Electronic, optical, and charge transport properties of

G Model CCLET 4561 No. of Pages 6

Chinese Chemical Letters xxx (2018) xxx–xxx

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Electronic, optical, and charge transport properties of A-p-A electron acceptors for organic solar cells: Impact of anti-aromatic p structures Yan Zenga,b , Ruihong Duanb , Yuan Guob , Guangchao Hanb , Qingxu Lia,* , Yuanping Yib,* a

School of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, China CAS Key Laboratory of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China b

A R T I C L E I N F O

Article history: Received 13 April 2018 Received in revised form 10 May 2018 Accepted 15 May 2018 Available online xxx Keywords: A-p-A electron acceptor Anti-aromatic structure Strong absorption Reorganization energy Organic solar cells

A B S T R A C T

Organic solar cells based on acceptor-p-acceptor (A-p-A) electron acceptors have attracted intensive attention due to their increasing and record power conversion efficiencies. To date, almost all of the reported A-p-A electron acceptors are based on aromatic p structures. Here, we have investigated the impact of anti-aromatization of the p-bridges on the optoelectronic properties of A-p-A electron acceptors by (time-dependent) density functional theory. Our calculations show that besides the frontier molecular orbitals corresponding to the aromatic p-bridge based acceptors (“aromatic” acceptors), additional and unique occupied and unoccupied frontier orbitals are found for the acceptors based on the anti-aromatic p-bridges (“anti-aromatic” acceptors). Moreover, by tuning isomeric structures of the p-bridges (e.g., fusion orientations or linking positions of thiophene moieties), the optical excitation energies for the transition between the additional occupied and unoccupied levels turn to be close to or substantially lower with respect to those for the transition between the “aromatic” frontier orbitals. The optical absorption of the “anti-aromatic” acceptors is thus either stronger or broader than the “aromatic” acceptors. Finally, the reorganization energies for electron transport are tunable and dependent on the p-bridge structures. These results indicate a great potential of “anti-aromatic” electron acceptors in organic photovoltaics. © 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.

Organic solar cells (OSCs) are regarded as a potential photovoltaic technology to convert sunlight into electricity due to their advantages of flexibility, light weight, large-area capability, and easy fabrication [1–6]. Typically, the active layer of an organic solar cell is a bulk- or bilayer-heterojunction consisting of two components, an electron-donating and an electron-accepting material [7–12]. Owing to the design of new active materials and optimization of processing conditions, great advances have achieved in the power conversion efficiencies (PCEs) of OSCs [13– 29]. Over the past twenty years, fullerene derivatives (e.g., PCBM and ICBA) were dominantly used as electron acceptors due to their superior electron affinity and high electron mobility [30–34]. However, these acceptor materials have some intrinsic limitations, including weak optical absorption, untunable energy levels, and high costs [35,36].

* Corresponding authors. E-mail addresses: [email protected] (Q. Li), [email protected] (Y. Yi).

In recent years, non-fullerene small-molecule acceptors have attracted increasing attention due to their strong and broad absorption and highly tunable electronic energy levels [35,37–39]. A large number of electron acceptors were developed on the base of fused or non-fused aromatics. Most strikingly, the indacenodithiophene (IDT)-based A-p-A electron acceptors, such as ITIC and IEIC [40,41], have achieved remarkable breakthrough in organic photovoltaics (OPVs). To date, the most efficient binary non-fullerene OPV devices using IEIC and ITIC as electron acceptor have gained PCEs up to 10% and 13.1% [29,42], respectively. The PCEs can be further improved by fabricating ternary or tandem OPV devices [42–45]. Relative to aromatic systems, anti-aromatics are characteristic of smaller energy gap and deeper electron affinity [46–53], which is beneficial for broadening optical absorption and increasing electron-accepting ability. For instance, quinoidal indenofluorenes (IFs) based on the s-indacene anti-aromatic framework exhibit electron affinity higher than PCBM [54–56]. What is more, IFs possess broad absorption with the absorption edge extending to the near infrared [57–60]. In addition, good and ambipolar charge

https://doi.org/10.1016/j.cclet.2018.05.029 1001-8417/ © 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: Y. Zeng, et al., Electronic, optical, and charge transport properties of A-p-A electron acceptors for organic solar cells: Impact of anti-aromatic p structures, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.05.029


