No abstract

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.

This letter reports dye-sensitized solar cells with a thin film of multiwall carbon nanotube/conducting poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composite as the counterelectrode. The composite thin film was prepared by spin coating the aqueous solution of the composite. The devices exhibited high performance with the energy conversion efficiency of 6.5%, short-circuit current of 15.5mAcm−2, open-circuit voltage of 0.66V, and fill factor of 0.63. This performance is close to the devices using conventional platinum as the counterelectrode and is significantly higher than the ones using a thin film of multiwall carbon nanotube/poly(styrenesulfonate acide) composite as the counterelectrode.

High-performance dye-sensitized solar cells (DSCs) with binder-free films of carbon nanotubes (CNTs), including single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs), as the counter electrode are reported. The CNT films were fabricated by coating gels, which were prepared by dispersing CNTs in low-molecular-weight poly(ethylene glycol) (PEG) through mechanical grinding and subsequent ultrasonication, on fluorine tin oxide (FTO) glass. PEG was removed from the CNT films through heating. These binder-free CNT films were rough and exhibited good adhesion to substrates. They were used as the counter electrode of DSCs. The DSCs with SWCNT or MWCNT counter electrodes exhibited a light-to-electricity conversion efficiency comparable with that with the conventional platinum (Pt) counter electrode, when the devices were tested immediately after device fabrication. The DSCs with an SWCNT counter electrode exhibited good stability in photovoltaic performance. The efficiency did not decrease after four weeks. On the other hand, DSCs with the MWCNT or Pt counter electrode exhibited a remarkable decrease in the photovoltaic efficiency after four weeks. The high photovoltaic performance of these DSCs is related to the excellent electrochemical catalysis of CNTs on the redox of the iodide/triiodide pair, as revealed by the cyclic voltammetry and ac impedance spectroscopy.

Polymer electrolytes are needed in many solid-state electronic and energy devices. But polymer electrolytes usually have a low ionic conductivity. This work reports the enhancement in the ionic conductivity of polyethylene oxide (PEO)-LiClO4 electrolyte by adding functionalized multiwalled carbon nanotubes (MWCNTs). MWCNTs are functionalized with carboxylic groups through oxidation with acids. They can be dispersed in acetonitrile solutions of PEO and LiClO4. The presence of functionalized MWCNTs can effectively enhance the ionic conductivity of the PEO-LiClO4 electrolyte, and the ionic conductivity enhancement depends on the loading of the functionalized MWCNTs. Enhancement by a factor of 3.3 was observed. The enhancement in the ionic conductivity is attributed to the functionalized MWCNT-induced decrease in the crystallinity of PEO and increase in the salt dissociation due to the Lewis acid–base interaction of the functionalized MWCNTs with PEO and LiClO4. The addition of functionalized MWCNTs can also effectively improve the mechanical properties of PEO films.