| Organic solar cells(OSCs)have great application potential in wearable devices and building integration due to their unique merits such as low-cost,light-weight,flexibility,and translucency,and have become one of the hotspots in the field of global green energy research.In recent years,the innovative development of narrow-bandgap nonfullerene acceptor materials and wide-bandgap polymer donor materials has greatly improved the power conversion efficiency(PCE)of OSCs.The maximum PCE of single-junction OSCs has exceeded 19%and that of the tandem OSCs has exceeded 20%.The key part of OSCs is the bulk-heterojunction(BHJ)active layer composed of donor and acceptor materials.Therefore,an in-depth understanding of the relationship between the molecular structure of materials and the morphology of the active layer and device performance is of great importance for OSCs to achieving high PCE.Accordingly,a series of studies have been carried out in this thesis to improve the photovoltaic performance of OSCs from the perspective of molecular structure design of donor and acceptor materials and morphology optimization of active layer.The main results of the studies are as follows:(1)The optimization of flexible side chains enhances the photovoltaic performance of polymer donor material.A new D-A conjugated polymer donor,PBT-SF,is designed and synthesized by introducing an alkylthio side chain into the conjugated thiophene side chain of the benzodithiophene(BDT)unit.The introduction of alkylthio side chains significantly downshifts the HOMO energy level of PBT-SF and enhances its absorption coefficient in the visible region.When blending with the acceptor IT-4F,the alkylthio substituted PBT-SF-based OSCs has high carrier mobilities,ordered molecular packing properties,and good phase separation morphology,thus achieving a PCE of 10.51%.In addition,PBT-SF has an outstanding application characteristic of semi-transparent devices.The PCE of the PBT-SF:IT-4F-based semi-transparent OSCs reaches 7.76%with an average visible transmittance(370-720 nm)of 31%.(2)Ternary copolymerization strategy tunes the photovoltaic properties of polymer donor material.Developing a series of random copolymers based on a ternary random copolymerization strategy that the benzo[1,2-c:4,5-c’]dithiophene-4,8-dione(BDD)acceptor unit is incorporated into PBN-Cl polymer with different contents.The introduction of BDD unit can effectively reduce the crystalline aggregation of polymers,thereby improving the molecular stacking and enhancing the hole mobility.Moreover,with the increase of the proportion of BDD unit,the morphology of blend films was further optimized,which effectively improved the exciton dissociation efficiency and suppressed charge recombination.Therefore,compared with the PBN-Cl:IT-4F-based device(PCE=11.21%),the PCE of the OSCs based on PBN-Cl-B80 with 80%BDD unit is significantly improved to 14.05%,and the fill factor(FF)also increases from 0.615 to 0.721.(3)Molecular design of polymerized small-molecule acceptors with defined structures.Designing and successfully isolating two thiophene-fused malononitrile isomer units(TCN-α and TCN-β)as linkages to develop two isomerically pure polymer acceptors(PBI-α and PBI-β)for efficient all-polymer solar cells(all-PSCs).The small difference in the molecular structure of these two monomers significantly impacts the absorption,energy levels,and device performance of the resulting polymer acceptors.PBI-β exhibits a higher LUMO energy level but blue-shifted absorption spectrum compared to PBI-α,which delivers a significantly different open-circuit voltage(Voc)(1.02 vs 0.93 V)and short-circuit current density(Jsc)(16.1 vs 19.0 mA cm-2)in their corresponding all-PSCs using PM6 as donor material.Meanwhile,PBI-β showed a more positive molecular packing effect to enhance the π-face-on orientation backbone stacking of polymer donor PM6 than PBI-α,leading to a higher FF of 0.684 in the PBIβ-based device compared to that of 0.646 for the PBI-α-based device.Overall,excellent PCEs of 11.4 and 11.3%were achieved in the PBI-α-and PBI-β-based all-PSCs,respectively(4)Two novel conjugated polymer acceptors(PIDT and PBDT)based on different linking units of indacenodithiophene(IDT)and BDT are developed.The different linking units have great impacts on the physicochemical and photovoltaic properties of the acceptors.Compared with PBDT,polymer PIDT shows stronger light absorption ability,up-shifted LUMO energy level,and more planar molecular structure.Meanwhile,the PIDT-based all-PSCs exhibits a high Voc of 1.10 V and an outstanding PCE of 10.19%when blending with the polymer donor PTQ10,due to the good charge transport properties and interpenetrating network structure.(5)The ternary strategy optimizes the morphology of the active layer.Three nonfullerene acceptor materials,BTPT,BTP-Th,and BTP-2Th,are developed for the optimization of the oversized phase separation morphology of PTQ10:Y6 binary blend films(PCE=16.41%,FF=0.742).The results show that the miscibility difference between the third component and the donor/acceptor has a significant impact on the crystallinity,molecular stacking,and morphology of the blend films and the device performance.BTPT,tending to miscible with Y6,slightly inhibited the self-aggregation of Y6,so it does not significantly decrease the phase separation size of blend film,and the ternary OSC obtained a moderate FF of 0.760 and PCE of 17.45%.Conversely,the good miscibility between BTP-2Th and PTQ10 made the phase separation size of the ternary blend film too small,thus achieving lower FF(0.751)and PCE(17.00%).The BTP-Th receptor molecule has good mutual compatibility with Y6 and PTQ10,which can prolong the crystallization time of the acceptor molecule and obtain more ordered molecular π-π stacking while achieving the optimal D/A phase separation morphology.Synergistically,the improved exciton dissociation and charge transport properties resulted in excellent FF(0.773)and PCE(17.65%)for the PTQ10:Y6:BTP-Th based OSCs.The three acceptors can also be used to optimize the blend film morphology and device performance of PTQ10:m-BTP-PhC6.An excellent PCE of 18.13%was obtained for the BTP-Th-based ternary OSCs. |
