Dim Indoor Light Photovoltaic Performance Improvement of Y-Series Acceptor via Halogenation of Terminal Group

Kim, Sunghyun; Baek, Ho Eon; Saeed, Muhammad Ahsan; Kim, Tae Hyuk; Chen, Shuhao; Ahn, Hyungju; Lee, Wooseop; Kim, Yun-Hi; Shim, Jae Won

doi:https://doi.org/10.1155/2023/3367579

International Journal of Energy Research

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Authors’ Contributions Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2023 | Article ID 3367579 | https://doi.org/10.1155/2023/3367579

Dim Indoor Light Photovoltaic Performance Improvement of Y-Series Acceptor via Halogenation of Terminal Group

Sunghyun Kim,¹Ho Eon Baek,²Muhammad Ahsan Saeed,¹Tae Hyuk Kim,¹Shuhao Chen,³Hyungju Ahn,⁴Wooseop Lee,⁴Yun-Hi Kim,²and Jae Won Shim¹

Academic Editor: Akshay Kumar Saha

Received06 Mar 2023

Revised28 Mar 2023

Accepted04 Apr 2023

Published21 Apr 2023

Abstract

The widespread use of Internet-of-things (IoT) devices has inspired researchers to adopt unique material design strategies to realize efficient indoor organic photovoltaic (OPV) systems. However, despite acceptor halogenation being an effective strategy for modulating OPV properties, studies on the systematic examination of nonfullerene acceptor- (NFA-) OPVs under dim indoor light using the halogenation approach are scarce. This study evaluates the performance of NFA-OPVs under indoor light by employing a halogenation approach with Y6-derivatives. The choice of the chlorination or fluorination unit in an NFA significantly affects the indoor performance of OPVs. The champion OPV devices with a chlorinated acceptor demonstrated excellent power conversion efficiency (PCE) of 25.5% compared to that of the fluorinated acceptor (PCE: 22.5%) under 1000-lx light-emitting-diode (LED) illumination. Moreover, suitable energy levels, satisfactory spectral matching, and improved surface morphology of the chlorinated acceptors resulted in the excellent indoor performance of the OPVs. In addition, acceptor chlorination resulted in high crystallinity and planarity, which facilitated suppressed trap-assisted recombination and low open-circuit voltage (V_OC) loss of OPV devices in an indoor environment.

1. Introduction

Although inorganic semiconductors such as silicon and III–IV compound semiconducting materials have long been the predominant technology for photovoltaic applications, they possess disadvantages that make them unsuitable for indoor use. The performance of organic photovoltaics (OPVs) has progressively increased with continuous advancements in material synthesis methodologies and the adoption of diverse device structural engineering. OPVs exhibit immense potential as autonomous off-grid energy sources for driving several wireless low-powered Internet-of-Things (IoT) devices [1–4]. Moreover, they have low electrical power requirements (~10 μW–~1 mW) in indoor environments. The prerequisites for high-performance OPVs for indoor applications differ from those under outdoor (1-sun) conditions. This is because indoor light sources, including light-emitting-diodes (LED), fluorescent lamps (FL), and halogen lamps, possess diverse characteristics such as narrowed radiation spectra, reduced light intensities, and various color temperatures, which render power generation in an indoor environment challenging [5]. Owing to the advancements in materials, the power conversion efficiency (PCE) of the emergent nonfullerene acceptor- (NFA-) based OPVs has increased by approximately 31%. However, this efficiency can be increased to an estimated 40% by using a suitable combination of photoactive layer materials [6–8].

