Enhancing the Photovoltaic Performance of Ladder-Type

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Enhancing the Photovoltaic Performance of LadderType Dithienocyclopentacarbazole-Based Non-Fullerene Acceptors through Fluorination and Side-Chain Engineering Qisheng Tu, Yunlong Ma, Xiaobo Zhou, Wei Ma, and Qingdong Zheng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b02355 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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Chemistry of Materials

Enhancing the Photovoltaic Performance of Ladder-Type Dithienocyclopentacarbazole-Based Non-Fullerene Acceptors through Fluorination and Side-Chain Engineering Qisheng Tu,†,‡ Yunlong Ma,† Xiaobo Zhou,§ Wei Ma,§ and Qingdong Zheng†, * State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou, Fujian 350002, China. †

‡

University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China.

§

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China.

ABSTRACT: Besides an aromatic polycyclic core with extended -conjugation, side-chains and terminal groups are the other two molecular-structure factors which can greatly affect the photovoltaic performance of non-fullerene acceptors. In this work, three dithienocyclopentacarbazole-based non-fullerene acceptors (HCN-C8, HCN-C16, and H2FCN-C16) have been designed and synthesized to investigate the effects of side-chain and fluorination on the photovoltaic properties of non-fullerene acceptors. Among the three acceptors, HCN-C8 and HCN-C16 were designed with the same terminal group of 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN), but with different side-chains of octyl and 2-hexyldecyl appended on the central nitrogen of the dithienocyclopentacarbazole unit, respectively. H2FCN-C16 and HCN-C16 share the same side-chain of 2-hexyldecyl, but the former is designed with the fluorinated INCN in the terminal groups. Although the side-chain has a neglectable effect on the bandgap and energy levels of the resulting non-fullerene acceptors, the replacement of linear octyl chain with the branched 2-hexyldecyl chain leads to a crystallinity change of the resulting acceptors from crystalline to amorphous thereby affecting their phase separation with donor polymers. On the other hand, the non-fullerene acceptor with fluorination on the ending groups shows a decreased optical bandgap with deepened energy levels in comparison with the counterpart without the fluorination. By using a p-type polymer (J71) as the donor material, the best-efficiency polymer solar cell based on H2FCN-C16 exhibited an impressive power conversion efficiency (PCE) of 11.18% with a high short circuit current density (JSC) of 18.62 mA cm-2, a fill factor (FF) of 66.7%, and an open circuit voltage (VOC) of 0.90 V. The PCE of 11.18 % is the highest among all dithienocyclopentacarbazole-based nonfullerene acceptors reported so far. However, the best-efficiency solar cells based on HCN-C8 and HCN-C16 showed low PCEs of 2.38% and 5.51%, respectively. We further elucidated the important structure-property relationships for these dithienocyclopentacarbazole-based non-fullerene acceptors. These results provide useful guidelines for enhancing the performance of non-fullerene acceptors through fluorination and side-chain engineering.

INTRODUCTION Polymer solar cells (PSCs) have distinctive features such as light-weight, good flexibility, semitransparency, and low-cost processing capability, which make themselves an attractive energy conversion technique in recent two decades.1-5 Generally, the active layer of bulkheterojunction (BHJ) PSC consists of a p-type polymer donor material and an n-type acceptor (fullerene or nonfullerene) material both of which determine the final performance of PSCs.6 Until 2015, the fullerene derivatives (such as PC61BM and PC71BM) were regarded as the primary electron acceptor materials for efficient PSCs due to their advantages of high electron mobility, fast photoinduced electron transfer, and isotropic charge transportation.3, 5 Although the highest power conversion efficiency (PCE) for the fullerene-based single-junction PSCs has exceeded 10%, further PCE improvement for this type of devices is severely impeded by the relatively fixed

energy levels and large bandgaps of the fullerene derivatives.2, 7, 8 In contrast, the energy levels and bandgaps of non-fullerene acceptors can be more rationally designed and adjusted according to the selection of p-type donor materials thereby leading to better energy level alinements and improved lightharvesting from UV-visible to near-infrared (NIR) region of the resulting PSCs.4, 9 With the synergistic developments in non-fullerene acceptors, polymer donor materials, and interface engineering, significantly progresses in non-fullerene PSCs have been made in the past five years.10-24 Currently, the most actively studied structural motifs for non-fullerene acceptor materials are acceptor-donoracceptor (A-D-A) quadrupolar molecules, in which the electron-deficient units (A) are connected to the fusedring-based electron-rich core (D) via C=C bonds.25-27 For the A-D-A configuration, ladder-type aromatic fused-ring

