Isochronal and isothermal diffusion experiments of gallium (Ga) in zinc oxide (ZnO) have been performed in the temperature range of 900-1050 C. The samples used consisted of a sputterdeposited and highly Ga-doped ZnO film at the surface of a single-crystal bulk material. We use a novel reaction diffusion (RD) approach to demonstrate that the diffusion behavior of Ga in ZnO is consistent with zinc vacancy (V Zn) mediation via the formation and dissociation of Ga Zn V Zn complexes. In the RD modeling, experimental diffusion data are fitted utilizing recent density-functional-theory estimates of the V Zn formation energy and the binding energy of Ga Zn V Zn. From the RD modeling, a migration energy of 2.3 eV is deduced for Ga Zn V Zn , and a total/effective activation energy of 3.0 eV is obtained for the Ga diffusion. Furthermore, and for comparison, employing the so-called Fair model, a total/effective activation energy of 2.7 eV is obtained for the Ga diffusion, reasonably close to the total value extracted from the RD-modeling.

Cuprous oxide (Cu₂O) is a promising material for large scale photovoltaic applications. The efficiencies of thin film structures are, however, currently lower than those for structures based on Cu₂O sheets, possibly due to their poorer transport properties. This study shows that post-deposition rapid thermal annealing (RTA) of Cu₂O films is an effective approach for improving carrier transport in films prepared by reactive magnetron sputtering. The as-deposited Cu₂O films were poly-crystalline, p-type, with weak near band edge (NBE) emission in photoluminescence spectra, a grain size of ~100 nm and a hole mobility of 2 - 18 cm²/Vs. Subsequent RTA (3 min) at a pressure of 50 Pa and temperatures of 600 - 1000 °C enhanced the NBE by 2-3 orders of magnitude, evidencing improved crystalline quality and reduction of non-radiative carrier recombination. Both grain size and hole mobility were increased considerably upon RTA, reaching values above 1µm and up to 58 cm²/Vs, respectively, for films annealed at 900 - 1000 °C. These films also exhibited a resistivity of ~50 - 200 Ω cm, a hole concentration of ~ 10¹⁵ cm^-3 at room temperature, and a transmittance above 80.

Epitaxial Cu2O films grown by reactive and ceramic radio frequency magnetron sputtering on single crystalline ZnO (0001) substrates are investigated. The films are grown on both O- and Zn-polar surface of the ZnO substrates. The Cu2O films exhibit a columnar growth manner apart from a ∼5 nm thick CuO interfacial layer. In comparison to the reactively sputtered Cu2O, the ceramic-sputtered films are less strained and appear to contain nanovoids. Irrespective of polarity, the Cu2O grown by reactive sputtering is observed to have (111)Cu2O||(0001)ZnO epitaxial relationship, but in the case of ceramic sputtering the films are found to show additional (110)Cu2O reflections when grown on O-polar surface. The observed CuO interfacial layer can be detrimental for the performance of Cu2O/ZnO heterojunction solar cells reported in the literature.

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Reliable monitoring of PV systems is essential to establish efficient maintenance routines that minimize the levelized cost of electricity. The existing solutions for affordable monitoring of commercial PV systems are however inadequate for climates where snow and highly varying weather result in unstable performance metrics. The aim of this work is to decrease this instability to enable more reliable monitoring solutions for PV systems installed in these climates.Different performance metrics have been tested on Norwegian installations with a total installed capacity of 3.3 MW: i) comparison of specific yield, ii) temperature corrected performance ratio, and iii) power performance index based on both physical modelling and machine learning. The most influential effects leading to instability are identified as snow, low light, curtailment, and systematic irradiance differences over the system. The standard deviation of all the performance metrics is reduced when filters targeting these four effects are applied. Compared to general low irradiance or clear sky filtering, a greater reduction in the variation of the metrics is achieved, and more data remains in the useful dataset. The most suitable performance metrics are comparison of specific yield and performance index based on machine learning modelling.The analysis highlights two paths to accomplish increased reliability of PV monitoring systems without increased hardware costs. First, better reliability can be achieved by selecting a suitable performance metric. Second, the variability of the performance metric can be reduced by utilizing filters that specifically target the origin of the variability instead of using standard literature thresholds.

DC voltage fields generated by RF plasmas and their influence on film growth morphology through static attraction to metal wetting layers: Beyond ion bombardment effects

Dopant diffusion of indium (In) in single crystal zinc oxide is studied by secondary ion mass spectrometry and is interpreted using a reaction-diffusion model that invokes predictions from density functional theory (DFT). An apparent activation energy of 2.2 eV is obtained for the diffusion of In, when the local Fermi-level position is about 0.2 eV below the conduction band edge. The diffusion of In is found to be significantly faster that that reported for the other group III donors, aluminum and gallium, with several orders of magnitude higher effective diffusivities, that can be assigned to a lower migration barrier for the diffusion of In. Furthermore, our results reveal self-consistency in previous DFT results of defect formation-and migration energies. From this, the diffusion of In is suggested to occur through mobile charged zinc vacancies -V Zn 2 that form intermediate mobile ( V In Zn Zn ) − pairs. The pairs in turn dissociate rather readily at the studied temperatures (850 °C-1150 °C), which results in distinct and abrupt diffusion fronts for the In depth distribution profiles.

This thesis explores various processing and materials aspects of an oxide solar cell. A tandem structure consisting of a Silicon (Si) bottom cell, and a Cuprous Oxide (Cu 2 O)/Zinc Oxide (ZnO) top cell is suggested, and several materials challenges relevant for the design is addressed. Using Flash Lamp Annealing, shallow p-type emitters are produced in Si by indiffusion of Boron (B) from a surface source. The B diffusion is shown to be enhanced by a transient of Si interstitials originating from the surface coating. The n ++-side of a Si tunneling junction is realized by sputter depositing highly Phosphorus (P) doped Si thin films on quartz. The P activation and thin film conductivity are highest after prolonged, high temperature annealing, and thin film roughness is significantly reduced after a Reactive Ion Etch treatment of the quartz substrates. A rectifying Cu 2 O/Si heterojunction is demonstrated, and most of its depletion layer is located in Si showing that the acceptor concentration in the sputtered Cu 2 O is significantly higher than 1.1×10 17 cm −3. Epitaxial growth of Cu 2 O is realized on single crystal ZnO, and XRD measurements reveal that the Cu 2 O grows in two separate [111]-oriented domains which are rotated 180 • relative to each other. Further, a thin, detrimental layer of CuO is evidenced by TEM at the interface between the sputtered Cu 2 O and the ZnO single crystal. Homoepitaxial sputter growth of ZnO is investigated in some detail, and the growth is found to depend on sputter target power density, substrate polarity, and growth ambient. Specifically, c-axis growth is promoted when growing on the Zn-face of ZnO, while slower, in-plane growth is promoted during O-face growth and during growth in a Nitrogen ambient. Treating sputter grown Cu 2 O thin films by Rapid Thermal Annealing is demonstrated by XRD, PL and Hall effect measurements to be highly beneficial for the thin film structural quality. Finally, two band gap defects in Cu 2 O are revealed using Temperature Dependent Hall effect measurements, and the defect ionization energy values are found to be dependent on sample strain.