## 2 Materials and Methods
Following this, the acetone was replaced, and the substrate was again submerged for 30 min with stirring. The substrate was then washed with fresh acetone and dried under a N\({}_{2}\) gas flow. This process gives strong attachment to the substrate, with manual scratching only succeeding in removing small portions of the film where the scratching took place. For the devices presented in this report, arc-discharge single-walled CNTs (SWCNTs) were used. The photovoltaic device substrates (see Figure 1) were produced by coating pieces of n-doped (phosphorous) silicon wafer (Active Business Company, Brunenthal, Germany) with a negative photoresist (AZ-1518, MicroChemicals, Ulm, Germany) by spin coating at 3000 rpm for 30 s using a Laurell WS-400B-6 NPP/LTE Spin Coater (Laurell Technologies Corp., North Wales, PA, USA). The resist was soft-baked at 100 \({}^{\circ}\)C for 50 s and was exposed to UV light through a patterned mask before developing in trimethyl ammonium hydroxide (MicroChemicals, Ulm, Germany) for 30 s. The patterned silicon was coated with 5 nm of Cr and 145 nm of Au before an acetone bath was used to dissolve the remaining resist. The result was an array of device substrates which consisted of a gold top electrode with a circular hole 0.079 cm\({}^{2}\) in area in the middle. This hole is the active area, prior to CNT film attachment a drop of buffered oxide etch (6:1 ratio of 40% NH\({}_{4}\)F and 49% HF) was used to dissolve the 100 nm oxide layer on the surface of the silicon. After CNT film attachment (see schematic in Figure 1b), the reverse side of each device was manually scratched with a diamond-tipped pen (to remove the 100 nm oxide layer), a GaIn eutectic was applied, and the device was attached to a steel plate with the eutectic acting as a contact between the silicon and the steel and as an adhesive. The devices were tested as prepared, after a 2% HF treatment, after allowing a droplet of SOCI\({}_{2}\) to evaporate from the surface, and after another 2% HF treatment. The testing was performed by applying contacts to the steel plate and the gold top contact. An applied voltage was varied from 1 V to -1 V and the current measured and converted to a current density measurement based on the active area of the device. This was done in complete darkness to test the diode properties of the device and under illumination by a 150 W Xenon lamp (Newport, Irvine, CA, USA) shone through an AM1.5G filter (Newport, Irvine, CA, USA). The light intensity was kept constant using a silicon reference cell (PV Measurements, NIST, Gaithersburg, MD, USA). The measurements were performed using a Keithley 2400 SourceMeter (Keithley Instruments, Inc., Cleveland, OH, USA) running through a custom program written in LabView. Figure 1: (**a**) Optical image of silicon/gold device substrate, the circle is the active area which will be covered by the single-walled carbon nanotubes (SWCNT) film. (**b**) a schematic of the SWCNT/Si device. Transmittance percentage values for CNT films attached to glass was determined by measuring the transmittance spectrum for a given film from 300 to 1100 nm using a Cary 60 UV/Vis spectrophotometer. The transmittance values at 450 nm and 800 nm averaged. The sheet resistance of these films was determined using a four-point probe (Keithlink, New Taipei City, Taiwan) attached to a Keithlink Gwinstek data acquisition unit. A set of four measurements were taken for three different orientations across the CNT film and the resultant 12 measurements were averaged to give the sheet resistance for the film. For experiments on glass, HCl was used in lieu of HF as HF would have dissolved the glass substrates. Atomic force microscopy (AFM) was performed in air with a Bruker multimode 8 AFM with a Nanoscope V controller (Bruker Corporation, Billerica, MA, USA) in standard tapping mode. The AFM probes used were silicon HQNSC15/AIBS Mikromasch (Tallinin, Estonia) probes (nominal tip diameter and spring constant is 16 nm and 40 N m\({}^{-1}\) respectively).