Research paperStrategical designing of diketopyrrolopyrrole-thiophene based donor-acceptor type organic oligomers and study their transport properties: A DFT/TD-DFT perspective
Graphical abstract
Introduction
Optoelectronic devices based on organic -conjugated semiconducting materials have recently attracted a wide range of attraction. Their potential application in organic photovoltaics (OPVs), organic light emitting diodes (OLEDs), organic field effect transistors (OFETs), sensors and other materials of plastic electronics have generated prominent interest [1], [2], [3], [4], [5]. Organic semiconductors are advantageous over the inorganic counter parts due to their diverse structure, tunable properties, low cost, flexibility, light weight and compactness. Because of imbalanced behaviour of p and n-type materials in organic semiconductors, they need to overcome some problems like instability in air, less solubility, difficulty in processing, etc. [2], [3], [4], [6].
Ambipolar devices where simultaneous or alternate transportation of electrons and holes occur can overcome the above difficulties. These can be constructed by metal deposition in bilayer, one with a low ionization potential (IP) and another with a high electron affinity (EA). Other constructing materials are polymers with narrow band gap or near-infrared absorbing dyes [7], [8], [9], [10], [11]. Polymers with -conjugated systems with low band gap and tunable electronic properties, based on alternating -electron rich donor (D) and -electron deficient acceptor (A) with donor acceptor (D-A) architecture, can fulfill this purpose [4], [12], [13], [14].
At the early stage, the fullerene derivatives (e.g. ([6], [6]-phenyl--butyric acid methyl ester), ([6], [6]-phenyl--butyric acid methyl ester) and ICBA (indene- bisadduct) were used as dominant electron acceptors in the D-A type architecture due to their high electron affinity and great electron transporting property [15]. However, they have also some disadvantages viz. weak absorption in solar spectrum, difficulty in tuning energy levels and difficulty to synthesize and purify. But, these drawbacks can be easily overcome by replacing the fullerene based acceptors and preparing D-A types of organic polymers [16], [17].
Naturally occuring dyes and pigments are deserving demand from thousands of years. Presently, many of these dyes and pigments have found their use in the flourishing field of organic electronic research. In this paper, we have chosen diketopyrrolopyrrole (DPP) [18], [19], [20], which have electron accepting property. Farmen et al. discovered diphenyl derivative of DPP pigment in 1974 with a number of characteristics that make it ideal for electronic devices [21]. To increase the solubility, the DPP block can also be modified by attaching alkyl side chains on the 2,5-positions (lactam N-atoms) [22]. Also, various substituents can be attached at various positions of the DPP unit to increase its performance [4], [22], [23]. Synthesis of DPP is cheap and effortless [19], [22]. DPP also exhibits extended -conjugated framework, good photochemical, mechanical and thermal stability, strong visible light absorption capacity and hence fulfills all essential requirements to use in stable organic devices. The optical properties, - stacking and intermolecular interactions and solid-state packing of the DPP unit depend primarily on the conjugated blocks adjacent to the DPP core [22], [24]. Thus DPP can be used as acceptor in the D-A type polymers [4], [22], [25].
In this paper, we have studied the electronic and structural properties of DPP (used as acceptor unit) and thiophene (used as donor unit) based D-A complex linked by an azomethine (-N=CH-) unit. The azomethine (-N=CH-) unit is used to link the donor and acceptor part to facilitate its charge transportation. It has been already reported that use of azomethine linkage is superior to vinyl type linkage [8], [26].
The main objective of this study is to investigate the effect of different electron withdrawing and donating substituents on the optoelectronic properties designed oligomers. Particularly, the molecular geometries, structural properties, HOMO and LUMO energy levels, ionization potentials (IP), electron affinities (EA), reorganization energies (), electronic coupling matrix element (V), charge transfer rate (), hopping mobility () values [6] and absorption spectra have been investigated. We have also calculated the -coherence length [27] of the oligomers by calculating the dipole moment (), reorganization energies () and excitation energies (). In this regard, a series of substituted derivative of the D-A complex have been studied. The structure of parent monomer is designated as A. On the basis of characteristics of the substituents, D-A complexes have been classified into two groups: derivative with electron donating substituents, and (compounds 1a-5b; Fig. 1) and with electron withdrawing substituent, and (compounds 6a-10b; Fig. 1). All the substituents have been placed at the lactam-N atom and 3rd position of the thiophene unit alternatively. The monomeric structures of the studied compounds are sketched in Fig. 1. The coordinates of all the studied monomers have been provided in Table S1 (in the supplementary material).
Section snippets
Computational details
All the calculations have been carried out using Gaussian 09 program package [28]. The ground-state geometries of all the compounds have been optimized by employing the Density Functional Theory (DFT) method. It is already reported that B3LYP (Becke’s three parameter functional and the Lee-Yang-Parr functional) [5], [18], [27], [29], [30], [31] is able to create the reliable geometries for medium sized organic molecules [29]. Hence, for the ground state calculations of all the oligomers we have
Theoretical methodology
We have calculated the vertical ionization potentials (IP) and electron affinities (EA) of the studied monomers using Eqs. (1), (2) respectively. It may be noted that the calculated vertical IP and EA values are determined from the calculations of the total energy on the neutral and ionic systems [1], [6], [42].where , and are the energies of the molecules in the neutral, cationic and anionic states respectively. Suitable IP and EA values are required for efficient
Dihedral angles and values
The dihedral angle has significant effect on the conjugation of the polymers, which in turn has effect on the optoelectronic properties. It is observed that structural properties of the oligomers are largely dependent on both the steric and electronic properties of the substituents. Table 1 summarizes the dihedral angles of all the oligomers along with their substituents in both the gas and solvent phases. The dihedral angle representation of a trimer is shown in Fig. 2. From Table 1 it is
Conclusions
In this paper, we have performed a series of DFT and TD-DFT calculations on variety of organic oligomers based on DPP and thiophene unit. To envisage the structural effect on the charge transport properties different electron donating and electron withdrawing substituents are attached to the 3rd position of the thiophene unit and the lactam-N atom of DPP unit. From the study we have found that both the electron withdrawing and donating substituents improve the optoelectronic properties of the
Declaration of Competing Interest
None.
Acknowledgment
The authors would like to acknowledge the Department of Science and Technology (SB/FT/CS-077/2013), India for the financial support. The authors would like to acknowledge the University Grants Commission for UGC-BSR Research startup-grant (NO.F.30.-122/2015(BSR)). The authors would also like to acknowledge the Gauhati University for providing research facilities and financial support. The authors are thank full to Suranjana Patowary, Dibash Kalita Bikash Ch. Mushahary, Tridip Chutia, Sujan Sen,
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