Intra-molecular Charge Transfer and Electron Delocalization in Non

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Intra-molecular Charge Transfer and Electron Delocalization in Non-Fullerene Organic Solar Cells Qinghe Wu, Donglin Zhao, Matthew Bryant Goldey, Alexander S. Filatov, Valerii Sharapov, Yamil J. Colon, Zhengxu Cai, Wei Chen, Juan J. de Pablo, Giulia Galli, and Luping Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18717 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Materials & Interfaces

Intra-molecular

Charge

Transfer

and

Electron

Delocalization in Non-Fullerene Organic Solar Cells Qinghe Wu†#*, Donglin Zhao‡#, Matthew B. Goldey§, Alexander S. Filatov‡, Valerii Sharapov‡, Yamil J. Colón◇§, Zhengxu Cai‡, Wei Chen◇§, Juan de Pablo◇§, Giulia Galli◇§, Luping Yu‡* †

Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural

Materials of Guangdong Province, Shantou University, Guangdong 515063, P. R. China ‡

Department of Chemistry and The James Franck Institute, The University of Chicago, 929 E 57th

Street, Chicago, IL 60637, USA ◇

Institute for Molecular Engineering and Materials Science Division, Argonne National Laboratory,

9700 Cass Avenue, Lemont, Illinois 60439, USA §

Institute for Molecular Engineering, the University of Chicago, 5747 South Ellis Avenue, Chicago,

Illinois 60637, USA KEYWORDS: Electron Delocalization, Intra-molecular Charge Transfer, Non-fullerene Electron Acceptor, Organic Solar Cell, Perylene diimide.

ABSTRACT. Two types of electron acceptors were synthesized by coupling two kinds of electron rich cores with four equivalent perylene diimides (PDI) at the α-position. With fully aromatic cores, TPB and TPSe have π-orbitals spread continuously over the whole aromatic conjugated backbone, unlike TPC and TPSi which contain isolated PDI units due to the use of a tetrahedron carbon or silicon linker. Density functional theory calculations of the projected density of states showed that the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals for TPB are localized in separate regions of space. Further, the LUMO of TPB shows a greater contribution from the orbitals belonging to ACS Paragon Plus Environment

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the connective core of the molecules than that of TPC. Overall the properties of the HOMO and LUMO point at increased intra-molecular delocalization of negative charge carriers for TPB and TPSe than for TPC and TPSi, and hence at a more facile intra-molecular charge transfer for the former. The film absorption and emission spectra showed evidences for the inter-molecular electron delocalization in TPB and TPSe, which is consistent with the network structure revealed by x-ray diffraction studies on single crystals of TPB. These features benefit the formation of charge transfer (CT) states and/or facilitate charge transport. Thus, higher electron mobility and higher charge dissociation probabilities under Jsc condition were observed in blend films of TPB:PTB7-Th and TPSe:PTB7-Th than those in TPC:PTB7-Th and TPSi:PTB7-Th blend films. As a result, the Jsc and FF values of 15.02 mA/cm2, 0.58 and 14.36 mA/cm2, 0.55 for TPB and TPSe based solar cell are observed, while those for TPC and TPSi are 11.55 mA/cm2, 0.47 and 10.35 mA/cm2, 0.42, respectively.

Introduction In the past several years, the rapid development of new electron acceptors has pushed the performance of non-fullerene organic solar cells (OSC) to the level that is comparable with or surpass that of fullerene OSC both in the individual parameters, such as fill factor (FF) and short circuit current (Jsc), and overall performance (PCE).1-14 Among various electron acceptors, compounds with an electron-rich core coupled with multiple electron-deficient moieties are very promising replacement for expensive fullerene derivatives because of their easy synthesis and tunable optical/electronic properties.15-34 In our previous communication, we reported development of an electron acceptor based on covalently bounded cluster of α-PDI (TPB), which was shown to exhibit excellent organic photovoltaic properties.19 To explain the interesting properties, a hypothesis was made that the built-in internal polarization and extended conjugation played important roles in enhancing organic photovoltaic (OPV) performances. It is well known that in bulky heterojunction solar cells, both donor and acceptor molecules absorb photons to generate excitons, which must undergo exciton-dissociation at interfaces between electron donor and acceptor materials to form charge transfer (CT) states.35,36

