Organic Semiconducting Alloys with Tunable Energy Levels

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Organic Semiconducting Alloys with Tunable Energy Levels Jin-hu Dou, Zhi-Ao Yu, Jun Zhang, Yu-Qing Zheng, Ze-Fan Yao, Zeyi Tu, Xinchang Wang, Shiliang Huang, Chengwen Liu, Junliang Sun, Yuanping Yi, Xiaoyu Cao, Yiqin Gao, Jie-Yu Wang, and Jian Pei J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13471 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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Journal of the American Chemical Society

Organic Semiconducting Alloys with Tunable Energy Levels Jin-Hu Dou,† Zhi-Ao Yu,† Jun Zhang,†,‡ Yu-Qing Zheng,∥ Ze-Fan Yao,† Zeyi Tu,§ Xinchang Wang,# Shiliang Huang,⊥ Chengwen Liu,┬ Junliang Sun,† Yuanping Yi,§ Xiaoyu Cao,# Yiqin Gao,†,‡ Jie-Yu Wang,† and Jian Pei*,† †Beijing

National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijng 100871, China. ‡Institute of Theoretical and Computational Chemistry, College of Chemistry and Molecular Engineering and Biodynamic Optical Imaging Center, Peking University, Beijing 100871, China. ∥Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA §Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. #State Key Laboratory of Physical Chemistry of Solid Surfaces, iChEM and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. ⊥Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621000, China. ┬Department of Biomedical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA KEYWORDS: organic semiconductors, energy level tuning, co-crystal, doping, n-type. ABSTRACT: Continuous band structure tuning, e.g. doping with different atoms, is one of the most important feature for inorganic semiconductors. However, this can hardly be realized in organic semicondutors. Here we report the first example of fine tuning organic semiconductors band structures by alloying structurally similar derivatives into one single phase. By incorporating halogen atoms on different positions of the backbone, BDOPV derivatives with complementary intramolecular or intermolecular charge distribution were obtained. To maximize the Coloumbic attractive interactions and minimize repulsive interactions, they form antiparallel cofacial stacking in monocomponent or in alloy single crystals, resulting in efficient π orbital overlap. Benefiting from self-assembly induced solid state “olefin metathesis” reaction, it was observed, for the first time, that three BDOPV derivatives co-crystallized in one single crystal. Molecules with different energy levels serve like the dopants in inorganic semiconductors. In consequence, as the total number of halogen atoms increased, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the alloy single crystals decrease monotonously in the range from -5.94 to -6.96 eV and from -4.19 to -4.48 eV, respectively.

INTRODUCTION The very basis of semiconductor devices is established on the electronic energy levels of conduction and valence bands. This also works with organic semiconductors.1–3 The remarkable success of organic semiconductor-based optoelectronic devices such as organic light-emitting diodes (OLEDs),4 organic photovoltaics (OPVs),5 and field-effect transistors (OFETs),6 crucially depends on the fine-tuning of energy levels of frontier molecular orbitals, i.e. the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).7,8 The alignment of HOMO level (for electron-rich component) or LUMO level (for electron-deficient one) with the metal electrodes is critical for reducing charge-injection/extraction barrier in organic optoelectronic devices.9 Although energy levels can be

changed through modification of molecular structures, it require extra synthetic procedures and continuous energy level tuning is hard to be realized. 10 In inorganic semiconductors, controllable energy level tuning can be realized by blending different materials into alloys (Figure 1a).11,12 In such strategy, the most demanding requirement is that semiconductor alloys have to be single phase with nearly perfect mixing, especially high-quality single crystals, to be useful13. This is however even more difficult to be realized in organic semiconductors, since the molecular packing in organic materials is determined by weak yet complex non-covalent interactions.1,14 Impurities introduced into organic semiconducting systems are usually assembled with host molecules in an uncontrolled way, thus leading to strongly disrupted solid-state packing structure,

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let alone forming perfect single crystals (Figure 1b).1 Such disrupted packing will probably result in less efficient energy level tuning and lower charge carrier mobility. Therefore, in order to realize in fine energy level tuning, besides elaborately choosing the host and doping molecules, attention should be paid to controlling the intermolecular interactions.15 The pioneering work by K. Leo and coworkers showed that thanks to the charge-quadrupole interactions between phthalocyanine (Pc)/halogenated Pc, successful

