Study On The Design Of High-efficiency Organic Small Molecule Photovoltaic Donor Materials And Device Optimization

Organic photovoltaic?OPV?devices have attracted great attentions in recent years due to their advantages of low cost,light weight,flexible,large-scale production,etc.In this dissertation,we focused on the improvement of power conversion efficiency?PCE?of small molecule based OPV devices from the perspective of donor material design and device optimization.Firstly,the strategies of enhancing the highest occupied molecular orbital?HOMO?and lowering the lowest unoccupied molecular orbital?LUMO?levels through changing donor unit and acceptor unit of acceptor-donor-acceptor?A-D-A?molecules,respectively,have been used to reduce the band gap of small molecules.Two low band gap small molecule donor materials,named as DR3TDTN and DTB3TCz,have been designed and synthesized for solution-processed OPVs.DR3TDTN,with a strong electron donating unit?dithienopyrrole?as the core,showed high-lying HOMO energy level of-4.74 eV,and narrow optical band gap with value of 1.49 eV.The device based on DR3TDTN:PC₇₁BM showed a PCE of 3.03%with a relatively low open voltage(V_oc)of 0.67 V.DTB3TCz with a weak electron donating unit?carbazole?as the core and a strong electron withdrawing unit?thiobarbituric acid?as the terminal,exhibited deep HOMO and LUMO energy levels,and narrow optical band gap of 1.61 eV.The device with DTB3TCz:PC₇₁BM achieved high V_oc of 0.91 V and short-circuit current density(J_sc)of 11.80 mA cm^-2,and a PCE of 5.26%.Secondly,the method of solvent vapor annealing?SVA?has been used in the ative layer morphology optimization with great success for small-molecule-based OPV devices.For DR3TBDTT:PC₇₁BM based blend films,four solvents?CS₂,CHCl₃,THF and CH₂Cl₂,?are employedas SVA solvents.Thesolvent quality-morphology-performance relationship has been established.Good solvents?CHCl₃ and CS₂?for donor and acceptor readily crystallize donor molecules,leading to increased length scale of phase separation and improved domain purity,contributing to enhanced device performance.SVA with CHCl₃ and CS₂ gave the best device performances with PCEs of 9.12%and 9.58%,respectively.Next,solvent vapor annealing together with thermal annealing was used to fine-tune the morphology of DRCN5T:PC₇₁BM active layer.Through two-step annealing treatment,the device exhibited a high PCE of 10.08%,which is the highest PCE for single-junction small molecule based organic solar cells reported to date.Thirdly,graphene quantum dots?GQDs?with uniform sizes and good conductivity have been prepared as hole transport layers?HTLs?to replace the traditional PEDOT:PSS.It was demonstrated that the GODs were excellent HTLs in both polymer-based solar cells?PSCs?and small-molecule-based solar cells?SMSCs?with the blends of P3HT:PC₆₁BM and DR3TBDT:PC₇₁M as the active layer,respectively.The PSCs and SMSCs based on GQDs yield PCEs of 3.51%and 6.82%,respectively,both comparable to those of solar cells with PEDOT:PSS as the HTLs.In addition,the devices with GQDs as HTLs exhibited much more reproducible and stable performance.These results indicate that GQDs can be a promising alternative of PEDOT:PSS for high-efficiency and stable organic photovoltaic cells.