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Papers by John Walls

Combined thin-film thickness measurement and surface metrology of photovoltaic thin films using Coherence Correlation Interferometry

ABSTRACT Accurate measurement of the thickness of transparent and semi-transparent thin films is ... more ABSTRACT Accurate measurement of the thickness of transparent and semi-transparent thin films is important in all thin film photovoltaic technologies. Surface roughness measurements are also important to interpret the performance of photovoltaic devices. Here we report on a new capability for coherence correlation interferometry that enables it to provide thin film thickness measurements with comparable accuracy to spectroscopic ellipsometry as well as measuring surface roughness with sub-nanometre precision. Applications of the technique&#39;s new capability for thin film thickness measurements are presented for a range of photovoltaic materials. In particular, we report on the close correlation between results obtained by coherence correlation interferometry and spectroscopic ellipsometry. Coherence Correlation Interferometry is a fast, sensitive, non-contacting, non-destructive metrology technique with the potential for use as an in-line quality assurance tool in the large scale production of photovoltaic modules.

Recent developments toward a one step thin-film PV interconnection process using laser scribing and inkjet printing

An optimized series interconnection process can provide many benefits for the manufacture of thin... more An optimized series interconnection process can provide many benefits for the manufacture of thin-film PV including lower panel transit times, lower capital equipment costs, smaller line foot print and less panel area wasted. The One Step Interconnect (OSI) process has been previously introduced [1]. It utilizes conventional laser scribing and inkjet additive manufacture to form the series interconnect. Good electrical performance has previously been achieved on CdTe mini-modules. Here the latest developments are presented. Further mini-module electrical results are shown with Fill Factors (FFs) of, on average, 70% and up to 80%. No loss of fill factor is seen as cells are connected in series. An extensive lifetime testing program is now underway. An encapsulation process has been found and verified using the IEC 61646 standard damp heat test for packaging integrity. Over 400 hours of thermal cycling has been completed on OSI interconnected modules with no degradation in power output.

Stacking faults in CdTe were studied using DFT simulations. Twin and tetrahedral stacking fault e... more Stacking faults in CdTe were studied using DFT simulations. Twin and tetrahedral stacking fault energies are significantly lower than previously suggested, strongly correlating with their high density observed experimentally. No long range ordering was found for tetrahedral stacking faults while a resistance for polytype clustering was calculated. All experimentally observed faults were shown to be electronically benign when considered in isolation but increased density may produce shallow electron trap states.

2017 IEEE 44th Photovoltaic Specialist Conference (PVSC), Jun 1, 2017

Magnesium-doped Zinc Oxide (MZO) was used as an alternative high resistance transparent layer for... more Magnesium-doped Zinc Oxide (MZO) was used as an alternative high resistance transparent layer for CdS/CdTe thin film solar cells. Thin films of MZO were deposited by RF magnetron sputtering and deposited on an Indium Tin Oxide contact (ITO). Thin film CdTe devices including a MZO high resistance transparent layer deposited at above 300 • C yielded a mean efficiency exceeding 10.5 %. This compares with an efficiency of 8.2 % without the MZO layer. The improvement in efficiency was due to a higher open circuit voltage and fill factor. Lowering the deposition temperature of MZO reduced the performance of the devices. Index Terms-magnesium-doped zinc oxide, high resistance transparent layer, thin film, solar cells, CdTe.

Journal of Physics: Condensed Matter, Mar 1, 2013

Modelling the growth of ZnO thin films by PVD methods and the effects of Modelling the growth of ... more Modelling the growth of ZnO thin films by PVD methods and the effects of Modelling the growth of ZnO thin films by PVD methods and the effects of post-annealing post-annealing PLEASE CITE THE PUBLISHED VERSION

