The Design, Synthesis And Property Of Thiophene-fused Diketopyrrolopyrrole Applied To Organic Semiconductor Materials

Compared with the inorganic semiconductor materials, organic semiconductor materials show some advantages, such as light-weight, low-cost, performance easy to adjust and capability to be fabricated into flexible devices with large-area production. The energy level and optical band gap of the materials can be easily and effectively regulated by using the electron-rich donor (D) and electron-deficient acceptor (A) to build organic conjugated backbone. This strategy has become the main idea to design and synthesize the organic solar cells (OSCs) and polymeric field-effect transistors (PFETs). OSCs and PFETs with D-A structure based on diketopyrrolopyrrole (DPP) as the acceptor unit has attracted wide attention and shown excellent optical and electrical performance. In this thesis, two new electron-deficient acceptors, TPTI with one thiophene ring fused with diketopyrrolopyrrole unit and BTI with two thiophene rings fused with diketopyrrolopyrrole unit, have been designed. Several new series of D-A type organic molecules based on TPTI for OSCs and D-A type copolymers based on BTI for PFETs have been synthesized and their photophysical and electrochemical properties have also been investigated. The corresponding device performances have been studied and the relationships between the structure and the properties have been explored. The main contents of this thesis are displayed as follows: In Chapter 1, the structural advantages and development of organic semiconductor materials and the common characteristics and development of the OSCs and PFETs materials were briefly depicted. Then the details of OSCs and PFETs were introduced, which included the device structures, working principles, material types and molecular designs. Finally, based on the wide application and research status of the DPP unit as the acceptor in OSCs and PFETs materials, the design strategies of this thesis were outlined. In Chapter 2, a new electron-acceptor TPTI with thiophene-fused DPP unit had been designed and synthesized. Through the fusion of one thiophene ring into DPP core, the conjugation backbone had been extended. Four A-D-A type small organic compounds (S1-S4) for OSCs had been designed and synthesized with TPTI unit as acceptor and terthiophene, benzo[1,2-b:4,5-b']dithiophene (BDT), thienyl-substituted BDT (BDTT) and dithienosilole (DTS) unit as donors, respectively. S1-S4 showed wide UV-vis absorption region in solution and thin film. The optical band-gap of S1-S4 was about 1.5 eV, belonging to the narrow band-gap organic semiconductor materials. The OSCs devices based on S1-S4 and PC71BM had been fabricated and tested. Among them, S4 (based on TPTI and DTS) had the best photovoltaic performance and its PCEmax reached 0.97% through the device optimization with the addition of 0.4% DIO. The results showed that the device performances based on all of four molecules were very poor with very low PCE and Jsc. The large steric hindrance of the long-chain alkyl group might hinder well-ordered intermolecular ?-? interaction, resulting in the poor charge transportation.In Chapter 3, considering the significant impact of the steric hindrance of the side alkyl chain on the opto-electronic properties of the materials, the structural adjustment of side alkyl chains for the BDTT-2TPTI and DTS-2TPTI system had been taken and four new molecules (S5-S8) containing different alkyl groups were obtained. Their photophysical and electrochemical properties indicated that the intermolecular ?-? interaction of the S5-S8 in solution and thin film was significantly strengthened and HOMO levels of the molecules were slightly enhanced. The OSCs devices based on S7 and S8 as electron-donating materials and PC71BM as electron-accepting material had been fabricated and tested. Compared with S3 and S4 containing same conjugated skeleton with long alkyl chain in chapter 2, the photovoltaic performances of S7 and S8 based devices were greatly improved and short-circuit current density was increased about 2-3 times. Especially, S8-based device exhibited the highest PCE of 4.28% after the device optimization with addition of 0.5% DIO additive.In Chapter 4, in order to reduce the HOMO level of the materials and enhance the Voc of the OSCs device, two new small molecules (S9 and S10) had been designed and synthesized through introducing ethynyl unit as a "?