Colloidal quantum dots (CQDs) are an inexpensive and solution processable photovoltaic(PV) material reaching modest efficiencies of 6%. However, doping quantum dots still remains a challenge. This thesis explores the level of doping in lead sulfide (PbS)CQDs by surface ligands and bulk doping within the quantum dot lattice using metals. In light of the knowledge that oxygen creates traps on the surface of PbS CQDs, we have turned to the use of oxygen-free fabrication. We find that under a nitrogen environment, PbS CQD films are n-type and tunable in doping by use of halide ions. We show for the first time control over the doping density of n-type CQD films over a wide range. We also show the ability to fabricate p-type PbS films with high doping density that are compatible with n-type films. This compatibility enabled us to make the world’s first CQD homojunction PV device.

Colloidal quantum dots (CQDs) are an inexpensive and solution processable photovoltaic(PV) material reaching modest efficiencies of 6%. However, doping quantum dots still remains a challenge. This thesis explores the level of doping in lead sulfide (PbS)CQDs by surface ligands and bulk doping within the quantum dot lattice using metals. In light of the knowledge that oxygen creates traps on the surface of PbS CQDs, we have turned to the use of oxygen-free fabrication. We find that under a nitrogen environment, PbS CQD films are n-type and tunable in doping by use of halide ions. We show for the first time control over the doping density of n-type CQD films over a wide range. We also show the ability to fabricate p-type PbS films with high doping density that are compatible with n-type films. This compatibility enabled us to make the world’s first CQD homojunction PV device.

ABSTRACT Organic light emitting diode (OLED) has significant scientific implication and technological potential. Using organic materials for tailored emitting color, threshold voltage reduction, and emission efficiency gain are the key points for the commercialization of OLED. An UV-Vis spectrophotometer was applied to obtain the absorption spectra of PVK, C6, and PRL, as well as their respective band gap (Eg) values of 3.49 eV, 2.32 eV, and 2.55 eV. The turn-on oxidation potential of cyclic voltammograms was reduced for HOMO energy at 5.64 eV, 5.21 eV, and 5.16 eV, respectively. The Eg subtracted from HOMO energy yielded the respective LUMO values of 2.15 eV, 2.89 eV and 2.61 eV. Excitation at 457 nm or 325 nm was applied to the freestanding films of PVK, PVK doped with C6 (10/1), and PVK doped with PRL (10/1). From the UV-Vis absorption spectra and Egs, we knew that 457 nm excitation did not generate photoluminescence (PL) of PVK. The PL spectra of the doped freestanding films were mostly attributed to the dye molecules of C6 or PRL. The PL spectra of doped freestanding films were insensitive to the excitation sources at 325 nm and 457 nm. There was a blue shift at the PL emission peak indicative of energy transfer from PVK to C6 or PRL for the doped films. Using spin-coating or vacuum deposition to fabricate PVK, C6, and PRL films onto an ITO substrate followed by evaporating Al (Ag) as the electron injector to form OLED devices. Because of the energy transfer between PVK and C6 or PRL, ITO/PVK:C6/Al homojunction OLED showed a smaller threshold voltage than that of ITO/C6/Al, from 9 V to 3.5 V. Likewise, ITO/PVK:PRL/Al homojunction OLED had a smaller threshold voltage than that of ITO/PRL/Al, from 8 V to 4.5 V. PVK was also used as the hole blocking layer to construct heterojunction OLED to balance electron-hole numbers in the emitting layer. The threshold voltage of ITO/C6/Al reduced from 9 V to 7 V with a heterojunction of ITO/PVK/C6/Al. A device of ITO/PRL/Al having a threshold voltage of 8V reduced to 6V with an ITO/PVK/PRL/Al heterojunction OLED. Coating a protective layer (Ag) on the metallic electron injector, or packaging the device in N2 could both decrease the decay and increase the life time of OLED.

