Elsevier

Synthetic Metals

Volume 281, November 2021, 116916
Synthetic Metals

The role of the π-bridge length in the performance of diketopyrrolopyrrole-based organic dyes for dye-sensitized solar cells

https://doi.org/10.1016/j.synthmet.2021.116916Get rights and content

Highlights

  • Novel DPP-based organic dyes with phenyl unit were synthesized.

  • The effect of π-bridge length on the properties of the DPP dyes was investigated.

  • A DPP-based dye with only one additional π-bridge unit is reported for the first time.

  • A maximum efficiency of 6.57% is achieved for the dye with a moderate π-bridge length.

Abstract

Three novel diketopyrrolopyrrole-based metal-free organic dyes (TPh, BPh and SPh) with different π-bridge lengths were designed and synthesized for dye-sensitized solar cells (DSSCs). The phenyl units were employed as the additional π-bridge for the three dyes, which located between the diketopyrrolopyrrole core and cyanoacrylic acid unit. Interestingly, it is the first time to report a diketopyrrolopyrrole-based dye with only one additional π-bridge unit (SPh). The effects of π-bridge length on photophysical, molecular planarity, energy level and photovoltaic properties of the dyes were systematically investigated. The results revealed that the prolongation of π-bridge length by introducing an additional phenyl unit results in an extra torsion in the structural framework and reduces the molecule planarity, which has a negative impact on improving the light-harvesting ability, while it is beneficial in lowering the tendency of aggregation and enhancing the excited electron injection efficiency. Finally, with a moderate π-bridge length (BPh), the best balance of overall performances is achieved, resulting in the highest power conversion efficiency (6.57%) without chenodeoxycholic acid.

Introduction

As a vital component of dye-sensitized solar cells (DSSCs), sensitizer largely affects the power conversion efficiency (PCE) and service lifespan, which can be divided into metal-complex dye and metal-free organic dye. Due to the irreplaceable advantages such as higher molar extinction coefficient, fewer environmental issues and more flexibility in molecular design, the metal-free organic dye has gained ever-increasing attention [1], [2].

It has been long known that the structural frameworks of metal-free organic dyes are usually composed of varieties of electron donor (D), π-bridge and acceptor (A) units. As a vital segment of the molecular structure of a dye, π-bridge connects the donor and acceptor and can largely affect the skeleton planarity, molecule size, energy level, photophysical and photovoltaic properties [3]. Thus far, many promising π-bridge units have been reported such as phenyl, thiophene, furan, pyrrole, oligothiophene, anthracene, Diketopyrrolopyrrole (DPP), 3,4-ethylenedioxythiophene, benzothiadiazole, benzotriazole, some large planar conjugated structures, etc. [4], [5], [6]. Among them, DPP chromophore was discovered as a reaction byproduct by Farnum and coworkers in 1974 [7]. Since then, it has been extensively utilized in the field of pigments, paints and inks owing to its strong light absorption ability, bright color, exceptional photochemical, weather and heat stability [8], [9]. Recently, Rajneesh Misra et al. summarized and discussed the photophysical and electrochemical properties of various donor/acceptor and metal functionalized DPP derivatives in detail [10], [11], [12]. Due to the unique characteristics and easy structure modification of DPP, it also has been used in some newly rising functional materials such as metal-free organic dyes in DSSCs [13], [14]. Because of the strong electron-withdrawing ability of DPP, the DPP-based dyes always show an efficient intramolecular charge transfer (ICT) process, allowing a broad and strong light absorption in the visible light region [15], [16]. However, only a part of them exhibit high photovoltaic performance while some of them show the PCEs even lower than 1% [17], [18], [19], [20], [21].

In the previous work, our group reported a series of DPP-based dyes and some critical results, noteworthy for improving the PCEs [22]. Firstly, the directly linking of donor and DPP unit can lead to a more efficient charge transfer. Secondly, 4,4′-bis(n-hexyloxy)triphenylamine (TPA) is found to be a better donor unit than 10-hexyl-10H-phenothiazine in the series of DPP-based dyes. Thirdly, the phenyl unit is proposed as an additional π-bridge besides the DPP unit, which can ideally tune the energy level and improve the anti-aggregation ability of the dye. However, an appropriate phenyl unit number, which determines the π-bridge length, still has not been discovered.

