High crystalline small molecule manipulates polymer-fullerene morphology and enables 20% improvement in fill factor and device performance

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High crystalline small molecule manipulates polymer-fullerene morphology and enables 20% improvement in fill factor and device performance Jianhua Chena,1, Chengcheng Tanga,1, Shigan Guoa, Zilong Wanga, Zhaoxia Hea, Ye-Jin Hwangb, Weibo Yana,∗, Hao Xina, Wei Huanga,∗∗ a Key Laboratory for Organic Electronics and Information Displays, Jiangsu Key Laboratory for Biosensors, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing, 210023, China b Chemical Engineering, Inha University, 100 Inha-ro, Yonghyun-dong, Nam-gu, Incheon, 22212, South Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Small molecule Ternary blend Phase separation Fill factor Organic solar cells

A morphology of bi-continuous donor-acceptor network absorber is critical to achieve high-performance organic solar cells and the rational control of the morphology is hard to realize. In this work, two high crystalline acceptor-donor-acceptor (A-D-A) conjugated small molecules (ITDCN, ITDCF) were synthesized and employed as the third component for PBDB-T/PC71BM system. Although ITDCN and ITDCF do not have favorable complementary absorption and cascade energy level alignment to PBDB-T and PC71BM, addition of ITDCN or ITDCF into PBDB-T:PC71BM binary system significantly improves device fill factor (up to 20%) and open circuit voltage (20–40 mV). Characterization using space-charge limited current (SCLC), AFM and TEM shows addition of ITDCN or ITDCF manipulates absorber film phase separation which leads to improved and balanced charge carrier mobility. A power conversion efficiency of 6.68% has been achieved in PBDB-T:ITDCN:PC71BM ternary solar cell with 15% ITDCN which is 24% higher than binary device. Our results suggest introducing high crystalline small molecule into polymer-fullerene system is an effective strategy to rationally control film morphology and improve polymer/fullerene solar cell performance.

1. Introduction Organic solar cells are considered as one of the most promising green energy technologies due to their light-weight, low-cost, flexibility, semitransparency, simple preparation and roll-to-roll production [1–6]. Recently, the efficiency of both single-junction [7–11] and tandem [12] organic solar cells (OSCs) are approaching the commercial silicon or thin film solar cells, demonstrating the great potential of OSCs for large-scale production in the near future. The power conversion efficiencies (PCEs) of OSCs highly depends on the morphology of absorber due to the excitonic nature of the organic semiconducting materials. A high crystalline donor-acceptor bi-continuous morphology with nanoscale domain size [13–17] plays a vital role in achieving efficient charge separation and transport and the morphology is greatly influenced by the crystallinity and miscibility properties of the materials involved. Generally, polymer donors are compatible with fullerene acceptors, however, their crystallinity is lower than small molecules which makes them hard to achieve significant phase separation and

thus impedes the improvement of fill factor (FF) and PCE. Meanwhile, compared with polymer donors, small molecules usually have higher hole mobility and crystallinity, which benefit charge transport but may lead to too large crystalline domains and limit charge dissociation [18,19]. Recently, adding small molecule to binary OSCs has been demonstrated to improve device performance by providing complementary absorption to increase short-circuit current density (JSC) and/or forming a cascade energy alignment to facilitate exciton diffusion and charge separation. For example, Sun and co-workers first reported 24% improvement in JSC and achieved 10.3% efficient ternary solar cell by using two different nonfullerene small molecule acceptors (ITIC-Th and SdiPBI-Se) and one conjugated copolymer (PDBT-T) for their complementary absorption [20]. Ge and co-workers reported 12.16% ternary solar cell by adding a novel acceptor ITCN into PBDBT:IT-M system to improve open circuit voltage (VOC) (from 0.937 V to 0.954 V) due to better energy level alignment as well as complementary absorption [21]. The effect of the small molecule on the morphology of phase separation of polymer/fullerene blend and device

∗

Corresponding author. Corresponding author. E-mail addresses: [email protected] (W. Yan), [email protected] (W. Huang). 1 contributed equally to this work. ∗∗

https://doi.org/10.1016/j.orgel.2019.105419 Received 31 May 2019; Received in revised form 14 July 2019; Accepted 23 August 2019 1566-1199/ © 2019 Published by Elsevier B.V.