Solar photovoltaic has become one of the potential solutions for current energy needs and for combating greenhouse gas emissions. The photovoltaics (PV) industry is booming, with a yearly growth rate well in excess of 30% over the last decade. This explosive growth has been driven by market development programs to accelerate the deployment of sustainable energy options and rapidly increasing fossil fuel prices. Currently, the PV market is based on silicon wafer solar cells (thick cells of around 150–300 μm made of crystalline silicon). This technology, classified as the first-generation of photovoltaic cells. The second generation of photovoltaic materials is based on the introduction of thin film layers of semiconductor materials. Unfortunately, the conversion efficiency of the current PV systems is low despite the lower manufacturing costs. Nevertheless, to achieve highly efficient silicon solar cell devices, the development of new high quality materials in terms of structure and electrical properties is a must to overcome the issues related to amorphous silicon (a -Si:H) degradation. Meanwhile, to remain competitive with the conventional energy sources, cost must be taken into consideration. Moreover, novel approaches combined with conventional mature silicon solar cell technology can boost the conventional efficiency and break its maximum limits. In our approach, we set to achieve efficient, stable and affordable silicon solar cell devices by focusing on the development of a new device made of epitaxial films. This new device is developed using new epitaxial growth phosphorous and/or boron doped layers at low processing temperature using plasma enhanced chemical vapor deposition (PECVD). The junction between the phosphorous or boron-doped epitaxial film of the device is formed between the film and the p or n-type crystalline silicon (c-Si) substrate, giving rise to (n epi-Si/p c-Si device or p epi-Si/n c-Si device), respectively. Different processing conditions have been fully characterized and deployed for the fabrication of different silicon solar cells architectures. The high quality epitaxial film (up to 400 nm) was used as an emitter for an efficient stable homojunction solar cell. Extensive analysis of the developed fine structure material, using high resolution transmission electron microscope (HRTEM), showed that hydrogen played a crucial role in the epitaxial growth of highly phosphorous doped silicon films. The main processing parameters that influenced the quality of the structure were; radio frequency (RF) power density, the processing chamber pressure, the substrate temperature, the gas flow rate used for deposition of silicon films, and hydrogen dilution. The best result, in terms of structure and electrical properties, was achieved at intermediate hydrogen dilution (HD) regime between 91 and 92% under optimized deposition conditions of the rest of the processing parameters. The conductivity and the carrier mobility values are good indicators of the electrical quality of the silicon (Si) film and can be used to investigate the structural quality indirectly. The electrical conductivity analyses using spreading resistance profile (SRP), through the detection of active carriers inside the developed films, are presented in details for the developed epitaxial film under the optimized processing conditions. Measurements of the active phosphorous dopant revealed that, the film has a very high active carrier concentration of an average of 5.0 x1019 cm-3 with a maximum value of 6.9 x 1019 cm-3 at the interface between substrate and the epitaxial film. The observed higher concentration of electrically active P atoms compared to the total phosphorus concentration indicates that more than half of dopants become incorporated into substitutional positions. Highly doping efficiency ηd of more than 50 % was calculated from both secondary ion mass spectroscopy (SIMS) and SRP analysis. A variety of proposed structures were fabricated and characterized on planar, textured, and under different deposition temperatures. Detailed studies of the photovoltaic properties of the fabricated devices were carried out using epitaxial silicon films. The results of these studies confirmed that the measured open circuit voltage (Voc) of the device ranged between 575 and 580 mV with good fill factor (FF) values in the range of 74-76 %. We applied the rapid thermal process (RTP) for a very short time (60 s) at moderate temperature of 750oC to enhance the photovoltaic properties of the fabricated device. The following results were achieved, the values of Voc, and the short circuit current (Isc) were 598 