The synthesis and design of photoactive materials and an understanding of their interdependence with device performance, molecular structures, and bulk heterojunction (BHJ) morphology are essential for modulating the photovoltaic parameters of high-performance OPVs under indoor lighting conditions [9, 10]. Thus, the halogenation of the terminal group in NFAs can be an effective design strategy for improving the intramolecular charge transfer effect and, consequently, the short-circuit current density (J_SC) of OPVs, as reported previously. For example, Zhao et al. recently developed fluorinated IT-4F (2,2-[[6,6,12,12-tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2,3-d]-s-indaceno[1,2-b:5,6-b]dithiophene-2,8-diyl]bis[methylidyne(5,6-difluoro-3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile]) by optimizing ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2,3-d]-s-indaceno[1,2-b:5,6-b]dithiophene) [11]. IT-4F exhibited a red-shifted absorption wavelength and downshifted molecular energy levels compared to those of nonfluorinated ITIC, which improved the J_SC and PCEs. Similarly, Cui et al. designed a low-bandgap acceptor using chlorinated Y6 (BTP-4Cl), which exhibited high crystallinity and J_SC owing to its high coherence length and red-shifted absorption wavelength, respectively [12]. Thus, it yielded an enhanced PCE compared to its counterpart. In indoor environments, the photoactive materials of OPVs should have high absorption coefficients in the visible wavelength region (400–750 nm). This is essential for effectively harvesting light-induced photons that are significantly low in number under these conditions. In addition, balanced charge transport characteristics related to the fill factor (FF) and J_SC of OPVs are highly desirable [13]. A favorable photoactive layer surface morphology is the premise for effectively suppressing trap-assisted recombination, which severely degrades the device’s performance under low-intensity light. Although NFA-based OPVs have achieved notable success in enhancing indoor PCEs, the impact of the halogenation of NFAs on the indoor photovoltaic performance of OPVs is yet to be appropriately investigated. Consequently, the development of unique photoactive materials for indoor light-energy harvesting has been hindered.

In this study, Y6-derivatives containing selenophene were modified using a halogenation approach to examine the indoor performance of OPVs [14]. The performance of OPVs was strongly influenced by the choice of chlorination or fluorination under dim indoor light. The face-on orientation and π–π stacking of the halogenated NFA with the Y6 acceptor contribute to improving the J_SC values. The modified NFAs with planar structures displayed broad absorption spectra, indicating their potential for harvesting light over a broader absorption wavelength range in indoor environments. Because indoor light sources have diverse light spectra, a wider absorption range is essential for obtaining an excellent spectral response [15–18]. For example, LEDs cover the visible wavelength region, while halogens have monotonically increased irradiation spectra. Moreover, the chlorinated OPV devices exhibited superior performance, with a high PCE greater than 25% compared to that of the fluorinated OPV devices (PCE: 22%) under 1000-lx LED lamps. The high performance of the chlorinated OPV devices can be attributed to the high crystallinity and planarity of the chlorinated acceptor, which reduces trap-assisted recombination and V_OC loss under indoor light.

2. Materials and Methods

2.1. Materials

All reactions were performed under a nitrogen atmosphere. All reagents and solvents were purchased from Alfa Aesar, Aldrich, and TCI. The catalysts were purchased from Umicore. All the starting materials were used as received without further purification, and all the solvents were further purified before use. 3,9-dinonyl-12,13-dihydroselenopheno[2,3:4,5]thieno[2,3:4,5]pyrrolo[3,2-g]selenopheno[2,3:4,5]thieno[3,2-b][1,2,5]thiadiazolo[3,4-e]indole was prepared as previously reported [19]. The PM6 was purchased from Brilliant Matters, while ZnO, MoO_x, and Ag were purchased from Aldrich.