Please cite this article as: Jianhua Chen, et al., Organic Electronics, https://doi.org/10.1016/j.orgel.2019.105419

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2.2. Synthesis

performance has not been paid enough attention. Compared to small molecules with symmetrical structure explored, asymmetrical structure usually produces larger intrinsic dipole-dipole moment and thus can induce intermolecular dipole interaction and antiparallel molecule arrangement, resulting in stronger π-π stacking [22,23], which can modify blend film morphology, inhibit bimolecular recombination and improve charge carrier mobility [24–26]. In this report, we have used asymmetrical indenothiophene [27,28] as core building block and strong electron withdrawing malononitrile or 5,6-difluoro-1,1-dicyanomethylene-3-indanone [29–31] as terminal group and synthesized two high crystalline small molecules ITDCN and ITDCF. The relatively weak electron-donating feature of indenothiophene unit is expected to increase ionization potential and improve device VOC [27,28]. The increase of electron withdrawing ability of the terminal group can reduce compound bandgap and we expected ITDCF will have narrower bandgap than ITDCN. Further, in order to achieve appropriate charge carrier mobility and crystallinity, we have selected thiophene as a bridge to connect core and terminal groups. We have used ITDCN and ITDCF as a third component in PBDB-T/PC71BM system to investigate their effects on device performance. We found the addition of the small molecule optimized polymer-fullerene blend phase separation which resulted in great improved and balanced charge carrier mobility and device performance within the ratio of 0.95:0.05:1 to 0.70:0.30:1 investigated. An achievement of 20% increase in FF and 24% increase in PCE was realized by adding 15% ITDCN to system without absorption complementary. Although ITDCF exhibits an undesirable energy level to PBDB-T/PC71BM system, 20% addition of ITDCF still achieved 12% improvement in FF solely due to more ideal phase separation and higher charge carrier mobility. Our results demonstrate addition of high crystalline small molecule to polymer/fullerene system is an effect strategy to tune absorber film morphology and improve OSCs efficiency.

