Influence of alkyl side chain length on the in-plane stacking of room temperature and low temperature cast poly(3-alkylthiophene) thin films
Graphical abstract
Introduction
Polythiophenes (PTs) are one of the important conjugated polymers that are widely used to fabricate light-emitting diodes, field-effect transistors and solar cells due to their unique optical and electrical properties [1], [2]. However, the insolubility of PTs in organic solvents such as chloroform, toluene, xylene, trichlorobenzene, etc., impedes the fabrication of low-cost electronic devices through large scale solution processing techniques such as, spin coating, drop casting, dip coating and printing. Generally the solubility of conjugated materials in organic solvents is readily obtained by the use of alkyl side chains grafted on the conjugated skeleton of the polymer and small molecules [3], [4], [5]. The solubility of polymer increases with the number of carbon atoms present in the alkyl side chain (CnH2n+1). Despite the improved processing that is thus obtained, the presence of side chains on the conjugated backbone impacts strongly on the crystallization and molecular packing of the conjugated backbones. In particular alkyl side chains can: (i) modify the orbital overlapping [4], [6], (ii) influence the flexibility of conjugated backbones [7], [8], (iii) increase of alkyl side chain flexibility and coil conformation with its length [4], [9], and (iv) increase of π–π stacking distance (it will be denoted as d020 hereafter) when the poly(3-alkylthiophene) (P3AT) is grafted with more than 10 number of hydrocarbons [10].
The other notable disadvantage of alkyl side chain grafting is the hindrance in charge transport along the direction of alkyl side chain stacking (lamellar stacking). This insulating property was found to increase with the alkyl side chain length [11]. Therefore, the organic semiconductors demand face-on and edge-on oriented crystallites for the realization of efficient organic solar cells [12] and organic field effect transistors (OFETs) [13], respectively. The face-on and edge-on oriented crystallites can be distinguished from the direction of π–π stacking which lie perpendicular and parallel to the substrate plane, respectively [14]. Fell et al. had first reported the existence of those two kinds of orientations in P3AT thin films [15]. Various methodologies were devoted to increase the content of edge-on crystallites of P3ATs on the substrate via epitaxial crystallization [16], substrate silanization [17], solvent vapor annealing (SVA) [18], post-growth annealing [19], [20], [21], etc. In addition to the above-mentioned orientations, the π–π stacking of polymers also plays a vital role on their optical and electrical characteristics [3], [13], [22]. So, designing the devices with strong interchain interaction is also essential for the charges to hop from one chain to another, where the growth along thiophene backbone axis of high molecular weight (HMW) P3AT is restricted due to backbone folding [20], [23]. Within the P3AT family, poly(3-hexylthiophene) (P3HT) is a most-widely studied system because of its remarkable electrical and optical properties. Morphological and structural studies on other P3ATs are relatively scarce [9], [10], [11], [24], [25], [26]. A few studies on P3ATs have been conducted to understand the role of alkyl side chains on the packing [24], [27], crystallization [28], influence of growth parameters on the structural anisotropy of P3ATs [29], physico-chemical properties of P3ATs (such as crystallization temperature, melting point and phase transition temperature) [30], and identification of best P3AT polymers for the fabrication of efficient OFETs [26] and solar cells [25].
Though quite considerable amount of structural studies were conducted on the P3ATs that are grafted with various alkyl side chains, most of the structural studies were conducted on the P3ATs that are grafted with even number of hydrocarbons [9], [24], [27], [31]. However, still further investigations are necessary to address the following: (i) reason for the random orientation of P3AT crystallites and increment in associated face-on crystallites on increasing the alkyl side chain length, which is commonly observed in the spin coated and drop cast thin films, [3], [20], [26] and (ii) the role of alkyl side chain length on the π–π stacking of P3AT crystallites. In order to address these issues, we have chosen a series of four P3AT polymers by sequentially increasing the alkyl side chain length from n-pentyl till n-octyl, namely poly(3-pentylthiophene) (P3PT), P3HT, poly(3-heptylthiophene) (P3HeptT), and poly(3-octylthiophene) (P3OT). This sequence is also essential to understand the real role of alkyl side chains on the packing of thiophene backbones. In addition, highly textured thin films were prepared by drop casting at −30 °C in order to understand the real role of alkyl side chains on the packing of P3ATs. X-ray and electron diffraction analyses were carried out under various geometries, mainly to investigate the structural properties of P3AT thin films and to assimilate their optical and electrical characteristics.
Section snippets
Experimental methods
HMW P3ATs with >99% regioregularity (rr) were synthesized using the well known method of Loewe et al. [32] The weight average molecular weight (Mw) and polydispersity index (PDI) of the P3AT polymers are presented in Table 1. The melting point, isothermal annealing temperature (Tis = melting point – 45 °C) and annealing duration of the investigated polymers were examined and reported in our previous articles [14], [20].
Effect of cast temperature and alkyl side chain length on the orientation of P3AT films
2D X-ray diffraction patterns of RT as-cast and annealed P3AT thick films (≈400 nm) are shown in Fig. 1. All the Bragg reflections were indexed using the monoclinic unit cell proposed by Kayunkid et al. (a = 16 Å, b = 7.8 Å, c = 7.8 Å and γ = 85°) [38] and Joshi et al. (a = 17.2 Å, b = 7.8 Å, c = 7.6 Å, α = γ = 90°, β = 105°) [19] for the HMW P3HT with slight modifications (a = 16.7 Å, b = 7.8 Å and c = 7.6 Å; α = β = 90° and γ = 85°). The calculated and the measured “d-spacing” values and the corresponding Miller indices of P3HT are
Conclusion
The thorough structural analysis conducted on the poly(3-alkylthiophene)s (P3ATs) by gradually increasing the alkyl side chain length from n-pentyl till n-octyl allow us to conclude that the alkyl side chains heavily impact the self-organization of P3ATs. Especially, the augment in solubility of P3ATs with the alkyl side chain length found to produce randomly oriented crystallites while casting at RT due to the latest initiation of nucleation in P3OT as compared to P3PT upon the evaporation of
Acknowledgements
Shabi Thankaraj Salammal is thankful to DAAD (German Academic Exchange Service) for the financial support. The authors are thankful to the beamline staffs from DELTA (BL9 beamline) for their experimental supports. Souren Grigorian thanks BMBF (Project No. 05K13PS4), Germany for financial support. Part of this work is supported by the research fund for international young scientists from the National Natural Science Foundation of China (Grant No: 51450110081) and China Postdoctoral Science
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