Study On The Performance Of Halogen Polar Molecules Treated PEDOT:PSS Electrode And Flexible Organic Solar Cells

Flexible Organic solar cells（OSCs）have become a popular research field due to their advantages of low coast,light weight,ease of fabrication and wearability.The power conversion efficiency（PCE）,stability and flexibility are the most important properties of the flexible OSCs.PEDOT:PSS has the characteristics of high light transmittance,solution processability,adjustable work function and excellent thermal stability.It is one of the ideal transparent electrodes to prepare flexible OSCs.However,the poor conductivity and work function of PEDOT:PSS flexible transparent electrodes can’t achieve flexible OSCs with high PCE.The problems are as follows:i)The current modified PEDOT:PSS electrode with lower crystallinity and contains too much sulfonate insulation,which causes the high sheet resistance;ii)Traditional high temperature and strong acid treatment can significantly improve the conductivity of PEDOT:PSS,but it could destroy the flexible substrate and can’t be used to fabricate the flexible OSC device;iii)the mismatched energy level between the PEDOT:PSS electrode,the hole transport layer and the donor of active layer,which increases the interface barrier and reduces the hole transport efficiency.Based on these key issues,this thesis describes the modification of PEDOT:PSS with halogen polar molecules,and systematically explained the regulation mechanism of halogen polar molecules on the conductivity,work function,wettability,morphology,composition and PEDOT molecular structure of PEDOT:PSS electrodes.This thesis also clarified the intrinsic relationship between the characteristics of PEDOT:PSS electrodes and the PCE and mechanical flexibility of flexible OSCs.The main research contents are as follows:（1）Trifluoromethanesulfonic（CF₃SO₃H）was used to modify the PEDOT:PSS film,and use the treated-PEDOT:PSS film as the transparent electrode for the preparation of flexible OSCs devices.By optimizing the concentration of CF₃SO₃H and the processing temperature,the PEDOT:PSS electrode can achieve a low sheet resistance of 35Ωsq^-1 through the 0.8 M CF₃SO₃H treated at 50℃.Moreover,the treated PEDOT:PSS electrode also exhibited a superior wettability and a raised work function about 5.0 e V.The high work function of PEDOT:PSS anode minimized the energy level mismatch with electron-donors of the active layers,thereby creating small potential batteries at interfaces.The optimized flexible OSCs based on the CF₃SO₃H-doped PEDOT:PSS anode exhibited an efficiency of 16.41%（maximum value:16.61%）.Furthermore,the solution-processed soft devices maintained a high flexibility.With 1,000 cyclic bending at radius of 1.5 mm,the flexible devices based on the CF₃SO₃H-doped PEDOT:PSS anodes maintained 94.4%of the initial efficiency.（2）A small amount of HClO₄ spraying treatment is proposed to modify the PEDOT:PSS films.HClO₄ is regarded as the strongest inorganic acid with ultrahigh acidity（p Ka:-10）,which can provide a strong protonation of H⁺to remove the insulating PSS and then make the PEDOT:PSS film more conductivity.Moreover,the abundant chlorine oxygen polar bonds of HClO₄ can polarize the PEDOTs in the polymeric matrices.Based on this,the HClO₄-treated PEDOT:PSS electrodes achieved a high work function about 5.39 e V.It can match well with the hole transporting layers and electron donors of active blends and leading to an enhanced carrier extraction.More importantly,the HClO₄ spray doping method avoids the direct contact between strong acids and plastic substrates to protect the soft substrates from damage.The metal oxide-free flexible OSCs based on the HClO₄-droped PEDOT:PSS anode yielded a mean PCE of 16.44%,which was comparable to that（PCE:16.89%）of the control OSCs fabricated on ITO/glass substrates.Besides,the solar cells retained 93.6%of the initial PCE in the 1000 cycle harsh flexing tests at a radius of 1.5 mm,demonstrating superior mechanical flexibility.（3）A halogen-containing polar solvent thionyl chloride（SOCl₂）was used to post treated PEDOT:PSS film at room temperature.PEDOT:PSS film achieved a high electrical conductivity of 2230 S cm^-1 through the SOCl₂ treatment.Different with traditional acid post-treatment,SOCl₂ has rich S-Cl and S=O polar covalent bonds,potentially reducing the Coulomb attractions between PEDOT and PSS chains,but didn’t significantly change the PEDOT structure and the amount of PSS.On the basis of the SOCl₂-treated PEDOT:PSS films as flexible transparent electrodes,the resultant flexible OSCs yielded the power conversion efficiency of 15.12%with a high fill factor of 71.9%.（4）Combining low-concentration CF₃SO₃H treatment and room temperature air drying technology,we achieved a highly transparent,highly conductive and highly flexible PEDOT:PSS electrode with the soft polyethylene（PE）film as the substrate.Then we developed an all annealing-free solution-manufacturing process and fabricated OSCs with high PCE and high flexiblility.Compared with the traditional PET substrate with a Young’s modulus of about 4.8 GPa,the PE substrate（about 24μm）have an extremely low Young’s modulus of only about 0.2 GPa and more transparent.During the device preparation process,all the functional layers including PEDOT:PSS anode,hole transport layer,active layer and electron transport layer are solution-processed without any thermal annealing.Based on the PE substrate and annealing-free process,the super flexible OSCs were achieved with a high PCE of14.66%,an improved P/W of 6.33 W g^-1 and a light weight of 2.27 mg cm^-2.In addition,the super flexible OSCs exhibit high mechanical flexibility witch under extreme bending conditions with a bending radius of 0.25 mm,the device efficiency remains above 90%after 1000 consecutive bends.