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continuous energy level tuning can be realized through mixing this pair at different ratios by evaporating them into thin films.10 However, it is still difficult to finely tune the energy levels while at the same time to arrange such multicomponent systems into single crystals (Figure 1c), which will be a great platform for fundamental structureproperty relationship study in complicated multicomponent systems.16

Figure 1. Conceptual diagram of (a) inorganic semiconductor lattice and dopant, (b) organic semiconductor matrix and impurity, and (c) organic semiconductor crystal with covalent dopant. (d) Diagram of BDOPV or F6-BDOPV (slipped stacking) and F3BDOPV (antiparallel stacking). (e) Diagram of BDOPV&F6-BDOPV co-crystal (antiparallel stacking dimer). Alkyl chains for all the derivatives are 2-ethylhexane. Recently, our group showed that controllable single crystal packing and tunable energy level can be simultaneously realized by introducing fluorine atoms to BDOPV backbones (Figure 1d).17 Due to the electron-withdrawing property of fluorine atoms, the energy levels of the desired compounds are decreased upon fluorine substitution.18 In the meanwhile, fluorine substitution changed the charge distribution symmetry, thus controlling the single crystal packing. For example, driven by maximizing attractive Columbic interactions and minimizing repulsive interactions, four fluorine atoms substituted BDOPV (F4-BDOPV) with centrosymmetric charge distribution formed a unique antiparallel cofacial stacking.17,19 However, all of these were only studied and realized in pure molecule single crystals. Inspired by the strategy of forming inorganic semiconductor alloys by using isovalent or isostructural

components, such as Si1−xGex,20 we envision that we can form an ordered organic alloy single crystal by combining two BDOPV derivatives with the same charge distribution symmetry but opposite potential (Figure 1e). Meanwhile, their energy levels can be changed due to different energy levels of each component (Figure S1). Herein, a series of halogenated BDOPV molecules with designed charge distribution and different energy levels are developed through aldol condensation (Figure 2). The alloy single crystals between BDOPV and X6-BDOPV (X=F, Cl) were obtained and they demonstrated antiparallel cofacial stacking with small displacement due to the opposite polarity of charge distribution. Thanks to the perfect mixing of molecules with different energy levels into crystals, the energy levels of single crystals based on monocomponent or multicomponents were systematically tuned. 2


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RESULTS AND DISCUSSIONS Synthesis and Physical Properties. To further study the influence of charge distribution on molecular packing in both monocomponent and bicomponent single crsytals, we designed and synthesized three new halogenated BDOPVbased small molecules, X3-BDOPV (X = F, Cl) and Cl6-BDOPV. For clarity, all the following X’s in X3-BDOPV and X6-BDOPV in this paper refer to F, Cl. Detailed synthetic procedures and data are shown in the Supporting Information. All the target molecules showed excellent thermal stability with decomposition temperatures over 330 °C and high melting points in the range of 200 °C to 300 °C (Figure S3 and S4). Rational Design of Molecular Charge Distribution. Electrostatic potential (ESP) has shown superiority in understanding the relationship between single crystal packing and noncovalent interactions of BDOPV-based small molecules.17 Due to the largest Pauling atomic

electronegativity of fluorine atoms, the ESP of F3-BDOPV around the lateral side where all three hydrogen atoms were replaced by fluorine atoms is negative, while positive on the other side, resulting in axisymmetry only along long-axis, which could be considered as intramolecular complementary charge distribution (Figure 1d and S5). Compared with fluorine atom, chlorine atom has weaker electronwithdrawing property, thus leading to less negative charge distribution around the substituted sites in Cl3-BDOPV, but the symmetry keeps the same as that of fluorine counterpart. In Cl6-BDOPV, even slightly positive potentials are observed around the two sites adjacent to alkyl chains, leading to symmetry “breaking” charge distribution compared to that of F6-BDOPV. Careful scrunity of ESPs of all the BDOPV-based molecules showed that BDOPV and X6-BDOPV had complementary charge distribution (same symmetry but different potential polarity), indicating their potentiality in forming organic alloy single crsytals (Figure S5).