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Y. Zeng et al. / Chinese Chemical Letters xxx (2018) xxx–xxx

transport properties have been found in the IFs based field-effect transistors [61–63]. Despite of these desirable properties, very few work was reported on the OPVs based on “anti-aromatic” electron acceptors [64–66]. In this contribution, we have designed a series of A-p-A smallmolecule electron acceptors, which consist of anti-aromatic sindacene based p-bridges and the electron-withdrawing groups of 2-(3-oxo-2,3-dihydroinden-1-ylidene) malononitrile (INCN). In order to assess the potential of the “anti-aromatic” acceptors in OPVs, we have calculated their electronic, optical, and charge transport properties in comparison with the corresponding “aromatic” electron acceptors by (time-dependent) density functional theory (the computational details see Supporting information). The chemical structures of the anti-aromatic and aromatic fused p-units are shown in Scheme 1. All the aromatic polycyclic p-units (1-4 and 40 ) contain a core of 1,5-dihydro-s-indacene (in blue). When the core is fused with two thiophene or benzene moieties, we get units 1 (indacenodithiophene, IDT) or 2 (indacenodibenzene, IDB). Further fusion of two thiophenes onto units 1 or 2 will return units 3 (indacenodithienothiophene, IDTT) or 4/40 (indacenodibenzothiophene). Isomers 4 and 40 are different in sulfur orientations of the fused thiophenes. Units 1 (IDT) and 3 (IDTT) have been reported as the backbone components of IEIC and ITIC, respectively. Correspondingly, the anti-aromatic polycyclic p-units (1a-4a and 40 a) are designed by replacing the aromatic 1,5dihydro-s-indacene core with the anti-aromatic s-indacene (in red), as shown in Scheme 1. The bond lengths of the optimized geometries for the p-units are shown in Fig. S1 (Supporting information). The bond length alternations are significant in the sindacene core. Overall, the C C bonds of benzene are longer in the anti-aromatic indacene than in the aromatic indacene; in contrast, the C C bonds of cyclopentene are much shorter in the antiaromatic indacene than the aromatic indacene (Fig. S2 in Supporting information). The calculated frontier orbital energy diagram and pictorial representation of these p-units are displayed in Fig. 1. The HOMOs (centrosymmetric, g) and LUMOs (anti-centrosymmetric, u) of all the aromatic p-units are delocalized over the whole backbones while the LUMO of 40 shows relatively weak distribution on the lateral atoms of the fused thiophenes. When replacing thiophene with benzene fused onto the aromatic indencene core, the HOMO level will descend appreciably by 0.31 eV but the LUMO level will ascend slightly by 0.11 eV, leading to an increment of 0.42 eV in the LUMO-HOMO energy gap (Egap) from 1 to 2. Similar energy changes are found from 3 to 4, but more apparent from 3 to 40 ; particularly, the increase of the LUMO energy from 4 to 40 even reaches 0.24 eV due to the relatively weakened delocalization. As expected, extension of the p conjugation from 1 (2) to 3 (4/40 ) results in lower LUMO and higher HOMO levels and smaller gaps. Interestingly, the anti-aromatic p-units 1a-4a and 40 a have two pairs of important frontier molecular orbitals. One pair of the frontier orbitals correspond to the HOMO and LUMO of the

