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.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 192-198). Available also in a digital version from Dissertation Abstracts.

The demand for compact, efficient and high performance electronic devices and sensor systems has become one of the primary driving force for rapid advancement in miniaturization of current technology. However, the attempt to push the limits of component length scales into the nano regime is being challenged by possibly unconventional laws of physics. One of the key design parameters for good performance of any system is its structural stability, defined by the strength of a material. The strength of a material is defined as its resistance to plastic (or permanent) deformation. In conventional metals plastic deformation is carried by the migration of lattice defects such as vacancies and dislocations. The barriers to the motion of these defects provide strength to metals, leading to an inverse power law scaling with inter barrier spacing, l  = Bl q where represents the nominal strength, B is a measure for strengthening capability of barriers and q represents the order of strengthening. The well known Hall Petch relation (q=0.5) expresses this effect for grain boundary strengthening, where grain boundaries (GBs) obstruct motion of dislocations during plastic deformation. Extensive research over past few decades has shown that grain size strengthening may be limited by GB mediated deformation processes in nanocrsytalline (nc) metals with grain sizes of ≤ 20 nm. The strength of nanocrsytalline metals saturates, or in some cases decreases, with a reduction in grain size. Molecular dynamic simulations have provided some indication of atomic scale activities that dominate deformation in nc metals. Although it is difficult to experimentally monitor the atomistic processes, in situ mechanical tests in synchrotron facilities have captured some mesoscopic features of deformation in nanocrsytalline metals. For instance, experiments have shown that the typically extended elastic plastic transition during deformation of nanocrsytalline metals could be classified into two regimes. For initial stages of deformation, in the microplastic regime, the width of various diffracting peaks decreases suggesting the dominance of processes leading to structural relaxation. However, at later stages of deformation, in a manner similar to conventional metals, the diffraction peak widths increased signifying the increasing importance of deformation processes that involve an accumulation of defects. It is well known that thermal annealing also causes relaxation of materials and during high temperature deformation continuous relaxation of stress concentrations could retard premature failure of metals. As the ductility in nanocrsytalline metals is limited to 3-5%, with very limited strain hardening, the processes of structural relaxation are very important. Thus, there is a fundamental need to understand the nature of structural relaxation during microplastic deformation in nanocrystals. It is well known that character of grain boundaries plays an important role in material properties. As the grain boundary area per unit volume varies inversely with grain size, nanocrsytalline metals contain a significant amount of grain boundary area. Moreover, due to small grain sizes, conventional Frank Read sources cannot operate for nucleating dislocations. Dislocations in nc metals are nucleated from GBs, traverse grains and gets absorbed in other GBs. The small volume of grains further restricts dislocation interactions. Thus, dislocation nucleation, propagation and absorption become possible rate controlling mechanisms in nc metals. Molecular dynamic simulations of nanocrsytalline and bicrystalline samples have shown that grain boundary structure could significantly affect these mechanisms. Simulations have shown further that apart from dislocation plasticity other grain boundary mediated process like GB sliding and GB diffusion become important with decreasing grain size. These processes are also influenced by GB character. Thus, it is important to understand the role of GB character in deformation of nc metals. On one hand where the structural need for high strength has encouraged reduction of internal microstructural length scales, miniaturization has also encouraged reduction in external length scale of device components. In modern electronic and sensor devices a typical component size varies from a few hundred microns to few tens of nanometer. Several studies have shown that free surfaces could reduce the constraints on deforming grains. With decreasing sample dimensions, the free surface to volume ratio increases and the internal microstructural length scale may become comparable to the external sample size. An increasing contribution from these two geometrical parameters can introduce external size effects in mechanical properties of materials. In the past, most of the external size effects have been attributed to strain gradient plasticity and deformation source starvation. However, a different external size effect has been observed during uniaxial test of polycrystalline metals where the strength of materials was found to deviate from their bulk values at smaller sample sizes. While most studies have shown a weakening effect, there have also been a few observations of strengthening with a reduction in sample size. In most studies, the external sample size was kept constant and the internal grain size was varied by thermal annealing to produce samples with different external to internal size ratios. As the mechanical properties of metals are sensitive to the internal length scales it is difficult to explicitly follow the external size effect during these experiments. Moreover, compared to internal size effects mechanistic understanding of external size effect is limited, and systematic experimental efforts are required for proper characterization of these effects. The present investigation was undertaken to improve the scientific understanding of internal and external length effects on mechanical properties of nanocrystalline and coarse grained (~16 – 140 µm) polycrystalline nickel. For studying the internal size effects free standing nanocrystalline nickel samples with ~30 nm grain size and two different textures were synthesized using galvanostatic pulsed electrodeposition technique from Watts and Sulfamate baths. The nanocrsytalline deposits from a Watts bath showed a strong <100> fiber texture (NiS) while deposits from a Sulfamate bath were relatively weak textured (NiW), with s and w representing strong and weak texture, respectively. In situ mechanical tests at the PSI synchrotron facility in Switzerland were used to understand the nature of relaxation processes during thermal annealing and deformation of nc metals. The diffraction peak analysis showed that thermal annealing at 423 K of strong textured deposits caused a significant reduction in root mean square strain with limited grain growth. Furthermore, no residual strains developed, suggesting a homogenous distribution of relaxation processes during thermal annealing. In contrast, during deformation, structural relaxation was highly biased due to dislocation activities. The grains contributing to <200> diffraction peak transverse to loading axis showed early yielding and faster relaxation during deformation. The inhomogeneous nature of deformation was also reflected in development of transverse tensile residual stresses in the <200> grains. These experiments showed that relaxation processes during thermal annealing and deformation differ in their respective length scales. Nanocrsytalline deposits with two different textures were also deformed under synchrotron to access the role of GB character. As direct quantification of GB character distribution is difficult in nc metals, texture was taken as a qualitative representative for GB character. Previous studies have shown that the fraction of low angle boundaries increases with increasing sharpness of texture in fiber textured materials. Thus, the two textured deposits represented materials with two different low angle GB fractions. In situ tests showed that during the initial stages of microplastic deformation dislocation mechanisms were favored in strongly textured NiS. The transversely oriented <200> grains showed early yielding, which caused a redistribution of stress among other grain families. However, for weakly textured NiW deposits, smaller length scale atomic activities preceded dislocation activities. All the grains supported larger elastic strains at lower stresses suggesting significant plastic activity at GB regions. At higher stresses transversely oriented <220> grains yielded plastically and transferred elastic loads to <200> grains. Thus, the nature of plastic deformation was observed to depend on the distribution of GB character. For understanding the external size effects on mechanical properties polycrystalline nickel samples with grain sizes of 16, 51 and 140 µm were tested uniaxially at various sample thicknesses. Three different deformation regimes were identified based on the thickness of the samples. At higher thicknesses, in regime I, no significant variation of flow strength was observed. Flow strengths in regime II, at intermediate thicknesses, showed a strengthening effect with a reduction in thickness. However at lower thicknesses in regime III, a weakening trend was observed with decreasing thickness. The cross over from strengthening to weakening was observed to depend on grain size and applied strain. Detailed microstructural analysis with electron back scattered diffraction (EBSD) imaging showed that intragranular lattice rotation increases with a reduction in sample thickness. As lattice rotations may be considered to be accommodated by geometrically necessary dislocations, a semi empirical phenomenological model based on strain gradient plasticity was developed to understand the mechanics of external size effect during uniaxial test of polycrystalline samples. Further application of the model to the present experimental results showed that the characteristic length for strain gradient decreased with increasing grain size and applied strain.

