Zongrui
Wang
*ab,
Renping
Li
c,
Kexiang
Zhao
d,
Fei
Yu
b,
Jianfeng
Zhao
*c,
Yonggang
Zhen
a and
Qichun
Zhang
*ef
aSchool of Materials Science and Engineering, Beijing University of Chemical Technology, Chaoyang District North Third Ring Road 15, Beijing 100029, P. R. China. E-mail: wangzr@mail.buct.edu.cn
bSchool of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore
cInstitute of Advanced Materials (IAM), Nanjing Tech University, Nanjing 210000, P. R. China. E-mail: iamjfzhao@njtech.edu.cn
dCollege of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
eDepartment of Materials Science and Engineering, City University of Hong Kong, Hong Kong SAR 999077, P. R. China. E-mail: qiczhang@cityu.edu.hk
fCenter of Super-Diamond and Advanced Films (COSDAF), City University of Hongkong, Hong Kong SAR 999077, P. R. China
First published on 20th October 2021
In this work, we demonstrated that the co-crystallization strategy has offered an efficient and promising alternative route to achieve high-performance n-type semiconductors through charge-transport switching from pristine p-type systems. By using a simple “green synthesis” process through molecular “doping” with F4TCNQ into a p-type planar azaacene derivative TMIQ (0.27 cm2 V−1 s−1) host, charge transport characteristic switching occurs with a high electron mobility of 0.12 cm2 V−1 s−1 under atmospheric conditions obtained for the D–A complex TMF4TQ (cocrystal). The reasons for such switching lie in the ingenious energy level and molecular packing arrangement tailoring. Specifically, the insertion of F4TCNQ molecules has led to packing transformation from herringbone stacking (TMIQ) to a dense 2D brick arrangement and the low-lying LUMO levels (−4.55 eV) aligned to gold electrodes, thereby facilitating efficient electron injection and transport, and ensuring the air-stable nature, which is further confirmed using theoretical calculations. We believe that our work would provide new insights into high-performance air-stable n-type organic semiconductors exploration.
Doping suitable molecules into OSCs is recognized as a “green” and effective method to fine-control the transistor behavior via regulating the molecular orbitals and carrier density.25–27 In addition to the well-known effect of improving the material conductivity and device stability, molecular doping was also reported to be able to switch the charge transport nature from p-type to n-type due to the electron trap passivation resulting from the trap filling effect.28 But the resulting n-channel device was extremely unstable under ambient conditions.28 Despite the fact that air-stability could be improved by tailoring the LUMO level, the further increase in the doping ratio tends to produce the disordered molecular arrangement and impede the inherent intermolecular interactions, which instead induces ineffective transport channels. On this basis, recently, an orderly molecular “doping” strategy, i.e., co-crystallization, where the host and guest (donor and acceptor) co-assembled with a fixed stoichiometric ratio and long-range ordered packing structure, has been emerging as a promising alternative way to exploit the novel optoelectronic properties such as n-type/ambipolar transport,29 light emission,30,31 ferroelectricity,32,33 photoconductivity,34,35 and so on.36,37 Unlike the traditional synthetic procedures that require harsh and cumbersome synthesis conditions in virtue of covalent bond breaking and formation, the co-crystallization strategy is a “green synthesis” strategy where noncovalent interactions (e.g., π–π stacking, hydrogen/halogen bonding, electrostatic interactions, charge transfer interactions) act as the driving forces for cocrystal formation, which not only eliminates tedious chemical synthesis, but also enables a flexible control of molecular arrangement and energy levels. Specifically, in D–A complexes, the hybridization between the frontier orbitals of appropriate donors (D) and acceptors (A) could bring about new LUMO hybrid orbitals more aligned with the Fermi levels of gold electrodes, and when coupled with the super-exchange mechanism enabling the indirect hole/electron pathway along alternate packing columns, a charge-transport switching from hole to electron transport would be possible. In the previous work, Zhang et al. achieved charge conversion from a p-type conjugated semiconductor 2,7-di-tert-butyl-10,14-di(thiophen-2-yl)phenanthro[4,5-abc][1,2,5]thiadiazolo[3,4-i]phenazine (DTPTP) to n-type when co-assembled with tetracyanoquinodimethane (TCNQ), but unfortunately, the resulting cocrystal DTPTP2-TCNQ exhibited a relatively low electron mobility (∼3 × 10−3 cm2 V−1 s−1), which is far from meeting the requirements.38 Thus, up to now, it is still regarded as a big challenge to acquire high-performance air-stable n-type OSCs via this molecule-level “green synthesis” pathway.
