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Halogenation on terminal groups of ITIC based electron acceptors as an effective strategy for efficient polymer solar cells
Solar Energy 195 (2020) 429–435
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Halogenation on terminal groups of ITIC based electron acceptors as an effective strategy for efficient polymer solar cells
T
Shujing Lua,1, Feng Lib,1, Kaili Zhanga, Jie Zhua, Wen Cuia, Renqiang Yangc, Liangmin Yud, ⁎ Mingliang Suna, a
School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Engineering, Qingdao University of Science & Technology, Qingdao 266042, China c CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China d Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100 b
A R T I C LE I N FO
A B S T R A C T
Keywords: Halogenation Non-fullerene acceptors Power conversion efficiencies
Acceptor halogenation is an excellent modification method for modulating molecular energy levels, improving optical absorption, shortening the intermolecular packing distance thus ameliorating morphologies and achieving effective exciton dissociation and charge transfer of organic photovoltaic device. In this work, F, Cl and Br terminated ITIC small molecule acceptor (SMA) derivatives (IT-2F, IT-2Cl and IT-2Br) are designed and synthesized to compare the halogen substitution effect. Among these three acceptors, IT-2F based PSCs devices show the highest power conversion efficiencies (PCEs) around 12.08%, but Br terminated ITIC show downshifted LUMO energy level. Brominated ITIC show comparable PCEs with Cl and F terminated compound, and moreover chlorinated and brominated ITIC show low synthesis cost and easy purification than fluorinated materials, which may be favorable for future commercialization.
1. Introduction Over the past decades, non-fullerene organic solar cells (NF-OSCs) with bulk heterojunction (BHJ) structures, which is consisted of electron donor and acceptors (polymers or small molecules), have attracted full attention for their high performance, low-cost, easy to process and thus large-scale application. (Lin et al., 2012; Heeger, 2014; Dou et al., 2015; Zhang et al., 2016a; Kumari et al., 2017; Liu et al., 2019) High performances OSCs materials must possess highly tunable optoelectronic properties for further obtaining improved power conversion efficiencies. Due to their strong and extensive absorption in the solar spectrum, and the adjustable molecular energy levels, crystalline properties of OSCs materials, and high performances OSCs devices can be achieved with improved open-circuit voltage (VOC), short circuit current density (JSC), and fill factor (FF). (Thompson & Frechet, 2008; Nielsen et al., 2015; Yao et al., 2017; Rostami et al., 2019) Previously, researchers have focus on the design strategies of polymer or smallmolecule (SM) electron donor materials (Beaujuge & Frechet, 2011; Duan et al., 2012; Henson et al., 2012; Zhang et al., 2016b) to match with fullerene acceptor in OSCs devices. The PCEs of fullerene-based
devices have been reached up to 12–13%. (Zhao et al., 2016b; Cui et al., 2017b; Zhao et al., 2017a; Zhao et al., 2017b) However, the application of fullerene-based OSCs has many limitations, which is attributed to their large voltage losses. (Bin et al., 2016; Liu et al., 2016b; Zhu et al., 2017b) Recently, non-fullerene SMA named Y6 have emerged and PCEs of over 16% have been recorded. (Bibi et al., 2019; Fan et al., 2019; Yuan et al., 2019) This is crucial for the further development of nonfullerene OSCs. At present, the classical structure of non-fullerene SMAs is so-called A-D-A type, which contain donor unit of strict coplanar and strong electron-withdrawing end groups. (Yao et al., 2015; Baran et al., 2016; Li et al., 2016a; Li et al., 2016b; Li et al., 2016c; Li et al., 2016d; Liu et al., 2016a; Zhao et al., 2016a; Li et al., 2017a; Liu et al., 2017; Zhang et al., 2017a; Choi et al., 2019) The emergence of this type SMAs have been regarded as promising candidates to break through high efficiencies in OSCs. In 2015, Zhan groups reported a non-fullerene SMA of narrow bandgap named ITIC, impressive PCEs of over 10% can be obtained by matching with varied polymer donor materials. Then they also reported about other indacenodithiophene (IDT)-based acceptors and the method to tune their crystallinity. (Lin et al., 2015; Holliday
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Corresponding author. E-mail address: [email protected] (M. Sun). 1 S. Lu and F. Li contribute equally to this work. https://doi.org/10.1016/j.solener.2019.11.074 Received 12 October 2019; Received in revised form 19 November 2019; Accepted 21 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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2. Results and discussions
et al., 2016; Lin et al., 2016) Results indicate that side chain variation have significant impact on devices performance. Researchers have discovered a variety of design strategies to effectively improve PCEs of OSCs. For example, introducing strong electron-withdrawing functional groups like carbonyl and halogen group, the orbital energy level of the molecular could be effectively down-shift. (Chen et al., 2009; Wang et al., 2018; Li et al., 2019) On the other hand, by introducing functional groups with weak electron-donating properties like alkoxy groups on the end-groups, the orbital energy level and absorption spectrum could be effectively modulated. (Huo et al., 2009; Yao et al., 2016; Nolasco et al., 2019) Researchers also have found that halogenation was one of the most excellent strategies and fluorinated molecules have a variety of advantages. (Tang et al., 2009; Tang & Bao, 2011; Bai et al., 2015; Zheng et al., 2015; Li et al., 2017b; Zhang et al., 2018b) As fluorine atom is highly electronegative and small in size, hence halogenation can effectively regulate properties of π electron while does not causing large steric hindrance for molecular packing. (Zhang et al., 2017b; Gao et al., 2019) In addition, fluorine-containing materials often have improved crystallinity and excellent mobility which is attributed to the noncovalent interactions of F···H, F···S, and etc. (Reichenbacher et al., 2005) In recent years chlorination also attracted much attentions, and chlorinated acceptor gradually become promising candidate to obtain high PCEs. Chlorinated materials have lower molecular energy level than fluorine-containing materials, (Liu et al., 2018; Zhang et al., 2018b) because the empty 3D orbit may be available to accept the π electrons and meanwhile low the effect of electronegativity. (Cui et al., 2017a; Zhang et al., 2018a) And, in comparison with fluorination, chlorination can effectively broaden the absorption spectrum due to the enhanced intramolecular charge transfer effect. But the size of the chlorine atom is larger than the fluorine atom, thus chlorination can impact on the intermolecular packing of materials. Furthermore, in terms of experimental cost, materials containing chlorine are cheaper than those containing fluorine, and the former is easier to be synthesized. (Mo et al., 2017) In recent years, a series of chlorinated donors or acceptors have been designed and synthesized, such as PBDB-T-2Cl, IT4Cl, IEICO-4Cl and etc. These materials also achieved excellent photovoltaic performance. The bromine, chlorine and fluorine atom belong to the same main group in the periodic table of chemical elements, and they have similar properties. In this work, we investigated halogenation effect of the star acceptor ITIC. More specifically, herein we synthesized and characterized three ITIC-based small molecules derivatives IT-2F, IT-2Cl and IT2Br (Scheme 1(a)), which contained core unit of DT-IDT (dithienoindaceno[1,2-b:5,6-b′]dithiophene) and end-groups with the halogen substituent is on the 5-/or 6- position. To provide an intuitional comparison, we selected the superior medium band-gap polymer PBDB-T as the electron donor. As the difference in molecular energy level of these three SMAs is minute, the energy levels of the donor and acceptor can be considered to be matched. These three small molecules revealed strong and similar absorption in dilute solution and slightly redshifted as well as different absorption range in thin film, indicating that diverse π-π interactions occurred in the state of thin films. (Li et al., 2017a; Zhu et al., 2017a) Three acceptor based OSCs devices yielded PCEs from 12.08% to 10.66%. And the short-circuit current density (JSC) are slightly decreased by substituting chlorine for fluorine (or bromine for chlorine). Interestingly, although the atomic size of bromine is the largest of these three, bromine-containing blend film show high fill factor of 0.71. Three blend films all exhibit relatively smooth surface with root-mean-squared (RMS) roughness of 2.03, 1.94 and 1.95 nm, and obviously aggregated domains are not observed in PBDB-T: IT-2F and PBDB-T: IT-2Cl based active layer, while slightly aggregates exist in the blend film of PBDB-T: IT-2Br. This is unfavorable to effective dissociation and dispersion of excitons on the donor/acceptor interfaces, which could be the main reason for the decreased external quantum efficiency (EQE) and JSC of IT-2Br based devices.