Indiana University-Purdue University Indianapolis (IUPUI) / Atomistic simulations are used in this project to study the deformation mechanism of polycrystalline and bicrystal of pure Al and Al-Mg alloys. Voronoi Tessellation was used to create three-dimensional polycrystalline models. Monte Carlo and Molecular Dynamics simulations were used to achieve both mechanical and chemical equilibrium in all models. The first part of the results showed improved strength, which is included the yield strength and ultimate strength in the applied tensile loading through the addition of 5 at% Mg to nanocrystalline aluminum. By viewing atomic structures, it clearly shows the multiple strengthening mechanisms related to doping in Al-Mg alloys. The strength mechanism of dopants exhibits as dopant pinning grain boundary (GB) migration at the early deformation stage. At the late stage where it is close to the failure of nanocrystalline materials, Mg dopants can stop the initiation of intergranular cracks and also do not let propagation of existing cracks along the GBs. Therefore, the flow stress will improve in Al-Mg alloy compared to pure Al. In the second part of our results, in different bicrystal Al model, ∑ 3 model has higher strength than other models. This result indicates that GB structure can affect the strength of the material. When the Mg dopants were added to the Al material, the strength of ∑5 bicrystal models was improved in the applied shear loading. However, it did not happen for ∑ 3 model, which shows Mg dopants cannot affect the behavior of this GB significantly. Analysis of GB movements shows that Mg dopants stopped GBs from moving in the ∑ 5 models. However, in the ∑ 3 GB, displacement of grain boundary planes was not affected by Mg dopants. Therefore, the strength and flow stress are improved by Mg dopants in ∑ 5 Al GBs, not in the ∑ 3 GBs.

