Benzo-fusedheterocycle Based Polymer:Synthesis And Application In Solar Cells

The absorption spectra and hole mobility of conjugated polymer donor materials are the major factors to affect the power conversion efficiency (PCE) of polymer solar cells (PSCs). This dissertation is focusing on the design and synthesis of new conjugated polymers with broader absorption band and higher hole mobility for improving the photovoltaic properties of the conjugated polymers. The main results are as follows:(1) A new low band-gap conjugated polymers—PBDFTT-C was synthesized from thieno-[3,4-b]thiophene and benzo[1,2-b:4,5-b’] difuran unit by Stille coupling reactions. The structure was verified by’H NMR and elemental analysis, the molecular weight was determined by gel permeation chromatography (GPC) and the thermal property was investigated by thermogravimetric analysis (TGA). PBDFTT-C shows low HOMO energy level of-5.27eV. The polymer film displayed broad absorption in the wavelength region from300nm to840nm with a low bandgap of1.48eV for PBDFTT-C. The field effect hole mobility of PBDFTT-C reached5.4×10-3cm2·V-1·s-1. By using1,8-diiodooctane (DIO) as the solvent additive, photovoltaic cells with the structure of ITO/PEDOT:PSS/PBDFTT-C:PC71BM (1:1.5, w/w)/Ca/Al demonstrated a power conversion efficiency of4.4%with a short circuit current of10.45mA·cm-2, open circuit voltage of0.66V and a fill factor of0.64, under the illumination of AM1.5G,100mW/cm2.(2) Three new low bandgap benzo[1,2-b:4,5-b’]difuran-based conjugated polymers, were synthesized by Stille coupling polymerization reactions. All of the polymers were found to be soluble in common organic solvents such as chloroform, tetrahydrofuran and chlorobenzene with excellent film forming properties. Their structures were verified by1H NMR and elemental analysis, the molecular weights were determined by gel permeation chromatography (GPC) and the thermal properties were investigated by thermogravimetric analysis (TGA). The polymer films exhibit broad absorption bands. The hole mobility of PBDFDODTBT:PC71BM (1:2, w/w) blend reached up to6.7×10-2cm2·V-1·s-1by the space-charge-limited current (SCLC) method. Preliminary photovoltaic cells based on the device structure of ITO/PEDOT:PSS/PBDFDODTBT:PC71BM(1:2,w/w)/Ca/Al showed an open-circuit voltage of0.69V, a power conversion efficiency of4.5%and a short circuit current of9.87mA·cm-2.(3) Two new alkylthienyl substituted benzodifuran(BDF)-based polymers, were synthesized by Stille coupling polymerization reactions. The two polymers were found to be soluble in common organic solvents with excellent film forming properties. The polymers exhibited tunable absorptions and energy levels on incorporation of different electron accepting units. The copolymers blends with PC7)BM exhibited mobility as high as10-1order by the space-charge-limited current (SCLC) method. Preliminary photovoltaic cells based on the device structure of ITO/PEDOT:PSS/PBDFTDTBT:PC71BM(l:1.5, w/w)/Ca/Al showed a power conversion efficiency of6.0%with a high open-circuit voltage (Voc) of0.76V and a short circuit current density(Jsc) of12.04mA·cm-2.(4) Three new alkylthienyl substituted benzodithiophene (BDT)-based polymers, were synthesized by Stille coupling polymerization reactions. All of the polymers were found to be soluble in common organic solvents such as chloroform, tetrahydrofuran and chlorobenzenewith excellent film forming properties. Their structures were verified by elemental analysis and NMR spectroscopy, the molecular weights were determined by gel permeation chromatography (GPC) and the thermal properties were investigated by thermogravimetric analysis (TGA). The polymers exhibited tunable absorptions and energy levels on incorporation of different electron accepting units. All the copolymers showed high field hole mobility up to10-2order, and their blends with PCBM exhibited mobility as high as10-1order by the space-charge-limited current (SCLC) method. Preliminary photovoltaic cells based on the device structure of ITO/PEDOT:PSS/PBDTTDTBO:PC71BM(1:1.5, w/w)/Ca/Al showed a power conversion efficiency of5.9%with a high open-circuit voltage (Voc) of0.84V and a short circuit current density(Jsc) of11.45mA·cm-2. To the best of our knowledge, this is the highest efficiency for dithienyl benzooxadiazole (DTBO)-based polymer solar cells.(5) A new building block of naphtho[1,2-b:5,6-b’]difuran(NDF)with alkyl chains was firstly designed and synthesized. NDF was used as the electron donor unit in constructing a new low band gap copolymer(PNDFDTBT)by a Stille cross-coupling reaction. PNDFDTBT was characterized by NMR spectroscopy, gel permeation chromatography, thermaogravimetric analysis, UV-vis absorption spectroscopy and cyclic voltammetry. Hole mobility and film morphology were investigated in detail. The hole mobility of PNDFDTBT:PC7,BM(1:2, w/w)blend is up to7.4×10"2cm2·V-1·s-1by the space-charge-limited current (SCLC) method. A polymer solar cell with the configuration of ITO/PEDOT: PSS/PNDFDTBT:PC7,BM(1:2, w/w)/Ca/Al demonstrates a promising power conversion efficiency (PCE) of4.5%under the illumination of AM1.5,100mW/cm2. The preliminary and encouraging results in this work show that NDF probably is a new and promising electron donor unit to develop high performance optoelectronic polymers.