Synthesis And Photovoltaic Properties Of Polymer Photovoltaic Materials Based On Thiadiazolo[3,4-c]pyridine Derivatives

As one of the third generation solar cells, polymer solar cells (PSCs) have beenwildely focused by many scientists all over the world, and have made remarkableprogress in recent years. At present, although the reported photoelectric conversionefficiency (PCE) of PSCs in literatures has reached10.6%, there is still a long way togo to realize real commercial production. In order to obtain polymers with abetter-matched spectral absorption of the visible region, a low-lying HOMO energylevel and higher hole mobility, and ultimately improve the PCEs of PSCs, we havedesigned and synthesized a series of polymers based on thiadiazolo[3,4-c]pyridine(PyT). For comparison, we also have synthesized the polymer with similar structuresbased on benzo[3,4-c]thiadiazole (BT). The structures of the intermediate and theas-synthesized polymers have been confirmed by1H-NMR,13C-NMR, FT-IR andMALDI-TOF. Simultaneously, the photophysical and electrochemical properties ofthe polymers have been investigated systematically by UV-vis spectra, PL spectra,and Cyclic Voltammetry. The main results are shown as follows:1、Two side-chain type copolymers (PBDT-T-DTPyT and PBDTT-T-DTPyT)were designed and synthesized, which contained benzo[1,2-b:4,5-b’]dithiophene(BDT) and thienylbenzo[1,2-b:4,5-b’]dithiophene (BDTT) as electron-donating unitsand4,7-bis(4-hexylthiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine (DTPyT)introduced via vinylene bonds as electron-withdrawing unit. Meanwhile, a main-chaintype copolymers (PBDT-DTPyT) was also synthesized in order to explore the effectsof PyT at different location (in backbone or side chain) on the photovoltaic propertiesof the polymers. All the polymers show good solubility and film-forming property.The research results show that the absorption spectrum of the main-chain typepolymer (PBDT-DTPyT) is redshifted110nm and73nm than that of correspondingside-chain type polymer PBDT-T-DTPyT and PBDTT-T-DTPyT, respectively. Itsuggests that the main-chain type polymer should have a stronger intramolecularcharge transfer (ICT) interactions than the side-chain type polymer. Although theHOMO energy levels of the three polymers are below-5.2eV, their Vocvalues of thePSCs are all below0.62V, which is much lower than theoretical Vocvalues. Apossible cause is that the interaction of PyT as a Lewis base and PSS as a Lewis acidshould be responsible for the non-ohmic contact on the interface and low Vocvalues ofthe PSCs. Therefore the Vocvalues of inverted PSC devices (without PSS in holeextracting layer) based on PBDTT-T-DTPyT is much higher than that of conventional PSC devices. Compared with the other two polymers, PBDTT-T-DTPyT presentsrelatively better photovoltaic performance. The conventional and inverted bulkheterojunction polymer solar cells based on PBDTT-T-DTPyT as electron donor and(6,6)-phenyl-C-butyric acid methyl ester (PC61BM) as electron acceptor exhibit powerconversion efficiencies of1.68%and2.08%, respectively.2、Two side-chain type polymers (PBDTT-T-DTPyT and PBDTT-T-DTBT) and amain-chain type polymer (PBDTT-DTPyT) were designed and synthesized byintroduction of BDTT as electron-donating unit and PyT or BT aselectron-withdrawing units. The research results show that the absorption spectrum ofthe side-chain type polymer (PBDTT-T-DTPyT) is redshifted by22nm than that ofPBDTT-T-DTBT, and the energy gap of the former is lower, which indicates PyT canlead to an enhanced π π stacking and ICT interactions of the polymer owing to itsstronger polarity and π-electron deficient property. Therefore, the PSC based onPBDTT-T-DTPyT has greater short-circuit current (Jsc) and PCE values. Although theabsorption spectrum of the main-chain type polymer (PBDTT-DTPyT) is wider andredshifted by91nm than the side-chain type polymer (PBDTT-T-DTPyT), the valuesof Jscand PCE of PSC based on PBDTT-DTPyT are much less than that of the laterbecause the hole mobility of the fomer is500times lower than the latter.