2.2. OPV Device Fabrication

The OPV devices were optimized using an inverted structure (ITO/ZnO/PM6:NFA/MoO_x/Ag). OPVs employing ZnO NPs as the electron transport layer (ETL) demonstrated maximum photovoltaic performance owing to suitable energy level alignment between the ETLs and the active layer (Tables S1 and S2 and Figure S1). After selecting ZnO NPs ETLs, the thickness of the active layer was established to the optimal conditions (Tables S3 and S4 and Figure S1). The ITO-coated glass was cleaned using deionized water with detergent in an ultrasonication bath for 20 min at 45°C. Then, the ITO-coated glass was successively cleaned with deionized water, acetone, and isopropyl alcohol under the same conditions. Subsequently, ZnO solution was coated on top of the ITO-coated glass via a spin coater using a 0.2 μm polytetrafluoroethylene filter and then annealed at 200°C for 30 min in air. The PM6:NFA solution was prepared via dilution in chloroform containing 0.5 vol% 1-chloronaphthalene to obtain a total concentration of 13.2 mg ml⁻¹ with a 1 : 1.2 ratio. The blended solution was stirred at 25°C for 4 h in a glovebox filled with N₂ gas. The photoactive layer (i.e., PM6:Y6) was spin-coated on top of the ITO/ZnO at 5000 rpm for 30 s without a filter and annealed at 100°C for 10 min. Finally, to deposit the HTL and top electrode, the samples were loaded into a vacuum thermal evaporation system connected to a glove box, and a 10 nm thick layer of MoO_x and a 150 nm thick layer of Ag were deposited through a shadow mask at a base pressure of Torr. The photoactive area of the device with the aperture was calculated to be approximately 0.045 cm².

2.3. OPV Device Characterization

The current density-voltage (J–V) characteristics under illumination and dark conditions were measured using a SourceMeter unit (2401, Keithley Instruments, Cleveland, OH, USA). To achieve the desired illumination conditions, an air-mass (1.5 G) solar simulator (McScience, Suwon, Republic of Korea) with an irradiance of mW cm⁻², LED lamps (McScience, Suwon, Republic of Korea) with an irradiance of mW cm⁻² under 1000 lx (color temperature 5600 K), and a halogen lamp (OSRAM ECO PRO CLASSIC 64543 A 240 V 46 W E27, OSRAM, Munich, Germany) with an irradiance of 7 mW cm⁻² under 1000 lx (color temperature 2700 K) were used. An incident photon-to-current efficiency measurement system (McScience, Suwon, Korea) was used to determine the external quantum efficiency (EQE of the OPVs. The thickness of each layer was measured using Alpha-Step IQ (KLA-Tencor, California, USA). The thin-film absorbance was determined using a UV-vis near-infrared spectrophotometer (Cary 5000, Agilent). The optical simulations required to obtain the maximum J_SC values at low illuminance levels were performed using Maxwell’s equation-based finite-difference time-domain (FDTD) simulations (Lumerical Inc.). The indoor irradiance measured the irradiance using equipment (PMD100D and THORLABS). A lux meter was placed below the light source and connected to equipment (TES 1330A). The illuminance and irradiance values were recorded. The work functions of ITO, ZnO, MoO_x, and Ag were measured using a Kelvin probe.