2.1. Synthesis and characterization The detailed synthetic routes of target materials IT-2F, IT-2Cl and IT-2Br are exhibited in Scheme 1 and the synthetic procedures are shown in the Experimental Section. As demonstrated in Scheme 1(c), these three ITIC-based derivatives, named IT-2F, IT-2Cl and IT-2Br, were synthesized though Knoevenagel condensation reaction. The core unit of DT-IDT-CHO was end-capped by halogen-modified end groups of DCI-F, DCI-Cl and DCI-Br, respectively. All of three acceptors were present glorious solubility and processability, and all can be dissolved in chloroform, chlorobenzene, and o-dichlorobenzene. Intermediates and target materials can be purified by column chromatography, which is attributed to the inequality of product polarity. The thermal stability of the three materials is determined by thermogravimetric analysis (TGA). The corresponding TGA plots is shown in Fig. 1a. These three small molecular materials show obvious weight loss at 330–350 °C, which may be derived from the decomposition of the side chain. After 370 °C, the TGA curve show the plateau period of weight loss slowdown, and then the second weight loss interval began to appear near 470 °C, which may be attributed to the decomposition of the small molecule material backbone. 2.2. Optical performance The normalized ultraviolet-visible (UV–vis) absorption spectra of the three small molecules in diluted chloroform (CF) solution and as solid film are exhibited in Fig. 1b and Fig. 1c. The related data is summarized in Table 1. All the three small molecule materials revealed strong absorption in the visible range of 500–700 nm, both in solution and in the film state. In dilute solution, the maximum main absorption peaks (λmax) of three small molecules are located around 690 nm with absorption onset around 740 nm. The absorption spectra of IT-2Br are similar to IT-2Cl, while the spectra of the IT-2F blue shifted by 5 nm compare with IT-2Cl. In the thin film, the absorption spectra of the three small molecule materials are all redshifted compared to their absorption spectra in solution, and the absorption range is widened, suggesting that diverse π-π interactions occurred in the thin films. However, the absorption peaks of the two small molecular materials of IT-2Cl and IT-2Br are red shifted by 44 nm, and IT-2F is red shifted by 36 nm, implying that IT-2Cl and IT-2Br exhibit stronger π-π interactions than IT-2F. Hence, in the thin film, the optical bandgap (Egopt) of IT-2Br is 1.52 eV which is identical with IT-2Cl (1.52 eV), and IT-2F has the Egopt of 1.54 eV. As shown in Fig. S2, the extinction coefficients in chloroform solution of three small molecules were calculated and the extinction coefficients of IT-2F, IT-2Cl and IT-2Br are 1.57 × 10−5, 1.80 × 10−5 and 1.49 × 10−5 M−1 cm−1, respectively. IT-2Br has the lowest extinction coefficient which may be attributed to molecules packing structures affected by twisted conjugated terminal groups and this is also related to the intramolecular aggregation. The absorption spectra of PBDB-T are provided in Fig. S3. The PBDB-T polymer exhibits absorption peak in 500–600 nm, while three small molecule materials show strong absorption in 600–800 nm. Therefore, this result indicated that solar cells with the polymer as a donor and the small molecule as acceptor exhibit complementary absorption in the solar spectrum, which is favorable to obtaining high short-circuit current density. 2.3. Electrochemical performance To further explore the impact of the halogen atoms on the molecular energy level and orbital distributions of these acceptor, theoretical calculations were carried out by using the density functional theory (DFT) method at the B3LYP/6-31G* level. With the purpose of reducing the computation time, here, four long n-hexyl side chains were replaced by the methyl group. The results shown that there is no apparent 430
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Scheme 1. (a) The chemical structures of the three acceptors (IT-2F, IT-2Cl and IT-2Br). (b) The structure of donor polymer PBDB-T. (c) Synthesis routes of IT-2F, IT2Cl and IT-2Br.
