Capacitance spectroscopy of GaAs p-i-n solar cells embedded with GaNAs quantum wells
- Authors: Venter, Danielle Ahlers
- Date: 2018
- Subjects: Solar cells , Photocatalysis Nanotechnology Fuel cells
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/21593 , vital:29713
- Description: The search for higher efficiencies in solar cell technology has brought forth competitive ideas, among them tandem solar cells (TSC) and intermediate-band solar cells (IBSC). These cells deliver higher efficiencies by absorbing a wider range of the electro-magnetic spectrum compared to conventional cells, but do come with unique challenges. This includes, amongst others, the need to find suitable material systems, which can fully realise the requirements behind the concept. In this study, the notion of using dilute nitrides in III-V systems as a candidate for the IBSC is considered. Incorporation of GaNAs QW structures into GaAs p-i-n solar cells are structurally, optically and electrically characterised. At a first estimate the photovoltaic properties of the material is obtained through current-voltage (I-V) measurements under illumination. It is observed that the open circuit voltage (𝑉𝑂𝐶), short circuit current (𝐼𝑆𝐶) and conversion efficiency decrease upon the incorporation of the QWs. Electrically active defect levels are notorious for reducing the life time of electron-hole pairs, directly impacting cell efficiency. In an effort to gain a clearer understanding of this behavior, the study of electrically active deep level center present in such devices were investigated. A comprehensive understanding of defects in semiconductors remains of fundamental importance and thus reinforces this approach. This was done using two of the most commonly used semiconductor defect spectroscopy techniques viz. admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS). Since in principle, these two techniques are similar, deep level related results were compared in order to verify the validity of the results. The devices under study, GaNAs/GaAs embedded QW p-i-n solar cells, were grown by molecular beam epitaxy (MBE). In particular, the doping of the quantum wells was varied and this effect on the electrical properties investigated. Four samples were studied and their electrical, optical and structural properties compared. The sample series consisted of a reference GaAs p-i-n diode that contained no embedded QWs and three GaAs p-i-n diodes each containing ten equally spaced and equally thick GaNAs QW layers. These layers were either Beryllium (Be) doped (p-type), un-doped or Silicon (Si) doped (n-type) respectively. Both AS and DLTS revealed deep level centers present in the devices. Each technique presented its own list of advantages and disadvantages and the collaborative use of both of them was found to be complementary in their determination of deep level defect centers. The correlation of these defects with the QWs is not clear as the structures were not optimized for capacitance spectroscopic measurements. NextNano++ simulation software was also used to theoretically model the electronic structure of the sample. The addition of the applied bias and its effect on the cross-over point of the Fermi level and the deep level energy, as well as the depletion width was investigated. This was a useful and essential tool for the interpretation of the results obtained and for the design of optimal structures for future studies.
- Full Text:
- Date Issued: 2018
- Authors: Venter, Danielle Ahlers
- Date: 2018
- Subjects: Solar cells , Photocatalysis Nanotechnology Fuel cells
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/21593 , vital:29713
- Description: The search for higher efficiencies in solar cell technology has brought forth competitive ideas, among them tandem solar cells (TSC) and intermediate-band solar cells (IBSC). These cells deliver higher efficiencies by absorbing a wider range of the electro-magnetic spectrum compared to conventional cells, but do come with unique challenges. This includes, amongst others, the need to find suitable material systems, which can fully realise the requirements behind the concept. In this study, the notion of using dilute nitrides in III-V systems as a candidate for the IBSC is considered. Incorporation of GaNAs QW structures into GaAs p-i-n solar cells are structurally, optically and electrically characterised. At a first estimate the photovoltaic properties of the material is obtained through current-voltage (I-V) measurements under illumination. It is observed that the open circuit voltage (𝑉𝑂𝐶), short circuit current (𝐼𝑆𝐶) and conversion efficiency decrease upon the incorporation of the QWs. Electrically active defect levels are notorious for reducing the life time of electron-hole pairs, directly impacting cell efficiency. In an effort to gain a clearer understanding of this behavior, the study of electrically active deep level center present in such devices were investigated. A comprehensive understanding of defects in semiconductors remains of fundamental importance and thus reinforces this approach. This was done using two of the most commonly used semiconductor defect spectroscopy techniques viz. admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS). Since in principle, these two techniques are similar, deep level related results were compared in order to verify the validity of the results. The devices under study, GaNAs/GaAs embedded QW p-i-n solar cells, were grown by molecular beam epitaxy (MBE). In particular, the doping of the quantum wells was varied and this effect on the electrical properties investigated. Four samples were studied and their electrical, optical and structural properties compared. The sample series consisted of a reference GaAs p-i-n diode that contained no embedded QWs and three GaAs p-i-n diodes each containing ten equally spaced and equally thick GaNAs QW layers. These layers were either Beryllium (Be) doped (p-type), un-doped or Silicon (Si) doped (n-type) respectively. Both AS and DLTS revealed deep level centers present in the devices. Each technique presented its own list of advantages and disadvantages and the collaborative use of both of them was found to be complementary in their determination of deep level defect centers. The correlation of these defects with the QWs is not clear as the structures were not optimized for capacitance spectroscopic measurements. NextNano++ simulation software was also used to theoretically model the electronic structure of the sample. The addition of the applied bias and its effect on the cross-over point of the Fermi level and the deep level energy, as well as the depletion width was investigated. This was a useful and essential tool for the interpretation of the results obtained and for the design of optimal structures for future studies.