3. Results and Discussion

Y6 derivative acceptors with selenophene, which can enhance intra- and intermolecular interactions, were synthesized via the Knoevenagel condensation with dichlorinate or difluorinated 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile (INCN) terminal groups [19]. The chemical structures of the modified Y6 derivatives ThSe-C9-C16-Cl and ThSe-C9-C16-F are shown in Figure 1(a), and detailed synthetic procedures and structural characterizations are described in the Supplementary Material (Figures S2–S7). The optical absorptions of the Y6 derivative acceptors and a donor (PM6) measured in the thin films are shown in Figure 1(b). The absorption spectra of the blended thin films (Figure S8a) and the NFA in the diluted solution (Figure S8b) are provided in the Supporting Information. The absorption peak of ThSe-C9-C16-F occurred at nm, whereas the peak was slightly red-shifted to nm for ThSe-C9-C16-Cl. This absorption shift was attributed to the strong dipole moment of chlorine, which has a larger atomic size and longer chlorine–carbon bonds than fluorine [16]. The major portion of the absorption band of the PM6:NFAs blend was located in the wavelength range of 600–900 nm, which covered the radiation spectra of AM1.5G (400–900 nm). Such excellent spectral coverage may yield high J_SC owing to sufficient charge dissociation in the active layer [20]. In contrast, a slight mismatch between the absorption spectra of the PM6:NFA-based active layer and the emission (400–750 nm) of the indoor light sources yielded a large V_loss because of insufficient exciton dissociation in the near-infrared range (Figure 1(c)) [21–23]. Next, cyclic voltammetry (CV) measurements were performed to calculate the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels (Figure S8c, Table S5). ThSe-C9-C16-F was characterized by a LUMO/HOMO level of -4.31/-5.64 eV, while the ThSe-C9-C16-Cl exhibited a slightly deeper LUMO/HOMO level of 4.35/-5.67 eV. Subsequently, V_OC was estimated from the difference between the HOMO and LUMO of the donor and acceptor, respectively, as shown in the energy level diagram (Figure 1(d)). Although ThSe-C9-C16-F was expected to provide a large V_OC owing to its deep HOMO level, it yielded a significantly lower V_OC than ThSe-C9-C16-Cl under indoor light sources (LED and halogen) and similar values under AM1.5G condition. This discrepancy in V_OC is explained in the subsequent section. Subsequently, grazing-incidence wide-angle X-ray scattering (GIWAXS) and atomic force microscopy (AFM) were performed to evaluate the morphological evolution of the crystallinity, molecular packing, and surface roughness of the active layer (Figure 2). The two-dimensional (2D) patterns of the PM6:ThTh-C9-C16-F and PM6:ThTh-C9-C16-Cl films are shown in Figure 2(a). The films of both active layers exhibit dominant strong (010) stacking peaks in the out-of-plane (OOP) direction, demonstrating a face-on orientation in the active layer [24, 25]. For an in-depth analysis of the orientation, one-dimensional (1D) GIWAXS patterns were plotted with the corresponding lines cut along the OOP (Figure 2(b)) and in-plane (IP) directions (Figure 2(c)). The 1D GIWAXS results indicate that the strong (010) stacking peaks are located at 1.80 Å^-1 (ThSe-C9-C16-F) and 1.72 Å^-1 (ThSe-C9-C16-Cl), whereas the (100) lamellar peaks occur located at 0.29 and 0.30 Å^-1, respectively. The stacking peaks in the OOP direction and lamellar peaks in the IP direction indicated a face-on orientation and dense intermolecular distance of the active layer with PM6 and modified NFAs and resulted in a high fill factor (FF) and balanced charge transfer [26, 27]. The crystal coherence length (CCL), which indicates the crystallinity of the PM6:NFAs films, was calculated using the Scherrer equation (, where is the full width at half maximum (FWHM) and is the shape factor (~0.9)) [28]. The PM6:ThSe-C9-C16-Cl film yielded a larger CCL (19.8 Å) in the OOP direction than PM6:ThSe-C9-C16-F (17.1 Å), indicating that the former exhibited higher crystallinity and better-ordered packing in film. This validated the improvement in electron transfer characteristics and FF of PM6:ThSe-C9-C16-Cl-comprised OPV devices. In contrast, PM6:ThSe-C9-C16-F exhibited a broad width around the peak, which implied weaker cofacial packing in the film and suggested that PM6:ThSe-C9-C16-F experienced a reduction in electron transfer and FF under low light intensities [29]. Subsequently, the surface roughness of the active layer was examined using AFM (Figure 2(d)). The mean-square surface roughness () values of ThSe-C9-C16-F and ThSe-C9-C16-Cl films were estimated at 2.89 and 1.55 nm, respectively. The large value of ThSe-C9-C16-F indicated irregularity and poor crystallinity in the active layer, which further increased interface recombination and reduced the FF. The surface analysis was further extended by determining the number of grains in the active layer (Figure S9). The numbers of grains in ThSe-C9-C16-Cl and ThSe-C9-C16-F were 163 and 273, respectively. The considerable number of grains elevated the grain boundaries, which induced recombination owing to poor surface morphology [30]. Crystallinity-dependent recombination is particularly important in low-light environments and is discussed in the subsequent sections [31, 32]. Furthermore, the performance of the OPV device was evaluated to examine the impact of NFA halogenation engineering under dim indoor light intensities. An OPV device was developed with an inverted structure of indium tin oxide (ITO)/zinc oxide (ZnO)/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):NFA (ThSe-C9-C16-F and ThSe-C9-C16-Cl)/molybdenum oxide (MoO_x)/silver (Ag) (Figure S10). The photovoltaic parameters are listed in Table 1. The current density-voltage (J-V) characteristic curves of the devices under simulated outdoor conditions (1-sun illumination, P_in: 100 mW/cm²) are shown in Figure 3(a). Under 1-sun illumination, ThSe-C9-C16-F devices exhibited a large PCE of % with a reasonably high FF of %, J_SC of mA/cm², and V_OC of mV. However, the ThSe-C9-C16-Cl devices yielded a slightly inferior PCE of % with substantially low FF of %, J_SC of mA/cm², and V_OC of mV. ThSe-C9-C16-Cl exhibited a slightly higher J_SC owing to its high absorption in the 400–600 nm range, where the absorption range of the blended film matched the highest range of radiation spectra of AM1.5G 1-sun illumination. In contrast, the ThSe-C9-C16-Cl devices yielded a significantly poor FF because of their high parasitic series resistance () (Eq. (1) and Table S6) [33].

