Zivayi Chiguvare - Academia.edu (original) (raw)
Papers by Zivayi Chiguvare
Wind shear coefficient (WSC) analysis and energy yield estimation were performed at a coastal and... more Wind shear coefficient (WSC) analysis and energy yield estimation were performed at a coastal and an inland location in Namibia using 10-minute average wind speeds collected at the two locations over a period of one year (January to December 2013). To enable proper understanding of wind shear trends, WSCs, and roughness lengths, wind data were observed at two heights (i.e., 20.88 m and 48.14 m above ground level (AGL) at Terrace Bay (TB) and (23.49 m and 50.49 m AGL) at Okanapehuri (OP). The computations resulted in overall mean values of WSCs of 0.0463 and 0.24 at TB and OP, respectively, and roughness lengths (z0) of 0.000000673 m and 0.526 m at TB and OP, respectively. The variations of WSCs and z0 by months, hours and direction were thoroughly analysed and described. At the coast, sea/land interface significantly influenced WSCs and z0, resulting in lower than expected mean values, whereas at OP, the transitional nature of the vegetation shaped the pattern observed. Based on the...
We studied charge injection and transport mechanisms in blends of poly(3hexylthiophene) (P3HT) an... more We studied charge injection and transport mechanisms in blends of poly(3hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM), by analyzing dark, temperature dependent current-voltage characteristics of the P3HT:PCBM blend thin films sandwiched between aluminium electrodes in a MIM configuration. We present a general method of interpreting Fowler Nordheim plots of metal/semiconductor/metal devices with pronounced non-linear characteristics by dividing them into several regions based on physical origins. We show that by applying appropriate electric fields it is possible to switch from electron-only conduction to hole-only conduction in a single Al/P3HT:PCBM/Al device. We affirm that electrons can be selectively transported through the lowest unoccupied molecular orbital of PCBM at low applied voltages and low temperatures; and alternatively holes can be transported through the highest occupied molecular orbital of P3HT at higher applied voltages and high temper...
Control of diameter and nitrogen doping in the synthesis of nitrogen doped carbon nanotubes (N-CN... more Control of diameter and nitrogen doping in the synthesis of nitrogen doped carbon nanotubes (N-CNTs) can be achieved by careful selection of catalyst, precursor mixture and chemical vapour deposition (CVD) synthesis temperature. A solution of methanol and acetonitrile (20% acetonitrile) was pyrolysed over a Fe/Al/MgO catalyst in a CVD reactor at 850 o C. The product was characterised by TEM, TGA and Raman spectroscopy. N-CNTs with well graphitised morphology, small diameter, and narrow diameter distribution were obtained. Catalyst support and metal particles were removed completely during purification without any change in morphology of the material. Organic solar devices were made from P3HT and the NCNTs synthesized.
Optical and Quantum Electronics, 2020
The asymmetric behavior in the dark current of any solar cell is essential for decoupling recombi... more The asymmetric behavior in the dark current of any solar cell is essential for decoupling recombination and charge extraction for efficient charge collection. Therefore current density dependence on applied voltage can be used to investigate the complex interplay between bulk charge transport, interface exchange effects and recombination mechanisms. In the present work, we investigate from dark current-density [J(V)] of bulk heterojunction solar cells, the dependence of mobility and charge injection mechanisms on different blend layer thicknesses. The active layer thickness has been established through varying the spin-coating speeds between 1000, 2000, 3000 and 4000 revolutions per minute (rpm) and confirmed by a dektak surface profilometer. The carrier mobility (μ) as a factor limiting the efficiency of organic solar cells was investigated from dark space charge limited current and trap free space charge limited current conduction mechanisms to distinguish between charge extraction and recombination. This approach allows the determination of the effects of threshold field through variation of the active layer thickness (ALT) on the potential barrier height (B) at the electrode contacts. Low values of charge carrier mobilities (10-6 cm 2 V −1 s −1) in the trap free space charge limited current conduction region have been correlated to the Langevin recombination constants. In the ohmic region, the highest dark carrier mobility corresponded to the 77.1 nm ALT. Further we observe a shift in the transition voltage at the inflection point of J-V curves with increasing film thickness in the forward bias.
