Design,Synthesis And Properties Of Novel Organic Photovoltaic Small Molecular Donor Materials

Solution-processed bulk heterojunction（BHJ）organic solar cells（OSCs）have attracted intense attention in recent years due to their noteworthy merits,such as low cost,light weight,ease of large-area fabrication by roll-to-roll printing and flexibility.Compared with the conjugated polymers,small molecules possess some unique advantages,such as well-defined molecular structures,high purity,easier energy level control,simple synthetic procedures and high reproducibility.They are supposed to be good candidates for investigating the relationship between chemical and structure-photovoltaic performance,which is highly important for further molecular design to achieve higher photovoltaic performance.Abstracts of each part are presented as following:Firstly,three new D₁-A-D₂-A-D₁ type small molecules DTBTBDTT,DTBTBDTO and DTBTDTS with 4,8-bis（5-（2-ethylhexyl）thiophen-2-yl）benzo[1,2-b:4,5-b’]dithiophene,4,8-bis（（2-ethylhexyl）oxy）benzo[1,2-b:4,5-b’]dithiophene and 4,4-dioctyl-4H-silolo[3,2-b:4,5-b’]dithiophene as core units,respectively,have been designed and synthesized.The influence of centeral donor units on their optical,thermal,electrochemical properties and device performance has been studied.All three compounds exhibit good stability and possess a wide absorption range and a high extinction coefficient in the visible-near-infrared region.The maximum absorption of DTBTDTS exhibits a significant red shift relative to the other two compounds,and its Highest Occupied Molecular Orbital（HOMO）level is also higher than those of the other two compounds because the electron donating ability of the DTS unit is stronger than that of BDT unit.Besides,compared with bithiophene,the trans bithiophene with vinyl group between the two thiophenes has a longer effective conjugation length and a narrower bandgap.However,the introduction of vinyl reduces the solubility of the three compounds,which leads to poor film during device fabrication and low power conversion efficiency（PCE）.Secondly,two novel A-D-A type small molecules DINDTS and DINCNDTS with 1,3-Indanthenone（INs）and its malononitrile derivatives（INCNs）as different end-capping groups have been designed and synthesized,and the effects of these two end-capping groups on their thermal,optical,electrochemical and photovoltaic properties have been systematically investigated.Both two compounds have good thermal stability,wherein DICNINDS has better crystallinity than compound DINDTS.The maximum absorption of the compound DINCNDTS shows a significant red shift relative to DINDTS.Its LUMO energy level is also lower and the band gap is narrower than that of DINDTS.The device based on DINDTS has a power conversion efficiency of 6.60%and a short-circuit current density(J_sc)of 13.50mA cm^-2.However,the PCE of the device based on DINCNDTS is only 0.58%,and its J_scc is only 1.82 mA cm^-2,which is mainly due to the large phase separation of the film morphology and too small?E_LUMOUMO of donor and acceptor.Thirdly,DTS-basedA-D-AtypesmallmoleculeDINTTDTSwith thieno[3,2-b]thiophene（TT）asπ-bridge has been designed and synthesized.DINTTDTS has good thermal stability and better crystallinity than that of DINDTS.DINTTDTS exhibits broad absorption extending to over 800 nm and its bandgap is lightly narrower than that of DINDTS.All-small-molecules solar cells were fabricated with DINDTS and DINTTDTS as donor materials respectively and IDT-C8 as acceptor materials.The PCE of the optimized DINTTDTS-based devices was 2.79%.The PCE of the the optimized DINDTS-based devices was 4.52%.J_ph-V_effff curves show that the exciton separation and charge collection efficiency of DINDTS-based device is higher than that of DINTTDTS-based device,which is closely related to its higher J_scc and FF.Besides,its external quantum efficiency in the wavelength range of300-800 nm is slightly higher than the DINTTDTS-based devices.Therefore,the PCE of DINDTS-based device is higher than that of DINTTDTS-based devices.