(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

(a)

(b)

(c)

(d)

Moreover, a low FF can be caused by the radiative recombination involved in bimolecular recombination, which will be explained later. Although PM6:ThSe-C9-C16-F exhibited a slightly poor surface morphology, its FF under 1-sun illumination was relatively high because its active layer absorbed sufficient light. This generated sufficient excitons, indicating a lower dependence on trap-assisted recombination under high light intensity. Next, EQE was measured to examine the discrepancies in the values of J_SC (Figure 3(b)). Overall, the EQE of the OPV devices exhibited similar behavior. However, the fluorinated OPV device yielded a slightly higher EQE in the wavelength range of 450–800 nm, with a maximum value of approximately 74% at a wavelength of 550 nm. The improved EQE of the fluorinated OPV device may arise from the high absorption of fluorine (Figure 1(b)), which improves the photovoltaic performance under 1-sun illumination.

The OPV device performance was subsequently evaluated under a low-light intensity of various light sources including LED (1000 lx, P_in: 0.23 mW/cm²) and halogen (1000 lx, P_in: 7.0 mW/cm²). In contrast to the outdoor illumination, the OPV devices exhibited significantly different tendencies under indoor light. The J-V characteristic curves of the devices under 1000 lx LED irradiation are shown in Figure 3(c), and the detailed parameters are listed in Table 1. Under the 1000 lx LED, the chlorinated OPV device yielded an excellent PCE of % with a corresponding high FF of %, J_SC of μA/cm², and a V_OC of mV. In contrast, the fluorinated OPV device with an FF of %, J_SC of μA/cm², and V_OC of mV yielded a low PCE of %. Typically, OPV performance exhibits a logarithmic drop in V_OC when the light intensity is changed from 1-sun to LED illumination. However, V_OC decreased significantly in the fluorinated OPV device owing to monomolecular recombination defects. In contrast, the chlorinated OPV device displayed a relatively high V_OC (an increase of more than 21 mV) because of reduced monomolecular recombination owing to its well-oriented morphological properties. Similarly, the fluorinated OPV device produced a comparatively low J_SC under low light intensity compared to the chlorinated OPV device, possibly owing to weak cofacial packing and downward crystallinity of the PM6:ThSe-C9-C16-F films [34]. Other photovoltaic parameters of the OPV under various light intensities are shown in Figure S11 and Table S7. To further understand the evolution of J_SC under indoor light, FDTD analysis was performed using the refractive index and extinction coefficient (Figure S12). The simulated J_SC was estimated by assuming internal quantum efficiency (IQE) of unity [35, 36]. The simulated J_SC under various light intensities exhibits a trend similar to that of the measured J_SC (Table S8). Similarly, the photovoltaic performances of the devices were investigated using a halogen lamp to determine the applicability of the halogenation approach (Figure 3(d)). Compared with LEDs, halogen lamps exhibit a monotonically increasing irradiation spectrum and higher light intensity, which are suitable for further exciton generation in the near-infrared (NIR) region (>800 nm) [37]. Under the 1000 lx halogen lamp, the chlorinated OPV device performed relatively better (PCE: 5.2%, output power density (P_max): ~402 μW/cm²) than the fluorinated OPV device (PCE: 4.2%, P_max: ~293 μW/cm²).