Journal of Renewable Energy, 2021
Evaluation of the maximum solar energy potential of a given area for possible deployment of solar... more Evaluation of the maximum solar energy potential of a given area for possible deployment of solar energy technologies requires assessment of clear sky solar irradiance for the region under consideration. Such localized assessment is critical for optimal sizing of the technology to be deployed in order to realize the anticipated output. As the measurements are not always available where they are needed, models may be used to estimate them. In this study, three different models were adapted for the geographical location of the area under study and used to estimate clear sky global horizontal irradiance (GHI) at three locations in the subtropical desert climate of Namibia. The three models, selected on the basis of input requirements, were used to compute clear sky GHI at Kokerboom, Arandis, and Auas. The models were validated and evaluated for performance using irradiance data measured at each of the sites for a period of three years by computing statistical parameters such as mean bi...
Optical and Quantum Electronics, 2020
Organic photovoltaic solar cells can offer advantages of being mechanically flexible and durable,... more Organic photovoltaic solar cells can offer advantages of being mechanically flexible and durable, large area devices, lightweight, made from a diversity of materials and low-cost fabrication. Their efficiency is, however, still too low for commercial exploitation. Empirical observations reveal that polymer-fullerene (P3HT:PCBM) based solar cell performance depends on thermal annealing processes employed, especially the annealing temperatures and durations. The annealing parameters are known to influence the energetics and kinetics of the blending process or morphology, but the associated physics is not fully understood. In this work, current density-voltage characteristics of P3HT:PCBM bulk heterojunction organic solar cells, thermally annealed at different temperatures, 65-160 °C post fabrication, were investigated under dark and illuminated conditions, and compared to their as-cast counterparts. In certain electrical regimes, as-cast devices showed higher values of current density in comparison to the corresponding annealed devices. Such performance was attributed to airborne chemical doping of the as-cast semiconductor layer, which creates electrically conductive percolation pathways within the as-cast devices. We propose that annealing of semiconductors must be a two-step process, which first initiates decrease in conductivity, followed by its increase. As-cast devices P3HT-PCBM bulk heterojunction solar cells prepared under atmospheric conditions were observed to have comparatively superior photovoltaic performance in comparison to thermally annealed devices. The efficiency drop in the annealed counterparts is attributed to dedoping due to thermal annealing. An annealing temperature of ~ 140 °C was found to be optimum for power conversion efficiency in the bulk heterojunction, 1:1 by mass, P3HT:PCBM based solar cells.
Catalysis Letters, 2020
This review focusses on the recent developments in designing Layered Double Hydroxides (LDHs) wit... more This review focusses on the recent developments in designing Layered Double Hydroxides (LDHs) with conductive, interlayer anion replacement, for efficient hydrogen fuel production by water splitting through Oxygen Evolution Reactions (OER) and Hydrogen Evolution Reactions (HER). Nickel nano structured catalysts improves OER performance are highlighted in detail in terms of compositional differences between transitional metal components, and challenges in future designing of rationalized Ni and Ni nano LDHs. The layered structure has exceptional flexibility of incorporating mixed valence transition metal ions into the LDHs structure in different compositions and this opens the massive potential to design high-performance LDHs catalysts on the molecular and nanometer scales. LDHs such as NiCoFe LDHs, Ni foam, Co Ni nano spheres, RuO 2 , Ir(dppe) 2 Cl, NiS 2 , Ni-N-Co-doped carbon nano fibers, NiCoSe 2 /cHRD are attracting increasing interest in the field of water splitting into hydrogen and oxygen due to their unique physicochemical properties. The highlighted summary will provide useful information in the development of novel Ni LDHs catalysts, which enables better understanding of OER properties valuable to address key issues. Increased fundamental understanding of water splitting catalysts would allow for rationallydirected improvements.
International Journal of Sustainability in Higher Education, 2018
Purpose This paper aims to present a new master’s programme for promoting energy access and energ... more Purpose This paper aims to present a new master’s programme for promoting energy access and energy efficiency in Southern Africa. Design/methodology/approach A transdisciplinary approach called “participatory integrated assessment of energy systems” (PARTICIPIA) was used for the development of the curriculum. This approach is based on the two emerging fields of “multi-scale integrated assessment” and “science for governance”, which bring innovative concepts and methods. Findings The application of the PARTICIPIA methodology to three case studies reveals that the proposed transdisciplinary approach could support energy and development policies in the region. The implementation of the PARTICIPIA curriculum in three higher education institutions reveals its ability to respond to the needs of specific contexts and its connection with existing higher education programmes. Practical implications Considering energy issues from a transdisciplinary approach in higher education is absolutely ...