Herein, the charge transport characteristics of a planar azaacene 8,8,18,18-tetramethyl-8,18-dihydroindolo[1,2,3-fg]indolo[3′,2′,1′:8,1]quinolino[2,3-b]acridine (TMIQ)34 were investigated and confirmed to be a p-type organic semiconductor with a good hole mobility of 0.27 cm2 V−1 s−1 (Fig. 1). It has been demonstrated that the large π-conjugated plane and N heteroatoms benefit from the effective intermolecular interaction, which is also conducive to guest molecule doping (co-crystallization). For this reason, electron-deficient 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecules were successfully assembled into the TMIQ host utilizing a simple and green solution-processed method (Fig. 1), where the noncovalent interactions including CT interactions, hydrogen bonding and π–π interactions act as the driving forces. An amazing charge-transport characteristic switching from hole to electron transport has been achieved through the ingenious energy level and molecular packing arrangement tailoring. The acceptor insertion on one hand has lowered the LUMO levels which favor the electron injection via the orbital hybridization of alternated arranged donors and acceptors due to the CT interactions, and also contributed to a dense two-dimensional (2D) brick stacking from the original herringbone structure. With the low-lying LUMO level (−4.55 eV) below the threshold for thermodynamically favorable electrochemical oxidation as well as the dense stacking by molecular rearrangement, a remarkable air-stable electron transport with mobility of up to 0.12 cm2 V−1 s−1 has been realized, which is among the best n-type performances in co-cocrystal systems. In-depth density functional theory (DFT) calculations confirm this charge transport transformation. This work informs that such a “green synthesis” method by molecular “doping” has offered an efficient and promising avenue for high-performance air-stable n-type OSC acquisition.
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Fig. 1 Schematic diagram of the synthetic procedure for cocrystal TMF4TQ and the corresponding chemical structures. |
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Fig. 2 (a) UV-vis-NIR absorption, (b) FTIR spectra, and (d) PXRD patterns of powders for TMIQ, F4TCNQ, and TMF4TQ cocrystals. (c) Full scan of XPS for the cocrystal TMF4TQ. |
Changes in the crystal structure and molecular packing with the incorporation of acceptors were performed and investigated by single-crystal X-ray diffraction (SCXRD, Fig. 3 and CCDC† number: 2111795). The crystallographic data are summarized in Table S2 (ESI†). Compound TMIQ crystallizes into the monoclinic system in space group P21/c with unit-cell parameters of a = 6.4601(11) Å, b = 22.900(3) Å, c = 8.6053(14) Å, α = 90°, β = 98.884(8)°, and γ = 90°. As shown in Fig. 3a, the TMIQ molecules adopt a herringbone arrangement with the π–π stacking along the a-axis direction, resulting in an efficient electronic coupling and a priority electronic channel. Because there is a relatively large structure twist in the TMIQ molecule with a dihedral angle of up to 16°, the vertical distance between the adjacent molecules along the π–π stacking is not uniform, where the shortest distance is 3.44 Å. Besides, the neighboring π–π stacking columns are communicated by the C–H⋯π interactions, providing a stable crystal structure and additional charge transport channels. After co-assembled with F4TCNQ acceptors, a very different molecular packing is obtained with the crystal structure of the cocrystal TMF4TQ adopted in space group P (triclinic system) with unit-cell dimensions of a = 8.9947(4) Å, b = 10.5212(4) Å, c = 10.7845(4) Å, α = 79.6858(13)°, β = 67.3649(12)°, and γ = 68.3715(14)°. Herein, the TMIQ and F4TCNQ molecules are alternately arranged along the c-axis in a stoichiometry of 1
:
1 via the D–A interactions and hydrogen bonding, and in the a-axis direction, the TMIQ molecules form close packing through π–π interactions, both of which resulted in a 2D brick stacking configuration that is quite different from that for a single component (Fig. 3b). Apart from this, the insertion of acceptors also leads to a tighter crystal packing structure, in which the average π–π stacking distance between the adjacent TMIQ molecules is reduced to 3.40 Å, indicating a stronger electronic coupling. Meanwhile, the good overlap between the alternate D/A pairs as well as the short intermolecular D–A distance of 3.35 Å also denote the formation of an efficient charge transport route. On this basis, it can be inferred that this kind of change in the molecular arrangement would have a great impact on the final electrical properties.
