Flexible and conductive multilayer films based on the assembly of PEDOT:PSS and water soluble polythiophenes
Graphical abstract
Introduction
π-conjugated polyelectrolytes (CPEs) have been the subject of tremendous interest in the recent years as they combine the semiconducting properties of conjugated polymers with charge-mediated behavior of polyelectrolytes, thanks to their π-conjugated backbone and ionic side chains [1]. Compared to neutral conjugated polymers, CPEs are soluble in polar solvents such as water and methanol, which allows their use in a wide range of applications such as biological sensors [2], [3], biomedical imaging [4], [5], photovoltaic cells [6] and organic light-emitting devices [7], [8]. Since the conformation of the polymer backbone directly influences the optoelectronic properties of CPEs, understanding and developing self-assembly strategies to control the CPE conformation is of crucial importance [9], [10].
Self-assembly is an elegant approach for obtaining conjugated polymer nanostructures with defined orientation and size [10], [11]. In the case of CPEs, their inherent amphilicity enables their self-assembly into ordered aggregates in both solution and solid phases [12], [13], [14]. Their self-assembly may also be manipulated by combining them with oppositely charged substances such as metal ions and complexes [15], [16], [17], [18], DNA [19], [20], surfactants [21], [22], [23] and polymers [24] for building complex supramolecular structures. In this respect, electrostatic layer-by-layer (LbL) deposition process based on the sequential adsorption of oppositely charged species represents an easy and convenient strategy for obtaining CPE films with controlled composition, architecture and properties at the nanometer scale [25], [26]. Such LbL-based deposited CPE films can be exploited into organic electronics as components of organic light-emitting devices [27], [28] and polymer solar cells [29], [30], [31].
Polythiophenes-based CPEs have emerged as a fascinating class of materials for organic electronics due to their high conductivity, stability and processability [32], [33], [34]. In addition, the availability of robust synthetic protocols (e.g. Kumada catalyst transfer polycondensation [35], [36], [37]), allows the preparation of multiple polymer topographies (homopolymers, random/block copolymers) with a high degree of control over the final structure and molecular weight. The first LbL self-assembly of polythiophene-based CPEs was reported by McCullough et al. leading to structurally ordered nano-layered sheets of conducting polymers exhibiting electrical conductivities of 0.04 S cm−1 [38]. More recently, Buriak's group combined a cationic pyridinium-based poly(3-hexylthiophene) (P3HT) CPE (P3HT-R) with a colloidal suspension of poly(3,4-ethylenedioxy thiophene):polystyrene sulfonate (PEDOT:PSS) through a LbL assembly process for inverted polymer solar cell device [29], [30], allowing to improve the film-forming property and stability of CPEs.
In this present work, we examined a broader array of CPEs based on a common polythiophene backbone, but bearing different ionic end groups (P3HT-R): poly(3-[N-(1-methylimidazolium-3-yl)hexyl]-thiophene-2,5-diyl) (P3HT-Im), poly(3-[6′-trimethylammonium)hexyl]-thiophene-2,5-diyl) (P3HT-NMe3) and poly(3-[6′-trimethylphosphonium)hexyl]-thiophene-2,5-diyl) (P3HT-PMe3) (Scheme 1). The growth of the electrostatic LbL films based on these cationic P3HT-R and the colloidal suspension of anionic PEDOT:PSS were investigated using a combination of in situ optical reflectometry, profilometry and atomic force microscopy (AFM). The electrical properties of these polyelectrolyte multilayer films deposited onto rigid and flexible substrates were then evaluated by van der Pauw method and Seebeck experiments. It was in particular showed that their conductivity is not modified by applying a bending stress onto the polymer flexible substrate.
Section snippets
Chemicals and materials
Cationic polythiophenes were synthesized as previously reported [20]. First, a Kumada catalyst-transfer polycondensation (KCTP) [39] was used to prepare the regioregular head-to-tail bromide-bearing polythiophene precursor polymer (P3HT-Br). The number-averaged molecular weight (Mn) was determined by SEC analysis (Mn ∼ 13 500 g mol−1, Đ = 1.27). Finally, the bromide precursor copolymers were converted to the conjugated polyelectrolytes (P3HT-R) by treatment with 1-methyl-imidazole,
Multilayer film growth: in situ investigations
Before measuring film intrinsic properties, the growth of multilayer films based on cationic CPEs P3HT-R and the negative (PEDOT:PSS)−Na+ complex was investigated using optical reflectometry. Ionic strength (I) of polyelectrolyte solutions was fixed at 0.003 M and pH was adjusted at 5.5. After the injection during 5 min of a polymer free solution, a P3HT-R solution was injected during 10 min leading to an increase of the reflectometry output (Fig. 1) and thus, indicating the adsorption of
Conclusions
Multilayered films were successfully prepared from cationic P3HT-based CPEs bearing different ionic side groups and anionic PEDOT:PSS by using electrostatic LbL assembly. The growth of the (P3HT-R/PEDOT:PSS) LbL films was found to be not dependent on the nature of the P3HT ionic side group as shown by in situ reflectometry analysis. On the contrary, the valence of the electrolyte ions was identified as a key parameter influencing the film growth since adsorbed amount of each polyelectrolyte
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