- Full Text:
- Date Issued: 2018
Wavelength-modulation spectroscopy for the evaluation of the photoresponse of solar cells
- Mandanirina, Nambinintsoa Roméoh Hasinjatovo
- Authors: Mandanirina, Nambinintsoa Roméoh Hasinjatovo
- Date: 2016
- Subjects: Gallium arsenide semiconductors , Solar cells , Modulation spectroscopy
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/7244 , vital:21312
- Description: This study describes the development of a wavelength-modulation spectroscopy technique for the evaluation of solar cell devices. In particular, the technique is used to investigate the sub-bandgap response associated with the incorporation of GaSb quantum rings into the active region of a conventional GaAs p-i-n solar cell. These GaSb/GaAs quantum ring solar cells are a class of third generation cells, with the potential to exceed the Shockley-Queisser efficiency limit of single junction devices. Wavelength-modulation spectroscopy (WMS) techniques involve the modulation of the wavelength of a pseudo-monochromatic light source, with the resulting variation in the measured photocurrent then being a measure of the differential optical response of the solar cell. Although the conventional photocurrent spectrum of a solar cell is a good measure of the optical response characteristics, the differential technique gives supplemental detail related to the absorption spectrum. In addition to the basic WMS setup, we also developed an in situ flux correction module to ensure that a constant excitation intensity is maintained during the wavelength modulation. The excitation source inherently has a spectral dependence that leads to an undesirable contribution to the photocurrent signal. The operation of the flux corrected WMS setup has been demonstrated by photocurrent and photo-capacitance response measurements to obtain the differential quantum efficiency and charging characteristics of the quantum ring solar cells.
- Full Text:
- Date Issued: 2016
- Authors: Mandanirina, Nambinintsoa Roméoh Hasinjatovo
- Date: 2016
- Subjects: Gallium arsenide semiconductors , Solar cells , Modulation spectroscopy
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/7244 , vital:21312
- Description: This study describes the development of a wavelength-modulation spectroscopy technique for the evaluation of solar cell devices. In particular, the technique is used to investigate the sub-bandgap response associated with the incorporation of GaSb quantum rings into the active region of a conventional GaAs p-i-n solar cell. These GaSb/GaAs quantum ring solar cells are a class of third generation cells, with the potential to exceed the Shockley-Queisser efficiency limit of single junction devices. Wavelength-modulation spectroscopy (WMS) techniques involve the modulation of the wavelength of a pseudo-monochromatic light source, with the resulting variation in the measured photocurrent then being a measure of the differential optical response of the solar cell. Although the conventional photocurrent spectrum of a solar cell is a good measure of the optical response characteristics, the differential technique gives supplemental detail related to the absorption spectrum. In addition to the basic WMS setup, we also developed an in situ flux correction module to ensure that a constant excitation intensity is maintained during the wavelength modulation. The excitation source inherently has a spectral dependence that leads to an undesirable contribution to the photocurrent signal. The operation of the flux corrected WMS setup has been demonstrated by photocurrent and photo-capacitance response measurements to obtain the differential quantum efficiency and charging characteristics of the quantum ring solar cells.
- Full Text:
- Date Issued: 2016
Characterization of cell mismatch in photovoltaic modules using electroluminescence and associated electro-optic techniques
- Authors: Crozier, Jacqueline Louise
- Date: 2012
- Subjects: Photovoltaic cells , Solar cells
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10545 , http://hdl.handle.net/10948/d1015059
- Description: Solar cells allow the energy from the sun to be converted into electrical energy; this makes solar energy much more environmentally friendly than fossil fuel energy sources. These solar cells are connected together in a photovoltaic (PV) module to provide the higher current, voltage and power outputs necessary for electrical applications. However, the performance of the PV module is limited by the performance of the individual cells. Cell mismatch occurs when some cells are damaged or shaded and produce lower current output than the other cells in the series connected string. The cell mismatch lowers the module performance and can result in further damage as the weak cells are reverse biased and dissipate heat. Bypass diodes can be connected into the module to increase the module current output and prevent further damage. Since cell mismatch results in a significant decrease in the performance of deployed modules it is important to fully understand and characterise its effect on PV modules. PV modules can be characterised using various techniques, each providing important information about the performance of the module. Most commonly the current-voltage (I-V) characteristic curve of a module is measured in outdoor, fully illuminated conditions. This allows performance parameters such as short circuit current (Isc), open circuit voltage (Voc) and maximum power (Pmax) to be determined. In addition to this the shape of the curve allows device parameters like series and shunt resistances to be determined using parameter extraction algorithms like Particle Swarm Optimisation (PSO). The extracted parameters can be entered into the diode equation to model the I-V curve of the module. The I-V characteristic of the module can also be used to identify poor current producing cells in the module by using the worst-case cell determination method. In this technique a cell is shaded and the greater the drop in current in the whole module the better the current production of the shaded cell. The photoresponse of cells in a module can be determined by the Large-area Light Beam Induced Current (LA-LBIC) technique which involves scanning a module with a laser beam and recording the current generated. Electroluminescence (EL) is emitted by a forward biased PV module and is used to identify defects in cell material. Defects such as cracks and broken fingers can be