters extracted from *J–V* curves measured in (**a**) forward and (**b**) reverse scan directions.
