All-Small-Molecule Organic Solar Cells Based on a Fluorinated Small Molecule Donor With High Open-Circuit Voltage of 1.07 V

Introduction

With the advantages of flexibility, light weight, roll to roll production and etc., bulk heterojunction (BHJ) organic solar cells (OSCs) have drawn extensive attention (Heeger, 2010; Qiu et al., 2017; Zhao F. et al., 2017; Cheng et al., 2018; Sun Y. et al., 2019). Significant progress has been obtained in the past decade, benefiting from the efforts in new material design and device optimization (Zhang Q. et al., 2014; He et al., 2015; Kan et al., 2015; Li Y. et al., 2018). Recently, non-fullerene acceptors (NFAs), especially acceptor-donor-acceptor (A-D-A) type small molecule acceptors, have demonstrated great success in OSCs due to the merits of facile synthesis, controllable absorption and fine-tuned energy levels (Lin et al., 2016; Zhao W. et al., 2017; Li T. et al., 2018; Zhang et al., 2018; Cui et al., 2019; Yuan et al., 2019).

From the perspective of donor materials, NFA based OSCs consist of polymer solar cells and small-molecule solar cells. Using polymer donor materials, both single junction and tandem OSCs have achieved the impressive power conversion efficiencies (PCEs) of 15–18% (Meng et al., 2018; An et al., 2019; Fan et al., 2019; Jiang et al., 2019; Lin et al., 2019; Sun H. et al., 2019; Wu Y. et al., 2019; Xiong et al., 2019; Yan et al., 2019). So far, all-small-molecule OSCs (ASM-OSCs), i.e., devices based on both small molecule donor and acceptor, have shown relatively lower performance compared with that of polymer donor based devices (Zhang Q. et al., 2014; Kan et al., 2015; Badgujar et al., 2016; Yang et al., 2017; Privado et al., 2018; Wang et al., 2018a; Duan et al., 2019). Nevertheless, with the advantages of versatile molecules design, less variation among different batches and thus easiness of property control for small molecules, ASM-OSCs are strongly believed to have the potential to achieve and even surpass the performance of polymer donor counterparts (Chen et al., 2013). To date, with the extensive study of small molecular design and device optimization for ASM-OSCs, the PCEs over 10% have been achieved (Bin et al., 2018; Wang et al., 2018a,b; Chen et al., 2019; Cheng et al., 2019; Gao et al., 2019; Ge et al., 2019; Wu H. et al., 2019; Yue et al., 2019; Zhou et al., 2019). Notably, a PCE over 14% has been obtained just recently by Wei et al. indicating the great potential of ASM-OSCs (Zhou et al., 2019).

Presently, compared with the comprehensive study of small molecule NFA, less attention has been paid to small molecule donors, which in fact play a vital role same as acceptors in OSCs. Among many properties that should be considered for designing active layer materials, matched energy levels between donor and acceptor not only contribute to charge transfer and exciton separation, but also have great influence on the open-circuit voltage (V_oc) of OSC devices (Tang et al., 2017, 2019). Therefore, it is important to efficiently tune energy levels for designing active layer molecules. Among various strategies for tuning the energy level, introducing fluorine atoms into the designing molecule has been proved to be a simple way to downshift the frontier molecular orbital energy level, which has been used successfully in molecular design (Tang et al., 2018; Chen et al., 2019; Ge et al., 2019; Wu H. et al., 2019; Yue et al., 2019), e.g., PM6, a well-known polymer donor synthesized by Hou's group (Li W. et al., 2018). Recently, we have reported an ASM-OSC based on a non-fluorinated donor DRTB-T and an acceptor F-2Cl, showing a PCE of 10.76%, but a relatively lower V_oc of 0.969 V compared with other ASM-OSCs (Wang et al., 2018b). With these considerations, we have designed and synthesized a new small molecule donor with an A-D-A structure, namely DRTB-FT (as shown in Figure 1A), in which fluorine atoms are introduced on the thienyl substituent of central benzodithiophene (BDT) unit (Zhang and Li, 2014). As expected, DRTB-FT shows a lower highest occupied molecular orbital (HOMO) energy level of −5.64 eV in contrast to DRTB-T (−5.51 eV) (Yang et al., 2017). Notably, DRTB-FT has nearly the same solid film absorption ranging from 300 to 600 nm compared with that of DRTB-T, which is well complementary to that of an A-D-A type acceptor F-2Cl (500–800 nm). Thus, ASM-OSC based on DRTB-FT:F-2Cl gave a PCE of 7.66% with a high V_oc of 1.070 V and a low energy loss (E_loss) of 0.47 eV. The results indicate that high V_oc of ASM-OSC device can be obtained through careful donor molecular optimization. The moderate performance was mainly attributed to the low FF (0.532) and J_sc (-13.46 mA cm⁻²) owing to the unmatched charge carrier mobilities and unfavorable active layer morphology.

