Controlling Molecular Packing and Orientation via Constructing a

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Energy, Environmental, and Catalysis Applications

Controlling Molecular Packing and Orientation via Constructing a Ladder-Type Electron Acceptor with Asymmetric Substituents for Thick-Film Non-Fullerene Solar Cells Shiyu Feng, Cai'e Zhang, zhaozhao bi, Yahui Liu, Pengcheng Jiang, Shouli Ming, Xinjun Xu, Wei Ma, and Zhishan Bo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19596 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

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ACS Applied Materials & Interfaces

Controlling Molecular Packing and Orientation via Constructing a Ladder-Type Electron Acceptor with Asymmetric Substituents for Thick-Film NonFullerene Solar Cells Shiyu Feng,‡a Cai’e Zhang,‡a Zhaozhao Bi,b Yahui Liu,a Pengcheng Jiang,a Shouli Ming,a Xinjun Xu,*a Wei Ma,*b Zhishan Bo*a a

Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing

Normal University, Beijing 100875, China b

State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an

710049, China KEYWORDS solar cells, asymmetric side chains, non-fullerene acceptor, face-on orientation, thick film

ABSTRACT

A non-fullerene acceptor, IDTT-OB, employing indacenodithieno[3,2-b]thiophene decorated with asymmetric substituents as the core, is designedly prepared. In comparison with the

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analogue IDT-OB, extending the five-heterocyclic indacenodithiophene core to sevenheterocyclic fused ring endows IDTT-OB with more broad absorption and elevated HOMO energy level. In addition, IDTT-OB shows a more intense molecular packing and a higher crystalline behavior with a strong face-on orientation in neat film and the PBDB-T:IDTT-OB blend film. Furthermore, an ideal nano-morphology with domain size of 19 nm can be obtained, which is in favor of the exciton diffusion and charge separation. Accordingly, PBDB-T:IDTTOB based polymer solar cells demonstrate a maximum power conversion efficiency (PCEmax) of 11.19% with an impressive FF of 0.74, comparable to the state-of-the-art acceptors with similar molecular backbones. More importantly, IDTT-OB based devices show good tolerance to the film thickness, which maintain a high PCE of 10.20% with a 250-nm-thick active layer, demonstrating that the asymmetric acceptor is profound for fabricating high efficiency thick film non-fullerene solar cells.

Introduction Non-fullerene polymer solar cells (PSCs) have developed by leaps and bounds in the past three years.1-6 The breakthrough of non-fullerene acceptors (NFAs) in PSCs mainly depends on their unique advantages of facile molecular modulation, tunable energy levels, good light absorption,

and

superior

morphological

stabilities.

Since

Zhan

et

al.

reported

indacenodithieno[3,2-b]thiophene (IDTT) and indacenodithiophene (IDT) based fused-ring electron acceptors (FREAs) in 2015,7-8 tremendous endeavors have been devoting to developing high-performance ladder-type FREAs, of which most prefer an acceptor-donor-acceptor (A-D-A) backbone structure.9-22


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To avoid strong self-aggregation caused by the central ladder-type fused rings, FREAs were decorated with several side chains sticking out of the planar central core. The substituents could profoundly influence the molecular packing, charge transport, molecular orientation and phase separation in the blends, which is vital for determining the maximum power conversion efficiency (PCE).23-30 It is noticeable that almost all FREAs incorporate symmetric side chains, such as alkylphenyl, alkoxylphenyl, alkylthiophenyl, alkylthienyl or alkyl substituents. Recently, we reported a simple acceptor named IDT-OB (Chart 1), which carries asymmetric substituents at the ladder-type IDT core.31 The introduction of asymmetric side chains endows IDT-OB with good solubility, which is conducive to controlling the nano-morphology of blend film to suppress large aggregations. In addition, benefiting from the asymmetric side chains, IDT-OB can keep packing closely which facilitates the transportation of electrons to obtain high fill factor (FF).31 As expected, when combining the classical donor polymer PBDB-T32, the IDT-OB-based PSCs gave a high PCE of 10.12%. Although, up to date, the state-of-the-art PCE value has already exceeded 14% in singlejunction non-fullerene PSCs,33-36 it usually decreases severely when the active layer film becomes thick, especially for film thicker than 200 nm.20,37 High-performance thick-film PSCs are highly desired because they are compatible with the future roll-to-roll manufacturing.38-42 To this end, some efforts have been made through the ternary blend strategy,43 the exploitation of new polymer donors,13,44-45 and the design of NFAs by tuning the terminal groups.21,46 As mentioned above, the central ladder-type fused rings have a crucial impact on NFA’s photovoltaic performance; unfortunately, molecular design of NFAs to obtain high-efficiency thick-film PSCs focused on modifying the central core unit is rarely reported. It is wellestablished that extending the conjugation length of the multi-fused acceptor is very useful for