The spectral mismatch between the irradiation spectrum of the halogen lamp and the absorbance range of the photoactive layer resulted in a low PCE. Interestingly, the chlorinated OPV showed a notable J_SC enhancement compared with the fluorinated OPV because of the red-shifted absorption wavelength of the chlorinated OPV. Furthermore, devices with large photoactive areas (1 cm²) were also fabricated (Figure S13). Under low-light illumination, the device exhibited a similar trend, demonstrating that the morphology and recombination properties also matched those of large-area indoor OPVs.

The dependence of V_OC and J_SC on the light intensity was examined to better understand the charge recombination dynamics under dim indoor light. The plot of V_OC versus light intensity (I_light) can be described using the expression , where , , , and denote the ideality factor, Boltzmann constant ( eV K⁻¹), temperature (300 K), and elementary charge ( C), respectively (Figure 4(a)). The drop in V_OC under indoor light conditions was strongly influenced by charge recombination. Under dim indoor light intensity (<0.18 sun), Shockley-Read-Hall (SRH) recombination, which is associated with trap-assisted and geminate recombination, results in a current loss [35]. The slope (s) of is close to 2 kT/q, which demonstrates the strong dominance of SRH recombination in the active layer, whereas a value of 1 kT/q indicates the dominance of bimolecular recombination [38, 39]. As shown in Figure 4(a), the chlorinated OPV device exhibits an value of 1.20, indicating the dominance of bimolecular recombination. This explains the lower dependence of the chlorinated OPV device on trap-assisted recombination, which results in a high FF under indoor light. In contrast, ThSe-C9-C16-F exhibited strong SRH recombination because of its high value (1.70), which resulted in significant FF and V_OC losses in the fluorinated OPV device. Furthermore, a significant difference in the s values between the NFAs may be caused by the variable surface roughness as it originates from a large phase separation, which is determined by the number of grains in the active layer. Consequently, this influences the photon-to-electron conversion related to trap-assisted recombination [30, 40]. In contrast, under 1-sun illumination, bimolecular recombination results in a current loss. Based on the exponent factor (α) of the light intensity against J_SC, bimolecular recombination can be numerically compared. As shown in Figure 4(b), ThSe-C9-C16-F exhibits a slightly higher α, and demonstrates a relatively high FF and PCE under 1-sun illumination.

(a)

(b)

(c)

Next, the exciton dissociation efficiency (η_diss) and charge collection efficiency (η_coll) were calculated to evaluate the improvement in the J_SC of the chlorinated OPV device under low-light intensity [41, 42]. Figure 4(c) shows a plot of the photocurrent density (J_ph) versus the effective voltage (V_eff) (V_eff was derived from , where is the applied voltage and is the voltage when J_ph is equal to 0). Similarly, J_ph was calculated as , where and are the current densities under light illumination (1-sun and LED 1000 lx) and in the dark, respectively. When V_eff approached approximately 2 V, the saturation photocurrent density (J_sat) indicated the dissociation of excitons and collection of free charges without bimolecular recombination. Here, η_diss is defined as J_SC/J_sat, and η_coll is defined as J_MPP/J_sat, where MPP is the maximum power point. Consequently, the ratios (η_diss/η_coll) were 97.6/81.8 and 96.7/77.4 for the chlorinated and fluorinated OPV devices, respectively. The higher ratio of chlorinated OPV devices validates the enhancement of J_SC under low light intensity.