Compound ITDCN. Compound A (300 mg, 0.43 mmol) and malononitrile (140 mg, 1.72 mmol) were added to a mixture of chloroform (30 mL) and pyridine (1 mL) and then deoxygenated by argon bubbling for 30 min. The mixture was heated to 80 °C and refluxed for 72 h under argon atmosphere. After cooled down to room temperature, the mixture was poured into methanol to precipitate as crude product, which was further purified by column chromatography using petroleum ether and dichloromethane as the eluent. Pure ITDCN (170 mg) was obtained as atrovirens solid with 50% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) = 7.77–7.76 (d, J = 8.0, 2H), 7.66–7.62 (dd, J = 8.0, 4.0 Hz, 2H), 7.61–7.55 (m, 2H), 7.47–7.46 (d, J = 4.0 Hz, 1H), 7.45 (s, 1H), 7.38–7.34 (m, 2H), 7.33–7.29 (m, 2H), 2.04–1.96 (d, J = 4.0 Hz, 2H), 1.35–1.25 (m, 6H), 1.05–0.76 (m, 8H), 0.74–0.54 (m, 18H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 167.74, 157.41, 154.78, 150.34, 149.94, 147.96, 144.32, 140.56, 137.90, 137.42, 134.11, 133.39, 132.33, 130.97, 128.87, 128.46, 125.27, 124.29, 124.00, 123.93, 122.56, 122.44, 121.19, 121.03, 120.98, 119.93, 114.42, 113.66, 75.91, 65.56, 54.61, 43.59, 35.17, 34.24, 33.77, 31.71, 30.58, 30.19, 29.71, 28.56, 27.50, 27.15, 22.81, 19.20, 14.00, 13.74, 10.71, 10.45. HRMS (MALDI) calcd. for C47H46N4S4, 794.260; found [M − H]-, 793.682. Compound ITDCF. The synthetic procedure of ITDCF was similar to that of ITDCN. Pure ITDCF was obtained as a black solid with 45.8% yield. 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.88–8.85 (s, 1H), 8.59–8.57 (dd, J = 16.0, 4.0 Hz, 2H), 7.85–7.81 (t, J = 4.0 Hz, 2H), 7.75–7.68 (m, 2H), 7.66–7.58 (m, 3H), 7.56–7.53 (t, J = 4.0 Hz, 1H), 7.51–7.47 (d, J = 8.0 Hz, 2H), 7.43–7.39 (m, 3H), 2.09–1.96 (d, J = 8.0 Hz, 4H), 1.44–1.23 (m, 5H), 1.11 (s, 1H), 1.06–0.78 (m, 10H), 0.76–0.56 (m, 14H). 13C NMR (100 MHz, CDCl3): δ (ppm) = 186.15, 186.03, 185.74, 169.48, 168.69, 167.72, 166.56, 163,94, 160,31, 158.13, 157.79,155.90, 154.95, 154.37, 153.56, 148.67, 147.75, 147.07, 145.36, 138.46, 137.61, 136.59, 135.43, 134.54, 132.60, 131.28, 130.96, 128.82, 125.33, 124.89, 122.94, 122.71, 121.18, 120.95, 120.31, 115.07, 114.90, 114.26, 114.07, 114.00, 112.76, 112.57, 110.97, 84.01, 79.55, 70.06, 65.59, 54.67, 51.58, 47.44, 43.72, 35.18, 34.29, 33.69, 31.96, 30.66, 29.76, 28.66, 28.28, 28.21, 27.56, 27.17, 22.92, 19.23, 14.14, 13.75, 10.84, 10.39. HRMS (MALDI) calcd for C66H50F4N4O2S4, 1122.280; found [M − H]-, 1121.618.

2. Experimental 2.1. Materials and methods All reagents were obtained from commercial sources and used without further purification unless specified. 5,6-difluoro-1,1-dicyanomethylene-3-indanone (98%) was purchased from Zhengzhou Alfachem Co., Ltd. PBDB-T and PC71BM were obtained from Solarmer Energy, Inc. Compound A was synthesized according to the literature [32]. 1H and 13 C NMR spectra were measured on a Bruker Daltonics Avance III NMR instrument operating at 400 MHz with tetramethylsilane (TMS) as internal reference. Relative molecular mass was measured by Bruker Daltonics autoflex speed MALDI-TOF. Thermogravimetric analysis (TGA) was performed on Netzsch TG209 at a heating rate of 10 °C·min−1 under nitrogen flow. Absorption spectra was acquired on a Lambda 35 UV/vis spectrophotometer. The electrochemical properties of the acceptors were measured on CHI660E electrochemical workstation with tetrabutylammonium hexafluorophosphate (Bu4NPF6) as electrolyte and acetonitrile as solvent. Glassy carbon, platinum wire and Ag/AgCl were used as working electrode, auxiliary electrode and reference electrode, respectively. X-ray diffraction (XRD) spectra were performed with a Bruker Daltonics D8 Advance A25 X-ray diffractometer. The samples for XRD measurement were prepared by dropcasting CH2Cl2 solution of small molecules on ITO/glass substrates. Atomic force microscope (AFM) height and phase images were measured on Icon dimension atomic force microscope. Transmission electron microscope (TEM) images were measured on HT7700. The current density-voltage (J–V) curves of the solar cells were measured under simulated 100 mWcm-2 AM 1.5 G irradiation by a Keithley 2400 source meter. The external quantum efficiency (EQE) of the solar cells were recorded on Enlitech QE-R3018 using calibrated Si (Enli technology Co. Ltd.) as reference.

