An Updated Strategy for Designing Non-Fullerene Acceptors by the

Subscriber access provided by IDAHO STATE UNIV

C: Energy Conversion and Storage; Energy and Charge Transport

An Updated Strategy for Designing Non-Fullerene Acceptors by the Lowest Singlet and Triplet States Excitation: Influence of Periodical Substitution from O, S, Se to Te for BAE Derivatives Ming-Yue Sui, Ming-Yang Li, Guang-Yan Sun, and Zhong-Min Su J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00498 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

An Updated Strategy for Designing Non-Fullerene Acceptors by the Lowest Singlet and Triplet States Excitation: Influence of Periodical Substitution from O, S, Se to Te for BAE Derivatives

Ming-Yue Sui,a Ming-Yang Li,a Guang-Yan Sun,a,b*, and Zhong-Min Sua,c*

a

Department of Chemistry, Faculty of Science, Yanbian University, Yanji, Jilin,

133002, China. *E-mail: [email protected] b

Faculty of chemical engineering and new energy materials, Zhuhai College of Jilin

University, Zhuhai, Guangdong, 519041, China. *E-mail: [email protected] c

Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal

University, Changchun, Jilin, 130024, China. *E-mail: [email protected]

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Trying to maintain original advantageous properties of molecule without increasing the difficulty of synthesis and processing, simultaneously, adding more predictable properties is a preferred choice for designing promising molecules. Here, we report the photovoltaic performance of bistricyclic aromatic enes (BAE) derivative acceptors as a function of heteroatom substitutions (O, S, Se, Te), and the range they can maintain. Beyond that, combining single-triplet exciton conversion with singlet fission (SF), an additional perspective to characterize molecules performance is predicted by employing T1-excitation. Theoretical calculation results showed that for small molecules (BAEs), heavy atoms substitution of the same group could increase conformation stability, electron acceptability and spin-orbit coupling. After expanding molecular size (DPP-BAE-DPP), the difference in molecular properties is mainly due to conformation type for singlet states excitation. Excitedly, for triplet states excitation, the degree of negative correlation between SF and single-triplet exciton conversion deceases, which is conducive to obtaining more T1 excitons, thus improves organic photovoltaic performances. As a result, A-S-DPP, A-Te-DPP, T-S-DPP, T-Se-DPP and T-Te-DPP not only have superior single-excitation performance but also potential triple-excitation possibilities, they are promising acceptors. These results provided some new evidences for designing non-fullerene acceptors and demonstrating the role of heavy atoms substitution in photovoltaic performance.

2


Page 2 of 34

Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction Maximizing organic photovoltaic (OPV) bulk-heterojunction (BHJ) device requires high open circuit voltage (VOC) and short circuit current (JSC) simultaneously according to the energy conversion efficiency (PCE) formula, PCE=VOC·JSC·FF / Pin. However, this situation is rare because they have a certain reverse correlation. Some researchers have shown the application of non-fullerene (NF) materials with low-cost building blocks, adjustable energy levels, and broad absorptions could reduce this interdependency as much as possible rather than fullerene system, its PCE has exceeded 13%.1-4 Face to higher requirements for material applications, researchers confront an unprecedented challenge to further develop new and excellent NF materials through effective strategies and methods.5-7 For instance, constructing push-pull

and

star

structures

increases

intramolecular

and

3D

transport,

respectively;8-10 extending conjugation to dimerization or trimerization reduces energy gap (Eg) and redshift spectrum;11,12 adding π bridge or ring-fusion obtains a twisted structure

to

hinders

self-aggregation;13,14

introducing

heteroatoms

and

electron-deficient/rich groups adjusts energy level and many more.15,16 Among them, heteroatom substitution is an unexceptionable strategy to minimal and/or more predictable changes geometry and performance of original molecule without increasing the cost of synthesis and processing difficulty. Recently, heavy atom substitutions such as selenium (Se) and tellurium (Te) have brought new opportunities and challenges in molecular properties. In general, 3



heavier atoms exhibit metalloid nature. Its large spin-orbit coupling (SOC) would lead to long-lived and long diffusion lengths triplet excited states through intersystem crossing, which is expected to facilitate charge separation. Strong interactions between heavy atoms could enhance interchain electronic coupling, which is conducive to morphology of active layer and charge transport. Therefore, some heavy atom-containing materials have been developed in OPV materials and have attention, especially group 16 heterocycles (thiophene, selenophene, tellurophene). 17-20 Grubbs group reported benzochalcogenophene-diketopyrrolopyrrole by replacing sulfur (S) to Se and Te, proving heavy-atom substitution is a viable approach to improve OPV performance.21 In the same year, they synthesized a tellurophene-containing polymer PDPPTe2T, compared with corresponding thiophene analog, which has longer wavelengths and smaller bandgap due to increased electronic coupling, the PCE of PDPPTe2T:PC71BM based BHJ devices reached 4.40 %.22 A series of diketopyrrolopyrrole-based copolymers with different chalcogenophene comonomers (thiophene, selenophene, and tellurophene) were synthesized by Ashraf and co-workers, the results indicated that bandgap was reduced and intermolecular heteroatom−heteroatom interactions were increased as group 16 enlarged, affording an excellent efficiency of 7.10−8.80% matching with fullerene derivatives acceptors.23 This strategy using heavy atoms is also applied to NF acceptors to improve photovoltaic performance of materials. Recently, Huang group reported three tellurophene-PDI based NF acceptors, demonstrating the existence and significance of 4


