# Effects of Co-Solvents on the Performance of PEDOT:PSS Films and Hybrid Photovoltaic Devices
## 3 Results
### Effect of Co-Solvents on the Electrical Properties of the PEDOT:PSS Films  
PEDOT:PSS dispersions with different co-solvent compositions were spin-coated on planar silicon substrates, as opposed to the textured substrates used for devices, at different spin conditions and their electrical properties were tested by the van der Pauw technique. The additional roughness introduced by the substrate texturing would cause a large error that would make the results not comparable among themselves. Table 1 shows the electrical properties of the various films tested.  
It is no surprise that lower spin speeds and correspondingly thicker films, produce films with lower sheet resistance. Although these results are not directly translatable to the films spin-coated on textured substrates, as these films are considerably less homogeneous, we confirm the significant enhancement of conductivity by using co-solvents. Furthermore, we can confirm that at the target thickness, for anti-reflective optimization, of 65 nm (2250 rpm), 7 wt% EG produces the lowest sheet resistance of all samples tested. Therefore at this spin condition, the anti-reflective properties of PEDOT:PSS and the sheet resistance of the film are optimized for a planar substrate; thicker films will have lower sheet resistance but will compromise the anti-reflective properties. However, as will be discussed in more detail in later sections, optimizing the anti-reflective properties, which are directly related to the thickness of the films, and the conductivity of films deposited on textured substrates is not as straightforward and requires a more empirical approach. It is also important to note that for irregular films the effective sheet resistance of the film will be limited by the thinner regions of the film. In other words, if the dispersion is more prone to accumulation in the valleys of the texturing then the thinner regions in the peaks of the pyramids will limit the overall conductivity of the film.  
Previous work has explored the mechanisms by which co-solvents affect the conductivity of PEDOT:PSS dispersions. In this case, the co-solvents modify the micelle-like morphology of the PEDOT:PSS complex, in which the flexible PSS molecules encapsulate the rigid PEDOT chains  
\begin{table}
\begin{tabular}{c c} \hline
**Recipe** & **Sheet Resistance (Ohm/\(\square\))** \\ \hline
7 wt\% DMSO 2250 rpm & 167.2 \\
7 wt\% DMSO 4000 rpm & 296.4 \\
7 wt\% EG 2250 rpm & 137.3 \\
7 wt\% EG 4000 rpm & 324.1 \\ Neat PEDOT:PSS 2250 rpm & 335.3 \\ Neat PEDOT:PSS 4000 rpm & 530.7 \\ \hline \end{tabular}
\end{table}
Table 1: The electrical properties of the PEDOT:PSS films spin-coated for 300 s on untextured substrates.  
Figure 3: The XPS survey data, with locations of relevant 1s, 2s, and 2p (triplet) peaks indicated. The peaks for oxygen, carbon, and sulfur, prominent in PEDOT:PSS, are indicated in black. No silicon peaks are discernable, indicating the complete or near-complete coverage of the surface by the PEDOT:PSS layer.  
forming a voluminous shell (this structure is commonly referred to as a gel) [7] (p. 115). In this structure, the doped conjugated PEDOT backbone is the only pathway for charge transport, as PSS is not electronically active and only contributes to doping the PEDOT and inducing its solubility. The addition of co-solvents decreases the thickness of the PSS shell and induces a rearrangement of the PEDOT chains, from the coil-like benzoid structure that may be present along the grain boundaries to a more linear or extended quinoid structure resembling stacked pancakes [4; 7; 8; 14; 15]. Combined, these structural modifications promote the closer packing of the PEDOT chains which in turn facilitates cross-chain charge transfer [4]. This, in turn, results in a higher bulk conductivity and lower sheet resistance.