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tuning the molecular energy levels and absorption spectra.47-48 However, this may cause aggregation problem.49-50 Our previously reported molecular design strategy of modifying the core unit by asymmetric substituents can both enhance the acceptor’s solubility and ensure the close molecular stacking,31 thus providing us with a possibility to design large fused-ring NFAs targeted for application in high efficiency thick film devices.

Chart 1. Molecular structures of several modified IDTs and IDTTs. Here, we synthesized a non-fullerene small molecular acceptor IDTT-OB (Scheme 1) via extending the previous five-heterocyclic to seven-heterocyclic ring. Extending the conjugation of the fused rings can enhance intermolecular − interactions, which is beneficial to charge transport. Organic photovoltaic materials with a high charge mobility can reduce the trapping and recombination possibility brought by increasing the thickness of active layer, facilitating to achieve high efficiency thick film PSCs. Compared with IDT-OB, IDTT-OB films showed more broad absorption, more intense − stacking with a strong face-on orientation, and an ideal nanoscale phase separation when blended with the donor polymer (PBDB-T). The IDTT-OBbased devices, as expected, have gained a high PCE of 11.19% with a comprehensive


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improvement of Jsc, Voc, and FF relative to the IDT-OB-based device. More importantly, IDTTOB-based devices are insensitive to the active layer thickness, which still demonstrate an average PCE of 10.20% even though the thickness being 250 nm. These results reveal that using a laddertype backbone structure with asymmetric side chains to construct the core of A-D-A-type NFAs is effective to access high performance thick film PSCs.

Scheme 1. (a) Syntheses of IDTT-OB and (b) molecular structures of IDT-OB, C8-ITIC, IT-M, ITIC and PBDB-T. Reagents and conditions: (i) 4-hexylphenyllithium, −78 °C, THF; (ii) concentrated HCl; (iii) KOBut, C8H17Br, DMSO; (iv) DMF, n-BuLi, THF; (v) CHCl3, pyridine, reﬂux.


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Results and Discussion Materials Synthesis. Scheme 1 illustrates how to synthesize IDTT-OB. Compound 1 was prepared by Stille coupling of compounds S2 and S3 (Scheme S1, Supporting Information). Then, reaction of 1 with an excess of freshly prepared 4-hexylphenyllithium provided an intermediate, followed by Friedel−Crafts-type cyclization reaction by use of concentrated hydrochloric acid to generate the ladder-type IDTT core 2. After subsequent treatment with 1bromooctane under strong basic condition, two alkyl chains were introduced onto the sp3carbons of 2 to afford compound 3. The two α-hydrogen atoms of 3 were then abstracted by the addition of n-butyllithium to form the dianion intermediate, which was subsequently quenched with dimethylformamide (DMF) to obtain the dialdehyde compound 4. The desirable acceptor IDTT-OB with asymmetric side chains was synthesized via Knoevenagel condensation of 4 and 2-(5 or 6-methyl-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (DCI-M). Like IDT-OB, IDTT-OB were also composed of several isomers due to the existence of asymmetric side chains and end groups. 1H and

13C

NMR spectroscopy and MALDI TOF mass spectroscopy were

utilized to verify the chemical structure of IDTT-OB (Supporting Information). This new acceptor can dissolve easily and thoroughly in toluene, dichloromethane (DCM) and odichlorobenzene (o-DCB), enabling facile solution process. In addition, IDTT-OB demonstrates good thermal stability up to 332 °C (with a 5% decomposition loss) as measured by thermogravimetric analysis (TGA, Figure S1). There was no obvious crystallization or melting peak in differential scanning calorimetry (DSC) curves (Figure S2).