Finally, the stabilities of the OPV cells were evaluated by examining the devices under continuous illumination with ambient light for 500 h (Figure S14). The chlorinated OPV device maintained 89% of its original PCE value and <7% J_SC loss compared with its initial value. This high stability may arise from reduced recombination owing to the improved surface morphology.

4. Conclusions

This study evaluated the photovoltaic performance of OPVs under dim indoor light conditions by modifying Y6-based NFA with selenophene and halogenation. The regulation of fluorination and chlorination strongly influenced the indoor performance of OPVs. The OPV device with the chlorinated acceptor-based OPV exhibited well-oriented crystallinity and high planarity, resulting in a relatively high V_OC and reduced trap-assisted recombination. In contrast, the fluorinated acceptor-based OPV exhibited weak cofacial packing and downward surface roughness, which resulted in slightly lower FF and V_OC under low intensity. Moreover, owing to optimization, the chlorinated OPV device yielded an excellent PCE of 25.5% (PCE of fluorinated OPV: 22.5%) with high FF (~75.1%) and J_SC (~116 μA/cm²) under 1000 lx LED illumination. Thus, improved morphology and reduced recombination contributed to the stability of the OPV performance. After 500 h, the chlorinated acceptor-based OPV device maintained 89% of its original PCE and a loss under 7% J_SC compared to its initial value under LED illumination. Thus, this study provides further insight into the molecular design of NFAs to realize efficient OPVs for dim indoor light energy-harvesting applications.

Data Availability

Data is available on request.

Conflicts of Interest

There are no conflicts to declare.

Authors’ Contributions

Sunghyun Kim and Ho Eon Baek equally contributed to this work. Sunghyun Kim, Ho Eon Baek, Jae Won Shim, and Yun-hi Kim conceptualized the research. Sunghyun Kim was in charge of configuration, measurement, and data curation related to all experiments. Ho Eon Baek carried out the material synthesis and manuscript review. Muhammad Ahsan Saeed analyzed the device performance and revised the manuscript, and Tae Hyuk Kim analyzed the device performance and performed grain counting. Shuhao Chen measured and analyzed material characteristics. Hyungju Ahn and Wooseop Lee performed GIWAXS and analyzed GIWAXS data. Jae Won Shim and Yun-Hi Kim oversaw the project administration and supervised. Sunghyun Kim and Ho Eon Baek equally contributed to this work.

Acknowledgments

This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (no. 2021R1A2B5B03086367, 2022R1A2C2009523, and 2022M3J7A1062940). This research was also supported by the Technology Innovation Program (20011336) funded by the Ministry of Trade, Industry, and Energy (Korea).

Supplementary Materials

The supplementary materials provided in this study consist of optimization of the ETL and thickness of the active layer in OPV devices (Figure S1, Tables S1-S4). The supplementary materials also provided detailed descriptions of the synthesis and measurement methods utilized for the nonfullerene acceptors, as well as the findings obtained from ¹H-NMR, ¹³C-NMR, and MALDI-TOF MS analyses of ThSe-C9-C16-F and ThSe-C9-C16-Cl (Figures S2-S7). Absorption spectra and cyclic voltammetry data are also included for both the nonfullerene acceptors and PM6:NFA films (Figure S8, Table S5), along with 2D AFM images and grain counts for PM6:NFA (Figure S9). A schematic diagram of the device structure is presented in Figure S10, while photovoltaic performance under varying light intensities is illustrated in Figure S10. FDTD simulation results are provided in Figure S12, and photovoltaic performance with a large active area is reported in Figure S13. Stability analyses for each parameter are detailed in Figure S14, and parasitic resistance parameters are summarized in Table S6. Photovoltaic parameters specific to PM6:NFA under various irradiances are given in Table S7, while calculated JSC values obtained from FDTD simulations are presented in Table S8. The GIWAXS parameters for the PM6:NFA films are presented in Table S9. (Supplementary Materials)

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Copyright © 2023 Sunghyun Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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