The Use of Gravity Filtration of Carbon Nanotubes from Suspension to Produce Films with Low Roughness for Carbon Nanotube/Silicon Heterojunction Solar Device Application

Featured Application

Transparent electrodes are critical in many applications. This work probes an approach to make highly conducting, highly transparent electrodes using carbon nanotubes.

Abstract

The morphology of carbon nanotube (CNT) films is an important factor in the performance of CNT/silicon (CNT/Si) heterojunction solar devices. Films have generally been prepared via vacuum filtration from aqueous suspensions. Whilst this enables strong films to be formed quickly, they are highly disordered on the micron scale, with many charge traps and gaps forming in the films. It has been previously established that lowering the filtration speed enables more ordered films to be formed. The use of slow gravity filtration to improve the morphology of CNT films used in the CNT/Si device is reported here. It was found that slow filtration causes significant macroscale inhomogeneity in the CNT films, with concentrated thick regions, surrounded by larger thinner areas. By using atomic force microscopy (AFM), scanning electron microscopy (SEM), and polarised Raman spectroscopy, it was determined that there was no large improvement in directional organisation of the CNTs on the microscale. However, the films were found to be much smoother on the microscale, with arithmetic and root mean square average height deviation values roughly 3 times lower for slow-filtered films compared to fast-filtered films. A comparison was performed with CNT-Si solar cells fabricated with both slow and fast-filtered single-walled CNTs (SWCNT) films. It was found that slow filtration can produce similar photovoltaic results with thinner films. The results demonstrate that film morphology, even without improved CNT alignment, can lead to significant improvement in device performance in some applications. However, slow filtration did not form films of uniform light transmittance over an extended area, causing an increase in the variation in performance between individual devices compared to fast-filtered films.

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 Wei 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, disfigurements 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. It is important, therefore, to look for methods to improve the nanoscale morphology of the CNT film and to improve the smoothness of the film to increase CNT/Si junction density. One such option is to focus on orienting the CNTs within the film to produce aligned films. Aligned CNT films do not suffer the same hole issues and provide an overall better coverage over the silicon. Many methods have been suggested and implemented for forming smooth, aligned CNT films such as physical shearing from highly concentrated CNT suspensions [18,19,20,21], physical shearing of dry CNT films [22,23], growth of upright CNTs followed by vertical collapse [24,25], drawing from highly aligned arrays [26], deposition of aligned CNTs using Langmuir-Blodgett [27] or Langmuir-Schaefer [28] deposition, horizontal growth on a patterned substrate [29], and self-assembly of CNTs from evaporation of suspension [30,31], drop-casting [32], and deposition on a slide pulled from solution [33]. This paper looks at a simple, novel method to produce flat, aligned CNT films over a large area through the use of ultra-slow filtration, first examined by He et al. [34].

It has been suggested that, in suspension, the CNTs will naturally align within the fluid, and a slow deposition process will allow this alignment to be maintained in the produced film and thus lead to the formation of smoother films. Slow filtration is also less labour-intensive and does not require the use of dangerous chemicals normally used to align CNTs. This would make slow filtration—a preferable method on both a cost and environmental basis. For CNT alignment to occur in slow filtration three conditions must be satisfied.

The surfactant concentration must be below the critical micelle concentration (CMC).

The CNT concentration must be below a threshold value.

The filtration process must be controlled at a low speed [34].

The alignment of the CNTs in suspension is driven by electrostatic interactions between anisometric particles in a colloidal solution as investigated by Onsager [35]. Song et al. determined that long MWCNTs form a lyotropic liquid crystalline phase [36] and Engel et al. used this property to form rows of aligned CNTs via solvent evaporation [31].

In this work, we produce and compare CNT films filtered quickly under vacuum and filtered slowly under gravity. We applied these films in CNT/Si solar cell devices. The films produced using gravity were smoother but more inhomogeneous over a large scale meaning solar cells using these films yielded more inconsistent results.

