Design,synthesis And Photovoltaic Properties Of Small Molecular Acceptors Based On Porphyrin And Perylene Diimides

Photovoltaic technology can directly convert light energy into electric energy.Compared with traditional silicon solar cells,bulk heterojunction(BHJ)organic solar cells(OSCs)based on solution processing have a series of advantages,such as low cost,light weight and roll-to-roll production.Since the development of the first reported organic solar cells in 1986,the power conversion efficiencies(PCEs)have been enhanced steadily.It is undoubted that with the emergence of new materials and technologies,this efficiency will be updated continuously.Porphyrin materials have been widely investigated in OSCs because of their excellent photochemical properties.Porphyrins as electron donor materials for OSCs have made great progress.However,the reports on porphyrin electron acceptor materials are still very limited.On the other hand,perylene diimides(PDI)groups have strong electron-withdrawing ability,and the acceptor materials based on PDI generally have good performances in photovoltaic devices.In this thesis,a series of novel porphyrin acceptor materials fucntinalized with PDI groups for OSCs are designed and synthesized.The effects of different molecular structures on the optical,electrochemical properties and photovoltaic properties of PDI based porphyrin acceptor materials are studied,which can provide guidelines for the future optimization on molecular design.The research contents include the following parts:In chapter 2,by connecting two electron-withdrawing dimeric perylene diimides(DPDI)groups to two of the meso positions at porphyrin ring(the electron-donating core)via acetylene bridges,three small molecular acceptors(Por-DPDI,TEHPor-DPDI and BBOPor-DPDI)without and with different side chains are designed and synthesized.These three small molecules are blended with the donor polymer PTB7-Th to construct the active layers,and OSCs devices are prepared.The small molecular acceptor Por-DPDI,which has no side chain can be synthesized with fewer steps but higher reaction yields,and the corresponding OSCs with PTB7-Th as the electron donor also show better performance with an optimized PCE of5.21%,a V_oc of 0.66 V,a J_sc of 13.54 m A cm^-2 and an FF of 58.29%,which can be contributed by the better?-?molecular packings compared with the small molecules TEHPor-DPDI and BBOPor-DPDI with side chains due to the lack of side chains Por-DPDI.This study demonstrates the effectiveness of the new electron-withdrawing group DPDI,as well as the advantages in the synthesis and performance of small molecules without side chain,providing guidelines for the molecular design of PDI based small molecular porphyrin acceptors.In chapter 3,by connecting electron-withdrawing mono or dimeric PDI groups to two or four meso positions at porphyrin ring via acetylene bridges,four small molecular acceptors TEH-PDI,TEH-DPDI,TPDI and TDPDI are designed and synthesized,which are blended with a porphyrin small molecular donor C19OD-DPP to prepare OSCs.This is the first time to report all-porphyrin OSCs.Our results show that the active layers of OSCs can also use all-porphyrin derivatives to achieve photovoltaic effect,just as chlorophyll in nature can complete the absorption of light energy by itself.Although the PCEs of the photovoltaic devices are not high,the energy levels,solubility and molecular coplanarity of small molecular acceptors can be adjusted by coordinating the type and number of electron-withdrawing groups as well as the connection mode with porphyrin core,which is expected to further improve the efficiencies of the all-porphyrin OSCs.This study opens up a way for the development of materials and the preparation of devices on all-porphyrin OSCs.In chapter 4,a porphyrin monomer C19OD-PDI and a porphyin dimer DPor-PDI are designed and synthesized,of which the dimer is constructed by connecting two same monomers via a single bond at the porphyrin meso positions.The OSCs with the monomer or dimer as the electron acceptors and PTB7-Th as the electron donor are prepared.Compared with C19OD-PDI devices,those based on DPor-PDI show a higher J_sc but a lower V_oc and a smaller FF,leading to a decreased PCE.Absorption spectroscopy and external quantum efficiency(EQE)measurements indicate that the incresed J_sc of dimer DPor-PDI devices is contributed by the significantly improved utilization of light energy.However,the two porphyrin units of DPor-PDI are not coplanar due to the introduction of the single bond,and it is not easy to form ordered intermolecular packings in thin films,hence the FF of the devices is low.Our results suggest that attentions should be paid to adjusting their LUMO levels to obtain high V_oc,and the connection mode of porphyrin dimers should also be changed to improve the coplanarity of molecules and the degree of ordered packings in films in the subsequent molecular design of small molecular acceptors based on porphyrin dimers.In recent years,the donor and acceptor materials in porphyrin-based BHJ solar cells are meso-substituted,but the electron-withdrawing carbonyl in chlorophylls is at the?position.Therefore,the functionation of porphyrin?positions has scientific significance in the study of OSCs.In chapter 5,a small molecular acceptor?-TPDI-Ni is designed and synthesized by connecting four PDI groups through acetylene bridges at the?positions of a porphyrin core,and the OSCs are prepared with?-TPDI-Ni as the acceptor and PTB7-Th as the donor.This is the first time that?-substituted porphyrin materials have been applied to BHJ organic solar cells with a V_oc of 0.65 V,a J_sc of 10.21 m A cm^-2,an FF of 45.97%and a PCE of 3.03%in devices.