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.

The present review gives an overview of four of the most promising classes of conjugated polymers for plastic solar cells. The latest developments on poly(2,7-carbazole)s, poly(1,4-diketopyrrolopyrrole)s, poly(thieno[3,4-b]thiophene)s, and poly(thieno[3,4-c]pyrrole-4,6-dione)s are reported. More precisely, the synthesis and the physical and electronic properties of the polymers are discussed. Devices characteristics such as the open-circuit voltage, the fill factor, the short-circuit current density and the power conversion efficiency are also addressed. In summary, this review wants to give the reader a highlight of the very latest improvements in the organic photovoltaic field.

Don't stand Stille: A direct heteroarylation polycondensation reaction was used for the synthesis of high‐molecular‐weight thienopyrroledione‐based polymers (see scheme) in an impressive yield (up to 96 %) and in only a few synthetic steps. This new method is an alternative to the standard Stille coupling reaction and thus avoids formation of toxic tin by‐products.

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Eleven new low bandgap diketopyrrolopyrrole-based copolymers have been prepared by Suzuki or Stille cross-coupling polycondensation reactions. Comonomers derived from thiophene, carbazoles, fluorene, dibenzosilole, and dithienylsilole have been investigated. The structural, thermal, optical, and electrochemical properties of all resulting copolymers have been characterized. These copolymers exhibit broad absorption extending into the near-infrared region with absorption maxima near 640-710 nm and optical band gaps ranging from 1.2 to 1.6 eV. HOMO energy levels of the copolymers vary between -5.6 and -5.2 eV whereas the LUMO energy levels are pinned between -3.9 and -3.8 eV. The combination of extending absorption into near-infrared region, optimal energy levels, and excellent mechanical and thermal properties makes this class of low bandgap copolymers very promising for photovoltaic applications.

The synthesis and characterization of new heterofluorene derivatives based on germanium are described. These germafluorene monomers have been polymerized with different aromatic comonomers. The resulting homopolymers and alternating copolymers have been characterized by size exclusion chromatography, thermal analyses (TGA and DSC), UV-vis-NIR absorption spectroscopy, X-ray diffraction, and cyclic voltammetry. These homopolymers and copolymers are air-stable and present bandgaps ranging from 3.0 to 1.6 eV. Some copolymers were tested in field-effect transistors (FETs) and bulk heterojunction photovoltaic cells (PCs). Best results in FETs were obtained with poly[2,7-(9,9-di-n-butylgermafluorene)-alt-3,6-bis(thiophen-5-yl)-2,5-dioctylpyrrolo[3,4-]pyrrole-1,4-dione], which shows a hole mobility up to 0.04 cm 2 (V 3 s) -1 with an I on /I off ratio of 1.0 Â 10 6 . For photovoltaic applications, the best results were obtained with poly[2,7-(9,9-di-n-octylgermafluorene)-alt-5,5-(4 0 ,7 0 -di-2-thienyl-2 0 ,1 0 ,3 0 -benzothiadiazole)] with a power conversion efficiency (PCE) of 2.8%.

A series of new dithieno[3,2-b:2′,3′-d]germole copolymers have been synthesized and characterized. The dithienogermole unit has been polymerized with different aromatic comonomers such as the benzothiadiazole (PGe1-C8 and PGe1-EH) and thieno[3,4-c]pyrrole-4,6-dione (PGe2). Suzuki and Stille coupling polymerizations under various conditions have been utilized. The polymers were then characterized by size-exclusion chromatography and thermal analyses (TGA, DSC), and their optical and electronic properties were investigated by UV–vis absorption spectroscopy and cyclic voltammetry. These low bandgap polymers (1.3–1.7 eV) have also been tested for photovoltaic applications; the best result was achieved with polymer PGe2, which shows a power conversion efficiency of 4.1%.

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