# Colloidal Quantum Dot:Organic Ternary Ink for Efficient Solution-Processed Hybrid Solar Cells

Authors: Hoon-Seok Jeong; Dongeon Kim; Seungin Jee; Min-Jae Si; Changjo Kim; Jung-Yong Lee; Yujin Jung; Se-Woong Baek
Journal: International Journal of Energy Research
 (2023)

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## Abstract

The fabrication of heterostructuresvia solution process is one of the essential technologies for realizing efficient advanced-generation optoelectronics. Hybrid structures comprising colloidal quantum dots (CQD) and organic semiconducting molecules are garnering considerable research interest because of their complementing optical and electrical properties. However, blending both the materials and forming a stable electronic ink are a challenge owing to the solubility mismatch. Herein, a CQD:organic ternary-blended hybrid solar ink is devised, and efficient hybrid solar cells are demonstrated via single-step spin coating under ambient conditions. Specifically, the passivation of the benzoic acid ligand on the CQD surface enables the dissolution in low-polar solvent such as chlorobenzene, which yields a stable CQD:organic hybrid ink. The hybrid ink facilitates the formation of favorable thin-film morphologies and, consequently, improves the charge extraction efficiency of the solar cells. The resulting hybrid solar cells exhibit a power conversion efficiency of 15.24% that is the highest performance among all existing air-processed CQD:organic hybrid solar cells.


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## Body

## 1. Introduction

Synthesis semiconductors using solution processes have received considerable attention due to the scalable manufacturing, low cost, and high yield [1, 2]. In addition to synthesis, the resultant semiconductor ink can be applied using various printing technologies such as roll-to-roll, spray, blade, and inkjet coating [3–5]. The emerging solution-processed semiconducting materials including organic molecules [6–10], colloidal quantum dots (CQD) [11, 12], and perovskites [13, 14] are major resources of future generation solar cells. All materials have been actively studied in terms of weight, cost, and flexibility/stretchability. In addition, hybrid structures of materials such as perovskite:CQD [15–19], CQD:organic [19–25], and perovskite:organic [26, 27] have been explored to utilize the advantages of both materials and thereby improve the device efficiency.Among them, CQD:organic hybrid structure has garnered considerable research interest owing to their complementary physical properties. CQD offers an advantage to tune its bandgap across the visible to infrared spectrum. However, it suffers from the unique absorption feature of CQD structure, the absorption dip near the first 1S excitonic peak [28]. This originates from a discrete atomic-like energy state of CQD. The reduced energy states near the band-edge result a low extinction coefficient, thereby forming an absorption dip near the excitonic peak [29, 30]. Although organic molecules display strong and distinctive absorption features, their absorption window is narrow. Therefore, several past studies have attempted to fabricate efficient CQD:organic hybrid structures for various optoelectronic applications such as the solar cell [19–21, 23], photodetector [22, 24], and light-emitting diode (LED) [25].Fundamentally, the development of an efficient bulk-heterojunction structure with CQD:organic hybridsvia solution processes is limited by the low exciton diffusion length of organics. However, the methods for fabricating a stable CQD:organic hybrid ink for solution-process have not yet been proposed. Prior research described various alternative methods to fabricate hybrid structures for efficient solar cells, such as the stacked structure or mixed structure using post-layer-by-layer (LBL) process [20, 21, 23, 31–33]. Nonetheless, these strategies are still limited for controlling the morphology of hybrid structures [34, 35]. Thus far, the most suitable CQD:organic hybrid solar cells deliver a power conversion efficiency (PCE) of 13.7%, which is less than that of CQD or organic-based solar cells [21].Herein, we propose a new method for fabricating CQD:organic hybrid ink, blending of CQD, polymer donor, and small-molecule acceptor as a solar ink was successfully determined to develop efficient hybrid structures. We carefully exchanged the CQD surface using benzoic acid (BA) ligand and, thereby, controlled the CQD surface polarity, resulting that the CQD can be completely dissolved in weakly polar solvents. This enables CQD to blend with organic molecules and forms a stable CQD:organic hybrid ink.Based on the CQD:organic ink, efficient hybrid solar cells were fabricated using a single-step spin cast under ambient conditions. The three light-absorbing materials enabled the hybrid device to perform the broadband light absorption, and the ink process facilitated the reduction of the surface roughness, resulting in higher charge extraction properties. Under the AM 1.5 illumination, the power conversion efficiency (PCE) of 15.24% was achieved, which is the most advanced solution-processed CQD:organic hybrid device to date that fabricates in ambient conditions.


## 2. Results and Discussion

### 2.1. Formation of CQD:Organic Ternary Ink

CQD:organic hybrid solar cells have been extensively studied with various polymers and small molecules [19–21, 23]. The major challenge of hybrid solar cell is to form a bulk heterojunction (BHJ) structure using both CQD and organic materials that are typically dissolved in weakly polar solvents such as chloroform (CF) and chlorobenzene (CB). In contrast, ligand-exchanged CQDs tend to be dispersed in high-polar solvents such as dimethylformamide (DMF) because of the nature of short inorganic surface ligands [36]. Therefore, mixing ligand exchanged CQD and organic molecules in a solvent as an ink is not feasible, and it limits the formation of efficient BHJ via solution process. In prior studies, solid-state ligand exchange process of CQDs [37–39] has been used for fabricating the CQD:organic hybrid solar cells. During the solid-state ligand-exchange process, the oleic acid-capped CQD:organic hybrid ink is deposited onto the substrate, followed by the direct ligand exchange process onto the hybrid layer that deteriorates the CQD:organic BHJ morphology. The bilayer structure causes inadequate charge separation and extraction at the CQD:organic interfaces, which degrades the performance.In this study, we directly prepared the CQD:organic hybrid ink based on weakly polar solvent (Figure1(a)). The current strategy is to exchange the OA to benzoic acid (BA) as a surface ligand of PbS CQD instead of the typical halide ligands earlier used for solar cells [40]. The BA-capped CQD (BA-CQD) was observed as well-dispersed in the weakly polar solvents. To form an efficient CQD:organic hybrid ink, we sought various p-type polymers for electron donor and n-type small molecules for electron acceptor with the consideration of complement light absorption and energy alignment for efficient charge transfer. Thus, PBDB-T-2F (PBDB-T-2F = poly[[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]-2,5-thiophenediyl], PM6) was selected as a polymer donor and BTP-4F (BTP-4F = 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile, Y6) as a small-molecule acceptor (Figure 1(b)). The absorption peaks were located at 615 nm for PM6, 828 nm for Y6, and 890 nm for CQD, indicating that three components exhibit complementary absorption (Figure 1(c)). The photographic image prominently depicts the solubility corresponding between BA-CQD and the organic materials (Figure 1(d)). Both halides-CQD and BA-CQD were prepared and mixed with two organic materials (5.5 mg) in CF (3 ml). The BA-CQDs were well-dissolved in the CF with organic molecules without aggregation, whereas halide–CQD:organic hybrid ink remains precipitated at the bottom of the vial (Figure 1(d)), implying that the BA-CQD:organic successfully formed a stable ternary ink. We examined the stability of hybrid ink using UV-vis spectroscopy [41, 42] and confirmed the clear peaks of PM6 and Y6 in hybrid ink even after 120 hours of storage under ambient conditions (Figure S6, Supporting Information).Figure 1
CQD:organic hybrid ternary ink. (a) Schematic of benzoic acid ligand exchange process using PbS CQDs. (b) Molecular structure of PM6 and Y6. (c) Absorbance spectra of PM6 (pink), BA-CQD (brown), and Y6 (blue). (d) Photographic image for comparison between hybrid inks: BA-CQD (left) and halide-CQD (right) with organics.
(a)(b)(c)(d)


