| Summary: | 博士 === 國立清華大學 === 化學工程學系 === 103 === Poly(cyclopentadithiophene-alt-benzothiadiazole) (PCPDTBT) was the first breakthrough on the third generation semiconducting polymer in organic photovoltaic (OPV) applications, featuring alternating donor-acceptor moieties and design of fused ring in the polymer main chain. This thesis involves two parts: the studies and improvements of PCPDTBT; the synthesis of new polymers and oligomers based on cyclopentadithiophene (CPDT) and benzothiadiazole (BT) units.
In the first part, we developed Pd-catalyzed direct arylation polymerization for the synthesis of PCPDTBT, being produced in 76% yield after Soxhlet with high number-average molecular weight (Mn) of 71700 in the presence of K2CO3, Pd(OAc)2, and pivalic acid (PivOH) in N-methylpyrrolidone (NMP) at 80 °C for 20 hours. Most importantly, OPV device of the best PCPDTBT shows improved power conversion efficiency (PCE) over the one by Suzuki coupling. The optimized reaction condition was also applied to other monomers to obtain various low-bandgap conjugated copolymers. This synthetic route allows scale-up synthesis of high performing polymers with fewer synthetic steps, higher yield, greener chemistry, and at a lower cost.
Then we worked on enhancing the stability of PCPDTBT OPV devices by synthesizing thermally crosslinkable group on the side chains on CPDT. The lifetime of the OPV was 51% enhanced over an OPV using standard PCPDTBT. This higher stability is accounted for the crosslinkable structure of the polymer after annealing. The crosslinking reaction was confirmed by solubility tests, Fourier transform infrared spectroscopy, and morphological analysis conducted by atomic force microscopy and simultaneous synchrotron grazing-incidence small-/wide-angle X-ray scattering.
We also studied on the effect of donor-acceptor ratio by synthesizing various and lengths of CPDT to BT repeating units, -[(CPDT)x-(BT)y]n-. The polymers showed optical band gap between 1.7 and 2.0 eV in film. Polymer with ratios of CPDT:BT (x:y = 1:1) units showed lowest energy bandgap, followed by x:y = 1:2, 2:1, 2:2 and 3:3. OPV devices of these polymers with PC61BM as the electron acceptor in the absence of any additives show the best PCE of 2.45% using the polymer with ratios of CPDT:BT (x:y = 2:2) units, followed by the ratios of x:y = 2:1, 1:1, 3:3 and 1:2, indicating relevance of ratios and length of donor and acceptor units in OPV performance.
In the second part, we started with the investigation of the X-ray single crystal structure of CPDT-BT-CPDT oligomer, which confirms that they are not entirely disordered, but are actually stacking directly across each other at the central BT units with an intermolecular distance of 3.61 Å, providing valuable insight into the polymer bulk structure. While showing moderate mobility of 5 × 10-3 cm2 V-1 s-1 and a PCE of 1.61%, one potential use for the oligomer could be as a sensitizer in a ternary blend with P3HT–PC61BM or PCPDTBT–PC61BM OPVs; the PCE can be relatively increased by 3–9% depending on concentration, primarily as a result of increased short circuit current density.
Then a series of polymers containing “CPDT-Acceptor-CPDT” structure has been designed and synthesized. We show that the increasing amount of thiophene in the polymer backbone can affect stepwise blue shift of the UV-vis absorption and better π-π stacking ability. When comparing with BT and TPD acceptor, polymers containing TPD unit showed about 40 nm blue shift of the UV absorption and 0.2 V deeper HOMO levels according to cyclic voltammetry measurement. Among them, polymer containing both TPD and BT shows best PCE of 3.0% with PC71BM in the presence of 1,8-octanedithiol.
Finally, we targeted on synthesis of fully conjugated block copolymer (BCP) by ring-opening metathesis polymerization (ROMP). Novel conjugated cyclophanes comprising electron donating cyclopentadithiophene-vinylene and accepting dithienobenzothiadiazole-vinylene have been synthesized by McMurry coupling. ROMP of the acceptor monomer and the subsequent addition of the donor monomer allows the preparation of a fully conjugated BCP. The BCP exhibits multiple optical and electrochemical functions, giving wide range of light absorption from 300 to 700 nm.
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