Page 4 of 34

Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


triplet exciton with long lifetime and diffusion length.24 It opens up a new perspective on the design of NF acceptors. However, almost all of studies for group 16 atoms substitution build on thiophene, selenophene and tellurophene, rather than on heteroatoms themselves, and most are donors. There is no detailed investigation with influence of oxygen (O), S, Se, Te (continuous periodic increase) atoms substitution on molecular properties and effective ranges for NF acceptors. Herein, combining the advantages of heteroatom substitution strategy and heavy atomic effects, the impact of atomic periodic changes (not in five-member ring) on molecular properties and influences range were explored in detail. We selected bistricyclic aromatic enes (BAE) derivatives owning multi-conformations changes (more microscopic considerations) as research objects to satisfy above purpose of inquiry. Some BAE derivatives (named A(T)-X) were designed using twisted (T) and anti-folded (A) as initial conformation, group 16 atom substitution (O, S, Se, Te) in X position concurrently (Fig. 1(a)). Besides, designing expanded size DPP-BAE-DPP type molecules (where the diketopyrrolopyrrole (DPP) unit is symmetrically connected to the periphery of BAE) explores the influence range of X substitution, named A(T)-X-DPP (Fig. 1(b)). Then continue to enlarge complexity of research system, build the donor/acceptor interface models (Fig. 1(c)) and characterize their performance. Research route is organized as follows: BAEs → DPP-BAE-DPPs → donor/acceptor models, from various conformations of single molecule to intermolecular charge separation and recombination. This work reveals the influence 5



rule of heavy atom substitution in the same group on molecular properties and the range can be affected. It provides more strategic options and theoretical guidance for NF molecules with high-performance and strong applications.

Figure 1. Molecule structures of BAE derivatives, named A(T)-X (a); DPP-BAE-DPP derivatives, named A(T)-X-DPP (b); and the starting interface geometry of the donor/A-O-DPP complex (c). Samples with different initial conformations (twisted (T) and anti-folded (A)), heteroatoms (O, S, Se, Te) at X positions and DPP connection locations are labeled systematically according to the format of A(T)-X-DPP.

2. Computational Methods All BAEs and DPP-BAE-DPPs were optimized by density functional theory (DFT) method at B3LYP/6-31G(d) level. The B3LYP method has been proved to be a good evaluation of geometric and electronic structure for BAE derivatives.25,26 The absorption spectra and excitation energies of all molecules were calculated at B3LYP/6-31G(d) level using time-dependent density functional theory (TD-DFT) method based on optimized ground state geometry.27 Above calculations were carried out in a vacuum. Moreover, B3LYP functional was used to calculate SOC constants 6


Page 6 of 34

Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of all molecules by ADF package.28,29 The natural transition orbital (NTO), electron-hole distributions and interfragment charge transfer (IFCT) were described by Multiwfn 3.6.30 For donor/acceptor interface models, the Marcus semi-empirical formula (calculation details were listed in Supporting Information) was used to investigate its optoelectronic properties and related parameters.31-33 P3HT derivative (P3HT-OCH3) was employed as the donor (D) for OPVs.26 The interface model with a face-on stacking34 and an initial separation distance of 3.5 Å35-37 between donor and acceptor have been widely used in the stacking. All interface models were optimized using CAM-B3LYP-D3/6-31G(d) method, where B3LYP-D3 is a functional that considers dispersion correction to describe the weak intermolecular interaction.38 Correlation excitation energy was calculated by the TD-DFT method at CAM-B3LYP/6-31G(d) level. CAM-B3LYP (long-rang correction)39,40 is more suitable for characterizing charge transfer excitation between donor and acceptor. Te atoms were represented by the LANL2DZ effective core potential and its corresponding basis set in the paper.22,41 All calculations mentioned above were performed in the Gaussian 09 software package.42

3. Results and Discussions 3.1 Effect of X atom on BAEs 3.1.1 Conformations The stability and rationality of each configuration were explored according to the multi-conformations features of BAEs. First, BAE molecules and transition states 7



(TS-X) were optimized to plot potential energy diagram (Fig. 2). TS-X states were correct by intrinsic reaction coordinates (IRC) verification (Fig. S1). As seen from Fig. 2, there are three conformation types: T (T-O), syn-folded (T-S, T-Se, T-Te) and A (A-Xs). Both syn-folded and A belong to the folded configuration. A-Xs with lower energy are more stable than T-Xs, which may be the reason why A-O has been discovered at experiment. T-O→TS-O→A-O (50.09 kJ/mol) and A-O→TS-O→T-O (72.81 kJ/mol) have the smallest energy barriers that are greater than 40 kJ/mol, indicating all conformations are stabilized. Replacing O with S atom, the conformation of T-S changes completely compared with T-O, from T to syn-folded, and the energy barrier is rapidly increased by more than two times, near or greater than 100 kJ/mol (It is necessary to convert conformation through some certain conditions). Continuing to increase X size to Se and Te, conformation type does not change, while energy barriers continue to increase, thereby making each conformation more stable. Therefore, for multi-conformation molecules, replacing with large-sized atoms in the same group may be a wonderful strategy to increase conformation stabilization, but it should be noticed whether a conformational change occurs.

8


Page 8 of 34

Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Schematic diagram of potential energy surfaces describing A-Xs, TS-Xs and T-Xs, calculated energy barriers and related optimization geometries. (kJ/mol, with the energy of the A-Xs taken as zero point).

3.1.2 Frontier molecular orbitals Frontier molecular orbital (FMO) energy level is the most primitive factor for the performance of OPV, which could affect absorption, charge separation ability and VOC.43,44 For smaller structure change of heteroatom-substituted, the electron cloud distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) could help to highlight the role of X position and its variation. In general, the more delocalized LUMO can be used to determine the functional position of the electronic excitation and electron transport by hopping for acceptors. Therefore, FMO energy level was analyzed and listed in Fig. 3 and Table S1. For A-X, the distributions at LUMO level are gradually reduced (small difference) on X position from O to Te, the distribution upon X is increasing and other parts (especially C9=C9') is gradually decreasing at HOMO level, indicating a more obvious electron transfer from X to other parts (especially C9=C9'). T-O has similar electron cloud distribution at HOMO and LUMO. From T-S to T-Te, HOMOs are distributed in parts other than C9=C9', mainly concentrated in X position and gradually increasing; LUMOs are distributed in areas other than X and C9=C9', mainly focusing on two carbons (C8a, C9a, C8a', C9a') connected to C9/C9', showing significant intramolecular charge transfer from X to C8a/C8a' and C9a/C9a'. These results manifest that conformational change between T-O and other molecules is the 9



main reason for electronic distribution difference. Hereinto, T-O has a unique conformation may be due to smaller difference between O and C atom (atomic radius and performance) weakens O influence as a whole, thus facilitates delocalization to form a T conformation. As a result, O substitution has significant differences in properties for organic molecules with higher carbon content compared to S/Se/Te substitution, so more detailed factors needs to be considered. The FMO energy level and Eg of BAEs have no change regularly. Note that X atom was located in a six-membered ring. If atomic replacement does not occur in the five-membered ring, then corresponding rule (tellurophene and selenophene replacement thiophene can reduce Eg22,23 and so on) would not be a general one. To characterize the ability of accept electrons for BAEs,45,46 LUMO energy levels for all molecules were listed in Fig. 3. When X atomic number increases, the quasi degenerate LUMO energy levels concentrate, thereby enhancing electron acceptability of molecules.