mV and 27.5 mA respectively, with a fill factor value of up to 76 % leading to an efficiency of 12.5 %. Efficiency enhancement by 13.06 % was achieved over the reference cell which was prepared without using RTP. Another way to increase the efficiency of the fabricated device is to reduce the reflections from its polished substrate. This was achieved by utilizing the light trapping technique that transforms the reflective polished surface into a pyramidical texturing using alkaline solutions. Further enhancements of both Voc and Isc were achieved with values of 612 mV and 31mA respectively, and a fill factor of 76 % leading to an increase in the efficiency by up to 13.8 %. A noticeable efficiency enhancement by ~20 % over the reference cell is reported for the developed devices on the textured surfaces. Moreover, the efficiency of the fabricated epitaxial silicon solar cells can be boosted by the deployment of silicon nanocrystals (Si NCs) on the top surface of the fabricated devices. In the course of this PhD research we found a way to achieve this by depositing a thin layer of Si NCs, embedded in amorphous silicon matrix, on top of the epitaxial film. Structural analysis of the deposited Si NCs was performed. It is shown from the HRTEM analysis that the developed Si NCs, are randomly distributed, have a spherical shape with a radius of approximately 2.5 nm, and are 10-20 nm apart in the amorphous silicon matrix. Based on the size of the developed Si NCs, the optical band gap was found to be in the region of 1.8-2.2 eV. Due to the incorporation of Si NCs layer a noticeable enhancement in the Isc was reported.

In this thesis, we study the physical properties of doped organic semiconductors. We first demonstrate the impact of doping on C60 films. In contrast to previous reports for organic thin films, the n-doped C60 films show a decrease of mobility with increasing doping levels; i.e., they follow the well-known Matthiessen rule which is generally observed in inorganic semiconductors. Using further strong organic donors and acceptors, we realize p-i-n homojunctions of several organic matrices: zinc-phthalocyanine, pentacene, and an iridium-complex TER004. We observe stable and reproducible diode characteristics, which can be described by the standard Shockley theory with an exception concerning the temperature dependence of the diode parameters. The current-voltage characteristics of the pentacene homojunctions under illuminated conditions indicate that the thermodynamic limitation of the open-circuit voltage is determined by the built-in voltage of 1.65 V, and that the recombination process is influenced by the distinct charge transport properties of electrons and holes. The very high built-in voltage of 2.2 V in the TER004 homojunction allows a red phosphorescent homo-OLED, which shows visible emission around 650 nm with low operation voltage. We examine the charge balance status in the homojunction structure, revealing that TER004 has superior electron transport properties.

Solar photovoltaic has become one of the potential solutions for current energy needs and for combating greenhouse gas emissions. The photovoltaics (PV) industry is booming, with a yearly growth rate well in excess of 30% over the last decade. This explosive growth has been driven by market development programs to accelerate the deployment of sustainable energy options and rapidly increasing fossil fuel prices. Currently, the PV market is based on silicon wafer solar cells (thick cells of around 150–300 μm made of crystalline silicon). This technology, classified as the first-generation of photovoltaic cells. The second generation of photovoltaic materials is based on the introduction of thin film layers of semiconductor materials. Unfortunately, the conversion efficiency of the current PV systems is low despite the lower manufacturing costs. Nevertheless, to achieve highly efficient silicon solar cell devices, the development of new high quality materials in terms of structure and electrical properties is a must to overcome the issues related to amorphous silicon (a -Si:H) degradation. Meanwhile, to remain competitive with the conventional energy sources, cost must be taken into consideration. Moreover, novel approaches combined with conventional mature silicon solar cell technology can boost the conventional efficiency and break its maximum limits. In our approach, we set to achieve efficient, stable and affordable silicon solar cell devices by focusing on the development of a new device made of epitaxial films. This new device is developed using new epitaxial growth phosphorous and/or boron doped layers at