Theoretical insights on the high and differential charge transfer performance of dithienyl-diketopyrrolopyrrole-based polymers as ambipolar semiconductors
Introduction
In comparison with inorganic semiconductors, organic semiconductors have advantages in the respects of low-cost, large-area, light-weight, flexible et al. They are promising materials as electronic devices including organic field-effect transistors (OFETs), organic light-emitting diodes (OLED), and organic photovoltaic cells (OPVs) [[1], [2], [3]]. Materials of π-conjugated small molecules, oligomer and polymers have received great attentions owing to their applications in thin‐film transistors [4,5]. Among these materials, a large number of conjugated oligomers containing electron-rich aromatic donor (D) and electron-deficient aromatic acceptor (A) with highly effective electron delocalization along the d-A oligomers chains have been synthesized [[6], [7], [8]]. However, most of the recent studied d-A conjugated oligomers mainly behave unipolar transport properties, or the mobility value of the ambipolar semiconductors is small. It is still necessary to explore new d-A conjugated oligomers based ambipolar materials with high charge transfer mobility for both hole and electron since ambipolar semiconductors have the potential applications as integrated circuits and complementary metal-oxide-semiconductor (CMOS) circuits [9,10].
Diketopyrrolopyrroles (DPPs) has attracted extensive attention of many scientists [[11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]].They have excellent aggregation properties, strong electron-deficient nature, and good planarity [[11], [12], [13]]. The oxygen atom in the DPP ring easily forms a hydrogen bond with the β hydrogen in the adjacent thiophene ring, [16]which enhances π-π interactions of its main chains and thus facilitates charge transport [17]. When thiophene (T) being flanked with DPP (dithienyl-DPP, DTDPP), the hydrogen bond further strengthens the planarity of the main chain, improves the intermolecular π-π stacking [7,18], and adjusts the band gap of the oligomer, which makes the oligomer exhibit prominent semiconductor properties as high efficiency field effect transistors or opto-electronic devices. Many literatures have reported the synthesis of DTDPP and bis-dithienyl-DPP (BDTDPP) compounds with thiophene being flanked and comprising various aromatic groups were designed and synthesized [16,19,20]. Terminal cyano group substituted BDTDPP (BDTDPPCN) with a hole mobility of 0.066 cm2V−1s−1 was reported by Wang and coworkers [14]. Core-expand BDTDPP-based derivatives BDTDPP benzothiadiazole(BTz) polymer (PBDTDPPBTz) exhibiting ambipolar semiconductor performances with hole and electron transfer mobility of 1.73 and 2.58 cm2V−1s−1 have been reported by Liu et al. [21] DTDPP with pyrene (Py), naphthodithiophene (NDT), benzodithiophene (BDT), dithieno[3,2-b:2′,3′-d]thiophene (DTT), and thieno [3,2-b] thieno [2′,3′:4,5]-thieno [2,3-d] thiophene (TTTT) donors showing ambipolar transport are reported by Choi et al. [8]. Recently, Liu et al. synthesized some copolymers of BDTDPP with bisthiophen (BT), thieno-[3,2-b]thiophene (TT), (E)-1,2-di(2,2’-bithiophen-5-yl)ethene (TVT), and BDT donors, which exhibit ambipolar transport characteristics with hole and electron mobility up to 4.16 and 3.01 cm2V−1s−1 for PBDTDPPTT-based OFET [19]. Therefore, DTDPP derivatives are feasible organic semiconductors and especially promising functional materials for electronic devices. Despite of the excellent potential as ambipolar semiconductor materials for these DTDPP-based polymers, the relationship between structure and property is not clear due to the hardness in accurately characterising the structure and packing models of polymers, in experimentally. Systematically studies on the structures, properties, and semiconductor performances of DTDPP-based oligomers with different conjugated length and D-A connection thus would be of great significance for understanding these semiconductors and designing new DTDPP-based semiconductors.
Many experimental and theoretical studies have indicated that introducing various aromatic donors and lengthening the π-conjugated backbone can influence the HOMO/LUMO energy level, the charge injection barrier, and molecular packing in crystals. In recent years, many studies have shown that donor units in conjugated oligomers repeat groups can affect polymer chain geometry, the effective conjugation, and solid state morphology, thereby regulating charge-transfer mobility [9,12,22]. Therefore, choosing the suitable donor and adjusting the length of the π-conjugated backbone appropriately can reduce the injection barrier, minimize the reorganization energy, and thus get high charge transfer mobilities [12]. However, the in-depth and systematic investigations about donor–acceptor DTDPP derivatives from a microscopic point of view are still lacking. Therefore, understanding the structure-property relationship is crucial for further developing high performance organic semiconductor materials.
In the present work, structures and charge transport properties of two series of total 24 donor–acceptor dithenyl-diketopyrrolopyrrole-based oligomers mDTDPPAr and nBDTDPPAr (m = 1–4, n = 1–2, Ar = TT, BT, BDT, TVT) (Scheme 1) are investigated theoretically through density functional theory and classical Marcus charge transfer theory in terms of geometries, frontier molecular orbitals, ionization potentials (IP) and electron affinities (EA), reorganization energies (λ), charge transfer integrals (V), crystal packing models, and anisotropic charge transfer mobilities.
Section snippets
Methodology and computational details
The development of quantum chemistry provides a theoretical basis for predicting organic crystal structures and properties. There are usually two mechanisms for the transfer of intrinsic charge transport in organic semiconductors: incoherent hopping mechanism [23] and coherent band mechanism [24]. At low temperature, the coherent band mechanism usually applies to highly ordered organic crystal structures in which the interaction between different sites is through strong valence bonds [25]. When
Molecular structures
The top and side views of the optimized molecular structures of all the studied oligomers in Scheme 1 are shown in Figure S1 (Supporting Information). As the oxygen atoms in the DPP ring form hydrogen bond with the β hydrogen in the adjacent thiophene (T) ring, DTDPP fragment is completely planar. The BDT, TVT, and TT groups are also planar. When BDT and TVT groups are attached to (B)DTDPP, the dihedral angles between the T of DTDPP and BDT or TVT are small that it is negligible in DTDPPBDT and
Conclusions
In summary, we have studied the molecular geometries, electronic structures and charge transport properties as organic semiconductors of two series of total 24 donor–acceptor dithienyl-diketopyrrolopyrrole-based oligomers mDTDPPAr and nBDTDPPAr (m = 1–4, n = 1–2, Ar = TT, BT, BDT, TVT) and revealed the effects of oligomer length, different Ar groups, and different Ar position on the structures and charge transfer properties. Expanding π-conjugate chain length is found to elevate the HOMO energy
Author statement
Ms Jing-Ai Qiao does most of the calculations and write the paper;
Yujing Jin does some calculations on reorganization energies and revises the paper;
Chang Liu optimizes some molecular structures and revises the paper;
Ling Luo does some calculations on IP and EA and revises the paper;
Xin Chi does some calculations on electronic transfer integral and revises the paper;
Yuexing Zhang* examines all the calculations and revises the paper throughout;
Ming-Hua Zeng* gives some suggestions on
Declaration of Competing Interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation for Distinguished Young Scholars of China (No. 21525101), the NSF of Hubei Province innovation group project (2017CFA006), the Science and Technology Research Project of Hubei Province Department of Education (D20191001), and the NSF of Hubei University.
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