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% v/v. This mixture was bath sonicated (≈50 W_RMS (root mean squared Watts) (Elmasonic S 30 H, Elma Schmidbauer, 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× 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 µm 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 µm 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 μm 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-pure water, 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 °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. 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 N₂ 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, Brunnthal, Germany) with a negative photoresist (AZ-1518, MicroChemicals, Ulm, Germany) by spin coating at 3000 rpm for 30 s using a Laurell WS-400B-6NPP/LITE Spin Coater (Laurell Technologies Corp., North Wales, PA, USA). The resist was soft-baked at 100 °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 cm² 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% NH₄F 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 SOCl₂ 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.

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). Set-point, scan rate, and gain values were chosen to optimise image quality. The AFM scanner was calibrated in x, y, and z directions using silicon calibration grids (Bruker model numbers PG: 1 μm pitch, 110 nm depth and VGRP: 10 μm pitch, 180 nm depth, Bruker Corporation, Billerica, MA, USA). AFM topography images have been flattened, and thickness and roughness measurements determined using Nanoscope Analysis 1.4 (Bruker Corporation, Billerica, MA, USA).

Scanning electron microscopy (SEM) images were obtained using an Inspect FEI F50 SEM (Hillsboro, OR, USA) with beam spot size and high voltage adjusted to optimise image quality. The image resolution was generally 2048 × 1768 with an imaging speed of 30 μs.

Polarised Raman spectroscopy was performed using a Horiba XploRA (Kyoto, Japan) scientific confocal Raman microscope. The laser used was a Nd:YAG 532 nm laser and the power used at the sample was 1.2 mW. A ×50 magnification objective was used. The data was collected at 25 locations in 5 × 5 grid 2 spectra were collected per location with a 30 s integration time per spectrum. The grating used was 1200 grooves per mm which gives spectral resolution of approximately 3 wavenumbers.

3. Results and Discussion

3.1. Surfactant and CNT Concentration

It was important to ensure that the sodium dodecyl sulfate (SDS) concentration was below the critical micelle concentration (CMC) to ensure that the SDS properly assists in suspending the SWCNTs [34]. The CMC of SDS is 8.2 mM [40].

It was found that the CNT concentration used in these experiments was 0.2 mg mL⁻¹.

3.2. Flow Rate

A variety of conditions were trialed to obtain the ideal flow rate of 1–2 mL hr⁻¹ (0.017–0.033 mL min⁻¹). The final process was to perform a brief vacuum filtration with just water to flatten the membrane and then to filter the CNT suspension completely under gravity, with no applied vacuum. An example of the flow rates of arc-discharge single-walled CNTs (SWCNTs) is shown in Figure 2a,b. Over time, the flow rate approaches the ideal parameters.

3.3. Macroscale Inhomogeneity

Figure 2c shows the macroscale inhomogeneity of CNT films filtered under gravity. This represents a common problem observed for all slow-filtered films. As can be seen, the dark regions of higher CNT concentration are smaller in area to the lighter regions. As such, for solar device application and future imaging, the lighter areas were used as more small films could be cut from this area and the transmittance and film conductivity could be more reliably calculated. The films produced by He et al. [34] were visually homogeneous across the whole film. The likely reason for this is that in their research a low, controlled vacuum was applied, whereas, in these experiments, gravity was used to pull the CNT suspension through the membrane.

3.4. Film Morphology

Figure 3 shows the microscale morphology of SWCNT films fabricated with both slow and fast filtration at different volumes. The only change visible with increasing SWCNT suspension volume (and thus SWCNT concentration) is an improvement in surface coverage. It is clear that directional SWCNT alignment was not achieved in any of the samples with the SWCNTs exhibiting random orientations in all of the slow and fast-filtered samples. However, a stark difference is observed in film roughness with slow filtration when compared to fast filtration. Large buckled SWCNT regions are seen in the fast-filtered films whereas these are not present in the slow-filtered films. From this alone, it is likely that slow-filtered films are preferable for use in CNT/Si devices, as smoother films will provide a better contact between the SWCNT film and the silicon substrate. In addition, polarised Raman spectroscopy was performed on slow-filtered films at two different orientations (see Appendix A). This showed no difference in peak intensity when the orientation angle was changed, further confirming the lack of CNT alignment in slow-filtered films.