The Coulombically-bound

electron-hole pairs can further separate into free charges or relax back to the ground state via ACS Paragon Plus Environment

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recombination.37-40 It was shown that when donor polymers exhibited suitable internal polarization, efficient exciton-dissociation and charge separation can be achieved, leading to high performance of solar cell devices.41-43 Our previous work indicated that internal polarization is equally important in designing electron acceptors.19,44 To further provide firm evidence for this notion, in this paper, we studied two series of electron acceptors with different degrees of molecular conjugation. The two series of molecules have marked differences in their optical and electronic properties, one contains extended delocalization and internal polarization in the photoexcited state and the other does not. The studies in solar cell performances, using PTB7-Th as donor polymer, showed that intra-molecular charge transfer and charge delocalization in electron acceptors influences charge dissociation and electron transport, and thus the corresponding OPV performance of non-fullerene solar cells. It is also known that interpenetrating networks of donor and acceptor molecules are crucial for charge transport in OPV devices. It was revealed that the ultrafast CT process and charge separation efficiency are strongly affected by the morphology and phase-separation structure in polymer/fullerene systems4548

. Revealing the charge separation process at the donor/acceptor interface and investigating its

relationship with the chemical structure of donor and/or acceptor are crucial for the rational design of semiconducting materials for OPVs. Herein, we also report the crystal structure of the TPB molecule, which provides an opportunity to deduce structural information and better understanding of the OPV properties. Result and discussion Materials Design, Synthesis, and Electronic Properties. Previously, our group reported the enhanced OPV performance using an acceptor with a BDT-Th core coupled with four PDIs.19,49 Herein, we designed and synthesized four compounds (Scheme 1) by employing two types of core linkers. One type of core is BDT-Th and BDT-Se which have the π-orbitals spreading continuously over the whole conjugated backbone. The other type of core is tetraphenylmethane and tetraphenylsilane which have the π-orbitals separated by a tetrahedral carbon or silicon center, respectively. TPB and TPSe have the ACS Paragon Plus Environment


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potential feature of allowing each PDI to be conjugated through the core with the other three PDIs. The four PDIs in TPC and TPSi are all isolated by the tetrahedron carbon or silicon center.

Scheme 1. Chemical structures of TPC, TPSi, TPB and TPSe. These molecules (TPC, TPSi, TPB and TPSe) are synthesized via the Suzuki coupling reaction of tetra-pinacolatoboron-substituted BDT-Th/BDT-Se/TPM/TPS with four equivalents of α-bromated PDIs in moderate yields with chemical structures shown in Scheme 1. All compounds show good solubility in common organic solvents such as methylene chloride, chloroform or chlorobenzene. Chemical structures of products were confirmed by 1H NMR, mass spectroscopy and high-resolution mass spectroscopy.

Figure 1 a) cyclic voltammograms of TPC, TPSi, TPB and TPSe films with Fc/Fc+ as the reference; b) Schematic of energy level of TPC, TPSi, TPB, TPSe and PTB7-Th; c) the solution and d) film

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absorption spectrum of TPC, TPSi, TPB and TPSe; e) the solution and f) film emission spectrum of TPC, TPSi, TPB and TPSe. Cyclic voltammetry measurement (CV) was used to determine the energy levels of these four compounds, and the results are shown in Figure 1a,b. TPC and TPSi exhibit the lowest unoccupied molecular orbital (LUMO) energy level of -3.75 and -3.77 eV, which are similar to those for TPB and TPSe (-3.79 and -3.82 eV, respectively). The highest occupied molecular orbital (HOMO) energy levels of TPC, TPSi, TPB and TPSe are -6.06 eV, -6.06 eV, -5.94 and -5.97 eV, respectively. The LUMO energy level of the four compounds is aligned with that of PTB7-Th, but with a sufficient energy offset to drive electron transfer. However, the HOMO energy level of the four compounds is much lower than that of PTB7-Th.