Figure 2. Molecular (a1-h1) and crystal structures of BDOPV derivatives. (a2-h2) Packing modes of two adjacent molecules along π-π-stacking direction placed in the stacking geometry and their corresponding longitudinal and transverse shift (all hydrogen atoms are omitted for clarity). (a3-h3) Molecular packing arrangements and corresponding calculated transfer integrals for electrons along π-stacking direction of nine BDOPV derivatives or co-crystals (for clarity, alkyl chains are omitted). Single-Crystal Packing and Analysis. Single crystals of X3BDOPV and co-crystals of BDOPV/X6-BDOPV were obtained by slow solvent vapor diffusion or solvent evaporation method (Figure 2 and S6). More detailed crystallographic data are summarized in the Supporting Information (Figure S7-S11, Table S1, and part 9). Interesting facts were observed when we grew the pure X3-BDOPV or BDOPV/X6-BDOPV cocrystals. When protic solvents were used, all the three components (BDOPV, X3-BDOPV, and X6-BDOPV) were found in final single crystal structures (X3-BDOPV-2 and BDOPV/X6BDOPV) (Figure S12). This means that in BDOPV derivative based systems, two or even three molecules can be perfectly

aligned in one single crystal. Detailed mechanism is included in the following discussion. In the single crystals of X3-BDOPV, the molecules form an antiparallel cofacial stacking along low longitudinal shifts (0.85 Å for F3-BDOPV and 1.16 Å for Cl3-BDOPV) and transverse shifts (1.10 Å for F3-BDOPV and 1.21 Å for Cl3BDOPV) between adjacent molecules (Figure 2b2 and 2d2), benefiting from Coulombic attractive interaction between the peripheral charges as expected (Figure S5). Although both X6-BDOPV molecules showed similar chemical structures with electron-withdrawing atoms on all pheriperal sites, they showed distinct single crystal packing modes:slipped column stacking in F6-BDOPV and antiparallel cofacial stacking in Cl63



BDOPV. This could be rationalized by the charge distribution symmetry “breaking” from highly symmetrical in F6-BDOPV to only centrosymmetric in Cl6-BDOPV as mentioned above from ESPs (Figure S5). Intermixing of BDOPV/X6-BDOPV successfully resulted in organic alloy single crsytals with alternating antiparallel cofacial stacking with small dislocation between adjacent molecules in order to maximize electrostatic attraction and minimize repulsion between two molecules with complementary charge distribution (Figure 2g2 and 2h2). Actually, when comparing charge distribution of BDOPV and X6-BDOPV, one can easily come to a conclusion that parallel cofacial stacking is also favoured when only focusing on Coloumbic interaction. Such antiparallel cofacial stacking is selected to avoid steric hindrance between alkyl chains that are substituted on BDOPV backbone in a staggered way.17 Therefore, to obtain organic alloy single crystals based on BDOPV derivatives, it is

required that not only the two molecules have opposite potential along terminals, but also charge distribution needs to be highly symmetric. Otherwise, minimizing steric effect and maximizing Coloumbic interaction could not be realized at the same time. The transfer integrals for electron transfer were calculated for the nearest neighbor pairs along the π direction for all the single crystals. All the molecules of antiparallel cofacial stackings with small displacements showed high transfer integrals over 150 meV (Figure 2), which are among the highest values in organic semiconductors, further proving the effectiveness of our charge distribution engineering strategy. The selected-area electron diffraction (SAED) patterns showed that the preferential growth direction is along the π-stacking direction for all the crystals (Figure S8). This is also in agreement with the Wulff plots simulated using the Bravais−Fredel−Donnay−Harker (BFDH) approach (Figure S13).

Figure 3. (a) HPLC spectrum and TLC result of BDOPV/F6-BDOPV co-crystal containing three peaks or spots. (b) 1H-NMR of BDOPV/F6-BDOPV co-crystal confirmed the existence of F3-BDOPV (*: F3-BDOPV peaks). (c, d) DFT calculations of the possible mechanism for fragment exchange. Self-assembly Induced “Olefin metathesis” Reaction. As mentioned above, under protic solvents conditions (Figure S6), in the final single crystals grown from pure X3-BDOPV or

mixture of BDOPV/X6-BDOPV, all the three molecules (BDOPV, X3-BDOPV, X6-BDOPV) were found by thin layer chromatography (TLC), high performance liquid 4