PSC devices were fabricated with a classical structure of ITO/ PEDOT:PSS/active layer/PDINO/Al. Active layer is comprised of the polymer PBDB-T as the donor material and IT-2F, IT-2Cl, and IT-2Br as acceptors material. The PBDB-T: ITIC based device were also fabricated as control to further demonstrate the effect of halogenation effect on device performance, and the corresponding data is summarized in the supporting information. The annealing temperature, processing additive and the weight ratio of donor/acceptor (D: A) were varied to optimize the device. The optimal conditions are as following. Chlorobenzene (CB) is used as the processing solvent and the optimized weight ratios for all PBDB-T: IT-2X (X = F, Cl, Br) blend systems are 1:1.5. Varied annealing temperatures and times were scanned and the optimal conditions are 80 °C for 10 min. The optimal photovoltaic data are in exhibit Table 2 and the corresponding current density-voltage (JV) curves are shown in Fig. 2a. As shown in Fig. 2a and Table 2, the device based on PBDB-T: IT-2F exhibits a VOC of 0.83 V, JSC of 18.62 mA cm−2, and FF of 0.71, which yields the highest PCEs of 12.08% among devices based on the three acceptors. The PBDB-T: IT-2Cl based device exhibits a relatively lower PCE of 10.77%, VOC of 0.81 V with a JSC of 18.07 mA cm−2, and FF of 0.70. The IT-2Br based devices present similar VOC of 0.83 V, JSC of 17.93 mA cm−2, FF of 0.71, and similar PCEs of 10.66%, respectively. However, the device based on PBDB-T: ITIC exhibits a VOC of 0.92 V, JSC of 17.28 mA cm−2, and FF of 0.71, PCE of 10.08% (Fig. S1 and Table S1). Obviously, the PCEs were significantly enhanced by halogenation. The improvement was mainly due to enhanced JSC of 17.93–18.62 mA cm−2. Compared to the control device based on PBDBT: ITIC, halogenation can effectively reduce the LUMO energy level of the acceptor material, resulting in a lower VOC, but it has an outstanding contribution to broadening the absorption spectrum and
difference on energy level and orbital distributions are not obviously affected by different halogenations. The calculated molecular energy levels and molecular conformation are shown in Fig. S4 and the calculated HOMO levels of IT-2F, IT-2Cl, and IT-2Br are −5.53, −5.56 and −5.56 eV, respectively. The calculated LUMO levels of IT-2F, IT2Cl and IT-2Br are −3.41, −3.47 and −3.47 eV, respectively. Electrochemical cyclic voltammetry (CV) was used to measure the oxidation/reduction potentials and further obtain the molecular energy levels of three small molecule materials and finally to calculate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the material. The relevant grams and data are summarized and shown in Fig. 1 and Table 1. Due to the electronwithdrawing effect of the halogen group, the LUMO level of the three small molecule materials is reduced compared to the ITIC. And it can be discovered by schematic energy-level diagrams that the LUMO energy levels of IT-2F, IT-2Cl, IT-2Br are gradually reduced, which is basically consistent with the result obtained from density functional theory (DFT) calculations. The oxidation onset potentials (Eox) of IT-2F, IT-2Cl and IT-2Br are 0.96, 0.97 and 1.05 V, respectively. The HOMO energy levels were calculated by the following equation: HOMO = -e (φox + 4.80 - φ1/2, FeCp2) eV, where the unit of potential is V (vs. SCE). Under the same conditions, the redox potential of the Fc/Fc+ internal reference was 0.35 V vs. SCE. Thus the HOMO levels of IT-2F, IT-2Cl and IT-2Br are: −5.41, −5.42 and −5.51 eV, respectively. According to the optical band-gaps, the corresponding LUMO energy levels are calculated to be −3.87, −3.90, and −3.98 eV, respectively. 2.4. Photovoltaic performance In order to investigate the acceptors photovoltaic properties, the 431
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Fig. 1. (a) TGA plots of IT-2F, IT-2Cl, and IT-2Br. (b) UV–vis absorption spectra of IT-2F, IT-2Cl, and IT-2Br in chloroform solution and (c) in the solid film state. (d) Cyclic voltammetry grams of IT-2F, IT-2Cl, and IT-2Br as thin films.
IT-2F based device exhibit highest EQE value among all the devices and a maximum EQE value of 71% was observed at 570 nm. The device based on PBDB-T: IT-2Br blend film exhibit similar spectral response range and VOC compare to PBDB-T: IT-2Cl blend film, thus, they display analogous optical performance, the former yield PCE of 10.66%, which is slightly lower than that of the latter. Moreover, PBDB-T: IT-2F based EQE curve performed slightly blue-shift than others, which is consistent with optical absorption spectra.
Table 1 Optical and electrochemical performance of the three molecules. Materials
λmax (nm)a
λmax (nm)b
λedge (nm)
Egopt (eV)
Eox (V)c
HOMO (eV)
LUMO (eV)
IT-2F IT-2Cl IT-2Br
685 690 691
721 735 735
804 813 817
1.54 1.52 1.52
0.96 0.97 1.05
−5.41 −5.42 −5.51
−3.87 −3.90 −3.98
a b c
Measured in dilute chloroform solution. Measured from a quartz film in a chloroform solution. Measured from cyclic voltammetry.
2.5. Charge generation Exciton dissociation efficiency (Pdiss) can be calculated from the relationship between the photocurrent density (Jph = JL − JD) and the effective voltage (Veff = V0 − Va), in which JL is the current densities under illumination and JD is the current densities in the dark, the value of Jph is the difference between JL and JD, V0 is defined as the voltage when Jph = 0, and Va is the applied voltage. As we all know, when the value of Jph reached its saturation (Jsat), the photo-generated excitons dissociate into free charge which can be collected by the electrodes. Therefore, Jph/Jsat is used to evaluate exciton dissociation and charge collection efficiency. As shown in Fig. 2c and Fig. S1, under short-circuit condition, the Jph/Jsat ratio for PBDB-T: IT-2F, PBDB-T: IT-2Cl, PBDB-T: IT-2Br and PBDB-T: ITIC are calculated to be 92.2%, 90.6%, 94.8%, and 93.3% respectively. This result indicated that both the
improving the short-circuit current JSC of the device, and finally obtains a higher PCE. In general, VOC is proportional to the energy level difference between the HOMO of the donor materials and the LUMO of the acceptor materials, and thus a higher LUMO level of small molecule is crucial to obtain higher VOC. However, VOC is also dependent on factors such as exciton dissociation and film morphology. In order to validate the JSC of devices, EQE measurements were measured and the corresponding curves are shown in Fig. 2b and Fig. S1. Three devices based on halogenated ITICs displayed strong photocurrent response from 300 to 850 nm, which are beneficial to obtain higher JSC. PBDB-T: ITIC based device show a relatively narrow photoresponse from 300 to 800 nm and low EQE value of 65%. The PBDB-T: Table 2 The photovoltaic parameters and mobilities of the devices. Blend
VOC (V)
JSC (mA cm−2)
FF
PCE (%)
μh (cm2 V−1 s−1)
μe (cm2 V−1 s−1)
PBDB-T: IT-2F PBDB-T: IT-2Cl PBDB-T: IT-2Br
0.83 0.81 0.83
18.62 18.07 17.93
0.71 0.70 0.71
12.08 10.77 10.66
7.0 × 10−5 5.8 × 10−5 6.4 × 10−5
28.7 × 10−5 63.4 × 10−5 27.3 × 10−5
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Fig. 2. (a) J-V curves of PBDB-T: IT-2F, PBDB-T: IT-2Cl and PBDB-T: IT-2Br based solar cells under optimal conditions and (b) EQE curves of the corresponding PSCs. (c) Photocurrent density (Jph) versus effective voltage (Veff) characteristics of the three devices based on PBDB-T: IT-2X blends. (d) The JSC versus light intensity for the three devices.