Applied Physics Letters, Aug 11, 2014

CdTe is a p--type semiconductor used in thin--film solar cells. To achieve high light--to-electri... more CdTe is a p--type semiconductor used in thin--film solar cells. To achieve high light--to-electricity conversion, annealing in the presence of CdCl 2 is essential, but the underlying mechanism is still under debate. Recent evidence suggests that a reduction in the high density of stacking faults in the CdTe grains is a key process that occurs during the chemical treatment. A range of stacking faults, including intrinsic, extrinsic and twin boundary, are computationally investigated to identify the extended defects that limit performance. The low--energy faults are found to be electrically benign, while a number of higher energy faults, consistent with atomic--resolution micrographs, are predicted to be hole traps with fluctuations in the local electrostatic potential. It is expected that stacking faults will also be important for other thin--film photovoltaic technologies.

ACS applied energy materials, May 4, 2018

Spray-deposition is a low-cost, roll-to-roll compatible technique that could potentially replace ... more Spray-deposition is a low-cost, roll-to-roll compatible technique that could potentially replace spin-coating for the deposition of highly efficient perovskite solar cells. Here, perovskite active layers were fabricated in air using an ultrasonic spray system and compared with equivalent spin-coated films. A chlorine-containing perovskite ink with a wide processing window coupled with an antisolvent extraction resulted in perovskite films with relatively rougher surfaces than those spin-coated. A power conversion efficiency (PCE) of 17.3% was achieved with an average of 16.3% from 24 devices. Despite observing differences in film roughness and structure, the performance of sprayed perovskite solar cells was comparable to that of the spincoated cells processed in an inert atmosphere, showing the versatility of perovskite processing.

Surface Engineering, Jul 12, 2018

The influence of the elemental composition of CuZnSn coatings deposited by electrodeposition on t... more The influence of the elemental composition of CuZnSn coatings deposited by electrodeposition on the formation of the Cu 2 ZnSnSe 4 crystal structure following a selenization process was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. This study showed that the optimum alloy ratio to achieve the Cu 2 ZnSnSe 4 crystal without impurity phases was in the range of 0.37-0.50 for Cu/(Zn+Sn) and 2.5-4 for Zn/Sn.

MRS Proceedings, 2011

A new magnetron sputtering strategy is introduced that utilizes high plasma density (~5mA.cm -2 )... more A new magnetron sputtering strategy is introduced that utilizes high plasma density (~5mA.cm -2 ) to avoid or reduce high temperature processing. The technique uses magnetrons of opposing magnetic polarity to create a "closed field" in which the plasma density is enhanced without the need for high applied Voltages. A batch system has been used which employs a rotating vertical drum as the substrate carrier and a symmetrical array of linear magnetrons. The magnetrons are fitted with target materials for each of the thin films required in the photovoltaic (PV) stack including the CdTe absorber layer, CdS window layer, metal contact using the conventional superstrate configuration. The "closed field" sputtering technology allows scale up not only for larger batch system designs but it is also configurable for "in-line" or "roll to roll" formats for large scale production. The morphology of each of the layers is characterized using a variety of structural and optical techniques including Field Emission Gun SEM and X-ray diffraction (XRD).

Solar Energy, Oct 1, 2018

CdS barrier to minimize Zn loss during CdCl2 treatment of Cd-Zn-Te CdS barrier to minimize Zn los... more CdS barrier to minimize Zn loss during CdCl2 treatment of Cd-Zn-Te CdS barrier to minimize Zn loss during CdCl2 treatment of Cd-Zn-Te absorbers absorbers PLEASE CITE THE PUBLISHED VERSION

Effect of varying deposition and substrate temperature on sublimated CdTe thin-film photovoltaics

A standardized process used for fabrication of CdTe solar cells was varied by increasing the subs... more A standardized process used for fabrication of CdTe solar cells was varied by increasing the substrate temperature during CdTe layer nucleation from approximately 460°C to 610°C and by increasing the CdTe sublimation vapor source temperature. Higher substrate temperatures increase device efficiency, but cause significant CdS re-sublimation. This effect was eliminated by using a Mg1-xZnxO window layer that also has higher transparency. Elevated CdTe source temperatures were found to increase contamination in the deposition system but did not further improve device efficiency. The improvement using high substrate temperatures is attributed to larger CdTe grains and better crystalline quality. TEM cross section analysis, X-ray diffraction measurements and device results are presented.

