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Design of a Highly Crystalline Low-Band Gap Fused-Ring Electron Acceptor for High-Efficiency Solar Cells with Low Energy Loss Xueliang Shi,† Lijian Zuo,† Sae Byeok Jo,† Ke Gao,† Francis Lin,‡ Feng Liu,*,∥ and Alex K.-Y. Jen*,†,‡,§ †

Department Department § Department ∥ Department ‡

of of of of

Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120, United States Chemistry, University of Washington, Seattle, Washington 98195-2120, United States Chemistry, City University of Hong Kong, Kowloon, Hong Kong Physics and Astronomy, Shanghai Jiaotong University, Shanghai 200240, China

S Supporting Information *

ABSTRACT: A fused-ring thiophene-thieno[3,2-b]thiophene-thiophene (4T)based low-band gap electron acceptor, 4TIC, has been designed and synthesized for non-fullerene solar cells. The utilization of the 4T center core enhances the charge mobility of 4TIC and extends its absorption band edge to ∼900 nm, which facilitates its function as a very efficient low-band gap electron acceptor. The rigid, π-conjugated framework of 4T also offers a lower reorganization energy to facilitate lower VOC energy loss. Femtosecond transient spectroscopy showed a level of polaron generation in 4TIC results in the more efficient transfer of energetic carriers higher than that seen with the benchmarked molecule, ITIC. Film morphology analysis has also shown that 4TIC has structural order that is more prominent than that of ITIC with a multiscale phase separation in the blend with donor polymer PTB7-Th. As a result, solar cells based on PTB7-Th and 4TIC exhibit a high power conversion efficiency of 10.43% and a relatively low non-ideal photon energy loss of 0.33 V. The low band gap and small energy loss make 4TIC suitable for tandem solar cells as a back-cell to reduce the transmission loss. As a demonstration, we fabricated series connection tandem solar cells incorporating 4TIC, which exhibts a high device performance of 12.62%. IDTT.13−16 The development of new central donor units has rarely been reported.17−19 Therefore, it is imperative to develop new and novel fused-ring center units that possess better geometrical and electronic properties to further improve the optical and electrical properties of NFAs and the subsequent OSCs. It is believed that the utilization of more electron-rich fused rings can yield lower-band gap NFAs with broadened optical absorption to cover the near-infrared (NIR) region (>800 nm) of the solar spectrum.20−22 In this work, we propose to utilize a rigid fused-ring thiophene-thieno[3,2b]thiophene-thiophene (4T) as the central donor unit and an electron-deficient 3-(dicyanomethylidene)indan-1-one (IC) as the terminal acceptor unit to construct a new low-band gap NFA, 4TIC (Figure 1a). It is expected that this molecule can function as a promising low-band gap NFA for the following reasons. (1) The rigid fused thiophene 4T will allow more efficient π-electron delocalization than IDT or IDTT because the resonance stabilization energy of thieno[3,2-b]thiophene is lower than that of benzene in IDT and IDTT (Figure 1a, highlighted in blue). (2) 4T has a rigid and extended π-conjugated framework,

I. INTRODUCTION Non-fullerene acceptors (NFAs) have recently been vigorously developed as active materials for high-performance organic solar cells (OSCs).1,2 Most of the reported efficient NFAs are based on the fused-ring conjugated molecules, exemplified by IEIC and ITIC, which enable OSCs to exhibit very high power conversion efficiencies (PCEs).3,4 The prevalent strategy for designing NFAs is based on using a planar acceptor−donor− acceptor (A−D−A) system with strong intramolecular charge transfer (ICT). In this type of structure, fused-ring aromatics such as indacenodithiophene (IDT) and indacenodithieno[3,2b]thiophene (IDTT) function as the central donor, and the electron-withdrawing unit, 3-(dicyanomethylidene)indan-1-one (IC), serves as the terminal acceptor. In addition, a spacer (usually an electron-rich or -deficient unit) is often employed to further enhance ICT and electron delocalization (Figure 1a). Guided by this design principle, significant efforts have been devoted to enhancing our understanding of structure−performance relationships and mechanisms to further improve the PCEs of OSCs. Different electron-withdrawing modalities and spacers have been explored to fine-tune the energy levels, absorption spectra, and morphology of NFA-based bulk heterojunction (BHJ) blends.5−12 However, the central donor units for high-performance NFAs reported so far are mainly based on indaceno-based fused rings such as IDT and © 2017 American Chemical Society