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Journal of the American Chemical Society chromatography (HPLC), and nuclear magnetic resonance (NMR) characterizations (Figure 3a, 3b, and S12). Different precursors resulted in different molar ratio in final single crystals and distinct single crystal packing mode. For example, using BDOPV/F6-BDOPV as precursors gave rise to three molecules with about 1:1:1 (BDOPV: F3-BDOPV: F6-BDOPV) molar ratio and antiparallel cofacial stacking, while using F3BDOPV as precursor gave a molar ratio of about 3:1:6 and slipped stacking (Figure 2c2).21,22

H3-isatin-EH leaving) are irreversible, serving as a thermodynamic driving force for the fragment-exchange reaction. In contrast, for H3-isatin-EH and BDOPV, the whole process is totally symmetric and reversible. The exchange reaction ends when the hydroxyl drops off (the reversed process for initial activation of BDOPV). This mechanism explains the thermodynamics-driven fragment exchange between BDOPV and F3-BDOPV, more discussion about the mechanism could be found in Supporting Inforamtion part 6.

In order to fully understand such results, detailed control experiments were carried out and we found that, i) in the intial stage of crystal growing when no single crystals were precipitated, no new molecules other than precursors could be found, ii) only when protic solvents, such as isopropanol, or the solvents that may potentially contain some proton, such as chloroform (trace acidic residue in it), were added in the precursor solutions, would this phenomenon happen. In contrast, when using non-protic solvents only, such as dichloroethane, carbon tetrachloride, chlorobenzene, or acetonitrile, to grow single crystals, there was no new molecule generated, iii) even if we directly mixed the F3isatin-EH with BDOPV and added p-toluenesulfonic acid to mimic the synthetic process of F3-BDOPV at room temperature, no new molecules were obtained. Based on these experimental clues, we hypothesized that there might be “olefin metathesis” reaction during the formation of single crystals.

Following this mechanism, it is intriguing to discuss why this “olefin metathesis” reaction rarely occurs in solution but expedited by self-assembly. The key point underlies the formation of the activated BDOPV (the reactant in Figure 3). BDOPV in solution is not so electrophile to be activated by hydroxyls. However, BDOPV(+) (electron-transferred donor), once transiently formed on the nascent self-assembled surface through charge transfer with F6-BDOPV(‒)(electrontransferred acceptor), is highly electrophilic. This can be ensured by the high transfer integral between BDOPV and F6BDOPV (Figure 2g3). Although difficult to calculate theoretically, it is reasonable to assume that the formation of BDOPV(+) increases its chance of being attacked by a nucleophile like hydroxyl. The active nucleophilic reactant is then formed after the electron doublet state is quenched. Therefore, the entire solid-state “olefin metathesis” reaction can be viewed as a novel self-assembly-induced self-catalysis.

The possible mechanisms for solid-state “olefin metathesis” reaction were investigated by DFT calculations using BDOPV/F6-BDOPV co-crystal as a example. The DFT calculation first ruled out the direct nucleophilic attack by BDOPV onto H3/F3-isatin-EH, since BDOPV (either as electron donor or acceptor) is not a good nucleophilic reagent (Figure 3c). Given the experimental observation that fragment exchange only occurs when nucleophile (eg. F6-BDOPV) is present, we assumed that BDOPV at the nascent crystal surface might be activated by some nucleophiles hence becomes a strong nucleophile. We then started step-wise mechanism calculations based on a possible activated BDOPV form (termed as reactant in Figure 3), which is formed after BDOPV being nucleophilically attacked by hydroxyl (from trace water in ambient condition). This nucleophilic reactant can attack either the F3-isatin-EH (Figure 3c) or H3isatin-EH (Figure 3d) through a rate-limiting step, and yields an intermediate complex. As F3-isatin-EH is more electrondeficient, we found that the intermediate complex formed from it (IM1) is relatively more stable than the one formed from H3-istain-EH (IM0). More importantly, IM1 spontaneously undergoes a proton transfer through intramolecular hydrogen bonding to reach IM2, which is an even more stable intermediate and a prerequisite for the subsequent exothermic leaving of the original H3-isatin-EH segment on BDOPV reactant. As seen in Figure 3, for F3isatin-EH and BDOPV, the last two steps (proton transfer and

Figure 4. (a) UPS spectra for nine BDOPV derivatives, 5



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ionization energy are depicted as short gray solid lines. (b, c) All the energy level data for nine BDOPV derivatives.

interactions could also be inferred from the solid-state “olefin metathesis” reaction on the surface.

