## 2 Materials and Methods  
CNT suspensions were prepared using previously published approaches [37; 38; 39]. A surfactant solution was formed by adding Triton X-100 to water at a concentration of 1% \(y\)/\(v\). This mixture was bath sonicated (\(\approx\)50 \(W_{\text{RMS}}\) (root mean squared Watts) (Elmasonic S 30 H, [PERSON], Singen, Germany) for 20 min before dry CNTs were added to the mixture at a concentration of 0.2 mg mL\({}^{-1}\). To suspend the CNTs in the solution, the mixture was bath sonicated in 3-4 20 min periods. The bath water was replaced after each period to keep the bath at around room temperature. Once the suspension was formed, it was centrifuged at 17,500\(\times\)\(g\) for 1 h to remove material that was not well dispersed. The top 2/3 of the centrifuged mixture was collected and the remaining 1/3 was centrifuged for an additional hour to increase yield, with the top 2/3 collected from this second run. The centrifuge used was a Beckmann-Coulter Allegra X-22 (Brea, CA, USA). Fast-filtered CNT films were produced from suspension using vacuum filtration. A Buchner flask was attached to a water aspiration vacuum via tubing, and a glass frit was placed in the top with a rubber bung providing the seal. A series of two filtration membranes were placed on the frit. The bottom membrane was a mixed cellulose ester (MCE) template membrane (VSWP, Millipore, 0.025 um pore size, Merck Millipore Ltd., Tullagreen, Carrigtwohill, Co., Cork, Ireland). This membrane was patterned with the shape desired for the final film. For device production a pattern consisting of 4 small holes was used to make 4 identical films. From these films one would be attached to glass and used for light transmittance and conductivity testing whilst the other three were used in device production. The second (top) membrane is an MCE membrane (HAWP, Millipore, 0.45 um pore size, Merck Millipore Ltd., Tullagreen, Carrigtwohill, Co., Cork, Ireland). A solution of approximately 250 mL of ultra-pure water containing an aliquot of CNT suspension was filtered through this set up. The flow rate of the solution was faster through the patterned holes in the lower membrane than through the 0.05 um pores. Thus, the nanotubes were deposited in the same pattern as the lower template paper. After the initial filtration, an additional filtration of 250 mL of ultra-pure water was performed in order to ensure any CNT or CNT/GO residue was washed from the glassware onto the film. Additionally, this step could assist in washing some surfactant from the CNTs in the film. Slow-filtered films were produced from a similar apparatus only on a small scale and with no patterned second membrane. In addition, there was no vacuum applied and the solution filtered through the membrane solely due to gravity. CNT films were attached to a variety of substrates (glass, silicon, or gold/silicon solar device substrates). The nanotube film was cut (using a shim hole punch) from the excess MCE membrane and was placed, nanotube side down, onto the desired substrate. The film was then wetted with ultra-purewater, with any excess water siphoned away via a pipette. The film was moved to the desired location on the substrate and was then dried by applying manual pressure to the wet film with absorbent paper. This pressure has the added advantage of ensuring any air pockets trapped between the film and the substrate were forced out prior to attachment. A piece of Teflon was placed over the film, followed by a piece of glass to form a substrate/CNT film/Teflon/glass sandwich. The Teflon was placed to prevent the MCE/nanotube film attaching to the upper layer of glass rather than the substrate. Once the sandwich was formed it was heated at 80 \({}^{\circ}\)C for 15 min. This heating dries the film, leading to strong attachment to the substrate. After heating, the film was left to cool for 2-5 min to prevent the film detaching from the substrate when the MCE membrane was dissolved. To dissolve the MCE, the substrate with attached film was submerged in a bath of acetone for 30 min.