Received: July 8, 2017 Revised: September 12, 2017 Published: September 13, 2017 8369

DOI: 10.1021/acs.chemmater.7b02853 Chem. Mater. 2017, 29, 8369−8376

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Chemistry of Materials

hexylphenyl)lithium to diester 3 followed by BF3·Et2O-induced Friedel−Crafts intramolecular cyclization afforded 4T in 55% yield. It was noted that the commonly used conditions for synthesizing IDT and IDTT with protic acid (acetic acid/HCl and acetic acid/H2SO2)-catalyzed ring closure gave only 4T in very low yield. This is because the use of protic acid often creates complicated oligomerization due to the electron-rich nature of 4T. 4T was then treated with Vilsmeier reagent to afford dicarbaldehyde 4T-CHO in 92% yield. Finally, 4TIC was obtained in 91% yield by Knoevenagel condensation reactions between 4T-CHO and IC. All the new compounds were fully characterized by 1H nuclear magnetic resonance (NMR), 13C NMR, and mass spectrometry (see the Supporting Information), and 4TIC was unambiguously confirmed by X-ray crystallographic analysis (vide inf ra). Photophysical and Electrochemical Properties. The ultraviolet−visible−NIR (UV−vis−NIR) absorption spectra of compounds 4TIC and ITIC were recorded in a dichloromethane (DCM) solution and in the solid state (Figure 2a). In

Figure 1. (a) Molecular design and chemical structures. (b) Synthetic route to 4TIC.

which provides more room in arranging substituted side chains to avoid twisting the backbone, leading to more efficient molecular packing and charge transport,23 as well as an enlarged polarization volume of π electrons at optical frequency.24,25 (3) 4T is more electron-rich compared than IDT and IDTT, which can induce stronger ICT with terminal acceptor IC to extend its absorption to the NIR. Considering the structural similarity between 4TIC and ITIC, a systematic comparison of these systems will provide valuable insights for understanding the influence of different central donors on the electrical and optical properties of the NFAs and OSC device performance. As a result, the devices based on 4TIC exhibit a high PCE of 10.43%, which surpasses that based on the ITIC system due to the high JSC and low VOC loss. First, the thin film absorption of 4TIC shows a broadened spectrum with a low energy onset at ∼900 nm, which is red-shifted ∼100 nm compared to that of ITIC, and contributed to the improved JSC. Second, 4TIC also shows an electron mobility that is higher than that of ITIC. 4TIC is strongly crystalline as evidenced by grazing incidence wide-angle X-ray spectroscopy (GIWAXS), which is important for the corresponding optoelectronic properties. Moreover, the results from transient absorption studies show distinctive charge transfer and high polaron generation efficiency between PTB7-Th and 4TIC in BHJ blends, which resulted in its low energy loss. In addition, the high device performance of a series connection tandem solar cell with an efficiency of 12.62% was achieved because of the long-range absorption of 4TIC for transmission loss suppression.

Figure 2. (a) UV−vis−NIR spectra of 4TIC and ITIC recorded in dichloromethane and in thin film. (b) Cyclic voltammograms of 4TIC and ITIC in DCM with 0.1 M Bu4NPF6 as the supporting electrolyte, Ag/AgCl as the reference electrode, and a Pt wire as the counter electrode and a scan rate at 20 mV/s. (c) Simulated molecular geometries and frontier molecular orbitals obtained by density functional theory calculations for 4TIC and ITIC.

solution, 4TIC exhibits a strong absorption peak in the region between 600 and 800 nm with an onset at 806 nm, which is red-shifted significantly compared to that of ITIC (band edge at ∼720 nm). The thin film spectrum of 4TIC further redshifted, reaching ∼900 nm, whereas the onsets of ITIC, IEIC, and other reported NFAs are usually at ∼800 nm, although they all have molecular and π-conjugation lengths that are longer than that of 4TIC.3,4,9,17−19 Simply replacing the benzene in IDT and IDTT core with thieno[3,2-b]thiophene enhances the electron-donating ability of the central core, 4T, because thiophene has a resonance stabilization energy that is lower than that of benzene, which results in more efficient electron delocalization in the 4T molecular framework.28,29 This explains why 4T has an electron-donating ability that is stronger than that of indaceno-based fused rings that have even longer molecular lengths.9,17−19