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cores are frequently adopted as D units because their large rigid coplanar structures can prolong the effective πconjugation length and enhance the π-electron delocalization. Depending on the conjugation length and electron-donating ability of the ladder-type aromatic core, the energy level, optical bandgap as well as carrier mobility of the resulting non-fullerene acceptor can be tailored. Therefore, increasing efforts have been made on the development of non-fullerene acceptors by using various ladder-type aromatic cores in their backbones.9, 28, 29 At the same time, efforts have also been made on the engineering of side-chains and ending groups of nonfullerene acceptors towards high-performance PSCs.30-36 It has been known that the extension in the -conjugated molecular backbone usually gives rise to poor solubility of the resulting materials which could be solved by introduction of long side-chains. However, too bulky side-chains attached on the backbone may be harmful to the ordered π-π stacking because of steric hindrance effects. Thus, there should be a trade-off between the planarity and the solubility (or aggregation) when designing efficient ladder-type fused-ring-based nonfullerene acceptors. Recently, some successful examples on side chain engineering of non-fullerene acceptors were reported in literatures.33-37 For examples, Bo and coworkers reported that the incorporation of asymmetric side chains can increase the solubility of acceptor molecules, and enable the acceptor molecules to pack closely, and form favorable phase separation upon blended with the polymer donor of PBDB-T, resulting in an improvement of PCE from 6.42% to 10.12%.37 When Yang et al. replaced 4-hexylphenyl groups with n-hexyl groups on an acceptor molecule, the resulting bestperformance PSC exhibited an enhanced PCE of 12.2% due to the increased electron mobility, and the formation of more ordered face-on packing of the acceptor.33 Besides the side chains, the ending groups can significantly affect the optical bandgaps, energy levels and morphology of the resulting non-fullerene acceptors.4, 38-43 The fluorinated ending groups have often been used for designing high-performance non-fullerene acceptors.23, 4446 The intermolecular interactions could be enhanced by the formation of non-covalent F-S and F-H bonds through the introduction of fluorine atoms on the resulting acceptor, which is in favor of an enhanced charge transportation. Furthermore, the strong electronegativity characteristic of fluorine atom could accelerate the intramolecular charge transfer in A-D-A molecules, and lead to bathochromically shifted absorption which is beneficial for an enhanced light-harvesting. Heptacenes have been known as promising building blocks for semiconducting materials although they are not very stable in ambient conditions.29 Since 2010, our group has successfully incorporated heteroatoms such as sulfur or nitrogen into the heptacene system to develop a series of ladder-type heteroheptacenes for the construction of various p-type semiconducting copolymers with improved carrier mobility, stability, and photovoltaic performances.47-52 As shown in Scheme 1,

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four different types of aromatic rings are included in the structure of dithienocyclopentacarbazole providing sites where solubilizing side-chains (R1 and R2) or additional functional motifs can be appended readily. With the electron-rich nature of dithienocyclopentacarbazole, it was first used as a donor unit in our research group to construct hole-transporting materials.48 Recently, we further demonstrated that in combination with suitable electron-deficient ending groups, dithienocyclopentacarbazole can be also used for building efficient n-type (acceptor) materials for PSCs.53 When the dithienocyclopentacarbazole group was connected with two 2-(3-oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile (INCN) units by two thiophene bridges, a new A-D-A non-fullerene acceptor denoted as CDTCN (Scheme 1 ) was first developed in our group.53 Unfortunately, the best-performance CDTCN-based device only delivered a moderate PCE of 6.23%. Almost at the same time, three other parallel independent efforts have also been made on these dithienocyclopentacarbazole-based materials for nonfullerene polymer solar cells.54-56 However, the best PCE for all the non-fullerene acceptors based on dithienocyclopentacarbazole have not surpassed 11% in the literatures.54-57 We may attribute the relatively inferior device performance of the dithienocyclopentacarbazolebased non-fullerene acceptors to the inappropriate design of side chains and ending groups. Scheme 1. Chemical structures of ladder-type tetra-pphenylene and heteroheptacenes

In this context, we present the design and synthesis of three dithienocyclopentacarbazole-based non-fullerene acceptors with different side-chains and ending groups (HCN-C8, HCN-C16, and H2FCN-C16 in Scheme 2), for the aim of achieving a better device performance. A wide bandgap (WBG) polymer (J71 in Scheme 2) was selected as the donor material to match the bandgaps as well as the energy levels of the non-fullerene acceptors synthesized in this work.58 The photophysical, charge transporting, and photovoltaic performances of the resulting nonfullerene acceptors were investigated in detail. HCN-C8 with an octyl chain on the nitrogen atom showed a rather low PCE of 2.38%. However, the PCE was improved to


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5.51% when the octyl side-chain was replaced with a longer 2-hexyldecyl chain. Furthermore, the non-fullerene acceptor with 2-hexyldecyl chain and fluorinated ending groups (H2FCN-C16) showed the highest efficiency of

11.18%, which is the highest dithienocyclopentacarbazole-based acceptors, to the best of our knowledge.

among all non-fullerene

Scheme 2. Chemical structure of the polymer donor J71, and synthetic routes for non-fullerene acceptors HCN-C8, HCN-C16 and H2FCN-C16

Reagents and conditions: i) CHCl3, pyridine, reflux; ii) CHCl3, pyridine, 50 oC.

