Design,Synthesis And Optoelectronic Properties Of Novel Conjugated Organic Materials Containing Benzodithiophene For Organic Solar Cells

Regarding to the unique advantages of light weight,low cost,solution processing,and potential fabrication of flexible devices,organic solar cells(OSCs)have attracted much interests from both academic and industrial community.As a result,a range of active layer materials have been developed for single junction BHJ organic solar cells that presented remarkable power conversion efficiencies(PCEs)over 12%.Typical photoactive layer of OSCs consists of the physical blends of the electron-donating semiconducting materials and the electron-accepting semiconducting materials to form the bulk-heterojunction structures,which can allow for efficient charge separation and transport in the bi-continuous percolated systems.Among the various reported conjugated materials,the cases based on benzodithiophene(BDT)as the donor unit has been extensively used and has been proved to be one of the most promising donor units for high-performance OSC materials.To achieve even higher PCEs for OSCs,a series of photovoltaic materials based on BDT were developed in this dissertation using novel molecular design strategy.In chapter 2,a series of novelelectron-donating building block of alkyltriazolyl substituted benzodithiophene were synthesized on the basis of the Cu(I)-catalyzedazide-alkyne cycloaddition.Alternating donor-acceptortype of ?-conjugated copolymers by using such alkyltriazolyl substituted benzodithiophene as the donor units and diketopyrrolopyrrole as the acceptor units were synthesized via Suzuki polymerization.The resultant copolymers exhibited good thermal properties and can be easily dissolved in various organic solvents.All copolymers show quite comparable absorption profiles in the range of 300-850 nm with the absorption onset at about 1.4 eV,where the copolymer with longer side chains exhibited slightly enhanced absorption coefficient.The cyclic voltammetry measurements indicated the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals were nearly independent with the size of the alkyl side chains.Polymer solar cells based on the resultant copolymers as the electron-donating materials and PC71BM as the electron-accepting material exhibited moderate photovoltaic performances.In chapter 3,we reported the synthesis of diethynylbenzo[1,2-b:4,5-b']dithiophene(DEBDT)-based small molecule and three new cross-conjugated copolymers that consist of DEBDT unit.The thermal,optophysical and electronic properties of these materials were systematically investigated.The small molecule exhibited higher absorption coefficient than the cross-conjugated copolymers.The energy levels of such three cross-conjugated copolymers were lower than that of the small molecule.Thanks to the unique molecular structure and electronic properties of cross-conjugated polymers,hole mobilities of the polymers were higher than that of the small molecule.Photovoltaic performances were investigated by fabricating organic solar cells with the configuration of ITO/PEDOT:PSS/donor:PC61BM/Ca/Al.The small molecule chromophore BDTTIPS exhibited a power conversion efficiency of 4.19%,which was much higher than those obtained from devices based on conjugated polymers as the photoactive layer.These observations indicated that the rational molecular design was required for the attainment of high photovoltaic performances.In chapter 4,we developed a series of acceptor-donor-acceptor(A-D-A)type small molecule acceptors utilizing BDT units with different side chain as the core and conjugated units as ?-bridge.The thermal,optophysical and electronic properties of these materials were systematically investigated.We found that the absorption spectra and energy levels of these resultant materials can be tuned by introducing different side chain and conjugated ?-bridge.Fullerene-free polymer solar cells based on PTB7-Th:BDTPhR blended films gave power conversion efficiency of 2.42%.In chapter 5,a serie of novel acceptor-pended conjugated polymers with n-type main chain were developed as electron-accepting materials for all-polymer solar cells.This design takes advantage of the well-established knowledge of nonlinear optical chromophores to optimize the absorptions pectra and energy levels of the resultant polymers.This work demonstrates a rational design for fine-tuned optical properties and energy levels of polymer acceptors,and reveals the high potential of all-PSCs through novel molecular design stratege.Bulk-heterojunction all-polymer solar cells based on these polymer accepters exhibited a moderate photovoltaic performance with a power conversion efficiency of 5.5%via a simple fabrication architecture without any additives or post-treatments.