On the basis of theoretical models and calculations, several alternating polymeric structures have been investigated to develop optimized poly(2,7-carbazole) derivatives for solar cell applications. Selected low band gap alternating copolymers have been obtained via a Suzuki coupling reaction. A good correlation between DFT theoretical calculations performed on model compounds and the experimental HOMO, LUMO, and band gap energies of the corresponding polymers has been obtained. This study reveals that the alternating copolymer HOMO energy level is mainly fixed by the carbazole moiety, whereas the LUMO energy level is mainly related to the nature of the electron-withdrawing comonomer. However, solar cell performances are not solely driven by the energy levels of the materials. Clearly, the molecular weight and the overall organization of the polymers are other important key parameters to consider when developing new polymers for solar cells. Preliminary measurements have revealed hole mobilities of about 1 x 10(-3) cm2 x V(-1) x s(-1) and a power conversion efficiency (PCE) up to 3.6%. Further improvements are anticipated through a rational design of new symmetric low band gap poly(2,7-carbazole) derivatives.

Harvesting energy directly from sunlight using photovoltaic cells (PCs) is a very important way to address growing global energy needs with a renewable resource while minimizing detrimental effects on the environment. For this purpose, the development of polymeric solar cells has received a great deal of attention from both academic and industrial laboratories. [1][2][3] Indeed, the utilization of semiconducting conjugated polymers as active components in bulk heterojunction photovoltaic devices offers significant potential advantages over existing inorganic materials in terms of ease of processing, formation of large surface areas, and costs. For example, poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV), [4] regioregular poly(3-hexylthiophene) (P3HT), [5,6] and other polythiophene derivatives [7] have been extensively studied over the last decade, resulting in PCs with a power conversion efficiency between 3.0 and 5.0 %. However, the performances of these polymers are somehow restricted by their relatively large bandgap and the limited possibilities to modulate their physical properties by synthetic methods. New low-bandgap polymers have been developed over the years to better harvest the solar spectrum, especially in the 1.4-1.9 eV region. Promising copolymers based on fluorene units have been proposed by Andersson/Inganäs [8,9] and Cao, [10] with power conversion efficiencies between 2.0 and 2.8 %. Interestingly, the physical properties of polyfluorene derivatives can be easily modulated through the design of various alternating copolymers.[11] However, relatively low hole mobilities were reported for low-bandgap polyfluorene derivatives. Therefore, besides those recent advances, there is still a need for new polymeric materials to go beyond the 5 % efficiency of actual materials. [3,12,13] Along these lines, poly(N-vinylcarbazole) (PVK) is well known as an excellent photoconductor. [14,15] Furthermore, studies have demonstrated that PVK photoconduction increases when doped with sensitizers like 2,4,7-trinitrofluorenone (TNF) or C 60 . [15,16] In parallel, oligo-and poly(2,7-carbazole) derivatives have been successfully used in polymer lightemitting diodes (PLEDs) [17] and organic field-effect transistors (OFETs), [17,18] demonstrating good p-type transport properties. Recently, Müllen and co-workers [19] have reported solar cells with an efficiency of 0.6 % with poly(N-alkyl-2,7-carbazole), whereas Leclerc and co-workers [20] have shown an efficiency of 0.8 % with poly(2,7-carbazolenevinylene) derivatives. Moreover, in contrast with the fluorene unit the carbazole moiety is fully aromatic, providing a better chemical and environmental stability. Taking all of these results into account, the development of new low-bandgap copolymers based on carbazoles should therefore lead to interesting features for photovoltaic applications. However, poly(N-alkyl-2,7-carbazole)s generally exhibit poor solubilities and low molecular weights. [21] To solve these problems, bulky side chains are us...

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A new soluble conjugated copolymer based on 2,7‐dibenzosilole and 4,7‐dithien‐2‐yl‐2,1,3‐benzothiadiazole units has been synthesized (PBSDTBT). Bulk heterojunction solar cell devices are fabricated using this material as the donor and [6,6]‐phenyl‐C61 butyric acid methyl ester (PCBM) as the acceptor. The power conversion efficiency is 1.6% under AM1.5 illumination. This material also shows a good VOC (0.97 V). The results are quite promising considering the relatively large bandgap (1.9 eV) of this polymer.magnified image