low processing temperature using plasma enhanced chemical vapor deposition (PECVD). The junction between the phosphorous or boron-doped epitaxial film of the device is formed between the film and the p or n-type crystalline silicon (c-Si) substrate, giving rise to (n epi-Si/p c-Si device or p epi-Si/n c-Si device), respectively. Different processing conditions have been fully characterized and deployed for the fabrication of different silicon solar cells architectures. The high quality epitaxial film (up to 400 nm) was used as an emitter for an efficient stable homojunction solar cell. Extensive analysis of the developed fine structure material, using high resolution transmission electron microscope (HRTEM), showed that hydrogen played a crucial role in the epitaxial growth of highly phosphorous doped silicon films. The main processing parameters that influenced the quality of the structure were; radio frequency (RF) power density, the processing chamber pressure, the substrate temperature, the gas flow rate used for deposition of silicon films, and hydrogen dilution. The best result, in terms of structure and electrical properties, was achieved at intermediate hydrogen dilution (HD) regime between 91 and 92% under optimized deposition conditions of the rest of the processing parameters. The conductivity and the carrier mobility values are good indicators of the electrical quality of the silicon (Si) film and can be used to investigate the structural quality indirectly. The electrical conductivity analyses using spreading resistance profile (SRP), through the detection of active carriers inside the developed films, are presented in details for the developed epitaxial film under the optimized processing conditions. Measurements of the active phosphorous dopant revealed that, the film has a very high active carrier concentration of an average of 5.0 x1019 cm-3 with a maximum value of 6.9 x 1019 cm-3 at the interface between substrate and the epitaxial film. The observed higher concentration of electrically active P atoms compared to the total phosphorus concentration indicates that more than half of dopants become incorporated into substitutional positions. Highly doping efficiency ηd of more than 50 % was calculated from both secondary ion mass spectroscopy (SIMS) and SRP analysis. A variety of proposed structures were fabricated and characterized on planar, textured, and under different deposition temperatures. Detailed studies of the photovoltaic properties of the fabricated devices were carried out using epitaxial silicon films. The results of these studies confirmed that the measured open circuit voltage (Voc) of the device ranged between 575 and 580 mV with good fill factor (FF) values in the range of 74-76 %. We applied the rapid thermal process (RTP) for a very short time (60 s) at moderate temperature of 750oC to enhance the photovoltaic properties of the fabricated device. The following results were achieved, the values of Voc, and the short circuit current (Isc) were 598 mV and 27.5 mA respectively, with a fill factor value of up to 76 % leading to an efficiency of 12.5 %. Efficiency enhancement by 13.06 % was achieved over the reference cell which was prepared without using RTP. Another way to increase the efficiency of the fabricated device is to reduce the reflections from its polished substrate. This was achieved by utilizing the light trapping technique that transforms the reflective polished surface into a pyramidical texturing using alkaline solutions. Further enhancements of both Voc and Isc were achieved with values of 612 mV and 31mA respectively, and a fill factor of 76 % leading to an increase in the efficiency by up to 13.8 %. A noticeable efficiency enhancement by ~20 % over the reference cell is reported for the developed devices on the textured surfaces. Moreover, the efficiency of the fabricated epitaxial silicon solar cells can be boosted by the deployment of silicon nanocrystals (Si NCs) on the top surface of the fabricated devices. In the course of this PhD research we found a way to achieve this by depositing a thin layer of Si NCs, embedded in amorphous silicon matrix, on top of the epitaxial film. Structural analysis of the deposited Si NCs was performed. It is shown from the HRTEM analysis that the developed Si NCs, are randomly distributed, have a spherical shape with a radius of approximately 2.5 nm, and are 10-20 nm apart in the amorphous silicon matrix. Based on the size of the developed Si NCs, the optical band gap was found to be in the region of 1.8-2.2 eV. Due to the incorporation of Si NCs layer a noticeable enhancement in the Isc was reported.