Figure 2. (a) J-V curves, (b) External quantum efficiency spectrum and (c) Photocurrent density (Jph) versus effective voltage (Veff) characteristics for solar cells with PTB7-Th as donor and TPC, TPSi, TPB or TPSe as acceptor. Table 1. Device parameters of solar cell fabricated with PTB7-Th as donor and TPC, TPSi, TPB and TPSe as acceptors.

µh (cm2/vs) µe (cm2/vs)

Acceptor

Jsc (mA cm-2)

Voc (V)

FF

Eff (%) (max)

TPC

11.55±0.15

0.85±0.004

0.47±0.00

4.60±0.04 (4.71)

4.0×10-4

1.8×10-5

TPSi

10.35±0.23

0.83±0.003

0.42±0.01

3.64±0.04 (3.68)

3.6×10-4

1.4×10-5

TPB

15.02±0.47

0.81±0.004

0.58±0.01

7.08±0.19 (7.38)

3.2×10-4

9.1×10-5

TPSe

14.36±0.26

0.81±0.004

0.55±0.01

6.32±0.25 (6.61)

3.0×10-4

5.6×10-5



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Average data for over ten devices. Solar

cell

performance.

Inverted

solar

cell

devices

with

configuration

of

ITO/ZnO/Acceptor:PTB7-Th/MoO3/Ag were fabricated to evaluate photovoltaic properties of TPC, TPSi, TPB and TPSe. Active layers around 90 nm thick were deposited by spin-casting warm PTB7-Th:acceptor chlorobenzene solution. The optimized donor/acceptor ratio for all four devices is 1 to 1.5 in weight. No solvent additive is added in processing the film for the purpose of comparison. Typical current density-voltage (J-V) curves of the four devices are shown in Figure 2a and averaged parameters over 10 devices are summarized in Table 1. The devices based on TPB and TPSe exhibited average power conversion efficiency (PCE) of 7.08 % and 6.36 %, while the efficiencies of TPC and TPSi based solar cells are 4.60 % and 3.64 %. The trend is very clear that electron acceptors with better conjugation show superior photovoltaic properties than acceptors with isolated PDI units. The TPB and TPSe based solar cell devices exhibited higher FF and Jsc values as well. The Jsc and FF values are 15.02 mA/cm2, 0.58 for TPB and 14.36 mA/cm2, 0.55 for TPSe, while those of TPC and TPSi are 11.55 mA/cm2, 0.47 and 10.35 mA/cm2, 0.42, respectively. Consistent with variations in their LUMO energy level, TPC and TPSi show slightly higher Voc values than those of TPB and TPSe. External quantum efficiency (EQE) spectra of the four devices are shown in Figure 2b. Integrated Jsc values from EQE spectrum are calculated to be 11.5 mA/cm2 (TPC), 10.9 mA/cm2 (TPSi), 14.1 mA/cm2 (TPB) and 13.0 mA/cm2 (TPSe). It should be noted that the Jsc values of TPC and TPSi devices integrated from EQE show negligible deviation from the Jsc values measured from J-V curves, while those of TPB and TPSe exhibit large negative deviation from JV measurements. This is because TPB and TPSe based devices are found to suffer efficiency drop during the UV-glue encapsulation procedure before devices are transferred out of glove box for EQE measurement.


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Optical Properties. To explain the experimental results and gain deeper insight, the optical absorption spectra of the four compounds were investigated both in solution and in films (Figure 1c, d). Solution absorption spectrums were taken in chlorobenzene at a concentration of 1.2×10-6 M. In solution, four acceptors show absorption spectra similar to PDI monomer, including three PDI characteristic peaks at 460, 495 and 530 nm, with intensities I00>I01>I02. The absorption coefficients of the four compounds at maximum absorption are very similar, around 2.3×105 M1

cm-1. Compared with that of TPC and TPSi, the spectrum of TPB and TPSe exhibited a weak