Figure 3. Calculated FMOs and the quasi degenerate LUMO energy levels of BAEs at B3LYP/6-31G(d) level.

3.1.3 Absorption spectra

10


Page 10 of 34

Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Whether it is donor excitation or acceptor excitation,47 a wider absorption range is advantageous to capture more photons, thereby increasing the JSC.48 Therefore, we studied the photoexcitation properties of BAE molecules to understand the physical processes of photocurrent generation. Simulated absorption spectra, corresponding absorption peaks, oscillator strengths and dominant excitation characters were listed in Fig. 4 and Table S2 respectively. All molecules (except T-O) have distinct multimodal characteristics and the same number of absorption peaks (under the same conformation) in 200-400 nm. While T-O has additional strong absorption at 658 nm arises from S0 →S1 transition, corresponding to the transition from HOMO→LUMO, which is consistent with its narrower Eg. There is some red shift with replacement from O→Te. A significant difference in absorption is between O and S/Se/Te substitution, which is consistent with previous FMO energy level analysis. Thereinto, S→Se→Te atomic substitutions made the absorption redshift conform to the general rule, which is conducive to enhancing JSC.

Figure

4.

Simulated

absorption

spectra

of

BAE

derivatives

(A/T-O~A/T-Te)

TD-B3LYP/6-31G(d) level. The value of the full-width at half-maximum is 3000 cm-1.

11


at


Page 12 of 34

3.2 Effect of X Atom on DPP-BAE-DPP 3.2.1 Characterization DPP-BAE-DPP based on singlet state exciton According

to

above

analysis,

some

crucial

photoelectric

properties

of

DPP-BAE-DPPs were explored and confirm the influence range of X through identical calculation and characterization methods, including FMO energy level and absorption spectra. Observing FMO map (Fig. S2), the difference in electron cloud distribution of each molecule is mainly affected by conformation type of BAE central unit. When BAE unit exhibits folded conformation, HOMO and LUMO have similar distributions, mainly localized on DPP groups and partially on core unit. For T-O-DPP, HOMO and LUMO are mainly distributed on BAE unit and partially on DPP groups. All molecules have significant charge transfer characteristics. X has a slight influence on electron clouds patterns, which could be negligible on entire molecule after further expanding molecule size. Also, the energy levels, VOC, the quasi degenerate LUMO levels, absorption and related characteristics were listed Table 1, Fig. S3-S5 and Table S3, respectively. For Table 1, except T-O-DPP, the data are similar with a maximum of no more than 0.06 under the same parameters. Surprisingly, the quasi degenerate LUMO orbital calculations showed expanded molecules (DPP-BAE-DPPs) still have same quasi degenerate increase as BAEs, but the trend is gently from O to Te substitution. Apart from this, the absorption range, maximum

wavelength,

waveform,

and

even

absorption

intensity

of

T-S-DPP~A-Te-DPP are alike. In contrast, higher HOMO and lower LUMO of T-O-DPP leads to reduction in Eg and VOC. 12


Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Calculated the FMO energy levels (eV) and energy gaps Eg (eV) of DPP-BAE-DPPs, the energetic driving forces ΔEL-L/ΔEH-H (eV), open circuit voltages VOC (V) for all DPP-BAE-DPPs and D at the B3LYP/6-31G(d) level in the vacuum. HOMO

LUMO

Eg

∆EL-L

∆EH-H

VOC

A-O-DPP

-4.89

-2.78

2.11

0.76

0.76

0.85

A-S-DPP

-4.93

-2.79

2.14

0.77

0.80

0.84

A-Se-DPP

-4.92

-2.81

2.11

0.79

0.79

0.82

A-Te-DPP

-4.91

-2.78

2.12

0.76

0.78

0.85

T-O-DPP

-4.45

-2.94

1.50

0.92

0.32

0.69

T-S-DPP

-4.92

-2.77

2.14

0.75

0.79

0.86

T-Se-DPP

-4.90

-2.76

2.14

0.74

0.77

0.87

T-Te-DPP

-4.90

-2.75

2.15

0.73

0.77

0.88

In order to further clarify the properties of excitons in molecules, electron and hole distributions49,50 before and after excited state, charge transfer amount by IFCT method51 and relevant parameters at S0→S1 were listed in Fig. 5 and Table 2, respectively. As can be seen from Table 2, D indexs (the total magnitude of charge transfer length) are small (0.1~0.256 Å), which is less than 1/4 of C-C bond. All Sr indexs (characterizing overlapping extent of hole and electron) are over 0.85, which corresponds to more than 85% of electrons and holes have perfectly overlap. The t indexs (measuring separation degree of hole and electron in charge transfer direction) are small (< 0), indicating electrons and holes having no significant separation. They displayed typical localized excitation (LE) feature. For Fig. 5, except for T-O-DPP, the contributions of holes, electrons and their overlaps exceed 40% on DPP units (localized charge transfer amount ~0.20 electrons), while the net charge transfer amount is 0, revealing typical LE of DPP. Fragment 2 as a bridge connecting DPP 13