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Fig. 3 Crystal structures of TMIQ and TMF4TQ: (a) molecular structure and crystal packing for TMIQ (herringbone stacking). (b) Molecular structure and crystal packing for TMF4TQ (2D brick stacking). |
To explore the effect of energy levels and molecular arrangement regulation by F4TCNQ “doping” on the inherent charge transport properties, OFETs based on the micro/nanocrystals of the single component TMIQ and cocrystal TMF4TQ were fabricated and investigated (Fig. 4). Above all, the self-assembled behaviors of these two compounds on OTS-modified SiO2/Si substrates was discussed. Single-crystalline micro-ribbons with lengths of several tens to hundreds of micrometers were obtained by dropping their CH2Cl2 solution on the substrates (Fig. S8, ESI†). As depicted in Fig. 4a, the out-of-plane XRD patterns for TMIQ micro-ribbons exhibit intense peaks at (020), (011) and (040) within their crystallographic data, suggesting that the crystals roughly grow along the π–π stacking direction, which is also a preferred charge transport direction. Whereas, for TMF4TQ, a strong and sharp peak that appeared in the XRD profile could be indexed as (010), which indicates that the crystals grow with the ac plane parallel to the substrate, thereby inferring the charge transport likely adopting a 2D mode (Fig. 4b). Based on the as-prepared micro-ribbons, top-contact bottom-gate OFETs were constructed via evaporating gold source/drain electrodes onto the target microcrystals through “organic ribbon masks”.46 All device fabrication and characterization studies were carried out in air (relative humidity range 50%–75%) and ambient temperature. As shown by the typical transfer and output characteristics in Fig. 4, devices based on TMIQ micro-ribbons exhibited a hole-transporting feature with the maximum mobility of up to 0.27 cm2 V−1 s−1 and a high on/off ratio of 106 (Fig. 4c and e), which is well consistent with the aforementioned energy level analysis. While in contrast, as predicted, the introduction of equal proportional F4TCNQ molecules into the host has brought about an amazing conversion in the charge-transport nature (Fig. 4d and f). The devices based on the cocrystal TMF4TQ displayed the electron transport properties with μe extracted from the saturated region up to 0.12 cm2 V−1 s−1 (and 0.09 cm2 V−1 s−1 from the linear region at VSD of 30 V), which is among the best n-type performances in cocrystal systems.29,47 Besides, all the transistors showed good air stability with smooth curves under atmospheric conditions as well as the almost no-shifted threshold voltage in repeated measurements (Fig. S9, ESI†), due to the low-lying LUMO level (−4.55 eV) tailored by the dopant molecules.
To more explicitly understand this transition of charge transport characteristics and their corresponding structure–property relationships, the effective electronic couplings strongly related to the charge transport were estimated through the molecular energy-splitting method.48,49 The detailed calculation approach to compute the intermolecular transfer integrals (t) along the stacking directions is illustrated in Fig. S10–S12 (ESI†). Since it has been speculated that the π–π stacking directions are the prominent transport path as previously discussed in the pure TMIQ crystal (Fig. 5a), the transfer integrals (i) are calculated to be 12.25 meV for electrons and 28.57 meV for holes (Fig. S10, ESI†). The apparently larger thole than telectron, combined with the high-lying HOMO level (Fig. 5b), accounts for its hole-dominant transport. By comparison, the cocrystal TMF4TQ adopts a 2D transport mainly at the ac plane (Fig. 5c), involving direct electronic couplings along the π–π stacking (D–D, ii) and mixed-stacking directions (D–A–D/A–D–A, iii) via the super-exchange effect.50,51 Surprisingly, the insertion of F4TCNQ molecules, along with the evident structural rearrangement, is proven to have a significant impact on effective transfer integrals. Specifically, an obviously larger estimated telectron (35.24 meV) is obtained in the D–D directions (ii, Fig. 5c and Fig. S11, ESI†) than that of thole (17.28 meV). In addition, along the mixed-stacking direction (iii, Fig. 5c and Fig. S12, ESI†) also, there is a similar trend that a distinctly greater telectron (21.22 meV) has been observed (thole: 4.218 meV), both of which illustrate the formation of an effective electron transmission channel and thereby induce a charge transport conversion from holes to electrons. Beyond this, the downward LUMO level (−4.55 eV) aligned with the gold electrodes (Fig. 5d) has reduced the electron injection barrier, thereby collectively ensuring the high-performance air-stable electron transport.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2111795. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1tc04610a |
This journal is © The Royal Society of Chemistry 2022 |