shoulder at 550-650 nm. Because the solution spectra were measured at very dilute concentration, the absorption shoulder indicated an intramolecular D-A charge transfer state rather than an aggregation-induced feature. The intramolecular D-A charge transfer state indicates internal polarization in compounds TPB and TPSe. The TPC and TPSi show film absorption spectra similar to the PDI monomer, and the intensities of the three major transitions are still I00>I01>I02. The onset of absorption is slightly redshifted from 550 nm to 569 nm. TPB and TPSe exhibit enhanced I01 and I02 absorption, where the intensity of I01 exceeds I00. The absorption onset is also redshifted from 553 nm to 584 nm. Both the enhanced I01/I00 values and the larger red-shift of absorption onset imply TPB and TPSe have stronger electron coupling in solid state compared to TPC and TPSi. Fluorescence emission of four compounds (Figure 1e, f) were measured both in 1.3 x 10-7 M dilute chlorobenzene solutions and films, excited at 495 nm. The TPC and TPSi exhibited relatively high emission quantum yields of 43 % and 15 % respectively, as measured by quantaQ integration sphere. The fluorescence of TPB and TPSe are, however, largely quenched and too dim to be determined by integration sphere, which is caused by strong intramolecular charge transfer. The film fluorescence spectra of TPC and TPSi exhibit a broad featureless peak with a

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maximum at 628 nm, while TPB and TPSe emission red shifted to 740 nm and 745 nm, respectively. The larger red shift of the fluorescence spectra of TPB and TPSe films indicates further extension of effective conjugation caused by strong intermolecular π-π interaction in solid states and excimer formation, which is the evidence of more delocalized electron wave function in TPB and TPSe films. The strong π−π intermolecular interaction are also observed in TPB single crystal, discussed below. Thus, both intramolecular and intermolecular charge delocalization are more pronounced in compounds with conjugated core.

Figure 3a) Projected density of states for TPB with PDI attachments in the alpha position of the PDI bay. Core and PDI denote the bi-(thiophene)-benzodithiophene center and the attached PDIs, correspondingly. The zero of energy here is set to the valence band maximum 3b) Projected density of states for TPC with PDI attachments in the alpha position of the PDI bay. Core and PDI denote the tetraphenyl carbon center and the attached PDIs, correspondingly. The zero of energy here is set to the valence band maximum. 3c) Contributions of orbitals centered in different chemical moieties of TPB to the HOMO-5 to HOMO-1 orbitals, the HOMO, and the LUMO to LUMO+3 states, as obtained from a projection onto atomic orbitals belonging to different moieties. Core and PDI denote the bi-(thiophene)-benzodithiophene center and the attached PDIs, correspondingly. Box widths denote the inner quartiles of the data, and whiskers denotes 1.5 times the quartiles. 3d) Contributions of orbitals centered in different chemical moieties of TPC to the HOMO-5 to HOMO-1 orbitals, the HOMO, and the LUMO to LUMO+3 orbitals, as obtained from a projection onto atomic orbitals belonging to different moieties. Core 8 ACS Paragon Plus Environment

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and PDI denote the tetraphenyl carbon center and the attached PDIs. Box widths denote the inner quartiles of the data, and whiskers denotes 1.5 times the quartiles. Electronic Structure Calculations. Detailed theoretical calculations also provided explanations for the above observations. Because of the similarity in chemical structure of TPB and TPSe, TPC and TPSi, only TPB and TPC were considered. Periodic DFT calculations were performed using the PBE density functional50 with a plane wave cutoff of 60 Ry and of the Quantum ESPRESSO package51. Due to the large number of configurations accessible in the condensed phase, electronic structure properties were averaged over 133 (132) configurations of TPB (TPC) obtained from classical molecular dynamics (MD). In particular we considered glassy snapshots from trajectories obtained after equilibrating a condensed phase simulation of pure TPB or TPC at 1000 K and cooling down the sample to 400 K; we used force fields obtained from the literature 52-57 and the GROMACS code with GPU acceleration 58-59. Using a projected density of states (PDOS) averaged over multiple molecular configurations, we identified several differences in the PDOS near the band edges of TPC and TPB, as shown in Figures 3a and 3b. The LUMO of both compounds is four-fold degenerate for TPB, the HOMO is isolated from other occupied states by 0.4 - 0.5 eV, which is distinct from TPC, whose HOMO is nearly degenerate with a large manifold of states. The projected density of electronic states (EDOS) allowed us to identify the contributions from atomic orbitals localized within different chemical moieties; the EDOS are shown in Figures 3a and 3b, and box whisker plots of the contributions from different moieties are shown in Figures 3c and 3d. Projections of the energy levels onto atomic orbitals demonstrate a very large contribution of the orbitals of PDI units to the LUMO for TPB (TPC) at 95±1 % (99.5±0.3 %) of the total 9 ACS Paragon Plus Environment