RESULTS AND DISCUSSION Synthesis and Characterization of Acceptors The chemical structures and synthetic routes for the dithienocyclopentacarbazole-based acceptors are displayed in Scheme 2, and the detailed synthetic procedures are described in the Experimental section. The key intermediates (M1 and M2) were prepared from a carbazole derivative according to the reported procedure.48, 53, 59 HCN-C8 and HCN-C16 were synthesized through a Knoevenagel condensation reaction at reflux between INCN and the aldehydes (M1 or M2) in 82% and 89% yields, respectively. Whereas, the Knoevenagel condensation between M2 and the fluorinated ending group of 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1ylidene) malononitrile (INCN2F) was carried out at a reduced temperature of 50 oC to afford H2FCN-C16 in 71% yield. For H2FCN-C16, the reflux temperature (61 oC) led to low synthetic yield of 50%. 1H NMR, high resolution mass spectrometry (HRMS), and elemental analysis were employed to confirm the chemical structures of the dithienocyclopentacarbazole-based acceptors. All the non-fullerene acceptors have good solubility in many common organic solvents, such as chloroform, toluene,

chlorobenzene, and dichloromethane at room temperature. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were carried out to evaluate the thermal properties for the synthesized non-fullerene acceptors. As shown in Fig. S1a, all the three acceptors showed excellent thermal stability with relatively high decomposition temperatures (Td) over 370 oC, which is a good attribute for their photovoltaic applications. Neither HCN-C16 nor H2FCN-C16 exhibited any thermal transition in the DSC measurement (Fig. S1cd), suggesting that both of them are of an amorphous nature. On the contrary, HCN-C8 with octyl side chain presented distinctively melting and crystallization transitions at 287 and 179 oC, respectively. These results indicate that the side chains on the -conjugated core have significant effect on the crystallinity of the resulting dithienocyclopentacarbazole-based acceptors. Optical and Electrochemical Properties The absorption spectra for HCN-C8, HCN-C16, and H2FCN-C16 in dilute chloroform solution and thin-film were measured, and the related data are outlined in Table 1. As presented in Fig. 1a, strong absorption bands located at 600-750 nm were observed for all the three nonfullerene molecules in solution which are mainly ascribed



to the intramolecular charge transfer between the dithienocyclopentacarbazole segment and the ending groups. HCN-C8 and HCN-C16 exhibited nearly identical absorption profiles with the absorption peaks (λmax) at 669 nm in solution despite their different side chains attached on the nitrogen atom of dithienocyclopentacarbazole. Furthermore, HCN-C8 and HCN-C16 show almost the same absorption ability with extinction coefficients (ε) of 1.85 × 105 M-1 cm-1 and 1.87 × 105 M-1 cm-1, respectively, suggesting the neglectable effect of side chain on the extinction coefficient of nonfullerene acceptors. With the fluorination on the ending groups, H2FCN-C16 showed 12 nm red-shifted absorption band with the largest extinction coefficient of 2.01 × 105 M-1 cm-1 among the three acceptors. As shown in Fig. 1b, in going from solution to thin-film, the corresponding absorption peaks for HCN-C8 and HCN-C16 exhibited bathochromic-shifts of 32 nm and 40 nm, respectively. In the case of H2FCN-C16, the value of bathochromic-shift increased to 53 nm. At the same time, a stronger shoulder peak at 670 nm appears in the thin film, suggesting a stronger intermolecular interaction in H2FCN-C16 film induced by the fluorination. Meanwhile, the absorption bands of the three non-fullerene acceptors match well with that of the WBG polymer donor J71, which is important to achieve high-performance PSCs. The optical bandgaps (Eopt g) for HCN-C8, HCN-C16 and H2FCNC16 are calculated to be 1.67, 1.66 and 1.59 eV, according to their absorption edges (λedge) in the pure films, respectively. As expected, the blend film based on J71:H2FCN-C16 (Fig. S2) has a strong and broad absorption in the region of 400-800 nm, which makes a high JSC value possible for the resulting device. However, the blend based on HCN-C8 or HCN-C16 has a narrower and more uneven absorption band in the region of 400600 nm, especially for the former. Considering the medium bandgap feature of all the three nonfullerene acceptors, both low bandgap and wide bandgap donor materials can be used to blend with them to achieve highperformance PSCs. In the work, a WBG polymer of J71 was selected as the donor material.

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The cyclic voltammetry (CV) measurements were carried out to investigate the impact of side-chain and ending groups on the energy levels of the three acceptors in thin-film. In the CV measurement, three-electrode system with an internal standard of ferrocene was used. Fig. 1c presents the CV curves for the three acceptors, and Table 1 summarizes the corresponding results. The onset oxidation/reduction potentials (ox/red) for HCN-C8, HCN-C16 and H2FCN-C16 are 0.81/-1.28 V, 0.81/-1.25 V and 1.13/-0.83 V, corresponding to electrochemical bandgaps of 2.09, 2.06 and 1.96 eV, respectively. The highest occupied molecular orbital (HOMO) energy levels (EHOMO) and lowest unoccupied molecular orbital (LUMO) energy levels (ELUMO) for the three acceptors were evaluated according to the formulas: EHOMO/ELUMO = (ox/red + 4.82) eV. Thus, the EHOMO/ELUMO values for HCN-C8 and HCN-C16 are -5.63/-3.54 eV and -5.63/-3.57 eV, respectively. It shows that the side-chain on the nitrogen atom has little influence on the energy levels of the resulting acceptors. However, the EHOMO/ELUMO values are down-shifted to -5.95/-3.99 eV for H2FCN-C16 which has four fluorine atoms attached on the ending groups. To further illuminate the fluorination effect on the energy levels of the resulting non-fullerene acceptors, density functional theory (DFT) calculations were performed at B3LYP/6-311G** level. The alkyl side chains on the phenyl groups and the central nitrogen atom were replaced with methyl units to simplify the DFT calculations. As shown in Fig. S3, the HOMO energy levels of HCN-C16 and H2FCN-C16 are mainly distributed on the dithienocyclopentacarbazole unit. Nevertheless, the electron densities of LUMO energy levels are extended from the quinoidal conjugation to the ending groups. With the fluorination, the calculated HOMO and LUMO energy levels were down-shifted from -5.71/-3.48 eV to 5.86/-3.65 eV with a relatively lower bandgap in agreement with the CV measurement results shown in Table 1. The energy diagram of the three acceptors as well as J71 was depicted in Fig. 1d. In general, the energy levels differences between J71 and the acceptors (HCN-C8, HCN-C16 and H2FCN-C16) are sufficient for efficient exciton dissociation.