2.3. Fabrication of organic solar cells Both binary and ternary solar cell devices with an inverted architecture of ITO/ZnO/active layer/MoO3/Ag were fabricated to investigate the effect of the addition of the small molecule on device performance. ITO glass substrates were washed with ethanol, acetone and isopropanol subsequently for 20 min under ultrasonic and dried with N2. The substrates were further treated by ozone plasma for 4 min. On top of ITO substrate, ZnO (30 nm) was fabricated by spin-casting precursor solution [6] and annealed at 200 °C for 60 min as the electron transporting layer. On top of ZnO, the active layer (100–200 nm) was formed by spin-coating a binary or trinary chlorobenzene solution with different concentrations at different speeds for 60 s. The film was annealed at 120 °C for 10 min. Then, MoO3 (8 nm) and Ag (100 nm) were successively evaporated on top of the active layer in a vacuum chamber with pressure below 2 × 10−4 Pa as electrodes. The active area of the devices was 4 mm2. 2.4. Fabrication and characterization of single-carrier devices The charge carrier mobility was measured using the space charge limited current (SCLC) method. The hole-only and electron-only devices were fabricated with the structures of ITO/PEDOT:PSS/active layer/ MoO3/Ag and ITO/ZnO/active layer/Ca/Al, respectively. The hole mobility (μh) and electron mobility (μe) were calculated from the dark J–V curves using single carrier SCLC model with the equation of 2

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Scheme 1. Synthetic routes of ITDCN and ITDCF.

−3.70 eV for ITDCN and −5.73 and −4.25 eV for ITDCF, respectively. The bandgap of ITDCN and ITDCF are thus calculated to be 1.82 and 1.48 eV, respectively.

J = 9ε0εrμV2/8 d3, where J is the current density, d is the thickness of the blend films, ε0 is the permittivity of free space, εr is the relative dielectric constant of the transport medium and μ is the charge carrier mobility. V = Vapp – Vbi, where Vapp is the applied voltage and Vbi is the offset voltage. The carrier mobility was calculated from the slope of the J1/2–V curves.

3.3. Optical properties UV–vis absorption spectra of ITDCN and ITDCF in solutions and as films are presented in Fig. 2. The absorption of polymer PBDB-T and fullerene derivative PC71BM that will be used as donor and acceptor in thin film solar cell are also shown. In solution, ITDCN exhibits a strong absorption band in wavelength range of 400–620 nm whereas ITDCF has absorption in much longer wavelength of 500–770 nm, which is consistent with their bandgaps measured from the CV. The molar absorption coefficients at the peak absorption are 8.26*104 M−1cm−1 for ITDCN and 1.17*105 M−1cm−1 for ITDCF (Fig. 2a). Compared to solution, films show broader and red-shifted absorption with ITDCN red shifted 66 nm (from 538 nm in solution to 604 nm in film) and ITDCF red-shifted 72 nm (from 663 nm in solution to 735 nm in film) (Fig. 2b) due to intermolecular interaction. The larger red shift of ITDCF compared to ITDCN indicates stronger molecular interaction. The absorption of film ITDCN falls to the wavelength range of PBDB-T and thus cannot complement light harvesting efficiently. On the contrary, the absorption of ITDCF film shows good complementation to PBDB-T at long wavelength range of 700–860 nm, having potential to achieve high current density. Some important optical and electronic data of the two small molecules were summarized in Table 1.

