Highly conductive sodium dodecyl sulphate treated poly (3,4-ethylendioxylthiophene) : polystyrene sulfonate (pedot: pss) containing copper nanoparticles for flexible organic solar cells
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Polymer Solar Cells (PSCs) have attracted great attention due to the advantages of their light weight, flexibility and low cost. The typical device architecture of polymer based solar cell consists Indium Tin Oxide (ITO) on glass as the transparent conducting electrode,Poly (3, 4-ethylenedioxythiophene): polystyrene sulfonate(PEDOT: PSS) as the hole transport layer, a photoactive layer and aluminium as the back contact. ITO is brittle and mechanically rigid making the solar cell device not to be flexible. This has motivated the search for alternatives to replace ITO. One such candidate is PEDOT:PSS. Although treatment with various solvents has demonstrated the ability to increase the conductivity of PEDOT: PSS, its conductivity has remained relatively low compared to that of ITO. In order to make PEDOT: PSS viable for use in ITO free solar cells, there is need to increase its conductivity and transparency. In this study, the optical, morphological and electrical properties of PEDOT: PSS films doped with sodium dodecyl sulphate (SDS) and copper nanoparticles (CuNPs) are investigated. Solutions of PEDOT: PSS containing different amounts of SDS and copper nanoparticles were prepared to determine the optimum loading levels. The solutions were sonicated and then deposited by spin coating onto pre-cleaned glass substrates. The films produced were characterized using Ultraviolet-Visible (UV-VIS) spectroscopy to determine the optical properties while atomic force microscopy (AFM) was used to evaluate morphological characteristics. Characterisation of the structural properties was done using X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Raman. Charge carrier mobility was characterised using the Hall Effect Measurements. Conductivity measurements were done using a four-point probe coupled to a source meter. The conductivity of PEDOT: PSS films were significantly enhanced from 0.56 +0.19 S/cm to 3772.86 + 53.18 S/cm by treatment with sodium dodecyl sulphate (SDS) and doping using copper nanoparticles. This high conductivity was as a result of the conformal change of the PEDOT:PSS form Benzoid (coiled) to Quinoid (linear) as confirmed by Raman. The AFM images showed segregated PEDOT and PSS regions at lower doping levels. The morphology evolved into a „honey comb‟ structure consisting inter-connected PEDOT regions which improved the charge transport hence conductivity. XRD peak at 41.6°corresponded to the presence of copper in PEDOT:PSS:SDS:CuNPs and the size of the crystal was calculated as 5.83nm. SEM images revealed a well-connected percolation network of CuNPs which formed a Face Centered Cubic (FCC) nanocrystal and nanosoldering structures hence enhancing conductivity and decreased the nanowire-nanowire(NW-NW) contact resistance by increasing the contact surface area under the assistance of the conducting polymer (PEDOT:PSS). The charge carrier mobility increased from 0.5cm2/Vs to 5.3cm2/Vs after the addition of SDS and copper nanoparticles. From these results, it was concluded that the tremendous increase in conductivity arose mainly by increased charge carrier mobility.
The University of Zambia