Novel dithieno[3,2-b:2′,3′-d]pyrrole-based organic dyes with high molar extinction coefficient for dye-sensitized solar cells
Graphical abstract
Introduction
Since the breakthrough made by Grätzel and O’regan [1], dye-sensitized solar cells (DSSCs) have been received considerable attention over the past two decades. Compared with the traditional silicon-based solar cells, DSSC is one of the most promising next-generation photovoltaic cells due to its simple, low cost synthesis and less environmental issues. In DSSCs systems, the sensitizers play an important role and many novel ruthenium (Ru) polypyridyl complexes are explored, such as cis-dithiocyanate-N,N′-bis(4-carboxylate-4-tetrabutylammonium-carboxylate-2,2′-bipyridine)-Ru(II) (N719) that can improve the cell performance and push the efficiency above 10% under Air Mass 1.5 global (AM 1.5 G) irradiation [2]. In spite of this, the main drawback of sensitizers is the lack of absorption in the red region of the visible spectrum and also relatively low molar extinction coefficient [3]. Therefore, it is still an urgent need to explore new types of sensitizers for high-performance solar cells. Our research is focused on increasing the molar extinction coefficient of sensitizers, so that DSSCs could be made thinner and thus more efficient because of reduced transport losses in the nanoporous environment. In quest of such sensitizers we have developed three high molar extinction coefficient sensitizers, constituted of different N-substituents of dithieno[3,2-b:2′,3′-d]pyrrole (DTP) groups with specific functionality.
Rasmussen and Ogawa pioneered the use of N-alkyldithieno[3,2-b:2′,3′-d]pyrrole as a very promising fused aromatic building block for electronic materials [4]. It has a completely flat crystal structure, indicating good π conjugation across the fused rings. The introduction of DTP units imparted enhanced conjugation, high conductivity, and high charge carrier mobility for electronic materials, such as organic light-emitting diodes (OLEDs) [5], organic photovoltaic devices (OPVs) [6], and field effect transistors (FETs) [7]. Upon polymerization, poly(N-alkyl dithieno[3,2-b:2′,3′-d]pyrroles) (PDTPs) exhibit excellent stability in their oxidized state, low band gaps, and efficient red fluorescence in solution [8]. DTP analoges, such as dithieno[3,2-b:2′,3′-d]thiophene (DTT) [9], 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT) [10] and dithienosilole (DTS) [11] have been employed in materials acting as the sensitizers in DSSCs, showing excellent photophysical and photochemical properties. However, only few investigated are the related dithieno[3,2-b:2′,3′-d]pyrroles which are emerging as a useful structure for DSSC. Small molecular dyes containing DTP with high molar extinction coefficient (79,000 M−1 cm−1) was utilized in DSSCs with a solar energy conversion efficiency (η) of 3.7%, for which the main reason may be the lack of efficient donors and too large dihedral angle between the DTP moiety and thiophene, resulting in less efficient intramolecular charge transfer (ICT) [12a]. Similarly, DTP as π-bridge and high molar extinction 68,000 M−1 cm−1 was attained by Wang et al. in 2012 [12c]. Based on the considerations above, we designed and synthesized a new series of DTP-based dyes (Scheme 1) as sensitizers for DSSCs. The purpose of incorporating two cyanoacetic acid groups in dyes can be ascribed to following several aspects: to obtain higher molar extinction coefficient, red-shifted absorption, higher photocurrent and better stability compared to its monobranched analoges [12], to prevent electron back transfer from TiO2 to the oxidized dye after electron injection by the delocalization of the positive charge in the molecule [13] and to provide intimate electronic coupling between its excited state wave-function and the conduction band manifold of semiconductor. Most importantly, dyes that contain two electron acceptors and anchoring groups can generate photoinduced intramolecular charge transfer (ICT), which can inject electrons to the TiO2 conduction band. Furthermore, the double anchoring groups containing dyes also cause a downward displacement of CB as compared to the single anchoring group containing dyes [14]. The rationale for using N-substituent fluorene is, firstly that the fluorene moiety exhibits excellent performance as donor part of DSSC dyes [9](a), [15] and to ensure greater resistance to degradation when exposed to light and high-temperature [16]; secondly, the hydrophobic alkyl chain can be used to improve solubility without leading to large torsion angles between the fused benzene units and neighboring monomers; finally, alkyl chain can reduce dyes aggregation and effectively suppresses the charge recombination of the injected electrons with the oxidized species in the electrolyte for higher Jsc [17].
In this paper, the structure–performance relationship was studied, especially the effect of the different N-substituted DTPs upon the photophysical, photochemical, electrochemical properties and solar-cell performances. As expected, the HOMO level tuning could be achieved through the choice of a suitable donor group. Our results indicate that the DTP unit as the π-bridge shows better photovoltaic performance due to its high molar extinction. Moreover, the incorporation of the phenyl-substituted DTP unit helps to inhibit sensitizer aggregation when adsorbed onto TiO2 film, resulting in relatively high Voc.
Section snippets
Experimental
THF, toluene were distilled over sodium/benzophenone. Other reagents were purchased from commercial sources and used without further purification. All glasswares were infrared dry, assembled hot, and cooled to room temperature in desiccator. Transfer of liquids was done by standard syringe techniques, and all reactions were performed under a stream of dry argon. Reactions were monitored by using TLC and chromatographic separations were performed using standard column methods with silica gel
Synthesis
The synthetic routes of three dyes are shown in Scheme 1. 4-Methyl-N-(4-nitrophenyl)-N-(p-tolyl)aniline (1) was synthesized via the Ullmann coupling reaction of 4-nitroaniline and 1-iodo-4-methylbenzene in the presence of copper powder, potassium carbonate and 18-crown-6. N1,N1-di-p-tolylbenzene-1,4-diamine (2) was obtained by reduction reaction of 1. 2,2′-Bithiophene (3) was prepared from 2-bromothiophene by Pd/C-catalyzed reductive coupling reaction in a H2O-alcohol system [19]. The building
Conclusions
In summary, we have designed and synthesized three high molar extinction coefficient organic sensitizers with planar dithienopyrrole linkers (FD1–3). The introduction of DTP as π-bridge in the molecular structure not only enhances the charge carrier mobility but also significantly improves the light-harvesting ability of dyes. Among the three dyes studied, FD1 with a branched alkyl chain exhibits an extremely high molar extinction coefficient (ε) of 14.5 × 104 M−1 cm−1 and a maximum power
Acknowledgements
We are grateful to the National Natural Science Foundation of China (21272079, 20873183, 21072064), the Natural Science Foundation of Guangdong Province, China (10351064101000000, S2012010010634) and the Fund from Guangzhou Science and Technology Project, China (2012J4100003) for the financial support.
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