units could obtain electrons from other fragments because electron contributions are greater than holes. As a result, BAE units (folded conformation) could break conjugation between two bistricyclics and highlight the role of DPP units in DPP-BAE-DPPs, showing mainly LE of DPP and partial charge transfer excitation from others to fragment 2. Plotted atom contribution of T-O-DPP makes analysis more clearly (Table S4 and Fig. S6), holes are provided by fragments 2, 3, and 5; electrons are provided by fragments 1, 2, 5, and 6. The overlaps of fragment 2 and 5 are relatively large, which is LE in BAE unit. In the BAE unit, electron transfer excitation mainly comes from C27, C32, C41, C46, X to C9=C9' (Fig. S6). In addition, charge density difference (diff) of BAE is less than 0 showing electron donating; the diff of DPP is greater than 0 showing electron accepting. It is also accompanied by a slight charge transfer excitation from BAE to DPP. Therefore, T-O-DPP weakens DPP effects due to increased BAE conjugation through good aromatic bridging property of C9=C9'. Above performance difference is caused by conformation type of BAE unit, T vs. folded. These conclusions also can be verified by the Hirshfeld method analyzing contribution of each target fragments in the HOMO and LUMO for BAEs and DPP-BAE-DPPs (Fig. S7). Particularly, based on FMO compositions and electron-hole distributions for atoms (Fig. S8-S9), confirming which X is more important in linking fragment and a non-linking fragment? For the HOMO and LUMO, the difference between two Xs was observed (not completely overlap), but mainly affected by the conformation type. 14


Page 14 of 34

Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


That is, for folded conformations, the two Xs are similar substantially (slightly different); while for T conformation, the X of linking fragment is more important. Before and after molecular excitation, BAE plays a major role in T-O-DPP, so that the effects of two Xs are not negligible, but their difference is not significant; in folded conformations, the two X effects are smaller, basically no difference. Above analysis is consistent with the conclusion that T conformation makes BAE unit play a major role, thereby amplifying X effects; in folded conformations, DPP groups are dominant.

Table 2. Computed the centroid distance D (Å), overlapping extent Sr index and separation degree t of hole and electron for DPP-BAE-DPPs. A-O-DPP A-S-DPP A-Se-DPP A-Te-DPP T-O-DPP T-S-DPP T-Se-DPP T-Te-DPP

D

Sr

t

0.102 0.118 0.256 0.188 0.104 0.234 0.259 0.227

0.856 0.856 0.859 0.858 0.869 0.857 0.857 0.858

-1.670 -3.628 -2.374 -2.340 -2.744 -1.675 -2.264 -1.605

15



Figure 5. Heat maps associated with the S1 for composition of fragment in hole and electron, as well as hole-electron overlap in various fragment spaces. Corresponding charge transfer amount and net electron transfer also presented in above panel.

3.2.2 D/acceptor interface models Further expand complexity of the system and build D/acceptor interface models. When photocurrent is generated, there are two main mechanisms on the interface model, i.e., exciton separation (kCS) and electron-hole recombination (kCR), which compete with each other to affect the JSC.52,53 High performance devices require the 16


Page 16 of 34

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


highest possible kCS and lowest possible kCR.54 First, valid charge transfer states were identified by the NTO method, that is, a significant charge transfer trajectory from D to acceptor in a pair of NTO,55,56 as depicted in Fig. 6. According to Marcus semi-empirical formula, the rates kCS, kCR, K (kCS/kCR) and associated parameters for all systems were listed in Table 3 and Table S5. The molecules with folded conformation have similar kCS, kCR, and K (1013~1015), and syn-folded exhibits a slight advantage over A. For exception D/T-O-DPP (T), which kCS is similar to that of other systems, but exploded kCR (9.49×1012) results in smallest K value of only 1.16×102. It could lead to a decrease in JSC. The difference in rate K is mainly due to conformation type, which is consistent with properties difference in DPP-BAE-DPPs. Fortunately, we have found that its conformation can be adjusted by replacing heavy atoms in the same group to manage corresponding photovoltaic performance. It is worth noting that the above results are based on single-excitation conditions. According to the special nature of heavy atoms, it is expected that molecules could also produce favorable triplet excitons. Therefore, the formation of triplet excitons in molecules and its influence on photovoltaic performance require systematically characterization and analysis.

17



Page 18 of 34

Figure 6. Charge transfer excited states of the D/acceptor complexs calculated at the B3LYP/6-31G(d) level. The hole and electron wave functions of each excited state come from a NTO analysis.

Table 3. Computed rates of charge recombination kCR (s-1), charge separation kCS (s-1) and ratio K (kCS/kCR) of D/acceptors at the B3LYP/6-31G(d) level. kCS

kCR 14

A-O-DPP

1.41×10

A-S-DPP

4.35×10

14

K 2

1.74×10

3

1.36×10

kCS 11

T-O-DPP

2.63×10

11

T-S-DPP

1.78×10

kCR

8.12×10 3.21×10

15 14

6.38×10

K 12

2.27×10

1

A-Se-DPP 1.50×1014 8.94×103 1.68×1010 T-Se-DPP 8.57×1014 1.64×102 A-Te-DPP 9.38×1014 3.88×103 2.41×1011 T-Te-DPP 2.07×1015 1.56×102

2

4.12×10

12

7.85×10

12

5.21×10

13

1.32×10

3.2.3 Triplet state excitons It is well known that heavier group 16 atoms Se and Te could have larger SOC to promote intersystem crossing into triplet excited states.41,57 Triplet excitons have longer lifetimes and diffusion lengths, which can effectively reduce recombination losses and increase photocurrent.58 There are two ways to form triplet excitons: single-triplet exciton conversion and singlet fission (SF). Single-triplet exciton conversion process requires smaller energy gap (∆EST) between S1 and T1, and larger 18


Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SOC. While SF is an inherent feature of molecule and could be simply characterized by parameter E(S1)-2E(T1)>0, which satisfies the driving force of S0+S1→T1+T1. In previous studies, 99'BF (the smallest BAE derivative) and DPP both possess SF characteristics,59-61 so designed molecules are promising herein. In addition, whether formed triplet excitons could be dissociated into free charges is an indicator to validate additional photocurrent, thereby improving PCEs of devices. Regarding to the problem, some theoretical and experimental researches support the view that the energy of generated T1 is higher than E(CT) (the energy difference between the HOMO of the donor and the acceptor LUMO), i.e. E(T1)>E(CT), they can separate into free charges, and vice versa.24,60,62 The parameters mentioned above were listed in Table 4. For BAEs, from O to Te substitution, SOC is increasing, ∆EST and E(S1)-2E(T1) are decreasing. It indicated that they are more likely to undergo intersystem crossing process and is inversely related to SF. Among them, the SOC of T-O and T-S are basically 0 (transition-forbidden). Simultaneously, only T-O (E(S1)-2E(T1)=1.96 eV) and T-S (E(S1)-2E(T1)=1.37 eV) could have SF characteristics, which are consistent with the large change of T1 structure (biradical nature) compared with S0 and S1 (Fig. S10). Based on the above DPP-BAE-DPPs analysis, the difference in performance mainly comes from conformation type after expansion size, and folded conformations are excellent. Therefore, the T1 of DPP-BAE-DPPs (except T-O) was further analyzed. All the values of E(S1)-2E(T1) are close to 0, which could occur SF compared with the corresponding BAEs; ∆EST 19



Page 20 of 34

values are similar (~0.90 eV). For SOC value, A-Se-DPP has a significant reduction, which is consistent with its developmental lesson (molecular performance could has a sharp drop).56 A/T-X-DPPs (X=S, Te) possess greater SOC values to accelerate intersystem crossing. In comparison, molecules with syn-folded conformations have a distinct advantage for SOC. Surprisingly, the degree of inverse correlation between SF and intersystem crossing is reduced, and occurrence rates are increased simultaneously, which is advantageous for obtaining more T1 excitons. The E(T1) are approximate 1.00 eV and E(CT) are about 1.30 eV, indicating generated T1 excitons cannot be separated into free carriers theoretically. However, it is not excluded that some triplet excitons could be separated due to adjacent E(T1) and E(CT). Thus, A-S-DPP, A-Te-DPP, T-S-DPP, T-Se-DPP and T-Te-DPP would have additional photocurrent contributions by triplet excitons, thereby increasing OPV performance. Of course, SF features requires more verification. This article is only a preliminary prediction, and the relevant research is continuing.

Table 4. Calculated the energy levels (eV), spin-orbit coupling SOC (cm-1), energy gap (∆EST) (eV) between S1 and T1, and charge transfer state E(CT) (eV) for BAEs and DPP-BAE-DPPs. E(S1)-2E(T1) SOC

∆EST

E(S1) E(T1) E(S1)-2E(T1) SOC ∆EST E(CT)

A-O

-0.85

0.2276 1.084

A-O-DPP

1.91

0.99

-0.08

0.0554 0.91

1.35

A-S

-1.00

0.4403 1.109

A-S-DPP

1.94

1.00

-0.05

0.2903 0.94

1.34

A-Se

-1.34

2.7592 0.898

A-Se-DPP

1.88

0.98

-0.08

0.0093 0.90

1.32

A-Te

-1.52

12.7909 0.804

A-Te-DPP 1.90

0.99

-0.07

0.0621 0.91

1.35

T-O

1.96

0.0001 2.168

T-O-DPP

─

─

─

T-S

1.37

0.0003 1.814

T-S-DPP

1.91

0.99

-0.06

0.3058 0.93

1.36

T-Se

-1.90

0.0206 0.551

T-Se-DPP

1.93

0.99

-0.05

0.1340 0.94

1.37

T-Te

-2.03

0.0272 0.353

T-Te-DPP

1.92

0.99

-0.05

0.6511 0.93

1.38

20


─

─

─

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. Conclusion In present work, from conformations to the D/acceptor interface models, we systematically characterized photovoltaic properties of molecules and influence range of the group 16 atoms by means of varying heteroatoms (O→S→Se→Te) in BAEs and DPP-BAE-DPPs. For BAEs, moving from O→Te, a series of regular increases occur as there are the increased energy barrier between conformations, the charge transfer from X to C9=C9' and the quasi-degenerate LUMO, which could enhance the stability of each conformation and acceptability of electrons. Also, increased SOC promoted single-triplet exciton conversion and reduced E(S1)-2E(T1) attenuated SF characteristics, they have strong inverse correlation. After expanding molecular size (DPP-BAE-DPPs), the change in quasi-degenerate LUMO can also be affected by X, but the degree is weakened. While other optoelectronic properties concentrated singlet excitation are mainly affected by conformational type instead of varying heteroatom. It is worth noting that if there is no X-induced conformational change previously, then there would be no performance changes in DPP-BAE-DPPs and interface models subsequently. Fortunately, A-S-DPP, A-Te-DPP, T-S-DPP, T-Se-DPP and T-Te-DPP have excellent performance and small differences for singlet excitation. Beyond that, they also have better SF and single-triplet exciton conversion, as well as smaller inverse correlation when excited by T1, there will be additional performance gains. We hope this work could provide an updated thinking and strategy for the design of NF acceptors. 21



Acknowledgement We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Project No. 21363025) and the Science and Technology Development Project Foundation of Jilin Province (20150101008JC).

Supporting Information Description Computational details of Marcus semi-empirical formula and calculated associated parameters; IRC of TS-X transition states, dihedral angles (C9a’-C9’-C9-C9a) of optimized molecular structures of BAE molecules; calculated FMO energy level, simulated absorption spectra and relevant information of BAEs and DPP-BAE-DPPs; electron-hole distributions and contribution of fragments 1-6 to holes and electrons for T-O-DPP; orbital contribution rate of all molecules for HOMO and LUMO by Hirshfeld method.

22


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References 1.

Gao, W.; Liu, T.; Ming, R. J.; Luo, Z. H.; Wu, K. L.; Zhang, L.; Xin, J. M.; Xie, D. J.; Zhang, G. Y.; Ma, W., et al. Near-Infrared Small Molecule Acceptor Enabled High-Performance Nonfullerene Polymer Solar Cells with over 13% Efficiency. Adv. Funct. Mater. 2018, 28, 1803128.

2.

Li, W. N.; Ye, L.; Li, S. S.; Yao, H. F.; Ade, H.; Hou, J. H. A High-Efficiency Organic Solar Cell Enabled by the Strong Intramolecular Electron Push-Pull Effect of the Nonfullerene Acceptor. Adv. Mater. 2018, 30, 1707170.

