# Effects of Co-Solvents on the Performance of PEDOT:PSS Films and Hybrid Photovoltaic Devices
## 1 Introduction
In designing the shadow mask for the contact, the primary goal is to minimize the shading caused by the metal while maintaining an optimal carrier collection. The efficiency of carrier collection is limited by transport losses through each of the materials and at the interfaces between them. For ohmic contacts, the transport losses at the interface are inversely proportional to the total contact area, such that there is a tradeoff between losses due to shading and those due to transport. In order to design the shadow mask, losses due to the metal contact resistance and shading, the contact resistance at the interface, and the resistance of the PEDOT:PSS must be quantitatively determined and optimized. This optimization was performed using the open access PV Lighthouse calculator [6]. In this case, silver was used as the front metal contact due to its high bulk conductivity and its reported compatibility with PEDOT:PSS, the specifics of which have not been adequately investigated previously. In this work, we quantitatively characterize the electrical properties of the Ag/PEDOT:PSS interface and used these results for our mask design. We demonstrate this interface develops ohmic characteristics with an even lower resistance than previously reported. It is not surprising that the performance of these hybrid solar cells is highly dependent on the properties of the PEDOT:PSS film, particularly its optical and electrical properties, and the behavior of the interfaces between the materials. PEDOT:PSS behaves like a degenerate p-type semiconductor which is why it induces a photoactive junction with n-type silicon, with the depletion region located exclusively in the active silicon layer. By using geometric considerations, the doping concentration  
Figure 1: The schematic of the hybrid photovoltaic device architecture. of PEDOT:PSS can be estimated to be 3 \(\times\) 10\({}^{20}\) cm\({}^{-3}\)[7] (p. 147). Due to the chemical nature of the PEDOT:PSS complex this number is unlikely to vary significantly and thus its contribution to the junction is predictable and reproducible. Despite having such a high carrier concentration, the conductivity of PEDOT:PSS is limited by the carrier mobility and results in a much lower conductivity than traditional metals and semiconductors, which has prompted numerous studies on improving it by different methods [3; 4; 5]. One such method is the addition of co-solvents to the PEDOT:PSS dispersion which modifies its structural morphology, promoting the closer packing of the micelle which in turn results in better conductivity of the deposited films [4]. Solutions of ethylene glycol (EG) and dimethyl sulfoxide (DMSO) have emerged as the most effective co-solvents for this purpose [8]. In this work, we aim to understand the implications of using the two co-solvents mentioned previously, EG and DMSO, in the design and fabrication of hybrid solar cells which employ a benzoquinone solution-based surface passivation treatment, a pyramidal textured silicon active layer, and a silver bus bar-finger front metal contact [9]. First, we validate the spin coating procedure for the PEDOT:PSS dispersions on textured substrates by analyzing the homogeneity and uniformity of the films. Second, we test the effects of these co-solvents on the electrical properties of PEDOT:PSS films. Third, we characterize the effects of the co-solvents on the photo collection efficiency of our device architecture. Fourth, we quantitatively characterize the electrical interfacial behavior between PEDOT:PSS and silver. Finally, we fabricate and characterize the hybrid solar cells, following the design described previously, using these co-solvents, and explore their effects on device performance.