As observed from SEM images (Figure 3) the fast-filtered films appeared much rougher on the microscale than slow-filtered films. The smoother a SWCNT film, the better contact it will make with the silicon surface and will thus lead to an increase in photocurrent production. The roughness of the film was analysed using AFM. Two areas on a film of each filtration type were imaged by AFM and the images are shown in Figure 4. Additionally, the root mean square average height deviation (R_q) and the arithmetic average of the surface height deviations (R_a) were taken from the image and are shown in

Table 1. Quantitative atomic force microscopy (AFM) roughness data for fast and slow-filtered films formed from 400 µL aliquots of arc-discharge SWCNT suspension.

	Image Z Range (nm)	Image Rq (nm)	Image Ra (nm)
Fast Film Spot 1	248.9	31.6	23.3
Fast Film Spot 2	307.7	40.1	28.9
Average	278.3	35.85	26.1
Slow Film Spot 1	84.7	10.7	7.54
Slow Film Spot 2	94.6	11.6	8.25
Average	89.65	11.15	7.90

, together these are quantitative measurements of the roughness of the film.

Both the AFM images themselves and the quantitative roughness data and Z-Range demonstrate clearly that the slow-filtered films are significantly flatter than fast-filtered films. The R_q values and R_a values for fast-filtered film were both 3–4 times higher than the slow-filtered film. Additionally, the Z-Range was also roughly 3 times higher. By observing the AFM images, large ridges can be seen in the fast-filtered film which dwarf the highest of ‘mounds’ visible in the slow-filtered film. These ridges are a product of the turbulent water flow caused by the vacuum filtration method and are the cause of the higher Z-range, R_q, and R_a values.

Slow and fast-filtered films were produced for use in CNT/Si devices. Both sets of films were produced with the same volume from the same SWCNT suspension. However, as slow filtration is known to produce non-homogeneous films it cannot be assumed that both fast and slow filtration setups will produce films of equal transmittance. The slow-filtered film had a transmittance of 78.5% whilst the fast-filtered film had a much lower value of 51.2%. This is a significant observation as a difference in transmittance of this size would be expected to give a large PCE difference. It is thus apparent that there is a difference in the light transmittance vs. film conductivity for SWCNT films produced via slow filtration. To further demonstrate this difference, the sheet resistance for both sets of films was measured and plotted here as Figure 5.

3.5. Initial CNT/Si Device Comparison

Slow and fast-filtered films of arc-discharge SWCNTs were produced from 400 µL aliquots in 10 mL of water and were attached to a CNT/Si device substrate with an active area 0.079 cm². For the fast-filtered films, the film areas were homogeneous on the macro scale and thus any area of the films could be used in CNT/Si devices. As discussed above, slow-filtered films are not homogeneous on the macroscale and thus it is important to keep consistent which areas of a film are chosen for solar device usage. All slow-filtered films have a small area of high CNT density and a larger area with a much lower density, for solar device applications, the high SWCNT density area was never used in order to keep the devices as consistent as possible.