Table 1. Optical, electrochemical and thermal properties of HCN-C8, HCN-C16 and H2FCN-C16 Acceptors

 [105 M-1 cm-1]

HCN-C8 HCN-C16 H2FCN-C16

1.85 1.87 2.01

a Estimated

maxsolution [nm] 669 669 681

maxfilm [nm] 701 709 734

Egopt [eV]a

HOMO [eV]b

LUMO [eV]b

Td [oC]c

1.67 1.66 1.59

-5.63 -5.63 -5.95

-3.54 -3.57 -3.99

372 395 377

by the absorption onset of thin film; b Energy levels obtained from CV; c Temperature with 5% weight loss.

Table 2. Device characteristics of the non-fullerene acceptors blended with J71 (1:1, w/w)a Acceptors HCN-C8 HCN-C16 H2FCN-C16 a

Voc [V] 1.01 (1.02±0.01) 1.03 (1.03±0.00) 0.90 (0.90±0.01)

Jsc [mA/cm2] 6.24 (5.81±0.21) 12.30 (12.08±0.25) 18.62 (18.64±0.09)

FF [%] 37.8 (37.6±0.41) 43.5 (42.1±0.99) 66.7 (66.2±0.37)

PCE [%]a 2.38 (2.23±0.10) 5.51 (5.24±0.20) 11.18 (11.10±0.06)

In the parentheses are average values based on more than 8 devices.


Rs [Ω cm2] 53.10 21.52 6.79

Rsh [Ω cm2] 400 340 1100

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Figure 1. a) UV−vis absorption spectra of HCN-C8, HCN-C16, H2FCN-C16 in CHCl3 solutions; b) absorption spectra of HCN-C8, HCN-C16, H2FCN-C16 and J71 in thin films; c) cyclic voltammetry of the non-fullerene molecules in Bu4NPF6 acetonitrile solution (0.1 M); d) energy level diagram of HCN-C8, HCN-C16, H2FCN-C16 and J71.

Figure 2. Photoluminescence spectra for the pure HCN-C8, HCN-C16, H2FCN-C16, J71 and the blend films of J71:acceptors (1:1, w/w): a) excited at 650 nm; b) excited at 650 nm; c) excited at 650 nm, d) excited at 540 nm. Photoluminescence To check the photo-induced exciton dissociation and charge transfer behaviors of the three non-fullerene acceptors in the blend films, photoluminescence (PL)

quenching experiments based on pure or blend films were performed. According to the absorption properties of HCN-C8, HCN-C16, H2FCN-C16 and J71 in pure film states, the wavelengths of 650 nm and 540 nm were



selected as the excitation wavelengths for the nonfullerene acceptors and polymer donor (J71) in the PL quenching experiments, respectively. Fig. 2a-d display the PL spectra of the pure non-fullerene acceptors and their blend films with J71. When excited at the wavelength of 650 nm, the non-fullerene acceptors show PL emission bands in the range from 700 to 870 nm. As shown in Fig. 2a, the PL emission of H2FCN-C16 is completely quenched by J71, suggesting efficient hole transfer from H2FCN-C16 to J71. However, for the blend of HCN-C16:J71, the corresponding PL emission is less quenched in comparison with H2FCN-C16:J71 counterpart. Moreover, as presented in Fig. 2c, the PL emission of the blend film based on HCN-C8:J71 is only ~70% quenched. The results suggest that the non-fullerene acceptor (H2FCN-C16) with branched side-chain and fluorine atoms in the ending groups can have the most efficient hole transfer with the donor polymer of J71 among all the three nonfullerene acceptors. Regarding the electron transfer, the PL emission band of J71 is located in the range of 600-850 nm when excited at 540 nm. The strong emission of J71 is also nearly all quenched by H2FCN-C16 in the H2FCNC16:J71 blend film, indicating the efficient electron transfer from J71 to H2FCN-C16. However, in both cases of HCN-C16:J71 and HCN-C8:J71, the PL emission of J71 was less completely quenched which could be partly attributed to the different morphologies of the blend films. Nonetheless, the results indicate that both the sidechain and fluorination have great effects on the exciton dissociation and charge transfer efficiencies of nonfullerene acceptors in the blend films which subsequently affect the performances of photovoltaic devices.

