Growth optimisation and laser processing of thin film phosphors for electroluminescent displays

This thesis presents results of a study of ZnS:Mn thin film phosphors used in Thin Film ELectroluminescent (TFEL) and Laterally Emitting TFEL (LETFEL) devices, examining techniques for phosphor growth optimisation and post deposition processing in order to strengthen development of novel TFEL devices. To achieve this, thin films of phosphor were deposited using RF magnetron sputtering to investigate the use of co-sputtering in order to optimise dopant concentration. 800 nm films of ZnS:Mn were simultaneously co-sputtered from ZnS and ZnS:Mn (1 wt.%) solid targets. The thin films were deposited at different manganese concentrations by varying the relative RF power applied to each target. The films were deposited directly onto 100 mm diameter (100) n-type silicon substrates, or onto a layer of 300 nm of Y2O3 to fabricate electroluminescent test devices. Luminescence from the phosphor films was characterised via photoluminescent excitation using a 337 nm pulsed N2 laser, with the photoluminescence (PL) optimum obtained at 0.38 ZnS:Mn power ratio. Electroluminescence (EL) from TFEL devices were excited by applying a sinusoidal waveform voltage at a frequency of 1 kHz with maximum luminance obtained at 0.36 ZnS:Mn power ratio

Process and Post-annealing Optimisation of SnS Thin Films with Alternative Buffer layers

Tin sulphide (SnS) is an environmentally friendly, Earth abundant and easy to fabricate thin film solar absorber for photovoltaic solar cell application. This work examines the properties of thermally evaporated SnS thin films, as a function of deposition parameters. Films were also subjected to a range of post-deposition treatments in vacuum, atmospheric pressure, chlorine and selenium ambient. SnS solar absorber layers were successfully deposited at low temperature (100 oC) to a thickness range from 100 to 3500 nm using thermal evaporation. Grain growth was partly dependent on the layer thickness where a progressive increase in grain size was noticed with increasing film thickness from 100 to 1500 nm; above 1500 nm thickness no further visible increase in the grains could be seen. Films grown to a thickness of 800 nm are found to be near stoichiometry with optimum energy bandgap compared to the thinner or thicker films. However, the SnS thin films showed strong dependence on substrate temperature. The temperature dependent study reveals that higher substrate temperatures lead to an increase in adatoms mobility, thereby promoting coalescences of smaller grains to form bigger grains. The increase in grain size with substrate temperature however stagnates after 350 oC such that further increasing the temperature does not induce further grain growth. Samples deposited at 350 oC substrate temperature were stoichiometric (Sn/S = 1.00) and with energy bandgap of 1.37 eV. Texture coefficient calculations showed that (111) orientation is more likely associated with the substrate temperatures 300 oC while, the (040) diffraction plane is related to higher temperatures (350 oC). Photoluminescence measurements demonstrated that controlling the film composition and optical bandgap is critical to produce a film that will luminesce, a requisite for any implementation in solar devices. On the other hand, the type of susbtrate material was found to significantly influence the properties of the SnS absorber films.The substrates studied include soda lime glass (SLG), quartz (Q), indium tin oxide (ITO) and fluorine-doped tin oxide (FTO) coated glass, molybdenum (Mo) coated SLG and quartz. ii Films composition remains stoichiometric (Sn/S = 1.00 0.01) across the range of substrates. For the Na-free samples, reduction in micro-strain followed an increase in grain size. Unlike kesterite or chalcopyrite materials, the absence of Na in the substrate induces a significant grain growth with the average grain size increasing from 0.14 μm on SLG to 0.32 μm on quartz, ITO and FTO. SnS absorber layers deposited at 350 oC (thickness of 800 nm) were subjected to heat treatment in diverse environments such as vacuum (P = 10-6 mbar, 60 min), nitrogen (P=1000 mbar, 60 min) and selenium (20 min under 10 mbar argon pressure) for temperatures greater than the growth temperature (400-500 oC). Vacuum annealing was ineffective in both inducing grain growth and achieving recrystallisation. Nitrogen ambient revealed a recrystallised structure with slight increase in grain sizes and ~6% decrease in the bandgap compared to the reference 1.37 eV for the as-grown layer due to loss of sulphur (Sn/S ratio increased from 1.00 to 1.27 following anneal). The incorporation of Se led to substantial increase in grains with an average grain size of ~2.0 µm compared to 0.14 µm for as-grown films, with a nearly complete sulphur substitution by selenium. In addition, Se incorporation minimised voids while reducing the bandgap to 1.28 eV, improving photoluminescence yield and the open circuit voltage. Finally, this thesis explores a range of n-type buffer layers in order to fabricate devices. Numerical simulations show that ZnS buffer layer has potential to replace conventional CdS in fabricating SnS-based solar cells as it offers the most appropriate band alignment. Working devices could only be fabricated when combining the selenium heat treatment and the ZnS buffer layer