Results are presented for the atomistic modelling of titania growth, specifically the rutile {110... more Results are presented for the atomistic modelling of titania growth, specifically the rutile {110} surface. Long time scale dynamics techniques are used to model the growth of thin films at realistic growth rates. Between deposition events, the system is evolved through an on-the-fly Kinetic Monte Carlo (otf-KMC) method, finding diffusion pathways and barrier heights without any prior knowledge of transitions. Otf-KMC allows thorough investigation of transitions and barriers observed during the film growth, giving a deeper understanding of growth mechanisms. An important rutile growth mechanism observed during all simulations, involves the upward diffusion of Ti interstitials below an O rich surface, with a barrier of 0.61 eV. The energy required for a single O ad-atom to diffuse on the surface is higher (between 0.65 eV -0.85 eV). Methods used also allow examination of the effects of varying the experimental parameters, such as substrate bias, plasma density and stoichiometry of the deposited material. Conclusions drawn from the film growth suggest that the evaporation process produces an incomplete structure with voids, which is interesting for dye cell use. The inclusion of a low energy ion-beam assist does however add enough kinetic energy to the substrate to enable the completion and densification of layers, producing a film with greater crystallinity. The sputtering process produces highly crystalline growth, which is useful for anti-reflection coatings.

Coatings, Feb 16, 2022

CdTe solar cells were produced using metal organic chemical vapour deposition (MOCVD), which empl... more CdTe solar cells were produced using metal organic chemical vapour deposition (MOCVD), which employed a (Zn,Al)S (AZS) high resistant transparent (HRT) layer at the transparent conducting oxide (TCO)/Cd(Zn)S emitter interface, to enable the higher annealing temperature of 440 • C to be employed in the chlorine heat treatment (CHT) process. The AZS HRT remained intact with conformal coverage over the TCO after performing the high CHT annealing, confirmed by cross-section scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy (STEM-EDX) characterisation, which also revealed the Cd(Zn)S emitter layer having been consumed by the CdTe absorber via interdiffusion. The more aggressive CHT resulted in large CdTe grains. The combination of AZS HRT and aggressive CHT increased open circuit voltage (V oc ) and improved solar cell performance.

IEEE Journal of Photovoltaics, 2018

Thin film Cu(In,Ga)(S,Se) 2 -based (generally referred to as CIGS) solar cells represent a promis... more Thin film Cu(In,Ga)(S,Se) 2 -based (generally referred to as CIGS) solar cells represent a promising alternative to conventional crystalline silicon solar cells due to their high efficiencies, reduced cost, and better material utilization. In recent years, it has been demonstrated that it is possible to form thin films by annealing nanoparticulate material such that the nanoparticles coalesce to form large grained thin films. In this paper, we present a 13.8% efficient CIGS solar cell derived from printed nanoparticle inks. The approach was successfully extended to fabricate monolithic devices on larger substrates. These results demonstrate that lowcost, nonvacuum printing of CIGS nanoparticles has great potential to achieve high efficiencies and reduce the performance gap with the more traditional vacuum co-evaporation and sputtering techniques. T HE photovoltaic market is currently dominated by sili- con wafer-based solar cells . In this type of solar cells, the active layer is made of single-crystal wafers produced by a process that involves fabricating and slicing high-purity, singlecrystal silicon ingots. This process is expensive and although much of the manufacturing and module equipment has become standardized, the production of crystalline silicon solar cells remains cost-intensive and is characterized by relatively poor material utilization. The high cost of crystalline silicon wafers has prompted both industry and academia to investigate alternative Manuscript