II. RESULTS AND DISCUSSION Synthesis. The synthetic route to 4T and 4TIC is depicted in Figure 1b. 4T is synthesized using a new method that affords a yield much higher than that reported previously.26 The Stille coupling between diethyl 2,5-dichlorothieno[3,2-b]thiophene3,6-dicarboxylate 127 and 2-(tributylstannyl)thiophene 2 gave diester 3 in 96% yield. The double nucleophilic addition of (48370

DOI: 10.1021/acs.chemmater.7b02853 Chem. Mater. 2017, 29, 8369−8376

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Chemistry of Materials The electrochemical properties of 4TIC and ITIC were investigated by performing cyclic voltammetry (CV) in a dry DCM solution (Figure 2b). Both compounds show one irreversible reductive wave and one reversible oxidative wave. The ionization potentials and electron affinities, which are also widely termed the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels,30,31 were estimated to be −5.28 and −3.87 eV for 4TIC and −5.43 and −3.81 eV for ITIC, respectively, based on the corresponding onset potential of the first oxidative and reductive wave. The electrochemical energy gaps of 4TIC and ITIC were thus determined to be 1.41 and 1.62 eV, respectively, which are slightly larger than their thin film optical energy gap (Table 1). Table 1. Summary of the Photophysical and Electrochemical Dataa compound

HOMO (eV)

LUMO (eV)

EgEC (eV)

EgOPT (eV)

4TIC ITIC

−5.28 −5.43

−3.87 −3.81

1.41 1.62

1.40 1.60

Figure 3. X-ray crystallographic structure of 4TIC. Hydrogen atoms are omitted for the sake of clarity.

a HOMO = −(4.8 + Eoxonset), and LUMO = −(4.8 + Eredonset), where Eoxonset and Eredonset are the onset potentials of the first oxidative and reductive waves, respectively. EgEC is the electrochemical band gap. EgOPT is the optical band gap estimated from the absorption onset.

one type of 4TIC molecular geometry was found in the solid state, presumably because of intimate [S···O] interactions (Figure 3, colored red in the top view). This is also consistent with the simulated molecular geometry from the DFT calculations discussed above (Figure 2c). It should be pointed out that this is the first time molecular geometry insight was provided in the single crystal of NFAs. Photovoltaic Properties. PTB7-Th is used as the donor polymer, and 4TIC and ITIC are used as acceptors to fabricate OSCs with ITO/PEDOT:PSS/PTB7-Th:4TIC and ITIC/C60bissalt/Ag device structures, respectively, where ITO is indium tin oxide and PEDOT:PSS is poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate). The initial test for PTB7Th:4TIC-based devices shows that it can give a high PCE of >9% without any optimization, which is better than the value reported for ITIC (∼7%). Figure 4a shows the optimized I−V characteristic of PTB7-Th:4TIC and PTB7-Th:ITIC, and the corresponding device performance is summarized in Table 2. It was found that a donor:acceptor weight ratio of 1:2 in a chlorobenzene solution with 0.5% diiodooctane (DIO) gave the best device performance after optimization with different processing solvents, donor:acceptor ratios, thicknesses, and annealing temperatures (see the Supporting Information). A high PCE of 10.43% can be obtained, which is ∼43% higher than that obtained for ITIC. The addition of DIO significantly improved the structural order and morphology of the active layer. Compared with that of a device based on PTB7Th:ITIC, a lower VOC was found for the device with PTB7Th:4TIC due to the reduced effective band gap of the 4TIC BHJ blend. However, considering the Shockley−Queisser limit of PTB7-Th:ITIC and PTB7-Th:4TIC, the non-ideal photon energy loss of PTB7-Th:4TIC is only ∼0.33 eV, which is much smaller than that from the PTB7-Th:ITIC system (0.49 eV).35 As expected, the PTB7-Th:4TIC-based device shows a shortcircuit current density that is higher than that of PTB7Th:ITIC, due to the lower band gap and better coverage of solar spectra. Moreover, the PTB7-Th:4TIC-based device shows a superior FF of 0.72, enabling a high PCE to be achieved in low-band gap OSCs.

