For the PDPP3T/PCBM system investigated here, atomic force microscopy, resonant soft X-ray scattering, and grazing incidence wide angle X-ray scattering are used as an initial set of tools to determine the surface texture, the bulk compositional morphology, and the crystallization behavior, respectively. We find systematic variations and relate them to device performance. A solvent mixture of DCB/CF/DIO = 76:19:5 (v/v/v) yields a PCE of 6.71%.

Introduction. Optoelectronic devices, including light-emitting diodes and photovoltaic cells, have strong application in many aspects. One electrode must be transparent for an optoelectronic device. Conventional transparent electrode materials are metal oxides, such as indium tin oxide (ITO). But these metal oxides have some problems in the optoelectronic application. One problem is the limitation of indium in earth. 1 Another problem is the rigidity of the metal oxides. 2 This rigidity badly affects the application of ITO in the flexible electronic devices, which are regarded as the next-generation electronic devices. [3][4][5] Hence, there is a strong demand for cheap and transparent thin films with high conductivity and high mechanical flexibility. Poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, chemical structure shown in Scheme 1) emerged as a promising conducting polymer to replace ITO in the optoelectronic application. It has many merits, including high transparency in the visible range, solution processability, high mechanical flexibility, and good thermal stability. [6][7][8] Though PEDOT:PSS has been extensively used in the optoelectronic devices, it is usually used as the material for the buffer layer between the electrode and the active layer other than as the electrode material. 9 This is related to the low conductivity of PEDOT:PSS. The as-prepared PEDOT:PSS film obtained from the PEDOT:PSS aqueous solution usually has a conductivity lower than 1 S cm -1 , remarkably lower than ITO. 7 This low conductivity badly affects the application of PEDOT: PSS in many aspects. Much effort has been made to improve the conductivity of PEDOT:PSS. One method recently developed is to add a high-boiling-point polar organic compound into the PEDOT:PSS aqueous solution or treat the PEDOT:PSS film with polar solvent, such as ethylene glycol or dimethyl sulfoxide. [10][11][12][13][14] This method can enhance the conductivity of the PEDOT:PSS film by a factor of several hundred.Here, we report a novel method to significantly enhance the conductivity of the PEDOT:PSS film by adding anionic surfactants into the PEDOT:PSS aqueous solution. The method is stimulated by the conductivity dependence of the conductive PEDOT film on the counteranions, 15,16 which are present in the conducting polymer to compensate the positive charges on the PEDOT chain. For example, PEDOT with p-toluenesulfonate as the counteranion (PEDOT:TsO) exhibits conductivity as high as 900 S cm -1 , which is significant higher than that of PEDOT: PSS. We discovered that the introduction of TsO anions into the PEDOT:PSS aqueous solution could significantly enhance the conductivity of PEDOT:PSS.