to absorption spectrum, the PL excitation wavelength is determined to be 610 nm for the polymer donor of PBDB-T and 690 nm for the acceptors. It is worth mentioning that the way of acceptor is quenched, when the acceptor is excited, there also occur a process of electrons transition and charge transfer, and then excitons is dissociated, therefore, the acceptor PL can also be quenched. When we measuring PL spectrum, we can only excite the donor or acceptor, and we also can excite them at the same time, as a result the donor and acceptor PL are simultaneously quenched. By contrast, we can find that PBDB-T: IT-2F based blend film exhibit excellent PL quenching efficiency (PLQE), which facilitate efficient photo-induced exciton separation and charge transfer. The PBDB-T: IT-2Br based blend film shows similar but slightly lower PLQE compared to the PBDB-T: IT-2Cl blend film, indicating that there is not effective charge transfer from PBDB-T donor to the acceptors, which is consistent with the AFM analysis.
processes of the exciton dissociation and charge collection are efficient. The higher Jph/Jsat ratio of PBDB-T: IT-2Br indicated a better exciton dissociation efficiency in this active layer, resulting in the high FF in the optimized device.
2.6. Film morphological characterization Because the surface and bulk morphology of the blend film seriously affected the photovoltaic performance of the device, atomic force microscope (AFM), transmission electron microscope (TEM) and photoluminescence (PL) spectroscopy measurements were applied to investigate the nanoscale morphology and miscibility of the three optimized blend films based on PBDB-T with IT-2F, IT-2Cl or IT-2Br, respectively. As shown in Fig. 3a-c, three blend films all exhibit relatively smooth surface with root-mean-squared (RMS) roughness of 2.03, 1.94 and 1.95 nm, Meanwhile, from the phase images (Fig. 3d-f), PBDBT: IT-2F and PBDB-T: IT-2Cl based blend films exhibit a mature fibrillike nanoscale phase separation with appropriate domain sizes and excellent interpenetrating network compared to PBDB-T: IT-2Br, which may lead to the relatively higher JSC and EQE of IT-2F and IT-2Cl based devices. Then, the bulk morphologies of three blend films are explored by transmission electron microscopy (TEM) (Fig. 3g-i). It can be seen that PBDB-T: IT-2F and PBDB-T: IT-2Cl based blend films present rather homogeneous morphology and no obvious aggregation domains, and this elucidate the good miscibility of the donor and acceptor components in the blend films, which contributes to charge transfer in donor/ acceptor interface. In contrast, the blend film of PBDB-T: IT-2Br showed slightly aggregates, such aggregates generate adverse effects on exciton diffusion. So, the relatively low JSC in the PBDB-T: IT-2Br device are attributed to the absence of effective separation of excitons. In addition, the PL spectra of pure films of IT-2F, IT-2Cl, and IT-2Br as well as PBDBT, and PBDB-T: acceptor blend films were shown in Fig. S5. According
2.7. Charge recombination mechanism The carrier mobilities were gained by forthputting the space-chargelimited current (SCLC) method. The hole and electron mobilities of blend films were measured with device structures of ITO/PEDOT:PSS/ PBDB-T:acceptor/MoO3/Al and ITO/ZnO /PBDB-T:acceptor/PDINO/ Al, respectively. The related data and corresponding curves are shown in Fig. S6 and Table 2. As shown, the hole and electron mobility values of the PBDB-T: IT-2Br blend film are found to be 6.4 × 10−5 and 27.3 × 10−5 cm2 V−1 s−1 with a μe/μh ratios of 4.26 which were similar to PBDB-T: IT-2F blend film. In contrast, the blend films based on PBDB-T: IT-2Cl show higher values of electron mobility but lower values of hole mobility. It demonstrates that the blend films based on IT2F and IT-2Br exhibit more balanced hole and electron mobility compared to blend films based on PBDB-T: IT-2Cl. For the control device based on PBDB-T: ITIC, the relatively low ratio of electron and hole 433
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Fig. 3. AFM height images (5 μm × 5 um) of (a) the PBDB-T: IT-2F and (b) the PBDB-T: IT-2Cl and (c) the PBDB-T: IT-2Br; AFM phase images (5 μm × 5 um) of (d) the PBDB-T: IT-2F and (e) the PBDB-T: IT-2Cl and (f) the PBDB-T: IT-2Br; TEM images of (g) the PBDB-T: IT-2F and (h) the PBDB-T: IT-2Cl and (i) the PBDB-T: IT-2Br; all blends are measured under their optimal conditions.
the relatively higher JSC and EQE of IT-2F and IT-2Cl based devices. Considering the synthesis and experimental cost, the introduction of chlorine and bromine atoms on end-group is much easier and cheaper than introduction of fluorine atoms. Halogenation may be as an effective strategy when we design and synthesize novel photovoltaic materials.
mobility (μe/μh = 3.15) suggests a balanced charge transport in PBDBT: ITIC blend film, and thus a high FF can be obtained in PBDB-T: ITICbased devices. To further investigate the behavior of charge recombination in optimal devices based on IT-2F, IT-2Cl and IT-2Br, we conducted measurement of photocurrent and photovoltaic properties dependence on light intensity (P). For organic photovoltaic cells, JSC and P follows power-law equation: JSC ∝ PS where S is a recombination exponent. When s is close to 1, the bimolecular recombination is efficiently inhibited. As shown in Fig. 2d, the linear relationship between JSC and light intensity are observed and S values of three devices based on IT2F, IT-2Cl and IT-2Br are 0.982, 0.938 and 0.981, which contribute to the high FF obtained by IT-2F-based and IT-2Br-based devices.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
3. Conclusions
The authors are deeply grateful to Fundamental Research Funds for the Central Universities (201822002), Natural Science Foundation of Shandong Province (ZR2018MEM023) and National Natural Science Foundation of China (U1806223).
