In this thesis we examine key structure-property relationships associated with eumelanin's photophysical properties. This has involved characterizing both the molecular and supramolecular structure of eumelanin, and examining how these relate to eumalanin's key optical properties that are relevant to their their role in the biosphere as photoprotectants. Using low-voltage high resolution transmission electron microscopy we definitively show that sheets of proto-molecules stack to form nanostructures. The inter-sheet spacings within these structures are between 3.7 and 4.0 Å consistent with non-covalent pie-pie stacking in heteroaromatic systems. Stacking interactions in similarly structured aromatic macromolecules play an important role in non-radiative energy dissipation and we propose that this may also be the case in the eumelanin system. We also examine the recently proposed hypothesis that excited state intramolecular proton transfer may play a role in the photophysics of 5,6-dihydoxyindole carboxylic acid, a key eumelanin monomer. The experimental results acquired in this study indicate that this hypothesis needs to be carefully re-examined and its justification would require more extensive experimental support. Key optical properties of 5,6-dihydoxyindole carboxylic acid are re-evaluated in an inert aprotic solvent and these have provided some insight into the electronic structure as well as the rates of radiative and non-radiative decay in this important eumelanin monomer. We go on to show how the structure of eumelanin can be manipulated to produce eumelanin thin films. These films display electrical conductivities comparable with amorphous silicon, as well as a host of other interesting and potentially useful optoelectronic properties. The results show great promise for eumelanin-based applications such as chemi-sensors (in a variety of architectures including organic field effect transistors with chemi-sensitive channels) and bolometric photon detectors.

In this thesis we examine key structure-property relationships associated with eumelanin's photophysical properties. This has involved characterizing both the molecular and supramolecular structure of eumelanin, and examining how these relate to eumalanin's key optical properties that are relevant to their their role in the biosphere as photoprotectants. Using low-voltage high resolution transmission electron microscopy we definitively show that sheets of proto-molecules stack to form nanostructures. The inter-sheet spacings within these structures are between 3.7 and 4.0 Å consistent with non-covalent pie-pie stacking in heteroaromatic systems. Stacking interactions in similarly structured aromatic macromolecules play an important role in non-radiative energy dissipation and we propose that this may also be the case in the eumelanin system. We also examine the recently proposed hypothesis that excited state intramolecular proton transfer may play a role in the photophysics of 5,6-dihydoxyindole carboxylic acid, a key eumelanin monomer. The experimental results acquired in this study indicate that this hypothesis needs to be carefully re-examined and its justification would require more extensive experimental support. Key optical properties of 5,6-dihydoxyindole carboxylic acid are re-evaluated in an inert aprotic solvent and these have provided some insight into the electronic structure as well as the rates of radiative and non-radiative decay in this important eumelanin monomer. We go on to show how the structure of eumelanin can be manipulated to produce eumelanin thin films. These films display electrical conductivities comparable with amorphous silicon, as well as a host of other interesting and potentially useful optoelectronic properties. The results show great promise for eumelanin-based applications such as chemi-sensors (in a variety of architectures including organic field effect transistors with chemi-sensitive channels) and bolometric photon detectors.

In this thesis we examine key structure-property relationships associated with eumelanin's photophysical properties. This has involved characterizing both the molecular and supramolecular structure of eumelanin, and examining how these relate to eumalanin's key optical properties that are relevant to their their role in the biosphere as photoprotectants. Using low-voltage high resolution transmission electron microscopy we definitively show that sheets of proto-molecules stack to form nanostructures. The inter-sheet spacings within these structures are between 3.7 and 4.0 Å consistent with non-covalent pie-pie stacking in heteroaromatic systems. Stacking interactions in similarly structured aromatic macromolecules play an important role in non-radiative energy dissipation and we propose that this may also be the case in the eumelanin system. We also examine the recently proposed hypothesis that excited state intramolecular proton transfer may play a role in the photophysics of 5,6-dihydoxyindole carboxylic acid, a key eumelanin monomer. The experimental results acquired in this study indicate that this hypothesis needs to be carefully re-examined and its justification would require more extensive experimental support. Key optical properties of 5,6-dihydoxyindole carboxylic acid are re-evaluated in an inert aprotic solvent and these have provided some insight into the electronic structure as well as the rates of radiative and non-radiative decay in this important eumelanin monomer. We go on to show how the structure of eumelanin can be manipulated to produce eumelanin thin films. These films display electrical conductivities comparable with amorphous silicon, as well as a host of other interesting and potentially useful optoelectronic properties. The results show great promise for eumelanin-based applications such as chemi-sensors (in a variety of architectures including organic field effect transistors with chemi-sensitive channels) and bolometric photon detectors.

