Experimental Investigation And Theoretical Calculation Of Conjugated Copolymers For Organic Photovoltaics

During the past few years, the efficiency of polymer solar cell has been developed dramatically. The polymer materials in active layer was the main driving force to promote the development, which have a significant impact on the spectral absorption, open circuit voltage, carrier mobility and other aspects. Moreover, fluorine-containing material has been widely used in polymer solar cell, including donor/accept materials in active layer and buffer layer. A fluorine atom, which is small with strong electron withdrawing ability, affect the polymer properties in term of band gap, energy levels, the dipole moment and the efficiency of photovoltaic devices. Density functional theory (DFT) calculations play many important roles in simulating the electronic structure and related properties of polymers. A series of research work were carried out on designing, synthesis and photovoltaic properties of fluorinated narrow band gap conjugated polymers. At the same time, combined with theoretical calculations, the structure of polymer, as well as the relation between structure and polymer property were also discussed in the text. The details are given follows:1. A novel donor-acceptor conjugated polymer containing benzodithiophene with lateral thiophenyl and fluorinated4,7-dithien-2-yl-2,1,3-benzothiadiazole was synthesized by Stille coupling polymerization. The photovoltaic performance of the donor-acceptor polymer was improved as compared to the non-fluorinated donor-acceptor polymer analogue. A power conversion efficiency of4.8%have been achieved in the device based on PTBDTffDTBT/PC61BM under AM1.5G illumination (100mW/cm2) without considerable optimization. Simultaneously enhanced Jsc (12.17mA/cm2) and Voc (0.86V) have also been obtained by fusing two-dimensional conjugated donor unit TBDT and electron-withdrawing fluorinated ffDTBT into the donor-acceptor conjugated polymer.2. The polymer PTBDTffDCgTBT and PTBDTffDC8TBT-FB were synthesized by introduce alkyl side chains and pentafluorophenyl end group to fomer polymer PTBDTffDTBT. And the properties of the new polymer were compared with similar polymer PTBDTffDTBT. Meanwhile, density functional theory (DFT) was applied in the simulation on the five kinds of polymers from the initial structure PBDTDTBT to PTBDTffDC8TBT-FB. The polymer PTBDTffDC8TBT and PTBDTffDC8TBT-FB have similar good thermal stability, wide spectral absorption (band gap of1.68eV), the molecular levels and the arrangement of the polymer chains. In compared with PTBDTffDTBT, the introduction of alkyl chains resulted in a) improved solubility and increased polymer molecular weight, b) slightly higher HOMO energy level, c) increased the distance of lamellar stacking. While the organization of the three polymers were not susceptible to the heat treatment. The PCE of the device based on PTBDTffDC8TBT/PC6iBM (1:1) under AM1.5G illumination (100mW/cm2) without considerable optimization achieved2.1%with Voc of0.80V, Jsc of8.31mA/cm2and FF of32%, while the PCE of PTBDTffDC8TBT-FB/PC6iBM (1:1) reached3.6%with the Voc of0.82V, Jsc of10.45mA/cm2and FF of42%, the difference between short-circuit current and the fill factor is the primary cause of the difference in efficiency. By AFM analysis, a more appropriate micro-phase separation morphology with phase separation size of about10-20nm was existed in PTBDTffDC8TBT-FB/PC61BM active layer, the surface roughness Sq (root mean square roughness) of which was much small than PTBDTffDCsTBT (0.62nm vs1.43nm). By the DFT calculations from the initial structure BDT, DTBT, the derivative structure of the monomers and the relevant polymers, it was found that:a) two conjugated thiophene side rings really helps to expand the electron delocalization; b) the introduction of fluorine to the backbone help to reduce energy levels and improve dipole moment; c) the alkyl side chains increase the torsion degree between monomers (dihedral angle increased approximately20°); d) especially that the introduction of pentafluorophenyl end group further change the dipole moment of the molecular chain, which promote exciton split and transport in the active layer, consequently increased the short circuit current and the device efficiency.3. The structure ffDTBT was polymerized with benzodithiophene (BDT) and diketopyrrolo[3,4-c]pyrrole (DPP) to synthesized dual receptor polymer PffDTBT-BDT-DPP (1:2:1). The polymer has good solubility, film-forming property and thermal stability. The UV-visible absorption of polymer broaden to800nm with optical band gap of1.46eV. Polymer has good crystallinity which sensitive to heat treatment may increase the crystallinity. The device PffDTBT-BDT-DPP/PC61BM (1:1) as active layer to obtain short-circuit current of 2.52mA/cm2, the open circuit voltage of0.73V, the fill factor of45.3%, a low short-circuit current is the main reason to obtained of0.8%resulting in a polymer. Compared to the counterpart D-A alternating copolymer PBDTDPP, the introduction of ffDTBT increased the optical bandgap improved the open circuit voltage of the device.4. A new conjugated polymer containing6H-phenanthro[1,10,9,8-cdefg]carbazole (PC) and4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) units, so as called PPCDTBT, is synthesized based on the further modification of carbazole moieties for poly[N-9’’-hepta-decanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)](PCDTBT). The resulted polymer exhibits narrow band gap with1.77eV resulting from the broader conjugation, while maintaining low-lying HOMO energy level. The polymer geometry is severely transformed by the large fused block phenanthrocarbazole (PC). Through the density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculation at the B3LYP/6-31G(d,p) level on the polymer dimer models, a big torsion angle is the main reason for breaking the backbone coplanarity, consequently the conjugation and organization. Moreover, a different transition from the HOMO-2orbital is responsible for the absorption shoulder at short wavelength. After ordinary optimization, the best power conversion efficiency (PCE) of2.3%is achieved with preferable Voc of0.80V and Jsc of7.9mA/cm2. Additionally, for holding extended conjugation from fused carbazole-like unit and suppressing the strong torsion, naphthocarbazole (NC) and counterpart alternative polymer of NC and DTBT (PNCDTBT) are proposed and simulated, which would be more planar for better intra-and inter-molecular interactions.