# Effects of Co-Solvents on the Performance of PEDOT:PSS Films and Hybrid Photovoltaic Devices
## 3 Results  
### PEDOT:PSS Coating Uniformity on Textured Substrates  
Due to the texturing of the substrate and the addition of co-solvents, the quality of the spin-coated PEDOT:PSS films had to be validated in order to confirm proper homogeneity and uniformity. Furthermore, the performance of an anti-reflective coating is dependent on the thickness of the film and the wavelength of light such that there is an optimal thickness for the solar spectrum that optimizes photo-collection and device performance. In the case of PEDOT:PSS, previous simulations performed using the online optical simulator OPAL 2 suggested an optimal thickness of 65 nm on a planar silicon substrate for the optimization of the anti-reflective properties of the coating at AM1.5g direct illumination conditions [12,13]. Previous ellipsometry studies on PEDOT:PSS with both EG and DMSO spin-coated on planar silicon substrates have confirmed the reproducible film thickness results for different spin conditions: 4000 rpm for 300 s having a thickness of 50 nm and 2250 rpm for 300 s for a thickness of 65 nm [12].  
Testing the thickness of PEDOT:PSS films spin-coated on textured surfaces poses a technical challenge as it is obvious that the texturing of the substrate induces additional roughness and inhomogeneity to the films. In fact, preliminary ellipsometry studies of films deposited on textured substrates confirm significant thickness variations throughout the deposition area, such that we are not confident in reporting the thickness of these films quantitatively. Due to the additional roughness and inhomogeneity introduced, it was important to confirm the complete coverage of the substrates as it is not immediately obvious that the spin coating procedure on these substrates guarantees complete coverage. It is also important to, at least qualitatively, understand the magnitude of the thickness variation of the film throughout the deposition area.  
PEDOT:PSS dispersions containing 7 wt% EG or DMSO were spin-coated on textured silicon wafers at 2250 rpm for 300 s. X-ray Energy Dispersive Spectroscopy (XEDS) and X-ray Photoelectron Spectroscopy (XPS) were conducted on the samples in order to confirm the complete coverage of the substrates and study the homogeneity of the films. Figure 2 shows the elemental mapping results of the XEDS analysis. The image reveals a greater concentration of C and S near the base of the pyramids of the substrate, indicating that during the spin coating, the PEDOT:PSS accumulates in the valleys of the texturing. It is difficult to establish complete PEDOT:PSS coverage of the silicon substrate based on the XEDS data alone due to the probing depth of the electron beam being thicker than the PEDOT:PSS films. XPS is a more surface sensitive technique and is capable of probing the first few nanometers of the sample. XPS analysis (Figure 3) revealed that no significant concentration of silicon was present at the surface of the samples, confirming complete coverage of PEDOT:PSS and indicating a lack of significant shunt through-holes in the PEDOT:PSS layer. Based on these results we can say that the reliability of the spin coating procedure translates well to the textured substrates and the use of co-solvents, but the accumulation phenomenon towards the valleys of the texturing is pervasive and its implications must be considered carefully. This accumulation phenomenon occurred in all the samples tested, however, from these techniques alone it is not possible to quantitatively characterize the irregularity of the film.  
Figure 2: (a) The composite XEDS image of PEDOT:PSS solution spun on textured Si wafers (b) Si K\(\alpha\)1 component peaks (c) C K\(\alpha\)1_2 component peaks, showing an accumulation of carbon in the valleys between the pyramids (d) S K\(\alpha\)1 peaks show a similar accumulation in the valleys, but a lower image contrast is visible.