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Photovoltaic Performance The photovoltaic performances of these non-fullerene acceptors were studied by fabricating PSCs with an inverted device structure of ITO/ZnO/J71:acceptor /MoO3/Ag where J71 is a donor material. The J71:acceptor blends in chloroform solution (12 mg mL-1) were spincoated onto the ZnO-covered ITO glass to form the active layers. Because the donor:acceptor (D:A) blend ratio plays an important role in determining the performance of PSCs, the blend ratio in this work was finely tuned from 1:0.9 to 1:1.2 to obtain the optimal weight ratio of 1:1. At the same time, various thermal annealing conditions were used to optimize the performance of J71:H2FCN-C16based device. The related parameters for the PSCs based on J71:H2FCN-C16 were summarized in Table S1. It can be seen that the fine tuning of weight ratio had little effect on the device performance. However, high-temperature thermal annealing (150-170 °C) played a vital role in achieving high-performance non-fullerene PSCs based on J71:H2FCN-C16 (see Table S1). As shown in Fig S4, the absorption of J71:H2FCN-C16 blend film with annealing was significantly enhanced in the range from 500 to 800 nm, which is favorable for an improved JSC of the resulting devices. Without any thermal annealing treatment, the PSC with a weight ratio of 1:1 showed a rather inferior PCE of 4.83% with a low JSC of 13.07 mA cm-2 and a low FF of 38.1% despite its relatively higher VOC of 0.97 V. Thus, the active layers based on the other two non-fullerene acceptors were also spin-coated from their chloroform solution with a fixed blend ratio of 1:1, and then thermal annealed at 160 °C for 8 min in this work.

Figure 3. a) The J-V curves of PSCs based on J71:acceptors (1:1, w/w) with thermal annealing at 160 oC for 8 min, under the irradiation of AM 1.5G, 100 mW cm-2; b) The corresponding EQE and integrated JSC curves for the optimal devices; c) Jph versus Veff curves for the optimal devices under AM 1.5G, 100 mW cm-2; d) The dependence of JSC for the optimal devices on the light intensity.


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Fig. 3a presents the J-V characteristic curves for the non-fullerene PSCs with a weight ratio of 1:1 and thermal treatment, and the related device parameters were listed in Table 2 for comparisons. The best-efficiency PSC based on HCN-C8 exhibited a PCE of 2.38% with a VOC of 1.01 V, a JSC of 6.24 mA cm-2 and a FF of 37.8%. However, the best-efficiency device based on HCN-C16 showed a VOC of 1.03 V, a high JSC of 12.30 mA cm-2, an increased FF of 43.5%, and a final PCE of 5.51% which is more than 131% larger than that of the HCN-C8-based device. Compared to the HCN-C16-based counterpart, the HCN-C8-based device showed a slightly lower VOC which can be partly ascribed to the poor morphology of HCN-C8-based active layer. The enhanced JSC value of the HCN-C16-based device can be attributed to the more even absorption spectrum of the HCN-C16:J71 blend compared to that of the HCN-C8:J71 blend (Fig. S2). Surprisingly, with fluorination on the ending groups, H2FCN-C16-based device exhibited the highest PCE of 11.18% with significantly increased values of both JSC and FF (18.62 mA cm-2 and 66.7%). A slightly decreased VOC of 0.90 V was found for the H2FCN-C16-based device which should be ascribed to the down-shifted LUMO energy level by the fluorination on the ending groups. To best of our knowledge, the PCE of 11.18% is the highest among all dithienocyclopentacarbazole-based non-fullerene acceptors reported so far.54-57, 60 We noticed that, the bestefficiency device based on J71:H2FCN-C16 showed a large energy loss of 0.69 eV. The 0.69 eV energy loss is rather high compared with other efficient organic solar cell systems, thus the PCE for H2FCN-C16-based devices could be further enhanced when other donor materials with more suitable energy levels are available. To evaluate the photo-response of the PSCs, we measured external quantum efficiency (EQE) spectra for the best-efficiency devices based on all the three nonfullerene acceptors. As shown in Fig. 3b, the EQE spectra are in agreement with the absorption profiles of the blend films (Fig S2) suggesting that both the acceptor and donor materials contribute to the generation of photocurrent in the devices. The photocurrent values calculated from the EQE data are 6.45 mA cm-2, 12.13 mA cm-2, and 18.02 mA cm-2 for the best-efficiency devices based on HCN-C8, HCN-C16 and H2FCN-C16, respectively, which are coincident with the measured values from the J-V curves. The maximum EQE value for the HCN-C16-based device is promoted to ~62% in comparison with a low value of 33% for the HCN-C8-based device. The value is further improved to 81% for the H2FCN-C16-based device in agreement with the PCE variation of the best-efficiency devices based on the three acceptors. The relatively high EQE response located in the region of 620-800 nm for the H2FCN-C16-based device could be mainly contributed to the absorption of H2FCN-C16, according to the individual spectra of pure J71 and H2FCN-C16. It also indicates a more efficient hole transport from H2FCN-C16 to J71 in comparison with the other two counterparts, which is also corroborated by the results from the PL quenching experiment.