Thesis (PhD)--University of South Australia, 1999

Transparent conductive oxides (TCOs) and polycrystalline silicon (poly-Si) thin-films are very promising for application in photovoltaics. It is extremely challenging to develop cheap TCOs and poly-Si films to make photovoltaic devices. The aim of this thesis is to study sputtered aluminum-doped ZnO TCO and poly-Si films by solid-phase crystallization (SPC) for application in low-cost photovoltaics. The investigated aspects have been (i) to develop and characterize sputtered aluminum-doped ZnO (ZnO:Al) films that can be used as a TCO material on crystalline silicon solar cells, (ii) to explore the potential of the developed ZnO:Al films for application in ZnO:Al/c-Si heterojunction solar cells, (iii) to make and characterize poly-Si thin-films on different kinds of glass substrates by SPC using electron-beam evaporated amorphous silicon (a-Si) [referred to as EVA poly-Si material (SPC of evaporated a-Si)], and (iv) to fabricate EVA poly-Si thin-film solar cells on glass and improve the energy conversion efficiency of these cells by post-crystallization treatments. The ZnO:Al work in this thesis is focused on the correlation between film characteristics and deposition parameters, such as rf sputter power (Prf), working gas pressure (Pw), and substrate temperature (Tsub), to get a clear picture of film properties in the optimized conditions for application in photovoltaic devices. Especially the laterally non-uniform film properties resulting from the laterally inhomogeneous erosion of the target material are investigated in detail. The influence of Prf, Pw and Tsub on the structural, electrical, optical and surface morphology properties of ZnO:Al films is discussed. It is found that the lateral variations of the parameters of ZnO:Al films prepared by rf magnetron sputtering can be reduced to acceptable levels by optimising the deposition parameters. ZnO:Al/c-Si heterojunction solar cells are fabricated and characterized to demonstrate the feasibility of the fabricated ZnO:Al films for application in heterojunction solar cells. In this application, expensive indium-tin oxide (ITO) is usually used. Under the standard AM1.5G spectrum (100 mW/cm2, 25 ??C), the best fabricated cell shows an open-circuit voltage of 411 mV, a short-circuit current density of 30.0 mA/cm2, a fill factor of 66.7 %, and a conversion efficiency of 8.2 %. This is believed to be the highest stable efficiency ever reported for this type of cell. By means of dark forward current density-voltage-temperature (J-V-T) measurements, it is shown that the dominant current transport mechanism in the ZnO:Al/c-Si solar cells, in the intermediate forward bias voltage region, is trap-assisted multistep tunneling. EVA poly-Si thin-films are prepared on four types of glass substrates (planar and textured glass, both either bare or SiN-coated) based on evaporated Si, which is a cheaper Si deposition method than the existing technologies. The textured glass is realized by the UNSW-developed AIT process (AIT = aluminium-induced texture). The investigation is concentrated on finding optimized process parameters and evaluating film crystallization quality. It is found that EVA poly-Si films have a grain size in the range 0.8-1.5 ??m, and a preferential (111) orientation. UV reflectance and Raman spectroscopy measurements reveal a high crystalline material quality, both at the air-side surface and in the bulk. EVA cells are fabricated in both substrate and superstrate configuration. Special attention is paid to improving the Voc of the solar cells. For this purpose, after the SPC process, the samples receive the two post-crystallization treatments: (i) a rapid thermal anneal (RTA), and (ii) a plasma hydrogenation. It is found that two post-crystallization treatments more than double the 1-Sun Voc of the substrate-type cells. It is demonstrated that RTA improves the structural material quality of the cells. Furthermore, a hydrogenation step is shown to significantly improve the electronic material quality of the cells. Based on the RTA???d and hydrogenated EVA poly-Si material, the first mesa-type EVA cells are fabricated in substrate configuration, by using sputtered Al-doped ZnO as the transparent front contact. The investigation is focused on addressing the correlation between the type of the substrate and cell performance. Optical, electrical and photovoltaic properties of the devices are characterized. It is found that the performance of EVA cells depends on the glass substrate topography. For cells on textured glass, the AIT texture is shown to have a beneficial effect on the optical absorption of EVA films. It is demonstrated that a SiN barrier layer on the AIT-textured glass improves significantly both the crystalline quality of the poly-Si films and the energy conversion efficiency of the resulting solar cells. For cells on planar glass, a SiN film between the planar glass and the poly-Si film has no obvious effect on the cell properties. The investigations in this thesis clearly show that EVA poly-Si films are very promising for poly-Si thin-film solar cells on glass.

