# Effects of Co-Solvents on the Performance of PEDOT:PSS Films and Hybrid Photovoltaic Devices
## 1 Introduction  
The application of Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) for photovoltaic applications has been a growing area of interest in recent years due to the opportunity for simple low-temperature processing and its anti-reflective properties [1; 2; 3; 4; 5]. PEDOT:PSS is perhaps the most popular electronically active complex of the polythiophene family. In this form, the PSS acts as a p-type doping counter ion which allows hole mobility along the PEDOT conjugated backbone and promotes the dispersion of the complex in polar solvents such as water due to the micelle-like morphology that the complex develops. PEDOT:PSS has been studied previously for a variety of applications, but its behavior in solid state electronic devices, such as diodes and transistors, is not yet completely understood. For understanding the behavior of PEDOT:PSS to be like that of a p-doped semiconductor, different photovoltaic device architectures have been studied with emphasis on all-organic devices. Nevertheless, due to its compatibility with traditional semiconductors, hybrid organic-inorganic devices have been developed that have shown better performance than the all-organic counterparts. One of the most promising of such hybrid devices uses PEDOT:PSS on n-type silicon as a front emitter. This cell structure has reached a maximum PCE of 14.6% on planar silicon substrates [1]. The general structure of this hybrid solar cell can be seen in Figure 1. In this structure, the PEDOT:PSS layer acts as a hole carrier selective contact which induces a photoactive junction with the n-type silicon and acts as an anti-reflective coating. This architecture also often involves a passivation treatment of the silicon surface previous to the PEDOT:PSS deposition which contributesto minimizing the surface defect density, minimizing the charge recombination, and the improving open circuit voltages. One of the main advantages of using PEDOT:PSS, besides its relatively low-cost and simple processing, is its potential to increase the photo collection efficiency of photovoltaic devices by acting as an anti-reflective coating. Texturing the active substrate is another common method used to improve the photo collection. In this work, we aim to take advantage of both of these techniques by using textured silicon substrates as our base in order to maximize the photo collection of the device. Achieving this effect, however, poses several unique design challenges. Just like any other anti-reflective treatment, the anti-reflective properties of PEDOT:PSS are optimized at a specific film thickness. Furthermore, the homogeneity of the film is crucial not only for the anti-reflective effect but also for proper device performance, as incomplete or otherwise faulty coatings will cause shunts that will drastically hinder the efficiency of the device. Therefore, for the fabrication of efficient hybrid devices, it is critical to accurately control the deposition procedure to achieve homogeneity and a proper thickness of the film. Spin coating has been the preferred casting method for PEDOT:PSS as it allows for very thin homogeneous films to be produced. In spin coating, the rheological properties of the fluid being cast, its interfacial interaction with the substrate, as well as the spin speed and time, ultimately determine the thickness and homogeneity of the resulting film. Spin coating parameters tend to produce highly reproducible results; however, this has only been validated for planar smooth substrates. Irregular or textured substrates, such as those used in this study, have not been studied as thoroughly in the context of spin coating thus additional rigor is required to validate the spin coating procedure in order to confirm the homogeneity and coverage of the films. Carrier collection is another major concern in designing solar cells. For our devices, we employ a silver fingers-bus bar scheme as a front contact deposited by electron beam physical vapor deposition with the use of a shadow mask.