The Design,Synthesis And Optoelectronic Properties Of Low Bandgap Conjugated Polymers With High Mobility Based On Side Chain Engineering Strategy

As the promising materials for the next generation of displays,field effect transistors and solar cells,organic semiconductors have attracted much attention.Great efforts have been devoted to developing various conjugated polymers,so as to improve the device performances.To date,the performances of these devices have been largely enhanced.For example,polymer solar cells(PSCs)have shown power conversion efficiencies(PCEs)of up to 13%.The mobilities up to 10 cm2/(V s)have achieved in conjugated polymer based organic field effect transistors(OFETs).The development of low bandgap conjugated polymers with high mobility are important and necessary because it could not only absorb more light to improve the PCEs,but also accelerate the charge transport,which can result in outstanding performance in thick-film PSCs.When designing novel conjugated polymers,selecting the proper side chains is as important as the selection of conjugated backbones.Side chain engineering is an effective approach to tune the properties of polymers and enhance the device performance.In chapter 2,we introduced some representative side chains used in organic semiconductors.Then we selected some examples in side chain engineering towards new conjugated polymers with improved device performances.In chapter 3,for desired large area and high speed roll-to-roll printing,an ideal active layer should not only display outstanding PCE at an optimal thickness,but also maintain enough high PCEs when a large thickness variation occurs.So we introduced siloxane-terminated side chain to the backbone of PFBT4T based on 5,6-difluoro[2,1,3]benzothiadiazole(FBT)and quarterthiophene(4T),resulting in a series of alternating and side-chain random conjugated copolymers with different ratios of the siloxane-terminated side chain.GIXD indicated that the siloxane-terminated side chain could induce side-chain random polymers with face-on orientations,giving high 3-D hole transport in neat films as supported by a high OFET hole mobility up to 2.46 cm2/(V s)and an SCLC hole mobility up to 5.9 × 10-2 cm2/(V s)in hole-only devices.The organic solar cell exhibited outstanding power conversion efficiencies(PCEs)>10%under large active layer variation from 270 nm to 600 nm.This is also the first PCE of more than 10%achieved by an active layer of a 600 nm thickness level in PSCs.Another notable feature is very high fill factors of more than 74%and 71%being achieved for very thick active layers of 420 and 600 nm,respectively.In chapter 4,in order to improve the stability ofpara-quinodimethane(p-QM),which act as an organic optoelectronic material building block but can’t be directly introduced to the backbone of conjugated polymers,we introduced a stable p-QM variant,namely p-azaquinodimethane(p-AQM)that incorporates nitrogen atoms in the central ring and alkoxy substituents on the periphery to increase the stability of the quinoidal structure.We firstly reported the chemical structure,synthesis and photoelectric properties of p-AQM as building block.The quinoidal character of the p-AQM unit endow the resulting polymers with narrow band gaps and high carrier transport mobilities of up to 0.54 cm2/(V s).The study of a series of copolymers employing different numbers of thiophene units revealed an unconventional trend in band gaps.Theoretical calculations have shed light on the nature of this trend,which may open the door to a unique class of conjugated polymers with promising optical and electronic properties.In chapter 5,we investigated and studied the influence of branching positions of side chains on the photoelectric properties of p-AQM-based polymers and the OFET device performance.A serial of p-AQM-based polymers with different side chain branching position were designed and synthesized.The results demonstrated the interesting odd-even effects in the yield of monomers,molecular weight,solubility and hole mobility of p-AQM-based polymers induced by the different branching position.After moving the branching point away from the backbones,the polymers showed gradually elevated HOMO levels,decreased bandgaps and transformation from a face-on orientation to an edge-on orientation.An exceptionally high mobility of 1.05 cm2/(V s)was achieved.Our study revealed that side chain engineering is a powerful strategy to achieve efficient transport in p-AQM-based polymers and provide a better understanding of the structure-property relationship.