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Figure 1. (a) Normalized absorption spectra and (b) energy-level diagram of materials used for the PSCs. Optical Properties. In dilute o-DCB solutions (see Figure S3), IDTT-OB exhibits a main absorption band ranged from 500 to 750 nm with an obvious peak at 682 nm, showing a bathochromic shift of 20 nm relative to IDT-OB. Such a red shift can be ascribed to the extended -conjugation and/or the enhanced intramolecular charge transfer (ICT) transition from electrondonating core (IDTT) to electron-withdrawing terminal group (DCI-M).3,51 Besides, a higher molar absorption coefficient (ε) of 2.03 × 105 M−1 cm−1 was observed in IDTT-OB dilute solution in contrast to that of IDT-OB (1.50 × 105 M−1 cm−1, Figure S4a). In the solid state, the maximum absorption of IDTT-OB was red-shifted to 715 nm. The optical bandgap of IDTT-OB is 1.59 eV following the formula Eg,opt = 1240/λedge, where the absorption edge (λedge) located at 778 nm. As illustrated in Figure 1a, IDTT-OB film shows a more complementary absorption spectrum with PBDB-T and a higher ε value of 1.24 × 105 cm−1 in contrast to IDT-OB (0.98 × 105 cm−1, Figure S4b), beneficial to better utilization of the sunlight. Accordingly, broader absorption and higher Jsc values can be expected in IDTT-OB based devices. Electrochemical Properties. Cyclic voltammetry (CV) was utilized to investigate the electrochemical behavior of IDTT-OB (Figure S5). The onset oxidation (Eox,onset) and reduction


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(Ered,onset) potentials were 0.89 V and −0.82 V, respectively. According to the equation EHOMO/LUMO = e [Eox/red,onset − E(Fc/Fc+) + 4.80],52 the LUMO and HOMO energy levels of IDTTOB were calculated to be −3.88 and −5.59 eV, respectively. As exhibited in Figure 1b, IDTT-OB and IDT-OB show almost similar LUMO energy levels, but the HOMO energy level of IDTTOB is about 0.18 eV higher than that of IDT-OB. The elevated HOMO energy level of IDTT-OB arises from the enhanced delocalized π-electron of the extended fused ring core, in accordance with the red-shifted absorption onset. In addition, we measured the absolute fluorescence quantum yields (FL) of IDTT-OB and IDT-OB in dilute chloroform solutions. A higher FL value of 10.10% was observed for IDTT-OB than that of 4.04% for IDT-OB (Figure S6). The reason is that extending the conjugation makes the molecular geometry of IDTT-OB become more planar and more rigid, which is helpful to restrain the non-radiative energy loss.16,53-55


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Figure 2. (a) J-V curves and (b) EQE curves of PSCs based on PBDB-T:IDTT-OB without and with DIO treatment at a 150-nm-thick active layer; (c) EQE curves and (d) photovoltaic parameters (Jsc, PCE, Voc, and FF) versus the active-layer thickness of PSCs based on PBDBT:IDTT-OB with DIO treatment (the error bar of each datum point was estimated by testing six devices). Table 1. The optimized photovoltaic parameters of the PSCs based on PBDB-T:IDT-OB and PBDB-T:IDTT-OB (0.3 v% DIO) blends. Device

Voc [V]

Jsc (Jcalca) [mA cm−2]

FF

PCEmax [%]

PCEaveb [%]

IDT-OBc

0.88 ± 0.00

15.91 ± 0.57

0.70 ±0.01

10.12

9.91 ± 0.15

IDTT-OB

0.91 ± 0.00

16.41 ± 0.23 (16.19)

