| 其他摘要 | Extending π-conjugation system and incorporating fluorine substituent into polymer chains are two effective methods to improve their photovoltaic properties. Currently thiophene is widely used to extend the π-conjugation system of conjugated polymers. But this method usually results in a higher highest occupied molecular orbital (HOMO) energy level, which is unfavorable to realizing a high open-circuit voltage (Voc) in photovoltaic devices. Incorporating fluorine substituent into polymer chains could down-shift their HOMO levels, but in many cases their optical bandgaps are increased due to the less down-shifts of the lowest unoccupied molecular orbital (LUMO) levels. Large bandgaps are unfavorable for polymers to harvest more light. This dissertation focuses on tuning the frontier molecular orbitals (HOMO and LUMO) of photovoltaic polymers: (i) To explore the method to simultaneously extend the π-conjugation system and down-shift the HOMO level of a polymer; (ii) To design a fluorinated polymer simultaneously owning a down-shifted HOMO and reduced optical bandgap (Egopt). The main contents are as follows.
1. Benzene was used to extend the π-conjugation system of the polymer based on alkylthienyl-substituted benzodithiophene. Compared with thiophene, benzene is π-deficient. Fusing benzene on the polymer down-shifted HOMO and LUMO levels from -5.36 and -3.60 eV to -5.47 and -3.73 eV, respectively. The Egopt of the polymer was reduced from 1.76 to 1.74 eV, leading to a red-shifted absorption edge of 713 nm. Additionally, the polymer with extended π-conjugation system showed noticeably enhanced intermolecular interactions and a slightly improved thermal decomposition temperature. Fusing benzene on the polymer significantly improved its photovoltaic properties. The short-circuit current density (Jsc) of the best-performing photovoltaic devides was increased from 9.83 to 12.93 mA/cm2, and the optimum power conversion efficiency (PCE) was increased from 5.35% to 7.30%.
2. Benzene was used to extend the π-conjugation system of the polymer based on alkoxyphenyl-substituted benzodithiophene. Fusing benzene on the polymer down-shifted HOMO and LUMO levels from -5.23 and -3.51 eV to -5.49 and -3.78 eV, respectively. The Egopt (1.71 eV) of the polymer was nearly unchanged. The photovoltaic properties of the polymer were slightly improved by fusing benzene. The Voc of the best-performing photovoltaic devides was increased from 0.83 to 0.84 V, and the Jsc was increased from 11.33 to 12.29 mA/cm2. Consequently, the optimum PCE was increased from 6.23% to 6.57%.
3. A novel polycyclic aromatic unit dithienobenzothiadiazole (fDTBT) was designed by fusing two thiophene rings onto benzothiadiazole (BT). fDTBT has a curved configuration and indacenodithiophene (IDT) has side chains stretched out of the backbone plane. A new polymer PIDT-fDTBT comprising alternating fDTBT and IDT was designed and it was an ideal polymer to study the effects of curvature on photovoltaic properties. The HOMO level of PIDT-fDTBT was down-shifted by 0.08 eV while the LUMO was up-shifted by 0.2 eV in comparison with those of the BT-containing polymer. As a result, PIDT-fDTBT exhibited an increased Egopt of 2.03 eV. The Voc of PIDT-fDTBT-based photovoltaic devices was pronouncedly enhanced, reaching 0.90 V. However, the Jsc and PCE were decreased due to the increased bandgap.
4. We replaced the sulphur atom in fDTBT with oxygen atom and synthesized another polycyclic aromatic compound dithienobenzooxadiazole (fDTBO). Compared with fDTBT, fDTBO also has a curved configuration but with stronger electron-withdrawing ability. Two polymers utilizing fDTBO as electron-deficient unit and alkoxy-substituted benzodithiophene (BDTO) and alkylthienyl-substituted benzodithiophene (BDTT) as electron-rich units were synthesized, respectively. The Egopt were 2.00 and 1.95 eV for PBDTO-fDTBO and PBDTT-fDTBO, respectively. PBDTO-fDTBO and PBDTT-fDTBO exhibited very deep HOMO energy levels of -5.58 and -5.60 eV, respectively. As a result, both of the photovoltaic devices based on PBDTO-fDTBO and PBDTT-fDTBO realized very high Voc values of over 1 V.
5. Fluorine was firstly applied in thiadiazolo[3,4-c]pyridine (PT)-containing polymer. The HOMO and LUMO levels of the fluorinated polymer PDTPT-2TF were -5.34 and -3.89 eV, down-shifted by 0.11 and 0.15 eV , respectively, in comparison with the none-fluorinated polymer PDTPT-2T. PDTPT-2TF exhibited a narrow Egopt of 1.45 eV, 0.04 eV smaller than that of PDTPT-2T. X-ray diffraction indicated that more ordered structure was formed in the solid film of PDTPT-2TF. Incorporation of fluorine into PT-containing polymer significantly improved the photovoltaic properties. The best-performing photovoltaic devices based on PDTPT-2T only gave a maximum PCE of 2.65%, with a Voc of 0.73 V, a Jsc of 5.34 mA/cm2, and a fill factor (FF) of 67.91%. In striking contrast, the maximum PCE of the devices based on PDTPT-2TF reached 8.01%, along with a slight higher Voc of 0.74 V, a significantly enhanced Jsc of 15.52 mA/cm2, and a slight higher FF of 69.73%. This efficiency is the highest one for PT-containing polymers and it was achieved without any processing additives or post-treatments.
The findings of this dissertation indicate that: (i) Extending π-conjugation system and increasing backone curvature of polymers are two effective strategies to simultaneously extend the π-conjugation system and down-shift the HOMO energy level of the polymer; (ii) Incorporating fluorine into DTPT-2T polymer can simultaneously down-shift the HOMO energy level and reduce the Egopt. This dissertation is of great significance in perfecting the theory of organic electronics. |
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