Procedia Manufacturing, 2017
physica status solidi (a), 2017
Here, it is shown that by placing a thin film of N‐CNTs on top of the transparent ITO hole collec... more Here, it is shown that by placing a thin film of N‐CNTs on top of the transparent ITO hole collecting electrode of a polymer solar cell, the short circuit current density and efficiency are enhanced by 25 and 50%, respectively, and the usual efficiency losses in polymer solar cells as a function of incident light illumination intensity are significantly reduced. Nanostructuring the anode improves the mobility and collection of holes toward the anode, and as a result reduces space‐charge accumulation in the polymer solar cell.
Proceedings of the ISES Solar World Congress 2011, 2011
MRS Proceedings, 2001
ABSTRACTWe studied the electronic transport properties of conjugated polymer/fullerene based sola... more ABSTRACTWe studied the electronic transport properties of conjugated polymer/fullerene based solar cells by means of temperature and illumination intensity dependent current-voltage characteristics, admittance spectroscopy and light-induced electron spin resonance. The short-circuit current density increases with temperature at all light illumination intensities applied, i.e., from 100 mW/cm2to 0.1 mW/cm2(white light), whereas a temperature independent behavior was expected. An increase of the open-circuit voltage from 850 mV to 940 mV was observed, when cooling down the device from room temperature to 100 K. The fill factor depends strongly on temperature with a positive temperature coefficient in the whole temperature range. In contrast, the light intensity dependence of the fill factor shows a maximum of 52% at intermediate illumination intensities (3 mW/cm2) and decreases subsequently, when increasing the intensity up to 100 mW/cm2. Further studies by admittance spectroscopy rev...
Zeitschrift für Naturforschung A, 2007
The effects of thermal annealing on the bulk transport properties of poly(3-hexylthiophene) (P3HT... more The effects of thermal annealing on the bulk transport properties of poly(3-hexylthiophene) (P3HT) were studied by analyzing room temperature current-voltage characteristics of polymer thin films sandwiched between indium tin oxide/poly[ethylene dioxythiophene]:poly[styrene sulfonate] (ITO/PEDOT:PSS) and aluminum electrodes, before and after a thermal annealing step. It was observed that annealing takes place in two steps: (1) Dedoping of the polymer of impurities such as oxygen, remnant solvent, water, leading to a decrease in the conductivity of the film, and (2) thermally induced motion of the polymer chains leading to closer packing, thus, stronger inter-chain interaction and, consequently, increase in conductivity. It was also observed that the ITO/PEDOT:PSS/P3HT hole injection barrier increases on annealing the ITO/PEDOT:PSS/P3HT/Al thin film devices. The implications of impurity dedoping and closer packing on the output characteristics of bulk heterojunction polymer-fullerene...
ABSTRACT Easily processable conjugated polymers for optoelectronics, in general, and for photovol... more ABSTRACT Easily processable conjugated polymers for optoelectronics, in general, and for photovoltaic energy conversion, in particular, is an attractive research field, in which the combined efforts of material science, device engineers, and spectroscopists are welcomed. To improve the efficiency of polymer solar cells, currently within the range of 2.5 and 3.5%, it is vital to understand which mechanisms control the current-voltage characteristics of a given device. Temperature and light intensity dependence of the main solar cell parameters is very informative for analysing the power efficiency losses. The strong temperature dependence of the J SC (Fig. 1a) reflects the transport losses due to trapping and recombination of charge carriers. Such behaviour was established recently for bulk-heterojunction devices based on absorber materials with low charge carrier mobility, polyparaphenylene-vinylene derivative, OC1C10-PPV blended with the fullerene PCBM, [1]. In contrast, in solar cells based on higher mobility polymers, poly (3-hexylthiophene 2.5-diyl) (P3HT) blended with PCBM, the J SC , being thermally activated at low temperatures, saturates and becomes nearly temperature independent at elevated temperatures (Fig. 1b), clearly indicating that the charge carriers traverse the active layer without significant losses [2,3].