detected as well as material features such as grain boundaries. These techniques are used to in conjunction to characterise the modules used in this study. The modules investigated in this study each exhibit cell mismatch resulting from different causes. Each module is characterised using a combination of characterisation techniques which allows the effect of cell mismatch be investigated. EL imaging enabled cracks and defects, invisible to the naked eye, to be detected allowing the reduced performance observed in I-V curves to be explained. It was seen that the cracked cells have a significant effect on the current produced by a string, while the effect of delaminated areas is less severe. Hot spots are observed on weak cells indicating they are in reverse bias conditions and will degrade further with time. PSO parameter extraction from I-V curves revealed that the effect of module degradation of device parameters like series and shunt resistances. A module with cracked cells and degradation of the antireflective coating has low shunt resistance indicating current losses due to shunting. Similar shunting is observed in a module with delamination and moisture ingress. The extracted parameters are used to simulate the I-V curves of modules with reasonable fit. The fit could be improved around the “knee” of the I-V curve by improving the methods of parameter extraction. This study has shown the effects of cell mismatch on the performance and I-V curves of the PV modules. The different causes of cell mismatch are discussed and modules with different cell configuration and damage are characterised. The characterisation techniques used on each module provide information about the photoresponse, current generation, material properties and cell defects. A comprehensive understanding of these techniques allows the cell mismatch in the modules to be fully characterized.
- Full Text:
- Date Issued: 2012
- Authors: Crozier, Jacqueline Louise
- Date: 2012
- Subjects: Photovoltaic cells , Solar cells
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10545 , http://hdl.handle.net/10948/d1015059
- Description: Solar cells allow the energy from the sun to be converted into electrical energy; this makes solar energy much more environmentally friendly than fossil fuel energy sources. These solar cells are connected together in a photovoltaic (PV) module to provide the higher current, voltage and power outputs necessary for electrical applications. However, the performance of the PV module is limited by the performance of the individual cells. Cell mismatch occurs when some cells are damaged or shaded and produce lower current output than the other cells in the series connected string. The cell mismatch lowers the module performance and can result in further damage as the weak cells are reverse biased and dissipate heat. Bypass diodes can be connected into the module to increase the module current output and prevent further damage. Since cell mismatch results in a significant decrease in the performance of deployed modules it is important to fully understand and characterise its effect on PV modules. PV modules can be characterised using various techniques, each providing important information about the performance of the module. Most commonly the current-voltage (I-V) characteristic curve of a module is measured in outdoor, fully illuminated conditions. This allows performance parameters such as short circuit current (Isc), open circuit voltage (Voc) and maximum power (Pmax) to be determined. In addition to this the shape of the curve allows device parameters like series and shunt resistances to be determined using parameter extraction algorithms like Particle Swarm Optimisation (PSO). The extracted parameters can be entered into the diode equation to model the I-V curve of the module. The I-V characteristic of the module can also be used to identify poor current producing cells in the module by using the worst-case cell determination method. In this technique a cell is shaded and the greater the drop in current in the whole module the better the current production of the shaded cell. The photoresponse of cells in a module can be determined by the Large-area Light Beam Induced Current (LA-LBIC) technique which involves scanning a module with a laser beam and recording the current generated. Electroluminescence (EL) is emitted by a forward biased PV module and is used to identify defects in cell material. Defects such as cracks and broken fingers can be detected as well as material features such as grain boundaries. These techniques are used to in conjunction to characterise the modules used in this study. The modules investigated in this study each exhibit cell mismatch resulting from different causes. Each module is characterised using a combination of characterisation techniques which allows the effect of cell mismatch be investigated. EL imaging enabled cracks and defects, invisible to the naked eye, to be detected allowing the reduced performance observed in I-V curves to be explained. It was seen that the cracked cells have a significant effect on the current produced by a string, while the effect of delaminated areas is less severe. Hot spots are observed on weak cells indicating they are in reverse bias conditions and will degrade further with time. PSO parameter extraction from I-V curves revealed that the effect of module degradation of device parameters like series and shunt resistances. A module with cracked cells and degradation of the antireflective coating has low shunt resistance indicating current losses due to shunting. Similar shunting is observed in a module with delamination and moisture ingress. The extracted parameters are used to simulate the I-V curves of modules with reasonable fit. The fit could be improved around the “knee” of the I-V curve by improving the methods of parameter extraction. This study has shown the effects of cell mismatch on the performance and I-V curves of the PV modules. The different causes of cell mismatch are discussed and modules with different cell configuration and damage are characterised. The characterisation techniques used on each module provide information about the photoresponse, current generation, material properties and cell defects. A comprehensive understanding of these techniques allows the cell mismatch in the modules to be fully characterized.
- Full Text:
- Date Issued: 2012
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