FIGURE 1

Figure 1. (A) Chemical structures of DRTB-T, DRTB-FT, and F-2Cl; (B) Top view and (C) Side view of the optimized molecular geometries of DRTB-FT at B3LYP/6-31G*; (D) Absorption spectra of DRTB-FT and F-2Cl.

Experimental Section

Materials and Synthesis

DRTB-FT was synthesized following the procedure in Scheme 1. F-2Cl was prepared according to the reported procedure (Wang et al., 2018b). The materials were commercially available and used without further purification. All manipulations and reactions were carried out following the standard Schlenk technique under argon protection.

SCHEME 1

Scheme 1. Synthetic route of DRTB-FT.

Compound 3. Under argon protection, the weighed compound 1 (0.40 g, 0.425 mmol) and 2 (0.64 g, 0.936 mmol) were dissolved in toluene(15 mL). Subsequently, Pd(PPh₃)₄ (50 mg) was added as catalyst. The solution was stirred under dark condition, keeping the temperature at 110°C for 20 h. After removing the solvent, compound 3 was obtained as a red solid by silica gel chromatography (PE/CH₂Cl₂ = 1:2 as eluent) (0.59 g, 76%). ¹H NMR (400 MHz, CDCl₃), δ (ppm): 9.71 (s, 2H), 8.10 (s, 2H), 7.51 (s, 2H), 7.47 (s, 2H), 7.30 (d, 2H), 7.19 (d, 4H), 6.93 (d, 4H), 2.94–2.90 (m, 12H), 1.77-1.72 (m, 6H), 1.48–1.38 (m, 48H), 1.04–0.95 (m, 36H) (Figure S1). ¹³C NMR (100 MHz, CDCl₃), δ(ppm): 183.74, 155.61, 153.11, 146.94, 146.47, 143.13, 141.48, 139.26, 138.90, 138.52, 137.77, 137.08, 135.78, 135.46, 133.95, 133.57, 133.48, 128.33, 126.07, 125.64, 125.51, 124.36, 123.06, 122.75, 122.57, 120.83, 118.14, 117.89, 41.35, 40.71, 34.38, 32.63, 32.59, 29.68, 29.53, 28.93, 25.86, 25.71, 23.06, 14.18, 10.87. MALDI-TOF MS: calcd for C₁₀₄H₁₂₀F₂O₂S₁₂ [M⁺], 1824.84; found: 1824.77.

Compound DRTB-FT. Compound 3 (150 mg, 0.082 mmol) and 3-ethyl-rhodanine (133 mg, 0.82 mmol) were added in CHCl₃ (20 mL). Under stirring the solution was deoxygenated using argon gas for 10 min. Subsequently, piperidine (0.1 mL) was added by syringe. After stirring at 60°C for 12 h, water (100 mL) was added and the product was extracted using CHCl₃ (2 × 80 mL). The combined CHCl₃ solution was washed twice using water and then dried by anhydrous Na₂SO₄. After removal of solvent, the crude product was chromatographied by silica gel (PE/CH₂Cl₃ = 1:3 as eluent), obtaining a powdery dark purple solid (142 mg, 82%). ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.66 (s, 2H), 7.43 (s, 2H), 7.34 (s, 2H), 7.30 (s, 2H), 7.26–7.23 (m, 4H), 7.16 (s, 2H), 6.94-6.92 (m, 4H), 4.02–4.00 (m, 4H), 2.96-2.87 (m, 12H), 1.81–1.78 (m, 6H), 1.57–1.42 (m, 48H), 1.19-1.16 (t, 6H), 1.06–0.92 (m, 36H). ¹³C NMR (100 MHz, CDCl₃), δ (ppm): 189.92, 164.84, 153.75, 151.21, 144.84, 144.50, 139.73, 136.68, 136.43, 136.06, 135.62, 135.52, 134.92, 134.35, 133.98, 131.78, 129.46, 126.51, 126.25, 123.64, 123.28, 122.73, 121.82, 121.09, 120.87, 120.68, 120.50, 118.85, 118.69, 116.27, 115.99, 39.52, 39.43, 38.78, 37.74, 32.60, 32.54, 30.88, 30.77, 27.77, 27.14, 27.08, 26.99, 23.98, 23.91, 23.81, 23.17, 21.29, 21.25, 12.38, 12.31, 10.39, 9.15, 8.99 (Figure S2). MALDI-TOF MS: calcd for C₁₁₄H₁₃₀F₂N₂O₂S₁₆ [M⁺], 2111.30; found: 2110.65 (Figure S3).