3.

Li, Y. X.; Zhong, L.; Wu, F. P.; Yuan, Y.; Bin, H. J.; Jiang, Z. Q.; Zhang, Z. J.; Zhang, Z. G.; Li, Y. F.; Liao, L. S. Non-Fullerene Polymer Solar Cells Based on a Selenophene-Containing Fused-Ring Acceptor with Photovoltaic Performance of 8.6%. Energy Environ. Sci. 2016, 9, 3429-3435.

4.

Liu, J.; Chen, S. S.; Qian, D. P.; Gautam, B.; Yang, G. F.; Zhao, J. B.; Bergqvist, J.; Zhang, F. L.; Ma, W.; Ade, H., et al. Fast Charge Separation in a Non-Fullerene Organic Solar Cell with a Small Driving Force. Nat. Energy 2016, 1, 16089.

5.

Lee, J.; Singh, R.; Sin, D. H.; Kim, H. G.; Song, K. C.; Cho, K. A Nonfullerene Small Molecule Acceptor with 3D Interlocking Geometry Enabling Efficient Organic Solar Cells. Adv. Mater. 2015, 28, 69-76.

23



6.

Liu, Z. T.; Wu, Y.; Zhang, Q.; Gao, X. Non-Fullerene Small Molecule Acceptors Based on Perylene Diimides. J. Mater. Chem. A 2016, 4, 17604-17622.

7.

Chen, W. Q.; Zhang, Q. C. Recent Progress in Non-Fullerene Small Molecule Acceptors in Organic Solar Cells (Oscs). J. Mater. Chem. C 2017, 5, 1275-1302.

8.

Suman; Bagui, A.; Datt, R.; Gupta, V.; Singh, S. P. A Simple Fluorene Core-Based Non-Fullerene Acceptor for High Performance Organic Solar Cells. Chem. Commun. 2017, 53, 12790-12793.

9.

Zhang, A. D.; Li, C.; Yang, F.; Zhang, J. Q.; Wang, Z. H.; Wei, Z. X.; Li, W. W. An Electron Acceptor with Porphyrin and Perylene Bisimides for Efficient Non-Fullerene Solar Cells. Angew. Chem. Int. Edit. 2017, 56, 2694-2698.

10.

Wu, M. L.; Yi, J. P.; Chen, L.; He, G. Y.; Chen, F.; Sfeir, M. Y.; Xia, J. L. Novel Star-Shaped Helical Perylene Diimide Electron Acceptors for Efficient Additive-Free Nonfullerene Organic Solar Cells. ACS Appl. Mater. Interfaces 2018, 10, 27894-27901.

11.

McAfee, S. M.; Topple, J. M.; Hill, I. G.; Welch, G. C. Key Components to the Recent Performance Increases of Solution Processed Non-Fullerene Small Molecule Acceptors. J. Mater. Chem. A 2015, 3, 16393-16408.

12.

Holliday, S.; Ashraf, R. S.; Wadsworth, A.; Baran, D.; Yousaf, S. A.; Nielsen, C. B.; Tan, C. H.; Dimitrov, S. D.; Shang, Z. R.; Gasparini, N., et al. 24


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


High-Efficiency and Air-Stable P3HT-Based Polymer Solar Cells with a New Non-Fullerene Acceptor. Nat. Commun. 2016, 7, 11585. 13.

Lin, Y. Z.; Wang, Y. F.; Wang, J. Y.; Hou, J. H.; Li, Y. F.; Zhu, D. B.; Zhan, X.

W.

A

Star-Shaped

Perylene

Diimide

Electron

Acceptor

for

High-Performance Organic Solar Cells. Adv. Mater. 2014, 26, 5137-5142. 14.

Sun, H.; Song, X.; Xie, J.; Sun, P.; Gu, P. Y.; Liu, C. M.; Chen, F.; Zhang, Q. C.; Chen, Z. K.; Huang, W. PDI Derivative through Fine-Tuning the Molecular Structure for Fullerene-Free Organic Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 29924-29931.

15.

Li, Y.; Xu, X. P.; Li, Z. J.; Yu, T.; Peng, Q. Two-Dimensional Photovoltaic Copolymers with Spatial D-A-D Structures: Synthesis, Characterization and Hetero-Atom Effect. Sci. China Chem. 2015, 58, 276-285.

16.

Suman; Gupta, V.; Bagui, A.; Singh, S. P. Molecular Engineering of Highly Efficient Small Molecule Nonfullerene Acceptor for Organic Solar Cells. Adv. Funct. Mater. 2016, 27, 1603820.

17.

Cameron, J.; Abed, M. M.; Chapman, S. J.; Findlay, N. J.; Skabara, P. J.; Horton, P. N.; Coles, S. J. Investigating the Effect of Heteroatom Substitution in 2,1,3-Benzoxadiazole and 2,1,3-Benzothiadiazole Compounds for Organic Photovoltaics. J. Mater. Chem. C 2018, 6, 3709-3714.

18.

Lee, J.; Sin, D. H.; Clement, J. A.; Kulshreshtha, C.; Kim, H. G.; Song, E.; Shin, J.; Hwang, H.; Cho, K. Medium-Bandgap Conjugated Polymers 25



Page 26 of 34

Containing Fused Dithienobenzochalcogenadiazoles: Chalcogen Atom Effects on Organic Photovoltaics. Macromolecules 2016, 49, 9358-9370. 19.

Wong, X. L.; Rahman, M. L.; Sarjadi, M. S. Benzodichalcogenophene-Based Conjugated Polymers as Photovoltaic Materials. Int. J. Electrochem. Sci. 2017, 12, 6315-6339.

20.

Kulshreshtha, C.; Son, J.; Pascher, T.; Kim, J. H.; Joo, T.; Lee, J.; Jeong, M. S.; Cho, K. Excitation Intensity Dependent Carrier Dynamics of Chalcogen Heteroatoms in Medium-Bandgap Polymer Solar Cells. Sci. Rep. 2017, 7, 836.

21.