Density functional theory (DFT) calculations at the B3LYP/ 6-31G(d,p) level were employed to understand the difference in energy levels between 4TIC and ITIC. In both cases, the electron densities of HOMO are predominantly localized at the central donor unit, whereas those of LUMO are mainly distributed along the quinoidal conjugation to the terminal acceptor unit, suggesting very efficient ICT in this A−D−A system (Figure 1c). Interestingly, the HOMO level of 4TIC is 0.088 eV higher than that of ITIC, while its LUMO level is 0.075 eV lower than that of ITIC, which collectively contribute to 4TIC’s smaller band gap. It should be noted that the widely used B3LYP functional normally overdelocalizes the wave functions and favors this kind of charge-transfer structure.31,32 Thus, DFT calculations at the LC-BLYP/6-311G(d,p) level were also conducted to plot the HOMO and LUMO of 4TIC and ITIC, because the LC mode has been demonstrated to describe more accurately the localization and delocalization of wave functions in π-conjugated electronic structures.31,32 It was found that very similar distributions of molecular geometries and HOMO and LUMO electron densities can be obtained in both modes (Figure S1). X-ray Crystallographic Analysis. Single crystals of 4TIC suitable for X-ray crystallographic analysis were obtained by slow diffusion of methanol into its toluene solution.33 The ORTEP drawings and three-dimensional (3D) packing structures are shown in Figure 3. The backbone of 4TIC is essentially planar, with four hexylphenyl groups oriented outward from the backbone. No obvious intermolecular interaction such as π stacking was found in the central 4T core, because of the bulky hexylphenyl groups. However, the terminal IC groups display a close π−π stacking with a distance of ∼3.60 Å (Figure 3, 3D packing colored green). This is very important for 4TIC to efficiently transport electrons.34 For most reported NFAs, their molecular structures are often arbitrarily drawn in two forms; i.e., either the O atom on IC points toward the S atom of neighboring thiophene ring, or the O atom points to the hexylphenyl groups. Interestingly, only 8371

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Figure 4. (a) J−V curves of the best performing devices. (b) EQE curves of the devices. (c) Energy levels of PTB7-Th, 4TIC, and ITIC.

Table 2. Photovoltaic Parameters of OSCs Based on PTB7-Th:4TIC and PTB7-Th:ITIC device

DIO (%)

VOC (V)

JSC (mA/cm2)a

FF (%)

PCE (%)b

PTB7-Th:4TIC PTB7-Th:4TIC PTB7-Th:ITIC

− 0.5 −

0.78 ± 0.01 (0.79) 0.77 ± 0.01 (0.78) 0.82 ± 0.01 (0.83)

16.8 ± 0.2 (17.2) 18.4 ± 0.3 (18.8) 14.92 ± 0.12 (15.18)

0.64 ± 0.02 (0.67) 0.70 ± 0.02 (0.72) 0.55 ± 0.01 (0.57)

8.39 ± 0.02 (9.13) 9.97 ± 0.25 (10.43) 6.73 ± 0.31 (7.28)

a

Calculated integrated current density from EQE spectra. bDevice fabrication: device area of 0.314 cm2, conventional ITO/PEDOT:PSS/active layer/V60-bissalt/Ag structure. More than 100 devices with 16 pixels on each device were fabricated to obtain the averaged data and standard deviation.

Figure 5. Photoinduced absorption spectra of 4TIC and its blend at excitation wavelengths of (a) 365 and (b) 850 nm. Kinetic traces of (c) ground state bleaching at 800 nm and (d) excited state absorption around 1100 nm. Relative comparison of polaron generation yields within neat and blend films for (e) 4TIC and (f) ITIC and (g and h) respective kinetic traces of excited state absorption. The pump−probe delays in panels a, e, and f are 2 ps. Signals in panels e and f were scaled to the ground state bleaching intensity. Numbers in parentheses are pump wavelengths except for those in panel b.

facilitates better charge extraction, resulting in a lower recombination loss.36 Compared with the device with ITIC, the device with 4TIC shows an improved FF, which is closely related to the carrier dynamics, e.g., charge recombination. Here, the light intensitydependent JSC and VOC were studied to examine the charge recombination behavior of the device. The JSC of the 4TIC system shows an index of 0.97 with light intensity, while the index of the ITIC system is ∼0.96 (Figure S3). The close to unity light intensity dependence of the 4TIC system indicates insignificant charge recombination loss during charge extraction. In addition, the VOC of the 4TIC device was observed to have a distinct light intensity dependence with a slope of 1.96 (kT/q), where the ITIC system shows a slope of 1.61 (kT/q). Together with small recombination loss, this result indicates most of the photoinduced carriers in 4TIC go through intrinsic monomolecular recombination in each phase rather than through Langevin recombination despite the higher carrier mobility, while the ITIC system suffers from loss through the bimolecular recombination channel. Therefore, the light