### 2.2. Morphology and Physical Properties of CQD:Organic Ternary Films

Surface morphology of BHJ is a critical factor for charge separation and extraction. The atomic force microscopy (AFM) was performed to study the surface morphology, as portrayed in Figures2(a)–2(c). Although hybrid films with ternary ink depict low roughness, the bare organic film without CQD displays a roughness of 3.341 nm in Figure 2(a), which agrees with previous reports [43, 44]. It indicates that the larger grain structures increased the surface roughness without CQD. For comparing the ternary ink and previous solid-state methods [37–39], the OA-CQD:organic hybrid film was spin-casted, followed by solid-state ligand exchange on the CQD:organic film using tetramethyl ammonium iodide (TMAI) dissolved in alcohol (Figure 2(b)). For CQD:organic hybrid films, two methods exhibit a notable difference. The TMAI-exchanged CQD:organic film exhibits the RMS of 3.070 nm, whereas the BA-CQD:organic hybrid film displays the RMS of 2.063 nm, indicating smoother morphology (Figure 2(c)). This result implies that the usage of high-polar solvent such as alcohol during the TMAI postligand exchange causes film morphology variations, yielding higher roughness. On the contrary, the BA-CQD:organic hybrid ink facilitates the formation of high-quality BHJ film because of the postligand exchange-free process.Figure 2
Hybrid ink-based film properties. (a) AFM surface images of5μm×5μm of organic binary film. (b) TMAI-CQD:organic ternary film. (c) BA-CQD:organic ternary film. (d) Normalized absorbance of organic binary film (yellow) and BA-CQD:organic ternary film (brown). (e) UPS spectra of BA-CQD (brown), Y6 (blue), and PM6 (pink), respectively. (f) Schematic energy band alignment of PM6 (pink), BA-CQD (brown), and Y6 (blue).
(a)(b)(c)(d)(e)(f)We further studied the optical properties of the CQD:organic hybrid film using UV-vis spectroscopy. The absorption of the CQD:organic hybrid film, comprising PM6, Y6, and CQD, is displayed in Figure2(d). The strong absorption peaks near 550–630 nm and 750–850 nm were those of polymer and small molecule, respectively. Compared to the bare organic film, the hybrid ternary film exhibited higher absorbance from 850 to 950 nm. Owing to the lower absorption coefficient of CQD at the first excitonic peak compared to that of the organic materials, it did not reveal any distinctive peak. Nonetheless, we verified that the absorbance of this regime increased with the concentration of CQD in the hybrid film, signifying the contribution of CQDs toward the absorption of the hybrid film (Figure S1, Supporting Information).Reportedly, the energy transfer between CQD and organics improves the photocarrier generation and enhances the device performance [20, 45]. Photoluminescence (PL) measurements of a pure PM6 film and a mixture of PM6 and CQD film were performed to verify the Förster resonance energy transfer (FRET) across the ternary system (Figure S2a, Supporting Information). A prior study has confirmed the existence of FRET between two organic molecules [46]. Additionally, further investigation is required for the FRET between CQD and both the organic molecules. Considering the absorption features of each component, the FRET can occur between two organics and CQD (Figure 1(c)). Indeed, for CQD:PM6 film, the PL red shift and quenching were observed within 665–671 nm compared to a bare PM6 film, implying the energy transfer from PM6 to CQD via FRET [20, 47] (Figures S2a and S2b, Supporting Information). Contrarily, the PL of CQD:Y6 did not reveal any spectroscopic evidence of FRET (Figure S3, Supporting Information). The absorption peak of CQD was located at approximately 900 nm, whereas the PL peak position of Y6 was observed at 1000 nm. Therefore, we speculate that this result originates from the low overlap between the CQD absorption and Y6 PL spectra.Furthermore, we explored the energy band alignment of the CQD:organic hybrid film to completely comprehend the charge transfer pathway. Ultraviolet photoelectron spectroscopy (UPS) was performed to identify the Fermi level (EF), and valence band maximum (VBM) (Figure 2(e)) and conduction band minimum (CBM) were determined by the bandgap of each material (Figure 2(f)). We observed that BA-CQD served as an additional donor with p-type configuration, and the VBM (4.93 eV) of the BA-CQD was shallower than the highest occupied molecular orbit (5.66 eV) level of the p-type donor PM6, implying that the BA-CQD can influence a hole transport channel.