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PDOS. The remainder of the LUMO originates from orbitals of the connective core of these compounds, and amounts to 5±1 % (0.5±0.3 %) for TPB (TPC) of the LUMO. The TPB HOMO is dominated by core contributions (70±5 %), and it is qualitatively different from slightly deeper occupied states (where the core is 15±6 % of the HOMO-5 to HOMO-1 energy levels) and from the occupied manifold of TPC, where 34±32 % of the HOMO originates from the core. The large standard deviation of core contributions for TPC reflects the structural variety accessed within glassy phases and emphasizes the need to combine electronic structure calculations and large scalemolecular dynamics simulations for studying of flexible, twisted multichromophores. The calculated asymmetry in the core participation in TPB/TPC occupied and unoccupied states was also found in B3LYP/6-31G* calculations of isolated chromophores, which showed that the HOMO orbital mainly localizes on the BDT-Th core and the LUMO orbital on PDI units for TPB19, while both the HOMO and LUMO orbitals mainly localize on PDI units for TPC (Figure 3d, Figure S7). These results demonstrate a higher propensity of photoexcited electrons to participate in intramolecular charge transfer in TPB than in TPC. Due to the high contributions of PDIs to the HOMO of TPC, dissociation of excitons into well-separated electron-hole pairs is expected to be less likely for TPC. The contribution of the connective core to the LUMO of TPB (5±1 %) is significantly higher than for TPC (0.5±0.3 %), an indication of possible coremediated intra-molecular charge transfer after electron-hole separation. Since these molecules also have qualitative differences in the core contributions to the LUMO, work is underway to predict how charge transport60 in TPB and TPC varies with molecular configuration, and which structural features, such as linkage twisting61, determine charge transport rates. Single crystal structure of TPB. To gain a better understanding of the three-dimensional packing of TPB molecules in the solid state, significant efforts were devoted to grow single 10 ACS Paragon Plus Environment

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crystals of this material. The TPB molecule is large (198 non-H atoms) and contains four PDI substituents connected via single bond linkages to the BDT-Th central unit. While crystallizing such a molecule posed a significant challenge, it was eventually found that TPB molecules can be grown into small single crystals by a very slow evaporation of its chloroform solution. The X-ray diffraction data were collected several times including two attempts using X-ray synchrotron radiation of the Advanced Photon Source at the Argonne National Lab (ChemMatCARS, Sector 15). Although best achieved results are still rather crude, the diffraction data allowed for a full resolution of all atoms and their refinement using isotropic thermal parameters (Figure 4a). In the molecule, the extended core is rather rigid and well resolved. Four PDI substituents form a cross-like molecular geometry with the angle of 75° between two mean planes passing through PDI subunits connected to thiophene substituents and two PDI subunits connected to BDT (Figure 4b). The dangling C7H15 alkyl side chains are extensively disordered due to high thermal motion as they located in the solvent filled voids. For refinement purposes, alkyl chains were introduced as fully constrained. The solvent accessible of 5886 Å3 (representing 1/2 of the unit cell volume of11643 Å3) and the electron count of 2074 electrons can account for 35 chloroform molecules crystallized in one-unit cell. It can be noted here that there are only three examples of single crystal structural analysis carried out with the same BDTTh substituted core62-64 and only one of them is of comparable complexity to the structure reported here.



