# Effects of Co-Solvents on the Performance of PEDOT:PSS Films and Hybrid Photovoltaic Devices
## 3 Results
### Characterization of the Ag/PEDOT:PSS Interface  
As mentioned earlier, the contact resistivity between Ag and PEDOT:PSS is a critical parameter in designing the front contact and it is not well understood or reported. In the case of the devices presented in this work, the major limiting loss factor was initially assumed to be the resistance of the Ag/PEDOT:PSS interface. [PERSON] et al. performed a contact resistivity measurement for Ag thermally deposited on PEDOT:PSS films (Clevious PH 1000, with a 5 wt% DMSO cosolvent and 0.1% Capstone FS-31 fluoro-surfactant) on planar glass substrates and found contact resistivities of 0.79 Ohm\(\cdot\)cm\({}^{2}\)[10]. Such large contact resistivities would make this interfacial resistance the largest source of power loss in the design of a front contact shadow mask. Furthermore, measurements from initial devices indicated that the total series resistance, as measured from the JV slope plateau beyond Voc, can reach down to 1 Ohm\(\cdot\)cm\({}^{2}\). By analyzing the specific contact grid design in the PV Lighthouse Grid calculator, an upper bound of 0.064 Ohm\(\cdot\)cm\({}^{2}\) can be deduced for the Ag contact resistivity in such devices. Clearly, the existing literature value for Ag contact resistivity was not applicable to the performance observed for most PEDOT:PSS devices on Si substrates. We used the transmission line measurement (TLM) to quantitatively characterize the electrical properties of this interface for the PEDOT:PSS films with co-solvents and use these results directly in our device design. Figure 5 shows the results obtained from the TLM measurements of PEDOT:PSS films with different co-solvents spin-coated on textured silicon substrates. Table 2 shows the results obtained from the TLM data by the general analytical approach described by [PERSON], including a correction factor to account for the edge effects of the contact pads [16]. Any nonlinearities in the fit can propagate into large errors in the estimation of the x- and y-intercepts used to calculate the contact resistance and transfer length. Here the effect of fitting errors was minimized by forcing the fit slope to pass through the first measured resistance data point. Using this approach, a low contact resistivity of 0.000133 Ohm\(\cdot\)cm\({}^{2}\) was found. The range of the possible values using the 95% confidence interval for the slope and y-intercept are also reported in Table 2. The contact resistivity obtained for this specific device design is drastically lower than the previously reported values, indicating that Ag/PEDOT:PSS contact resistivity is not the series resistance bottleneck and can be largely neglected in future shadow mask design. These results indicate much lower shading ratios than can be achieved with the 10.4% mask used in this paper. Future devices could utilize masks with  
Figure 5: The measured resistance vs. the pad spacing to determine the sheet resistance and contact resistivity. 4.8% shading ratios where front grid power losses are limited primarily by the sheet resistance of the PEDOT:PSS layer itself. The extent to which the choice of co-solvents modifies the contact resistivity for this interface has yet to be studied. In addition to measuring contact resistivity, TLM can provide a reliable measurement of sheet resistance, since errors in the fit slope are typically small. For the case of a film with 7% DMSO spin-coated at 4000 rpm for 300 s, the measured sheet resistance was 374 Ohm/\(\square\). This sheet resistance measurement is in good agreement with the 296 Ohm/\(\square\) measured via the van der Pauw technique. The TLM measurements were performed on textured partially conductive (1-5 Ohm-cm) samples, which give a sheet resistance more relevant to the actual device conditions, but do not allow for the full isolation of the PEDOT:PSS conductivity.