### 2.3. Performance of CQD:Organic Hybrid Solar Cells

Thereafter, we investigated the solar cell performance using the BA-CQD:organic hybrid film. We fabricated the solar cell devices with the following device architecture: glass/indium tin oxide (ITO)/zinc oxide (ZnO)/CQD:organic hybrid film/MoO3/Ag (Figure 3(a)). Subsequently, the concentration of BA-CQD was controlled in hybrid film from 0 to 10 wt% to determine an optimized condition. The current density (J)–voltage (V) curves were tested under the illumination of AM1.5 100 mW·cm−2, as depicted in Figure 3(b), and the solar cell parameters are summarized in Table 1. Moreover, the J–V characteristics were determined based on the concentration of BA-CQD, and the best performances were obtained at 5 wt% of BA-CQD hybrid ink. The augmentation of the short-circuit current density (Jsc) and fill factor (FF) in comparison to the hybrid film without BA-CQD yields the maximum PCE of 15.24% with an enhancement of approximately 8.3% (Table 1). Compared to organic only device, hybrid device showed a FF enhancement of 5%, the largest contribution in efficiency increase. This result was the highest PCE among the CQD:organic hybrid solar cells, which have been reported up to date (Figure 3(b) and Table S1, Supporting Information) [20, 23, 31–33, 39, 48–53]. Recently, the bare PM6:Y6-based organic solar cell has delivered a PCE of 17–18% [54–56]. However, the current result was obtained under ambient conditions, which is comparable to the existing CQD:organic hybrid devices [19–21, 23].Figure 3
CQD:organic hybrid solar cell performance. (a) Schematic of hybrid solar cell. (b)J–V curves with respect to various weight ratios of BA-CQDs in hybrid inks. (c) PCE history of CQD:organic hybrid solar cells based on previous studies. (d, e) Photovoltaic parameter dependence with respect to varying weight ratios of BA-CQDs in hybrid inks. (f, g) EQE results under zero-biased (gray), negative-biased (black), and estimated IQE spectra (red dashed) of BA-CQD:organic hybrid device (f) without and (g) with BA-CQD.
(a)(b)(c)(d)(e)(f)(g)Table 1
Solar cell parameters of hybrid solar cells with respect to different concentrations of BA-CQDs.
Voc (V)Jsc (mA/cm2)FF (%)PCE (%)/wo BA-CQD0.839±0.00824.87±0.7366.13±0.9713.72±0.78Hybrids (BA-CQD: 2 wt%)0.851±0.00125.29±0.3968.25±0.5814.66±0.38Hybrids (BA-CQD: 5 wt%)0.851±0.00225.66±0.1569.36±0.4915.11±0.13Hybrids (BA-CQD: 10 wt%)0.830±0.00524.63±0.3867.26±1.2013.33±0.35Best cell (BA-CQD: 5 wt%)0.85025.6869.8415.24The solar cell parameters relying on the various concentrations of CQDs were investigated as well (Figures3(d) and 3(e)). More specifically, Jsc and FF were enhanced as the concentration increased, but the performances were rather inferior to those of the reference organic film because the BA-CQD concentration was larger than 5 wt%. In the case of Voc, no significant differences were observed up to 5 wt% of BA-CQD, but it decreased at 10 wt% condition. We observed that all the photovoltaic parameters decreased in the 10 wt% condition, signifying that the high concentration of CQD in the hybrid film can hinder the charge transport properties, regardless of the CQD improving the absorption of the hybrid film under this condition (Figure S1, Supporting Information).In addition, spectral quantum efficiency (QE) measurement was performed to further study the origin ofJsc enhancement of CQD:organic hybrid structure (Figures 3(f) and 3(g)). Based on the quantum efficiency measurement, we were able to identify whether the origin of Jsc enhancement was optical or electrical gain. The CQD:organic hybrid cells displayed high external quantum efficiency (EQE) over 80% up to 820 nm, yielding an integrated Jsc of 25.68 mA·cm−2. Without BA-CQD, the QE decreases below 80% from 760 nm to 1000 nm. To verify the higher QE at the longer wavelength region, we carried out IQE measurements, which can be obtained by EQE/absorption. Applied negative voltage enabled the complete depletion of the interior photoactive layer to extract approximately 100% of absorbed photons. Therefore, the QE obtained from the negative bias can be considered the absorption of the device [57, 58]. We applied -2 V to all device conditions and finally obtained an IQE by using EQE (0 V)/EQE (-2 V). For the hybrid device without BA-CQD, the EQE (-2 V) increased from 700 to 900 nm that implied the imperfection of the estimated IQE. In contrast, the hybrid device with 5 wt% of BA-CQDs displays EQE (0 V)/EQE (-2 V) value approx. 1, which indicates that the hybrid device produces an almost perfect charge extraction efficiency. We additionally studied the exciton dissociation efficiency and charge extraction efficiency to understand the origin of IQE enhancement based on the previous works [59, 60]. As displayed in Figure S4, the exciton dissociation efficiency and charge extraction efficiency for hybrid solar cells are 0.99 and 0.87, respectively, which are higher than those of organic-only device (0.95 and 0.79, respectively). This result indicates that the CQD:organic hybrid solar cell shows enhancement of both exciton dissociation and charge extraction efficiency, which coincides with previous QE analysis.