Filmes finos de Cu2ZnSnS4 para PV: comparação de métodos de crescimento

Mestrado em Engenharia FísicaThis work focuses on a comparison between Cu2ZnSnS4 thin films with precursors grown exclusively by evaporation or by evaporation and RF Magnetron Sputtering. On the films which were grown using the second method was either sputtered ZnS or elemental Zinc. The morphology and composition of the samples was studied by SEM/EDS and their structure by XRD. Both methods were successful in producing thin films containing Cu2ZnSnS4. The samples which had their precursors grown exclusively through evaporation exhibited the most compact morphology but also were the ones that had more undesirable crystalline phases. Regarding the remaining samples, in the case where elemental Zinc was sputtered no diffusion issues were observed, whereas the ones with ZnS presented a layer of this material on the surface. This report is divided into six chapters which contain the introduction, information relative to semiconductors, Cu2ZnSnS4 solar cells, the growth and characterization techniques, the experimental procedure, results and their analysis and ends with the conclusion.Com o presente trabalho pretende-se efectuar uma comparação entre filmes finos de Cu2ZnSnS4 cujos precursores foram crescidos exclusivamente por evaporação ou por evaporação e pulverização catódica RF com magnetrão. A morfologia e composição das amostras foram estudadas por SEM/EDS e a sua estrutura por DRX. Com ambos os métodos conseguiu-se crescer filmes finos de Cu2ZnSnS4. As amostras cujos precursores foram crescidos exclusivamente através de evaporação apresentavam uma morfologia mais compacta, contudo eram as que apresentavam maior número de fases cristalinas indesejadas. Relativamente às restantes amostras, no caso em que Zinco foi depositado por pulverização catódica, não foram observados problemas de difusão, contudo o mesmo não se verificou para as que continham ZnS, sendo que estas apresentavam uma camada deste material na superfície dos filmes. Este documento encontra-se dividido em seis capítulos que incluem a introdução, informação relativa a semicondutores, células solares de Cu2ZnSnS4, as técnicas de crescimento e caracterização, o procedimento experimental, os resultados e a sua análise e termina com a conclusão

Thin film CDTE solar cells deposited by pulsed DC magnetron sputtering

Thin film cadmium telluride (CdTe) technology is the most important competitor for silicon (Si) based solar cells. Pulsed direct current (DC) magnetron sputtering is a new technique has been developed for thin film CdTe deposition. This technique is industrially scalable and provides uniform coating. It is also possible to deposit thin films at low substrate temperatures. A series of experiments are presented for the optimisation of the cadmium chloride (CdCl2) activation process. Thin film CdTe solar cells require CdCl2 activation process to improve conversion efficiencies. The role of this activation process is to increase the grain size by recrystallisation and to remove stacking faults. Compaan and Bohn [1] used the radio-frequency (RF) sputtering technique for CdTe solar cell deposition and they observed small blisters on CdTe layer surface. They reported that blistering occurred after the CdCl2 treatment during the annealing process. Moreover, void formation was observed in the CdTe layer after the CdCl2 activation process. Voids at the cadmium sulphide (CdS)/CdTe junction caused delamination hence quality of the junction is poor. This issue has been known for more than two decades but the mechanisms of the blister formation have not been understood. One reason may be the stress formation during CdTe solar cells deposition or during the CdCl2 treatment. Therefore, the stress analysis was performed to remove the defects observed after the CdCl2 treatment. This was followed by the rapid thermal annealing to isolate the CdCl2 effect by simply annealing. Small bubbles observed in the CdTe layer which is the first step of the blister formation. Using high resolution transmission electron microscopy (HR-TEM), it has been discovered that argon (Ar) working gas trapped during the deposition process diffuses in the lattice which merge and form the bubbles during the annealing process and grow agglomeration mainly at interfaces and grain boundaries (GBs). Blister and void formation were observed in the CdTe devices after the CdCl2 treatment. Therefore, krypton (Kr), neon (Ne) gases were used as the magnetron working gas during the deposition of CdTe layer. The results presented in this thesis indicated that blister and void formation were still existing with the use of Kr an Ne. Xe, which has a higher atomic mass than Kr, Ne, Ar, Cd and Te, was used as the magnetron working gas and it resulted in surface blister and void free devices

