侧链修饰对基于芴-吡咯并吡咯二酮小分子给体光伏性能的影响

Side-chain modification effect on photovoltaic properties of fluorenediketopyrrolopyrrole based small molecule donor

本文分别将正辛基和正辛氧基苯基作为Flu(nDPP)_(2)和Flu(phDPP)_(2)的侧链,设计合成了2种基于芴-吡咯并吡咯二酮的有机小分子给体材料Flu(nDPP)_(2)和Flu(phDPP)_(2).以此来探讨侧链修饰对分子光伏性能的影响.通过紫外-可见光吸收谱图、循环伏安曲线和光伏器件的J-V曲线和外量子效率谱图等测试方法对两者的光学性质、电化学性质以及光伏性能等方面进行表征.结果表明,Flu(nDPP)_(2)和Flu(phDPP)_(2)在氯仿溶液状态下呈现了相似的光吸收谱图,但是在薄膜状态下,Flu(nDPP)_(2)的边缘吸收波长比Flu(phDPP)_(2)的红移程度更大,表明Flu(nDPP)_(2)在薄膜状态下形成了更为紧密的堆积方式.此外,Flu(nDPP)_(2)和Flu(phDPP)_(2)分别获得了1.86和1.88eV较为理想的电化学带隙.将2种给体材料分别与PC_(71)BM进行混合制备了有机光伏器件,结果显示,基于Flu(nDPP)_(2):PC_(71)BM和Flu(phDPP)_(2):PC_(71)BM的有机太阳能电池分别获得了4.10%和2.80%的能量转换效率,并且都实现了0.94V的高开路电压.两者能量转换效率的差异主要是因为基于Flu(nDPP)_(2):PC_(71)BM的共混膜比基于Flu(phDPP)_(2):PC_(71)BM的共混膜表现出更有效的光-电响应、更高以及更平衡的载流子迁移率,这也使得基于Flu(nDPP)_(2):PC_(71)BM的器件实现了14.42mA·cm^(-2)的较高的短路电流密度.研究表明,相比于正辛氧基苯基,正辛基在增强分子固体堆积和提升基于芴-吡咯并吡咯二酮给体的光伏性能等方面更有优势.

In this work,two novel A-π-D-π-A type donors,namely Flu(nDPP)_(2) and Flu(phDPP)_(2) were designed and synthesized to fully investigate the effect of side-chain on the photovoltaic properties of fluorene-DPP based small molecule donor(SMDs).Fluorene is employed as donor unit due to its good planarity and deep HOMO energy level,facilitating electron delocalization and achieving a high open-circuit voltage(VOC)in organic solar cells(OSCs).In order to enhance the intermolecular interaction and optical absorption,DPP was introduced into the molecule as accepter unit because of its intense absorption in the visible region and intermolecular interaction,such as hydrogen bonding and π-πinteractions.In addition,ethynyl functionalized thiophene was used as π-bridge bond linking fluorene and DPP.Remarkably,n-octyl was used as side chains of Flu(nDPP)_(2) while Flu(phDPP)_(2) was developed by replacing n-octyl with n-octyloxyphenyl as side chains.The relationship between side-chain structure and photovoltaic properties was studied by UV-Vis absorption spectra,cyclic voltammetry curves,current density-voltage(J-V)measurements and EQE spectra of OSCs employing Flu(nDPP)_(2) and Flu(phDPP)_(2) as donors and PC_(71)BM as acceptor.Consequently,from solution to films,the absorption edge wavelength(λfilm edge)for Flu(nDPP)_(2) exhibits much larger redshift than that of Flu(phDPP)_(2),indicating that Flu(nDPP)_(2) can form tight molecular packing in film.The electrochemical bandgap(ECV g)of Flu(nDPP)_(2) and Flu(phDPP)_(2) are 1.86 and 1.88eV,providing a desirable bandgap as donors.More importantly,Flu(nDPP)_(2) and Flu(phDPP)_(2) were employed as electron donors blending with PC_(71)BM as electron acceptor for solution-processable OSCs to test their photovoltaic performance.As a result,OSCs based on Flu(nDPP)_(2):PC_(71)BM and Flu(phDPP)_(2):PC_(71)BM achieve power conversion efficiencies(PCEs)of 4.10%and 2.80% with high VOCof 0.94V.The difference in PCE can be attributed to more effective photon-electron response,higher and more balanced charge carrier mobility of devices for Flu(nDPP)_(2):PC_(71)BM than that of Flu(phDPP)_(2)-based devices,which helps the former achieves a short-circuit current density(JSC)of 14.42 mA·cm^(-2).This work reveals that n-octyl is more effective in enhancing the packing order and the photovoltaic properties of fluorene-DPP backbone based donors compared with n-octyloxyphenyl.