Charge Carrier Recombination

Generation,

Transport

and

To know the exciton dissociation, carrier transportation, and carrier collection efficiencies of the non-fullerene solar cells in this work, we investigated the relationships between photocurrent density (Jph) and effective voltage (Veff). The corresponding results are presented in Fig. 3c. The photocurrent density is given by Jph = JL − JD, in which JD and JL are current densities of the device in the dark and under the irradiation conditions, respectively. The effective voltage is given by Veff = V0 − Vbias, in which Vbias is the voltage applied on the device, and V0 is the voltage when Jph is equal to zero, respectively. As presented in Fig. 3c, Jph arrives at saturation state with a relatively high Veff (Veff ≈ 1.6 V) to attain the saturation current density (Jsat) for the H2FCN-C16-based device. Thus, the photoinduced excitons are dissociated to free carriers which subsequently arrive at the corresponding electrodes efficiently. In general, the figure of merit for exciton dissociation probability (Pdis) is given by Pdis = Jph/Jsat. The Pdis in the short-circuit condition is calculated to be as high as 98% for the H2FCN-C16-based device, which is obviously higher than that for the device based on HCNC16 (87%). The results are consistent with the measured JSC values of corresponding PSCs. The Pdis value is further dropped to 81% for the device based on HCN-C8:J71 which might be due to the deteriorated charge generation and extraction probability with the replacement of the 2hexyldecyl side-chain by the octyl side-chain. In addition, the HCN-C8-based device showed a larger series resistance (Rs) of 53.10 Ω cm2 in comparison with the Rs of 21.52 Ω cm2 for the HCN-C16-based device despite their comparable shunt resistances (Rsh) (400 vs 340 Ω cm2). Among the three devices, the H2FCN-C16-based PSC exhibited the best ohmic contact in the device with the smallest Rs value of 6.79 Ω cm2 while the largest Rsh value of 1100 Ω cm2. The dependence of JSC for the device under different light intensities (P) was evaluated in order to obtain indepth insight into the influences of side-chain and fluorination on the carrier recombination process in the PSCs based on the three acceptors. In principle, the correlation between P and JSC could be depicted by using the formula of JSC ∝ Pα, where α represents the degree of bimolecular recombination. The optimal device should have an α value approaching 1 which suggests the neglectable bimolecular recombination in the device. As shown in Fig. 3d, the linear fitted α value is up to 0.986 for the PSC based on J71:H2FCN-C16 in comparison to a lower value of 0.939 for the HCN-C16-based device. The result indicates that the bimolecular recombination could be remarkably suppressed in the device based on the nonfullerene acceptor with fluorination, thus leading to an improved FF. However, the α value is as low as 0.908 for the HCN-C8-based device, which partially explains its lower FF of 37.8%. The charge carrier transport properties of the J71:acceptor-based devices were investigated by using the space charge-limited current (SCLC) method in order to



learn the molecular structure-property relationships of the non-fullerene acceptors. The device configurations of ITO/ZnO/J71:acceptor/Ca/Al and ITO/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/J71:acceptor/Au were used in the electronand hole-only devices, respectively. As shown in Table 3 and Fig. S5, the electron (μe) and hole (μh) mobilities of J71:HCN-C8-based devices were measured to be 9.44 × 10-7 and 1.46 × 10-7 cm2 V-1 s-1, respectively. On the contrary, the HCN-C16-based devices exhibited μe/μh mobilities of 1.29 × 10-6/6.23 × 10-7 cm2 V-1 s-1. Compared to the devices based on HCN-C8 or HCN-C16, the device based on H2FCNC16 not only exhibited higher μe/μh mobilities of 7.53 × 106/6.67 × 10-6 cm2 V-1 s-1, but also showed a more balanced μe/μh ratio of 1.13, all of which are in favor of less carrier recombination, higher values in JSC and FF of the corresponding PSC. In general, the SCLC results agree well with the photovoltaic performance variations in the PSCs based on the acceptors in going from HCN-C8, HCN-C16 to H2FCN-C16. Table 3. Electron- and hole-mobilities of J71:acceptor blends (1:1, w/w) measured by using the SCLC methoda Acceptors HCN-C8 HCN-C16 H2FCN-C16

h [10-7cm2 V-1 s1]

1.46 (1.35±0.11) 6.23 (6.07±0.22) 66.7 (63.3±4.1)

e [10-7cm2 V-1

s-1] 9.44 (9.12±0.37) 12.9 (12.1±0.8) 75.3 (73.8±1.9)

e/h 6.46 2.07 1.13

In the parentheses are average values based on 8 devices. a

Morphology In order to better explain the reasons for the significant variation in the photovoltaic performance among the devices, the morphologies of blend films were studied by atomic force microscopy (AFM) with a tapping-mode.

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The blend films were prepared according to the same fabrication condition as that used for the PSCs. As presented in Fig. 4a, b, the blend film based on H2FCNC16 has a quite smooth surface with a relatively smaller root mean square (RMS) roughness of 0.69 nm compared to the HCN-C16-based blend film with an RMS roughness of 0.97 nm. A smooth surface may be in favor of forming a better ohmic contact between the active layer and the buffer layer. As presented in Fig. 4d, e, clear nanoscale phase separation could be observed for the blend films based on H2FCN-C16 and HCN-C16. However, the average domain sizes of blend film based on H2FCN-C16 are relatively smaller in comparison with those of the HCN-C16-based blend film. The result suggests that the fluorination would positively promote formation of phase separation with optimal domain sizes in BHJ active layer. The well-developed interpenetrating network with appropriately phase-separated domains for the H2FCNC16-based blend film would afford plenty of continuous percolating path for efficient charge dissociation transportation and collection thereby leading to higher JSC and FF values for the corresponding PSCs. It should be noted that the as-cast blend film based on H2FCN-C16 is relatively rough with an RMS of 0.92 nm (Fig. S6). Compared to that of the blend film based on H2FCN-C16 or HCN-C16, the surface of J71:HCN-C8 blend film is rougher with an increased RMS roughness of 3.39 nm. Large-size aggregations can be clearly observed from both the topography and phase images (Fig. 4c, f), which might be related to the good crystallinity of HCN-C8. Considering that the exciton diffusion length of general organic semiconducting materials is less than 10 nm, the J71:HCN-C8 blend film with large-sized aggregates would have an increased carrier recombination rate which agrees with the low PCE of 2.38% for the HCN-C8-based device.