Fine Energy Level Tuning. To evaluate the energy level changes among all the single crystals with different components, UV-vis spectroscopy (UV), cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS) and theoretical simulation were all conducted in order to crossvalidate (Figure 4, S14-S18, and Table S2). Samples for UPS measurement were prepared by thermal evaporation of corresponding single crystals onto silicon substrates. To verify the correlation between thin films and single crystals, high-resolution X-ray photoelectron spectra (XPS) were performed for all these evaporated thin film samples. Inspection of the O(1s), C(1s), N(1s), F(1s) and Cl(2p) regions of the high-resolution X-ray photoelectron spectra (Figure S19) evidenced that the chemical environments and component are consistent with single crystals in all materials. Ionization energy (IE) obtained from UPS characterization increases as more halogen atoms are introduced from 5.94 eV for BDOPV, to 6.38 eV for BDOPV/F6-BDOPV and 6.51 eV for BDOPV/Cl6-BDOPV, and further to 6.94 eV for F6-BDOPV, while keeping the edge shapes of the spectra almost identical (Figure 4a and S16 and S17). Electron-withdrawing property of halogen atoms stabilize the HOMO level by decreasing the electron density, thus showing higher IE energy. It is noticed that the energy level shift obtained from UPS is larger than that from CV measurement. Such phenomenon may be due to both the different charge-quadruple interaction between different crystals and the difference between UPS and CV measurement. We calculated the quadruple moments of BDOPV, F6-BDOPV, and Cl6-BDOPV (Table S3). It is found that the quadruple moments of BDOPV are much stronger than those of F6-BDOPV and Cl6-BDOPV. Thus, the chargequadruple interactions would be stronger for BDOPV, leading to larger polarization energy and up-shift of the HOMO level.10 The difference between UPS and CV has been carefully studied in previous works and the reasons for it include both the solvent effect and image charge effect in the conductive working electrode23–25.Although the tuning range of HOMO levels between pure molecules and alloys is not wide enough, yet this is the first time energy level tuning was realized in organic alloy single crystals, which creates a new strategy towards rational molecular design for both single crystal engineering and energy level tuning. The HOMO distributions of neat single crystals are fitted by single Gaussian functions, while those of alloy single cryrstals are described by a superposition of multiple Gaussian peaks. The molar ratio of multicomponenets determined by HPLC was used as the intensity ratio of these peaks. Fitting results showed that the difference between IE of BDOPV and X6BDOPV in alloy single crystals is smaller than the difference in neat single crystals. This could be explained by the reason that electrostatic interactions between two adjacent molecules shift the on-site energies of the HOMOs of BDOPV and X6-BDOPV towards each other. Such strong electrostatic

Although fluorine atom has the largest Pauling electronegativity, compared with F3-BDOPV-1 (Figure 2b2), Cl3-BDOPV-1 (Figure 2d2), with the same single crystal packing mode, showed even lower HOMO level, due to the fact that empty 3d orbital of chlorine can accept π-electrons from conjugated backbone. In contrast, Cl6-BDOPV showed HOMO level higher than that of F6-BDOPV. IE has been reported to be orientation dependent, originating from different charge-quadrupole interactions.10 The antiparallel cofacial stacking of Cl6-BDOPV results in stronger intermolecular interaction and more efficient π-orbital overlap, thus leading to reduced bandgap and higher HOMO level. Therefore, another advantage of chlorinated BDOPV series, if compared with fluorinated one, is that all of them showed the same antiparallel cofacial stacking in single crystals thanks to the charge distribution engineering.

