Synthesis And Properties Of Novel Low Band Gap Polymers For Organic Solar Cells

Organic photovoltaic technology provides an essential way for the effective utilization of solar energy. The advantages of polymer solar cells (PSCs) include low cost, light weight, easy fabrication, flexible and tunable properties. There is increasing interest in PSCs during the last few years, however, the power conversion efficiency (PCE) of PSCs far beyond practical requirements. Up on this, Design and synthesis of appropriately low band gap polymers and probing into the mechanisms become the extremely interesting topics in the field of high efficiency PSCs. Based on these, several novel low band gap polymeric materials were successfully prepared. Besides, the PCE of the materials were also studied and some positive and original results were obtained.This thesis was divided into eight parts, as follows:Chapter1:A serials of cyclopentadithiophene-based conjugated polymers with varied alkyl side-chain patterns and fluor-substitutions were successful designed and synthesized. All of the polymers exhibit strong absorptions and extremely narrow band gaps (Eg<1.5eV). The poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b’]dithiophene)-alt-4,7-(monofluoro-2,1,3-benzo-thiadiazole)](PCPDTFBT) with short side chain and mono-fluro substitution had the shortest π-π stacking distances (3.8A) and the highest mobility (0.014cm2V-1s-1). The bulk heterojunction solar cell devices based on this polymer showed the highest PCE of6.6%in single junction solar cells and8.2%in double junction solar cells.Chapter2:Two fused-rings ladder-type conjugated polymers were designed and synthesized. The fully fused PIDTCPDT-DFBT possesses lower band-gap, better planarity and lower reorganizational energy, and one-order higher hole-mobility. The charge separated configuration of PIDTCPDT-DFBT was found to be lived up to1.52ns in the chlorobenzene. The solar cells made from PIDTCPDT-DFBT also showed higher power conversion efficiency of6.46%. The short circuit current (Jsc) also increased～40%from10.40mA/cm2for a partially fused reference polymer, PIDTT-T-DFBT to14.59mA/cm2. This is among one of the highest Jsc reported for the ladder-type polymers. These results showed the strategy of extending conjugation length in fused-ring ladder-type polymers was an effective way to reduce band-gap and improve charge transport for polymers to obtain higher photovoltaic efficiencies.Chapter3:Three two-dimension polymers PBDT-DPP, PBDTTT-DPP and PBDT-TTDPP were successfully designed and synthesized. The change from thiophene to thieno[3,2-b]thiophene (TT) in the side chain and bridge caused variety of absorption, electrochemical and hole transport property. The results from UV-Vis measurements showed that enhanced absorption coefficient could be obtained when TT unit introduced as a bridge, which in turn could absorb light efficiently. The photovoltaic properties of these polymers were investigated using the device configuration of ITO/PEDOT:PSS/polymer:PC71BM/Bis-C6o/Ag. The highest achievable PCE for PBDT-DPP, PBDTTT-DPP, and PBDT-TTDPP were4.06%,4.47%, and5.34%, respectively. It was worth noting that the quality of the blending films played an important role in exciton separation and diffusion. The morphology of these films improved significantly due to the co-solvent processing, which leading to the remarkably increased PCE of these devices.Chapter4:A new Se-containing electron deficient building block monofluro-2,1,3-benzoselenadiazol (FBSe) was developed. By using a microwave-assisted palladium-catalyzed Stille polymerization, two novel FBSe-based low band gap polymers, PBDT-T-FBSe and PIDT-T-FBSe were successfully synthesized. Both of these two polymers showed narrow band gap of1.60and1.58eV for PBDT-T-FBSe and PIDT-T-FBSe, respectively. The hole mobility of PBDT-FBSe and PIDT-FBSe were1.1×10-4and3.0×10-3cm2V-1s-1, the PCE of PBDT-T-FBSe and PIDT-T-FBSe were4.65%and5.00%.Chapter5:Three quinoxaline-based conjugated polymers were successfully synthesized. All of these polymers showed good solubility in common organic solvent. An increased HOMO levels were observed in the alkoxy functionalized polymers and believed to be the result of the strong donating nature of alkoxyphenyl side-chains. In addition, the longer alkoxyphenyl side-chains resulted in large steric hindrance, which prohibited the exiton separation and dissociation. Up on this, the PSCs devices based on poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta-[2,1-b;3,4-b’]dithiophene)-alt-6,7-difluoro-2,3-bis-(3"-octyloxyphenyl) quinoxaline](PCPDT-DFPhQ-O) only exhibited low Jsc of2.52mA/cm2and PCE of0.94%. On the contrary, the polymer of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b’]dithiophene)-alt-6,7-diflu oro-2,3-bis-(3"-phenyl) quinoxaline](PCPDT-DFPhQ) with short side chains showed high Jsc of12.05mA/cm2and PCE of5.30%. Chapter6:Two novel region-regular alternating conjugated polymers with a D-A1-D-A2structure in which CPDT acts electron donor and DFBT/DPP or DFBT/TPD as electron acceptor were prepared. The incorporation of two electron-deficient units in the structure of terpolymer could enhance solubility, adjust band gap, optimize stack property, and change the electron distribution in the system. Both of these polymers had good solubility and abroad absorption. The PSCs devices based on these two polymers showed the PCE of3.15%and3.11%for PCPDT-DFBT-TPD and PCPDT-DFBT-DPP. These results provided new insight into designing new generation CPs for light-emitting diodes, field-effect transistors, solar cells and other optoelectronic devices.Chapter7:The results from chapter2to chapter7were summarized.