3. Results and discussion 3.1. Synthesis Scheme 1 shows the synthetic routes of ITDCN and ITDCF. The skeleton block of compound A was synthesized according to the literature [32]. Under the catalysis of piperidine, precursor A reacted with terminal group dicyanopropane and 2-(5,6-difluoro-3-oxo-indan-1-ylidene)-malononitrile to produce ITDCN and ITDCF with a yield of 50% and 45.8%, respectively. Both small molecules show good solubility at room temperature in common organic solvents such as dichloromethane, tetrahydrofuran, chlorobenzene and toluene, which is favorable for solution-process strategy. 3.2. Thermogravimetric and electrochemical properties Thermal stability and energy levels of the small molecules are investigated by thermogravimetric analysis and cyclic voltammetry and the results are shown in Fig. 1a. The decomposition temperature of ITDCN and ITDCF are 180 and 160 °C, respectively, higher than the annealing temperature of active layer (120 °C) used in this work. According to cyclic voltammogram curves in Fig. 1b, the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are calculated to be −5.52 and

3.4. Crystallinity X-ray diffraction (XRD) was performed to investigate the molecular packing of ITDCN and ITDCF. As shown in Fig. 3, sharp (100)

Fig. 1. (a) Thermogravimetric curves and (b) cyclic voltammogram of ITDCN and ITDCF. 3

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Fig. 2. UV–vis absorption spectra of ITDCN, ITDCF, PBDB-T and PC71BM in toluene solutions (a) and as thin films (b).

diffraction peaks are observed in both films with ITDCN at 2θ of 7.13° and ITDCF at 4.13°, which correspond to the lamellar stacking with intermolecular distances of 12.38 Å for ITDCN and 21.37 Å for ITDCNF, respectively. The (010) diffractions stemming from intermolecular π-π stacking are detected for both films with ITDCN appears at 2θ of 24.48° and ITDCF appears at 25.97°, corresponding to distances of 3.63 Å and 3.43 Å, respectively. ITDCF film not only shows shorter π-π stacking distance (3.43 Å) than ITDCN (3.63 Å) but also has much stronger and sharper diffraction peaks, indicating higher crystallinity of this film than ITDCN. The XRD results indicate aromatic group enhances indenothiophene based small molecule self-crystallization. The high crystallinity of both molecules are expected to tune film morphology and improve device performance when added into polymer-fullerene binary active layers [33,34].

3.5. Photovoltaic properties The effect of the small molecules ITDCN and ITDCF as an additive (third component) to the photovoltaic performance of PBDB-T:PC71BM binary system are investigated by fabrication of inverted devices with structure of ITO/ZnO/active layer/MoO3/Ag. The molecular chemical structures, device configuration, and energy level diagram of the materials used in this study are given in Fig. 4. The photovoltaic performances of the ternary solar cells were investigated with varied PBDBT:ITDCN/ITDCF:PC71BM ratio and the results were shown in Table 2, S4 and Fig. 5. It can be seen that with increase of ITDCN versus PBDB-T from 5% to 30%, the average JSC first increases then decreases, reaching a maximum value of 12.57 mA/cm2 at 15% whereas FF continuously increases and reaches 66.05% at 30% replacement, 20% enhancement compared to PBDB-T:PC71BM binary system. The VOC is slightly improved and remains invariable at ~0.83 V. We note the thickness of the ternary blend is about 170–180 nm and the thick absorber is beneficial for industrial applications. The JSC values were verified by external quantum efficiency (EQE) (Fig. 5c). The JSC of PBDB-T:ITDCN:PC71BM system mainly originated from the wide EQE response from 300 to 700 nm and the EQE curve begins to exceed than PBDB-T:PC71BM system with 15% ITDCN addition. However, large decline of EQE response happens in 350–500 nm and 550–650 nm over 20% ITDCN addition. Considering the inferior absorption ability of ITDCN than PBDBT (Fig. 2b), the replacement of small amount of PBDB-T by ITDCN

Fig. 3. The XRD patents of ITDCN and ITDCF films.

