Synthesis And Properties Of Organic Semiconductor Materials Based On Benzotrithiophenes

Organic solar cells(OSCs) are promising and competitive photovoltaic devices in the international solar cell research field, due to its lightweight,flexibility, excellent processability and large area applications. In the last decade, bulk heterojunction(BHJ) based on the blend of conjugated organic semiconductor/PCBM has been explored deeply. Now power conversion efficiency of5-9%has been achieved for BHJ OSCs based on polymer donors and fullerene acceptors,3-7%for BHJ OSCs based on small molecular donors and fullerene acceptors respectively. There are two reasons leading to low efficiency:(1) band gap of donors and acceptors can not fit solar spectrum which results in low light absorption efficiency; energy level of donors does not match that of acceptors which results in low output voltage.(2) The restriction of exciton diffusion length and charge recombination during charge transport, unorderly morphology of active layer cause low carrier mobility and output current. In order to obtain donor materials with strong and wide absorption in visible light zone, tunable energy level and band gap which can match that of acceptors (e.g. PCBM) great solution processibility and carrier transport property, this study design two noval donor materials based on benzotrithiophenes (BTT), which can be applied to polymer solar cells and organic small molecule solar cells respectively. The research on the synthesis, characterization and properties of BTT based materials has been carried out as following.Compound7with potential quinoid structure were designed and synthesized. The structure of target molecule and monomers were identified by1H and13C NMR spectra and mass spectrum. Compound7can be used as a electronic rich monomer in D-A alternative polymer with narrow band gap. The polymer can obtain extra stabilization energy through monomer7's quinoid structure leading to low band gap, which contributes to the promotion of light absorption of active layer in organic solar cells.BTTh-BTD-1and BTTh-BTD-2were designed and synthesized with2D planar π-conjugated star shape structure. The chemical structure, thermal properties, absorption spectra and electronic structures were studied via1H and13C NMR, TG, DSC, UV-vis, FL and CV. BTTh-BTD-1and BTTh-BTD-2embrace good thermal stability and solubility in common organic solvent. They have a wide absorption in the visible region and their thin film materials demonstrate large red shift when compared with their solutions, which indicates strong intramolecularπ-π stacking in films. Compared to the chemical structure of BTTh-BTD-1, BTTh-BTD-2has enlarged π congjugation in planar with one more thiophene unit inserted into the branch conjugation, which greatly affect the optical and electronic properties. The protons on conjugated backbone of BTTh-BTD-2show obvious passivation and the chemical shifts move downfield.λmax and λonset from UV-vis spectra both in solution and film exhibit greater red shifts, which resulting in a lower band gap of1.88eV to1.99eV of BTTh-BTD-1. The fluorescence of BTTh-BTD-2in CH2C12solution diminishes dramatically. Cyclic voltammetry (CV) test indicates the HOMO level of BTTh-BTD-2locating at-5.35eV,0.15eV higher than that of BTTh-BTD-1(-5.5eV). Thereby, the energy level and band gap of these star-shaped molecule donors can be tuned by adjusting the ratio of benzo[2,1,3]thiadiazole group and thiophene group on the branched backbone. These series of semiconductor materials with2D planar structures will show different optical and electronic properties from1D linear semiconductor materials.