Transparent conductive oxides (TCOs) and polycrystalline silicon (poly-Si) thin-films are very promising for application in photovoltaics. It is extremely challenging to develop cheap TCOs and poly-Si films to make photovoltaic devices. The aim of this thesis is to study sputtered aluminum-doped ZnO TCO and poly-Si films by solid-phase crystallization (SPC) for application in low-cost photovoltaics. The investigated aspects have been (i) to develop and characterize sputtered aluminum-doped ZnO (ZnO:Al) films that can be used as a TCO material on crystalline silicon solar cells, (ii) to explore the potential of the developed ZnO:Al films for application in ZnO:Al/c-Si heterojunction solar cells, (iii) to make and characterize poly-Si thin-films on different kinds of glass substrates by SPC using electron-beam evaporated amorphous silicon (a-Si) [referred to as EVA poly-Si material (SPC of evaporated a-Si)], and (iv) to fabricate EVA poly-Si thin-film solar cells on glass and improve the energy conversion efficiency of these cells by post-crystallization treatments. The ZnO:Al work in this thesis is focused on the correlation between film characteristics and deposition parameters, such as rf sputter power (Prf), working gas pressure (Pw), and substrate temperature (Tsub), to get a clear picture of film properties in the optimized conditions for application in photovoltaic devices. Especially the laterally non-uniform film properties resulting from the laterally inhomogeneous erosion of the target material are investigated in detail. The influence of Prf, Pw and Tsub on the structural, electrical, optical and surface morphology properties of ZnO:Al films is discussed. It is found that the lateral variations of the parameters of ZnO:Al films prepared by rf magnetron sputtering can be reduced to acceptable levels by optimising the deposition parameters. ZnO:Al/c-Si heterojunction solar cells are fabricated and characterized to demonstrate the feasibility of the fabricated ZnO:Al films for application in heterojunction solar cells. In this application, expensive indium-tin oxide (ITO) is usually used. Under the standard AM1.5G spectrum (100 mW/cm2, 25 ??C), the best fabricated cell shows an open-circuit voltage of 411 mV, a short-circuit current density of 30.0 mA/cm2, a fill factor of 66.7 %, and a conversion efficiency of 8.2 %. This is believed to be the highest stable efficiency ever reported for this type of cell. By means of dark forward current density-voltage-temperature (J-V-T) measurements, it is shown that the dominant current transport mechanism in the ZnO:Al/c-Si solar cells, in the intermediate forward bias voltage region, is trap-assisted multistep tunneling. EVA poly-Si thin-films are prepared on four types of glass substrates (planar and textured glass, both either bare or SiN-coated) based on evaporated Si, which is a cheaper Si deposition method than the existing technologies. The textured glass is realized by the UNSW-developed AIT process (AIT = aluminium-induced texture). The investigation is concentrated on finding optimized process parameters and evaluating film crystallization quality. It is found that EVA poly-Si films have a grain size in the range 0.8-1.5 ??m, and a preferential (111) orientation. UV reflectance and Raman spectroscopy measurements reveal a high crystalline material quality, both at the air-side surface and in the bulk. EVA cells are fabricated in both substrate and superstrate configuration. Special attention is paid to improving the Voc of the solar cells. For this purpose, after the SPC process, the samples receive the two post-crystallization treatments: (i) a rapid thermal anneal (RTA), and (ii) a plasma hydrogenation. It is found that two post-crystallization treatments more than double the 1-Sun Voc of the substrate-type cells. It is demonstrated that RTA improves the structural material quality of the cells. Furthermore, a hydrogenation step is shown to significantly improve the electronic material quality of the cells. Based on the RTA???d and hydrogenated EVA poly-Si material, the first mesa-type EVA cells are fabricated in substrate configuration, by using sputtered Al-doped ZnO as the transparent front contact. The investigation is focused on addressing the correlation between the type of the substrate and cell performance. Optical, electrical and photovoltaic properties of the devices are characterized. It is found that the performance of EVA cells depends on the glass substrate topography. For cells on textured glass, the AIT texture is shown to have a beneficial effect on the optical absorption of EVA films. It is demonstrated that a SiN barrier layer on the AIT-textured glass improves significantly both the crystalline quality of the poly-Si films and the energy conversion efficiency of the resulting solar cells. For cells on planar glass, a SiN film between the planar glass and the poly-Si film has no obvious effect on the cell properties. The investigations in this thesis clearly show that EVA poly-Si films are very promising for poly-Si thin-film solar cells on glass.