### 2.4. Electrical Properties of CQD:Organic Hybrid Solar Cells

The electrical properties were further explored to determine the significance of BA-CQDs in the CQD:organic hybrid system. Accordingly, we performed the transient photocurrent (TPC) measurements to evaluate the photocarrier dynamics (Figure4(a)). The diminishing photocurrent decay time under short-circuit condition represents an improved charge carrier extraction capability and reduced extraction period [61]. According to the TPC results, 5 wt% of BA-CQDs in the hybrid device reduces the photocurrent decay time from 4.5 μs to 3.1 μs, thereby implying that the BA-CQDs facilitate the carrier extraction and charge transport of the hybrid solar cells. In addition to TPC, the hole transport properties of the hybrid structure were tested using the space–charge limited current (SCLC) method [62, 63]. The hole mobilities have evident continuities with the addition of CQDs. The ternary hybrid device exhibits a greater hole mobility of 9.2×10−5 V−1·cm2·s−1 compared to the hybrid structure without CQD (6.4×10−5 V−1·cm2·s−1) (Figure 4(b)). We confirmed that the addition of CQD improved the morphology of the organic film as shown in Figure 2. It was known that the mobility can increase as the surface roughness of the organic film is smoother [64–67]. Therefore, we concluded that the hybrid film shows enhanced charge mobility, agreeing well with TPC measurement. Moreover, it is well-known that carrier mobility is related to the fill factor of solar cell [68]. Hence, an enhanced mobility can be one of the reasons of a fill factor improvement.Figure 4
Photoelectrical properties of hybrid solar cell. (a) Normalized transient photocurrent spectra of CQD:organic hybrids without (yellow) and with BA-CQD (brown). (b)J–V spectra of CQD:organic hybrids without (yellow) and with BA-CQD (brown) under dark conditions to investigate hole mobilities according to the SCLC method. (c, d) Measured electrical properties under various light intensities: CQD:organic hybrids without (yellow) and with BA-CQD (brown).
(a)(b)(c)(d)Subsequently, we investigated the charge recombination properties of the hybrid device by testing theJsc and Voc under various light intensities (Figure 4(c)). As depicted in the following equation, Voc decreased algebraically as the intensity of light decreased.
(1)V′oc=Voc+nkBTqlnX,where kB, q, T, and X denote the Boltzmann constant, elementary charge, temperature, and light intensity, respectively [69, 70]. Under the low light intensity, trap-assisted recombination loss rates were strongly influenced by the shunt resistance of the device. The ideality factor n can be obtained by calculating Voc against light intensity. The slopes of the semilogarithmic plots of Voc against light intensity for the hybrid solar cell with 5 wt% and 0 wt% were 1.361 kBT/q and 1.473 kBT/q, respectively (Figure 4(c)). This concludes that BA-CQDs reduce the recombination rate and, therefore, facilitate the charge extraction (Figures 4(a) and 4(b)).ForJscvs. light intensity, Equation (2) can be abbreviated as the following equation:
(2)Jsc∝Xα,where α denotes a parameter that can be derived from the slope of log–log plot of Jsc against light intensity. In particular, the α value of an ideal solar cell without the second-order bimolecular recombination is equal to 1. The α values of 0.883 and 0.875 were obtained for the CQD:organic hybrid device with and without BA-CQD, respectively (Figure 4(d)). This corresponds with the aforementioned results that the BA-CQDs improve the charge extraction, reduce the recombination, and enhance the solar cell efficiency [40].


## 2.1. Formation of CQD:Organic Ternary Ink

CQD:organic hybrid solar cells have been extensively studied with various polymers and small molecules [19–21, 23]. The major challenge of hybrid solar cell is to form a bulk heterojunction (BHJ) structure using both CQD and organic materials that are typically dissolved in weakly polar solvents such as chloroform (CF) and chlorobenzene (CB). In contrast, ligand-exchanged CQDs tend to be dispersed in high-polar solvents such as dimethylformamide (DMF) because of the nature of short inorganic surface ligands [36]. Therefore, mixing ligand exchanged CQD and organic molecules in a solvent as an ink is not feasible, and it limits the formation of efficient BHJ via solution process. In prior studies, solid-state ligand exchange process of CQDs [37–39] has been used for fabricating the CQD:organic hybrid solar cells. During the solid-state ligand-exchange process, the oleic acid-capped CQD:organic hybrid ink is deposited onto the substrate, followed by the direct ligand exchange process onto the hybrid layer that deteriorates the CQD:organic BHJ morphology. The bilayer structure causes inadequate charge separation and extraction at the CQD:organic interfaces, which degrades the performance.In this study, we directly prepared the CQD:organic hybrid ink based on weakly polar solvent (Figure1(a)). The current strategy is to exchange the OA to benzoic acid (BA) as a surface ligand of PbS CQD instead of the typical halide ligands earlier used for solar cells [40]. The BA-capped CQD (BA-CQD) was observed as well-dispersed in the weakly polar solvents. To form an efficient CQD:organic hybrid ink, we sought various p-type polymers for electron donor and n-type small molecules for electron acceptor with the consideration of complement light absorption and energy alignment for efficient charge transfer. Thus, PBDB-T-2F (PBDB-T-2F = poly[[4,8-bis[5-(2-ethylhexyl)-4-fluoro-2-thienyl]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl]-2,5-thiophenediyl[5,7-bis(2-ethylhexyl)-4,8-dioxo-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-1,3-diyl]-2,5-thiophenediyl], PM6) was selected as a polymer donor and BTP-4F (BTP-4F = 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile, Y6) as a small-molecule acceptor (Figure 1(b)). The absorption peaks were located at 615 nm for PM6, 828 nm for Y6, and 890 nm for CQD, indicating that three components exhibit complementary absorption (Figure 1(c)). The photographic image prominently depicts the solubility corresponding between BA-CQD and the organic materials (Figure 1(d)). Both halides-CQD and BA-CQD were prepared and mixed with two organic materials (5.5 mg) in CF (3 ml). The BA-CQDs were well-dissolved in the CF with organic molecules without aggregation, whereas halide–CQD:organic hybrid ink remains precipitated at the bottom of the vial (Figure 1(d)), implying that the BA-CQD:organic successfully formed a stable ternary ink. We examined the stability of hybrid ink using UV-vis spectroscopy [41, 42] and confirmed the clear peaks of PM6 and Y6 in hybrid ink even after 120 hours of storage under ambient conditions (Figure S6, Supporting Information).Figure 1
CQD:organic hybrid ternary ink. (a) Schematic of benzoic acid ligand exchange process using PbS CQDs. (b) Molecular structure of PM6 and Y6. (c) Absorbance spectra of PM6 (pink), BA-CQD (brown), and Y6 (blue). (d) Photographic image for comparison between hybrid inks: BA-CQD (left) and halide-CQD (right) with organics.
(a)(b)(c)(d)