Figure 5. Summary of carrier mobility, calculated transfer integral from single crystal structures, and LUMO levels of the BDOPV derivatives. Single Crystal Packing and Energy Level-Dependent OFET Device Performance. In order to evaluate the influence of energy level and molecular packing mode on OFET device performance, FET devices with single crytals as channel materials were fabricated (Figure S20, S21 and Table S4). Single crystals grown from solutions were directly deposited onto BCB coated SiO2/Si surfaces through spin-coating. Channel direction is along the preferential growth direction of the single crystal, which is also the π-π stacking direction of the molecules. Au was used as the source and drain electrodes, the work function of which is around 4.9 to 5.1 eV.26 The operation process of OFET devices starts from charge injection from source electrode and followed by charge transport through the semiconductor layer. Therefore, the apparent carrier mobilities obtained from OFETs are highly related to both the energy levels and the π orbital overlap. Over twenty devices were measured for each single crystal. The histograms of electron mobilities for them were shown in Figure S21. All the BDOPV derivatives with antiparallel cofacial stacking showed relatively higher transfer integral than other ones with slipped column stacking, including BDOPV, F3-BDOPV-2 and F6-BDOPV (Figure 5). As a consequence, the electron mobilities of these single crystals 6


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Journal of the American Chemical Society measured under ambient conditions also followed the similar trend, yet some carrier mobility data cannot be solely explained by transfer integral levels. For example, although F3-BDOPV-1 has slightly higher transfer integral of 182.2 eV than that of Cl3-BDOPV-1(174.9 eV), yet its carrier mobility (1.13 cm2 V-1 s-1) is obviously lowered than its chlorinated counterpart (1.95 cm2 V-1 s-1). This is due to the fact that LUMO level is also determinative to apparent mobility. When two single crystals showed similar transfer integral, the one with lower LUMO level will facilitate the electron injection from Au electrode, thus realizing in higher FET carrier mobility, such as the case with F3-BDOPV-1 and Cl3-BDOPV1. In the comparison between F6-BDOPV and F3-BDOPV-1, low LUMO level of F6-BDOPV caused by more fluorine atoms substitution compensate the negative effect of its smaller transfer integral caused by the slipped molecular packing mode, and therefore comparable carrier mobilities of around 1.15 cm2 V-1 s-1 were observed. Such phenomenon indicates the equal importance of packing mode-determined transfer integral and also the chemical structure-determined energy level in the FET carrier mobility. It is noticed that different from the semiconducting properties of all the other single crystals, Cl6-BDOPV single crystals exhibit conductivity of 0.059 S/cm (average value of 0.046 S/cm) and a mobility of 0.1 cm2 V-1 s-1. The conducting property is probably due to different electronic strcutures or unespected doping from the significantly lowered LUMO levels of Cl6-BDOPV. As for the low mobility of Cl6-BDOPV, at relatively lower doping concentration, the Coulomb potential caused by dopant ions will inhibit efficient charge transfer. On the other hand, cocrystals between F6-BDOPV/Cl6-BDOPV and BDOPV did not show FET performance, which may be caused by the discontinuous energy level distribution between adjacent molecules.27 In-depth investigation of these phenomena is ongoing in our lab, yet beyond the scope of this paper. CONCLUSION In summary, multicomponents organic semiconducting alloy single crystals are successfully obtained by charge distribution engineering strategy. Synergistic effects of energy level and transfer integral are revealed from the systematic study on a series of carefully designed molecules. Through introducing halogen atoms onto different positions of BDOPV backbone, molecules with the same charge distribution symmetry but opposite charge at terminals, such as BDOPV and F6-BDOPV, packed into antiparallel cofacial stacking mode driven by Coloumbic interactions. It is because of the high charge transfer between adjacent molecules that solid-state “olefin metathesis” reaction occurs and three molecules pack orderly into sinlge phase. Thanks to the highquality single-phase alloy formation, the energy levels of these single crystals were readily tuned, but the shapes of UPS spectra did not change. Single crystal pakcing mode and LUMO level both played vital roles to the final carrier mobilities of OFET devices. Our work provides not only a

variety of organic semiconducting alloy single crystals, but also a perfect platform to understand the charge transfer and doping mechanism in organic small molecules.

ASSOCIATED CONTENT Supporting Information Additional experimental details and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes

The authors declare no competing financial interests.

ACKNOWLEDGMENT This work is supported by National Key R&D Program of China (2017YFA0204701), National Natural Science Foundation of China (21420102005, 21790360, 21722201), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020200). The authors thank beamline BL17B1 and BL17U1 (Shanghai Synchrotron Radiation Facility) for providing the beam time. We thank Dr. Nan Zheng at South China University of Technology and Ms. Xiaoge Wang at Peking University for assistance with the GIWAXS experiments.

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Organic Semiconducting Alloys with Tunable Energy Levels

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