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Cuprite is well known in the scientific literature as a p-type oxide with a direct band gap of around 2.17eV and a cubic crystal structure with a space group of Oh4 = pn3m (61, 62). Visually

speaking, this form of copper oxide has a red appearance. The p-type nature of this oxide is thought to derive from a departure from the stoichiometric oxide structure (63), due to a naturally occurring metal deficiency (or possibly a surplus of interstitial oxygen ions) providing holes in the valence band. Using calculations from first principles, Raebiger et al. found that the p-type nature of Cu2O was majoritively due to Cu ion vacancies and that

stable concentrations of Cu vacancies can reach up to 1020 cm-3, yielding hole concentrations of around 1018 cm-3(64).

2.1.2.1.1 Applications of Cu2O

Cu2O is used in a wide range of electronic and photochemical devices because of its band-

gap, intrinsic p-type nature, ease of fabrication and low cost. Described below are a few examples of current uses of Cu2O.

Li et al. fabricated Cu2O/Cu/TiO2 nanotube heterojunction arrays from Cu/Cu2O

nanoparticles and anodised TiO2 nanowires. These were used to promote the

photocatalysis of water at the liquid/device interface to produce H2 gas under illumination

33 photocatalytic structures the H2 production rate was increased due to an increased light

absorption range induced by the surface plasmon response of the Cu/Cu2O interface (65).

Using similar techniques, many other groups have used Cu2O for the production of

hydrogen via photocatalysis of water under UV irradiation. Depositing the p-type Cu2O onto

n-type nanostructured materials such as TiO2 (66), ZnO (67) and Si (68), typically by wet

deposition methods, these groups were able to optimise the photocatalytic activity of the devices by tuning the geometrical properties of the substrate nanostructures.

Liu et al. deposited graphene sheets onto nanocubes of Cu2O producing electrochemical

sensors to detect glucose and hydrogen peroxide. Cu2O nanocube powder was synthesised

via the reduction of aqueous Cu(OH)4. The addition of graphene oxide in the solution

resulted in a coating of graphene over the Cu2O nanocubes which in turn increased the

sensitivity of the devices and prolonged their lifetime(69).

Jiang et al. produced a Cu2O homojunction structure and using two electrodeposition steps

deposited both n-type and p-type copper oxide onto FTO. This device was tested for its ability to sense acetaldehyde and other gases at different concentrations in air. Surface Photovoltage (SPV) measurements using a chopped light source at 532nm and 405nm and a lock-in amplifier showed that these devices could achieve high sensitivity for the detection of acetaldehyde at room temperature (70).

Lv et al. produced magnetron sputtered n-ZnO/i-ZnO/p-Cu2O heterojunction PV devices

using the insulating ZnO layer to attempt to reduce leakage current. The group investigated the effect of different thicknesses of i-ZnO and what effects this had on the resulting device characteristics measured without illumination. The active layers were deposited onto indium-zinc oxide (IZO)/glass substrates and electrical contacts were made using metallic indium. It was found that for increasing the i-ZnO thickness from 0nm to 200nm in 50nm

34 increments, the leakage current improved significantly. The I-V characteristics for the 0nm i-ZnO device showed very high leakage and did not display the expected rectifying behaviour. For higher thicknesses rectifying behaviour was observed along with increasing threshold voltage, from around 0.5V to 0.9V for 100nm and 200nm i-ZnO layers. The Ideallity factor for the dark curves was found to increase from 3.6 to 8.9 with increasing thickness, where and ideallity factor of 1-2 is considered optimal (71,72).

Nam et al. constructed Cu2O based thin-film transistors using RF sputtering to deposit

varying thicknesses of Cu2O on to p-type Si substrates. The Cu2O layer formed the active

channel, the Si substrate and SiO2 dielectric acted as the gate whilst the source and drain

were formed from Ni top contacts. The group found that an optimal Cu2O layer thickness of

45nm resulted in the best transfer function. For thinner layers it was found that the on current was quite low whilst for thicker layers the turn-on voltage increased and a subthreshold slope hump appeared (73).

2.1.2.1.2 Electronic Properties of Cu2O

Cu2O has several advantageous properties when considering applications such as

photovoltaics or photocatalysis. Various values within a relatively large range have been measured for the bandgap of Cu2O, produced by numerous different methods, usually

between 1.7eV and 2.6eV (72,73)_{. Most commonly the bandgap is quoted as around 2.1eV.}

This corresponds to a wavelength of ~590nm, allowing the material to absorb radiation at this or higher energies, and to use this energy to promote electrons to a useable state in the conduction band. Essentially this means that the material is well suited to absorption of a large portion of the visible light spectrum (from green down to UV). Several other electronic properties play an important role in the application of this material in energy generation settings, the resistivity (comprised of the minority carrier diffusion length, mobility and concentration), and the optical absorption properties. Gupta et al. give the

35 absorption coefficient as ~105cm-1(72), but a more detailed analysis by Malerba et al. gives the extinction coefficients (related to the absorption coefficient by , where = absorption coefficient, = extinction coefficient and = wavelength(74)) for bulk and thin film Cu2O at different wavelengths and substrate temperatures (75). In their paper they list

the extinction coefficients of the material for light with wavelengths of 250nm to 2.5µm, part of which is shown below in Figure 7.

Figure 7: Absorption Coefficient for Cu2O, calculated from ref 75.

The mobilities, carrier concentrations and diffusion lengths for Cu2O vary quite significantly

depending on the growth method and particular growth parameters. Typical values for the mobility are around 1-10 cm2/Vs (76), although some growth methods can yield higher values in the 50-90 cm2/Vs range (72,73). Carrier concentrations can also vary significantly, 1016 cm-3was reported by Munos-Rojas et al. for thin films grown by fast atmospheric ALD

(76)

, while for Cu2O grown by electrodeposition Jiang et al. reported values of between

6.7x1015cm-3 and 7.9x1017cm-3 depending on the pH of the deposition process (77). The minority carrier lifetime (related to the minority carrier diffusion length, the diffusivity and the recombination rate) also plays an important role in assessing the performance of this material in energy generation applications. Bhattacharyya et al. give the lifetimes of carrier in sputtered Cu2O at ~250ns (78). Values for the minority carrier diffusion length also vary

0 100 200 300 400 500 600 700 800 900 1000 250 350 450 550 650 A b sor p tion Co e ff ic ie n t, α (c m -1) x10 3 Wavelength, λ (nm)

36 significantly, with Gupta et al. reporting ~3.5μm (72), Dimitriadis reporting between 1µm and 4µm (79), and Sculfort et al. reporting lengths of 0.5µm (80). As these widely varying values indicate, the electrical properties of Cu2O are highly dependent on the methods and growth

conditions used to produce the oxide.