0.74 ± 0.01

11.19

11.01 ± 0.15

aJ calc

was calculated by integrating the product of the EQE with the AM1.5G spectrum. bThe

average PCE values were calculated from six devices. cFrom ref. [31]. Photovoltaic Properties. Solution-processed devices were fabricated to appraise the photovoltaic performance of IDTT-OB as an electron acceptor, using PBDB-T as the donor with a device architecture of ITO/ZnO (30 nm)/active layer (150 nm)/MoO3 (8.5 nm)/Ag (100 nm). The detailed optimizing processes, including the weight ratios of donor to acceptor (D/A), spincoating rates and additive concentrations can be found in the Supporting Information (Table S1, S2). PBDB-T:IDTT-OB based pristine devices (see Figure 2a), fabricated with a D/A weight ratio of 1:1 (polymer concentration: 4.0 mg/ml in o-DCB solutions), gave an impressive PCE of 10.30% with a Voc of 0.91 V, a Jsc of 16.19 mA cm−2 and an FF of 0.70. After processed with DIO additive (0.3%, by volume), the PCE was further improved to 11.19%, with a similar Voc of 0.91 V, a slightly promoted Jsc of 16.58 mA cm−2 and an excellent FF of 0.74. In contrast with


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the previous reported PBDB-T:IDT-OB based devices,31 which obtained an average PCE of 9.91% as shown in Table 1, the higher PCEs (PCEave: 11.01%) of IDTT-OB based devices were manifested as a slightly comprehensive improvement of Voc, Jsc and FF. Since IDTT-OB possesses a more rigid coplanar backbone relative to IDT-OB, it tends to reduce vibrational energy dissipation, and consequently lead to a smaller energy loss of 0.68 eV compared to the 0.78 eV for IDT-OB. In addition, the extended effective conjugation in IDTT-OB also promotes close intermolecular π−π stacking, in favor of the electron transport (vide infra). External quantum efficiency (EQE) measurements of IDTT-OB based devices without and with DIO treatment were also carried out. PBDB-T and IDTT-OB have made synergistic contribution to the broad photoresponse (see Figure 2b). The EQE curves of the pristine devices and the DIOtreated devices were almost the same from 450 to 650 nm with a maximum EQE of 75%, where the light absorption mainly comes from the donor component PBDB-T. However, in the range of 650 through 800 nm, the highest EQE value of the DIO-treated device was close to 78%, slightly higher than that of the pristine device (72%). In this wavelength region, because IDTT-OB makes dominant contribution to the light absorption, the increase of EQE values can thus be attributed to the optimization of acceptor phase brought by DIO addition, which leads to a more effective photoelectron conversion process and a higher Jsc in the DIO-treated device. Jsc values estimated by integrating EQE spectra were within 4% error compared to those acquired from J−V curves. As we know, the film thickness of PSCs tends to fluctuate during the mass roll-to-roll manufacture, thus a well-maintained high photovoltaic performance is desired for the activelayer thickness from 100 to 300 nm.38,41 To test whether IDTT-OB is suitable to fabricate wellperforming PSCs with thick film, we carefully researched the influence of film thickness on the