Transparent conductive oxides (TCOs) and polycrystalline silicon (poly-Si) thin-films are very promising for application in photovoltaics. It is extremely challenging to develop cheap TCOs and poly-Si films to make photovoltaic devices. The aim of this thesis is to study sputtered aluminum-doped ZnO TCO and poly-Si films by solid-phase crystallization (SPC) for application in low-cost photovoltaics. The investigated aspects have been (i) to develop and characterize sputtered aluminum-doped ZnO (ZnO:Al) films that can be used as a TCO material on crystalline silicon solar cells, (ii) to explore the potential of the developed ZnO:Al films for application in ZnO:Al/c-Si heterojunction solar cells, (iii) to make and characterize poly-Si thin-films on different kinds of glass substrates by SPC using electron-beam evaporated amorphous silicon (a-Si) [referred to as EVA poly-Si material (SPC of evaporated a-Si)], and (iv) to fabricate EVA poly-Si thin-film solar cells on glass and improve the energy conversion efficiency of these cells by post-crystallization treatments. The ZnO:Al work in this thesis is focused on the correlation between film characteristics and deposition parameters, such as rf sputter power (Prf), working gas pressure (Pw), and substrate temperature (Tsub), to get a clear picture of film properties in the optimized conditions for application in photovoltaic devices. Especially the laterally non-uniform film properties resulting from the laterally inhomogeneous erosion of the target material are investigated in detail. The influence of Prf, Pw and Tsub on the structural, electrical, optical and surface morphology properties of ZnO:Al films is discussed. It is found that the lateral variations of the parameters of ZnO:Al films prepared by rf magnetron sputtering can be reduced to acceptable levels by optimising the deposition parameters. ZnO:Al/c-Si heterojunction solar cells are fabricated and characterized to demonstrate the feasibility of the fabricated ZnO:Al films for application in heterojunction solar cells. In this application, expensive indium-tin oxide (ITO) is usually used. Under the standard AM1.5G spectrum (100 mW/cm2, 25 ??C), the best fabricated cell shows an open-circuit voltage of 411 mV, a short-circuit current density of 30.0 mA/cm2, a fill factor of 66.7 %, and a conversion efficiency of 8.2 %. This is believed to be the highest stable efficiency ever reported for this type of cell. By means of dark forward current density-voltage-temperature (J-V-T) measurements, it is shown that the dominant current transport mechanism in the ZnO:Al/c-Si solar cells, in the intermediate forward bias voltage region, is trap-assisted multistep tunneling. EVA poly-Si thin-films are prepared on four types of glass substrates (planar and textured glass, both either bare or SiN-coated) based on evaporated Si, which is a cheaper Si deposition method than the existing technologies. The textured glass is realized by the UNSW-developed AIT process (AIT = aluminium-induced texture). The investigation is concentrated on finding optimized process parameters and evaluating film crystallization quality. It is found that EVA poly-Si films have a grain size in the range 0.8-1.5 ??m, and a preferential (111) orientation. UV reflectance and Raman spectroscopy measurements reveal a high crystalline material quality, both at the air-side surface and in the bulk. EVA cells are fabricated in both substrate and superstrate configuration. Special attention is paid to improving the Voc of the solar cells. For this purpose, after the SPC process, the samples receive the two post-crystallization treatments: (i) a rapid thermal anneal (RTA), and (ii) a plasma hydrogenation. It is found that two post-crystallization treatments more than double the 1-Sun Voc of the substrate-type cells. It is demonstrated that RTA improves the structural material quality of the cells. Furthermore, a hydrogenation step is