It is apparent that, on average, the devices with fast-filtered films outperform those with slow-filtered films with an average PCE of 7.21% vs. 6.51% although this includes a low performing 2.26% device in the slow-filtered batch (see Figure 6). Without this outlier device the slow-filtered films average rises to 8.64% albeit from only 2 cells. To address the large offset between devices in the slow-filtered samples a further set of 3 devices were fabricated and tested. This data is shown in Appendix B. The additional set of devices did not serve to improve the stability of the average PCE, with one device failing to produce a photocurrent and two other devices of 6.51% and 2.25%. If the three top performing devices from the slow and fast sample sets are averaged the slow-filtered films outperform the fast-filtered films with an average of 7.93% compared to 7.21%. However, the existence of several poorer performing devices in the slow-filtered film sets highlights the inhomogeneity of the films produced. The full sets of device data after final 2% HF etch can be seen in

Table 2. Solar device data for fast-filtered and slow-filtered films, bold text is the best-recorded result with the average and standard deviation in brackets.

	Fast-Filtered Films	Slow Gravity Filtered Films
PCE (%)	8.06 (7.21 ± 0.84)	9.64 (5.71 ± 3.24)
Jsc (mA cm−2)	24.76 (22.26 ± 1.33)	27.46 (19.84 ± 6.55)
Voc (V)	0.532 (0.501 ± 0.038)	0.532 (0.495 ± 0.044)
FF	0.66 (0.647 ± 0.012)	0.67 (0.554 ± 0.172)
Ideality	2.34 (2.53 ± 0.26)	2.40 (2.89 ± 0.71)
Saturated Current Density (mA cm−2)	0.00198 (0.005 ± 0.003)	0.0008 (0.025 ± 0.033)
Series Resistance (Ω)	51.4 (53 ± 1.66)	50.5 (128 ± 115)
Shunt Resistance (Ω)	25,300 (10,456 ± 12,857)	17,200 (7665 ± 6775)

. It is noticeable that, for most parameters, the slow-filtered devices best performance was superior to or equal to the best performance of the fast-filtered devices. However, for every parameter (except shunt resistance) the variation between individual data points was greater for the devices with slow-filtered films. The high variability between devices produced with slow-filtered SWCNT films is likely due to the macroscale inhomogeneity of the films (Figure 2c).

An important observation from Table 2 is that the best performing device with a slow-filtered film produced a PCE of 9.6%, which is higher than the best performing device with a fast-filtered film of 8.1%. However, the large inhomogeneity in the devices with slow-filtered films led to a significantly lower average of PCE (5.7% vs. 7.2%) and a much higher error value of 3.2% vs. 0.8%. Thus, slow-filtered films have the potential to improve the performance of the CNT/Si device but the inhomogeneity in performance, likely due to the macroscale variant in CNT density, makes the improvement difficult to achieve.

The slow-filtered films used for solar devices let through 30% more light than the fast-filtered films but have twice the sheet resistance. Together, the sheet resistance and transmittance can be combined to produce a ratio of DC electrical conductivity (σ_dc) and optical conductivity (σ_OP). This ratio is a good way to directly compare thin SWCNT films with differing resistance and transmittance properties [41,42]. The ratio is defined relative to the transmittance as shown in Equation (1) and this equation can be rearranged to directly give the σ_dc:σ_OP ratio with an experimental transmittance at 550 nm and an experimental sheet resistance (Equation (1)), both of which have been achieved for the slow and fast arc-discharge SWCNT films.

Equation (1): σ_dc:σ_OP ratio equation.

$$\begin{gathered} {\%\mathrm{T}}_{550}={(1+\frac{1}{2{\mathrm{R}}_{\mathrm{s}}}\ast \sqrt{\frac{{\mathsf{\mu }}_0}{{\mathsf{\epsilon }}_0}}\ast \frac{{\mathsf{\sigma }}_{\mathrm{OP}}(550)}{{\mathsf{\sigma }}_{\mathrm{dc}}})}^{-2} \\ \frac{{\mathsf{\sigma }}_{\mathrm{dc}}}{{\mathsf{\sigma }}_{\mathrm{OP}}\left(550\right)}=\frac{1}{2{\mathrm{R}}_{\mathrm{s}}}\ast \frac{\sqrt{\frac{{\mathsf{\mu }}_0}{{\mathsf{\epsilon }}_0}}}{{\mathrm{T}}^{-1/2}-1} \end{gathered}$$