In summary, we have synthesized and characterized several typical ITIC-based acceptors IT-2F, IT-2Cl and IT-2Br via halogenation to systematically investigate the effect of halogen substituents in non-fullerene small molecule acceptors. The device of donor: acceptor show superior PCEs from 10.66%~12.08%. Due to the electron-withdrawing effect of the halogen group, three materials demonstrate decreased LUMO level from −3.98 eV to −3.87 eV and slightly different absorption spectrum. In addition, changing the halogen end group could show impact on bulk morphology. The results of AFM and TEM measurements exhibit that the blend films based on PBDB-T: IT-2F and PBDB-T:IT-2Cl have rather uniform morphology and appropriate domain sizes compared to the blend film of PBDB-T: IT-2Br, which lead to
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Halogenation on terminal groups of ITIC based electron acceptors as an effective strategy for efficient polymer solar cells
T
Shujing Lua,1, Feng Lib,1, Kaili Zhanga, Jie Zhua, Wen Cuia, Renqiang Yangc, Liangmin Yud, ⁎ Mingliang Suna, a
School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Engineering, Qingdao University of Science & Technology, Qingdao 266042, China c CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China d Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100 b
A R T I C LE I N FO
A B S T R A C T
Keywords: Halogenation Non-fullerene acceptors Power conversion efficiencies
Acceptor halogenation is an excellent modification method for modulating molecular energy levels, improving optical absorption, shortening the intermolecular packing distance thus ameliorating morphologies and achieving effective exciton dissociation and charge transfer of organic photovoltaic device. In this work, F, Cl and Br terminated ITIC small molecule acceptor (SMA) derivatives (IT-2F, IT-2Cl and IT-2Br) are designed and synthesized to compare the halogen substitution effect. Among these three acceptors, IT-2F based PSCs devices show the highest power conversion efficiencies (PCEs) around 12.08%, but Br terminated ITIC show downshifted LUMO energy level. Brominated ITIC show comparable PCEs with Cl and F terminated compound, and moreover chlorinated and brominated ITIC show low synthesis cost and easy purification than fluorinated materials, which may be favorable for future commercialization.
1. Introduction Over the past decades, non-fullerene organic solar cells (NF-OSCs) with bulk heterojunction (BHJ) structures, which is consisted of electron donor and acceptors (polymers or small molecules), have attracted full attention for their high performance, low-cost, easy to process and thus large-scale application. (Lin et al., 2012; Heeger, 2014; Dou et al., 2015; Zhang et al., 2016a; Kumari et al., 2017; Liu et al., 2019) High performances OSCs materials must possess highly tunable optoelectronic properties for further obtaining improved power conversion efficiencies. Due to their strong and extensive absorption in the solar spectrum, and the adjustable molecular energy levels, crystalline properties of OSCs materials, and high performances OSCs devices can be achieved with improved open-circuit voltage (VOC), short circuit current density (JSC), and fill factor (FF). (Thompson & Frechet, 2008; Nielsen et al., 2015; Yao et al., 2017; Rostami et al., 2019) Previously, researchers have focus on the design strategies of polymer or smallmolecule (SM) electron donor materials (Beaujuge & Frechet, 2011; Duan et al., 2012; Henson et al., 2012; Zhang et al., 2016b) to match with fullerene acceptor in OSCs devices. The PCEs of fullerene-based
devices have been reached up to 12–13%. (Zhao et al., 2016b; Cui et al., 2017b; Zhao et al., 2017a; Zhao et al., 2017b) However, the application of fullerene-based OSCs has many limitations, which is attributed to their large voltage losses. (Bin et al., 2016; Liu et al., 2016b; Zhu et al., 2017b) Recently, non-fullerene SMA named Y6 have emerged and PCEs of over 16% have been recorded. (Bibi et al., 2019; Fan et al., 2019; Yuan et al., 2019) This is crucial for the further development of nonfullerene OSCs. At present, the classical structure of non-fullerene SMAs is so-called A-D-A type, which contain donor unit of strict coplanar and strong electron-withdrawing end groups. (Yao et al., 2015; Baran et al., 2016; Li et al., 2016a; Li et al., 2016b; Li et al., 2016c; Li et al., 2016d; Liu et al., 2016a; Zhao et al., 2016a; Li et al., 2017a; Liu et al., 2017; Zhang et al., 2017a; Choi et al., 2019) The emergence of this type SMAs have been regarded as promising candidates to break through high efficiencies in OSCs. In 2015, Zhan groups reported a non-fullerene SMA of narrow bandgap named ITIC, impressive PCEs of over 10% can be obtained by matching with varied polymer donor materials. Then they also reported about other indacenodithiophene (IDT)-based acceptors and the method to tune their crystallinity. (Lin et al., 2015; Holliday
⁎
Corresponding author. E-mail address: [email protected] (M. Sun). 1 S. Lu and F. Li contribute equally to this work. https://doi.org/10.1016/j.solener.2019.11.074 Received 12 October 2019; Received in revised form 19 November 2019; Accepted 21 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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2. Results and discussions
et al., 2016; Lin et al., 2016) Results indicate that side chain variation have significant impact on devices performance. Researchers have discovered a variety of design strategies to effectively improve PCEs of OSCs. For example, introducing strong electron-withdrawing functional groups like carbonyl and halogen group, the orbital energy level of the molecular could be effectively down-shift. (Chen et al., 2009; Wang et al., 2018; Li et al., 2019) On the other hand, by introducing functional groups with weak electron-donating properties like alkoxy groups on the end-groups, the orbital energy level and absorption spectrum could be effectively modulated. (Huo et al., 2009; Yao et al., 2016; Nolasco et al., 2019) Researchers also have found that halogenation was one of the most excellent strategies and fluorinated molecules have a variety of advantages. (Tang et al., 2009; Tang & Bao, 2011; Bai et al., 2015; Zheng et al., 2015; Li et al., 2017b; Zhang et al., 2018b) As fluorine atom is highly electronegative and small in size, hence halogenation can effectively regulate properties of π electron while does not causing large steric hindrance for molecular packing. (Zhang et al., 2017b; Gao et al., 2019) In addition, fluorine-containing materials often have improved crystallinity and excellent mobility which is attributed to the noncovalent interactions of F···H, F···S, and etc. (Reichenbacher et al., 2005) In recent years chlorination also attracted much attentions, and chlorinated acceptor gradually become promising candidate to obtain high PCEs. Chlorinated materials have lower molecular energy level than fluorine-containing materials, (Liu et al., 2018; Zhang et al., 2018b) because the empty 3D orbit may be available to accept the π electrons and meanwhile low the effect of electronegativity. (Cui et al., 2017a; Zhang et al., 2018a) And, in comparison with fluorination, chlorination can effectively broaden the absorption spectrum due to the enhanced intramolecular charge transfer effect. But the size of the chlorine atom is larger than the fluorine atom, thus chlorination can impact on the intermolecular packing of materials. Furthermore, in terms of experimental cost, materials containing chlorine are cheaper than those containing fluorine, and the former is easier to be synthesized. (Mo et al., 2017) In recent years, a series of chlorinated donors or acceptors have been designed and synthesized, such as PBDB-T-2Cl, IT4Cl, IEICO-4Cl and etc. These materials also achieved excellent photovoltaic performance. The bromine, chlorine and fluorine atom belong to the same main group in the periodic table of chemical elements, and they have similar properties. In this work, we investigated halogenation effect of the star acceptor ITIC. More specifically, herein we synthesized and characterized three ITIC-based small molecules derivatives IT-2F, IT-2Cl and IT2Br (Scheme 1(a)), which contained core unit of DT-IDT (dithienoindaceno[1,2-b:5,6-b′]dithiophene) and end-groups with the halogen substituent is on the 5-/or 6- position. To provide an intuitional comparison, we selected the superior medium band-gap polymer PBDB-T as the electron donor. As the difference in molecular energy level of these three SMAs is minute, the energy levels of the donor and acceptor can be considered to be matched. These three small molecules revealed strong and similar absorption in dilute solution and slightly redshifted as well as different absorption range in thin film, indicating that diverse π-π interactions occurred in the state of thin films. (Li et al., 2017a; Zhu et al., 2017a) Three acceptor based OSCs devices yielded PCEs from 12.08% to 10.66%. And the short-circuit current density (JSC) are slightly decreased by substituting chlorine for fluorine (or bromine for chlorine). Interestingly, although the atomic size of bromine is the largest of these three, bromine-containing blend film show high fill factor of 0.71. Three blend films all exhibit relatively smooth surface with root-mean-squared (RMS) roughness of 2.03, 1.94 and 1.95 nm, and obviously aggregated domains are not observed in PBDB-T: IT-2F and PBDB-T: IT-2Cl based active layer, while slightly aggregates exist in the blend film of PBDB-T: IT-2Br. This is unfavorable to effective dissociation and dispersion of excitons on the donor/acceptor interfaces, which could be the main reason for the decreased external quantum efficiency (EQE) and JSC of IT-2Br based devices.