Journal of Applied Physics, 2014
The superiority of the electronic transport properties of single-walled carbon nanotube (SWNT) ro... more The superiority of the electronic transport properties of single-walled carbon nanotube (SWNT) ropes over SWNT mats is verified from low temperature and frequency-dependent transport. The overall change of resistance versus in nanotube mats shows that 3D variable range hopping is the dominant conduction mechanism within the 2–300 K range. The magneto-resistance (MR) is found to be predominantly negative with a parabolic nature, which can also be described by the hopping model. Although the positive upturn of the MR at low temperatures establishes the contribution from quantum interference, the inherent quantum transport in individual tubes is suppressed at elevated temperatures. Therefore, to minimize multi-channel effects from inter-tube interactions and other defects, two-terminal devices were fabricated from aligned SWNT (extracted from a mat) for low temperature transport as well as high-frequency measurements. In contrast to the mat, the aligned ropes exhibit step-like features...
Proceedings from the International Conference on Advances in Engineering and Technology, 2006
ABSTRACT Polymer photovoltaics has become a very interesting area of research given the success o... more ABSTRACT Polymer photovoltaics has become a very interesting area of research given the success of polymer electronic devices like displays, field effect transistors and light emitting diodes. We developed bulk heterojunction polymer-fullerene solar cells, with a poly(3hexyl)thiophene and a soluble fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester, P3HT:PCBM blend as the active layer, that yielded above 2.5 % energy conversion efficiencies at standard test conditions. The solar cells were characterized by analyzing current-voltage characteristics at various temperatures, from 80 to 400 K, and measuring their external quantum efficiencies at room temperature. We discuss the origin of open circuit voltage observed and estimate that the upper limit of open circuit voltage for P3HT: PCBM based polymer heterojunction solar cells is 1.2 V. The highest current density measured was 8.5 mA/cm 2 , while the energy conversion efficiency gave 2.71 % for 100 mW/cm 2 white light intensity, AM 1.5 spectrum at 300 K. A maximum external quantum efficiency of 65 % was also determined for monochromatic light of wavelength 550 nm. Some of the major challenges in efficiency improvement of such solar cells are discussed.
Physical Review B, 2010
Based on magneto-resistance studies performed over a wide range of magnetic fields as well as tem... more Based on magneto-resistance studies performed over a wide range of magnetic fields as well as temperatures, we establish that correction to conductivity in heavily nitrogen doped ultrananocrystalline diamond (UNCD) films is dominated by anisotropic weak localization (WL) in three dimensions associated with a propagative Fermi surface. Also, low temperature electrical conductivity can show weakly localized transport in 3D combined with the effect of electronelectron interactions in these materials, which is remarkably different from the conductivity in 2DWL or strong localization regime. The corresponding dephasing time of electronic wavefunctions in these systems described as ~ T-p with p < 1, follows a relatively weak temperature dependence compared to the generally expected nature for bulk dirty metals having p 1. The temperature dependence of Hall (electron) mobility together with an enhanced electron density has been used to interpret the unusual magneto-transport features and show delocalized electronic transport in these n-type UNCD films, which can be described as low-dimensional superlattice structures.
![Research paper thumbnail of Current Conduction in Poly(3-Hexylthiophene) and in Poly(3- Hexylthiophene) doped [6,6]-Phenyl C61-Butyric Acid Methylester Composite Thin Film Devices](https://attachments.academia-assets.com/116934721/thumbnails/1.jpg)
](Zeitschrift für Naturforschung A, 2012
Transport properties of poly(3-hexylthiophene) (P3HT), and of its blend with [6,6]-phenyl C61- bu... more Transport properties of poly(3-hexylthiophene) (P3HT), and of its blend with [6,6]-phenyl C61- butyric acid methylester (PCBM), were studied by analysing temperature dependent current-voltage characteristics of spin cast thin films sandwiched between aluminium electrodes in a metal-insulator- metal (MIM) configuration. It was found that in Al/P3HT/Al devices, the current is limited by space charge that accumulates near the hole injecting electrode due to the poor bulk transport properties of P3HT. At low temperatures and high applied electric fields the current density obeys a power law of the form J _ Vm, characteristic of space charge limited current (SCLC) in the presence of exponentially distributed traps within the band gap. These traps are filled by charge that is injected by quantum mechanical tunnelling, which is adequately described by the Fowler-Nordheim (FN) theory. By calculating the majority charge carrier mobility in Al/P3HT/Al and Al/P3HT:PCBM/Al devices from the Ohmi...