(1)

where T is transmittance at 550 nm in this case, R_s is the sheet resistance, µ₀ (4π × 10⁻⁷ N A⁻²) is the free space permeability and ε₀ (8.854 × 10⁻¹² C² N⁻¹ m⁻²) is the free space permittivity. Equation (1) shows that a high σ_dc:σ_OP ratio will be produced for a film with low sheet resistance and high light transmittance. Thus, a higher σ_dc:σ_OP ratio indicates a more optimal film for use in a CNT/Si device [43].

Table 3. σ_dc: σ_OP ratio calculations for slow and fast films used in solar device production. Note that HCl was used in lieu of HF as HF would have dissolved the glass on which the CNT films were attached.

Treatment Point	Slow-Filtered Films			Fast-Filtered Films
	T550	Sheet Resistance (Ω)	σdc:σOP Ratio Slow-Filtered	T550	Sheet Resistance (Ω)	σdc:σOP Ratio Fast-Filtered
As Prepared	0.79	440	0.379425	0.53	220	0.621004
HCl 1	0.79	427	0.390997	0.53	200	0.682166
Thionyl Chloride	0.79	319	0.524073	0.53	142	0.962568
HCl 2	0.79	325	0.513056	0.53	143	0.95487

shows that the σ_dc:σ_OP ratio increases with treatment, indicating an improvement in overall film performance. This is expected as the treatment improves the film conductivity, the thionyl chloride doping also removes the VHS transitions visible in the 600–800 nm regions of the spectrum and thus reduces light absorbance (increasing transmittance). Overall, Table 3 also shows that the σ_dc:σ_OP is higher for the fast-filtered films than for the slow-filtered films. This indicates that the fast-filtered films are superior for CNT/Si device use as their lower sheet resistance outweighs their increased light absorbance.

3.6. CNT/Si Devices with Films of Similar Transmittance

Due to the difference between the transmittance values, another experiment was performed with CNT films with similar transmittance of about 95%. Whilst thinner films were used, the slow-filtered films were again inhomogeneous on the macroscale and the obviously thicker regions were avoided when sections were cut for use in the solar cells. An attempt was made to produce slow and fast-filtered films of roughly equivalent transmittance.

Figure 7 shows that both sets of devices have the same maximum PCE of around 5.4%. In this case, however, the slow-filtered films produced a higher average PCE, with a higher average J_sc, and V_oc, but a lower average FF.

As per the films used previously, the slow-filtered film segments used for solar device application were still thinner than the fast-filtered films. There was some variance between the films and so two films were used in each case. The fast-filtered films were found to be between 85% and 95% and the slower filtered films were from 95% to 97%. It is expected that this difference in thickness for the fast-filtered films accounts for the variance in device performance with these films. It is known that, when the transmittance of the CNT films approaches 90% and above that the devices will decline in performance as the ability of super thin films to transfer current is poor as the CNT network becomes patchy as thickness decreases. This explains the high variability in PCE for devices with fast-filtered films (

Table 4. Device data for fast and slow-filtered films formed from 500 µL aliquots of CNT suspension. Best performing values are bolded, with the average and error in brackets.

	Fast-Filtered Films	Slow Gravity Filtered Films
PCE (%)	5.38 (4.21 ± 1.01)	5.39 (4.86 ± 0.76)
Jsc (mA cm−2)	23.47 (22.77 ± 0.62)	25.21 (24.17 ± 1.47)
Voc (V)	0.451 (0.440 ± 0.009)	0.501 (0.497 ± 0.0064)
FF	0.53 (0.417 ± 0.098)	0.43 (0.40 ± 0.042)
Ideality	3.72 (3.82 ± 0.16)	2.03 (2.74 ± 0.92)
Saturated Current Density (mA cm−2)	0.0950 (0.143 ± 0.046)	0.00068 (0.0087 ± 0.013)
Series Resistance (Ω)	13.3 (113 ± 34.2)	103 (259 ± 254)
Shunt Resistance (Ω)	2820 (1482 ± 1172)	41,300 (14,360 ± 23,331)