No description available.

The cost of photovoltaic electricity needs to be significantly reduced in order to achieve a high electricity market penetration. Thin-film solar cells have good potential to achieve such cost savings though (i) large-area deposition onto low-cost foreign substrates, (ii) more streamlined processing, (iii) monolithic cell interconnection, and very efficient use of the expensive semiconductor material. Polycrystalline silicon (poly-Si) on glass is a promising technology for the cost-effective large volume production of PV modules since it (i) makes use of an abundant raw material, (ii) is non-toxic, (iii) does not suffer from light-induced degradation, and (iv) does not rely on TCO layers. Usually plasma enhanced chemical vapour deposition (PECVD) is used for the layer formation. This thesis explores the use of e-beam evaporation as deposition method since it is potentially much faster and cheaper than PECVD. The resulting solar cells are referred to as EVA (from EVAporation). Two inherent shunting mechanisms in EVA cells are demonstrated to be shunting through sub-micron sized pinholes when the back electrode is deposited and shunting between the emitter and the absorber layer at the glass-side electrode. Through the improved understanding of these shunting mechanisms it was possible to develop a suitable metallisation scheme for EVA cells using an aligned deposition of emitter and back surface field line contacts and a specially developed shunt mitigation etching technique. For the first time appreciable efficiencies of up to 5.2% were demonstrated on this material. It was also shown that only very lightly doped absorber layers can lead to the required high short-circuit currents in EVA cells. The resulting cells are currently completely limited by space charge region recombination occurring with comparatively low ideality factors of only ~ 1.4 This thesis also demonstrates the usefulness of Jsc-Suns measurements and investigates optical loss mechanisms in the current devices. Advanced modelling of distributed series resistance effects, influencing Suns-Voc, m-Voc and Jsc-Suns curves, is employed. PC1D modelling is used to extract relevant device parameters. In this work it was found that the diffusion length in the best EVA cells is longer than the absorber layer and that insufficient light trapping is currently the major hurdle to higher cell efficiencies. From the obtained results it can be concluded that EVA solar cells are promising candidates for the low-cost and high-volume production of solar modules.

The cost of photovoltaic electricity needs to be significantly reduced in order to achieve a high electricity market penetration. Thin-film solar cells have good potential to achieve such cost savings though (i) large-area deposition onto low-cost foreign substrates, (ii) more streamlined processing, (iii) monolithic cell interconnection, and very efficient use of the expensive semiconductor material. Polycrystalline silicon (poly-Si) on glass is a promising technology for the cost-effective large volume production of PV modules since it (i) makes use of an abundant raw material, (ii) is non-toxic, (iii) does not suffer from light-induced degradation, and (iv) does not rely on TCO layers. Usually plasma enhanced chemical vapour deposition (PECVD) is used for the layer formation. This thesis explores the use of e-beam evaporation as deposition method since it is potentially much faster and cheaper than PECVD. The resulting solar cells are referred to as EVA (from EVAporation). Two inherent shunting mechanisms in EVA cells are demonstrated to be shunting through sub-micron sized pinholes when the back electrode is deposited and shunting between the emitter and the absorber layer at the glass-side electrode. Through the improved understanding of these shunting mechanisms it was possible to develop a suitable metallisation scheme for EVA cells using an aligned deposition of emitter and back surface field line contacts and a specially developed shunt mitigation etching technique. For the first time appreciable efficiencies of up to 5.2% were demonstrated on this material. It was also shown that only very lightly doped absorber layers can lead to the required high short-circuit currents in EVA cells. The resulting cells are currently completely limited by space charge region recombination occurring with comparatively low ideality factors of only ~ 1.4 This thesis also demonstrates the usefulness of Jsc-Suns measurements and investigates optical loss mechanisms in the current devices. Advanced modelling of distributed series resistance effects, influencing Suns-Voc, m-Voc and Jsc-Suns curves, is employed. PC1D modelling is used to extract relevant device parameters. In this work it was found that the diffusion length in the best EVA cells is longer than the absorber layer and that insufficient light trapping is currently the major hurdle to higher cell efficiencies. From the obtained results it can be concluded that EVA solar cells are promising candidates for the low-cost and high-volume production of solar modules.

Thesis (Ph. D.)--University of Missouri-Columbia, 1998. / Typescript. Vita. Includes bibliographical references (leaves 107-111). Also available on the Internet.