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efficiency of PBDB-T:IDTT-OB-based devices (see Table S3). As indicated in Figure 2c, the EQE value increases slightly as the active layer becomes thicker, mainly owing to the enhanced light absorption of PBDB-T (Figure S7). Figure 2d shows that the Voc value is insensitive to the thickness of active layer and Jsc value is in accordance with the EQE result. Meanwhile, the FF value drops down slowly as the active-layer thickness increases, but it still maintains a desirable value of 0.69 with a 250-nm-thick active layer. Thus, the corresponding device exhibits a high average PCE of 10.20% at this thickness. Compared with IDT-OB, which exhibits a PCE of 9.17% with a 210-nm-thick film, the tolerance of IDTT-OB against the film thickness is better. The well-maintained high photovoltaic performance of IDTT-OB-based devices with thick-film active layer implies its potential to be used in the large-scale roll-to-roll manufacture of PSCs. Furthermore, to make a performance comparison between IDTT-OB and other state-of-the-art NFAs (shown in Figure S8), we fabricated and optimized PSCs using PBDB-T:NFA as the active layer. It’s worth noting that the chemical structures of IT-M,56 ITIC57 and C8-ITIC10 are similar to IDTT-OB (Scheme 1b), but bearing different side chains. Specifically, IT-M and ITIC possess symmetric alkylphenyl side chains, C8-ITIC incorporates symmetric linear alkyl side chains, and IDTT-OB bears asymmetric side chains in combination alkylphenyl with linear alkyl substituents. Considering the different molecular weight of PBDB-T and different device structures, we didn’t directly adopt the reported fabricated conditions to evaluate the photovoltaic performance of other reported non-fullerene acceptors (IT-M, ITIC and C8-ITIC). Instead, when employing the same device structure and the same batch of polymer, the performance of PBDB-T:ITIC, PBDB-T:IT-M and PBDB-T:C8-ITIC-based devices were systematically optimized in our lab (Table S4), showing the optimal PCEs of 9.78%, 10.39% and 10.68%, respectively. In comparison, PBDB-T:IDTT-OB-based devices have achieved a


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comparable maximum PCE of 11.19%, indicating that IDTT-OB is a promising non-fullerene acceptor. It is noted that the PCEs of our control devices were slightly lower than the reported ones, which might be assigned to the different batch of polymers (Figure S12).39,58-60 Film Morphology and Molecular Stacking. Tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM) were utilized to observe morphologies of active layers, which are of vital importance to the performance of device. As indicated in Figure S9, the pristine (no additive) PBDB-T:IDTT-OB blend film exhibited a smooth and homogeneous surface with a root-means-square (RMS) roughness value of 1.61 nm. After processed with DIO additive (0.3%, by volume), the blend film showed slightly increased phase separation with RMS value of 1.68 nm. As for the TEM images, no evident difference was observed and both demonstrated well-dispersed nanofibrillar networks.

Figure 3. (a) 2D GIWAXS patterns for pristine PBDB-T:IDTT-OB blend film, PBDB-T:IDTTOB blend film processed with DIO additive (0.3%, by volume), neat IDTT-OB film and neat IDT-OB film, and (b) the corresponding in-plane (IP) and out-of-plane (OOP) profiles.


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Aiming to obtain an in-depth investigation of the molecular stacking, grazing-incidence wideangle X-ray scattering (GIWAXS) was employed in observing films of the pure acceptors and the blends. As displayed in Figure 3, the IDT-OB neat film exhibits two diffraction peaks along both the out-of-plane (OOP) and in-plane (IP) directions, with π–π stacking distances (d010) of 4.39 and 3.59 Å, respectively. The coherence lengths (CL) are 12.23 and 17.01 Å, respectively. In contrast, the IDTT-OB crystallites exhibit a preferential OOP π–π stacking reflection with a shorter d010 value of 3.52 Å and longer CL value of 28.75 Å, indicative of a higher crystallinity in IDTT-OB films with a preferential face-on orientation (the relative crystallinity parameters are summarized in Table S5). Meanwhile, compared with IDTT-OB, ITIC bearing bulky alkylphenyl side chains showed a longer d010 value of 3.70 Å and shorter CL value of 19.6 Å according to the reported literature, with face-on and edge-on crystallites coexisting in the ITIC neat film.28 As for the C8-ITIC bearing linear alkyl side chains, it exhibited too strong crystallization tendency and tended to crystallize in ordered 3D structures, showing sharp diffraction spots along the vertical, horizontal and off-axis directions.10,61 Therefore, the relatively close π–π stacking, proper crystallinity, dominant face-on orientation, and suitable microphase structures in IDTTOB film are conducive to promoting the electron transport, especially in the thick films. More importantly, the predominant face-on orientation is maintained in the as-cast PBDB-T:IDTT-OB blend film with d010 = 3.64 Å and CL = 22.91 Å (Table 2). When treated with 0.3% DIO, PBDB-T:IDTT-OB blend film shows enhanced intermolecular associations with d010 = 3.59 Å and increased coherence length with CL = 26.59 Å. In addition, the DIO-treated PBDB-T:IDTTOB blend film also shows increased CL value of 149.65 Å for IP (100) peak compared with the pristine film (CL = 131.73 Å), which are conducive for higher charge carrier mobility.