2.1. Synthesis and characterization The detailed synthetic routes of target materials IT-2F, IT-2Cl and IT-2Br are exhibited in Scheme 1 and the synthetic procedures are shown in the Experimental Section. As demonstrated in Scheme 1(c), these three ITIC-based derivatives, named IT-2F, IT-2Cl and IT-2Br, were synthesized though Knoevenagel condensation reaction. The core unit of DT-IDT-CHO was end-capped by halogen-modified end groups of DCI-F, DCI-Cl and DCI-Br, respectively. All of three acceptors were present glorious solubility and processability, and all can be dissolved in chloroform, chlorobenzene, and o-dichlorobenzene. Intermediates and target materials can be purified by column chromatography, which is attributed to the inequality of product polarity. The thermal stability of the three materials is determined by thermogravimetric analysis (TGA). The corresponding TGA plots is shown in Fig. 1a. These three small molecular materials show obvious weight loss at 330–350 °C, which may be derived from the decomposition of the side chain. After 370 °C, the TGA curve show the plateau period of weight loss slowdown, and then the second weight loss interval began to appear near 470 °C, which may be attributed to the decomposition of the small molecule material backbone. 2.2. Optical performance The normalized ultraviolet-visible (UV–vis) absorption spectra of the three small molecules in diluted chloroform (CF) solution and as solid film are exhibited in Fig. 1b and Fig. 1c. The related data is summarized in Table 1. All the three small molecule materials revealed strong absorption in the visible range of 500–700 nm, both in solution and in the film state. In dilute solution, the maximum main absorption peaks (λmax) of three small molecules are located around 690 nm with absorption onset around 740 nm. The absorption spectra of IT-2Br are similar to IT-2Cl, while the spectra of the IT-2F blue shifted by 5 nm compare with IT-2Cl. In the thin film, the absorption spectra of the three small molecule materials are all redshifted compared to their absorption spectra in solution, and the absorption range is widened, suggesting that diverse π-π interactions occurred in the thin films. However, the absorption peaks of the two small molecular materials of IT-2Cl and IT-2Br are red shifted by 44 nm, and IT-2F is red shifted by 36 nm, implying that IT-2Cl and IT-2Br exhibit stronger π-π interactions than IT-2F. Hence, in the thin film, the optical bandgap (Egopt) of IT-2Br is 1.52 eV which is identical with IT-2Cl (1.52 eV), and IT-2F has the Egopt of 1.54 eV. As shown in Fig. S2, the extinction coefficients in chloroform solution of three small molecules were calculated and the extinction coefficients of IT-2F, IT-2Cl and IT-2Br are 1.57 × 10−5, 1.80 × 10−5 and 1.49 × 10−5 M−1 cm−1, respectively. IT-2Br has the lowest extinction coefficient which may be attributed to molecules packing structures affected by twisted conjugated terminal groups and this is also related to the intramolecular aggregation. The absorption spectra of PBDB-T are provided in Fig. S3. The PBDB-T polymer exhibits absorption peak in 500–600 nm, while three small molecule materials show strong absorption in 600–800 nm. Therefore, this result indicated that solar cells with the polymer as a donor and the small molecule as acceptor exhibit complementary absorption in the solar spectrum, which is favorable to obtaining high short-circuit current density. 2.3. Electrochemical performance To further explore the impact of the halogen atoms on the molecular energy level and orbital distributions of these acceptor, theoretical calculations were carried out by using the density functional theory (DFT) method at the B3LYP/6-31G* level. With the purpose of reducing the computation time, here, four long n-hexyl side chains were replaced by the methyl group. The results shown that there is no apparent 430
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Scheme 1. (a) The chemical structures of the three acceptors (IT-2F, IT-2Cl and IT-2Br). (b) The structure of donor polymer PBDB-T. (c) Synthesis routes of IT-2F, IT2Cl and IT-2Br.