Actually, the π-bridge length significantly influences the performance of the dyes. It is well-known that the extension of π-conjugation is one of the common strategies to enhance the light-harvesting ability, including broadening of UV–vis absorption range and enhancing molar extinction coefficients (εmax), which always can improve the PCE [3], [23]. Nevertheless, the extension of π-bridge length may have some undesirable impacts such as π-stacked aggregation on TiO2 surface, hampering of electron injection, increase in molecular size and reduction of sensitizer uptake amount [24], [25]. Thus, it is quite necessary to find an ideal π-bridge length to ensure a good balance between the photophysical and photovoltaic performances. However, only limited attention has been paid to it in the case of DPP-based dyes until now. To the best of our knowledge, for the DPP-based dyes, most of them contain three additional π-bridge units (e.g., phenyl, thiophene and furan), while the dyes with two or even one additional π-bridge units are rarely reported [26], [27].

Considering the above points, three DPP-based dyes with different π-bridge lengths (TPh, BPh and SPh) were designed and prepared. As shown in Fig. 1, these dyes employed DPP unit as the π-bridge and introduced phenyl unit as the additional π-bridge, and used TPA and cyanoacrylic acid units as the donor and acceptor, respectively. The structure of these dyes is different from most of the reported DPP-based dyes, in which the additional π-bridge units are always located at both sides of the DPP ring [14], [21]. TPh has three phenyl units between the DPP core and cyanoacrylic acid unit, thus, the TPA and DPP units in TPh can be linked directly. Two or one phenyl units were inserted separately in BPh and SPh molecules. Notably, we are the first one to introduce only one additional π-bridge unit in DPP-based dyes [4], [13], [19]. In the present study, the effects of different π-bridge lengths on the molecular structure, photophysical, electrochemical and photovoltaic performances of the dyes were investigated in detail, which will indicate the potential direction for the further optimization of molecular design of DPP-based dyes.

Section snippets

Reagents and conditions

(a) tert-Butyl cyanoacetate, ammonium acetate, acetic acid, toluene, Ar, reflux, 5 h; (b) Bis(pinacolato)diboron, Pd(dPPf)Cl2, KOAc, 1,4-dioxane, Ar, 90 °C, 18 h; (c) Potassium tert-butanolate, 2-Ethylhexyl bromide, NMP, 60 °C, Ar, 24 h; (d) 4-Formylphenylboronic acid or compound 2, Pd(PPh3)4, 2 M aqueous solution of K2CO3, toluene, Ar, 80 °C, 24 h; (e) 2 M aqueous solution of hydrochloric acid, THF, 60 °C, 2 h; (f) Pd(OAc)2, (tBu)3P (10 wt% in hexane), tBuONa, toluene, Ar, 90 °C, 8 h; (g)

Conclusion

In summary, three novel DPP-based dyes (TPh, BPh and SPh) with different π-bridge lengths were synthesized, which employed 4,4′-bis(n-hexyloxy)triphenylamine as the donor, phenyl unit as the additional π-bridge and cyanoacrylic acid as the acceptor, respectively. It is found that the photophysical, electrochemical and photovoltaic performance of the dyes are evidently tuned by altering the π-bridge length. On extending the π-bridge by three phenyl units, the molecular framework of the dye (TPh)

CRediT authorship contribution statement

Xu-Feng Zang: Conceptualization, Methodology, Funding acquisition, Writing – review & editing. Haoliang Cheng: Investigation, Formal analysis. Min Chen: Investigation, Visualization. Yingying Zhang: Investigation. Tao Huang: Resources. Hui-ling Xia: Data curation, Resources, Writing – original draft.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No. 11947410), and the Zhejiang Provincial Natural Science Foundation of China (LQ21B030004).

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      The metal-free organic sensitizers received ever-increasing attention recently due to the characteristics of high molar extinction coefficients, structure diversity and low preparation cost, which generally arrange in a push-pull donor-π bridge-acceptor (D-π-A) structure. To better absorbing the sunlight, various electron-rich chromophores are incorporated into the metal-free organic sensitizing dyes as the electron donor (D), such as triarylamine [7–9], carbazole [10–12], phenothiazine/phenoxazine [13–15], indoline [16,17] and coumarin [18,19] etc. The vast majority of donors are the well-known arylamine units owing to their excellent electronic properties and favorable photoelectrochemical behavior.

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