) but it is surprising to see that the devices with slow-filtered films performed well and with low variation considering that they all had transmittances above 90%. This performance again highlights the importance of the film smoothness. The sheet resistance values for these films were all an order of magnitude higher than that used previously. In addition, the values were much more varied and values on the order of ×10⁶ Ω were occasionally reported. These issues are all likely due to how thin the films were, thinner films will conduct poorly compared to thick films and will have more charge gaps and holes throughout the network. The outlying data points were removed when average sheet resistance was calculated. The sheet resistance and transmittance measurements were combined to produce σ_dc:σ_OP ratios for the slow and fast-filtered films to allow for a better comparison.

Table 5. σ_dc:σ_OP ratio calculations for slow and fast-filtered films of similar %T. Sheet resistance values calculated after a thionyl chloride dope.

	%T(450–800)	%T(550)	Sheet Resistance after Thionyl Chloride Doping (Ω)	σdc:σOP Ratio
Slow Films	95	95.3	2240 ± 1760	0.081953
Slow Films	97	97.9	2870 ± 930	0.064665
Fast Films	85	84.7	3930 ± 960	0.043994
Fast Films	95	97.9	4300 ± 151	0.043187

shows a different result to Table 3, with the slow-filtered films producing higher σ_dc:σ_OP ratios than the fast-filtered films. Both fast-filtered films produced σ_dc:σ_OP ratios significantly lower than the thicker fast-filtered films in Table 3 which is expected given the thinness of the films. This indicates that the slow-filtered films were superior thin conductive films to the thin fast-filtered films.

On the macroscale, films produced via the slow filtration method were less homogeneous than with standard filtration methods, with small areas of high CNT density surrounded by larger areas of low CNT density. The data in Appendix C clearly shows that over smaller areas the slow-filtered films are uniform. Fast-filtered films, on the other hand, were homogeneous on the macroscale to the naked eye.

For improved reproducibility for the slow filtered films, some degree of stirring during gravity filtration could be used. However, this may also decrease the benefits seen on the microscale for slow filtration as it could disrupt the natural ordering of the CNTs in suspension. Alternatively, the use of a controllable low vacuum system instead of gravity filtration may lead to improved macroscale homogeneity. The low vacuum would ensure that a uniform and controllable filtration rate, which would provide tunable alignment and film thickness. The importance of the filtration rate was demonstrated by He et al. [34].

4. Conclusions

It was apparent that smoother slow-filtered films (T = 80%) have the potential to produce better performing CNT/Si devices than devices with fast-filtered films (T = 50%). However, inhomogeneity in the results meant that, on average, devices with fast-filtered films performed better. When thinner CNT films were used, slow-filtered films were found to produce CNT/Si devices that performed at a similar level to devices produced with fast-filtered films despite the slow-filtered films being thinner than the fast-filtered films. This indicates that the smooth nature of the films allows for an improved CNT-Si contact and improved charge production and extraction capability despite a thinner film being used. The slow-filtered SWCNT films were found to produce superior σ_dc:σ_OP ratios at high transmittance (>90%), indicating that when the CNT films are very thin slow-filtration produces a better thin film conductor.

This is an important conclusion as it has been determined from previous research that the performance of the CNT/Si device rapidly drops off at film transmittance above 90% as the sheet resistance increases [44]. Clearly, highly transparent and still highly conducting films are of great value and this work shows that a slow filtration approach has the potential to produce high quality transparent films. With improvements in reproducibility, it is clear that the CNT films could be used as thin film conductors for other applications such as gas sensing, display screens, and OLEDs.