## 2.2. Morphology and Physical Properties of CQD:Organic Ternary Films

Surface morphology of BHJ is a critical factor for charge separation and extraction. The atomic force microscopy (AFM) was performed to study the surface morphology, as portrayed in Figures2(a)–2(c). Although hybrid films with ternary ink depict low roughness, the bare organic film without CQD displays a roughness of 3.341 nm in Figure 2(a), which agrees with previous reports [43, 44]. It indicates that the larger grain structures increased the surface roughness without CQD. For comparing the ternary ink and previous solid-state methods [37–39], the OA-CQD:organic hybrid film was spin-casted, followed by solid-state ligand exchange on the CQD:organic film using tetramethyl ammonium iodide (TMAI) dissolved in alcohol (Figure 2(b)). For CQD:organic hybrid films, two methods exhibit a notable difference. The TMAI-exchanged CQD:organic film exhibits the RMS of 3.070 nm, whereas the BA-CQD:organic hybrid film displays the RMS of 2.063 nm, indicating smoother morphology (Figure 2(c)). This result implies that the usage of high-polar solvent such as alcohol during the TMAI postligand exchange causes film morphology variations, yielding higher roughness. On the contrary, the BA-CQD:organic hybrid ink facilitates the formation of high-quality BHJ film because of the postligand exchange-free process.Figure 2
Hybrid ink-based film properties. (a) AFM surface images of5μm×5μm of organic binary film. (b) TMAI-CQD:organic ternary film. (c) BA-CQD:organic ternary film. (d) Normalized absorbance of organic binary film (yellow) and BA-CQD:organic ternary film (brown). (e) UPS spectra of BA-CQD (brown), Y6 (blue), and PM6 (pink), respectively. (f) Schematic energy band alignment of PM6 (pink), BA-CQD (brown), and Y6 (blue).
(a)(b)(c)(d)(e)(f)We further studied the optical properties of the CQD:organic hybrid film using UV-vis spectroscopy. The absorption of the CQD:organic hybrid film, comprising PM6, Y6, and CQD, is displayed in Figure2(d). The strong absorption peaks near 550–630 nm and 750–850 nm were those of polymer and small molecule, respectively. Compared to the bare organic film, the hybrid ternary film exhibited higher absorbance from 850 to 950 nm. Owing to the lower absorption coefficient of CQD at the first excitonic peak compared to that of the organic materials, it did not reveal any distinctive peak. Nonetheless, we verified that the absorbance of this regime increased with the concentration of CQD in the hybrid film, signifying the contribution of CQDs toward the absorption of the hybrid film (Figure S1, Supporting Information).Reportedly, the energy transfer between CQD and organics improves the photocarrier generation and enhances the device performance [20, 45]. Photoluminescence (PL) measurements of a pure PM6 film and a mixture of PM6 and CQD film were performed to verify the Förster resonance energy transfer (FRET) across the ternary system (Figure S2a, Supporting Information). A prior study has confirmed the existence of FRET between two organic molecules [46]. Additionally, further investigation is required for the FRET between CQD and both the organic molecules. Considering the absorption features of each component, the FRET can occur between two organics and CQD (Figure 1(c)). Indeed, for CQD:PM6 film, the PL red shift and quenching were observed within 665–671 nm compared to a bare PM6 film, implying the energy transfer from PM6 to CQD via FRET [20, 47] (Figures S2a and S2b, Supporting Information). Contrarily, the PL of CQD:Y6 did not reveal any spectroscopic evidence of FRET (Figure S3, Supporting Information). The absorption peak of CQD was located at approximately 900 nm, whereas the PL peak position of Y6 was observed at 1000 nm. Therefore, we speculate that this result originates from the low overlap between the CQD absorption and Y6 PL spectra.Furthermore, we explored the energy band alignment of the CQD:organic hybrid film to completely comprehend the charge transfer pathway. Ultraviolet photoelectron spectroscopy (UPS) was performed to identify the Fermi level (EF), and valence band maximum (VBM) (Figure 2(e)) and conduction band minimum (CBM) were determined by the bandgap of each material (Figure 2(f)). We observed that BA-CQD served as an additional donor with p-type configuration, and the VBM (4.93 eV) of the BA-CQD was shallower than the highest occupied molecular orbit (5.66 eV) level of the p-type donor PM6, implying that the BA-CQD can influence a hole transport channel.


## 2.3. Performance of CQD:Organic Hybrid Solar Cells

Thereafter, we investigated the solar cell performance using the BA-CQD:organic hybrid film. We fabricated the solar cell devices with the following device architecture: glass/indium tin oxide (ITO)/zinc oxide (ZnO)/CQD:organic hybrid film/MoO3/Ag (Figure 3(a)). Subsequently, the concentration of BA-CQD was controlled in hybrid film from 0 to 10 wt% to determine an optimized condition. The current density (J)–voltage (V) curves were tested under the illumination of AM1.5 100 mW·cm−2, as depicted in Figure 3(b), and the solar cell parameters are summarized in Table 1. Moreover, the J–V characteristics were determined based on the concentration of BA-CQD, and the best performances were obtained at 5 wt% of BA-CQD hybrid ink. The augmentation of the short-circuit current density (Jsc) and fill factor (FF) in comparison to the hybrid film without BA-CQD yields the maximum PCE of 15.24% with an enhancement of approximately 8.3% (Table 1). Compared to organic only device, hybrid device showed a FF enhancement of 5%, the largest contribution in efficiency increase. This result was the highest PCE among the CQD:organic hybrid solar cells, which have been reported up to date (Figure 3(b) and Table S1, Supporting Information) [20, 23, 31–33, 39, 48–53]. Recently, the bare PM6:Y6-based organic solar cell has delivered a PCE of 17–18% [54–56]. However, the current result was obtained under ambient conditions, which is comparable to the existing CQD:organic hybrid devices [19–21, 23].Figure 3
CQD:organic hybrid solar cell performance. (a) Schematic of hybrid solar cell. (b)J–V curves with respect to various weight ratios of BA-CQDs in hybrid inks. (c) PCE history of CQD:organic hybrid solar cells based on previous studies. (d, e) Photovoltaic parameter dependence with respect to varying weight ratios of BA-CQDs in hybrid inks. (f, g) EQE results under zero-biased (gray), negative-biased (black), and estimated IQE spectra (red dashed) of BA-CQD:organic hybrid device (f) without and (g) with BA-CQD.
(a)(b)(c)(d)(e)(f)(g)Table 1
Solar cell parameters of hybrid solar cells with respect to different concentrations of BA-CQDs.
Voc (V)Jsc (mA/cm2)FF (%)PCE (%)/wo BA-CQD0.839±0.00824.87±0.7366.13±0.9713.72±0.78Hybrids (BA-CQD: 2 wt%)0.851±0.00125.29±0.3968.25±0.5814.66±0.38Hybrids (BA-CQD: 5 wt%)0.851±0.00225.66±0.1569.36±0.4915.11±0.13Hybrids (BA-CQD: 10 wt%)0.830±0.00524.63±0.3867.26±1.2013.33±0.35Best cell (BA-CQD: 5 wt%)0.85025.6869.8415.24The solar cell parameters relying on the various concentrations of CQDs were investigated as well (Figures3(d) and 3(e)). More specifically, Jsc and FF were enhanced as the concentration increased, but the performances were rather inferior to those of the reference organic film because the BA-CQD concentration was larger than 5 wt%. In the case of Voc, no significant differences were observed up to 5 wt% of BA-CQD, but it decreased at 10 wt% condition. We observed that all the photovoltaic parameters decreased in the 10 wt% condition, signifying that the high concentration of CQD in the hybrid film can hinder the charge transport properties, regardless of the CQD improving the absorption of the hybrid film under this condition (Figure S1, Supporting Information).In addition, spectral quantum efficiency (QE) measurement was performed to further study the origin ofJsc enhancement of CQD:organic hybrid structure (Figures 3(f) and 3(g)). Based on the quantum efficiency measurement, we were able to identify whether the origin of Jsc enhancement was optical or electrical gain. The CQD:organic hybrid cells displayed high external quantum efficiency (EQE) over 80% up to 820 nm, yielding an integrated Jsc of 25.68 mA·cm−2. Without BA-CQD, the QE decreases below 80% from 760 nm to 1000 nm. To verify the higher QE at the longer wavelength region, we carried out IQE measurements, which can be obtained by EQE/absorption. Applied negative voltage enabled the complete depletion of the interior photoactive layer to extract approximately 100% of absorbed photons. Therefore, the QE obtained from the negative bias can be considered the absorption of the device [57, 58]. We applied -2 V to all device conditions and finally obtained an IQE by using EQE (0 V)/EQE (-2 V). For the hybrid device without BA-CQD, the EQE (-2 V) increased from 700 to 900 nm that implied the imperfection of the estimated IQE. In contrast, the hybrid device with 5 wt% of BA-CQDs displays EQE (0 V)/EQE (-2 V) value approx. 1, which indicates that the hybrid device produces an almost perfect charge extraction efficiency. We additionally studied the exciton dissociation efficiency and charge extraction efficiency to understand the origin of IQE enhancement based on the previous works [59, 60]. As displayed in Figure S4, the exciton dissociation efficiency and charge extraction efficiency for hybrid solar cells are 0.99 and 0.87, respectively, which are higher than those of organic-only device (0.95 and 0.79, respectively). This result indicates that the CQD:organic hybrid solar cell shows enhancement of both exciton dissociation and charge extraction efficiency, which coincides with previous QE analysis.


