Structure-properties relationships in triarylamine-based push-pull systems-C60 dyads as active material for single-material organic solar cells
Introduction
Organic photovoltaic (OPV) cells have been subject to a continuing research effort for almost forty years [1]. Starting with Shottky diodes delivering short-circuit current densities of a few micro-amps, the performances have been progressively improved, marked by several milestones such as the first vacuum deposited donor-acceptor bilayer planar heterojunction in 1986 [2], the invention of solution-processed polymer-based bulk heterojunction (BHJ) [3], the use of fullerenes as acceptor materials [4], the introduction of soluble molecular donor materials [5] and the recent emergence of non-fullerene molecular acceptors [6,7]. Thanks to these cumulative synergistic research efforts, the PCE of BHJ cells is now approaching that of silicon solar cells with values close to 18% [8]. In spite of this impressive progress, the industrialization of OPV cells is still hindered by several problems such as the cost and scalability of active materials [9,10] and the insufficient stability of BHJ cells [11,12]. The fabrication of highly efficient BHJ cells implies the achievement of optimal nano-phase separation of the donor (D) and acceptor (A) materials by optimization of many parameters such as composition of feed solution, solvent, conditions of film processing, additives and application of thermal and/or solvent annealing [[13], [14], [15]]. However, the optimized morphology of multi-component BHJs is thermodynamically unstable and undergoes a progressive phase separation of D and A accompanied by a decrease of PCE [[11], [12], [13]].
Single material organic solar cells (SMOSCs) combining D and A parts capable to ensure the elemental processes of light absorption, exciton dissociation and charge-transport can represent a definitive solution to the problem of morphological instability. Furthermore, SMOSCs present major potential advantages in terms of simplicity of fabrication and cost of OPV cells [16,17]. However, the extremely complex fundamental and technical problems posed by the design of active single materials and the widely accepted opinion that fast charge-recombination and inefficient charge transport definitely limits the efficiency of SMOSCs have strongly inhibited research effort in this direction. The design of SMOSC materials resort to two main approaches which basically differ by the degree of intramolecular interaction of the D and A blocks. Fully conjugated molecular D-A systems probably represent the ultimate stage of simplification of SMOSCs and hence of OPV cells but very few examples are known with PCE of 0.40–0.80% [[18], [19], [20]] while the best value reported so far is only of ~1% [21]. Until now, the most widely investigated and most efficient SMOSCs are based on the BHJ model, namely with D and A blocks linked by a flexible insulating spacer allowing the self-organization and nano-scale segregation of the D and A phases. Due to a limited research effort, the highest efficiency of SMOSCs has for a long time stagnated to values of 1.00–1.50%. However, the past few years have witnessed an acceleration of research and several recent publications reported PCE above 3.0% [[22], [23], [24], [25]] while quite recently, Park et al. reported a PCE of 5.28% for a cell based on a D-A block copolymer [26], while Weiwei Li and coworkers synthesized double-cable polymers leading to SMOSCs with PCE up to 6.30% [27,28].
In a constant search of scalable active OPV materials combining simple structure and low synthetic complexity we have developed a systematic analysis of structure-properties relationships on simple model systems based on small push-pull molecules involving arylamine donor blocks connected to an electron-withdrawing group by a thienyl spacer [29]. For instance, we have shown that replacing a phenyl group of triphenylamine by a β-naphtyl group (Nph-A) (Chart 1) leads to a five-fold increase of hole-mobility and improves the PCE of a simple D/A bi-layer planar heterojunction from 2.50 to 3.40% [30]. In the frame of the extension of this approach to the synthesis of active materials for SMOSCs, we recently reported a dyad with a fullerene C60 acceptor unit linked to a donor block derived from Nph-A through a cyanoester group (ECV-C60) (Chart 1). Evaluated in a simple device (ITO/ECV-C60/Al), this compound gives a short-circuit current density (Jsc) of 1.90 mA cm−2 and a PCE of 0.40% [31], while similar results were obtained with other analog dyads [[31], [32], [33]]. In an attempt to improve these results, we report herein the synthesis of a dyad based on Nph-A and C60 but using a different mode of connection of the D and A blocks. Thus, instead of attaching the C60 unit at the 2-position of thiophene through a cyanoester group like in ECV-C60, we use the 3-position of the thiophene ring. Although this strategy requires a more complex synthetic approach, it presents two major advantages namely i) the problem of E/Z isomerism around the double bond connecting the thienyl ring and the cyanovinyl ester [34] is eliminated and ii) the presence of a free aldehyde group at the 2-position of thiophene during the synthesis (Scheme 1) offers the possibility to modulate the electronic properties of the donor block. This approach is illustrated here with DCV-C60 in which the cyanoester of ECV-C60 is replaced by the stronger electron-withdrawing dicyanovinyl group. The synthesis of the new dyad is described and its electronic properties are analyzed by UV–Vis spectroscopy, cyclic voltammetry and theoretical calculations. The results of a preliminary evaluation of DCV-C60 as active SMOSC material are discussed with reference to ECV-C60 and model compounds of the two donor blocks.
Section snippets
Synthesis
The synthesis of the target compound DCV-C60 is depicted in Scheme 1. Bromination of 3-thienylmethanol 10 with NBS gave the dibromo compound 9 in 62% yield. Reaction of 9 with tert-butyldimethylsilylchloride (TBDMSCl)afforded tert-butyl((2,5-dibromothiophen-3-yl)-methoxy) dimethyl-silane (8) in 99% yield. This compound was then reacted with n-BuLi and DMF to give the corresponding carbaldehyde 7 in 40% yield. Suzuki coupling of the commercially available boronic ester 6 with aldehyde 7 gave
Conclusion
A new molecular dyad consisting of a triarylamine-thienyl-based push-pull π-conjugated system linked to fullerene C60 by esterification of an hydroxymethyl group attached at a β-position of the thienyl unit has been synthesized in good yield. Comparison with previously reported SMOSCs based on parent compounds containing similar constitutive blocks shows that this synthetic approach offers further possibilities of tuning of the electronic properties of the donor block and of control of the cell
Declaration of competing interest
No conflict of interest.
Acknowledgments
This work was financially supported by the project SMOSCs, ID: 37_220, Cod MySMIS:103509 funded by the Romanian Ministry for European Funds through the National Authority for Scientific Research and Innovation (ANCSI) and co-funded by the European Regional Development Fund/Competitiveness Operational Programme 2014–2020 (POC) Priority Axis 1/Action 1.1.4.
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