would decrease the total absorption of the ternary blend layer. The best average PCE was obtained at the weight ratio of 0.85:0.15:1 for PBDBT:ITDCN:PC71BM ternary system, and the champion device achieved 6.68% PCE with JSC of 12.78 mA/cm2, VOC of 0.83 V and FF of 0.63. As for the PBDB-T:ITDCF:PC71BM ternary system, as shown in Fig. 5b and Table S4, with the increase of ITDCF versus PBDB-T from 5% to 30%, the FF and VOC follow the same trend as that of PBDBT:ITDCN:PC71BM system, that is, FF increases almost linearly while VOC slightly increases, indicating similar effect of the two small molecules on film morphology. Different from PBDB-T:ITDCF:PC71BM system, the JSC of PBDB-T:ITDCN:PC71BM is lower than PBDB-T:PC71BM binary system. This is because with the increase of ITDCF, the film thickness gradually drops from 140 to 102 nm, which is thinner than PBDBT:ITDCN:PC71BM ternary system fabricated at the same concentration. We attribute the reduction of active layer thickness to ITDCF's too strong crystallinity. If a thicker film can be fabricated by optimizing film fabrication condition, better device performance can be expected. What's more, the Jsc values were verified by external quantum efficiency (EQE) (Fig. 5d). It can be seen that addition of ITDCF at any ratio

Table 1 Optical and electronic properties of ITDCN and ITDCF. Small Molecule ITDCN ITDCF a b

λmax/sol (nm) 538 663

εmax/sol (M−1cm−1) 4

8.26*10 1.17*105

λonset/film (nm)

a

698 864

1.77 1.43

Eg = 1240/λonset. Determined by cyclic voltammetry. 4

Eg (eV)

b

HOMO (eV)

−5.52 −5.73

b

LUMO (eV)

−3.70 −4.25

b

Eg (eV)

1.82 1.48

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Fig. 4. (a) Chemical structures of PBDB-T, PC71BM, ITDCN and ITDCF; (b) solar cell device structure; (c) energy level diagram of the four materials in (a).

increase in VOC.

all leads to EQE decline at absorption region from 300 to 700 nm, which mainly stems from the gradually thinning film thickness, but results in wider EQE response reaching 800 nm due to the absorption of ITDCF. This explains why the Jsc value of PBDB-T:ITDCF:PC71BM ternary system is only slightly lower than that of PBDB-T:PC71BM binary system. Energy level of PBDB-T, ITDCN/ITDCF and PC71BM is shown in Fig. 4c. It can be seen that the energy levels of ITDCN are located between PBDB-T and PC71BM forming energy cascade. For ITDCF, due to its deeper-lying LUMO energy level than that of PC71BM, so it cannot form energy cascade between PBDB-T and PC71BM. Similar improvement of VOC in PBDB-T:ITDCN/ITDCF:PC71BM ternary system demonstrates that energy level is not the key issue to influence the device performance. The photovoltaic performance suggests that the proper addition of ITDCN/ITDCF can both improves the PCE of the OSCs devices.

