Fully visible-light-harvesting conjugated polymers with pendant donor-π-acceptor chromophores for photovoltaic applications
Graphical abstract
Highlights
► Two D-π-A chromophores pended polymers bearing different acceptors were synthesized. ► The polymers exhibit intense absorptions in the whole visible light region. ► The polymers were blended with PC71BM to fabricate BHJ solar cells. ► A maximum PCE of 2.83% was obtained for the BHJ solar cell based on these polymers.
Introduction
Polymeric bulk-heterojunction (BHJ) solar cells have attracted increasing attention recently from both academic community and application-oriented companies because of their potential for fabrication of low-cost, low-weight and flexible photovoltaic devices through solution processing [1], [2], [3], [4], [5], [6], [7], [8], [9]. Considerable advances have been achieved in this field, as witnessed by the power conversion efficiencies (PCEs) improving from nearly 1% initially to more than 9% recently [10]. Polymeric BHJ solar cells usually involve the use of a nanoscale phase-separated blend of donor and acceptor materials as the active layers [1]. Generally, conjugated polymers are used as donors, while fullerene derivatives of (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM) or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) [1], [11], for instance, are used as acceptors. Because of the limited light-harvesting ability of fullerene-derived acceptors, the development of conjugated polymers with narrow band gap (NBG) for strong light-absorbing ability in a broad range to harvest as much solar radiation as possible is critical to achieve high-efficiency BHJ solar cells [3], [4], [5], [6]. Up to now, the most effective approach to obtain NBG conjugated polymers is to incorporate electron-rich (donor, D) and electron-deficient (acceptor, A) segments alternatively along the polymer main chain to afford the so-called “D–A conjugated polymer” (see Scheme 1a). Through the push–pull interaction, efficient photoinduced intramolecular charge transfer (ICT) takes place from donor to acceptor moieties upon photoexcitation, and consequently generates an absorption band in the low energy region [12], [13]. In order to extend the absorption of D–A conjugated polymers, considerable effort has been devoted to the synthesis of three-component random D–A copolymers (see Scheme 1b). Through introducing two different kinds of acceptor segments into conjugated backbone and adjusting the donor/acceptor ratios, the absorption spectra of the resulting copolymers could be effectively extended, and some copolymers with absorptions covering almost full visible light region were reported [14], [15], [16], [17], [18], [19]. However most of these three-component D–A conjugated polymers were made by one-pot reaction of different monomers through coupling reaction, it is very challenging to reproducibly synthesize the resulting polymers with well-defined chemical structure, due to the distinct reactivity of different acceptor segments for carbon-carbon coupling reaction. Consequently, the distribution of different acceptor segments along the conjugated main chain is hard to control, resulting in random structures which may greatly affect the packing of conjugated polymers and hence the charge transporting ability and device performance of the resulting polymers.
Recently, we have reported a new strategy to make NBG copolymers containing pendant D-π-A chromophores conjugated side chains (see Scheme 1c) [20]. Different from traditional linear D–A conjugated polymers, these polymers' main chains consist of alternating donor segments (such as fluorene, carbazole, silafluorene, triphenylamine, etc.), while their acceptor groups are located at the end of the side chains, which are connected with the main chains through a π-bridge. Thus, the band gaps and energy level of the resulting D-π-A chromophores pended polymers could be easily modulated by adjusting the donors on the main chains, the π-bridge or the acceptors on the side chains. Moreover, their acceptors are usually introduced by the Knoevenagel condensation between the aldehyde-functionalized precursor polymers and electron-withdrawing acceptor groups [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], which offer a useful way to develop fully visible light absorbing polymers by simply introducing different kinds of acceptors onto the same polymer main chain, as depicted in Scheme 1d. Different from previously reported linear three-component D–A conjugated polymers, different acceptors can be sequentially introduced and their contents can be well controlled by adjusting the reaction conditions. Moreover, the influence of introduced pendant acceptors on the conjugated main chains will also be greatly decreased. Based on this strategy, herein we reported the development of two kinds of D-π-A chromophores pended copolymers PFMIX and PCzMIX (corresponding to Scheme 1d), where malononitrile and 2-(1,2-dihydro-1-oxoinden-3-ylidene) malononitrile (DCNIO) were used as the acceptors simultaneously to extend the resulting polymers' absorption. For comparison, the polymers PFDCNIO and PCzDCNIO (corresponding to Scheme 1c) with DCNIO acceptor were also developed. The optical properties, energy levels, charge transporting abilities and photovoltaic properties of the resulting polymers were investigated in detail.
Section snippets
Synthesis
All reactions were carried out under argon atmosphere. 2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dioctylfluorene (M1) [31], N-(9-heptadecanyl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)carbazole (M2) [32], 2-[2-[4-[N,N-di(4-bromophenyl)amino]phenyl]ethenyl]thien-5-aldehyde (M3) [20] and DCNIO [33] were prepared according to the reported methods. Malononitrile and Pd(PPh3)4 were purchased from Alfa Aesar Chemical Co., and they were used as received. All the solvents
Synthesis and characterization
The chemical structures and the synthetic route to the target polymers were demonstrated in Scheme 2, and the reaction yields were listed in Table 1. The diboronic ester monomers M1 and M2, as well as aldehyde-containing dibromide monomer M3 were synthesized according to the previously reported method [20], [31], [32]. The two key intermediate polymers, PFCHO and PCzCHO, were prepared via Suzuki polycondensation with equimolecular amount of the mixture of respective monomers, with the yields of
Conclusions
In conclusion, derived from two aldehyde-containing conjugated copolymers based on triphenylamine alternating with fluorene and carbazole, the D-π-A chromophores pended NBG conjugated polymers PFDCNIO, PCzDCNIO, PFMIX and PCzMIX bearing pendant acceptor groups of malononitrile and DCNIO were synthesized via Knoevenagel condensation. Through introducing strong electron-withdrawing groups onto their side chains, the absorption spectra of PFDCNIO and PCzDCNIO were extended into NIR region.
Acknowledgments
The work was financially supported by the Natural Science Foundation of China (No. 50990065, 51010003, 51073058 and 20904011), the Ministry of Science and Technology, China (MOST) National Research Project (No. 2009CB623601) and the Fundamental Research Funds for the Central Universities, South China University of Technology, the World Class University (WCU) program through the National Research Foundation of Korea under the Ministry of Education, Science and Technology (R31-10035) and the DOE
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