Thin Solid Films, 2014

Scanning White Light Interferometry is a well-established technique for providing accurate surfac... more Scanning White Light Interferometry is a well-established technique for providing accurate surface roughness measurements and three dimensional topographical images. Here we report on the use of a variant of Scanning White Light Interferometry called coherence correlation interferometry which is now capable of providing accurate thickness measurements from transparent and semi-transparent thin films with thickness below 1 μm. This capability will have many important applications which include measurements on optical coatings, displays, semiconductor devices, transparent conducting oxides and thin film photovoltaics. In this paper we report measurements of thin film thickness made using coherence correlation interferometry on a variety of materials including metal-oxides (Nb 2 O 5 and ZrO 2 ), a metal-nitride (SiN x :H), a carbon-nitride (SiC x N y :H) and indium tin oxide, a transparent conducting oxide. The measurements are compared with those obtained using spectroscopic ellipsometry and in all cases excellent correlation is obtained between the techniques. A key advantage of this capability is the combination of thin film thickness and surface roughness and other threedimensional metrology measurements from the same sample area.

Effect of Texture on the Efficiency of Polycrystalline Thin Film Cdte Solar Cells

Chlorine is known to have numerous effects on the electronic performance of cadmium telluride (Cd... more Chlorine is known to have numerous effects on the electronic performance of cadmium telluride (CdTe) solar cells, such as doping the CdTe absorber material and pacifying crystal defects. However the mechanisms by which the element improves device efficiency following the cadmium chloride treatment are still not fully understood. In this work the distributions of chlorine in a high efficiency CdTe device are tracked over large areas and in three dimensions by high resolution dynamic SIMS measurements. The results give new insights into the role of chlorine and defects on the performance of CdTe solar cells, particularly when combined with correlative backscatter diffraction measurements.

The microstructure of thin film CdTe absorber layers deposited by pulsed dc magnetron sputtering

High rate pulsed DC magnetron sputtering has been used to fabricate thin film CdTe photovoltaic d... more High rate pulsed DC magnetron sputtering has been used to fabricate thin film CdTe photovoltaic devices. Fabrication of thin film photovoltaic devices using magnetron sputtering has the advantages of excellent coating uniformity and use of lower substrate temperatures during deposition. The CdTe and CdS layers were deposited from compound targets. The substrates were held at 200°C during deposition, a process condition previously found to minimize the stress in the coatings. The devices were deposited using rotation to provide coatings with exceptionally uniformity. This enabled the CdTe absorber thickness to be reduced to ∼1μm. The microstructure of the as-deposited material is dense and columnar. The cadmium chloride activation treatment results in an increase in grain size and it also removes the majority of planar defects. However, void formation has been observed which becomes more severe with increasing time of the activation process. In some cases the void formation leads to catastrophic failure and delamination at the CdS/CdTe junction. The microstructure of the films has been characterized before and after activation using a number of techniques including High Resolution Transmission Electron Microscopy of device cross-sections. The observations made have been correlated to device performance.

2022 IEEE 49th Photovoltaics Specialists Conference (PVSC)

In-situ Cd-saturated growth of polycrystalline CdTe:As thin film was performed by metal organic c... more In-situ Cd-saturated growth of polycrystalline CdTe:As thin film was performed by metal organic chemical vapour deposition at a low temperature of 350 °C, to investigate the impact on As doping and device V OC . Device characterization showed conversion efficiency of ∼14%, and V OC of 772 mV, which is an improvement to the baseline device with CdTe:As absorber layer grown at 390 °C under non-saturated conditions. When the low temperature Cd-saturated growth was combined with chlorine heat treatment at a higher temperature of 440 °C (in contrast with the standard 420 °C) for 10 min, device efficiency improved to ∼17% with a high V OC of 877 mV. As a result, ∼100 mV boost in V OC from baseline is demonstrated with Cd-saturated CdTe:As device. Micro-photoluminescence and time-resolved photoluminescence measurements performed on these Cd-saturated CdTe:As devices confirmed that minority carrier lifetime significantly improved.

IEEE Journal of Photovoltaics