3.7. Morphology of absorber film Based on the above analysis, we believe that morphological control may be the key factor affecting device performance. To further explain the device characteristics, the morphology of the active layer was investigated by AFM and TEM (Fig. 7, S1). AFM height images of PBDBT:ITDCN:PC71BM blend film present that the surface root-mean square roughness (RMS) gradually diminished from 6.45 to 2.18 nm with increasing ITDCN amount from 0% to 20% (Fig. 7a–e). With further addition of ITDCN to 30% and 100%, film RMS increased dramatically to 13.9 nm and 36.6 nm (Fig. 7f and g). This means appropriate amount of ITDCN addition can effectively reduce surface roughness of active layer, which is beneficial for device performance. In the respect of AFM phase and TEM images, with increasing ITDCN amount from 0% to 20% (Fig. 7a'-7e', 7a"-7e"), the phase separation and interconnected structure become clearer. In the meantime, the domain size becomes smaller. This confirms that addition of high crystalline ITDCN can adjust the morphology of the active layer and improve the FF. Further increase ITDCN to 30% (Fig. 7f'), the domain size grows to 50 nm, which is far beyond ideal dimension, therefore the device performance starts to decline with increased FF. Obviously, variation of morphology is closely related to the change of FF. The fine continuous interpenetrating phase separation with domain size of 20–30 nm which is close to the exciton diffusion length (10–20 nm) could be observed at 15% addition amount of ITDCN, in the case of which efficient exciton dissociation and more balanced μe/μh can occur, resulting in high FF and the best efficiency. We also studied the film morphology of ITDCN:PC71BM binary blend (Fig. 7g'), and at this extreme case, 200 nm long nanofibers were formed and the devices did not work. As for the PBDB-T:ITDCF:PC71BM system (Fig. S1), the surface RMS has similar variation features with the addition of ITDCF compared to PBDB-T:ITDCN:PC71BM system. The RMS gradually diminished from 10.50 to 2.78 nm with increasing ITDCF amount from 5% to 20%. With further addition of ITDCF, film RMS increased just like PBDB-T:ITDCN:PC71BM system. In addition, as is shown in AFM phase and TEM images, with increasing ITDCF amount from 0% to 20%, clearer phase separation and interconnected structure can be observed. These phenomena indicate that proper addition ITDCF of ternary active layer has positive impact on the morphology as well as ITDCN. The best film morphology appeared at 20% addition amount of

3.6. Charge carrier mobility The hole (μh) and electron mobility (μe) of blend films of PBDBT:ITDCN/ITDCF:PC71BM with different composition were investigated via SCLC method and the J-V curves are shown in Fig. 6. The mobilities extracted from the J-V curves are summarized in Table 3. It's clear that with the addition of ITDCN or ITDCF into PBDB-T:PC71BM binary system, μh and μe of two ternary system both increase significantly. Specifically, μh increases from 6.98*10−5 (0% addition) to the maximum of 3.56*10−4 cm2V−1s−1 (15% ITDCN addition) and μe increases from 2.56*10−4 (0% addition) to the maximum of 4.02*10−4 cm2V−1s−1 (20% ITDCN addition), and the minimum μe/μh ratio of 1.09 is achieved at a weight ratio of 0.85:0.15:1 for PBDBT:ITDCN:PC71BM ternary system. The improved charge carrier mobility and more balanced μe/μh ratio lead to continuous increase in FF, which reaches a maximum of 68.44%. For PBDB-T:ITDCF:PC71BM ternary system, μh reaches the maximum of 3.05*10−4 cm2V−1s−1 and the minimum μe/μh ratio of 1.14 is achieved at a weight ratio of 0.80:0.20:1. Meanwhile, as well as the former ternary system, FF increases a lot and reaches a maximum of 62.87%. These results strongly indicate that the addition of ITDCN/ITDCF can effectively enhance the μe and μh, and also promote their balance in the ternary system of PBDB-T:ITDCN/ITDCF:PC71BM, which are beneficial for charge transfer and collection, resulting in significant improvement in FF and slight Table 2 Photovoltaic performance of ternary OSCs with different composition. PBDBT:ITDCN :PC71BM

Thickness (nm)

VOC (V)

1:0:1 0.95:0.05:1 0.90:0.10:1 0.85:0.15:1 0.80:0.20:1 0.70:0.30:1

140 170 170 180 175 178

0.80 0.83 0.82 0.83 0.82 0.84

± ± ± ± ± ±

10 8 6 5 8 9

± ± ± ± ± ±

0.02 0.01 0.02 0.01 0.01 0.01

5

JSC (mAcm−2)

FF (%)