EQE measurements were conducted, and the corresponding curves are presented in Figure 4b. The two devices have comparative EQE values as high as 69% beyond 600 nm. The PTB7-Th:4TIC device shows the EQE response from 350 to 900 nm, which extended ∼100 nm compared with that of the PTB7-Th:ITIC system. Thus, the JSC values of the PTB7Th:4TIC device calculated from the EQE curves are significantly higher than that from the PTB7-Th:ITIC device, consistent with the improved JSC. The electron mobilities of 4TIC and ITIC were measured using the space-charge-limited-current (SCLC) method with an ITO/ZnO/active layer/C60-bissalt/Ag device structure (Figure S2). A built-in potential of 0.3 V is corrected to calculate the mobility because of the energy difference between ZnO and C60-bissalt. An electron mobility of 6.2 × 10−4 cm2 V−1 s−1 can be obtained in the vertical direction of the PTB7-Th:4TIC blended film. In contrast, the ITIC film exhibits a lower electron mobility of 2.4 × 10−4 cm2 V−1 s−1. The higher mobility of 4TIC is due to the more ordered packing in the solid state. The higher charge mobility of PTB7-Th:4TIC 8372

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Figure 6. (a) Grazing incidence X-ray diffraction patterns of ITIC, 4TIC, and their corresponding blends. (b) In-plane (dotted lines) and out-ofplane (solid lines) line-cut profiles of GIXD results. (c) Resonant soft X-ray scattering of BHJ blends.

intensity dependence of VOC and JSC indicates that the 4TIC system could effectively prevent energy loss by reducing extra nonradiative loss compared to that of the ITIC system. Transient Absorption Analysis. Femtosecond transient absorption spectroscopy (fs-TAS) was performed to investigate the charge generation behavior and reduced energy loss in the 4TIC system. Initially, ground state bleaching (GSB) was tracked with an explicit excitation of the donor and acceptor in blended films. Two pump wavelengths (365 and 850 nm) were selected on the basis of steady state absorption spectra of each material. Panels a and b of Figure 5 show photoinduced absorption spectra at a pump−probe delay of 2 ps. Two discernible GSB signals were clearly observed because of the different band edges of the donor and acceptor. Distinctive excited state absorptions (ESAs) in the NIR range were assigned to the energetic manifolds of excitons (broad peak at 1450 nm for the donor singlet) and polarons (peak at 1100 nm for both positive and negative charges). As shown in panels c and g of Figure 5, the apparent elongation of both the GSB signal and the polaron absorption signal with excitation exclusive to each phase clearly indicates efficient charge transfer both from the donor to the acceptor and from the acceptor to the donor.37 Furthermore, slower polaron decay in spite of the higher polaron density of the 4TIC system compared to that of the ITIC system is in good agreement with light intensitydependent device analysis (Figure 5d). Surprisingly, in addition to the recombination behavior observed in the 4TIC system, 4TIC and ITIC also show a difference in the initial generation of polarons. Via comparison of the polaron yield within neat and blended films (Figure 5e,f), efficient formation of negative polarons was observed even without the aid of any energetic offsets in both NFAs. In the 4TIC system, however, significantly larger portions of GSBs were composed of those ESAs rather than bound excitons, of which their binding energies are the primary source of energy loss during charge generation. This trend can also be corroborated by comparing the sensitivity of polaron kinetic traces to the presence of the donor (Figures 5g and 5h). Considering that photoinduced charge-transfer behavior occurs within a few picoseconds, the dielectric behavior during the process is primarily governed by the π-electron polarization rather than permanent molecular dipoles.38 In this regard, despite the minimal dipole moment that is caused by molecular symmetry, superior π-electron delocalization in 4TIC can