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Figure 4. a) Side and b) top view of TPB single molecule; c) intermolecular packing along b and c axis, the intermolecular π−π interactions are marked with blue lines; d) 3D packing of TPB in single crystal. The obtained single crystal structure provides interesting structural insights into the charge transport pathway. The crystal packing is dominated by π-stacking between the individual PDI moieties (Figure 4c). All four PDIs are involved in inter-molecular interactions with an interplane separation starting at ca. 3.3 Å. These interactions lead to the formation of infinite 2D sheets. Dangling alkyl side chains along with the solvent media fill the space between the sheets. Such strong π−π inter-molecular interactions may represent charge carriers transporting channels65facilitating the electron delocalization over more TPB molecules. This is consistent with the observation of the film absorption and emission spectrum. To further evaluate the packing variation of TPB in pure film and blend film with PTB7-th, the pure and blend film absorption and emission spectrum were measured and summarized in Figure S8 and Figure S9. The absorption of TPB in blend film shows three vibronic peaks that are nearly identical with that of pure film absorption. Although the photoluminescence of TPB are largely quenched by excited at 500 nm, the shape of weak emission spectra is still similar with that of pure film 12 ACS Paragon Plus Environment

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emission with maximum peaks both at 738 nm. Both absorption and emission spectrum provide evidence that the TPB in blend film should maintain the same packing as in pure film. Thus, it can be proposed that when TPB is mixed with polymers, a continuous charge transport pathway can be established via intermolecular π−π interactions and intramolecular BDT linkage. This should be in contrast with TPC and TPSi. Active layer Characterization. Both GIWAXS and AFM studies offered minimal insight into the blend film structures. The 2D GIWAXS patterns are shown in Figure 5 and the out-ofplane/in-plane line cuts from GIWAXS patterns are exhibited in Figure S10. Although the GIWAXS pattern of TPC, TPSi, TPB and TPSe neat films show three low-intensity arc-shapes scattering, indicating a weak crystalline nature and molecular orientation isotropy for all four compounds. The four blend films show similar features, where the scattering peaks due to small molecule acceptors all merge with that of donor polymer. The broad and weak diffusion peaks mean amorphous nature of blend films. The films topography (Figure 6) of four blend films from the atomic force microscopy (AFM) studies are all smooth with surface roughness value of between 0.5 and 0.6 nm, and show very similar topographic feature, which do not account for their OPV performance variation. This characteristic of non-fullerene BHJ blend film poses a challenge to morphological studies due to their lack of contrast.



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Figure 5. 2D GIWAXS patterns of films on ZnO-modified Si substrates. a) neat TPC film; b), neat TPSi film; c), neat TPB film; d), neat TPSe film; e), blend film of TPC:PTB7-Th; f), blend film of TPSi:PTB7-Th; g), blend film of TPB:PTB7-Th; h), blend film of TPC:PTB7-Th.

Figure 6. AFM of films of a) TPC:PTB7-Th; b) TPSi:PTB7-Th; c) TPB:PTB7-Th and d) TPSe:PTB7-Th. Charge carrier mobility. Electron and hole mobility were measured in four devices (See in Figure S11). The four blend films exhibited similar hole mobility, calculated to be 4.0×10-4 cm2/vs for TPC:PTB7-Th, 3.6×10-4 cm2/vs for TPSi:PTB7-Th, 3.2×10-4 cm2/vs for TPB:PTB7Th and 3.0×10-4 cm2/vs for TPSe:PTB7-Th, respectively. Large differences were observed in electron mobilities, which reflect the properties of the acceptor molecule. The TPC and TPSi blend films have low electron mobilities of 1.8×10-5 cm2/vs and 1.4×10-5cm2/vs. The TPB and TPSe blend films show higher mobility values of 9.1×10-5 cm2/vs and 5.6×10-5 cm2/vs, clearly due to intramolecular conjugation and network due to strong intermolecular π−π stacking. The higher electron mobility is beneficial to FF values, thus PCE values. Exciton dissociation and charge generation. The structural differences in electron delocalization of these acceptors were also manifested in their changes in saturation current density (Jsat) and charge dissociation probabilities P(E,T) of devices in combination with PTB714 ACS Paragon Plus Environment

