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It was found that initial increases in CuO layer thickness produced a noticeable reduction in resistivity. With increases in thickness beyond 10 nm however resis- tivity showed a slight increase (Figure 8.5). A possible explanation for this increase is that at layer thicknesses of more than 10 nm the distance between the higher mobility SnO layers is too great, meaning that there is reduced carrier movement from the CuO into the SnO. Equally, CuO layers of less than 10 nm may provide fewer carriers. 0 5 10 15 20 25 30 35 40 45 1 2 3 4 Layer thickness (nm) Resistivit y (x10 − 2 Ω.cm)

Figure 8.5: Resistivity against CuO layer thickness for unequal layer thickness films

Transmission was found to show no obvious pattern other than that the films with 40 nm CuO layers showed somewhat reduced transmissions (Figure 8.6). Film band gaps were all nearly identical at about 1.47 eV, with the exception of the equal thickness 2.5 nm film, which had a slightly narrower band gap of 1.35 eV (Figure 8.7).

CHAPTER 8. MODULATION DOPED METAL OXIDES 0 5 10 15 20 25 30 35 40 45 0 20 40 60 80 100 Layer thickness (nm) Av erage transmission (%)

Figure 8.6: Average transmission against CuO layer thickness for unequal layer thickness films 0 5 10 15 20 25 30 35 40 45 1 1.2 1.4 1.6 1.8 2 Layer thickness (nm) Band gap (eV)

CHAPTER 8. MODULATION DOPED METAL OXIDES

8.3

Conclusions

The results presented in this preliminary study demonstrate that modulation dop- ing is an effective technique for reducing the resistivity of p-type metal oxides. TEM analysis demonstrates that these films consist of thin multilayer stacks, and that the layered structure is both remarkably consistent in thickness and shows good long range lateral stability. There are however some unresolved concerns regarding film delamination.

Resistivity was found to drop from 3.9x10−2 Ω.cm for the single layer control sam- ple to a low of 2.88x10−2 Ω.cm for films with equal layer thickness. By keeping SnO layers constant at 2.5 nm and increasing CuO layer thickness to 10 nm, resis- tivity was reduced further to a minimum of 2.46x10−2 Ω.cm. Further increases in CuO layer thickness were found to increase the resistivity. Selected sample anal- ysis using the Seebeck effect was inconclusive regarding the cause of the reduced resistivity, but showed that both single layer and multilayer films have an astonish- ingly high carrier concentration of nearly 3x1022 _/cm3_{. This is offset by mobilities}

of around 5 to 6x10−3 cm2_{/V.s. Film band gaps and transmission were found to}

Chapter 9

Conclusions

Metal oxides are a very important class of materials with a wide range of techno- logical applications, and are widely used in thin film photovoltaics. Despite the abundant published data however there is still a lot that is either not known or is poorly understood. In particular there are two areas that would benefit from fur- ther research. These are the identification and development of ways in which the hole localisation effect of oxygen can be countered so as to provide good mobility in p-type oxides, and exploration of techniques to further optimise and reduce the resistivity of n-type TCOs without adversely affecting the transmission. Identi- fying techniques for increasing hole mobility could have profound implications in terms of both the development of high quality low resistivity p-type TCOs and in the realisation of all-oxide solar cells with a reasonable efficiency. Further re- ductions in the resistivity of n-type TCOs would provide gains in all areas of use, and in photovoltaic applications could both increase current collection efficiency and remove the need for metal bus-bars, thereby increasing the total cell efficiency as areas of the cell would no longer be shaded. A series of studies are presented in this work which provide a useful basis for continuation of research into these areas.

AZO is a well-known n-type TCO and alternative to ITO consisting entirely of Earth-abundant materials. The study presented here examines the impact of tem-

CHAPTER 9. CONCLUSIONS

perature and deposition time on the optical, electrical and structural properties of sputtered AZO with 0.5 % aluminium doping. The targets were synthesised using AZO powder formed through an emulsion detonation process developed by the Portuguese company Innovnano S.A. Unlike many commercial targets, this material is pre-doped, with the aluminium incorporated into the crystal structure prior to target manufacture. Films were found to show good optical transmission, and resistivity was found to decrease with both deposition time and increasing temperature up to 300◦C. Temperatures above 300◦C were found to be detrimen- tal to film formation, with increasing amounts of damage to the crystal structure and consequent increases in the resistivity.

Several photovoltaic technologies, and particularly CdTe cells, have significant problems with formation of ohmic back contacts. In the case of CdTe this is be- cause of the relatively large electron affinity, which when added to the band gap gives a work function of 5.9 eV. It has generally been assumed that in order to prevent the formation of a Schottky barrier, a high work function material is re- quired to form the back contact. Molybdenum oxide is known to have been used successfully. The resistivity of the pure oxide is typically very high, potentially resulting in an increase in the cell series resistance and hence reduced efficiency. The study presented here investigates the effect of alloying molybdenum oxide with molybdenum nitride through reactive sputtering in a mixed oxygen-nitrogen atmosphere. Unexpectedly, all mixed films were found to show p-type behaviour. Films deposited on glass showed high levels of stress, with significant delamination often visible to the naked eye. This is a result of the lattice mismatch between the alloy and the amorphous glass substrate. Resistivity was found to improve with increased nitrogen content, in contrast to optical transmission, which re- duced with increasing nitrogen. A selection of compositions was deposited onto CdTe cells as back contacts. These cells showed an increase in efficiency with increasing nitrogen content, which was contrary to expectations. UPS conducted by the Stephenson Laboratory at Liverpool University showed that as expected work function increased with increasing oxygen content, but that all work func- tions were low. Resistivity was found to correlate strongly with efficiency, which