provide a favorable dielectric environment for separating excitons as well as stabilizing polarons in each domain at the optical frequency (>1014 Hz). Therefore, it can be deduced that the lowered resonance energy in the 4T core also provides an intrinsic benefit for reducing energetic loss in BHJ-based OSCs, which can be an important design criterion for NFAs. Film Morphology Analysis. The morphology of thin films plays a critical role in determining the device performance of OSCs. Therefore, a systematic morphological study was performed, including structure order, phase segregation, and surface topology. Shown in Figure 6a are the two-dimensional (2D) grazing incidence X-ray diffraction patterns of ITIC, 4TIC, and their corresponding BHJ thin films; the line-cut profiles are summarized in Figure 6b. It can be clearly seen that ITIC in a neat film is highly disordered, with quite weak diffraction signals. It showed a weak (100) packing at ∼0.33 Å−1 (1.90 nm) and π−π stacking at ∼1.76 Å−1 (0.36 nm). An obvious amorphous halo was seen at ∼1.35 Å−1. Thus, the structural order of ITIC is quite weak, which is most probably due to the bulky side chain substitution and weak IDT centerto-center interactions. Replacing IDT with the 4T center dramatically changes the manner of molecular packing in the solid state. The as-cast 4TIC thin films showed quite good crystalline order in a pure film. As seen from the 2D pattern, a well-developed (100) peak was observed at 0.46 Å−1, corresponding to a distance of 1.36 nm. Thus, the length and bulky side chains were arranged and packed tightly. As seen from the line-cut profiles, a quite broad (100) peak is seen in the out-of-plane direction and a quite sharp (100) peak is seen in the in-plane direction. Thus, large crystals take a face-on orientation, and tiny crystals take an edge-on orientation. The (100) crystal coherence lengths for face-on and edge-on crystals are estimated to be 22.7 and 10.2 nm, respectively. A sharp π−π stacking peak can be observed in the out-of-plane direction at 1.84 Å−1, corresponding to a stacking distance of 0.34 nm, which is quite consistent with the π−π stacking distance observed in its solid state single crystal (Figure 3). It is quite obvious that the change from IDTT to 4T strongly changes the molecular behavior and self-assembly. It is well documented that fused thiophenes have a strong tendency to form highly ordered packing;39 the current case of fusing thiophene together to form 4T strongly enhances intermolecular interactions, aided by the quinoid resonance that forms 8373

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Chemistry of Materials a highly flat structure. The better packing and crystallization in 4TIC allow better transport in the material. In ITIC BHJ blends, PTB7-Th diffraction dominates the 2D pattern. The broad scattering of the PTB7-Th (100) peak was seen at ∼0.29 Å−1, agreeing well with previous reports.40 The π−π stacking of PTB7-Th is strong but broad and shown in the out-of-plane direction. Thus, PTB7-Th showed quite small (100) and π−π ordered structures. The 4TIC BHJ blends showed similar GIXD results, except that some 4TIC (100) peak signals can be noticed (shown as a hump), and thus, 4TIC can still form ordered structures but with lower crystalline content. This is because 4TIC and PTB7-Th could form good mixtures to retard 4TIC crystallization. The length scale of phase separation of BHJ blends was characterized using resonant soft X-ray scattering (RSoXS), and the results are shown in Figure 6c. It is seen that PTB7Th:ITIC showed a quite quick decay in intensity, and not much structure could be observed. Thus, PTB7-Th and ITIC could form a quite good mixture in blends, which reduces the FF and PCE in solar cells. However, PTB7-Th:4TIC showed a quite different RSoXS profile, with broad and elevated intensities from 0.003 to 0.01 Å−1. Therefore, a quite broad length scale (63−210 nm) of phase separation exists in PTB7Th:4TIC blends.41−43 The broad length scale of phase separation and better structural order in blends contribute to the improved JSC and FF in solar cells. The surface morphologies of PTB7-Th:4TIC and PTB7Th:ITIC are similar (Figure S4). However, with a 0.5% DIO treatment, the film surface of PTB7-Th:4TIC becomes rougher; moreover, fiberlike crystalline domains were formed on the surface (Figure S4b), indicating that the 4TIC molecule reorganized into a highly crystalline morphology in the presence of DIO. Additionally, the adhesion image of PTB7Th:4TIC shows a phase separation scale of ∼10 nm (Figure S4d). These results suggested a multiscale phase segregation in the PTB7-Th:4TIC system, which is consistent with what was observed via RSoXS (Figure 6c). Thus, the small amount of DIO additive could help 4TIC material reorganize into a highly crystalline structure and undergo multiscale phase segregation to facilitate both exciton dissociation and charge transport. It is also expected to improve the structural order in the bulk, as noticed by diffraction hump in Figure 6b for blend line-cut profiles. These results show that the active layer morphology is significantly improved with a higher crystallinity, which can be the reason for the improved carrier mobility and FF. Tandem Solar Cells. The long-range absorbing and lowenergy loss properties of 4TIC make it suitable for tandem solar cells to reduce the transmission loss. As a demonstration, here we fabricated the tandem solar cells (device architecture shown in the inset of Figure 7); details regarding device fabrication can be found in our previous work,44 where a relatively large band gap material PBDB-T:ITIC developed by Hou et al. was used as front cell active layers. The PBDBT:4TIC single-junction cells exhibited a high PCE of 10.60%, with a VOC of 0.92 V, a FF of 71%, and a JSC of 16.10 mA/cm2 (I−V curves shown in Figure 7 and device parameters summarized in Table 3). The high VOC of PBDB-T:ITIC solar cells makes them suitable as front subcells for reducing the thermalization loss of high-energy photons in the solar spectra. As a result of a better compromise between thermalization and transmission loss, the designed tandem solar cell exhibited a high PCE of 12.62%, which ranked among the highest for organic photovoltaics.

