All-Conjugated Poly (3-allkylthiophene) Diblock Copolymers: Synthesis, Self-Assembly And Photovoltaic Performances

Polymer bulk heterojunction (BHJ) solar cell is widely recognized as one of the most promising approaches for producing lightweight, flexible, low-cost devices to harvest solar energy. Extensive effort in designing new conjugated polymers and optimizing device architectures has been devoted to improving the film morphology, photovoltaic performance and stability of polymer BHJ solar cells. Conjugated block copolymer (BCP), including rod-coil and rod-rod block copolymer, exhibits excellent flexibility for tuning the bandgap of semiconducting polymers, regulating the molecular organization of donor (and/or acceptor) units, templating the film morphology of active layers, and achieving well-defined BHJ architectures. In this thesis, we focus on the synthesis, self-assembly, and utilization of all-conjugated poly(3-alkylthiophene) (P3AT) BCPs for solar energy conversion, and elucidate the correlation between the molecular organization, film morphology and photovoltaic property in poly(3-butylthiophene)-b-poly(3-hexylthiophene) (P3BT-b-P3HT, P3BHT) BCPs.First, we synthesized a series of all-conjugated P3BHT diblock copolymers with different block ratios using a modified multi-step Grignard metathesis (GRIM) polymerization, wherein the molar ratios of P3BT block to P3HT block are 2:1,1:1, and 1:2, respectively, marking as P3BHT21 (P3BT:P3HT=2:1 mol/mol, Mn= 8300 g/mol, PDI=1.3), P3BHT11 (P3BT:P3HT=1:1 mol/mol, Mn= 11400 g/mol, PDI=1.3), P3BHT12 (P3BT:P3HT=2:1 mol/mol, Mn= 15100 g/mol, PDI=1.3). The GRIM polymerization is a quasi-living chain-growth synthesis and thereby enables block ratio to be controlled by adjusting the molar ratio of respective monomers. The monomers used here were 2-bromo-5-iodo-3-butylthiophene and 2-bromo-5-iodo-3-hexylthiophene, both of which have good selectivity of activation by isopropylmagnesium chloride and hence ensure the narrow PDI.Second, we explored the self-assembly of newly synthesized P3BHT BCPs in mixed selective solvents of anisole/chloroform, and demonstrated that the use of mixed selective solvents provides an effective mean to control self-assembly of the all-conjugated P3BHT BCPs into nanostructured morphologies. Anisole is poor solvent for both P3HT and P3BT blocks, while chloroform is good solvent for them. Depending on the anisole/chloroform solvent ratio, P3BHT chains experience different kinetic pathways, yielding nanowires at a low anisole/chloroform ratio (?2:1) and nanorings at a high anisole/chloroform ratio (?6:1). The solvent and temperature effects on the self-assembly of P3BHT during cooling and subsequent crystallization were systematically explored by means of UV-vis spectroscopy, XRD, and TEM. The nanowires are formed as a direct consequence of strong interchain?-?stacking, while the formation of nanorings are governed by solvophobic interactions between conjugated blocks and the poor solvent anisole to minimize the unfavorable contacts between the P3BT block (?50?) and later P3HT (?35?) block and anisole.Third, we fabricated the all-conjugated P3BHT BCPs into polymer bulk heterojunction (BHJ) solar cells, blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). We claimed that control over the ratio of two blocks in P3BHT provides a facile approach to precisely tune the molecular organization and nanoscale morphology in polymer BHJ solar cells. In stark contrast to the power conversion efficiency, PCE, of 1.08% in P3BT/PC71BM and 3.54% in P3HT/PC71BM solar cells, an attractively high PCE of 4.02% was achieved in a P3BHT21/PC7,BM BHJ device. The ratio of P3BT and P3HT blocks was found to exert a noteworthy influence on the molecular organization of P3BHT, the film morphology of P3BHT/PC71BM blend, and the final performance of P3BHT/PC71BM photovoltaic devices. This enhanced performance reflected a synergy of finer phase separation of P3BHT21 and PC71BM and the formation of respective percolation networks of electron donor P3BHT and electron acceptor PC71BM. The P3HT block rendered the P3BHT21 chains favorable chemical compatibility for the diffusion of PC71BM molecules, allowing for finer phase separation between P3BHT crystalline domains with PC71BM domains at the nanoscale and maximizing the interfacial area of P3BHT21/PC71BM for improved charge generation. The P3BT block facilitated the self-assembly of chains into sufficient interpenetrating pathways for efficient charge transport and collection.Finally, the effects of thermal annealing and solvent vapor annealing on the performance of P3BHT21/PC71BM BHJ solar cells were theoretically analyzed by the single diode model and experimentally explored by UV-vis absorption, XRD, and AFM studies. Thermal annealing achieved a better balance between increased ordered packing of P3BHT21 chains and finer nanoscale phase separation in the P3BHT21/PC71BM blend film. In the case of CHCl3 vapor annealing, although the treatment further improved the crystallization of P3BHT21 chains as compared to thermal annealing, larger crystalline P3BHT21 domains suppressed the dispersion of PC71BM molecules in the blend film, resulting in unbalanced respective electron and hole transports and thus increased charge recombination. The present integrated study via experimental measurements and theoretical modeling may provide much physical insight into the mechanism responsible for charge generation and transport in the polymer BHJ solar cells. The single diode model demonstrated in this work can be implemented to guide the optimization of processing parameters for fabrication of high-efficiency photovoltaic devices.In conclusion, we designed and synthesized all-conjugated poly(3-alkylthiophene) BCPs with different block ratios using a quasi-living GRIM polymerization. Compared to P3AT homopolymers, a rational control over the formation of large-area ordered nanostructure and attractive electronic activity has been realized with P3BHT BCPs by judiciously controlling the compositions of two alkyl side chains in P3BHT BCPs, leading to completely different self-assembling kinetic pathways, varied stacking of polymer chains, and promoted photovoltaic performances from their homopolymer counterparts.