Synthetic Metals, 2003
Electrical and optical design and characterisation of regioregular poly(3-hexylthiophene-2,5diyl)... more Electrical and optical design and characterisation of regioregular poly(3-hexylthiophene-2,5diyl)/fullerenebased heterojunction polymer solar cells.
Wind shear coefficient (WSC) analysis and energy yield estimation were performed at a coastal and... more Wind shear coefficient (WSC) analysis and energy yield estimation were performed at a coastal and an inland location in Namibia using 10-minute average wind speeds collected at the two locations over a period of one year (January to December 2013). To enable proper understanding of wind shear trends, WSCs, and roughness lengths, wind data were observed at two heights (i.e., 20.88 m and 48.14 m above ground level (AGL) at Terrace Bay (TB) and (23.49 m and 50.49 m AGL) at Okanapehuri (OP). The computations resulted in overall mean values of WSCs of 0.0463 and 0.24 at TB and OP, respectively, and roughness lengths (z0) of 0.000000673 m and 0.526 m at TB and OP, respectively. The variations of WSCs and z0 by months, hours and direction were thoroughly analysed and described. At the coast, sea/land interface significantly influenced WSCs and z0, resulting in lower than expected mean values, whereas at OP, the transitional nature of the vegetation shaped the pattern observed. Based on the...
We studied charge injection and transport mechanisms in blends of poly(3hexylthiophene) (P3HT) an... more We studied charge injection and transport mechanisms in blends of poly(3hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM), by analyzing dark, temperature dependent current-voltage characteristics of the P3HT:PCBM blend thin films sandwiched between aluminium electrodes in a MIM configuration. We present a general method of interpreting Fowler Nordheim plots of metal/semiconductor/metal devices with pronounced non-linear characteristics by dividing them into several regions based on physical origins. We show that by applying appropriate electric fields it is possible to switch from electron-only conduction to hole-only conduction in a single Al/P3HT:PCBM/Al device. We affirm that electrons can be selectively transported through the lowest unoccupied molecular orbital of PCBM at low applied voltages and low temperatures; and alternatively holes can be transported through the highest occupied molecular orbital of P3HT at higher applied voltages and high temper...
Control of diameter and nitrogen doping in the synthesis of nitrogen doped carbon nanotubes (N-CN... more Control of diameter and nitrogen doping in the synthesis of nitrogen doped carbon nanotubes (N-CNTs) can be achieved by careful selection of catalyst, precursor mixture and chemical vapour deposition (CVD) synthesis temperature. A solution of methanol and acetonitrile (20% acetonitrile) was pyrolysed over a Fe/Al/MgO catalyst in a CVD reactor at 850 o C. The product was characterised by TEM, TGA and Raman spectroscopy. N-CNTs with well graphitised morphology, small diameter, and narrow diameter distribution were obtained. Catalyst support and metal particles were removed completely during purification without any change in morphology of the material. Organic solar devices were made from P3HT and the NCNTs synthesized.
Optical and Quantum Electronics, 2020
The asymmetric behavior in the dark current of any solar cell is essential for decoupling recombi... more The asymmetric behavior in the dark current of any solar cell is essential for decoupling recombination and charge extraction for efficient charge collection. Therefore current density dependence on applied voltage can be used to investigate the complex interplay between bulk charge transport, interface exchange effects and recombination mechanisms. In the present work, we investigate from dark current-density [J(V)] of bulk heterojunction solar cells, the dependence of mobility and charge injection mechanisms on different blend layer thicknesses. The active layer thickness has been established through varying the spin-coating speeds between 1000, 2000, 3000 and 4000 revolutions per minute (rpm) and confirmed by a dektak surface profilometer. The carrier mobility (μ) as a factor limiting the efficiency of organic solar cells was investigated from dark space charge limited current and trap free space charge limited current conduction mechanisms to distinguish between charge extraction and recombination. This approach allows the determination of the effects of threshold field through variation of the active layer thickness (ALT) on the potential barrier height (B) at the electrode contacts. Low values of charge carrier mobilities (10-6 cm 2 V −1 s −1) in the trap free space charge limited current conduction region have been correlated to the Langevin recombination constants. In the ohmic region, the highest dark carrier mobility corresponded to the 77.1 nm ALT. Further we observe a shift in the transition voltage at the inflection point of J-V curves with increasing film thickness in the forward bias.
