## 1 Introduction  
Carbon nanotubes were first incorporated into photovoltaic devices as nanoscale fillers in a polymer matrix to provide an electron transport path, or as a transparent conducting electrode for hole collection [1; 2; 3; 4; 5]. In these applications, the nanotubes did not participate in the generation of a photocurrent; they merely assisted in the transport of already generated photoelectrons [2; 3; 4]. In 2007 [PERSON] et al. [2] investigated a device architecture in which carbon nanotubes (CNTs) not only transport charge carriers but also assist in forming the junction to separate and collect photovoltaic charge carriers [2; 4; 6]. In such devices, the Fermi level equilibrates at the junction between the nanotubes and the silicon, causing band bending of the silicon and the generation of a depletion layer. This built-in potential separates electron-hole pairs (excitons), which are produced when photons are absorbed by the silicon [7]. These devices are generally called carbon nanotube/silicon heterojunction solar devices (CNT/Si heterojunction devices or CNT/Si devices). The first such devices used DWCNTs deposited via water expansion and aqueous film transfer of a film made of CVD DWCNTs [2; 6]. In less than a decade, the device efficiency was improved by a factor of 10 from an initial percent current efficiency (PCE) of 1.3% to 17% in 2015 [8; 9]. This rapid improvement, coupled with potential cost savings and the relative simplicity of the junction structure, compared with multilayer DSSCs and perovskite devices, are the reasons for much excitement around this device design. In addition, these devices are exciting for future research as their manufacturing process is both simple and scalable [4]. In addition to their application in CNT/Si devices, CNT films (or mats) have been used for a range of applications. These include the growth of somatosensory neurons [10], as gas sensing membranes [11], as current injection for blue light-emitting diodes [12], as field effect transmitters [12], and as organic light-emitting diodes (OLEDs) [13]. Transparent electrodes in general are vital for technologies such as display screens and thin CNT films have the potential to replace the expensive and brittle ITO [14]. Research is currently ongoing in the use of CNT mats as filters to remove and neutralise a close relative of the SARS-CoV-2 virus that causes COVID-19 from the air [15; 16]. The morphology of the CNT films used in CNT/Si devices is of the utmost importance when it comes to their photocurrent production. When films are produced via standard vacuum filtration, they are homogeneous on the macroscale. On the micro/nano-scale, however, the films are highly disordered and form a complex network of nanotubes, with many holes and clumps distributed across the film. Recent work on vacuum filtration [17] has demonstrated that this process can produce carbon nanotube films, but while potential applications are discussed, there was no demonstration of application. Holes in the film are a problem for device performance, as they reduce CNT/Si contact area which introduces variations in the depletion region. Additionally, disfigurenents in the film will retard the film's ability to efficiently transport charge carriers across the film from the CNT/Si heterojunction to the gold top contact. Previous work has reported efforts to use smoother films in the devices usually through alignment of the CNTs [18]. Improved performance was observed, but the procedures to produce the aligned films are laborious and involve dangerous chemicals, so are not a viable path for large scale production of highly conducting, transparent films. Whilst the presence of holes in the film can be reduced by producing a thicker film, this leads to its own problems. A thicker film will reduce the amount of light incident on the silicon layer of the device, lowering photocurrent production.