Synthesis Of P-type D-A Conjugated Copolymers Based On Benzodithiophene And Properties Of All-polymer Solar Cells

Recently,all-polymer solar cells(all-PSCs),utilizing a p-type polymer as donor and an n-type polymer as acceptor,have attracted much attention recently due to the advantages of stability and continually improved conversion efficiency(PCE).The n-type conjugated polymer acceptors mostly used in the all-PSCs are naphthalene imide(NDI)-containing polymers because of-their high electron mobility,high electron affinity,and broad light absorption.Besides designing NDI-containing polymers,selecting matched donor/acceptor pairs is crucial.Firstly,the energy level of the donor/acceptor were matched to satisfy the high open circuit voltage and driving current.Secondly,the absorption of'the donor/acceptor under visible range should be complementary and fully absorbed by sunlight to achieve high current.Thirdly,the donor/acceptor have appropriate crystallinity and compatibility to ensure that the active layer exhibited good morphology with a suitable phase separation scale.The balance of crystallinity and compatibility between the donor and acceptor is key to obtain optimal nano-scale bulk heterojunction.In our provious work,the D-A polymer(PBDT(T)-HT-TPD)based on thienopyrrole and a series of NDI-containing polymer acceptors(x-NDI)were synthesized and optimized to obtain the matched donor/acceptor pair.In this work,the polymer donors were synthesized with the introduction of thiophene ?-bridge or modified side chains.Compared with PBDT(T)-TPD,the planarity of the main chain and crystallinity of polymers decreased,as well as the absorption band gap blue shifted because of the thiophene ?-bridge.Meanwhile,a series of NDI-containing polymer acceptors with different crystallinitity were synthesized.By selecting proper donor/acceptor pairs,the photo-electrical(photovoltaic)properties and crystallinity were studied in order to obtain the well matched donor/acceptor pair with good absorption complementary and compatibility.To obtain all-PSCs with high PCE value,the works were following:1.A series of donor materials PBDT(T)-T-TPD,PBDT(T)-HT-TPD and PBDT(ST)-HT-TPD were synthesized by the stille coupling reaction using thienopyrodazole(TPD)and benzodithiophene(BDT)as acceptor and donor unit.Two NDI-containing polymer acceptors P(NDI20D-T)and P(NDI20D-Se)with longer side chains were synthesized.These polymers were characterized by TGA,UV-Vis,CV,GPC and XRD.From UV-Vis and XRD.It was found that the bandgap of the polymers were widen and crystallinity were weaken when introducing thiophene ? bridge.By studying the device performance,the better matched donor/acceptor combination were selected,as follows:PBDT(T)-HT-TPD:P(NDI20D-T),PBDT(T)-HT-TPD:P(NDI20D-Se),PBDT(ST)-HT-TPD:P(NDI20D-T),PBDT(ST)-HT-TPD:P(NDI20D-Se).The photoelectric conversion efficiencies(PCEs)based thess devices were shown 4.55%,5.97%,3.87%,and 4.22%,respectively.The results revealed that the donor polymer PBDT(T)-HT-TPD(PBDT(ST)-HT-TPD)matched with P(NDI20D-T)(P(NDI20D-Se)).2.The two NDI-containing polymer acceptors P(NDI2HD-T)and P(NDI2HD-Se)with shorter chain chains were synthesized by the stille coupling reaction.They were characterized by TGA,UV-Vis,CV,GPC and XRD.All polymers have very good thermal properties.The results showed that shortening the side chain of'the NDI unit significantly reduced the optical bandgap and widened absorption range.Thus,the acceptors and wide-bandgap donors synthesized in chapter 1 possessed better complementary absorption in the wide wavelength region.The complementary absorption and broad absorption of polymer donor/acceptor blend film is desirable for enhancing light harvest so that increasing the short-circuit current density(Jsc)of the device.The energy levels also match each other,which shortening the side chain of the NDI unit will cause the LUMO level to shift upwards,and the Voc of the device will increase.Finally,no significant phase separation or large pure polymer domains in the blend film can be obtained by photoluminescence analysis,indicating that the two polymers can be well mixed without large-scale phase separation,which is helpful for device performance.Therefore,we can get a good match between the two NDI-containing polymers with shorter side chains and the wide bandgap donor polymer.3.Based on above work,PBDT(T)-HT-TPD and PBDT(ST)-HT-TPD were selected as donor.To obtain the excellent properties,the devices based PBDT(T)-HT-TPD:P(NDI2HD-Se)were optimized by process of the device,including the proportion of the acceptor,the concentration of the solution,the amount of the additive,annealing temperature,the cathode buffer layer,non-halogen solvent.The PCE of four devices based(PBDT(T)-HT-TPD:P(NDI2HD-T),PBDT(T)-HT-TPD:P(NDI2HD-Se),PBDT(ST)-HT-TPD:P(NDI2HD-T),and PBDT(ST)-HT-TPD:P(NDI2HD-Se),showed 5.21%,6.80%,4.88%and 4.63%,respectively.The structure and properties of device performance were characterized by atomic force microscopy(AFM),external quantum efficiency(EQE),electron and hole mobility.External quantum efficiency results revealed that these two donor-acceptor blends possessed good complementary absorption in the visible range responding to sunlight.Meanwhile,the important reason for the high efficiency based PBDT(T)-HT-TPD:P(NDI2HD-Se)device was that the active layer formed a significant fibrous reciprocal transmission network structure and lower roughness,which was conducive to the contact between the active layer and the electrode interface for higher charge transfer.