CHAPTER 9. CONCLUSIONS

showed a significant increase as resistivity dropped. I-V data showed that the key parameter was cell voltage, which increased significantly with reduced resistivity. This indicates that at least for p-type materials, work function may not be as im- portant as resistivity. All but one cell showed the roll-over effect characteristic of Schottky barrier formation however. It is recommended that further work should include investigation into the effects of pure molybdenum nitride as a back contact material as well as other high-conductivity p-type materials.

The development of p-type oxides requires the use of metals which are capable of hybridising the oxygen p-orbitals that make up the valence band in a typical metal oxide. Both experimental and theoretical studies have shown that copper d-states have this ability. The copper oxides however both have relatively narrow band gaps, limiting their uses as TCOs. Cupric oxide (CuO) in particular has a near- ideal band gap for use as a photoabsorber (eg, [181]). A series of investigations were undertaken involving studying the behaviour of intrinsic and sodium-doped CuO, and examining the effects of alloying with other wider band gap materials. Finally, a preliminary study into increasing the mobility through modulation doping was carried out.

It was found that temperature and deposition environment both play an important part in determining the properties of CuO films, with low temperatures and high oxygen partial pressures producing the lowest resistivities, but higher temperatures and reduced oxygen resulting in improved structure. The band gap was found to be highly susceptible to variation in oxygen partial pressure, with high oxygen contents causing an increase from 0.8 to 1.42 eV. Sodium doping was found to reduce the resistivity by up to four orders of magnitude as compared to equivalent intrinsic films. High oxygen content sodium-doped films were found to have carrier concentrations two orders of magnitude higher than that typically shown by ITO. Mobility however was found to be very low, at around 10−3 to 10−4 cm2/V.s. Combining CuO with tin dioxide (SnO2) was found to have a profound impact

on film properties. These materials were identified as a new suite of highly mis- matched alloys, and DFT simulation indicated that no crystal structure was possi-

CHAPTER 9. CONCLUSIONS

ble. The band gap was found to decrease with increasing copper content, showing a curve of the type demonstrated by other mismatched alloy systems such as Ga(As,N) or (In,Ga)As. Resistivity and average transmission however showed lit- tle change up to a copper content of 50 %. Further increases in copper caused a dramatic reduction in both. DFT analysis showed that this is caused by a sud- den shift from a tin-dominated character to copper domination. The resistivity decrease is caused by a greater degree of valence band hybridisation and a si- multaneous increase in the number of charge percolation pathways resulting from significant reduction in the energetic disorder. A concurrent increase in the number of sub-gap states resulting from unfilled copper d-states is the cause of the reduced transmission. This sudden change in character is referred to as a compositional mobility edge. The link between valence band hybridisation, reduced disorder and increased sub-gap states implies that synthesising a CuO-based amorphous mate- rial which is both transparent and conductive is likely to be very difficult, perhaps even impossible. The principles developed in this study however could readily be applied to other p-type mixed oxide materials, and could provide a route for the development of new p-type TCOs. The identification of the compositional mobility edge in particular shows that varying the material parameters can have dramatic effects on certain properties whilst having minimal impact on others.

Alloys of CuO with zinc oxide (ZnO) were found to show very different behaviour from CuO-SnO2. All films were found to be crystalline, with zinc-rich films showing

the typical XRD pattern of sputtered ZnO. Increased copper content caused a sudden shift in structure to a single broad CuO peak. Further increases resulted in a second structural change, with two other CuO peaks appearing. These variations in structure were found to have a dramatic effect on the optical and electrical properties, with both shifts causing a significant increase in both transmission and band gap. The second change resulted in a dramatic increase in resistivity, but the first showed no significant impact. Ignoring the structurally induced shifts, the general trend for resistivity, band gap and transmission is for a decrease with increasing copper content. It is clear from this study that for crystalline films, the precise crystal structure has a highly significant impact on the optical and

CHAPTER 9. CONCLUSIONS

electrical properties. As with the observed compositional mobility edge for the Cu-Sn-O sequence, this implies that careful control of the composition can be used to tailor the various material properties to obtain the optimum balance for a given application.

The final study into copper-based oxide materials involved the use of modulation doping in an attempt to increase the film mobility. It was found that reducing the individual layer thickness reduced the resistivity significantly, with films made up of layers of less than 5 nm thickness showing the best resistivities, down to a low of 2.88 x 10−2 Ω.cm. Films with unequal layer thicknesses were found to perform even better, with films made up of 2.5 nm SnO2 and 10 nm CuO layers giving

the best resistivities at 2.46 x 10−2 Ω.cm. This demonstrates that the technique provides a successful approach to reducing resistivity.

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