12.13 ± 0.13 11.76 ± 0.21 11.62 ± 0.16 12.57 ± 0.21 10.22 ± 0.30 9.35 ± 0.37

54.99 58.89 62.54 62.93 65.72 66.05

± ± ± ± ± ±

PCEave/PCEmax (%) 0.93 1.07 1.44 1.30 1.68 2.39

5.34 5.75 5.96 6.56 5.51 5.19

± ± ± ± ± ±

0.05/5.39 0.06/5.81 0.10/6.06 0.12/6.68 0.28/5.79 0.38/5.57

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Fig. 5. J–V curves of ternary devices for PBDB-T:ITDCN:PC71BM (a), PBDB-T:ITDCF:PC71BM (b); EQE spectra of ternary devices for PBDB-T:ITDCN:PC71BM in a (c), for PBDB-T:ITDCF:PC71BM in b (d).

Fig. 6. J-V characteristics of hole-only (a, c) and electron-only (b, d) devices of PBDB-T:ITDCN:PC71BM (a, b) and PBDB-T:ITDCF:PC71BM (c, d) blend films with different addition of ITDCN and ITDCF.

6

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Table 3 The charge carrier mobility of PBDB-T:ITDCN(ITDCF):PC71BM ternary system with different addition proportion of ITDCN(ITDCF). addition amount

PBDB-T:ITDCN:PC71BM 2 −1 −1

μh (cm V 0% 5% 10% 15% 20% 30%

−5

6.98*10 1.67*10−4 2.45*10−4 3.56*10−4 3.22*10−4 2.87*10−4

s

)

PBDB-T:ITDCF:PC71BM 2 −1 −1

μe (cm V

s

μe/μh

)

−4

2.56*10 2.4*10−4 3.65*10−4 3.88*10−4 4.02*10−4 3.55*10−4

3.67 1.60 1.49 1.09 1.25 1.24

μh (cm2V−1s−1) −5

6.98*10 7.22*10−5 9.65*10−5 1.64*10−4 2.85*10−4 3.05*10−4

μe (cm2V−1s−1) −4

2.56*10 2.01*10−4 2.15*10−4 2.68*10−4 3.24*10−4 4.10*10−4

μe/μh 3.67 2.78 2.23 1.63 1.14 1.34

Fig. 7. AFM height (a–g), phase (a'–g') and TEM (a"–g") images of PBDB-T:ITDCN:PC71BM blend films with ITDCN/PC71BM ratio of 0%, 5%, 10%, 15%, 20%, 30% and 100%.

Declaration of competing interest

ITDCF, behaving with a RMS of 2.78 nm and a domain size of 50–80 nm. No ideal phase separation was obtained, which is attributed to the strong self-crystalline property of ITDCF and its thin thickness of active layer, leading to the inferior device performance.

The authors declare no competing financial interests. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Grant nos. 21571106 and 61704087) and the National Natural Science Foundation of Jiangsu Province (BK20161514 and BK20160887).

4. Conclusions In summary, we have designed and synthesized two A-D-A conjugated small molecules ITDCN and ITDCF. Characterizations of TGA, UV–vis, and XRD demonstrates that both molecules exhibit good thermal stability, strong absorption, excellent solubility and high crystallinity. With unfavorable absorption and energy level alignment, addition of ITDCN or ITDCF into PBDB-T:PC71BM binary system significantly improves device FF (up to 20%) with 20–40 mV increase in VOC. With 15% addition of ITDCN into PBDB-T:PC71BM, a power conversion efficiency of 6.68% has been achieved in the ternary device which is 24% higher than binary device due to enhancement of FF from 55.0% to 63.0% and VOC from 0.80 to 0.83 V. Addition of ITDCF shows similar improvement in FF and VOC but does not lead to obvious improvement in PCE owing to thinner active layer under the same film fabrication condition. Characterization using SCLC, AFM and TEM reveals addition of ITDCN or ITDCF manipulates bend film morphology which leads to greatly improved and more balanced charge carrier mobility. Our results demonstrate introducing high crystalline small molecule into polymer/fullerene system is a promising strategy to optimize absorber film morphology and improve achieve device photovoltaic performance.

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