## 2.4. Electrical Properties of CQD:Organic Hybrid Solar Cells

The electrical properties were further explored to determine the significance of BA-CQDs in the CQD:organic hybrid system. Accordingly, we performed the transient photocurrent (TPC) measurements to evaluate the photocarrier dynamics (Figure4(a)). The diminishing photocurrent decay time under short-circuit condition represents an improved charge carrier extraction capability and reduced extraction period [61]. According to the TPC results, 5 wt% of BA-CQDs in the hybrid device reduces the photocurrent decay time from 4.5 μs to 3.1 μs, thereby implying that the BA-CQDs facilitate the carrier extraction and charge transport of the hybrid solar cells. In addition to TPC, the hole transport properties of the hybrid structure were tested using the space–charge limited current (SCLC) method [62, 63]. The hole mobilities have evident continuities with the addition of CQDs. The ternary hybrid device exhibits a greater hole mobility of 9.2×10−5 V−1·cm2·s−1 compared to the hybrid structure without CQD (6.4×10−5 V−1·cm2·s−1) (Figure 4(b)). We confirmed that the addition of CQD improved the morphology of the organic film as shown in Figure 2. It was known that the mobility can increase as the surface roughness of the organic film is smoother [64–67]. Therefore, we concluded that the hybrid film shows enhanced charge mobility, agreeing well with TPC measurement. Moreover, it is well-known that carrier mobility is related to the fill factor of solar cell [68]. Hence, an enhanced mobility can be one of the reasons of a fill factor improvement.Figure 4
Photoelectrical properties of hybrid solar cell. (a) Normalized transient photocurrent spectra of CQD:organic hybrids without (yellow) and with BA-CQD (brown). (b)J–V spectra of CQD:organic hybrids without (yellow) and with BA-CQD (brown) under dark conditions to investigate hole mobilities according to the SCLC method. (c, d) Measured electrical properties under various light intensities: CQD:organic hybrids without (yellow) and with BA-CQD (brown).
(a)(b)(c)(d)Subsequently, we investigated the charge recombination properties of the hybrid device by testing theJsc and Voc under various light intensities (Figure 4(c)). As depicted in the following equation, Voc decreased algebraically as the intensity of light decreased.
(1)V′oc=Voc+nkBTqlnX,where kB, q, T, and X denote the Boltzmann constant, elementary charge, temperature, and light intensity, respectively [69, 70]. Under the low light intensity, trap-assisted recombination loss rates were strongly influenced by the shunt resistance of the device. The ideality factor n can be obtained by calculating Voc against light intensity. The slopes of the semilogarithmic plots of Voc against light intensity for the hybrid solar cell with 5 wt% and 0 wt% were 1.361 kBT/q and 1.473 kBT/q, respectively (Figure 4(c)). This concludes that BA-CQDs reduce the recombination rate and, therefore, facilitate the charge extraction (Figures 4(a) and 4(b)).ForJscvs. light intensity, Equation (2) can be abbreviated as the following equation:
(2)Jsc∝Xα,where α denotes a parameter that can be derived from the slope of log–log plot of Jsc against light intensity. In particular, the α value of an ideal solar cell without the second-order bimolecular recombination is equal to 1. The α values of 0.883 and 0.875 were obtained for the CQD:organic hybrid device with and without BA-CQD, respectively (Figure 4(d)). This corresponds with the aforementioned results that the BA-CQDs improve the charge extraction, reduce the recombination, and enhance the solar cell efficiency [40].


## 3. Conclusion

In this study, we demonstrated a CQD:organic ternary ink and displayed high-quality hybrid film with high-performance solar cells. To fabricate ternary blended ink, we performed the ligand exchange of CQD using BA and then dissolved it in weakly polar solvent. Introducing BA-CQD improves hybrid film morphology and charge extraction. Based on this work, the relationship between CQD and polymer hybrid morphology should be studied further as future research. This work led to a maximum PCE of 15.24%, which is the best result among previous CQD:organic hybrid solar cells processed in ambient conditions. This research is valuable, because it can be utilized for various polymer materials with CQD in the future. This indicates that the hybrid blending method for fabricating the CQD:organic BHJ ink in this study provides a new avenue for manufacturing the hybrid solar cells beyond the conventional solid-state layer-by-layer process.


