Study The Doping And Modification Of PEDOT:PSS Film And Its Application In Organic Solar Cells

At present, the major disadvantage to organic solar cells is the low efficiency. How to improve its power conversion efficiency(PCE) has become crucial in its industrial development and competitiveness against traditional inorganic photovoltaics devices. In this paper, as anode decoration layer, the doping and modification of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) film and its application in organic solar cells have been studied. The results are shown as follows:1. The PEDOT:PSS doped with multi-walled carbon nanotubes(MWCNTs), sorbital, and bromine transparent conducting thin films are fabricated separately on quartz substrates by blending-spin coating method. The effects of different dopants on the optical and electrical properties of PEDOT:PSS film have been investigated.①The optical transmission of PEDOT:PSS film in the wavelength region 550-850nm decreases after the addition of MWCNTs, while the sorbitol and bromine doping favor the improvement of PEDOT:PSS film optical transmittance②The electronic conductivity of PEDOT:PSS films doped with MWCNTs, sorbital, bromine are superior to that of the pristine PEDOT:PSS film. The conductivity of PEDOT:PSS film can be 400 times acvtive by doping of sorbitol (8wt%). The conductivity with bromine(6wt%) is appropriatly 300 times higher. The conductivity of MWCNTs (0.20wt%) doped PEDOT:PSS film is about 40 times higher than that of the non-doped film.2. The effects of the incorporation of three different types of dopants(MWCNTs, sorbitol and bromine) on corresponding changes in conductivity of PEDOT:PSS films have been studied and results was obtained by carrying out X-ray diffraction (XRD), scanning electron microscope (SEM), atom force microscope (AFM), X-ray photo-electron spectrum (XPS), etc.①The mechanism of conductivity enhancement in PEDOT:PSS film doped with MWCNTs mainly attribute to two effects:the "π-πinteraction" effect and the "net" effect. The former effect results fromπ-πinteractions between the thiophene rings of PEDOT backbone and MWCNTs when the individual nanotubes are dispersed in the PEDOT:PSS matrix at 0.04wt%, and at the same time the nanotubes are not contacting each other. The electronic density transfer occurs from PEDOT to MWCNTs in MWCNTNs-PEDOT:PSS help the charge more delocalized on the PEDOT chains. The latter stems from the formation of some conductive MWCNTs channels in the PEDOT:PSS matrix when the concentration of MWCNTs is 0.10wt%. These two effects can help charge transport and enhance the conductivity of composite films. Moreove, concentration of MWCNTs higher than 0.20wt% leads to a decrease in film conductivity resulting from the high contact resistance of MWCNTs each other.②The mechanism of conductivity enhancement in PEDOT:PSS film doped with sorbitol attributeds to the change of the resonant structure of PEDOT chain from a 'benzoid' to a 'quinoid'structure, which represents a conformational change of the PEDOT chains from the coil structure into expanded-coil or linear structure. The more partially ordered structures in the expanded-coil or linear structure over the coil structure may yield additional benefits to the charge transfer leading to an improvement of the conductivity of PEDOT:PSS films.③The mechanism of conductivity enhancement in PEDOT:PSS film doped with bromine is affected by the oxidative characteristics of hydrobromic acid and bromine oxidation on PEDOT chains. Bromine reacts with water gives hydrobromic acid when bromine is added to PEDOT:PSS aqueous solutions, and there are some bromine molecules in Br2-saturated aqueous solutions. Hydrobromic acid and bromine molecules function as relatively weak oxidants and produce two effects on PEDOT chains. The first effect is necessary to achieve high hole carrier concentration by releasing more electrons from PEDOT chains which gave an increase in film conductivity cosequently, the other effect is because oxidantion of molecules damages the p-conjugation of PEDOT resulting in the loss of PEDOT:PSS film conductivity to some extent. The conductivity of PEDOT:PSS film is impoved once the former effect has a much greater impact on mechanism. Otherwise the conductivity of PEDOT:PSS film decreases.3. We also investigated the effects of MWCNTs, sorbitol and bromine doping on the photovoltaic performance and stability of polymer photovoltaic devices based on blends of poly(3-hexylthiophene) and [6,6]-phenyl-C61-buytyric acid methyl ester have been investigated separately.①The device performance is limited by lower shunt resistance due to the leakage current, the current gradually increased with increasing concentration of MWCNTs and sorbitol. However, the bromine doping of film can not affect the device leakage current and shunt resistance.②The efficiency of polymer PVs have been improved by reducing the device series resistance by doping PEDOT:PSS films with MWCNTs, sorbitol and bromine. For optimized bromine concentration(6wt%), the device exhibits pronounced improvement with ISC=11.27 mA/cm2, VOC=0.60V, and FF=51.1%. The corresponding PCE is 3.16%, which is higher about 49% than PCE of device with pure PEDOT:PSS film. The addition of the sobitol(8wt%) increased both the device short-circuit current(ISC=10.31 mA/cm2) and fill factor(FF=49.1%) resulting in an increase in efficiency by about 38.2% compared to the device with pure PEDOT:PSS film, although open-circuit voltage (VOC=0.53V)decreases a little after the addition of sobitol. With 0.4mg of MWCNTs(0.04wt%) added, the ISC and FF increase to 9.52 mA/cm2 and 44.8 respectively, while Voc remains nearly constant at 0.55V. And, thus, the PCE also increases to 2.35%, which is more about 13% than PCE of device without MWCNTs doping.③Solar stability experiments show that the stability of the device doped with MWCNTs(0.04wt%) and sorbitol(8wt%) are unchanged within 20 days at room temperature under high-purity argon atmosphere with less than 1 ppm oxygen and moisture in a glove box, while there is a decrease in the efficiency of the device doped with bromine(6wt%) after 12 days at the same experimental conditions.