Measurements and Instruments

The ¹H and ¹³C nuclear magnetic resonance (NMR) spectra were taken on by the Bruker AV400 Spectrometer. Matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass were measured using a Bruker Autoflex III instrument. X-ray diffraction (XRD) experiments were carried out with a Bruker Model D8 FOCUS X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) at a current of 40 mA and a generator voltage of 40 kV. The UV-vis spectra were performed on a JASCO V-570 spectrophotometer. Cyclic voltammetry (CV) experiments were carried out using a LK98B II Microcomputer-based Electrochemical Analyzer and a conventional three-electrode configuration with a saturated calomel electrode (SCE) as the reference electrode, a Pt wire as the counter electrode, and a glassy carbon electrode as the working electrode at room temperature. The measurement was carried out employing acetonitrile solution of tetrabutylammonium phosphorus hexafluoride (n-Bu₄NPF₆, 0.1 M) as the supporting electrolyte with the scan rate of 100 mV s⁻¹. The LUMO (lowest unoccupied molecular orbital) and HOMO values were obtained following the formula E_LUMO = – (4.80 + E_re^onset) and E_HOMO = – (4.80 + E_ox^onset), where E_re^onset and E_ox^onset can be estimated from the CV curves, respectively. The geometry structures of DRTB-FT was optimized through Density functional theory (DFT) calculations (B3LYP/6-31G^*) according to Gaussian 16 (Lee et al., 1988; Frisch et al., 2016).

The current density-voltage (J-V) characteristics of ASM-OSC devices were measured with a Keithley 2400 source-measure unit. Using a SAN-EI XES-70S1 AAA class solar simulator, photocurrent was performed under illumination with simulated 100 mW cm⁻² AM 1.5G irradiation. The external quantum efficiency (EQE) curve was obtained by a QE-R Solar Cell Spectral Response Measurement System.

Atomic force microscope (AFM) and Transmission electron microscopy (TEM) images were obtained on a Bruker MultiMode 8 in tapping mode and Philips Technical G2 F20 at 200 kV, respectively. Space charge limited current (SCLC) mobility was performed by a diode configuration of glass/ZnO/active layer/Al for electron mobility and ITO/PEDOT:PSS/active layer/Au for hole mobility, respectively. The results were fitted via a space charge limited form, being described as the equation:

$\begin{array}{l}J=\frac{9{\epsilon }_0{\epsilon }_r{\mu }_0{V}^2}{8L^3}\text{exp}\left(0.89\beta \sqrt{\frac{V}{L}}\right)\end{array}$

where J is the current density, ε₀ is the permittivity of free space (8.85 × 10⁻¹² F m⁻¹), ε_r is the relative dielectric constant of the transport medium, μ₀ is the hole or electron mobility, V (= V_appl – V_bi) is the internal voltage in the device, where V_appl is the applied voltage to the device and V_bi is the built-in voltage due to the relative work function difference of the two electrodes, L is the film thickness of the active layer.

Fabrication of OSC Devices

ASM-OSC devices were fabricated using a conventional structure of glass/ITO/PEDOT:PSS/Donor:Acceptor/PDINO/Al, in which the electronic transport material PDINO was designed by Zhang et al. (2014). As a common procedure: the cleaned ITO-coated glasses were treated by ultraviolet-ozone for 15 min and spin-coated with PEDOT:PSS solution at 4,800 rpm for 20 s. Subsequently, the glasses were baked at 150°C for 20 min and transferred into the glove box with argon gas. Chloroform solution of both donor and acceptor was spin-coated on the PEDOT:PSS at 1,700 rpm for 20 s. Afterward, PDINO was spin-coated on the active layer at 3,000 rpm for 20 s with the concentration of 1.0 mg/ml using EtOH as solvent. Finally, under the high vacuum, a cathode material Al was deposited for 50 nm onto PDINO layer. The work area of each device is about 0.04 cm².

Results and Discussion

Materials Synthesis and Characterization

The synthesis of DRTB-FT is illustrated in Scheme 1. By Stille coupling reaction between compound 1 and 2, two commercially available BDT derivatives, compound 3 was synthesized. Subsequent Knoevenagel condensation reaction between compound 3 and 3-ethylrhodanine gave the final product in high yield. As shown in the experimental section above, DRTB-FT was characterized through NMR spectroscopy and MALDI-TOF MS. DRTB-FT exhibits good solubility in common organic solvents such as chlorobenzene, dichloromethane and chloroform. As shown in Figures 1B,C, the optimized geometry of DRTB-FT based on DFT (B3LYP/6-31G^*) displays a good coplanar conformation. The structural ordering of pristine DRTB-FT and DRTB-T film was investigated by XRD analysis. According to the XRD results (Figure S4), a weak diffraction peak (100) at 4.44 (2θ) was observed for the DRTB-T film. In contrast, DRTB-FT film exhibits a strong diffraction peak (100) at 4.46 (2θ), indicating stronger stacking and crystallinity than DRTB-T.

