Design, Synthesis And Photovoltaic Properties Of Donor-acceptor Polymers With Low Band Gaps And Deep HOMO Energy Levels

Polymer solar cells (PSCs) have many merits such as low weight, low cost, easy fabrication, flexible and tunable properties. Polymers are the material foundation for PSCs. Design and synthesis of new polymers are important for improving the performance of PSCs. In this thesis, we systematically synthesized a series of new donor-acceptor polymers with low bangaps and low HOMO energy levels. Studies were focused on the polymers with electron deficient imine or lactam group. The relationship between structures, photoelectric properties and photovoltaic performance were also studied. The works are divided as follows:1. Three new low bandgap conjugated polymers with 3,4-ethylenedioxythiophene (EDOT) as donor and 2,3-bis(4-octyloxyphenyl)-quinoxaline (P1), 2,3-bis(4-octyloxyphenyl)-thiadiazolquinoxaline (P2, P3) as acceptors were synthesized by Stille cross-coupling reaction (Chapter 2). Their optical, electrochemical and photovoltaic properties were studied. Photovoltaic devices were fabricated based on polymer and fullerene derivatives PC₆₁BM. The device measurements revealed the best performance of power conversion efficiency (PCE) of 0.60% with an open-circuit voltage (V_oc) of 0.52 V, a short-circuit current density (J_sc) of 3.24 mA/cm².2. The HOMO energy level of polymer can be reduced by introducing electron withdraw group on acceptor unit. In Chapter 3, the strong electron-withdrawing acceptor unit (phthalimide) which contained imide group was designed and synthesized. Two new donor-acceptor polymers PBDTPh and PBDTPhBT with alternating benzo[1,2-b:4,5-b']dithiophene (BDT) and phthalimide (Ph) units were synthesized by Stille coupling reaction. The UV-vis spectra indicated that polymers have strong absorption in visible range. The cyclic voltammetry showed the polymers have low HOMO energy levels. The bulk heterojunction solar cell devices based on these polymers as donors and PC₇₁BM as acceptor showed the high open circuit voltages of about 0.89-0.93 V and the highest power conversion efficiency of 1.54%.3. In order to make the absorption of polymer more redshifted than polymer PBDTPh and PBDTPhBT. In Chapter 4, new donor/acceptor polymer PBDTDPT1 and PBDTDPT2 with alternating benzo[1,2-b:4,5-b']dithiophene and N-alkylated dioxopyrrolo-thiophene were synthesized. The UV-vis spectra showed that absorption of the polymers were broader than those of phthalimide-based polymers in Chapter 3. The new polymers had low HOMO energy levels of -5.42 and -5.44 eV for PBDTDPT1 and PBDTDPT2, respectively. The bulk heterojunction solar cell devices based on these polymers as donors and PC₇₁BM as acceptor exhibited high V_oc of 0.91-0.97 V. The preliminary devices based on PBDTDPT1 and PBDTDPT2 achieved the J_sc of 6.58 and 10.34 mA/cm², the PCE of 3.42% and 4.79%, respectively.4. The band gap of polymer can usually be reduced when introduce extra donating units around the acceptor. In Chapter 4,New donor/acceptor polymer PBDTTPT1 and PBDTTPT2 with alternating benzo[1,2-b:4,5-b']dithiophene (BDT) and bisthiophene-dioxopyrrolothiophene (TPT) units were also synthesized by Stille coupling reaction. However, the results indicated the optical band gaps did not decrease significantly in these systems. The polymers had the optical band gaps of 1.82 and 1.78 eV. The polymer solar cell devices based on these polymers as donors and PC₇₁BM as acceptor showed the open circuit voltage values as high as 0.90 V and power conversion efficiency of 2.43% and 2.68% for PBDTTPT1 and PBDTTPT2, respectively.5. In Chapter 5, the strong electron-withdrawing isoindigo unit which contained lactam group was designed and synthesized. A series of new isoindigo-based low band bap polymers, containing thiophene, thieno[3,2-b]thiophene and benzo[1,2-b:4,5-b']dithiophene as donors, have been synthesized by Stille cross-coupling reaction. Their photo-physical, electrochemical and photovoltaic properties have been investigated. The isoindigo-based polymers have strong absorption in the whole visible region, low HOMO energy levels, suitable LUMO energy levels and ideal optical band gaps. Preliminary investigation on the bulk heterojunction solar cell devices based on these polymers as donors and PC₇₁BM as acceptor exhibited the PCE as high as 1.91%.6. In Chapter 6, another high electron-withdrawing unit 3,6-diaryl-2,5-dihydropyrrolo[3,4-c] pyrrole-1,4- dione (DPP) which contained lactam group was chosen as the acceptor. The DPP unit has well-conjugated structure, which leads to strongÏ€-Ï€interaction. The lactam structure makes the DPP unit exhibit a strong electron-withdrawing effect. Polymer was synthesized based on DPP and thieno[3,2-b]thiophene units. The polymer had strong absorption in the entire visible region and absorption extended to near infrared region. The polymer has low HOMO energy level. The bulk heterojunction solar cell devices based on the polymer as donor and PC₇₁BM as acceptor showed the PCE of 1.60%.