Park, Y. S.; Kale, T. S.; Nam, C. Y.; Choi, D.; Grubbs, R. B. Effects of Heteroatom Substitution in Conjugated Heterocyclic Compounds on Photovoltaic Performance: From Sulfur to Tellurium. Chem. Commun. 2014, 50, 7964-7967.

22.

Park, Y. S.; Wu, Q.; Nam, C. Y.; Grubbs, R. B. Polymerization of Tellurophene

Derivatives

by

Microwave-Assisted

Palladium-Catalyzed

Ipso-Arylative Polymerization. Angew. Chem. Int. Ed. 2014, 53, 10691-10695. 23.

Ashraf, R. S.; Meager, I.; Nikolka, M.; Kirkus, M.; Planells, M.; Schroeder, B. C.; Holliday, S.; Hurhangee, M.; Nielsen, C. B.; Sirringhaus, H., et al. Chalcogenophene

Comonomer

Comparison

in

Small

Band

Gap

Diketopyrrolopyrrole-Based Conjugated Polymers for High-Performing Field-Effect Transistors and Organic Solar Cells. J. Am. Chem. Soc. 2015, 137, 1314-1321. 26


Page 27 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24.

Yang, L.; Gu, W. X.; Lv, L.; Chen, Y. S.; Yang, Y. F.; Ye, P.; Wu, J. F.; Hong, L.; Peng, A. D.; Huang, H. Triplet Tellurophene-Based Acceptors for Organic Solar Cells. Angew. Chem. Int. Ed. 2017, 57, 1096-1102.

25.

Fan, Y. L.; Barlow, S.; Zhang, S. Y.; Lin, B. P.; Marder, S. R. Comparison of 3d Non-Fullerene Acceptors for Organic Photovoltaics Based on Naphthalene Diimide and Perylene Diimide-Substituted 9,9'-Bifluorenylidene. RSC Adv. 2016, 6, 70493-70500.

26.

Sui, M. Y.; Geng, Y.; Sun, G. Y.; Wang, J. P. Construction of a 9,9'-Bifluorenylidene-Based Small Molecule Acceptor Materials by Screening Conformation, Steric Configuration and Repeating Unit Number: A Theoretical Design and Characterization. 2017, 5, 10343-10352.

27.

Kim, H. U.; Kim, J. H.; Suh, H.; Kwak, J.; Kim, D.; Grimsdale, A. C.; Yoon, S. C.; Hwang, D. H. High Open Circuit Voltage Organic Photovoltaic Cells Fabricated Using 9,9’-Bifluorenylidene as a Non-Fullerene Type Electron Acceptor. Chem. Commun. 2013, 49, 10950-10952.

28.

Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Towards an Order-N Dft Method. Theor. Chem. Acc. 1998, 99, 391-403.

29.

Pan, Q. Q.; Li, S. B.; Wu, Y.; Sun, G. Y.; Geng, Y.; Su, Z. M. A Comparative Study of a Fluorene-Based Non-Fullerene Electron Acceptor and Pc61bm in Organic Solar Cell at a Quantum Chemical Level. RSC Adv. 2016, 6, 81164-81173. 27



30.

Lu, T.; Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580-592.

31.

Li, S. B.; Duan, Y. A.; Geng, Y.; Li, H. B.; Zhang, J. Z.; Xu, H. L.; Zhang, M.; Su, Z. M. A Designed Bithiopheneimide-Based Conjugated Polymer for Organic Photovoltaic with Ultrafast Charge Transfer at Donor/PC71BM Interface: Theoretical Study and Characterization. Phys. Chem. Chem. Phys. 2014, 16, 25799-25808.

32.

Pan, J.; Yin, H.; Xie, Y. Z.; Sun, G. Y.; Su, Z. M. The Conversion of Donor to Acceptor and Rational Design for Diketopyrrolopyrrole-Containing Small Molecule Acceptors by Introducing Nitrogen-Atoms for Organic Solar Cells. RSC Adv. 2017, 7, 31800-31806.

33.

Yin, H.; Sui, M. Y.; Pan, Q. Q.; Sun, G. Y.; Geng, Y. A Series of Bowl-Shaped Pdi Dimers Designed for Organic Photovoltaic Cells through Engineering N-Annulated Bridge Towards Potential Alternatives of PDI Bridged Dimer Acceptors. Dyes Pigments 2018, 148, 394-404.

34.

Rivnay, J.; Toney, M. F.; Zheng, Y.; Kauvar, I. V.; Chen, Z. H.; Wagner, V.; Facchetti, A.; Salleo, A. Unconventional Face-on Texture and Exceptional in-Plane Order of a High Mobility N-Type Polymer. Adv. Mater. 2010, 22, 4359-4363.

28


Page 28 of 34

Page 29 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35.

Mothy, S.; Guillaume, M.; Idé, J.; Castet, F.; Ducasse, L.; Cornil, J.; Beljonne, D. Tuning the Interfacial Electronic Structure at Organic Heterojunctions by Chemical Design. J. Phys. Chem. Lett. 2012, 3, 2374-2378.

36.

Sui, M. Y.; Pan, Q. Q.; Yin, H.; Sun, G. Y.; Geng, Y.; Su, Z. M. Design of Hexabenzocoronene Derivatives as Non-Fullerene Acceptors in Organic Photovoltaics by Bridging Dimers and Modulating Structural Twists. Solar RRL 2017, 1, 1700060.

37.

Pan, Q. Q.; Li, S. B.; Duan, Y. C.; Wu, Y.; Zhang, J.; Geng, Y.; Zhao, L.; Su, Z. M. Exploring What Prompts Itic to Become a Superior Acceptor in Organic Solar Cell by Combining Molecular Dynamics Simulation with Quantum Chemistry Calculation. Phys. Chem. Chem. Phys. 2017, 19, 31227-31235.

38.

Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456-1465.

39.

Leng, C.; Qin, H. M.; Si, Y. B.; Zhao, Y. Theoretical Prediction of the Rate Constants for Exciton Dissociation and Charge Recombination to a Triplet State in PCPDTBT with Different Fullerene Derivatives. J. Phys. Chem. C 2014, 118, 1843-1855.

40.

Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid Exchange–Correlation Functional Using the Coulomb-Attenuating Method (Cam-B3LYP). Chem. Phys. Lett. 2004, 393, 51-57. 29



41.

Jahnke, A. A.; Howe, G. W.; Seferos, D. S. Polytellurophenes with Properties Controlled by Tellurium-Coordination. Angew. Chem. Int. Ed. 2010, 49, 10140-10144.

42.

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Revision D.01; Gaussian, Inc., Wallingford, CT, 2013.

43.

Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Solar Cells-Towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789-794.

44.

Ku, J.; Lansac, Y.; Jang, Y. H. Time-Dependent Density Functional Theory Study on Benzothiadiazole-Based Low-Band-Gap Fused-Ring Copolymers for Organic Solar Cell Applications. J. Phys. Chem. C 2011, 115, 21508-21516.

45.

Liu, T.; Troisi, A. What Makes Fullerene Acceptors Special as Electron Acceptors in Organic Solar Cells and How to Replace Them. Adv. Mater. 2012, 25, 1038-1041.

46.

Li, H. Y.; Earmme, T.; Ren, G. Q.; Saeki, A.; Yoshikawa, S.; Murari, N. M.; Subramaniyan, S.; Crane, M. J.; Seki, S.; Jenekhe, S. A. Beyond Fullerenes: Design of Nonfullerene Acceptors for Efficient Organic Photovoltaics. J. Am. Chem. Soc. 2014, 136, 14589-14597.

30


Page 30 of 34

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47.

Nielsen, C. B.; Holliday, S.; Chen, H. Y.; Cryer, S. J.; McCulloch, I. Non-Fullerene Electron Acceptors for Use in Organic Solar Cells. Acc. Chem. Res. 2015, 48, 2803-2812.

48.

An, Q. S.; Zhang, F. J.; Zhang, J.; Tang, W. H.; Deng, Z. B.; Hu, B. Versatile Ternary Organic Solar Cells: A Critical Review. Energy Environ. Sci. 2016, 9, 281-322.

49.

Chen, S. W.; Ullah, N.; Zhang, R. Q. Exciton Self-Trapping in sp2 Carbon Nanostructures Induced by Edge Ether Groups. J. Phys. Chem. Lett. 2018, 9, 4857-4864.

50.

Lu, T. Multiwfn Manual. version 3.6(dev), Section 3.21.1.

51.

Lu, T. Multiwfn Manual. version 3.6(dev), Section 3.21.8.

52.

Brédas, J. L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular Understanding of Organic Solar Cells: The Challenges. Acc. Chem. Res. 2009, 42, 1691-1699.

53.

Borges, I.; Aquino, A. J. A.; Köhn, A.; Nieman, R.; Hase, W. L.; Chen, L. X.; Lischka, H. Ab Initio Modeling of Excitonic and Charge-Transfer States in Organic Semiconductors: The PTB1/PCBM Low Band Gap System. J. Am. Chem. Soc. 2013, 135, 18252-18255.

54.

Li, S. B.; Duan, Y. A.; Geng, Y.; Gao, H. Z.; Qiu, Y. Q.; Su, Z. M. Theoretical Design and Characterization of Pyridalthiadiazole-Based Chromophores with

31



Fast Charge Transfer at Donor/Acceptor Interface toward Small Molecule Organic Photovoltaics. RSC Adv. 2015, 5, 29401-29411. 55.

Joseph, S.; Ravva, M. K.; Brédas, J. L. Charge-Transfer Dynamics in the Lowest Excited State of a Pentacene-Fullerene Complex: Implications for Organic Solar Cells. J. Phys. Chem. Lett. 2017, 8, 5171-5176.

56.

Shen, X. X.; Han, G. C.; Yi, Y. P. The Nature of Excited States in Dipolar Donor/Fullerene Complexes for Organic Solar Cells: Evolution with the Donor Stack Size. Phys. Chem. Chem. Phys. 2016, 18, 15955-15963.

57.

Lv, L.; Wang, X. F.; Wang, X. L.; Yang, L.; Dong, T.; Yang, Z.; Huang, H. Tellurophene-Based N-Type Copolymers for Photovoltaic Applications. ACS Appl. Mater. Interfaces 2016, 8, 34620-34629.

58.

Carrera, E. I.; Seferos, D. S. Semiconducting Polymers Containing Tellurium: Perspectives toward Obtaining High-Performance Materials. Macromolecules 2015, 48, 297-308.

59.

Mauck, C. M.; Hartnett, P. E.; Margulies, E. A.; Ma, L.; Miller, C. E.; Schatz, G. C.; Marks, T. J.; Wasielewski, M. R. Singlet Fission Via an Excimer-Like Intermediate in 3,6-Bis(thiophen-2-yl)diketopyrrolopyrrole Derivatives. J. Am. Chem. Soc. 2016, 138, 11749-11761.

60.

Kawata, S.; Furudate, J.; Kimura, T.; Minaki, H.; Saito, A.; Katagiri, H.; Pu, Y. J. Controlling the Excited-State Energy Levels of 9,9'-Bifluorenylidene

32


Page 32 of 34

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Derivatives by Twisting Their Structure to Attaining Singlet Fission Character in Organic Photovoltaics. J. Mater. Chem. C 2017, 5, 4909-4914. 61.

Shen, L.; Tang, Z. F.; Wang, X. M.; Liu, H. Y.; Chen, Y. L.; Li, X. Y. Effects of Aromatic Substituents on the Electronic Structure and Excited State Energy Levels of Diketopyrrolopyrrole Derivatives for Singlet Fission. Phys. Chem. Chem. Phys. 2018, 20, 22997-23006.

62.

Jadhav, P. J.; Brown, P. R.; Thompson, N.; Wunsch, B.; Mohanty, A.; Yost, S. R.; Hontz, E.; Van Voorhis, T.; Bawendi, M. G.; Bulovic, V., et al. Triplet Exciton Dissociation in Singlet Exciton Fission Photovoltaics. Adv. Mater. 2012, 24, 6169-6174.

33



TOC Graphic

34


Page 34 of 34

An Updated Strategy for Designing Non-Fullerene Acceptors by the

Recommend Documents