High Voltage Organic Photovoltaic Cells Based On Benzotriazole Non-Fullerene Acceptors

In recent years,the power conversion efficiency(PCE)has been improved in organic solar cells(OSCs)by design and development of the polymer donor and nonfullerene acceptor(NFAs)materials.Compared to the inorganic silicon and perovskite solar cells,the PCE of organic solar cells is still lower.Therefore,how to further reduce the voltage loss of OSCs and investigate the mechanism of voltage loss is a hot research topic in the field.In order to achieve effective exciton dissociation in fullerene systems,the driving force should traditionally be larger than 0.3 eV.For some non-fullerene systems,the exciton dissociation can be effective with very small energy shifts between the donor and acceptor.Reducing the driving force for exciton dissociation to the charge transfer state is an effective method of reducing voltage losses.However,there is a lack of systematic studies on how to reduce the voltage loss by matching the donor and acceptor while ensuring effective exciton dissociation.In this research context,six molecules with A2-A1-D-A1-A2 as the main chain were designed and synthesized to prepare organic photovoltaic devices by co-blending with the classical D(Donor)A(Acceptor)type narrow bandgap polymer donor PTB7-Th and wide bandgap polymer donor PTQ10,respectively.The effect of fine-tuning the acceptors structure on the performance and energy loss of D-A type polymer-based devices was systematically investigated.The main research include:(1)Fluorination and side-chain engineering were used to design three types of acceptor materials with A2-A1-D-A1-A2 as backbone,named as BTA45,F-BTA45 and F-BTA5,where the D-unit is IDT,A1-unit is benzotriazoleand(BTA)and A2-unit is dicyanomethylene rhodamine with benzyl groups.F-BTA45 is the introduction of fluorine atoms into Ai-unit of BTA45 and F-BTA5 is the replacement of alkoxy groups with alkyl groups on D-unit of F-BTA45.Compared with BTA45,F-BTA45 has the higher maximum extinction coefficients and the lower LUMO energy level.However,the introduction of fluorine atoms into the acceptor causes a drop in its energy level,so the F-BTA45-based device obtains a VOC below 1.0 V.To compensate for the voltage loss,we applied side chain engineering to replace the alkoxy group of F-BTA45 with an alkyl group,named F-BTA5.To prepare the devices,the classical narrow bandgap D-A tape polymer donor material PTB7-Th was chosen.F-BTA5-based organic solar cell achieved a high VOC of 1.03 V,Jsc of 15.29 mA cm-2,FF of 0.66 and a PCE of 10.36%.The device based on PTB7-Th:BTA45 obtained a PCE of 7.97%with a VOC of 1.06 V,Jsc of 12.62 mA cm-2 and FF of 0.59.The device based on PTB7-Th:FBTA45 achieved PCE of 8.48%with VOC of 0.99 V,Jsc of 12.73 mA cm-2 and FF of 0.67.The results show that side-chain engineering and fluorination can be used to optimize the A2-A1-D-A1-A2 structure of the non-fulleren acceptors to achive high voltage and efficiency.(2)In the second work,we designed three acceptor materials based on the results of the first work,named as BTA5,BTA5-Cl,and Cl-BTA5.The backbone of acceptors is A2-A1-D-A1-A2 where the IDT as D unit,the BTA as A1 unit,and the dicyanomethylene rhodamine with benzyl groups as A2 unit.BTA5-C1 is the introduction of chlorine atoms into the A2 unit of BTA5,while Cl-BTA5 is the introduction of chlorine atoms into the A1 unit of BTA5.In contrast to the D-A tape narrow bandgap polymer donor PTB7-Th,PTQ10 is structurally simple,easy to synthesise and is more widely used in high efficiency systems of organic photovoltaic.The introduction of fluorine or chlorine atoms into most small molecule acceptors will reduce the HOMO and LUMO energy levels.Chlorine,a cognate of fluorine,is also strongly electronegative and the chlorine-carbon bond has a larger dipole than the fluorine-carbon bond.Moreover,the synthesis of chlorine-containing compounds is simpler than fluorine-containing compounds with lower costs.The device based on PTQ10:Cl-BTA5 obtained a high VOC of 1.17 V,Jsc of 13.31 mA cm-2,FF of 0.68 and the highest PCE of 10.73%.The device based on BTA5 yield a PCE of 4.63%(VOC=1.26 V,Jsc=6.23 mA cm-2,and FF of 0.59).The device based on BTA5-Cl obtained a PCE of 8.38%(VOC=1.21 V,JSC=10.49 mA cm-2,and FF=0.66).The work shows that compared with BTA5,chlorine substitution of BTA pulls down the LUMO energy level of the acceptor and reduces the VOC,while forming a better phase separation shape with PTQ10,and greatly enhancing the Jsc.Moreover,the degree of improvement was different for different positions of chlorine substituted BTA.Therefore,chlorine substitution at different positions in A2-A1-D-A1-A2 type acceptors is a simple and effective method to obtain high performance and high voltage.