Journal of Renewable Energy, 2021
Evaluation of the maximum solar energy potential of a given area for possible deployment of solar... more Evaluation of the maximum solar energy potential of a given area for possible deployment of solar energy technologies requires assessment of clear sky solar irradiance for the region under consideration. Such localized assessment is critical for optimal sizing of the technology to be deployed in order to realize the anticipated output. As the measurements are not always available where they are needed, models may be used to estimate them. In this study, three different models were adapted for the geographical location of the area under study and used to estimate clear sky global horizontal irradiance (GHI) at three locations in the subtropical desert climate of Namibia. The three models, selected on the basis of input requirements, were used to compute clear sky GHI at Kokerboom, Arandis, and Auas. The models were validated and evaluated for performance using irradiance data measured at each of the sites for a period of three years by computing statistical parameters such as mean bi...
Optical and Quantum Electronics, 2020
Organic photovoltaic solar cells can offer advantages of being mechanically flexible and durable,... more Organic photovoltaic solar cells can offer advantages of being mechanically flexible and durable, large area devices, lightweight, made from a diversity of materials and low-cost fabrication. Their efficiency is, however, still too low for commercial exploitation. Empirical observations reveal that polymer-fullerene (P3HT:PCBM) based solar cell performance depends on thermal annealing processes employed, especially the annealing temperatures and durations. The annealing parameters are known to influence the energetics and kinetics of the blending process or morphology, but the associated physics is not fully understood. In this work, current density-voltage characteristics of P3HT:PCBM bulk heterojunction organic solar cells, thermally annealed at different temperatures, 65-160 °C post fabrication, were investigated under dark and illuminated conditions, and compared to their as-cast counterparts. In certain electrical regimes, as-cast devices showed higher values of current density in comparison to the corresponding annealed devices. Such performance was attributed to airborne chemical doping of the as-cast semiconductor layer, which creates electrically conductive percolation pathways within the as-cast devices. We propose that annealing of semiconductors must be a two-step process, which first initiates decrease in conductivity, followed by its increase. As-cast devices P3HT-PCBM bulk heterojunction solar cells prepared under atmospheric conditions were observed to have comparatively superior photovoltaic performance in comparison to thermally annealed devices. The efficiency drop in the annealed counterparts is attributed to dedoping due to thermal annealing. An annealing temperature of ~ 140 °C was found to be optimum for power conversion efficiency in the bulk heterojunction, 1:1 by mass, P3HT:PCBM based solar cells.
Catalysis Letters, 2020
This review focusses on the recent developments in designing Layered Double Hydroxides (LDHs) wit... more This review focusses on the recent developments in designing Layered Double Hydroxides (LDHs) with conductive, interlayer anion replacement, for efficient hydrogen fuel production by water splitting through Oxygen Evolution Reactions (OER) and Hydrogen Evolution Reactions (HER). Nickel nano structured catalysts improves OER performance are highlighted in detail in terms of compositional differences between transitional metal components, and challenges in future designing of rationalized Ni and Ni nano LDHs. The layered structure has exceptional flexibility of incorporating mixed valence transition metal ions into the LDHs structure in different compositions and this opens the massive potential to design high-performance LDHs catalysts on the molecular and nanometer scales. LDHs such as NiCoFe LDHs, Ni foam, Co Ni nano spheres, RuO 2 , Ir(dppe) 2 Cl, NiS 2 , Ni-N-Co-doped carbon nano fibers, NiCoSe 2 /cHRD are attracting increasing interest in the field of water splitting into hydrogen and oxygen due to their unique physicochemical properties. The highlighted summary will provide useful information in the development of novel Ni LDHs catalysts, which enables better understanding of OER properties valuable to address key issues. Increased fundamental understanding of water splitting catalysts would allow for rationallydirected improvements.
International Journal of Sustainability in Higher Education, 2018
Purpose This paper aims to present a new master’s programme for promoting energy access and energ... more Purpose This paper aims to present a new master’s programme for promoting energy access and energy efficiency in Southern Africa. Design/methodology/approach A transdisciplinary approach called “participatory integrated assessment of energy systems” (PARTICIPIA) was used for the development of the curriculum. This approach is based on the two emerging fields of “multi-scale integrated assessment” and “science for governance”, which bring innovative concepts and methods. Findings The application of the PARTICIPIA methodology to three case studies reveals that the proposed transdisciplinary approach could support energy and development policies in the region. The implementation of the PARTICIPIA curriculum in three higher education institutions reveals its ability to respond to the needs of specific contexts and its connection with existing higher education programmes. Practical implications Considering energy issues from a transdisciplinary approach in higher education is absolutely ...
