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.
Highlights
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]
This paper looks at a simple, novel method to produce flat, aligned carbon nanotube (CNT) films over a large area through the use of ultra-slow filtration, first examined by He et al [34]
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 single-walled carbon nanotubes (SWCNT) [34]
Summary
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]. Wei et al [2] investigated a device architecture in which carbon nanotubes (CNTs) transport charge carriers and 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. 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]. In less than a decade, the device efficiency was improved by a factor of 10 from an initial percent current efficiency (PCE)
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