Abstract
"Metal halide perovskite solar cells (PSCs) are an emerging technology and present an exciting prospect for high power conversion efficiency thin film photovoltaic devices with low fabrication costs. Over the last decade, they have made a staggering improvement in power conversion efficiency (PCE) from 3.8 % to 25.5 %. However, the stability of PSCs is a key obstacle to commercialisation. In addition, the charge extraction of the hole transporting layer (HTL) in planar devices remains the rate limiting step for the PCE. One predominant factor causing instability in PSCs, is the intrinsic sensitivity of the perovskite to moisture. Vertically aligned carbon nanotubes (VACNTs) present a promising route for improving hole transport efficiency and stability within the PSC architecture. Their predetermined orientation, p-type nature, and exceptional charge transport capabilities along the tube length enhance their suitability as a potential HTL to increase charge extraction within the device. Furthermore, due to their hydrophobic nature, they have the potential to protect the perovskite layer from moisture related degradation. However, the growth of VACNTs, using conventional chemical vapour deposition, typically requires high temperatures (>700 °C) and is therefore considered unsuitable to facilitate growth on temperature sensitive transparent conductive oxide (TCO) glass substrates, used in the standard fabrication of PSCs.
In this thesis, the growth of VACNTs onto temperature sensitive TCO substrates at low temperatures is demonstrated. The benefit of reflection of the substrate, within the infrared region of the electromagnetic spectrum, and the effect surface roughness has on the resulting VACNT forests are studied. Following the optimisation of growth conditions for VACNT forests on TCO substrates, optical lithography was used to form patterned arrays in an effort to increase optical transmission and active surface area of the VACNT forests. The array patterned VACNT forests are then used in inverted planar PSC devices. A significant increase in charge extraction and larger grain sizes in the perovskite film results. Consequently, there was an increase in device efficiency with a champion device of 9.01 % without additional hole transporting layers (HTLs). With an additional complimentary integrating HTL, a champion device efficiency approaching 16 % was produced. The stability of the devices was then studied to investigate the effect of including VACNT forests within the architecture. Devices were stored in the dark for 1000 hours under ambient conditions and measured periodically over time. Devices that included the patterned VACNT forests showed an impressive increase in PCE to almost double their original value within the first 200 hours, which was then maintained for up to 500 hours. Even after 1000 hours the value did not drop below their original PCE, whereas the HTL free control devices dropped to below 70 % of their original PCE after 1000hrs. When studying the stability of devices incorporating a poly-TPD HTL, with and without patterned VACNT forests, the device stability was similar over time indicating that the poly-TPD was the primary factor affecting stability in these devices.
To further understand the role of the CNTs, in relation to device performance and stability, the effect of the periodicity and size of the patterned CNT towers on the substrate, acting as templated HTLs, was investigated. For this purpose, 50 µm × 50 µm, 100 µm × 100 µm, 200 µm × 200 µm, and 400 µm × 400 µm tower features were fabricated and used in the inverted planar heterojunction architecture, without any additional HTLs. The CNT towers showed that they could reduce the intrinsic strain, typically induced by a variation in perovskite grain size. However, they also led to more localised distortions within the lattice. The performances were compared showing, on average, 50 µm features produced the best device performance followed by th"