The polymer light-emitting electrochemical cell (LEC) is an alternative method for producing electroluminescence (EL) from conjugated luminescent polymers. The in situ electrochemical doping process that leads to a dynamic p-n junction makes the devices highly insensitive to device thickness and relatively insensitive to electrode materials. These characteristics make an extremely large planar configuration accessible for observing the cross-section of the device and watching it turn on dynamically. By cooling the device to freeze ionic motion, the junction can be stabilized and photovoltaic (PV) characteristics investigated. In the planar configuration, the p-n junction was found to make up a small fraction of the inter-electrode spacing. Enabled by the insensitivity to electrode materials, small metallic particles embedded in the LEC film led to a large number of p-n junctions that could be turned on in series and parallel. This alleviates the issue of low specific emitting area suffered by planar devices and leads to improved EL effciency as well as a high open circuit voltage (Voc) when operated as a PV cell. The bulk homojunction fabrication process has been optimized by segregating the metallic particles to eliminate large aggregates. A new technique to achieve highly uniform EL from large planar LECs is also presented here. By the evaporation of a thin gold or silver film on top of an LEC, independent islands form that act as doping initiation sites across the device width. A bulk homojunction is turned on in the top layer of the LEC with a high applied bias. Island diameters and spacings are several orders of magnitude smaller than the particles in previously-reported bulk homojunction devices. Both island and particle devices had their interelectrode spacings scaled down by at least a factor of 10. The successful scaling is a promising result for the possibility of a sandwich configuration bulk homojunction device. In the case of silver island devices, cooling a 50-micron wide device after turn-on resulted in a PV cell with an open circuit voltage of 8.3 V, several times the band gap of the luminescent polymer used. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-09-12 12:21:12.949

[pt] O estudo a respeito de homojunções e heterojunções se apresenta como de grande interesse científico e tecnológico, pois os mecanismos de formação e de atuação dessas estruturas ainda não são plenamente conhecidos. Essas junções estão na base de diferentes tecnologias, tais como, diodos, transistores, capacitores e supercapacitores, varistores, células fotovoltaicas, detectores de luz UV, diversos tipos de sensores, catalisadores e fotocatalisadores, entre outros. A presente tese de doutorado visa contribuir para o desenvolvimento de sistemas cerâmicos policristalinos (micro e nanoestruturados) à base de ZnO e SnO2 e para a compreensão dos mecanismos de formação das homojunções e heterojunções presentes nesse sistema material e suas relações com o comportamento varistor, em termos da estabilidade e degradação dessas junções. Microscopia eletrônica de varredura, espectroscopia de raios-X por dispersão de energia e difração de raios-X foram utilizadas para a caracterização microestrutural. Análises térmica e dilatométrica foram utilizadas para a determinação dos parâmetros e mecanismos de densificação e sinterização que dão origem às junções consideradas. Para a determinação das características elétricas foi utilizada a análise de capacitância e levantamento das curvas de polarização. Em função das composições químicas avaliadas foram obtidas microestruturas composta por homojunções e heterojunções, com diferentes níveis de densificação e características varistoras, ou seja, comportamento não-linear entre tensão e corrente elétrica, com tensões de chaveamento de diferentes magnitudes, permitindo relacionar o comportamento eletrotérmico dos varistores com as características das homojunções e heterojunções consideradas. / [en] The study about homojunctions and heterojunctions has scientific and technological value, because the mechanisms of formation and performance of these structures are not fully known. These junctions are in the base of different technologies, such as diodes, transistors, capacitors and supercapacitors, varistors, photovoltaic cells, detector of UV light, many kinds of sensors, catalysts and photocatalysts, among others. The present thesis aims to contribute to the development of polycrystalline ceramic systems (micro and nanostructured) based on ZnO and SnO2 and to the understanding ofthe mechanisms of formation of homojunctions and heterojunctions present in these systems and their relations with the varistor behavior, in terms of stability and degradation. Scanning electron microscopy, X-ray energy dispersive spectroscopy and X-ray diffraction were used to the microstructural characterization. Thermal and dilatometric analyses were used to determine the parameters and mechanisms of densification and sintering that give rise to the junctions considered. For the determination of the electrical characteristics, analysis of capacitance and polarization curves were used. Depending on the chemical compositions a variely of microstructures were obtained containing homojunctions and heterojunctions, with different densification levels and distinct varistors characteristics, that it, nonlinear behavior between voltage and electric current, with different magnitudes of switching voltages permitting to correlate the electrothermal behavior of varistors with the characteristics of homojunctions and heterojunctions considered.