Procedia Manufacturing, 2017
physica status solidi (a), 2017
Here, it is shown that by placing a thin film of N‐CNTs on top of the transparent ITO hole collec... more Here, it is shown that by placing a thin film of N‐CNTs on top of the transparent ITO hole collecting electrode of a polymer solar cell, the short circuit current density and efficiency are enhanced by 25 and 50%, respectively, and the usual efficiency losses in polymer solar cells as a function of incident light illumination intensity are significantly reduced. Nanostructuring the anode improves the mobility and collection of holes toward the anode, and as a result reduces space‐charge accumulation in the polymer solar cell.
Proceedings of the ISES Solar World Congress 2011, 2011
MRS Proceedings, 2001
ABSTRACTWe studied the electronic transport properties of conjugated polymer/fullerene based sola... more ABSTRACTWe studied the electronic transport properties of conjugated polymer/fullerene based solar cells by means of temperature and illumination intensity dependent current-voltage characteristics, admittance spectroscopy and light-induced electron spin resonance. The short-circuit current density increases with temperature at all light illumination intensities applied, i.e., from 100 mW/cm2to 0.1 mW/cm2(white light), whereas a temperature independent behavior was expected. An increase of the open-circuit voltage from 850 mV to 940 mV was observed, when cooling down the device from room temperature to 100 K. The fill factor depends strongly on temperature with a positive temperature coefficient in the whole temperature range. In contrast, the light intensity dependence of the fill factor shows a maximum of 52% at intermediate illumination intensities (3 mW/cm2) and decreases subsequently, when increasing the intensity up to 100 mW/cm2. Further studies by admittance spectroscopy rev...
Zeitschrift für Naturforschung A, 2007
The effects of thermal annealing on the bulk transport properties of poly(3-hexylthiophene) (P3HT... more The effects of thermal annealing on the bulk transport properties of poly(3-hexylthiophene) (P3HT) were studied by analyzing room temperature current-voltage characteristics of polymer thin films sandwiched between indium tin oxide/poly[ethylene dioxythiophene]:poly[styrene sulfonate] (ITO/PEDOT:PSS) and aluminum electrodes, before and after a thermal annealing step. It was observed that annealing takes place in two steps: (1) Dedoping of the polymer of impurities such as oxygen, remnant solvent, water, leading to a decrease in the conductivity of the film, and (2) thermally induced motion of the polymer chains leading to closer packing, thus, stronger inter-chain interaction and, consequently, increase in conductivity. It was also observed that the ITO/PEDOT:PSS/P3HT hole injection barrier increases on annealing the ITO/PEDOT:PSS/P3HT/Al thin film devices. The implications of impurity dedoping and closer packing on the output characteristics of bulk heterojunction polymer-fullerene...
ABSTRACT Easily processable conjugated polymers for optoelectronics, in general, and for photovol... more ABSTRACT Easily processable conjugated polymers for optoelectronics, in general, and for photovoltaic energy conversion, in particular, is an attractive research field, in which the combined efforts of material science, device engineers, and spectroscopists are welcomed. To improve the efficiency of polymer solar cells, currently within the range of 2.5 and 3.5%, it is vital to understand which mechanisms control the current-voltage characteristics of a given device. Temperature and light intensity dependence of the main solar cell parameters is very informative for analysing the power efficiency losses. The strong temperature dependence of the J SC (Fig. 1a) reflects the transport losses due to trapping and recombination of charge carriers. Such behaviour was established recently for bulk-heterojunction devices based on absorber materials with low charge carrier mobility, polyparaphenylene-vinylene derivative, OC1C10-PPV blended with the fullerene PCBM, [1]. In contrast, in solar cells based on higher mobility polymers, poly (3-hexylthiophene 2.5-diyl) (P3HT) blended with PCBM, the J SC , being thermally activated at low temperatures, saturates and becomes nearly temperature independent at elevated temperatures (Fig. 1b), clearly indicating that the charge carriers traverse the active layer without significant losses [2,3].
