Surface morphology of PEDOT
Surface morphology of PEDOT:PSS with and without treatment on n-Si is observed by SEM and is shown in figure 4. When coated on n – Si without any treatment, conducting PEDOT is not uniformly distributed as observed for sample S1. Separate treatment with DMSO and HNO3 (samples S2 and S3 respectively) resulted in an increase in PEDOT grain size and partial removal of insulating PSS on the film. In sample S4, PEDOT:PSS grains are interconnected and uniformly distributed but possess high surface roughness. Whereas sample S5 exhibits a smooth and continuous surface which is uniform and without any cracks or holes on a large scale. Morphology studies from SEM clearly indicate that sample S5 has low roughness from AFM studies, high relative intensity in the Raman specra and high electrical conductivity from hall measurement than other samples(Table 2).
The role of co-solvents, HNO3 and DMSO, in improving the conductivity of PEDOT:PSS layer could be formulated from the as observed morphology of the samples with and without treatment. It could be postulated that the addition of a co-solvent leads to an increased grain size of PEDOT nanocrystals, which is in turn induces an internal re-ordering of crystals.31 The increase in grain size and clustering of PEDOT limits the amount of PSS on the surface. The addition of a co-solvent hence leads to a rearrangement of PSS chains and also stabilizes them, resulting in reduced defect density at the interface and hence better device performance. In the absence of any co-solvent, PSS matrix chain surrounds PEDOT nanocrystals and a high content of PSS chains provides poor ohmic contact between the top electrode and the grain boundaries reducing the carrier collection and hence efficiency of the device. Surface topography and root mean square (RMS) roughness were recorded by non-contact mode AFM as shown in Figure 5 and tabulated in Table 2. The roughness of PEDOT:PSS layers in both S2 and S4 is high as compared to sample S1, as a result of an increased grain size and phase separation upon DMSO addition 2. The observed surface properties could be explained as follows: the spin-coated PEDOT:PSS film is generally composed of PEDOT:PSS grains, with a hydrophobic and highly conductive PEDOT-rich core and a hydrophilic insulating PSS-rich shell. The typical morphology of pristine PEDOT:PSS exhibits bright (positive) phase shifts corresponding to PEDOT-rich grains and dark (negative) phase shifts corresponding to PSS-rich grains 12,14-17. The size of the disconnected conducting PEDOT-rich grains is small in untreated layer. For PEDOT:PSS treated with DMSO, the size and area of the PEDOT-rich grains become bigger than that of pristine PEDOT:PSS 14. For samples S3 and S5, HNO3 treatment lowers the roughness when compared to other samples; this is because PSS rich domains has disappear after the HNO3 treatment 12 and it also suggests that the PEDOT particles on HNO3 treated PEDOT:PSS film surface are thoroughly distributed all over the film surface. A comparison of resistivity values of these PEDOT:PSS layers stresses on the role of HNO3 vapour treatment of PEDOT:PSS layer which further modifies the resistance post DMSO treatment. It is also responsible for the lower resistivity of S5 samples. The removal of PSS chains from the surface of the sample accounts for the elongated grains observed in the AFM micrographs for DMSO treated samples. The surface roughening of the samples also indicate towards phase separation and removal of PSS chains on DMSO treatment. The molecular structure at the surface of pristine PEDOT: PSS, DMSO and HNO3 treated PEDOT:PSS films are investigated by Raman spectroscopy as shown in Figure 6. Conformational changes of DMSO and HNO3 treated PEDOT:PSS films are also investigated by using Raman spectroscopy. These changes are induced by DMSO and HNO3 treatment of PEDOT:PSS. The vibrational modes of PEDOT are placed at 1255 cm-1, 1368 cm-1, 1440 cm-1 and 1506 cm-1 and assigned to the C? ? C? inter-ring stretching, C? ? C? stretching, C? ? C? symmetrical and C? ? C? asymmetrical vibrations respectively. The vibrational modes of PSS are located at 991 cm-1, 1110 cm-1 and 1568 cm-1 respectively. The relative intensity of Raman scattering peaks for the treated films are intense and narrow. The removal of PSS on DMSO and HNO3 treatment of the samples as observed and confirmed from AFM images is