Novel D–A–D type dyes based on BODIPY platform for solution processed organic solar cells
Graphical abstract
Introduction
Organic solar cells (OSCs) are booming as one of the promising technologies for mitigation of the energy crisis because of their potential low-cost fabrication, lightweight, color-tunable feature, and mechanical flexibility [1], [2], [3], [4]. Solution processed small molecule organic solar cells have attracted more and more attention benefited from their definite molecular weight, easy purification, and well-reproducible device performances [5], [6], [7], [8], [9], [10]. For the traditional fullerene based organic solar cell, the visible and near-infrared solar photons are normally absorbed by the blended donor material because the fullerene acceptor, typically, PC61BM or PC71BM absorbs the solar photons at the near-ultraviolet and visible range. In the solar spectrum ca. 50% of solar photons is located in the wavelength of 600–1000 nm and the highest photon flux of the solar spectrum is distributed around 600–800 nm [11]. Therefore, designing narrow-band gap (<1.6 eV, or with onset absorption beyond 800 nm) small-molecule donors are required in the field of small-molecule organic solar cells.
Boron dipyrromethene (BODIPY) dyes have attracted considerable interest in recent years owing to a unique combination of facile synthesis, stability, high absorption coefficient, and high photoluminescence efficiency [12], [13], [14], [15], [16], along with the spectroscopic and photophysical properties of BODIPYs can be fine-tuned by attachment of ancillary residues at the appropriate positions of the BODIPY core [17], [18]. The BODIPY derivatives commonly possess solution-state absorption spectra with absorption maxima exceeding 500 nm, even up to 1000 nm, by a suitable modification [17], [18], [19], [20], [21]. In addition, BODIPY inherently bears deep HOMO energy levels (<5.2 eV) [17]. Therefore, the relatively high extinction coefficient of the BODIPY core combined with deep HOMO energy levels and propensity to π-stack in solid-state, make BODIPY-based materials attractive candidates for organic solar cells [17]. However, to our knowledge, there are only several reports on organic solar cells based on BODIPY to date. Roncali et al. reported the first examples of BODIPY donors involving two styryl units along with PC61BM as the electron acceptor for solution-processed bulk-heterojunction (BHJ) OSCs [22], [23]. Subsequently, other groups reported BHJ OSCs based on BODIPY small molecules and polymers [16], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], and the best PCE based on BODIPY small molecules was only 5.05% [35]. Therefore, the development of novel BODIPY based small molecule materials with versatile structures and further obtaining insight into their potential photovoltaic application has aroused our interest.
Inspired by the strategy for constructing low-band gap materials with conjugated donor–acceptor (D–A) system [2], herein, the highly flexible BODIPY platform was selected for the development of D–A structural small molecular-based panchromatic donors. In this contribution, we present four novel D–A–D type BODIPY dyes, BDP1–4, consisting of an electron-withdrawing BODIPY as central acceptor, and different electron-donating aromatic groups, including thiophene, bithiophene, fluorine and carbazole, symmetrically flanked at 2,6-positions of BODIPY nucleus as terminal conjugated donors (Scheme 1). We expect that these chromophores would be significant in broadening of absorption and improving light-harvesting ability in the whole sunlight region. The photophysical properties and electrochemical behaviors of these dyes were studied systematically to reveal the relationships between the structures and the photoelectric characteristics. Time-dependent density functional theory (TD-DFT) calculations were performed to further understanding their structural and electronic features. Furthermore, the photovoltaic performances of these dyes were investigated in detail to insight their potential OSCs applications.
Section snippets
Materials
2,2′-((4-(Octyloxy)phenyl)methylene)bis(1H-pyrrole) (3), 5,5-difluoro-10-(4-(octyloxy)phenyl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (4) and 2,8-dibromo-5,5-difluoro-10- (4-(octyloxy)phenyl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (5) were prepared following the reported procedures [34], [36], [37]. Compound 6, 7, 8 and 9 were synthesized according to the literature [38], [39], [40]. THF and toluene were dried and distilled over sodium and benzophenone.
Synthesis and thermal property
The synthetic routes for the four novel D–A–D type BODIPY dyes are depicted in Scheme 1. For the construction of these dyes, we utilized the Suzuki and Stille cross-coupling reaction to connect two electron-donating moieties covalently to the 2,6-positions of BODIPY nucleus, where a bisbromated BODIPY 5 should be prerequisite for the synthesis. Thus, our synthetic approach started with the preparation of 5 from commercially available 4-hydroxybenzaldehyde 1 through a four-step sequence
Conclusions
In summary, a series of novel symmetrical D–A–D structured BODIPY-based dyes, BDP1–4, featuring an electron-withdrawing BODIPY motif as the central core, and several types of electron-donating segments, including thiophene, bithiophene, fluorene and carbazole, flanked at its 2,6-positions as terminal groups were successfully synthesized and characterized. These BDPs are soluble in common organic solvents, possess panchromatic absorption covering the wavelength range 300–900 nm, and have
Acknowledgments
We greatly acknowledge the financial support from the Nature Science Foundation of Guangdong Province (2014A030313630), the National Natural Science Foundation of China (51476036, 21342003), Project for High-level Personnel of University in Guangdong Province ([2013]246), the Production-Study Research Combinational Project of Guangdong Province and Ministry of Education (2012B091100296), the University Science and Technology Innovation Key Project of Guangdong Province (cxzd1148) and the
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