Design And Synthesis Of New Electron Donor/Acceptor Materials For Polymer Solar Cell Application

Polymer solar cells (PSCs) have recently attracted considerable attention due to their advantages of low cost, light weight, flexibility, and facile large-area solution processability. Though impressive progresses have been made in this field over the past ten years, it is still far away from the industrial commercialization. The ideal active layer materials including the electron donor polymer and the electron acceptor should meet the requirements such as broad absorption, high carrier mobility, well matched energy levels, good compatibility with each other and low production cost. To date, fullerene derivatives are the most successful electron acceptors in PSCs. However, they still have many drawbacks such as high production cost, difficulty to tune the energy levels and poor absorption in the visible-to-near-infrared ranges. The photovoltaic performances of a few low band-gap donor-acceptor (D-A) polymers have exceed that of the most used poly(3-hexythiophene)(P3HT), such as the poly(benzodithiophene-alt-thieno[3,4-b]thiophene)(PTB) series discovered by L.P. Yu. However, the synthetic routes toward almost all excellent D-A polymers are very complicated which is not beneficial for industrial commercialization. With the above considerations in mind, in this dissertation, we designed and synthesized a series of new electron donor/acceptor polymers through few reaction steps. The chemical structure, thermal stability, optical absorption, HOMO/LUMO energy levels and photovoltaic properties of obtained polymers were studied. The polymers showed broad absorption, optimized energy levels and good photovoltaic performances. The works are listed as follows:1. A series of low band-gap conjugated polymers based on2,7-carbazole and arylene diimides (naphthalene diimide NDI and perylene diimide PDI) or dithienyl-arylene diimides were synthesized via Suzuki cross-coupling reaction. Through cyclic voltammetry measuremnts, it was found that all copolymers were n-type materials and their LUMO energy levels were higher than that of [6,6]-phenyl-C61butyric acid methyl ester (PCBM). Photovoltaic properties of the copolymers blended with P3HT as electron donor were investigated. Among four copolymers, the copolymer PCDTPDI alternating2,7-carbazole and dithienyl-perylene diimide exhibited the best photovoltaic performance with a power conversion efficiency (PCE) of0.68%.2. Considering that the HOMO energy level could be efficiently reduced by introducing the electron-accepting groups into the polymer side chain, the easily synthesized thiophene-3-carboxylate (CT) was chosen as the electron deficient units to copolymerize with electron-rich units like bithiophene (DT) and benzodithiophene (BDT). Thus, two new ester group functionalized polythiophene derivatives, PCTDT and PCTBDT, were obtained. The copolymers exhibited broad and strong absorptions in the visible region, which was similar to that of P3HT. Both copolymers showed lower HOMO energy levels than P3HT. Photovoltaic properties of the copolymers blended with PCBM as electron acceptor were investigated. PSC made from PCTBDT:PCBM blend showed a PCE up to4.03%with an open circuit voltage (Voc) of0.78V, a short circuit current (Isc) of8.19mA/cm2, a fill factor (FF) of63.2%.3. Three random ester-functionalized polythieno[3,4-b]thiophene derivatives (P1-P3) were synthesized by incorporating different contents of thiophene-3,4-dicarboxylate moiety into the polymer backbone. The copolymers exhibited low optical band gaps of1.23-1.42eV. The HOMO energy levels of the copolymers gradually decreased with increasing the content of the thiophene-3,4-dicarboxylate moiety. Preliminary photovoltaic properties of the copolymers blended with PCBM as electron acceptor were investigated. Among the three copolymers, PI exhibited the best photovoltaic performance with a PCE of1.02%and a Voc up to0.71V was achieved in the solar cell based on P3:PCBM blend.4. Two new ester-functionalized diketopyrrolopyrrole (DPP) containing polymers (random PCTDPP and regular PDCTDPP) were synthesized by incorporating thiophene-3,4-dicarboxylate moiety into the polymer backbone. The copolymers exhibited low optical band gaps below1.40eV and relatively low HOMO energy levels. Photovoltaic properties of the copolymers blended with PCBM as electron acceptor were investigated. PSC made from PCTDPP:PCBM blend showed a PCE of3.52%and a Voc up to0.84V was achieved in the solar cell based on PDCTDPP:PCBM blend.5. A new conjugated polymer (PDTPTPD) alternating dithienopyrrole (DTP) and thienopyrroledione (TPD) units was synthesized by Stille coupling reaction. The resulting copolymer showed both a low optical band gap of1.62eV and a low HOMO energy level of-5.09eV owing to the electronegativity of TPD moiety. Preliminary photovoltaic properties of the copolymers blended with PCBM as electron acceptor were investigated. PSC made from PDTPTPD:PCBM blend shows a PCE of1.9%and a Voc up to0.74V could be achieved.6. A series of-(D-A1-D-A2)n-type diketopyrrolopyrrole (DPP) containing polymers P4-P10were synthesized by incorporating both strong and weak electron-accepting units into the polymer backbone. The resulting copolymers exhibited both low optical band gap of1.27-1.48eV and deep HOMO energy levels of-5.25～-5.44eV. Preliminary photovoltaic properties of the copolymers blended with PCBM as electron acceptor were investigated. All photovoltaic devices featured high Voc (0.72-0.91V) and P8exhibited the best photovoltaic performance with a PCE of1.33%. Furthermore, we found the above molecule design is an efficient strategy to obtain high performance ambipolar field-effect transistors. P7showed quite balanced and high hole and electron field-effect mobilities with both around1.0cm2/V/s.