In this thesis, we study the physical properties of doped organic semiconductors. We first demonstrate the impact of doping on C60 films. In contrast to previous reports for organic thin films, the n-doped C60 films show a decrease of mobility with increasing doping levels; i.e., they follow the well-known Matthiessen rule which is generally observed in inorganic semiconductors. Using further strong organic donors and acceptors, we realize p-i-n homojunctions of several organic matrices: zinc-phthalocyanine, pentacene, and an iridium-complex TER004. We observe stable and reproducible diode characteristics, which can be described by the standard Shockley theory with an exception concerning the temperature dependence of the diode parameters. The current-voltage characteristics of the pentacene homojunctions under illuminated conditions indicate that the thermodynamic limitation of the open-circuit voltage is determined by the built-in voltage of 1.65 V, and that the recombination process is influenced by the distinct charge transport properties of electrons and holes. The very high built-in voltage of 2.2 V in the TER004 homojunction allows a red phosphorescent homo-OLED, which shows visible emission around 650 nm with low operation voltage. We examine the charge balance status in the homojunction structure, revealing that TER004 has superior electron transport properties.

In this work, homojunction interfacial workfunction internal photoemission (HIWIP) detectors based on GaAs, and heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors based mainly on the Gallium Arsenide/Aluminum Gallium Arsenide material system are presented. Design principles of HIWIP and HEIWIP detectors, such as free carrier absorption, photocarrier generation, photoemission, and responsivity, are discussed in detail. Results of p-type HIWIPs based on GaAs material are presented. Homojunction detectors based on p-type GaAs were found to limit their operating wavelength range. This is mainly due to band depletion arising through carrier transitions from the heavy/light hole bands to the split off band. Designing n-type GaAs HIWIP detectors is difficult as it is strenuous to control their workfunction. Heterojunction detectors based on Gallium Arsenide/Aluminum Gallium Arsenide material system will allow tuning their threshold wavelength by adjusting the alloy composition of the Aluminum Gallium Arsenide/Gallium Arsenide barrier, while keeping a fixed doping density in the emitter. The detectors covered in this work operate from 1 to 128 micron (300 to 2.3 THz). Enhancement of detector response using resonance cavity architecture is demonstrated. Threshold wavelength extension of HEIWIPs by varying the Al composition of the barrier was investigated. The threshold limit of approximately 3.3 THz (92 micron), due to a practical Al fraction limit of approximately 0.005, can be overcome by replacing GaAs emitters in Gallium Arsenide/Aluminum Gallium Arsenide HEIWIPs with Aluminum Gallium Arsenide/Gallium Arsenide emitters. As the initial step, terahertz absorption for 1 micron-thick Be-doped Aluminum Gallium Arsenide epilayers (with different Al fraction and doping density) grown on GaAs substrates was measured. The absorption probability of the epilayers was derived from these absorption measurements. Based on the terahertz absorption results, an Aluminum Gallium Arsenide/Gallium Arsenide HEIWIP detector was designed and the extension of threshold frequency (f0) to 2.3 THz was successfully demonstrated. In a different study, switching in Gallium Arsenide/Aluminum Gallium Arsenide heterostructures from a tunneling dominated low conductance branch to a thermal emission dominated high conductance branch was investigated. This bistability leads to neuron-like voltage pulses observed in some heterostructure devices. The bias field that initiates the switching was determined from an iterative method that uses feedback information, such as carrier drift velocity and electron temperature, from hot carrier transport. The bias voltage needed to switch the device was found to decrease with the increasing device temperature.