PSC devices were fabricated with a classical structure of ITO/ PEDOT:PSS/active layer/PDINO/Al. Active layer is comprised of the polymer PBDB-T as the donor material and IT-2F, IT-2Cl, and IT-2Br as acceptors material. The PBDB-T: ITIC based device were also fabricated as control to further demonstrate the effect of halogenation effect on device performance, and the corresponding data is summarized in the supporting information. The annealing temperature, processing additive and the weight ratio of donor/acceptor (D: A) were varied to optimize the device. The optimal conditions are as following. Chlorobenzene (CB) is used as the processing solvent and the optimized weight ratios for all PBDB-T: IT-2X (X = F, Cl, Br) blend systems are 1:1.5. Varied annealing temperatures and times were scanned and the optimal conditions are 80 °C for 10 min. The optimal photovoltaic data are in exhibit Table 2 and the corresponding current density-voltage (JV) curves are shown in Fig. 2a. As shown in Fig. 2a and Table 2, the device based on PBDB-T: IT-2F exhibits a VOC of 0.83 V, JSC of 18.62 mA cm−2, and FF of 0.71, which yields the highest PCEs of 12.08% among devices based on the three acceptors. The PBDB-T: IT-2Cl based device exhibits a relatively lower PCE of 10.77%, VOC of 0.81 V with a JSC of 18.07 mA cm−2, and FF of 0.70. The IT-2Br based devices present similar VOC of 0.83 V, JSC of 17.93 mA cm−2, FF of 0.71, and similar PCEs of 10.66%, respectively. However, the device based on PBDB-T: ITIC exhibits a VOC of 0.92 V, JSC of 17.28 mA cm−2, and FF of 0.71, PCE of 10.08% (Fig. S1 and Table S1). Obviously, the PCEs were significantly enhanced by halogenation. The improvement was mainly due to enhanced JSC of 17.93–18.62 mA cm−2. Compared to the control device based on PBDBT: ITIC, halogenation can effectively reduce the LUMO energy level of the acceptor material, resulting in a lower VOC, but it has an outstanding contribution to broadening the absorption spectrum and
difference on energy level and orbital distributions are not obviously affected by different halogenations. The calculated molecular energy levels and molecular conformation are shown in Fig. S4 and the calculated HOMO levels of IT-2F, IT-2Cl, and IT-2Br are −5.53, −5.56 and −5.56 eV, respectively. The calculated LUMO levels of IT-2F, IT2Cl and IT-2Br are −3.41, −3.47 and −3.47 eV, respectively. Electrochemical cyclic voltammetry (CV) was used to measure the oxidation/reduction potentials and further obtain the molecular energy levels of three small molecule materials and finally to calculate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the material. The relevant grams and data are summarized and shown in Fig. 1 and Table 1. Due to the electronwithdrawing effect of the halogen group, the LUMO level of the three small molecule materials is reduced compared to the ITIC. And it can be discovered by schematic energy-level diagrams that the LUMO energy levels of IT-2F, IT-2Cl, IT-2Br are gradually reduced, which is basically consistent with the result obtained from density functional theory (DFT) calculations. The oxidation onset potentials (Eox) of IT-2F, IT-2Cl and IT-2Br are 0.96, 0.97 and 1.05 V, respectively. The HOMO energy levels were calculated by the following equation: HOMO = -e (φox + 4.80 - φ1/2, FeCp2) eV, where the unit of potential is V (vs. SCE). Under the same conditions, the redox potential of the Fc/Fc+ internal reference was 0.35 V vs. SCE. Thus the HOMO levels of IT-2F, IT-2Cl and IT-2Br are: −5.41, −5.42 and −5.51 eV, respectively. According to the optical band-gaps, the corresponding LUMO energy levels are calculated to be −3.87, −3.90, and −3.98 eV, respectively. 2.4. Photovoltaic performance In order to investigate the acceptors photovoltaic properties, the 431
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Fig. 1. (a) TGA plots of IT-2F, IT-2Cl, and IT-2Br. (b) UV–vis absorption spectra of IT-2F, IT-2Cl, and IT-2Br in chloroform solution and (c) in the solid film state. (d) Cyclic voltammetry grams of IT-2F, IT-2Cl, and IT-2Br as thin films.
IT-2F based device exhibit highest EQE value among all the devices and a maximum EQE value of 71% was observed at 570 nm. The device based on PBDB-T: IT-2Br blend film exhibit similar spectral response range and VOC compare to PBDB-T: IT-2Cl blend film, thus, they display analogous optical performance, the former yield PCE of 10.66%, which is slightly lower than that of the latter. Moreover, PBDB-T: IT-2F based EQE curve performed slightly blue-shift than others, which is consistent with optical absorption spectra.
Table 1 Optical and electrochemical performance of the three molecules. Materials
λmax (nm)a
λmax (nm)b
λedge (nm)
Egopt (eV)
Eox (V)c
HOMO (eV)
LUMO (eV)
IT-2F IT-2Cl IT-2Br
685 690 691
721 735 735
804 813 817
1.54 1.52 1.52
0.96 0.97 1.05
−5.41 −5.42 −5.51
−3.87 −3.90 −3.98
a b c
Measured in dilute chloroform solution. Measured from a quartz film in a chloroform solution. Measured from cyclic voltammetry.
2.5. Charge generation Exciton dissociation efficiency (Pdiss) can be calculated from the relationship between the photocurrent density (Jph = JL − JD) and the effective voltage (Veff = V0 − Va), in which JL is the current densities under illumination and JD is the current densities in the dark, the value of Jph is the difference between JL and JD, V0 is defined as the voltage when Jph = 0, and Va is the applied voltage. As we all know, when the value of Jph reached its saturation (Jsat), the photo-generated excitons dissociate into free charge which can be collected by the electrodes. Therefore, Jph/Jsat is used to evaluate exciton dissociation and charge collection efficiency. As shown in Fig. 2c and Fig. S1, under short-circuit condition, the Jph/Jsat ratio for PBDB-T: IT-2F, PBDB-T: IT-2Cl, PBDB-T: IT-2Br and PBDB-T: ITIC are calculated to be 92.2%, 90.6%, 94.8%, and 93.3% respectively. This result indicated that both the
improving the short-circuit current JSC of the device, and finally obtains a higher PCE. In general, VOC is proportional to the energy level difference between the HOMO of the donor materials and the LUMO of the acceptor materials, and thus a higher LUMO level of small molecule is crucial to obtain higher VOC. However, VOC is also dependent on factors such as exciton dissociation and film morphology. In order to validate the JSC of devices, EQE measurements were measured and the corresponding curves are shown in Fig. 2b and Fig. S1. Three devices based on halogenated ITICs displayed strong photocurrent response from 300 to 850 nm, which are beneficial to obtain higher JSC. PBDB-T: ITIC based device show a relatively narrow photoresponse from 300 to 800 nm and low EQE value of 65%. The PBDB-T: Table 2 The photovoltaic parameters and mobilities of the devices. Blend
VOC (V)
JSC (mA cm−2)
FF
PCE (%)
μh (cm2 V−1 s−1)
μe (cm2 V−1 s−1)
PBDB-T: IT-2F PBDB-T: IT-2Cl PBDB-T: IT-2Br
0.83 0.81 0.83
18.62 18.07 17.93
0.71 0.70 0.71
12.08 10.77 10.66
7.0 × 10−5 5.8 × 10−5 6.4 × 10−5
28.7 × 10−5 63.4 × 10−5 27.3 × 10−5
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Fig. 2. (a) J-V curves of PBDB-T: IT-2F, PBDB-T: IT-2Cl and PBDB-T: IT-2Br based solar cells under optimal conditions and (b) EQE curves of the corresponding PSCs. (c) Photocurrent density (Jph) versus effective voltage (Veff) characteristics of the three devices based on PBDB-T: IT-2X blends. (d) The JSC versus light intensity for the three devices.