aromatic units 1-4 and 40 , which we name as “aromatic” orbitals. Compared with 1-4 and 40 , the “aromatic” unoccupied orbital energies are significantly up-shifted while the “aromatic” occupied orbital energies are hardly changed for 1a-4a and 40 a. The other pair of frontier orbitals of 1a-4a and 40 a originate from the antiaromatic s-indacene, which are named as “anti-aromatic” orbitals. These “anti-aromatic” orbitals display a quinoidal p-conjugation character dictated by the significant bond length alternation in the s-indacene core. The “anti-aromatic” unoccupied orbital levels (centrosymmetric, g) are lower than the “aromatic” unoccupied orbitals by ca. 1.3–2.0 eV. As a result, the LUMO energies are substantially (at least 1.2 eV) decreased for the anti-aromatic units relative to the aromatic units, indicating that introducing antiaromatic structures can enhance electron-accepting abilities of the p-units. In contrast, the energy differences are much smaller between the “anti-aromatic” and “aromatic” occupied orbitals, ca. 0.6–0.7 eV for 2a, 4a and 40 a and even only 0.07 eV and 0.16 eV for 1a and 3a, respectively. Both the occupied and unoccupied “antiaromatic” orbitals are mainly localized on the s-indacene core for 1a and 3a, but more extended for 2a, 4a and 40 a. Also, the electron distributions of the “anti-aromatic” orbitals can be modulated by the fused thiophene orientations; higher electron densities are found on the peripheral atoms of the fused thiophenes for 40 a relative to 4a. The UV–vis absorption spectra of all the p-units are shown in Fig. S3 (Supporting information), and the electronic transition properties of the lowest excited singlet states (S1) are summarized in Table S1 (Supporting information). For the aromatic p-units, only a single absorption peak appears in the UV region, corresponding to the S1 excitation dominated by the HOMO ! LUMO transition. Consistent with the ordering of Egap, the wavelength at the maximum absorption is red-shifted as follows 2 < 1 and 40 < 4 < 3. For the anti-aromatic p-units, the absorption from the transition between the “aromatic” occupied and unoccupied frontier orbitals is significantly blue-shifted and much stronger due to larger energy gap with respect to the aromatic p-units. Besides, an additional weaker absorption appears in the visible region owing to the lower-energy transition between the “anti-aromatic” occupied and unoccupied frontier orbitals. It should be noticed that the S1 excitation for the anti-aromatic units is dominated by the transition between the “aromatic” occupied orbital and “anti-aromatic” unoccupied orbital and is symmetry forbidden. To summarize, both isomerization and antiaromatization have important impact on the electronic and optical properties of the p-units. The chemical structures of all the A-p-A acceptor molecules are shown in Scheme 2. For the acceptors containing the short fusedring units (A1/A1a, A2/A2a, and A20 /A20 a), the p-units are extended by linking two additional thiophene moieties. The structure difference between A2 and A20 or A2a and A20 a is the linking positions on the fused-ring units. Most of those molecules show a completely flat backbone (Figs. S4 and S5 in Supporting information). In the case of A2/A20 and A2a/A20 a, the fused-ring

Scheme 1. Chemical structures of the aromatic fused p-units (top) and corresponding anti-aromatic fused p-units (bottom).




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Fig. 1. Energy diagram and pictorial representation for the frontier orbitals of the aromatic 1-4/40 and anti-aromatic 1a-4a/40 a p-units.

core and the thiophene moieties exhibit a twisted angle of 22.4 / 26.6 for the aromatic molecules and 22.7 /21.3 for the antiaromatic molecules. Fig. 2 and Fig. S6 (Supporting information) show the frontier molecular orbitals and corresponding energies for the acceptors based on the short and long fused units, respectively. As seen from Fig. S7 (Supporting information), the HOMO/LUMO energies of INCN (9.00 eV/1.65 eV) are substantially lower than the “aromatic” frontier levels of the p-units, especially for the HOMO. As a result, the HOMO (g-symmetry) of the “aromatic” acceptors is determined by the p-bridges. Consistent with the trends in the p-units, the HOMO levels of the acceptors are successively decreased as follows: A1 > A2 > A20 and A3 > A4 > A40 (Fig. 3). In contrast, the LUMO (u-symmetry) and LUMO + 1 (g-symmetry) of the “aromatic” acceptors are dominated by the two terminal INCN units. Compared with A1 (IEIC) or A3 (ITIC), the LUMO electron densities are more localized on the INCN groups for A2/A20 or A4/ A40 , in particular for A20 and A40 . Because of the reduced electronic

delocalization, the LUMO level is gradually up-shifted by nearly 0.3 eV (A1 < A2 < A20 and A3 < A4 < A40 ). Relatively, the LUMO + 1 electron densities and energy levels are slightly changed. Thus, the LUMO and LUMO + 1 energies are almost degenerate for A20 and A40 . The increased LUMO levels are beneficial for achieving high open circuit voltages. Compared to the “aromatic” acceptors, the “anti-aromatic” acceptors have one more anti-centrosymmetric occupied frontier orbital and one more centrosymmetric unoccupied frontier orbital (Figs. 2 and S6), which are brought about by the anti-aromaticity of the s-indacene core. The “aromatic” occupied frontier orbitals of the “anti-aromatic” acceptors are similar to the HOMOs of the “aromatic” acceptors; except A4a, the orbital energies are slightly higher (Fig. 3). The “anti-aromatic” occupied frontier orbitals are localized on the fused-ring cores for A1a/A2a and A3a/A4a while extended to the INCN groups for A20 a and A40 a due to effective couplings of the fused-ring units with the rest moieties caused by the large electron densities on the linking atoms; the energy

Scheme 2. Chemical structures of the “aromatic” and “anti-aromatic” acceptors.



