Synthesis And Photovoltaic Properties Of Fused Perylene Diimide-containing Electron Acceptors

The continue quest for sustainable energy sources has promoted the accelerated growth in research of clean solution-processed bulk-heterojunction organic solar cells(BHJ-OSCs).In the research process of OSCs,the soluble fullerene electron acceptors have long been the most important leading role and delivered remarkable performance in BHJ OSCs,however,their intrinsic drawbacks in the limited visible light absorption,energy level tunability and morphological stability restrict the further improvement in efficiency and have forced researchers to seek potential replacements-non-fullerene electron acceptors(NFAs).Perylene diimide(PDI)and its derivatives have long been widely used as building block in the NFAs design.Their features,such as ease of functionalization,high electron affinity,suitable optoelectronic properties,and excellent thermo-stability,have established their competitiveness as ideal NFAs.However,the reported efficiency of the current state-of-the-art PDI-based non-fullerene OSCs(NF-OSCs)is still low,and one of the key challenges is how to maintain the balance between strong crystallinity and excellent carrier transport properties in the active layer considering the strong aggregation tendency come from the rigid polycyclic aromatic PDI core.To address this problem,one successful and feasible design strategy is to construct a quasi-two-dimensional(quasi-2D)fused perylene diimide dimer(FPDI)unit,which can efficiently prohibit the strong aggregation tendency of PDI units without adversely weakening the charge transport ability of the resulting non-fullerene acceptors,thanks to the small built-in twisted molecular structure in FPDI unit.In order to develop a fundamental understanding of the structure-property relationship,semiconductor pair matching rules,and operating mechanisms for expanding the use of the new fused aromatic imide FPDI in NF-OSCs,this dissertation designed and synthesized different series of quasi-2D FPDI-based electron acceptors.The research work illustrated in this dissertation consist of two aspects:the first part is to explore the effectiveness and applicability of FPDI moiety in constructing small molecule acceptor materials,and comprehensively investigate the effect of molecular geometry regulation on the performance of FPDI-based NF-OSCs.The second part is mainly to study the all-polymer solar cell(all-PSCs)devices by designing different quasi-2D FPDI-based polymer acceptors.The details are as follows:Two asymmetric and symmetric FPDI-based small-molecule acceptors(FPDI-Se and FPDI-2Se)were designed and synthesized by incorporating the selenophene heterocycle into the bay position of FPDI building block.The influence of this Se heterocycle in the opto-electronic characters were further investigated and discussed.With PTB7-Th as the polymer donor,the symmetric annulated acceptor FPDI-2Se-based device only gave a power conversion efficiency(PCE)of 4.45%with a poor fill factor(FF)of 39.5%,whereas the asymmetric annulated acceptor FPDI-Se-based device,however,showed the remarkable enhanced PCE of 6.61%with an increased short-circuit current density(Jsc)of 14.8 m A/cm~2 and an improved FF of 56.1%in NF-OSCs.These results indicated that the design and development of the heteroannulated-FPDI electron acceptors with asymmetrical configuration is an effective strategy by which to improve the photovoltaic performances of NF-OSCs.Two pairs of FPDI acceptors,one in the form of FPDI-π-bridge-FPDI(T-FPDI and TT-FPDI)with theπ-bridge being either an axisymmetric thiophene(T)or a centrosymmetric thienothiophene(TT)unit,and the other being the corresponding ring-fusion counterpart(FT-FPDI and FTT-FPDI),were constructed by adopting linker-regulation and ring fusion engineering.The T-FPDI-based NF-OSC exhibited a PCE of5.50%,while a higher PCE of 7.17%with a simultaneous enhancement in the open circuit voltage(Voc),Jsc and FF was achieved for the TT-FPDI-based solar cells.Furthermore,the devices based on the ring-fused FT-FPDI and FTT-FPDI molecules delivered PCEs of 6.75%and 7.66%,respectively,both higher than those of the corresponding non-fused counterparts.Finally,a novel FPDI-based electron acceptor(SF-FPDI)with quasi-2D configuration was further designed by using spirobifluorene(SF)as the centralπbridge,and found the corresponding device showed an impressive PCE of 6.24%with a low energy loss(Eloss)0.76 V.It is worth noting that the above NF-OSCs do not need any solvent additives.Two A-D-A typed small molecular electron acceptors(BT-FPDI and f BT-FPDI)were developed by molecular modification of the conjugated centralπbridge.BT-FPDI has a highly twisted geometry with the assistance of the quasi-2D configuration of the FPDI unit and modest dihedral angles between FPDI and BT units,while f BT-FPDI shows a relatively planar geometry with symmetrically aligned FPDI molecular wings.When paired with polymer donor PTB7-Th,the intrinsic configuration characteristic of BT-FPDI guaranteed an impressive PCE of 8.07%.In contrast,the optimal device employing f BT-FPDI as an electron acceptor delivered a PCE of only 5.89%.The results show that,unlike traditional PDI molecules,the ring-fusion molecular design did not produce the desired positive effect in FPDI derivatives,and also provide a new insight into the regulation of the molecular geometry based on FPDI acceptors with an intrinsic quasi-2D structure.In order to extend the quasi-2D FPDI building block to all-PSCs,a series of N-type polymeric electron acceptors(PFPDITT,PFPDI-T,PFPDI-Se,and PFPDI-BDF)were designed and sythesized.The results show that the optical,electronic and even photovoltaic properties of FPDI-based polymer acceptors are mainly determined by the FPDI unit with a small effect from the comonomers,which is significantly different from the others reported rylenediimide-based polymer acceptors.The conventional all-PSCs based on the above polymer acceptors and PTB7-Th both gave remarkable device performace with over 6%PCEs,and favorable inter-molecular stacking behavior and morphology compatibility(FF>60%).Among this,PFPDI-Se-based all-PSC achieved the highest PCE of 6.58%with a Jsc of 14.0 m A/cm~2,an Voc of 0.76 V,and a FF of 62.0%.This work not only further investigated the structure-property relationship of quasi-2D FPDI-based polymer acceptors,but also elucidate the importance of polymer compatibility in the design of efficient FPDI-based all-PSCs.