Synthesis and Opto-electrical Investigation of Linear and Hyperbranched Polythiophene Derivatives Containing Diketopyrrolopyrroles as Linking Groups
|摘要:||本研究之目的在合成出含吡咯併吡咯二酮作為連接基之線型及超分枝聚噻吩衍生物，並探討其熱性質、光學性質、電化學及光伏特性。本研究同時製備聚(3-己基噻吩)以作為對照之用。所有高分子皆以通用格林納金屬催化途徑合成，其數量平均分子量經測定介於2.14–4.03×104 g/mol之間，而重量平均分子量則介於3.92–9.98×104 g/mol之間，分子量分佈則為1.81–2.85。使用核磁共振光譜儀分析所有高分子之立體規則度皆達96%以上。利用熱重分析儀及示差掃描卡計探討高分子熱性質，所有高分子於300 oC呈現第一階段熱損失溫度；此外含有吡咯併吡咯二酮之高分子經高溫加熱後，相較於聚(3-己基噻吩)其熱重損失量較小，顯示確實增強材料的熱穩定性。所合成之高分子於薄膜態的紫外–可見光吸收光譜皆與聚(3-己基噻吩)大致相符。而含有吡咯併吡咯二酮之高分子之螢光放射產生衰減，說明在這些材料中激子不易經由再結合而放光；易言之，應用於太陽能元件時載子有更多機會傳遞至兩端電極以增加電流值。利用循環伏安計量法分析電化學特性，得知引入吡咯併吡咯二酮團基能降低材料之最高已填滿分子軌域及最低未填滿分子軌域。以高分子作為主動層製作單電洞元件，並利用空間電荷限制電流公式計算得知直鏈型聚噻吩系列以P1-50的遷移率最高，達到2.1–2.2×10-4 cm2/Vs，超分枝聚噻吩系列則以P2-10的遷移率最高，達到1.7–1.8×10-4 cm2/Vs。最後，將所有高分子用於製作反結構太陽能元件之主動層，結構為ITO/ZnO nanorods/P2-PF6/Polymer:PC61BM/PEDOT/WO3/Au，其中ZnO nanorods作為電子傳輸層，P2-PF6作為潤濕層，PEDOT作為電洞傳輸層，以及WO3作為電洞萃取層。經測量得知直鏈型聚噻吩系列作為主動層之元件其開路電壓為0.55–0.58 V、短路電流為8.62–16.21 mA/cm2、填充因子為37–41%及光電轉換效率值為1.73–3.74%。超分枝聚噻吩系列作為主動層之元件其開路電壓、短路電流、填充因子及光電轉換效率則分別為0.55–0.58 V、9.49–11.87 mA/cm2、36–38%及2.01–2.49%。以上結果顯示含吡咯併吡咯二酮之直鏈型高分子相較於超分枝高分子作為高分子太陽能元件之發展潛力較高。|
The goal of this research is to synthesize the linear and hyperbranched polythiophene derivatives containing diketopyrrolopyrrole as linking groups, and to investigate the thermal, optical, electrochemical, and photovoltaic properties of those derivatives. Poly(3-hexylthiophene) (P3HT) was also synthesized for comparison in this study. All polymers were synthesized via the Universal Grignard metathesis. The number-average molecular weights of polymers were measured to be in the range of 2.14–4.03×104 g/mol, while their weight-average molecular weights were in the range of 3.92–9.98×104 g/mol, with polydispersity index of 1.81–2.85. The regioregularity of all polymers were analyzed by nuclear magnetic resonance spectrometer to be higher than 96%. The thermal characteristics of polymers were investigated by differential scanning calorimeter and thermogravimetric analyzer, showing a first-stage weight loss at about 300 oC; in addition, polymers containing diketopyrrolopyrrole groups possess less weight loss than P3HT after heating, indicative of enhanced thermal stabilities. The UV-vis absorption spectra of the synthesize polymers are similar to that of P3HT in film state. Polymers containing diketopyrrolopyrrole groups show distinct attenuation in fluorensencent emission, indicating that excitons are not easy to recombine to emit light in those materials; in other words, there are more opportunities for carriers to transport to both electrodes to increase current. The electrochemical properties of polymers were analyzed by cyclic voltammetry, revealing that introduction of diketopyrrolopyrrole groups may result in decreasing HOMO and LUMO levels of polymers. All polymers were used as active layers for fabrication of hole only devices and applied space-charge-limited-current (SCLC) model to extract the charge carrier mobility. P1-50 exhibited a highest hole mobility of 2.1–2.2 × 10−4 cm2/Vs of linear polythiophene series; P2-10 exhibited a highest hole mobility of 1.7–1.8 × 10−4 cm2/Vs of hyperbranched polythiophene series. Finally, all polymers were used as active layers for fabrication of inverted solar devices with the configuration of ITO/ZnO nanorods/P2-PF6/polymer:PCBM/PEDOT/WO3/Au, using ZnO nanorods as electron transporting layer, P2-PF6 as wetting layer, PEDOT as hole transporting layer, and WO3 as hole extraction layer. The devices based on linear polythiophene series as active layers were measured to show the open-circuit voltage (VOC) of 0.55–0.58 V, the short-circuit current (JSC) of 8.62–16.21 mA/cm2, the fill factor (FF) of 37–41%, and the power conversion efficiency (PCE) of 1.73–3.74%. The devices based on hyperbranched polythiophene series as active layers showed VOC, JSC, FF, and PCE values of 0.55–0.58 V, 9.49–11.87 mA/cm2, 36–38%, and 2.01–2.49%, respectively. The above results demonstrate that the linear polymers containing diketopyrrolopyrrole showed better potential use in polymer solar devices than hyberbranched ones.