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Fig. 2. Frontier orbital energies and pictorial representation for the A-p-A acceptors based on (a) the aromatic (A1, A2 and A20 ) and (b) the anti-aromatic s-indacene cores (A1a, A2a, and A20 a).

change trend appears to be opposite to the “aromatic” occupied frontier orbitals. Consequently, the HOMO is “aromatic” for A1a and A3a but “anti-aromatic” for A2a/A20 a and A4a/A40 a. The anticentrosymmetric unoccupied frontier orbitals are similar to those of the “aromatic” acceptors while their energies are a bit lower. In the case of the centrosymmetric unoccupied frontier orbitals, the “anti-aromatic” orbitals are concentrated on the s-indacene core and isolated from the INCN-dominant ones for A1a and A3a; on the contrary, the “anti-aromatic” and INCN-dominant components are hybridized for A2a/A20 a and A4a/A40 a. Especially for A20 a and A40 a, large energy splitting (>0.6 eV) is found between these two centrosymmetric unoccupied orbitals due to strong electronic interaction; hence the LUMO becomes to be of g-symmetry. The remarkable changes in the frontier molecular levels are expected

Fig. 3. Frontier orbital energies of the A-p-A acceptors based on (a) the aromatic and (b) the anti-aromatic s-indacene cores (red: u-symmetry, black: g-symmetry).

to have profound influence on the optical absorption properties of the electron acceptors. The UV–vis absorption spectra of all the A-p-A acceptors are shown in Fig. 4 and corresponding excitation properties are summarized in Table 1. All the acceptors have similar and moderate absorption in the UV range of 250–350 nm, which is composed of many high-energy excitations. Here we focus on the absorption spectra at the long wavelengths above 350 nm. As seen from Fig. 4a, the “aromatic” acceptors A1, A2, and A20 exhibit a single absorption peak, which is attributed to the S1 excitation. From A1 to A2 and A20 , the contribution to the S1 excitation is decreased for the HOMO ! LUMO transition while increased for the other higher-energy transitions (Table 1); along with the enlarged energy gap (Fig. 3), the absorption peak is then obviously blueshifted with decreased intensity. Similar trend in the absorption is found from A3 to A4 and A40 . We noticed that the absorption of A40 arises from both S1 and S3 excitations, which have close energies and consist of the same main transitions with different percentages. Because of additional “anti-aromatic” frontier molecular levels, more excitations can contribute to the absorption spectra of the “anti-aromatic” acceptors. For A1a, A2a, A3a, and A4a, the optically allowed transitions have similar energy gaps between the occupied and unoccupied orbitals (Fig. 3), so the corresponding excitation energies show small variation and are located near the S1 excitation of the “aromatic” acceptors. Hence the profiles of the absorption spectra for these “anti-aromatic” acceptors are similar to the related “aromatic” acceptors. However, owing to the




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Fig. 4. UV–vis absorption spectra and oscillator strengths of the excited states for the A-p-A acceptors.

additional contributions of the excitations from the “antiaromatic” levels, the absorption intensities are enhanced. Interestingly, for A20 a and A40 a, the energy gap is much smaller for the “anti-aromatic” HOMO!LUMO transition with respect to the other optically allowed transitions, arising from strong electronic coupling between the fused-ring units and the rest moieties for the “anti-aromatic” frontier orbitals. Consequently, besides the absorption at the wavelengths similar to A20 and A40 , an extra absorption appears at much longer wavelengths; thus A20 a and A40 a are hopeful to be a panchromatic sunlight absorber. For organic solar cells, high charge carrier mobility will facilitate charge separation to improve short-circuit current, fill factor, and hence power conversion efficiency. Reorganization energy is one of

the key parameters to determine charge transport performance in organic semiconductors; small reorganization energy is beneficial for achieving high mobility. Here, we are interested in the reorganization energies for electron transport in the A-p-A acceptors. The calculated results are shown in Fig. 5. The reorganization energies of A1 (IEIC) and A3 (ITIC) are ca. 0.14 eV and 0.16 eV; these values are similar to those of fullerenes and twice smaller than those of perylenediimides [67]. When the fused thiophene on s-indacene is replaced by benzene, the reorganization energies are decreased for A2/A20 and A4/A40 . Moreover, the decrease depends on the linking positions or fusion orientations of thiophene onto indacenodibenzene, and the A20 and A40 acceptors exhibit the smallest reorganization energies of