Elsevier

Polymer

Volume 53, Issue 7, 22 March 2012, Pages 1465-1472
Polymer

Synthesis of novel narrow-band-gap copolymers based on [1,2,5]thiadiazolo[3,4-f]benzotriazole and their application in bulk-heterojunction photovoltaic devices

https://doi.org/10.1016/j.polymer.2012.02.014Get rights and content

Abstract

Two novel conjugated alternating copolymers with [1,2,5]thiadiazolo[3,4-f]benzotriazole as acceptor and 9,9-dioctylfluorene or N-9’-heptadecanyl-carbazole as donors respectively, were synthesized by Suzuki polycondensation. Both of the two copolymers have nearly ideal band gaps and show excellent absorption spectra in near infrared region. Polymer solar cells based on the blends of them and [6,6]-phenyl-C71 butyric acid methyl ester show excellent performance when using a water/alcohol soluble conjugated polymer as cathode interlayer, which exhibit a maximum power conversion efficiency of 3.17% with the short-circuit current density of 8.50 mA/cm2, the open-circuit voltage of 0.70 V and the fill factor of 41%. Our results demonstrate that [1,2,5]thiadiazolo[3,4-f]benzotriazole is a promising acceptor unit for low band gap polymer donor materials design.

Introduction

In the past decade, polymer solar cells (PSCs), especially bulk-heterojunction (BHJ) PSCs, have attracted considerable attention due to their advantages over traditional silicon-based solar cells for their low cost, light weight, and the facility for solvent process [1], [2], [3], [4], [5], [6]. Huge progress has been made in the past decade as the power conversion efficiencies (PCEs) of PSCs have been improved from 1% to more than 9% [7]. To achieve PSCs with high performance, the BHJ active layer materials, especially the polymer donor materials are very important. An ideal polymer donor material should have suitable low band gap and good π–π stacking to maximize sunlight absorption and consequently high short circuit current (Jsc), high charge carrier mobility to get high fill factor (FF). Furthermore, the highest occupied molecular orbital (HOMO) of the polymer should be low, which will enhance the resulting PSCs’ open circuit voltage (Voc). Lastly, the polymer should have excellent solubility for compatibility with solution process. To meet all these requirements, one common strategy of donor material design is the use of donor–acceptor (D–A) copolymer structure, in which an electron donor unit and an electron acceptor unit form an alternating copolymer to effectively lower the band gap of the resulting polymer [8]. Moreover, the energy levels and band gaps of the designed D–A copolymers can be readily tuned by controlling the intramolecular charge transfer (ICT) from the donor units to the acceptor units. By this approach, many D–A copolymer donor materials have been developed and exhibited promising photovoltaic performances with PCEs in the range of ∼3–7% [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24].

Benzo[1,2,5]thiadiazole (BT) has been widely used as the electron acceptor unit in D–A copolymer design due to its existence of the heteroatom interactions, excellent π–π stacking, and simple synthesis [25]. However, the electron withdraw capability of BT is not strong and the band gaps of the BT-based D–A copolymers are relative large and not optimal for efficient sunlight harvesting [21], [24], [26]. Moreover, the solubilities of many BT based D–A copolymers are relatively poor, which limit their compatibility with solution process. Thereby, much effort has been put into modification of the BT-based acceptor units to obtain D–A conjugated materials with optimal band gaps and improved solubility for solar cell applications. Bo et al. synthesized alkoxyl BT based D–A copolymers which showed better solubility and thereby improved photovoltaic performance compared to those BT-based analogous D–A polymers [27], [28], [29]. Recently, You et al. reported the D–A copolymers based on a fluorine substituted BT, which exhibited promising photovoltaic performance with PCEs exceeded 7% [30]. Besides, benzo[1,2-c;4,5-c’]bis[1,2,5]thiadiazole, [1,2,5]thiadiazolo[3,4-g]quinoxaline and their derivatives have also been developed which exhibited much stronger electron withdrawing capability than BT. However, the strong electron affinities of the resulting polymers affected the charge separation efficiencies between the polymers and [6,6]-phenyl-C61 butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM), leading to a poor device performance despite of their dramatically lowered band gaps [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]. Thereby, it is critical to find a balance between the band gaps and the electron affinities of the donor materials for PSCs application.

Benzotriazole (BTA) is another promising electron-withdrawing building block [41] for PSC and polymer light emitting device (PLED) applications. Although its electron-withdrawing capability is weaker compared with BT [42], [43], [44], the BTA has a better solubility because of the modifiability of the N atom on the triazole ring, which enable the linkage long alkyl side chains for a good solubility. To take the advantages of BT and BTA units, here we use a new electron-withdrawing building block [1,2,5]thiadiazolo[3,4-f]benzotriazole (TZBTTT), which combined both the merits of BT and BTA [45], as the acceptor in D–A system. Two novel D–A copolymers: poly [2,7-(9,9-dioctylfluoren)-alt-5,5-(4’,8’-di-2-thienyl)-6-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-f]benzotriazole] (PF-TZBTTT) and poly [N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,8’-di-2-thienyl)-6-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-f]benzotriazole] (PCZ-TZBTTT) were developed. It was found that the band gaps of the resulting copolymers were effectively lowered and their solubilities were also improved due to the linked alkyl side chains on the triazole rings. The photovoltaic properties of the resulting copolymers were investigated and the PSC devices based on these polymers exhibited good performances.

Section snippets

Measurement and characterization

1H and 13C NMR spectra were measured on Bruker DRX 300 and Varian INOVA 500NB spectrometers operating at 300 MHz and 75 MHz respectively. Chemical shifts were reported as δ value (ppm) relative to an internal tetramethylsilane (TMS) standard. The number-average molecular weights (Mn), weight-average molecular weights (Mw) and polydispersity index (PDI) were determined at 150 °C by a PL-GPC 220 type in 1,2,4-trichlorobenzene using a calibration curve with standard polystyrene as a reference.

Synthesis and characterization

The synthetic routes of the monomers and copolymers are shown in Scheme 1, Scheme 2, respectively. Monomers 4,7-dibromo-5,6-dinitro-benzo[1,2,5]thiadiazole (2), 5,6-dinitro-4,7-bis(thiophen-2-yl)-benzo[1,2,5]thiadiazole (3), 5,6-diamino-4,7-bis(thiophen-2-yl)-benzo[1,2,5]thiadiazole (4), 2,7-bis(4’,4’,5’,5’-tetramethyl-1’,3’,2’-dioxaborolan-2’-yl)-9,9-dioctylfluorene (8) and 2,7-bis(4’,4’,5’,5’-tetramethyl-1’,3’,2’-dioxaborolan-2’-yl)-N-9’- heptadecanyl-carbazole (9) were prepared according to

Conclusions

In summary, two novel [1,2,5]thiadiazolo[3,4-f]benzotriazole-based copolymers, PF-TZBTTT and PCZ-TZBTTT have been successfully synthesized. The band gaps of the resulting copolymers were effectively narrowed to ∼1.3 eV compared to previously reported BT based analogous polymers because of the induced extra electron deficient triazole groups on the benzene ring. Moreover their solubilities were also improved due to the linked alkyl side chains on the triazole rings. The photovoltaic properties of

Acknowledgments

The work was financially supported by the Natural Science Foundation of China (No. 21125419, 50990065, 51010003, 51073058, and 20904011), the Ministry of Science and Technology, China (MOST) National Research Project (no. 2009CB623601).

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