Fluorinated Side Chain Engineering Of Twodimensional Benzodithiophene Based Wide-Bandgap Polymer Dnonors

Compared with traditional inorganic solar cells,organic solar cell（OSCs）has the advantages of light weight,relatively simple preparation process and easy to manufacture large area flexible devices.So far,the efficiency of OSCs has reached 20.2%.During this process,many molecular design strategies for constructing efficient polymer donors have been developed,and the halogenation strategies is one of the widely recognized effective strategies for regulating the optoelectronic properties of active layer materials.The paper systematically studied the effect of the number of fluorine atoms on the performance of thiophene side chains using halogenation strategies,and further studied the effect of the proportion of doped monomer BD2FT on material properties using the strategy of constructing ternary polymers.Among various halogen atoms,fluorine atom is an element with the strongest electronegativity and the smallest atomic radius,which can be effectively functionalized into electron withdrawing substituents and deepen its HOMO energy level,so as to obtain higher open circuit voltage(V_OC)in OSCs without increasing the spatial hindrance.On the other hand,fluorine substitution on conjugated frameworks can regulate the planarity of molecules and improve intermolecular stacking through non-covalent interactions within/between molecules,such as F··H and F··S,thereby enhancing charge transport properties.In the second chapter,a new wide band gap polymer PBD2FTBDD was obtained by copolymerization of a difluoro substituted alkyl thiophene side chain with the 1,3-bis（thiophen-2-yl）-5,7-bis（2-ethylhexyl）benzo[1,2-c:4,5-c′]dithiophene-4,8-dione（BDD）unit.,The optical,electrochemical,film stacking and photovoltaic performance of the polymer were systematically studied and compared with PM6 and PBDB-T.As the number of fluorine atoms increased,the energy level of the polymers was gradually deepened,and the molar extinction coefficient was firstly increased for PM6 and then decreased for PBD2FTBDD.And the inter chain stacking of the polymer was weakened,while theπ-πstacking was gradually strengthened.Blending PBD2FTBDD,PM6,and PBDB-T with the small molecule acceptor Y6 to fabricate OSCs,it was found that as the number of fluorine atoms increased,the open circuit voltage(V_OC)of the device gradually increased.Finally,the device based on PBD2FTBDD:Y6achieved a high open circuit voltage(V_OC)of 0.897 V and an efficiency（PCE）of 9.64%.In Chapter 3,in order to further regulate the energy level structure of polymer donor materials,a ternary copolymerization strategy was adopted to introduce BD2FT as the third component into PM6,and three ternary copolymers with different doping ratios,P2FM1,P2FM2,and P2FM3,were constructed.Compared with PM6,the absorption of the ternary copolymer shows a slight blue shift,deepening the energy level,and slightly increasing the band gap.As the content of the third component unit gradually increases（from 10%to 30%）,the HOMO energy level of the polymer gradually increases,the molar extinction coefficient gradually increases,the absorption performance enhances,and the inter chain stacking of the polymer increases,but it is still weaker than PM6;Theπ-πstacking gradually increases and is stronger than PM6.By blending P2FM1,P2FM2,and P2FM3 with receptor small molecule Y6 to prepare devices,it was found that as the doping ratio increased,the recombination probability in the device increased,while the exciton dissociation rate gradually decreased.Finally,the highest 11.85%of PCE,0.865 V of V_OC,22.94 m A·cm^-2 of short circuit current density(J_SC)and 59.73%of Fill factor（FF）were obtained based on P2FM1 photovoltaic devices.In order to further investigate the impact of fluorine side group engineering on the photovoltaic performance of high-efficiency polymer donor materials,BD2FT was introduced as the third component into polymer D18 using a ternary copolymerization strategy in Chapter4.Three ternary copolymers P2FD1,P2FD2,and P2FD3 were synthesized.Two homopolymers D18 and PBD2FTDTBT were used as control groups to systematically study the photovoltaic properties of five polymer donor materials.Through the comparison of the performance tests of five polymers,it can be seen that the thermal stability of the three ternary polymers P2FD1,P2FD2,and P2FD3 is not significantly different,and has been improved compared to PBD2FTDTBT.All four polymers can meet the requirements of later device processing and have strong absorption light stability.The PBD2FTDTBT with difluoro substitution on the side chain has a wider bandgap compared to the D18 with monofluoro substitution,while the bandgap sizes of the three ternary polymers are between the two.The absorption of P2FD1,P2FD2,and P2FD3 in solution and film is very similar,indicating that the change in doping ratio has little effect on the absorption of these three ternary polymers.PBD2FTDTBT has stronger absorption capacity than D18,while the absorption capacity of the ternary polymer for light is further strengthened.As the doping ratio increases,its molar extinction coefficient gradually increases,and the absorption performance also enhances.In terms of crystallinity,comparing D18 and PBD2FTDTBT,it can be seen that an increase in the number of fluorine atoms enhances the inter chain packing of the polymer,whileπ-πpacking weakens.Introducing a third component to construct a ternary polymer reduces the inter chain packing andπ-πpacking of the polymer compared to D18.However,as the proportion of BD2FT impurities increases,the inter chain packing andπ-πpacking of the polymer gradually strengthens,and the inter chain packing of P2FD2 and P2FD3 is significantly stronger than D18,However,it is still weaker than PBD2FTDTBT.In terms of photovoltaic performance,the substitution of difluoride on the side chain improves the V_OC of the device,but the lower current and Fill factor make the PCE of the device based on PBD2FTDTBT much lower than D18,while the V_OC of the ternary copolymers P2FD1,P2FD2 and P2FD3 is between PBD2FTDTBT and D18,and retains a higher FF and good J_SC,making its PCE higher than D18.Based on the P2FD1 photovoltaic device,a maximum of PCE of 14.28%,V_OC of 0.861 V,J_SC 25.20 m A·cm^-2,and FF of 65.84%were obtained.Further research on exciton dissociation and carrier behavior in devices has revealed that,the difluoro substitution on the side chain enhances the bimolecular recombination of PBD2FTDTBT:Y6 devices,while reducing trap recombination,and its exciton dissociation rate is much lower than that of D18.Compared with P2FD1,P2FD2,and P2FD3,it was found that as the doping ratio increased,bimolecular recombination increased,but all were weaker than bimolecular recombination in D18:Y6.Compared to D18:Y6,trap recombination has been effectively suppressed,and as the doping ratio increases,trap recombination becomes stronger and stronger.The exciton dissociation of the device gradually decreases with the increase of doping ratio,and the exciton dissociation based on three ternary polymers and Y6 devices is similar to D18:Y6,while higher open circuit voltage and lower recombination make its efficiency higher than D18:Y6.Further research on the photostability of the device revealed that as the doping ratio increased,the photostability of the device gradually decreased.Blending P2FD1 with Y6 with a 10%impurity ratio resulted in a normalized PCE of 78%,with the strongest photostability.The blending of PBD2FTDTBT and Y6 without impurities also achieved 78%of PCE photostability.