Design And Properties Of Organic Small Molecule Photovoltaic Donor Materials With Spirobifluorene As Core

Over the last two decades,a tremendous number of interdisciplinary approaches have been put into bulk-heterojunction(BHJ)organic solar cells(OSCs)research that have led to unprecedented progress and breakthroughs.The flexibility,lightness and wearability of organic solar cells promise remarkable and innovative applications that are difficult to achieve with conventional solar cells.By virtue of clear molecular structure,uncomplicated synthesis and easy modification,organic small molecule photovoltaic materials have become one of the most promising photovoltaic materials.This work aims to develop organic small molecule photovoltaic donor materials with good photoelectric properties by means of structure design.Spirobifluorene has been regarded as a potential electron donor unit for organic photovoltaic materials due to its unique three-dimensional structure,excellent photoelectric properties and good structural modifiability.In this paper,the spirobifluorene(SF)unit is selected as the electron donor core,and the diketopyrrolopyrrole(DPP)and benzothiadiazole(BT)unit are selected as the electron acceptor units of the materials.Based on the strategy that acetylene?-bridge and terminal carbazole units are introduced separately or simultaneously in the molecular skeleton to extend the two arms,nine novel organic small molecule photovoltaic donor materials were designed and synthesized.According to the different electron acceptor unit,they are divided into spirobifluorene-diketopyrrolopyrrole(SF-DPP)materials and spirobifluorene-benzothiadiazole(SF-BT)materials.Herein,density functional theory calculation was preliminarily performed to guide material design and predict the properties of target compounds.Performance characterizations such as light absorption,electrochemistry,thermogravimetry and X-ray diffraction have been applied to explore the relationship between molecular structure and properties.The photovoltaic performance of devices based on the synthesized materials has been investigated in detail.Due to the high planarity and strong electron withdrawing effect of DPP group,all of SF-DPP materials possess narrow band gap,broad and strong optical absorption,high charge mobility,etc.Among them,SF(DPPCz)2 with carbazole units as the end groups exhibits the broadest absorption with the absorption edge at 839 nm.By introducing both acetylene?-bridge and carbazole terminals,SF(aDPPCz)2 exhibits the strongest optical absorption with a molar extinction coefficient(?)of 1.67×10⁵ M^-1 cm^-1.The photovoltaic device based on SF(DPPCz)2 has obtained a power conversion efficiency(PCE)of 8.12%after optimization,which is the highest value obtained in this work.Compared with SF-DPP materials,SF-BT materials have good solubility and film-forming property.By introducing carbazole units as end groups,SF(BTCz)2 exhibits a broad absorption and a narrow band gap of 1.50 eV.SF(aBTCz)2 with the introduction of acetylene?-bridge and carbazole end groups possesses the highest?of 9.3×10⁴ M^-1 cm^-1 in this series of materials.The photovoltaic device based on SF(BTCz)2 has obtained a PCE of 6.28%after optimization.From the point of view of material properties,the excellent device performance could be attributed to narrow band gap,broad and strong optical absorption and other good properties of materials.The introduction of acetylene?-bridge enhances the planarity and optical absorption of materials.And the introduction of carbazole end groups broadens the conjugate plane of materials,which not only enhances and broadens the absorption but promotes intramolecular charge transfer and narrows the band gap.While acetylene?-bridge and carbazole end groups could construct large conjugated planar molecules with strong absorption.It could be proved that designing two-arm extended molecules with three-dimensional spirobifluorene as central core is an effective strategy for constructing organic small molecule donor materials with outstanding photoelectric properties.