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.

A new alternating copolymer of dithienosilole and thienopyrrole-4,6-dione (PDTSTPD) possesses both a low optical bandgap (1.73 eV) and a deep highest occupied molecular orbital energy level (5.57 eV). The introduction of branched alkyl chains to the dithienosilole unit was found to be critical for the improvement of the polymer solubility. When blended with PC(71)BM, PDTSTPD exhibited a power conversion efficiency of 7.3% on the photovoltaic devices with an active area of 1 cm(2).

We have studied the utilization of PCDTBT, an alternating poly(2,7-carbazole) derivative, in organic bulk heterojunction solar cells. The effect of polymer molecular weight, PCDTBT:[60]PCBM ratio, and active layer thickness on the device performance is reported. The best performance was obtained when the number-average molecular weights (M n ) are around 20 kDa with a polydispersity index around 2.2. Both PCDTBT:[60]PCBM ratio and active layer thickness affect the light absorption and the charge transport properties. By optimizing these two parameters, power conversion efficiency (PCE) up to 4.35% was reached under calibrated AM1.5G illumination of 100 mW cm À2 . When blended with [70]PCBM, PCDTBT exhibited a PCE up to 4.6%.

spectra were recorded on a Varian AS400 apparatus in appropriated deuterated solvent solution at 298 K. Chemical shifts were reported as δ values (ppm) relative to internal tetramethylsilane. Differential scanning calorimetry (DSC) was performed on Perkin-Elmer DSC7 module in conjunction with the Perkin-Elmer thermal analysis controller TAC7/DX at a heating rate of 2°C/min. UV-visible absorption spectra were recorded on a Hewlett-Packard diode-array spectrophotometer (model 8452A) using 1 cm path length quartz cells. Optical band gap was calculated from the onset of the UV-visible absorption band. For the solid state measurement, oligomer solution in THF was cast on a quartz plates. Cyclic voltammetry was carried out in a two-compartment cell with a Solartron potentiostat (model 1287) with a platinum electrodes at a scan rate of 50 mV/s against SCE reference electrode in an anhydrous and argonsaturated solution containing tetrabutylammonium perchlorate (TBAP) (10 -1 M) and oligomer (10 -4 M) in THF. Materials.All starting organic compounds were purchased from Aldrich Co. and used without further purification. All solvents used in the reactions were freshly distilled under argon. The supporting electrolyte tetrabutylammonium perchlorate was previously dried under vacuum at 60°C. Syntheses of 2,5-dimethyl-4-methoxyphenylboronic acid, 1 Pd(PPh 3 ) 4 , 2 PdCl 2 (dppf) 3 have been already reported in the literature. 2,5-dimethyl-4-(2'-nitrophenyl)anisole (1).A two-necked 500 mL flask fitted with condenser, magnetic stirring bar was charged with 15.33 g (88 mmol) of 2,5-dimethyl-4methoxyphenylboronic acid, 16.18 g (80 mmol) of 1-bromo-2-nitrobenzene, 120 mL of toluene and 80 ml of K 2 CO 3 2M. The resulting solution was degassed with a vigorous flow of argon for 30 min, then 0.92 g (1 mol%, 0.8 mmol) of Pd(PPh 3 ) 4 was added under argon and the mixture was refluxed