to absorption spectrum, the PL excitation wavelength is determined to be 610 nm for the polymer donor of PBDB-T and 690 nm for the acceptors. It is worth mentioning that the way of acceptor is quenched, when the acceptor is excited, there also occur a process of electrons transition and charge transfer, and then excitons is dissociated, therefore, the acceptor PL can also be quenched. When we measuring PL spectrum, we can only excite the donor or acceptor, and we also can excite them at the same time, as a result the donor and acceptor PL are simultaneously quenched. By contrast, we can find that PBDB-T: IT-2F based blend film exhibit excellent PL quenching efficiency (PLQE), which facilitate efficient photo-induced exciton separation and charge transfer. The PBDB-T: IT-2Br based blend film shows similar but slightly lower PLQE compared to the PBDB-T: IT-2Cl blend film, indicating that there is not effective charge transfer from PBDB-T donor to the acceptors, which is consistent with the AFM analysis.
processes of the exciton dissociation and charge collection are efficient. The higher Jph/Jsat ratio of PBDB-T: IT-2Br indicated a better exciton dissociation efficiency in this active layer, resulting in the high FF in the optimized device.
2.6. Film morphological characterization Because the surface and bulk morphology of the blend film seriously affected the photovoltaic performance of the device, atomic force microscope (AFM), transmission electron microscope (TEM) and photoluminescence (PL) spectroscopy measurements were applied to investigate the nanoscale morphology and miscibility of the three optimized blend films based on PBDB-T with IT-2F, IT-2Cl or IT-2Br, respectively. As shown in Fig. 3a-c, three blend films all exhibit relatively smooth surface with root-mean-squared (RMS) roughness of 2.03, 1.94 and 1.95 nm, Meanwhile, from the phase images (Fig. 3d-f), PBDBT: IT-2F and PBDB-T: IT-2Cl based blend films exhibit a mature fibrillike nanoscale phase separation with appropriate domain sizes and excellent interpenetrating network compared to PBDB-T: IT-2Br, which may lead to the relatively higher JSC and EQE of IT-2F and IT-2Cl based devices. Then, the bulk morphologies of three blend films are explored by transmission electron microscopy (TEM) (Fig. 3g-i). It can be seen that PBDB-T: IT-2F and PBDB-T: IT-2Cl based blend films present rather homogeneous morphology and no obvious aggregation domains, and this elucidate the good miscibility of the donor and acceptor components in the blend films, which contributes to charge transfer in donor/ acceptor interface. In contrast, the blend film of PBDB-T: IT-2Br showed slightly aggregates, such aggregates generate adverse effects on exciton diffusion. So, the relatively low JSC in the PBDB-T: IT-2Br device are attributed to the absence of effective separation of excitons. In addition, the PL spectra of pure films of IT-2F, IT-2Cl, and IT-2Br as well as PBDBT, and PBDB-T: acceptor blend films were shown in Fig. S5. According
2.7. Charge recombination mechanism The carrier mobilities were gained by forthputting the space-chargelimited current (SCLC) method. The hole and electron mobilities of blend films were measured with device structures of ITO/PEDOT:PSS/ PBDB-T:acceptor/MoO3/Al and ITO/ZnO /PBDB-T:acceptor/PDINO/ Al, respectively. The related data and corresponding curves are shown in Fig. S6 and Table 2. As shown, the hole and electron mobility values of the PBDB-T: IT-2Br blend film are found to be 6.4 × 10−5 and 27.3 × 10−5 cm2 V−1 s−1 with a μe/μh ratios of 4.26 which were similar to PBDB-T: IT-2F blend film. In contrast, the blend films based on PBDB-T: IT-2Cl show higher values of electron mobility but lower values of hole mobility. It demonstrates that the blend films based on IT2F and IT-2Br exhibit more balanced hole and electron mobility compared to blend films based on PBDB-T: IT-2Cl. For the control device based on PBDB-T: ITIC, the relatively low ratio of electron and hole 433
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Fig. 3. AFM height images (5 μm × 5 um) of (a) the PBDB-T: IT-2F and (b) the PBDB-T: IT-2Cl and (c) the PBDB-T: IT-2Br; AFM phase images (5 μm × 5 um) of (d) the PBDB-T: IT-2F and (e) the PBDB-T: IT-2Cl and (f) the PBDB-T: IT-2Br; TEM images of (g) the PBDB-T: IT-2F and (h) the PBDB-T: IT-2Cl and (i) the PBDB-T: IT-2Br; all blends are measured under their optimal conditions.
the relatively higher JSC and EQE of IT-2F and IT-2Cl based devices. Considering the synthesis and experimental cost, the introduction of chlorine and bromine atoms on end-group is much easier and cheaper than introduction of fluorine atoms. Halogenation may be as an effective strategy when we design and synthesize novel photovoltaic materials.
mobility (μe/μh = 3.15) suggests a balanced charge transport in PBDBT: ITIC blend film, and thus a high FF can be obtained in PBDB-T: ITICbased devices. To further investigate the behavior of charge recombination in optimal devices based on IT-2F, IT-2Cl and IT-2Br, we conducted measurement of photocurrent and photovoltaic properties dependence on light intensity (P). For organic photovoltaic cells, JSC and P follows power-law equation: JSC ∝ PS where S is a recombination exponent. When s is close to 1, the bimolecular recombination is efficiently inhibited. As shown in Fig. 2d, the linear relationship between JSC and light intensity are observed and S values of three devices based on IT2F, IT-2Cl and IT-2Br are 0.982, 0.938 and 0.981, which contribute to the high FF obtained by IT-2F-based and IT-2Br-based devices.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements
3. Conclusions
The authors are deeply grateful to Fundamental Research Funds for the Central Universities (201822002), Natural Science Foundation of Shandong Province (ZR2018MEM023) and National Natural Science Foundation of China (U1806223).
In summary, we have synthesized and characterized several typical ITIC-based acceptors IT-2F, IT-2Cl and IT-2Br via halogenation to systematically investigate the effect of halogen substituents in non-fullerene small molecule acceptors. The device of donor: acceptor show superior PCEs from 10.66%~12.08%. Due to the electron-withdrawing effect of the halogen group, three materials demonstrate decreased LUMO level from −3.98 eV to −3.87 eV and slightly different absorption spectrum. In addition, changing the halogen end group could show impact on bulk morphology. The results of AFM and TEM measurements exhibit that the blend films based on PBDB-T: IT-2F and PBDB-T:IT-2Cl have rather uniform morphology and appropriate domain sizes compared to the blend film of PBDB-T: IT-2Br, which lead to
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