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Moreover, the microphase separation of PBDB-T:IDTT-OB blend films is investigated by resonant soft X-ray scattering (R-SoXS, see Figure 4a). The resonant energy of 285.2 eV was picked to cement contrast. A median domain size was 14 nm for the pristine PBDB-T:IDTT-OB blend film. When treated with 0.3% DIO, the median domain size of the blend film was slightly increased to 19 nm (Table 2). Both are within the exciton diffusion length (10−20 nm) in organic materials. Via integrating scattering profiles, the relative domain purity of the pristine and DIOtreated PBDB-T:IDTT-OB-based blends were calculated to be 0.85 and 1.00, respectively. Therefore, 0.3% DIO promotes the average domain purity of blend films, which is beneficial to suppress bimolecular recombination and promote charge transport, thus in favor of the improvement of PCE. Table 2. Experimental data of PBDB-T:IDTT-OB blend films obtained from GIWAXS and RSoXS characterization. PBDBT:IDTT-OB As-cast 0.3% DIO

IP (100) peak location (Å-1) 0.29 0.29

d-spacing (Å) 21.52 21.43

coherence length (Å) 131.73 149.65

OOP (010) peak location (Å-1) 1.73 1.75

d-spacing (Å) 3.64 3.59

coherence length (Å) 22.91 26.59

domain purity 0.85 1

domain size 14 19


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Figure 4. (a) R-SoXS profiles, (b) Jph versus Veff characteristics, (c) Jsc dependence on light intensity, and (d) Voc dependence on light intensity. Charge Generation, Dissociation, Transport and Recombination Properties. To obtain thickness insensitive PSCs, photoactive components with a high charge mobility are required, which can reduce the trapping and recombination possibility brought by increasing the thickness of active layer.28,38,43,62 As a verification, the charge transport abilities of the active layers in IDTT-OB based devices were characterized by the space charge limited current (SCLC) method (Figure S10). The pristine PBDB-T:IDTT-OB blend film shows hole (μh) and electron (μe) mobilities of 4.87×10−4 and 4.13×10−4 cm2 V−1 s−1, respectively, with an μh/μe of 1.18. After the blend film processed with DIO additive (0.3%, by volume), μh and μe are enhanced to 7.26×10−4 and 6.49×10−4cm2 V−1 s−1, respectively, with a more balanced μh/μe of 1.12, contributing to the improved FF and Jsc. We attribute the increased mobilities to the increased crystallinity of both PBDB-T and IDTT-OB in the blend film after the addition of 0.3% DIO, enabling them to form purer domains as confirmed by GIWAXS and R-SoXS. Besides, for the purpose of observing the influence of active-layer thickness on the charge transport, we also measured the DIO-treated PBDB-T:IDTT-OB blend film with a thickness of 250 nm. The corresponding μh and μe were still kept at high values of 5.52×10−4 and 4.06×10−4cm2 V−1 s−1, respectively, with a μh/μe of 1.36.


