# Effects of Co-Solvents on the Performance of PEDOT:PSS Films and Hybrid Photovoltaic Devices
## 2 Materials and Methods  
N-type silicon (100) textured substrates (doped with phosphorous, CZ) with a back surface field (BSF) were fabricated at Arizona State University (Solar power lab, ASU, Tempe, AZ, USA). These wafers are 145 microns thick with 1-5 ohm-cm resistivity. Random texturization was performed on both sides using potassium hydroxide (2% KOH yielding pyramid sizes of about a 3-5 um base size) alkaline etching. To create BSF n+ layer on the N-type wafers, phosphorous oxychloride diffusion was performed at 820 \({}^{\circ}\)C for 15 min with a POCl\({}_{3}\) carrier gas flow rate of 1500 sccm (standard cubic centimeters per min). This step is for the phosphosilicate glass (PSG) growth and drive-in. Finally, the wafers were then exposed to a 10 min buffered oxide etch to remove the PSG. The sheet resistance value of the BSF side is 55 ohm/square. Double-side polished intrinsic silicon substrates were purchased from Virginia Semiconductors (Fredericksburg, Virginia, USA). These polished wafers are 300 microns thick and have a resistivity >5000 ohm-cm. PEDOT:PSS PH1000 (Heraeus Clevios, Vandalia, OH, USA) was used as the stock polymer dispersion. The solvents ethylene glycol (EG), dimethyl sulfoxide (DMSO), and surfactant Triton X-100 (non-ionic surfactant) were purchased from Fischer Chemical (Waltham, MA, USA) (>95%) and used as purchased. All wafers were cleaned following the procedures described by [PERSON] and [PERSON] et al. : the substrates were cleaned using a Piranha etch (H\({}_{2}\)SO\({}_{4}\):H\({}_{2}\)O\({}_{2}\) = 4:1) for 5 min, followed by a 5-min DI water rise and a 2-min immersion in hydrofluoric acid (HF, 2 wt%) [10]. After cleaning, the substrates were quickly dried by blowing nitrogen and immediately placed in the passivation solution of benzoquinone (BQ) and methanol (MeOH) [9]. The wafers are allowed to stay in the sealed bag containing a passivation solution for 2 h in ambient light conditions. Passivation studies performed in the group previously have suggested that benzoquinone (BQ) does an effective job of passivating the surface. PEDOT:PSS does not possess notably good passivation properties and, hence, the role played by the BQ in these cells is critical. Post-2-hour immersion in BQ, the wafers were removed and given a quick rinse with methanol. The wafers were dried and put on a spinner chuck for spin coating. PEDOT:PSS dispersions were prepared in advance of the spin coating procedure by adding the appropriate co-solvent amounts as well as 0.25 wt% Triton X-100 as a surfactant. The dispersions were then deposited by spin coating on the appropriate wafers at different spin conditions in a HeadwayResearch (Garland, TX, USA) spin coater. All spin-coated samples were baked on a hot plate at 135 \({}^{\circ}\)C for 15 min immediately following the PEDOT:PSS deposition. Textured silicon wafers of 1 cm \(\times\) 1 cm were used as substrates for the validation study of the spin coating procedure. An X-ray energy dispersive spectroscopy (XEDS) analysis was conducted on a JEOL JSM-7400F (Peabody, MA, USA) scanning electron microscope (SEM). X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Physical Electronics (PHI) model 5600 X-ray photoelectron spectrometer (Chanhassen, MN, USA) using an Al 7 mm filament monochromated source with an X-ray energy of 1486.6 eV. Double-side polished intrinsic silicon wafers were used as substrates for the PEDOT:PSS conductivity measurements. Conductivity measurements were carried out by the van der Pauw technique on a custom built room temperature resistivity and Hall Effect system [11]. The resistivity of the films was measured twice (bi-directional by switching the four contacts between voltage and current probes) and the resulting values were averaged. Device reflectivity measurements were carried out on 2 cm \(\times\) 2 cm textured silicon substrates with different PEDOT:PSS spin-coated recipes.