Molecular Design And Characterization Of High-Efficiency Organic Photovoltaic Materials

Polymer solar cells(PSCs),which have unique features of simple device structure,easy fabrication by solution-processing,and capability to be fabricated into flexible and semi-transparent devices,offer a promising and renewable energy technology.How to improve the power conversion efficiency(PCE)is still the focus of research in this field.Among them,the design,synthesis,and application of novel organic materials with superior photovoltaic properties are the key components determining the photovoltaic performance of PSCs and play vital roles in improving the PCEs of these devices.Therefore,understanding the structure-property relationships is of great importance to design highly efficient organic photovoltaic materials.In this thesis,by utilizing efficient molecular design strategies,basic characteristics,microstructure characteristics,and photovoltaic properties of the materials were modulated,and excellent photovoltaic performance was recorded in the PSCs.The main results are as follows:(1)A novel narrow-bandgap conjugated polymer acceptor(PN1)based on an electron-withdrawing MOITIC16 as core building block and thiophene as linking unit,was developed.The absorption spectrum,molecular energy levels and charge transport performance of the polymer acceptor are regulated by the advandages of the planar molecular backbone,high electron affinity and strong intramolecular charge transfer of electron-withdrawing MOITIC16.The PN1 exhibits a broad and strong absorption in the region of 550-800 nm with a high maximum extinction coefficient of 1.28×105 cm-1,a high electron mobility of 9.39×10-4 cm2 v-1 s-1,relatively higher LUMO energy level of-3.85 eV.By matching with the wide bandgap polymer donor PM6,the blend films with 3%CN form an ideal interpenetrating network structure and nano-scale phase separation morphology and the corresponding all-PSC exhibited a PCE of 10.5%with a large open-circuit voltage(Voc)of 1.0 V,a high short circuit current density(Jsc)of 15.2 mA cm-2 and a superior fill factor(FF)of 0.69.(2)A novel small molecular acceptor(SMA)named IDTT8-N based on indacenodithienothiophene(IDTT)core is designed and synthesized.Compared with the counterpart molecule IDTN with indacenodithiophene(IDT)core,IDTT8-N with the extended ?-conjugation length of IDT core exhibits not only a redshift of ca.42 nm in optical absorption and enhanced the molecular interaction,but also little change on its lowest unoccupied molecular orbital(LUMO)energy level.Therefore,the optimized PM6:IDTT8-N based devices exhibit an extraordinary PCE of 14.1%with a Voc of 0.94 V,a Jsc of 20.98 mA cm-2 and an FF of 0.72.(3)Developing a series of donor polymers(named PM1,PM2 and PBFTz)based on a random terpolymerization strategy that the acceptor unit thiophene-thiazolothiazole(TTz)is incorporated into PM6 polymer with different contents.The effects of TTz unit on the photoelectric properties,aggregation properties and photovoltaic performances of the polymers were systematically studied.By including 20%of TTz unit into the PM6 polymer backbone,the intrinsic properties and blending morphologies of the resulting polymer(PM1)were synergistically optimized,enabling highly efficient non-fullerene OSCs with efficiencies reaching 17.6%with a Voc of 0.87 V,a Jsc of 25.9 mA cm-2 and an FF of 0.78,while PM6:Y6-based device obtained a moderate PCE of 15.8%with a Voc of 0.86 V,a Jsc of 25.5 mA cm-2,and an FF of 0.72,indicating the further improvement of three important photovoltaic parameters and PCE.Furthermore,the PM1 polymer exhibits excellent batch-to-batch reproducibility,indicating a promise to become the new "work-horse" material for the field of OSCs.