Investigations into the carbon nanotube/silicon heterojunction solar device

Author: Tom Grace

Grace, Tom, 2020 Investigations into the carbon nanotube/silicon heterojunction solar device, Flinders University, College of Science and Engineering

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The search for novel solar cell designs as an alternative to standard silicon solar cells is important for the future of renewable energy production. One such alternative design is the carbon nanotube/silicon (CNT/Si) heterojunction solar device. In this thesis, various aspects of the CNT/Si device are investigated. The history of these devices is catalogued, with a focus on the electronic properties of the heterojunction and the use of chemical dopants, conductive interlayers and light trapping layers to improve those properties.

For the first experimental investigation, a comparative study was performed between single walled, double walled, and multi walled CNTs (SWCNTs, DWCNTs, and MWCNTs) with different wall diameters. The majority of previous research on the CNT/Si device has used SWCNTs, although some research has looked into CNTs with multiple walls. However, there has not been much comparative work done to determine the optimal number of walls to use. Previous work used different techniques to suspend different CNT species and thus created an additional variable making it difficult to directly compare the effect of CNTs with different wall number on device efficiency. The work presented in this thesis found that large diameter SWCNTs give the best CNT/Si device performance. The main reason, in this study, for their superior performance was an ability to form a homogeneous, concentrated suspension in an aqueous solution with a 1 % surfactant concentration. Additionally, it was found that, whilst chemical p-doping gave significant conductivity improvement to SWCNT and DWCNT films, the effect is muted somewhat on MWCNT films, likely due to the shielding effect the outer tube has on the inner tubes.

The morphology of CNT films is an important factor in device performance. Previously, the films have 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. Research is presented in this thesis on the use of slow, gravity filtration to improve the morphology of CNT films used in the CNT/Si device. A comparison was performed with devices fabricated with both slow and fast filtered SWCNT films. It was found that slow filtration can produce similar photovoltaic results with thinner films. However, it was also found that slow filtration causes significant macroscale inhomogeneity of the CNT films, with concentrated thick regions, surrounded by larger thinner areas. Thus, 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. By using atomic force microscopy (AFM) and scanning electron microscopy (SEM), 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 major focus of this thesis is on scaling up the active area of CNT/Si devices. This is an important step on the road to commercial use. The vast majority of research has been on CNT/Si devices with small active areas (<< 1 cm2). This is suitable to test the viability of CNT/Si devices, to analyse junction properties and to examine the effect of various chemical dopants, interlayers and coatings. It is notable, however, that the best performance results were achieved with devices with smaller active areas. Thus, it is important to analyse the effect of increasing the active area. To this end, a CNT/Si device design was produced with an active area of 1.5 × 1.5 cm (2.25 cm2). Initially, this design was found to perform significantly worse than the smaller area devices. The addition of a gold grid across the CNT/Si junction to enhance the CNT film’s ability to transport charge was found to provide significant improvement. The addition of an aluminium back contact was also performed to improve homogeneity between devices.

The biggest factor limiting the performance of large area devices compared to small area devices was a significantly lower short circuit current density. This was determined to be due to multiple factors, the largest of which was inhomogeneity in the light spot used to illuminate the devices for testing. A 2D map of irradiance was measured for the spot, and it was found that irradiance decreased radially outwards from the centre. This phenomenon had a more pronounced negative effect on the large area devices as more of the light spot was required for total illumination. Inherent device performance decreases due to power loss over the active area were another cause of lower short circuit current densities.

In order to further improve the performance of the large area CNT/Si devices, a series of materials were trialled for use as an interlayer between the CNT film and the silicon surface. Various materials have been used in previous CNT/Si device research to improve junction performance, the current carrying properties of the CNT film, or both. The interlayer materials studied in this thesis were two conducting polymers which have been previously used in CNT/Si devices: Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and polyaniline (PANI). A novel organic material, 4,10-bis(bis(4-methoxyphenyl)amino)naptho[7,8,1,2,3-nopqr]tetraphene-6,12-dione (DAD) and two inorganic materials: copper thiocyanate (CuSCN), and molybdenum oxide (MoOx) were also implemented. Additionally, devices were produced with a thin gold film in place of the CNT film to test the effect of the interlayers with an alternative conductive film. Overall, the effects of each interlayer were examined and compared with physical properties of the interlayers determined from SEM and AFM imaging. The interlayer thickness was varied for each material to determine an optimal thickness for device performance.

Keywords: nanotechnology, carbon, nanotubes, carbon nanotubes, solar cells, solar, solar devices

Subject: Nanotechnology thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Joe Shapter