-bridge" into BDTT-2TTI and DTS-2TPTI conjugated system via Sonogashira coupling reaction. Their photophysical and electrochemical properties indicated that the absorption spectra were partially blue-shifted and the HOMO levels of molecules were effectively reduced. The OSCs devices based on S9 and S10 as electron-donating material and PC71BM as electron-accepting material had been fabricated and tested. Compared with similar BDTT-2TTI (S7) and DTS-2TPTI (S8) system in chapter 3, the Voc of the OSCs based on S9 and S10 was significantly enhanced with a maximum increment of 0.16 V. However, the electron-withdrawing ability of ethynyl unit might exert a negative effect on the carrier transportation, thus greatly reduced the JSc of the OSCs device. The optimizations had been performed to improve the photovoltaic performance for S9 and S10 based devices. The PCEmax of the S9 reached 1.90% by addition of 0.5% DIO and the PCEmax of the S10 reached 2.83% by addition of 0.3% DPE.In Chapter 5, with TPTI as acceptor unit and fluorene, carbazole, triphenylamine and IDT as donor units respectively, four non-fullerene electron-accepting materials (S11-S14) had been designed and synthesized to match the P3HT as electron-donating material for OSCs. Their photophysical and electrochemical properties indicated that the HOMO-LUMO levels of S11-S14 can basically match with the P3HT. The OSCs devices based on S11-S14 as electron-accepting material and P3HT as electron-donating material were fabricated and the photovoltaic performances were measured. All the devices exhibited relative high Voc but much low Jsc, and the maximum efficiency only 0.138%. The AFM images showed that the poor phase separation or excessive phase separation size was the main factor affected the device performance. At same time, the OSCs devices based on S11-S14 and PC71BM were also fabricated and the photovoltaic performances were measured. All of them showed some photoelectric conversion efficiency. After addition of 0.7% DIO for S14, the phase separation of active layer was obviously improved, and the photoelectric conversion efficiency was also greatly improved and the highest PCE reached 2.20%. In Chapter 6, with two thiophene rings fused into DPP unit, a new acceptor BTI had been designed and synthesized. Based on BTI as acceptor and combined with thiophene, bithiophene, terthiophene and thieno[3,2-b]thiophene as donors respectively, five D-A conjugated polymers P1-P5 had been synthesized. All the polymers showed large molecular weight (Mn) and good thermal stability. Their photophysical and electrochemical properties indicated that all of them exhibited relative high HOMO energy levels and broad ICT absorption with narrow optical bandgap. Through PFETs device tests, five polymers all showed hole transporting properties and polymer P2, P4 and P5 showed relative high hole mobility at room temperature spun. The P2 showed the highest hole mobility of 0.45cm2V-1s-1 and Ion/off of 106. The big steric hindrance between the alkyl chains of the polymer P1 affected the intermolecular ?-? interaction between the conjugated main chains. The solubility of the polymer P3 was poor, which affected its solubility and film-forming property. Therefore, the hole-mobility of P1 and P3 was relatively low.In Chapter 7, to reduce the HOMO energy level of the polymers, the weak electron-donating group of benzene and the strong electron-deficient group of benzothiadiazole (BT) derivates had been chosen to construct the conjugated polymers (P6-P9) with BTI unit. In addition to the polymer P7 due to the greater rigidity and low molecular weight, the rest polymers had large Mn. Compared with P6 and P7, P8 and P9 showed an obvious shoulder peak in the ICT absorption band, indicating the presence of strong intermolecular ?-? stacking in the solution and film state due to the introduction of another thiophene bridge into the conjugated main-chain. The HOMO energy levels of the polymers had been significantly reduced by the introduction of the electron-deficient BT group. The PFETs devices tests indicated that all of them showed hole transporting properties and polymer P8-CF showed the best performance with the hole mobility of 0.14 cm2 V-1s-1 and Ion/off of 107 at room temperature spun among them.