shown to significantly improve the electronic material quality of the cells. Based on the RTA???d and hydrogenated EVA poly-Si material, the first mesa-type EVA cells are fabricated in substrate configuration, by using sputtered Al-doped ZnO as the transparent front contact. The investigation is focused on addressing the correlation between the type of the substrate and cell performance. Optical, electrical and photovoltaic properties of the devices are characterized. It is found that the performance of EVA cells depends on the glass substrate topography. For cells on textured glass, the AIT texture is shown to have a beneficial effect on the optical absorption of EVA films. It is demonstrated that a SiN barrier layer on the AIT-textured glass improves significantly both the crystalline quality of the poly-Si films and the energy conversion efficiency of the resulting solar cells. For cells on planar glass, a SiN film between the planar glass and the poly-Si film has no obvious effect on the cell properties. The investigations in this thesis clearly show that EVA poly-Si films are very promising for poly-Si thin-film solar cells on glass.

Transparent conductive oxides (TCOs) and polycrystalline silicon (poly-Si) thin-films are very promising for application in photovoltaics. It is extremely challenging to develop cheap TCOs and poly-Si films to make photovoltaic devices. The aim of this thesis is to study sputtered aluminum-doped ZnO TCO and poly-Si films by solid-phase crystallization (SPC) for application in low-cost photovoltaics. The investigated aspects have been (i) to develop and characterize sputtered aluminum-doped ZnO (ZnO:Al) films that can be used as a TCO material on crystalline silicon solar cells, (ii) to explore the potential of the developed ZnO:Al films for application in ZnO:Al/c-Si heterojunction solar cells, (iii) to make and characterize poly-Si thin-films on different kinds of glass substrates by SPC using electron-beam evaporated amorphous silicon (a-Si) [referred to as EVA poly-Si material (SPC of evaporated a-Si)], and (iv) to fabricate EVA poly-Si thin-film solar cells on glass and improve the energy conversion efficiency of these cells by post-crystallization treatments. The ZnO:Al work in this thesis is focused on the correlation between film characteristics and deposition parameters, such as rf sputter power (Prf), working gas pressure (Pw), and substrate temperature (Tsub), to get a clear picture of film properties in the optimized conditions for application in photovoltaic devices. Especially the laterally non-uniform film properties resulting from the laterally inhomogeneous erosion of the target material are investigated in detail. The influence of Prf, Pw and Tsub on the structural, electrical, optical and surface morphology properties of ZnO:Al films is discussed. It is found that the lateral variations of the parameters of ZnO:Al films prepared by rf magnetron sputtering can be reduced to acceptable levels by optimising the deposition parameters. ZnO:Al/c-Si heterojunction solar cells are fabricated and characterized to demonstrate the feasibility of the fabricated ZnO:Al films for application in heterojunction solar cells. In this application, expensive indium-tin oxide (ITO) is usually used. Under the standard AM1.5G spectrum (100 mW/cm2, 25 ??C), the best fabricated cell shows an open-circuit voltage of 411 mV, a short-circuit current density of 30.0 mA/cm2, a fill factor of 66.7 %, and a conversion efficiency of 8.2 %. This is believed to be the highest stable efficiency ever reported for this type of cell. By means of dark forward current density-voltage-temperature (J-V-T) measurements, it is shown that the dominant current transport mechanism in the ZnO:Al/c-Si solar cells, in the intermediate forward bias voltage region, is trap-assisted multistep tunneling. EVA poly-Si thin-films are prepared on four types of glass substrates (planar and textured glass, both either bare or SiN-coated) based on evaporated Si, which is a cheaper Si deposition method than the existing technologies. The textured glass is realized by the UNSW-developed AIT process (AIT = aluminium-induced texture). The investigation is concentrated on finding optimized process parameters and evaluating film crystallization quality. It is found that EVA poly-Si films have a grain size in the range 0.8-1.5 ??m, and a preferential (111) orientation. UV reflectance and Raman spectroscopy measurements reveal a high crystalline material quality, both at the