CDS-CU(_x)S single crystal and thin film solar cells

The work presented in this thesis is concerned with photovoltaic cells formed by plating CdS single crystals and thin films, and Cd(_y) Zn(1 _ y)S single crystals, with copper sulphide. An electroplating technique has been used to control the phase of copper sulphide by changing the electric field during its formation. Different phases of Cu(_x)S have been identified directly using Reflection High Energy Diffraction (RHEED), and indirectly from spectral response measurements. A dramatic change in the spectral response accompanying the reduction in the covellite response associated with an increase in that from chalcocite following argon heat treatment has been achieved. The change from the djurleite phase to that of chalcocite has also been obtained by using argon heat treatment for 5 minutes at 200 C. This effect was found to be reversible in that layers of chalcocite were converted to djurleite when air was used as the ambient for the heat treatment. C-V measurements have demonstrated that with increasing plating bias the donor concentration decreases at first before it assumes a constant value. This led to the effect of decreasing the junction capacitance as the width of the depletion region changed. The problem of the stability of the CdS-Cu(_2)S photovoltaic devices formed by wet plating" is addressed by studying the combined effects of the substrate onto which the CdS is deposited and the ambient used during annealing. Thin film cells have been prepared on both Ag/Cr and SnO substrates, and the device characteristics for each have been investigated as a function of annealing ambient. The results have shown that devices formed on Ag/Cr substrates were more stable following annealing in air than in argon, while the converse was true for cells fabricated on SnO(_x) substrates. The degradation effects of CdS-Cu(_2) S photovoltaic cells have been investigated. While devices stored in the dark showed little or no degradation, those maintained under illumination exhibited a significant deterioration in all operational parameters over a four week period. As far as the combined effect of temperature and ambient on the stability of cells are concerned, it was found that the ageing of devices in argon at room temperature in the dark was negligible, and moreover the fill factor was observed to improve marginally. When the devices were stored in the same ambient conditions at 50 C, they showed a significant improvement in the fill factor, but simultaneously exhibited a considerable reduction in the short circuit current. This process was reversible, since the sensitivity of degraded devices could be restored by annealing them in a hydrogen/nitrogen mixture. By comparing Electron Spectroscopy for Chemical Analysis (ESCA) studies with solar cell device characteristics, it has been shown that the formation of copper oxide on the Cu(_2)S surface plays a significant role in the degradation of CdS-Cu(_2) S devices. The extent of the cross-over between the dark and light J-V characteristics is a function of the period of etching used prior to junction formation. The variation of current and diode factor has been established as a function of the bias value. The dependence of forward current on the temperature at fixed forward voltage has also been investigated. Finally this work has shown that an increase in V(_oc) can be achieved when Cd(_0◦8)Zn(_0◦2)S is used as a base material for solar cells instead of CdS. Different traps were identified through a photocapacitance investigation. An important trap was found at 0.78eV below the conduction band. It has been demonstrated that the effect of this level was found to be diminished much more slowly when the annealing was carried out in argon rather than in air. This level may play an important role in the Cd(0◦8) Zn(0◦2)S-Cu(_2)S solar cell properties

Electrooptic modulation of infrared radiation in planar cadmium sulphide waveguides

Abstract available p. 6-

Characterisation and optimisation of alternating current thin film electroluminescent displays

This Thesis presents research undertaken to investigate the electro-optical characterisation and optimisation of Thin Film Electroluminescent (TFEL) devices and Laterally Emitting Thin Film Electroluminescent (LETFEL) devices with respect to device lifetime and aging. Post deposition localised laser annealing as an alternative to thermal annealing has been previously described in the literature. The effects of laser annealing on various devices is investigated and described within this Thesis. In particular, the novel use of ArF laser annealing at a wavelength of 193nm as a post deposition annealing process for ZnS:Mn thin films deposited by RF magnetron sputtering has been presented and compared to KrF laser annealing (248nm wavelength). Additionally the use of KrF laser annealing on a new deposition method, High Target Utilisation Sputtering (HiTUS) is presented, with successful results obtained on heat sensitive substrates. Results presented show that the use of KrF produces slightly better performance in respect to maximum luminance, however the use of ArF laser annealing can allow for achievement of higher luminance at lower applied voltages