Effects of π-spacer and fluorine loading on the optoelectronic and photovoltaic properties of (X-DADAD)n benzodithiophene-based conjugated polymers
Graphical abstract
Introduction
Organic solar cells (OSCs) are actively investigated over the past two decades due to their potential to become lightweight, flexible and low cost source of solar energy for a broad range of applications [1]. The power conversion efficiency (PCE) of OSCs depends mainly on the properties of the materials used in the active layer, e.g. conjugated polymers. Various approaches are currently implemented to design high performance conjugated polymers for photovoltaic applications [[2], [3], [4], [5], [6]]. Incorporation of alternating electron donor (D) and electron acceptor (A) moieties into the polymer backbone represents one of the most widely used strategies to design absorber materials with tailored optical and electronic properties. In particular, this so-called “push-pull” approach allows one to control frontier energy levels, band gap, charge carrier mobility of conjugated polymers as well as nanoscale morphology of their blends with acceptor components.
Within the last years, a particular attention is paid to the introduction of electron-withdrawing fluorine substituents in the polymer backbone while modifying either donor or acceptor units or even both of them [7]. Fluorine substitution was shown to be an efficient approach to optimize the optoelectronic properties of conjugated polymers, e.g. by lowering HOMO energy and thus enhancing the open-circuit voltage (VOC) of organic solar cells [[8], [9], [10], [11], [12]].Moreover, introduction of the fluorine atoms makes polymers less miscible with acceptor counterparts thus suppressing the formation of disordered mixed phases contributing strongly to the recombination of charge carriers [13]. Thus, using fluorine-loaded polymers improves active layer morphology and photovoltaic performance (mainly due to increased current density) owing to the blocked loss channel. Difluorobenzothiadiazole represents one of the most popular fluorine-containing building blocks for conjugated copolymers due to its availability, high photostability and attractive optoelectronic properties [14,15].
The introduction of additional π-spacers between the D and A units is commonly used to modulate and optimize planarity of the conjugated backbone, aggregation behavior of polymeric chains and miscibility of the electron donor polymers with the acceptor counterparts, e.g. fullerene derivatives [[16], [17], [18], [19], [20], [21], [22]]. Conjugated π-spacers crucially affect the geometry of the macromolecules and hence their optical, electrochemical, charge transport and photovoltaic properties [23].
In our previous works, we reported the design of (X-DADAD)n conjugated polymers with the extended DADAD donor-acceptor molecular framework. These polymers demonstrate considerably improved optoelectronic properties as compared to the well-known and easily accessible (X-DAD)n structures [24]. Moreover, (X-DADAD)n polymers usually demonstrate good photostability comparable to that of PCDTBT known as one of the most robust conjugated polymers [25,26]. According to the Scharber theoretical model, this type of donor materials with optimal optoelectronic properties can provide 13–15% efficiency in single-junction OSCs [27].
In this work, we synthesized four novel (X-DADAD)n polymers based on benzodithiophene (X), thiophene (D), and benzothiadiazole (A) units and investigated the effects of the thiophene π-spacer and fluorine loading in the polymer backbone on the optoelectronic and photovoltaic properties of the designed materials. The aim of the study was to evaluate the effects of each of the aforementioned modifications and find a proper balance to enable further rational design of promising materials for efficient and stable organic photovoltaics.
Section snippets
Results and discussion
The synthesis of key monomers is shown in Scheme 1. Briefly, compounds 1a and 1b were prepared using Stille cross-coupling reactions between the monomers M1 or M2 [28,29] and tributyl(thiophen-2-yl)stannane (i). Further bromination of 1a-b with N-bromosuccinimide (ii) in 1,2-dichlorobenzene afforded the monomers M3 and M4.
Conjugated polymers P1-P4 were synthesized via palladium-catalyzed (i) Stille polycondensation reaction using monomers M1-M4 and D1 (Scheme 2). The synthesis of D1 was
Conclusion
We synthesized and investigated four novel conjugated polymers based on benzodithiophene, thiophene and 2,1,3-benzothiadiazole or 5,6-difluoro-2,1,3-benzothiadiazole. The effects of fluorine substitution and introduction of π-spacer on the optoelectronic properties, charge carriers mobility, blend morphology, and photovoltaic performance were revealed. While comparing the photovoltaic characteristics of polymers P1 and P2, one can conclude that incorporation of fluorine substituents into
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.
Acknowledgement
This work was funded by the Russian Science Foundation (grant No. 18-13-00205).
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