Figure 4. AFM topography images (a, b, c) and phase images (d, e, f) of J71:acceptor blend films with tappingmode: (a, d) J71:H2FCN-C16 (1:1, w/w), (b, e) J71:HCN-C16 (1:1, w/w), (c, f) J71:HCN-C8 (1:1, w/w). The scan size is 500 × 500 nm.


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J71:H2FCN-C16 As-cast

(b)

qxy [Å-1]

(d)

J71:HCN-C8

qz [Å-1]

J71:HCN-C16

qxy [Å-1]

in plane out of plane J71:H2FCN-C16 As-cast

Intensity (a. u.)

qxy [Å-1]

(c)

(e)

J71:H2FCN-C16

qz [Å-1]

qz [Å-1]

(a)

qz [Å-1]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


J71:H2FCN-C16

J71:HCN-C16

J71:HCN-C8

qxy [Å-1]

Q vector [Å-1]

Figure 5. 2D-GIWAXS patterns of the blend films: a) as-cast J71:H2FCN-C16, b) annealed J71:H2FCN-C16, c) annealed J71:HCN-C16, d) annealed J71:HCN-C8, and e) the corresponding 1D line-cuts in the out of plane and in-plane directions. The grazing incidence wide-angle X-ray scattering (GIWAXS)61 experiments were carried out in order to know the the difference in the molecular packing of the blend films based on J71:acceptors. Fig. 5 depicts the corresponding 2D GIWAXS patterns and 1D cut-line curves, and Table S2 summaries the related data. From the figure, one can find that the molecules in all four blend films show face-on orientations with respect to the substrate because their lamellar stacking peaks (100) are found in the in-plane (IP) direction, and their π−π stacking peaks (010) are observed in the out-of plane (OOP) direction. For the as-cast J71:H2FCN-C16 blend film, thermally annealed J71:H2FCN-C16, J71:HCN-C16 and J71:HCN-C8 blend films, the crystal coherence lengths (CCLs) of π-π stacking are 16.4, 26.7, 15.3, and 17.1 Å, respectively, and the CCLs of (100) lamellar packing are 48.2, 81.9, 71.2, and 62.4 Å, respectively. The results indicate that among all the four blends, the annealed J71:H2FCN-C16 blend film has the strongest crystallinity which is beneficial for efficient charge transport and enhanced PCE of the corresponding device (11.18 % PCE). For the blend based on the fluorinated acceptor (J71:H2FCN-C16) with annealing, the π-π stacking distance (dπ) and CCL calculated from (010) diffraction peak in OOP direction are 3.54 and 26.7 Å, respectively. While for the blend based on the nonfluorinated acceptor (J71:HCN-C16) with annealing, the dπ and CCL values are 3.60 and 15.3 Å, respectively. The more compact π-π stacking and improved crystallinity for J71:H2FCN-C16 in comparison with J71:HCN-C16, explain the greatly increased hole- and electron-mobilities of the former which can reduce the charge recombination rate of the device. As a result, J71:H2FCN-C16 showed a much higher

PCE of 11.18%, compared to J71:HCN-C16 which exhibited a PCE of 5.51%. The values of (100) lamellar packing distance (dl) and dπ were simultaneously enlarged for the acceptor in going from HCN-C16 to HCN-C8. Furthermore, many other diffraction peaks appear in IP direction for HCN-C8-based blend film, which would be unfavorable for efficient charge transport thereby leading to deteriorated device performance (2.38%).

CONCLUSION In summary, three A-D-A non-fullerene acceptors (HCNC8, HCN-C16 and H2FCN-C16) based on a heteroheptacene of dithienocyclopentacarbazole have been designed and synthesized. Different side-chains and ending groups were introduced into the dithienocyclopentacarbazole-based molecular backbone in order to investigate the structure-photovoltaic property relationships of the non-fullerene acceptors. The sidechain on the central nitrogen atom of dithienocyclopentacarbazole barely affect the bandgap and energy levels of the final non-fullerene acceptors, but it can influence the crystallinity, mobility, miscibility and charge transfer with the polymer donor of J71. Thus, HCN-C16 with 2-hexyldecyl side-chain exhibited a PCE of 5.51% which is more than 131% higher than that for the non-fullerene acceptor with octyl side-chain (HCN-C8). With the combined effects from the branched 2hexyldecyl side-chain and the fluorination on the ending groups, H2FCN-C16 exhibited red-shifted absorption, enhanced mobility, improved charge transfer and phase separation morphology with J71 all of which resulted in a best-performance device with a remarkable PCE of 11.18%,



a high JSC of 18.62 mA cm-2, a FF of 66.7%, and a VOC of 0.90 V. The PCE of 11.18% is the highest among all dithienocyclopentacarbazole-based non-fullerene acceptors. The results suggest that the fluorination on the ending groups combined with suitable side-chain engineering is a useful strategy to boost the photovoltaic property of heteroheptacene-based non-fullerene acceptors.

