VARIABLES PRINCIPALES QUE INFLUYEN EN LA CIANURACIÓN

thick transition metal oxide layer to optimise interfacial energy level alignment and light coupling into the device. The most commonly used oxides are TiOx, WOx,

MoOx, ZnO although V2O5 can also be used. Transition metal oxides may be

deposited by evaporation203,204 and/or from solution.200,201,202 Solution deposition processing typically requires a high temperature post-deposition anneal to drive off the solvent and decompose the organometallic sol from which they are formed. The oxides are typically spin-cast from an alcohol solution of the precursors (e. g. Ti[OCH(CH3)2]300, and MoO2(OH)(OOH)200,299). These compounds undergo

hydrolysis even at room temperature with the formation of TiOx and MoOx. Mild

heating (100 – 150 °C) accelerates the evaporation of the solvent which reduce the thickness of the overlayer.200, 301, 300 The most dramatic changes in film structure occur below 200 °C when a film of amorphous oxide is formed. However, further annealing is known to improve the electrical properties of these films200 and promote crystallisation and crystallite sintering. For example, TiOx is often annealed at 450

°C in the context of DSSCs at which temperature the anatase and rutile start to crystallise.300 As shown previously, ultra-thin Au electrodes can withstand temperatures up to 300 °C before the onset of aperture formation. To investigate if

the continuity of these films can be preserved at higher temperatures when buried beneath an oxide layer, 8.4 nm Au films supported on an APTMS:MPTMS nanolayer with thin overlayers of TiOx, MoOx and WOx were tested. The layers of

TiOx and MoOx were spin-cast from solution. WOx was evaporated under high

vacuum. All three oxides were deposited according to established procedures reported in the literature with thicknesses of ~ 10 nm and annealed at 500 °C under nitrogen together with an 8.4 nm Au reference electrodes. The morphologies of these films before and after annealing are depicted in Figure 4.9.

Figure 4.9: AFM morphologies of 8.4 nm Au films without an overlayer (a); and with overlayers of: MoOx (b); WOx (c) and TiOx (d) all annealed to 500 °C for 30

As expected, the uncapped film shows a high density of holes of different diameters. Conversely, films with MoOx and WOx are smooth with an Rrms ~1 nm,

whilst films with a TiOx overlayer have a very smooth surface comparable to that of

unannealed Au (Rrms ~ 0.4 nm) and are defect-free. Since MoOx can also be

evaporated, the robustness of 8.4 nm Au / 10 nm MoOx with an evaporated MoOx

was also tested. Au films capped with an evaporated MoOx layer were not as robust

as those capped with the other three types of oxide layer. When annealed at temperatures above 400 °C the film catastrophically breaks down (Figure 4.10).

Figure 4.10: AFM morphologies of 8.4 nm Au film with evaporated MoOx overlayer

annealed to 500 °C.

Collectively these results indicate that a thin oxide layers deposited from solution are very effective at making optically thin Au films more robust towards elevated temperature. This is attributed to the gradual hardening of these films through solvent loss, and the mobility of the oxide particles immediately after deposition, which helps to minimise strain in the oxide overlayer.

The sheet resistance of films with an oxide overlayer annealed to 500 °C is ~7 Ω sq-1

. Such a low resistance is the lowest value for this thickness, and was previously only achieved for films without an oxide overlayer when annealed at 300

°C, where the conductivity is enhanced as a result of the increase in crystallinity without the deleterious influence of forming apertures. To test if apertures do actually form in Au films buried underneath the oxide overlayer when annealed to 500 oC but are obscured by the oxide, the following experiment was performed: An 8.4 nm Au film was deposited onto two APTMS:MPTMS nanolayer derivatised glass substrates. Both films were annealed at 500 °C which resulted in aperture formation (Figure 4.11 (a)). One of these annealed films (with apertures) along with one previously unannealed film (without apertures) was then covered with a TiOx

overlayer. Both films were then annealed at 500 °C and their morphology was examined using AFM. The apertures formed during the first annealing step are still well-pronounced (Figure 4.11 (b)), indicating that the TiOx overlayer is of uniform

thickness and does not hide the apertures. If the aperture formed underneath the TiOx

they would be evident as indentations. However the film which was not annealed prior to TiOx deposition is smooth and remains aperture free (Figure 4.11 (c)) which

indicates that 8.4 nm Au film annealed with TiOx overlayer retain intact continuity

up to 500 °C.

Figure 4.11: AFM morphology of: (a) 8.4 nm Au film annealed to 500 °C; (b) 8.4 nm Au film annealed to 500 °C and then again with TiOx overlayer; (c) 8.4 nm Au

Apart from the very low sheet resistance and ultra-smooth surface these films show also excellent optical properties, due to the anti-reflective role played by the oxide overlayer, which increases in the mean transparency of the metal / metal oxide bi-layers as compared to the metal films without an oxide overlayer: MoOx - ~71%,

WO3 – ~73%, TiOx – ~76%, compared to ~ 65% before annealing (Figure 4.12).

Importantly this improvement narrows the gap between these metal electrodes and ITO coated glass (mean transparency ~85%) without adding an extra processing step to OPV fabrication. It is also notable that the shape of the absorption spectra with a thin oxide layer is very similar to that of the unannealed metal film, which is further evidence that the metal film is continuous after annealing.

Figure 4.12: Far-field transparency spectra of 8.4 nm Au films supported on APTMS:MPTMS with and without different oxide overlayers annealed to 500 °C; for comparison the spectrum of a not annealed Au film and ITO on glass is also shown.

The transparency at 600 nm of thin Au films on glass, in the air, with different oxide overlayer has been simulated using MacLeod software. The simulated transparencies as a function of thickness of Au and TiOx/MoOx/WOx oxide are

presented in Figure 4.13: The results remain in good agreement with experiment and can be used for further optimisation of the optical properties of thin metal films.

Figure 4.13: Calculated contour plots of transmittance for metal/metal oxide bi-layer on glass upon variation of layer thicknesses. The simulations were performed for different oxides; (a) TiOx; (b) WOx; (c) – MoOx.

4.3.4 Thermal stability of optically thin Ag films capped with an oxide