Journal of Applied Physics, 2014
The superiority of the electronic transport properties of single-walled carbon nanotube (SWNT) ro... more The superiority of the electronic transport properties of single-walled carbon nanotube (SWNT) ropes over SWNT mats is verified from low temperature and frequency-dependent transport. The overall change of resistance versus in nanotube mats shows that 3D variable range hopping is the dominant conduction mechanism within the 2–300 K range. The magneto-resistance (MR) is found to be predominantly negative with a parabolic nature, which can also be described by the hopping model. Although the positive upturn of the MR at low temperatures establishes the contribution from quantum interference, the inherent quantum transport in individual tubes is suppressed at elevated temperatures. Therefore, to minimize multi-channel effects from inter-tube interactions and other defects, two-terminal devices were fabricated from aligned SWNT (extracted from a mat) for low temperature transport as well as high-frequency measurements. In contrast to the mat, the aligned ropes exhibit step-like features...
Proceedings from the International Conference on Advances in Engineering and Technology, 2006
ABSTRACT Polymer photovoltaics has become a very interesting area of research given the success o... more ABSTRACT Polymer photovoltaics has become a very interesting area of research given the success of polymer electronic devices like displays, field effect transistors and light emitting diodes. We developed bulk heterojunction polymer-fullerene solar cells, with a poly(3hexyl)thiophene and a soluble fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester, P3HT:PCBM blend as the active layer, that yielded above 2.5 % energy conversion efficiencies at standard test conditions. The solar cells were characterized by analyzing current-voltage characteristics at various temperatures, from 80 to 400 K, and measuring their external quantum efficiencies at room temperature. We discuss the origin of open circuit voltage observed and estimate that the upper limit of open circuit voltage for P3HT: PCBM based polymer heterojunction solar cells is 1.2 V. The highest current density measured was 8.5 mA/cm 2 , while the energy conversion efficiency gave 2.71 % for 100 mW/cm 2 white light intensity, AM 1.5 spectrum at 300 K. A maximum external quantum efficiency of 65 % was also determined for monochromatic light of wavelength 550 nm. Some of the major challenges in efficiency improvement of such solar cells are discussed.
Physical Review B, 2010
Based on magneto-resistance studies performed over a wide range of magnetic fields as well as tem... more Based on magneto-resistance studies performed over a wide range of magnetic fields as well as temperatures, we establish that correction to conductivity in heavily nitrogen doped ultrananocrystalline diamond (UNCD) films is dominated by anisotropic weak localization (WL) in three dimensions associated with a propagative Fermi surface. Also, low temperature electrical conductivity can show weakly localized transport in 3D combined with the effect of electronelectron interactions in these materials, which is remarkably different from the conductivity in 2DWL or strong localization regime. The corresponding dephasing time of electronic wavefunctions in these systems described as ~ T-p with p < 1, follows a relatively weak temperature dependence compared to the generally expected nature for bulk dirty metals having p 1. The temperature dependence of Hall (electron) mobility together with an enhanced electron density has been used to interpret the unusual magneto-transport features and show delocalized electronic transport in these n-type UNCD films, which can be described as low-dimensional superlattice structures.
Zeitschrift für Naturforschung A, 2012
Transport properties of poly(3-hexylthiophene) (P3HT), and of its blend with [6,6]-phenyl C61- bu... more Transport properties of poly(3-hexylthiophene) (P3HT), and of its blend with [6,6]-phenyl C61- butyric acid methylester (PCBM), were studied by analysing temperature dependent current-voltage characteristics of spin cast thin films sandwiched between aluminium electrodes in a metal-insulator- metal (MIM) configuration. It was found that in Al/P3HT/Al devices, the current is limited by space charge that accumulates near the hole injecting electrode due to the poor bulk transport properties of P3HT. At low temperatures and high applied electric fields the current density obeys a power law of the form J _ Vm, characteristic of space charge limited current (SCLC) in the presence of exponentially distributed traps within the band gap. These traps are filled by charge that is injected by quantum mechanical tunnelling, which is adequately described by the Fowler-Nordheim (FN) theory. By calculating the majority charge carrier mobility in Al/P3HT/Al and Al/P3HT:PCBM/Al devices from the Ohmi...
Synthetic Metals, 2003
Electrical and optical design and characterisation of regioregular poly(3-hexylthiophene-2,5diyl)... more Electrical and optical design and characterisation of regioregular poly(3-hexylthiophene-2,5diyl)/fullerenebased heterojunction polymer solar cells.