Figure 7. I−V characteristic curves of PBDB-T:ITIC and PTB7Th:4TIC single-junction cells and tandem solar cells. The inset shows the device architecture of a tandem solar cell.

Table 3. Photovoltaic Parameters of OSCs Based on PBDBT:4TIC and PTB7-Th:ITIC device

VOC (V)

JSC (mA/cm2)a

FF (%)

PCE (%)b

PBDB-T:ITIC PTB7-Th:4TIC tandem cell

0.92 0.78 1.65

16.10 18.75 10.62

0.71 0.72 0.71

10.60 10.43 12.62

a

Calculated integrated current density from EQE spectra. bAverage PCE values obtained from 10 devices are shown in parentheses.

III. CONCLUSION In summary, a novel fused-ring electron acceptor 4TIC has been rationally designed and synthesized by using a thiophenethieno[3,2-b]thiophene-thiophene (4T) core as the central donor unit. A systematic comparison between 4TIC and the benchmarked ITIC revealed that the utilization of the 4T core in 4TIC endows the acceptor with a significantly narrowed band gap (1.41 eV), better molecular packing, and a higher SCLC charge mobility (6.7 × 10−4 cm2 V−1 s−1). The thin film absorption band edge of 4TIC extends to ∼900 nm, which is ∼100 nm red-shifted compared to that of ITIC. The higher electron mobility of 4TIC can be ascribed to the better structural order of 4TIC in BHJ blends and a proper phaseseparated morphology. These distinctive features enable the device based on PTB7-Th:4TIC to reach a very high PCE of 10.43% with a high JSC of 18.75 mA/cm2 and a high FF of 0.72, which surpass those of PTB7-Th:ITIC-based devices. The higher JSC and FF of PTB7-Th:4TIC are quite consistent with the extended absorption and superior charge mobility of 4TIC. The non-ideal photon energy loss of PTB7-Th:4TIC is quite small (∼0.33 eV). As a demonstration, integrating the PTB7Th:4TIC system into a tandem architecture further improved the device performance to 12.62% due to the reduced transmission loss, which is among the highest of the OPVs. Moreover, the results from transient absorption studies showed distinctive charge transfer and efficient polaron generation originating from the molecular structure difference between these two NFAs, which further validates the advantage of the 4T core in facilitating easier charge generation and reduced photon energy loss. 8374

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02853. Synthetic procedures and characterization data for all new compounds, general experimental method, additional spectroscopic data, OPV fabrication, and measurement device (PDF) Crystallographic data (CIF) checkCIF/PLATON report (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xueliang Shi: 0000-0002-4577-2288 Author Contributions

X.S., L.Z., and S.B.J. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.-Y.J. acknowledges the financial support from the Asian Office of Aerospace R&D (FA2386-15-1-4106) and the Office of Naval Research (N00014-17-1-2201). F. Liu was supported by the Young 1000 Talents Global Recruitment Program of China. We thank Dr. Werner Kaminsky for the crystallographic analysis, Ting Zhao for the atomic force microscopy study, and Dr. Tullimilli Y. Gopalakrishna for the DFT calculations. Portions of this research were performed at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, Lawrence Berkeley National Laboratory, which was supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences.

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