EXPERIMENTAL SECTION Synthesis of HCN-C8. Under N2 atmosphere, compound M1 (0.15 g, 0.13 mmol) and INCN (0.20 g, 0.8 mmol) were added to a flask with dry chloroform (30 mL) in one portion. After the mixture was deoxygenated with N2 for 20 min, 0.5 mL of dry pyridine was syringed into the mixture. Then, the reaction was stirred and heated at reflux overnight. Subsequently, the reaction solvent was removed to give a crude product which was then purified by using column chromatography on silica gel with the eluent of petroleum ether/dichloromethane (1:1, v/v). A black crystalline solid was obtained (0.16 g, 82%). 1H NMR (CDCl3, 400 MHz, ppm): 8.77 (s, 2H), 8.69 (m, 2H), 7.97 (s, 2H), 7.87 (m, 2H), 7.77 (m, 4H), 7.68 (s, 2H), 7.34 (s, 2H), 7.24 (d, J = 8.0 Hz, 8H), 7.14 (d, J = 8.0 Hz, 8H), 3.87 (s, 2H), 2.58 (t, J = 8.0 Hz, 8H), 1.72 (s, 2H), 1.43-1.15 (m, 40 H), 0.88 (s, 17H). HRMS (MALDI) m/z: calc. for C104H101N5O2S2: 1515.7391; found: 1515.7347. Elemental analysis (%) calc. for C104H101N5O2S2: C, 82.34; H, 6.71; N, 4.62; found: C, 82.27; H, 6.76; N, 4.60. Synthesis of HCN-C16. HCN-C16 was synthesized by following the same procedure as HCN-C8. Yield: 89%. 1H NMR (CDCl3, 400 MHz, ppm): 8.92 (s, 2H), 8.70 (m, 2H), 7.93 (s, 4H), 7.77 (m, 4H), 7.72 (s, 2H), 7.64 (s, 2H), 7.16 (d, J = 8.0 Hz, 8H), 7.09 (d, J = 8.0 Hz, 8H), 4.26 (d, J = 8.0 Hz, 2H), 2.55 (t, J = 8.0 Hz, 8H), 2.25 (s, 1H), 1.52-1.15 (m, 56H), 0.93-0.78 (m, 18H). HRMS (MALDI) m/z: calc. for C112H117N5O2S2: 1627.8643; found: 1627.8600. Elemental analysis (%) calc. for C112H117N5O2S2: C, 82.56; H, 7.24; N, 4.30; found: C, 82.81; H, 7.25; N, 4.31. Synthesis of H2FCN-C16. Compound M2 (0.15 g, 0.12 mmol) and INCN2F (0.22 g, 0.96 mmol) were dissolved in dry chloroform (30 mL). After the mixture was bubbled with N2 for 20 min, 0.5 mL of dry pyridine was syringed in one portion. Under N2 protection, the mixture was stirred and heated at 50 oC for 8 h. After the removal of solvents, the crude product was quickly purified through column chromatography using petroleum ether/dichloromethane (3:2, v/v) as eluent to obtain gloss black solid (0.14 g, 71%). 1H NMR (CDCl , 400 MHz, ppm): 8.90 (s, 2H), 8.56 (m, 3 2H), 7.93 (s, 2H), 7.74 (s, 2H), 7.69 (t, J = 7.2 Hz, 2H), 7.66 (s, 2H), 7.15(d, J = 8.0 Hz, 8H), 7.09 (d, J = 8.0 Hz, 8H), 4.25 (d, J = 8.0 Hz, 2H), 2.55 (t, J = 8.0 Hz, 8H), 2.25 (s, 1H), 1.54-1.15 (m, 56H), 0.92-0.77 (m, 18H). HRMS (MALDI) m/z: calc. for C112H113F4N5O2S2: 1699.8266; found: 1699.8237. Elemental analysis (%) calc. for C112H113F4N5O2S2: C, 79.07; H, 6.70; N, 4.12; found: C, 79.26; H, 6.69; N, 4.09.

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ASSOCIATED CONTENT Supporting Information The detailed experimental procedures including materials and general methods, device fabrication and characterization, TGA and DSC curves, DFT calculations, J1/2V characteristics of hole- and electron-only devices, absorption spectra of blend films, AFM images and photovoltaic parameters of devices fabricated under different conditions are provided in supporting information.

AUTHOR INFORMATION Corresponding Author * E-mail:[email protected]

ACKNOWLEDGMENT We thank the support from the National Natural Science Foundation of China (Nos. U1605241, 21704082, 51561165011, 21875182), and the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (No. QYZDB-SSWSLH032). We acknowledge the support from the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract (No. DE-AC0205CH11231) for the X-ray data acquired at beamlines 7.3.3 at the Advanced Light Source.

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Enhancing the Photovoltaic Performance of Ladder-Type

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