air-side surface and in the bulk. EVA cells are fabricated in both substrate and superstrate configuration. Special attention is paid to improving the Voc of the solar cells. For this purpose, after the SPC process, the samples receive the two post-crystallization treatments: (i) a rapid thermal anneal (RTA), and (ii) a plasma hydrogenation. It is found that two post-crystallization treatments more than double the 1-Sun Voc of the substrate-type cells. It is demonstrated that RTA improves the structural material quality of the cells. Furthermore, a hydrogenation step is shown to significantly improve the electronic material quality of the cells. Based on the RTA???d and hydrogenated EVA poly-Si material, the first mesa-type EVA cells are fabricated in substrate configuration, by using sputtered Al-doped ZnO as the transparent front contact. The investigation is focused on addressing the correlation between the type of the substrate and cell performance. Optical, electrical and photovoltaic properties of the devices are characterized. It is found that the performance of EVA cells depends on the glass substrate topography. For cells on textured glass, the AIT texture is shown to have a beneficial effect on the optical absorption of EVA films. It is demonstrated that a SiN barrier layer on the AIT-textured glass improves significantly both the crystalline quality of the poly-Si films and the energy conversion efficiency of the resulting solar cells. For cells on planar glass, a SiN film between the planar glass and the poly-Si film has no obvious effect on the cell properties. The investigations in this thesis clearly show that EVA poly-Si films are very promising for poly-Si thin-film solar cells on glass.

The optical properties of zinc oxide crystals and thin films prepared by different methods are investigated. Single crystal zinc oxide samples prepared by melt and hydrothermal growth techniques were obtained. The influence of polarity and growth method on the optical properties were studied and correlated with their electronic properties. Thin films prepared by molecular beam epitaxy (MBE) and sputtering were studied and the influence of growth conditions and post growth treatment on the optical properties of the films was investigated. The photo-luminescence (PL) of bulk zinc oxide was examined at high resolution. Line widths of less than 0.1 meV were observed. More than a dozen different transitions in the near band edge region (NBE 360-380 nm) were noted, several of which displayed a separation of <0.5 meV which goes some way to illustrating the complexity of the system. Attempts were made, with some success, to reconcile the two main competing identification systems of the NBE transitions and explanations for some of the discrepancies are provided. The controversial deep level transitions in the visible part of the spectrum are fit with 3 Gaussians and their identities discussed with relation to the available literature. The presence of copper impurities was detected in annealed films and a model to explain their behaviour under annealing conditions is hypothesised. Films grown by MBE here at the University of Canterbury are shown to have PL line widths of as little as 2.2 meV, the ratio of active oxygen species in the growth chamber during deposition is shown to effect the optical quality of the films. It is shown that annealing can improve the optical quality of the films and various other methods of influencing the films properties are discussed. Reactive, magnetron, direct current sputtering is shown to be the optimal method of growth for maximising both optical and piezo-electric properties. Optimum annealing temperatures were found at 900 and 1100 ℃ with a local minimum at 1000 ℃. X-ray diffraction, atomic force and scanning electron microscopy measurements in addition to optical PL measurements show the influence of annealing on the polycrystalline sputtered ZnO films. Films grown on glass, silicon, sapphire and quartz were shown to display similar behaviour under annealing conditions. It was found that zinc oxide based devices were liable to be chemically unstable at temperatures above 1100 ℃. The piezo electric properties of the films were examined and attempts were made to prepare a zinc oxide film optimised for both optical quality and piezoelectric properties for possible future applications of a hybrid opto-mechanical coupled devices.

Thesis (M.S.)--Rochester Institute of Technology, 1989. / Includes bibliographical references (leaves 74-76).