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Th. The Jsat will only be limited by total amount of absorbed incident photons if all the photogenerated excitons are dissociated into free charge carriers and collected by electrodes at a high Veff (i.e. Veff = 2.5 V). Here, photocurrent density (Jph) is defined as Jph = JL - JD, where JL and JD are the current densities under illumination and dark, respectively. Effective voltage (Veff) is defined as Veff = V0 - Vapp, where V0 is the voltage where Jph equals zero and Vapp is applied bias voltage. Figure 2c illustrates characteristic curves of Jph versus Veff for the four devices. The Jsat values varied for the four devices from 13.18 mA cm-2 (TPC), 13.31 mA cm-2 (TPSi), 16.59 mA cm-2 (TPB) to 15.41 mA cm-2 (TPSe), respectively. The results suggested that photogeneration of charge is more efficient in TPB and TPSe devices than those in TPC and TPSi devices. The P(E, T) values calculated by normalizing Jph with Jsat

for the four devices under short circuit

condition were 85 % (TPC), 78 % (TPSi), 92 % (TPB) and 92 % (TPSe), respectively. Acceptors with conjugated cores (TPB and TPSe) exhibit more efficient exciton dissociation and charge generation inside solar cell devices because the delocalization of electron wave-functions facilitates charge transfer (CT) states overcoming Coulombic attraction and results in higher Jsc values.39,40 Conclusion Two types of electron acceptors were synthesized by coupling two kinds of electron rich cores with four equivalent PDIs attached at the α-position. Comprehensive studies indicated that TPB and TPSe with π-orbitals delocalized over the whole aromatic conjugated backbone exhibit higher electron mobilities and higher charge dissociation probabilities under Jsc condition, and thus higher solar cell performances than those of TPC and TPSi; the later contain isolated PDI units originating from the use of a tetrahedral carbon or silicon linker. Our results show that the



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localization properties of excited electron wave-functions play a key role in determining the properties of non-fullerene acceptor materials. ASSOCIATED CONTENT The supporting information is available. It includes synthesis details, absorption spectra, photoluminescence spectra, theoretical calculations and device fabrication and characterization details. “CCDC 1551964” contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.” Obtaining such data required several attempts using synchrotron radiation, however, the intrinsic problems of crystals (large size of disordered alkyl chains and large unite cells with large pores filled with disordered solvent) prevented obtaining significantly high-quality data. We specifically concentrated on getting the structure of TPB with these large dangling chains (knowing the degrading outcome on the quality) to correctly evaluate the crystal packing and 3D structure in the solid state rather than to evaluate fine details of molecular structure. All A and B level alerts in chekCIF were carefully looked at and are a consequence of these issues rather than wrongly described solution or a simply overlooked issue. Since these issues may be of importance to a reader of the paper, they were included and discussed in the main body of the manuscript. AUTHOR INFORMATION Corresponding Author Corresponding author: [email protected]; [email protected] Author Contributions


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#

These authors contributed equally. L. Yu conceptualized the project. Q. Wu designed and

synthesized the non-fullerene acceptors. D. Zhao and V. Sharapov performed the device experiments. M. Goldey, Y. Colón, J. Pablo and G. Galli performed the calculation. Z. Cai and W. Chen performed 2D-GIWAXS experiments. A. Filatov performed X-ray diffraction experiment and data analysis. ACKOWLEDGMENT This work was supported by NIST via CHIMAD program (QHW, MG, YC, JDP, GG and LPY). DOE via the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-SC0001059 (VS) and U. S. National Science Foundation grant (DLZ, NSF DMR-1263006), NSF MRSEC program at the University of Chicago (WYL, DMR- 1420709), STU Scientific Research Foundation for Talents (NTF17005) and Youth Program of National Natural Science Foundation of China (21702133). W.C. gratefully acknowledges financial support from the US Department of Energy, Office of Science, Materials Sciences and Engineering Division. Use of the Advanced Photon Source (APS) at the Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. ChemMatCARS Sector 15 is principally supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE-1346572. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357.



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Generalized Gradient Approximation Made 24


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Intra-molecular Charge Transfer and Electron Delocalization in Non

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