## 4. Experimental Section

### 4.1. Preparation of Benzoic Acid-Capped PbS CQD

The Pb(OA)2 solution was diluted in 80 ml of ODE at 96°C under a vacuum of 2×10−2 Torr. For an injection temperature of 72°C, the solution composed of (TMS)2S (0.851 ml) and an additional ODE (2 ml) was injected. Thereafter, the PbS crude solution was emerged and cooled to 30°C, followed by gradual heating. To clean the residue, 27 ml of acetone was injected into CQDs as an antisolvent and centrifuged at 8000 rpm for 3 min. After washing the antisolvent, 7.5 ml of toluene was injected into CQDs and vortexed for dispersion. In particular, the rinsing processes were repeated 3 times [71]. Subsequently, the resulting PbS CQDs were redispersed in octane for 55 mg·ml−1 that reveals the first excitonic peak at 890 nm. The BA ligand solution was prepared by dissolving the BA ligand (0.3 M) into toluene, and 1 ml of the BA ligand solution was added to 1 ml of OA-CQD solution (20 mg·ml−1 in toluene) with stirring for 10 min. Moreover, 2.5 ml of hexane was added for precipitating the exchanged CQDs and centrifuged at 7000 rpm for 2 min. For purifying the CQDs, the precipitated CQDs were redispersed in 1 ml of CF and 1.5 ml of hexane was added to precipitate the CQDs. The rinsing process was repeated for three instances to ditch the residual ligands and OA byproducts.


### 4.2. Hybrid Device Fabrication

The ITO substrates were cleaned with detergent, deionized water, acetone, and isopropyl alcohol with ultrasonication for 20 min, respectively. Thereafter, the ZnO sol–gel was prepared by dissolving zinc acetate dihydrate (1.0 g) and ethanolamine (0.28 g) in 2-methoxyethanol (10 ml) with stirring for 24 h. After UV–O3 treatment for 15 min, the ZnO sol–gel was deposited onto the ITO glass. Subsequently, the as-prepared CQD:organic ternary inks were spin-casted onto the ZnO-coated ITO glass at 4000 rpm for 30 s. For CQD:organic hybrid inks, PM6:Y6 (ITECH, Seongnam, Korea) (D:A=1:1.2, 11 mg·ml−1 in total) was dissolved in CF with the solvent additive of 1-chloronaphthalene (CN) (0.5% v/v). Ultimately, the polymer solution and CQD were mixed at a ratio of PM6:BA-CQD:Y6 (D:D:A=0.95:0.05:1.2 wt%, 11 mg·ml−1 in total). Furthermore, MoO3 (10 nm) and Ag (120 nm) were thermally evaporated through a patterned mask. For fabrication of the hole-only devices, MoO3 (20 nm) was deposited onto the ITO glass to obstruct the electron transport. Thereafter, the prepared hybrid inks were spin-casted at 4000 rpm for 30 s onto the MoO3-deposited ITO glass. Ultimately, MoO3 (20 nm) and Ag (120 nm) were thermally evaporated through a patterned mask. All processes were performed under the ambient condition.


### 4.3. Spectroscopic Analysis

To recognize PL spectra, PM6 (2.5 mg·ml−1), PM6:BA-CQD 5 wt% (2.5 mg·ml−1 in total), Y6 (3 mg·ml−1), and Y6:BA-CQD 5 wt% (3 mg·ml−1 in total) hybrid inks were prepared and deposited onto slide glasses. The PL characterization was performed using a Horiba Fluorolog system, and the PL spectra were acquired by illuminating the samples using a monochromatic Xe lamp. The excitation wavelength was 450 nm, and the emission was scanned from 600 to 850 nm with 5 nm slit size. The absorbance spectra were measured using a UV-vis-NIR spectrophotometer (Cary 5000, Agilent Technologies). More specifically, the UPS measurement was performed using a helium discharge source (HeI α, hv=21.22 eV) and spectra were obtained at a –5 V bias relative to the spectrometer to efficiently assimilate the low kinetic-energy electrons. The TPC measurements were performed by placing the device across a resistor and biasing the device with the desired voltage and background light intensity. Additionally, a transient light pulse was applied at a wavelength of 532 nm Nd:YAG green laser and the light intensity was 100 W/m2. The transient photocurrent decay was measured using an oscilloscope in parallel with a resistor, and the hole mobilities were evaluated by fitting the J–V curves according to the Mott–Gurney Law.


### 4.4. Device Characterization

TheJ–V characteristics were recorded using a Keithley 2401 source meter under simulated solar light illumination (100 MW·cm−2) generated by a solar simulator equipped with an air mass 1.5G filter (K3000, Class AAA, McScience Inc.). Prior to each measurement, the light intensity was calibrated with a reference cell, and the EQE measurements were conducted using a spectral measurement system (K3100, McScience Inc.). After the EQE measurements under zero bias, the light intensity calibration was performed relative to a reference cell. The EQE spectra under –0.7 V reverse bias progressed after sufficient stabilization with the applied voltage. The effective voltage (Veff) and photocurrent density (Jph) analyses were performed by J-V characteristics under the dark and 1 sun illumination conditions. Jph is defined by current density under 1.5 AM illumination (JL) minus current density under dark condition (JD). Veff is defined by a difference between applied voltage (V) and compensation voltage (V0), which is applied voltage when Jph is zero. Exciton dissociation efficiency and charge extraction efficiency were obtained by Jph∗/Jsat and Jph#/Jsat, respectively. Jph∗, Jph#, and Jsat denote Jph at short circuit condition, maximum power point, and saturated condition, respectively. Between each measurement, the devices were stored in ambient air, and each measurement was progressed via the patterned mask and the aperture size of each device was 0.045 cm2.


### 4.5. Morphology Characterization

To identify the AFM surface morphology, PM6:Y6 (5 mg of PM6 and 6 mg of Y6 in 1 ml CF) and PM6:BA-CQD 5 wt%:Y6 (4.75 mg of PM6, 0.25 mg of BA-CQD, and 6 mg of Y6 in 1 ml CF) inks were spin-casted onto the ITO substrate. The AFM measurements were conducted using XE-100 (Park Systems) in a tapping mode with 1 Hz scan speed. The root mean square (Rms) surface roughness of the films was quantified from their AFM topographic images of 5μm×5μm.