Optical and Electrochemical Properties

The UV-vis absorption spectra of DRTB-FT obtained in chloroform solution and in solid state are shown in Figure 1D. In chloroform solution, DRTB-FT exhibits an absorption maximum at 547 nm, and it shows a bathochromic-shifted and broadened absorption in the solid state ranging from 300 to 630 nm, which is well complementary to that of F-2Cl film. The optical bandgap (E_g^opt) obtained from DRTB-FT film absorption onset (622 nm) is 1.99 eV. Moreover, a stronger shoulder peak was observed in film absorption spectrum in comparison to the non-fluorinated counterpart DRTB-T, which reveals the stronger intermolecular stacking and π-π interactions in DRTB-FT solid film. The electrochemical behavior of DRTB-FT was investigated by CV experiments. As shown in Figure S5, the estimated reduction and oxidation potentials of DRTB-FT are −1.17V and 0.84V (vs Fc/Fc⁺), respectively. Therefore, the first oxidation event of DRTB-FT occurs at a higher oxidation potential compared with DRTB-T, leading to a deeper HOMO of −5.64 eV, which is in favor of obtaining a higher V_oc in the devices when combined with appropriate acceptor. The optical and electrochemical data of DRTB-FT and DRTB-T are listed in Table 1.

TABLE 1

Table 1. Optical and electrochemical data of DRTB-T and DRTB-FT.

Photovoltaic Properties

ASM-OSC devices with a conventional structure of ITO/PEDOT:PSS/active layer/PDINO/Al were fabricated. The devices performance were optimized by varying the donor to acceptor weight ratio (D/A, w/w) in the active layer, applying solvent vapor annealing (SVA) and thermal annealing (TA) treatment and etc. The detailed device parameters are provided in Tables S1–S3. The photovoltaic parameters of the as-cast and optimal devices are listed in Table 2 and the corresponding J-V characteristics are shown in Figure 2A. For the as-cast device, a PCE of 5.72% with a high V_oc of 1.10 V was obtained. An improved photovoltaic performance with a PCE of 7.66% and a high V_oc of 1.07 V was achieved after TA treatment at 100°C for 10 min, and the corresponding E_loss is as low as 0.47 eV calculated via the equation E_loss = E_g^opt-eV_oc, where E_g^opt is the optical bandgap of F-2Cl film (1.54 eV). As expected, OSCs based on DRTB-FT:F-2Cl demonstrated a high V_oc owing to the low-lying HOMO energy level of the DRTB-FT donor. In contrast, the DRTB-T:F-2Cl based device has a V_oc of 0.969 V. However, limited by the low J_sc (13.46 mA cm⁻²) and FF (0.532), the optimized device based on DRTB-FT:F-2Cl gave a moderate PCE (7.66%). The unsatisfactory device performance can be ascribed to the unfavorable active layer morphology and unmatched charge carrier mobilities, as will be discussed in the following section.

TABLE 2

Table 2. Photovoltaic parameters of OSCs based on DRTB-FT:F-2Cl and DRTB-T:F-2Cl.

FIGURE 2

Figure 2. (A) Current density-voltage (J-V) curves of BHJ devices and (B) EQE curves of the optimal BHJ device based on DRTB-FT:F-2Cl.

The EQE spectrum of the optimal device is presented in Figure 2B, which exhibits a broad photo-to-current response from 300 to 790 nm. However, the maximum efficiency is only 58% at 471 nm, indicating a lower photoelectron conversion process in comparison to that of DRTB-T based device (Wang et al., 2018b). The calculated J_sc value obtained from the integration of the EQE curve is 13.01 mA cm⁻², which is in good agreement with the J_sc value from the corresponding J-V curve with about 3% mismatch.