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To gain more insight into the influence of additive and thickness on exciton dissociation process, the characterization of photocurrent density (Jph) versus effective voltage (Veff) was carried out (see Figure 4b). Jph is determined by JL – JD (JL and JD represent the current density under the light and in the dark), and Veff is defined as V0 – Va (V0 is the voltage where Jph = 0, and Va is the bias voltage). The exciton dissociation probability is governed by the ratio of Jph/Jsat, where Jsat stands for the saturation current density. Under short-circuit conditions, the Jph/Jsat value of the as-cast device is calculated to be 0.965. After the addition of 0.3% DIO, it increases to 0.980, indicating more effective exciton dissociation. Furthermore, the thick device (0.3% DIO, 250nm) still maintains a high Jph/Jsat value of 0.958, suggesting that the exciton dissociation is efficient in the thick active layer. Figure 4c and 4d show the dependence of Jsc and Voc values on the light intensity (Plight), which reflect the charge recombination process. In principle, the relationship between Jsc and Plight is usually expressed as Jsc ∝ Plight α. When the bimolecular recombination in the device is negligible, the exponential factor α should be close to 1.63,64 The values of α for the pristine(150 nm) and DIO-treated devices (150 nm and 250 nm) are 0.977, 0.983 and 0.970, respectively, indicative of the weak bimolecular recombination in these active layers even in the thick film. In addition, the plot of Voc versus the natural logarithm of Plight helps us further understand the degree of trap-assisted recombination. A slope of kBT/q suggests that the bimolecular recombination is predominant, where q is elementary charge, kB is Boltzmann’s constant and T is temperature. Conversely, a stronger dependence of Voc on Plight (2 kBT/q) implies the trap-assisted monomolecular recombination.65,66 As revealed in Figure 4d, slope values for the as-cast (150 nm) and DIO treated (150 nm and 250 nm) devices are estimated to be 1.19, 1.15 and 1.21 kBT/q, respectively, indicating that the addition of DIO can effectively suppress traps and the influence


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of film thickness on the trap-assisted recombination is relatively small in this system. The above results indicate that the thick devices still possess relatively efficient exciton dissociation, weak recombination and high charge transport ability, which can partially explain the slow downward trend of FF and well-maintained photovoltaic performance in thick-film PSCs. Conclusions In conclusion, we have developed a seven-heterocyclic fused ring acceptor IDTT-OB with asymmetric substituents. In comparison with the five heterocyclic fused ring acceptor IDT-OB, IDTT-OB exhibits a broad absorption (500-800 nm), a narrow bandgap (1.59 eV) and an elevated HOMO energy level (-5.59 eV). Specially, GIWAXS characterizations demonstrated that IDTT-OB in thin film shows a predominant face-on orientation with a longer CL value and a shorter − stacking distance. In addition, ITIC bearing bulky side chains shows a longer − stacking distance with face-on and edge-on crystallites coexistence, and C8-ITIC bearing linear side chains exhibited too stronger crystallization tendency along undesired directions. In contrast, asymmetric side-chain modification endows IDTT-OB with close π–π stacking and proper crystallinity, which is beneficial for forming suitable microphase structures especially in the thick blend films. As expect, the PBDB-T:IDTT-OB blend films exhibit a good nanoscale phase separation with appropriate domain size of 19 nm, beneficial for the exciton diffusion and charge transport. Consequently, a high PCE of 11.19% was achieved in the PBDB-T:IDTT-OB based device, with a comprehensive improvement of Voc, Jsc and FF. Also, its PCE value is comparable to the state-of-the-art acceptors including ITIC, IT-M, and C8-ITIC. More importantly, at the 250-nm-thick active layer, IDTT-OB based devices still feature efficient exciton dissociation, weak recombination and high charge transport behaviors. As expected, IDTT-OB based devices maintained an average PCE of 10.20% at the 250-nm-thick film,


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implying its potential for the future mass fabrication of PSCs by roll-to-roll process. The above results demonstrate that constructing a ladder-type backbone structure with asymmetric side chains is promising in designing high performance thickness insensitive acceptors. ASSOCIATED CONTENT Supporting Information. Synthesis and characterization of IDTT-OB, AFM and TEM images, molecular weight and molecular weight distribution of PBDB-T, SCLC and OPV measurement. AUTHOR INFORMATION Corresponding Author * Email: [email protected]; [email protected]; [email protected] Author Contributions Shiyu Feng and Cai’e Zhang made a fair contribution to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21574013, 51673028, 21421003, 51003006, and 21734009) and Program for Changjiang Scholars and Innovative Research Team in University. X. X. thanks the support from the Fundamental Research Funds for the Central Universities. W. M acknowledges the support from Ministry of science and technology (No. 2016YFA0200700), NSFC (21504066 and 21534003).


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Controlling Molecular Packing and Orientation via Constructing a

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