## 4.1. Preparation of Benzoic Acid-Capped PbS CQD

The Pb(OA)2 solution was diluted in 80 ml of ODE at 96°C under a vacuum of 2×10−2 Torr. For an injection temperature of 72°C, the solution composed of (TMS)2S (0.851 ml) and an additional ODE (2 ml) was injected. Thereafter, the PbS crude solution was emerged and cooled to 30°C, followed by gradual heating. To clean the residue, 27 ml of acetone was injected into CQDs as an antisolvent and centrifuged at 8000 rpm for 3 min. After washing the antisolvent, 7.5 ml of toluene was injected into CQDs and vortexed for dispersion. In particular, the rinsing processes were repeated 3 times [71]. Subsequently, the resulting PbS CQDs were redispersed in octane for 55 mg·ml−1 that reveals the first excitonic peak at 890 nm. The BA ligand solution was prepared by dissolving the BA ligand (0.3 M) into toluene, and 1 ml of the BA ligand solution was added to 1 ml of OA-CQD solution (20 mg·ml−1 in toluene) with stirring for 10 min. Moreover, 2.5 ml of hexane was added for precipitating the exchanged CQDs and centrifuged at 7000 rpm for 2 min. For purifying the CQDs, the precipitated CQDs were redispersed in 1 ml of CF and 1.5 ml of hexane was added to precipitate the CQDs. The rinsing process was repeated for three instances to ditch the residual ligands and OA byproducts.


## 4.2. Hybrid Device Fabrication

The ITO substrates were cleaned with detergent, deionized water, acetone, and isopropyl alcohol with ultrasonication for 20 min, respectively. Thereafter, the ZnO sol–gel was prepared by dissolving zinc acetate dihydrate (1.0 g) and ethanolamine (0.28 g) in 2-methoxyethanol (10 ml) with stirring for 24 h. After UV–O3 treatment for 15 min, the ZnO sol–gel was deposited onto the ITO glass. Subsequently, the as-prepared CQD:organic ternary inks were spin-casted onto the ZnO-coated ITO glass at 4000 rpm for 30 s. For CQD:organic hybrid inks, PM6:Y6 (ITECH, Seongnam, Korea) (D:A=1:1.2, 11 mg·ml−1 in total) was dissolved in CF with the solvent additive of 1-chloronaphthalene (CN) (0.5% v/v). Ultimately, the polymer solution and CQD were mixed at a ratio of PM6:BA-CQD:Y6 (D:D:A=0.95:0.05:1.2 wt%, 11 mg·ml−1 in total). Furthermore, MoO3 (10 nm) and Ag (120 nm) were thermally evaporated through a patterned mask. For fabrication of the hole-only devices, MoO3 (20 nm) was deposited onto the ITO glass to obstruct the electron transport. Thereafter, the prepared hybrid inks were spin-casted at 4000 rpm for 30 s onto the MoO3-deposited ITO glass. Ultimately, MoO3 (20 nm) and Ag (120 nm) were thermally evaporated through a patterned mask. All processes were performed under the ambient condition.


## 4.3. Spectroscopic Analysis

To recognize PL spectra, PM6 (2.5 mg·ml−1), PM6:BA-CQD 5 wt% (2.5 mg·ml−1 in total), Y6 (3 mg·ml−1), and Y6:BA-CQD 5 wt% (3 mg·ml−1 in total) hybrid inks were prepared and deposited onto slide glasses. The PL characterization was performed using a Horiba Fluorolog system, and the PL spectra were acquired by illuminating the samples using a monochromatic Xe lamp. The excitation wavelength was 450 nm, and the emission was scanned from 600 to 850 nm with 5 nm slit size. The absorbance spectra were measured using a UV-vis-NIR spectrophotometer (Cary 5000, Agilent Technologies). More specifically, the UPS measurement was performed using a helium discharge source (HeI α, hv=21.22 eV) and spectra were obtained at a –5 V bias relative to the spectrometer to efficiently assimilate the low kinetic-energy electrons. The TPC measurements were performed by placing the device across a resistor and biasing the device with the desired voltage and background light intensity. Additionally, a transient light pulse was applied at a wavelength of 532 nm Nd:YAG green laser and the light intensity was 100 W/m2. The transient photocurrent decay was measured using an oscilloscope in parallel with a resistor, and the hole mobilities were evaluated by fitting the J–V curves according to the Mott–Gurney Law.


## 4.4. Device Characterization

TheJ–V characteristics were recorded using a Keithley 2401 source meter under simulated solar light illumination (100 MW·cm−2) generated by a solar simulator equipped with an air mass 1.5G filter (K3000, Class AAA, McScience Inc.). Prior to each measurement, the light intensity was calibrated with a reference cell, and the EQE measurements were conducted using a spectral measurement system (K3100, McScience Inc.). After the EQE measurements under zero bias, the light intensity calibration was performed relative to a reference cell. The EQE spectra under –0.7 V reverse bias progressed after sufficient stabilization with the applied voltage. The effective voltage (Veff) and photocurrent density (Jph) analyses were performed by J-V characteristics under the dark and 1 sun illumination conditions. Jph is defined by current density under 1.5 AM illumination (JL) minus current density under dark condition (JD). Veff is defined by a difference between applied voltage (V) and compensation voltage (V0), which is applied voltage when Jph is zero. Exciton dissociation efficiency and charge extraction efficiency were obtained by Jph∗/Jsat and Jph#/Jsat, respectively. Jph∗, Jph#, and Jsat denote Jph at short circuit condition, maximum power point, and saturated condition, respectively. Between each measurement, the devices were stored in ambient air, and each measurement was progressed via the patterned mask and the aperture size of each device was 0.045 cm2.


## 4.5. Morphology Characterization

To identify the AFM surface morphology, PM6:Y6 (5 mg of PM6 and 6 mg of Y6 in 1 ml CF) and PM6:BA-CQD 5 wt%:Y6 (4.75 mg of PM6, 0.25 mg of BA-CQD, and 6 mg of Y6 in 1 ml CF) inks were spin-casted onto the ITO substrate. The AFM measurements were conducted using XE-100 (Park Systems) in a tapping mode with 1 Hz scan speed. The root mean square (Rms) surface roughness of the films was quantified from their AFM topographic images of 5μm×5μm.



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*Source: 4911750-2023-02-06.xml*