To explore the reason for lower J_sc and FF of the ASM-OSC, the exciton dissociation and extraction process in the active layer of optimal device was investigated. Figure 3A shows the relation between photocurrent density J_ph (J_ph = J_L – J_D) and effective voltage V_eff (V_eff = V₀ – V_a), where J_L, J_D, V₀, and V_a are the light current density, the dark current density, the voltage at J_ph = 0 and the applied voltage, respectively. It can be seen that the J_ph curve of DRTB-FT:F-2Cl based device is hard to reach the saturation current density (J_sat) even at a high V_eff. Lower J_ph/J_sat values of 85.9% and 61.0% were obtained under the short-circuit and maximal output power conditions, respectively, which demonstrated a poor exciton dissociation and charge extraction processes and limited J_sc and FF. For further study, the bimolecular recombination degree in active layer was estimated on the basis of the formula of J_sc ∝ P^α, where α is the exponential factor. When α value was closer to 1, the less bimolecular recombination in blend film. The fitted slope (α) is calculated to be 0.964 for the devices based on DRTB-FT as shown in Figure 3B, which is relatively lower compared with that of DRTB-T based device (0.985) (Wang et al., 2018b), indicating more bimolecular recombination in DRTB-FT:F-2Cl active layer.

FIGURE 3

Figure 3. (A) Photocurrent density vs. effective voltage (J_ph-V_eff) and (B) double logarithmic plots of J_sc as a function of incident light intensity for the optimal device based on DRTB-FT:F-2Cl.

The influence of active layer morphology on device performance was investigated by AFM and TEM technology. Figure 4A shows that the surface topography of the as-cast DRTB-FT:F-2Cl blend film is relatively rough in terms of the large root-mean-square (RMS) roughness (9.2 nm), which reveals the existence of excessive aggregation in the as-cast film due to the strong stacking and crystallinity of donor molecules. For the optimal blend film (Figure 4B), a relatively smaller RMS roughness of 4.4 nm was observed, which is beneficial to form better contact between the film and the electrode, and thus to promote charge extraction. TEM images are in consistent with the AFM, from Figure 4C, it can be seen that an oversized phase separation and more defects in the as-cast blend film, and the morphology becomes better obviously after TA treatment (Figure 4D). However, there was still no clear fiber structure or suitable phase separation in the optimized blend film, which might account for the low J_sc and FF.

FIGURE 4

Figure 4. Tapping-mode AFM height images of the active layers of the (A) as-cast and (B) optimal DRTB-FT:F-2Cl blend films. TEM images of the (C) as-cast and (D) optimal DRTB-FT:F-2Cl blend films.

To investigate the charge-transporting behavior of the studied devices, the electron and hole mobilities (μ_e and μ_h) were measured by the SCLC method as described in experimental section. According to Figure S6, the electron mobility of μ_e=2.11 × 10⁻⁴ cm⁻²V⁻¹s⁻¹ and hole mobility of μ_h = 4.51 × 10⁻⁵ cm⁻²V⁻¹s⁻¹ were obtained for the as-cast device. After TA treatment, the charge mobilities were raised to μ_e= 4.12 × 10⁻⁴ cm⁻²V⁻¹s⁻¹ and μ_h = 8.56 × 10⁻⁵ cm⁻²V⁻¹s⁻¹, respectively. The relatively lower and unbalanced hole and electron mobilities could lead to more charge recombination, thus the lower FF and J_sc.

Conclusion

In summary, we have designed and synthesized a new small molecule and A-D-A type donor named DRTB-FT, taking the fluorothienyl-substituted benzodithiophene as the central unit. The introduction of fluorine atoms enables DRTB-FT to get a low HOMO of −5.64 eV. When combined with an acceptor F-2Cl, the optimal ASM-OSC device gave a PCE of 7.66%, especially a high V_oc of 1.070 V and a low E_loss of 0.47 eV. The results indicate that high V_oc of ASM-OSCs can be obtained through careful donor molecular optimization. On the other hand, morphology control also plays a critical role in ASM-OSCs. In this case, the low J_sc and FF are caused by the unfavorable active layer morphology with large aggregation of small molecule donors. It is believed that high performance ASM-OSCs will be achieved through synergistic study of molecular design with complementary absorption, matched energy levels and morphology control.

Data Availability Statement

All datasets generated for this study are included in the article/Supplementary Material.

Author Contributions

CLiu synthesized the DRTB-FT and measured the optical-electric properties of DRTB-T and DRTB-FT. NQ fabricated and optimized the devices, and wrote the manuscript. YS contributed to the morphology characterization. XK ran the calculations of DRTB-FT, HZ, CLi, XW, and YC conceived and directed the project. All authors contributed to the whole work.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors acknowledge the support from the NSFC of China (51773095), the Research Fund of Jining University (2019BSZX01), and the Young Innovative Talents Introduction & Cultivation Program for Colleges and Universities of Shandong Province: Innovative Research Team on Optoelectronic Functional Materials.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2020.00329/full#supplementary-material

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