Matching Items (4)
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- All Subjects: Photovoltaic Cells
- All Subjects: Transmission Electron Microscopy
- Creators: Alford, Terry L.
Description
As the 3rd generation solar cell, quantum dot solar cells are expected to outperform the first 2 generations with higher efficiency and lower manufacture cost. Currently the main problems for QD cells are the low conversion efficiency and stability. This work is trying to improve the reliability as well as the device performance by inserting an interlayer between the metal cathode and the active layer. Titanium oxide and a novel nitrogen doped titanium oxide were compared and TiOxNy capped device shown a superior performance and stability to TiOx capped one. A unique light anneal effect on the interfacial layer was discovered first time and proved to be the trigger of the enhancement of both device reliability and efficiency. The efficiency was improved by 300% and the device can retain 73.1% of the efficiency with TiOxNy when normal device completely failed after kept for long time. Photoluminescence indicted an increased charge disassociation rate at TiOxNy interface. External quantum efficiency measurement also inferred a significant performance enhancement in TiOxNy capped device, which resulted in a higher photocurrent. X-ray photoelectron spectrometry was performed to explain the impact of light doping on optical band gap. Atomic force microscopy illustrated the effect of light anneal on quantum dot polymer surface. The particle size is increased and the surface composition is changed after irradiation. The mechanism for performance improvement via a TiOx based interlayer was discussed based on a trap filling model. Then Tunneling AFM was performed to further confirm the reliability of interlayer capped organic photovoltaic devices. As a powerful tool based on SPM technique, tunneling AFM was able to explain the reason for low efficiency in non-capped inverted organic photovoltaic devices. The local injection properties as well as the correspondent topography were compared in organic solar cells with or without TiOx interlayer. The current-voltage characteristics were also tested at a single interested point. A severe short-circuit was discovered in non capped devices and a slight reverse bias leakage current was also revealed in TiOx capped device though tunneling AFM results. The failure reason for low stability in normal devices was also discussed comparing to capped devices.
ContributorsYu, Jialin (Author) / Jabbour, Ghassan E. (Thesis advisor) / Alford, Terry L. (Thesis advisor) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2011
Description
The state of the solar industry has reached a point where significant advancements in efficiency will require new materials and device concepts. The material class broadly known as the III-N's have a rich history as a commercially successful semiconductor. Since discovery in 2003 these materials have shown promise for the field of photovoltaic solar technologies. However, inherent material issues in crystal growth and the subsequent effects on device performance have hindered their development. This thesis explores new growth techniques for III-N materials in tandem with new device concepts that will either work around the previous hindrances or open pathways to device technologies with higher theoretical limits than much of current photovoltaics. These include a novel crystal growth reactor, efforts in production of better quality material at faster rates, and development of advanced photovoltaic devices: an inversion junction solar cell, material work for hot carrier solar cell, ground work for a selective carrier contact, and finally a refractory solar cell for operation at several hundred degrees Celsius.
ContributorsWilliams, Joshua J (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Goodnick, Stephen M. (Thesis advisor) / Williamson, Todd L. (Committee member) / Alford, Terry L. (Committee member) / King, Richard R. (Committee member) / Arizona State University (Publisher)
Created2016
Description
In this dissertation research, conventional and aberration-corrected (AC) transmission electron microscopy (TEM) techniques were used to evaluate the structural and compositional properties of thin-film semiconductor compounds/alloys grown by molecular beam epitaxy for infrared photo-detection. Imaging, diffraction and spectroscopy techniques were applied to TEM specimens in cross-section geometry to extract information about extended structural defects, chemical homogeneity and interface abruptness. The materials investigated included InAs1-xBix alloys grown on GaSb (001) substrates, InAs/InAs1-xSbx type-II superlattices grown on GaSb (001) substrates, and CdTe-based thin-film structures grown on InSb (001) substrates.
The InAsBi dilute-bismide epitaxial films were grown on GaSb (001) substrates at relatively low growth temperatures. The films were mostly free of extended defects, as observed in diffraction-contrast images, but the incorporation of bismuth was not homogeneous, as manifested by the lateral Bi-composition modulation and Bi-rich surface droplets. Successful Bi incorporation into the InAs matrix was confirmed using lattice expansion measurements obtained from misfit strain analysis of high-resolution TEM (HREM) images.
Analysis of averaged intensity line profiles in HREM and scanning TEM (STEM) images of the Ga-free InAs/InAs1-xSbx type-II strained superlattices indicated slight variations in layer thickness across the superlattice stack. The interface abruptness was evaluated using misfit strain analysis of AC-STEM images, electron energy-loss spectroscopy and 002 dark-field imaging. The compositional profiles of antimony across the superlattices were fitted to a segregation model and revealed a strong antimony segregation probability.
The CdTe/MgxCd1-xTe double-heterostructures were grown with Cd overflux in a dual-chamber molecular beam epitaxy with an ultra-high vacuum transfer loadlock. Diffraction-contrast images showed that the growth temperature had a strong impact on the structural quality of the epilayers. Very abrupt CdTe/InSb interfaces were obtained for epilayers grown at the optimum temperature of 265 °C, and high-resolution imaging using AC-STEM revealed an interfacial transition region with a width of a few monolayers and smaller lattice spacing than either CdTe or InSb.
The InAsBi dilute-bismide epitaxial films were grown on GaSb (001) substrates at relatively low growth temperatures. The films were mostly free of extended defects, as observed in diffraction-contrast images, but the incorporation of bismuth was not homogeneous, as manifested by the lateral Bi-composition modulation and Bi-rich surface droplets. Successful Bi incorporation into the InAs matrix was confirmed using lattice expansion measurements obtained from misfit strain analysis of high-resolution TEM (HREM) images.
Analysis of averaged intensity line profiles in HREM and scanning TEM (STEM) images of the Ga-free InAs/InAs1-xSbx type-II strained superlattices indicated slight variations in layer thickness across the superlattice stack. The interface abruptness was evaluated using misfit strain analysis of AC-STEM images, electron energy-loss spectroscopy and 002 dark-field imaging. The compositional profiles of antimony across the superlattices were fitted to a segregation model and revealed a strong antimony segregation probability.
The CdTe/MgxCd1-xTe double-heterostructures were grown with Cd overflux in a dual-chamber molecular beam epitaxy with an ultra-high vacuum transfer loadlock. Diffraction-contrast images showed that the growth temperature had a strong impact on the structural quality of the epilayers. Very abrupt CdTe/InSb interfaces were obtained for epilayers grown at the optimum temperature of 265 °C, and high-resolution imaging using AC-STEM revealed an interfacial transition region with a width of a few monolayers and smaller lattice spacing than either CdTe or InSb.
ContributorsLu, Jing (Author) / Smith, David J. (Thesis advisor) / Alford, Terry L. (Committee member) / Crozier, Peter A. (Committee member) / McCartney, Martha R. (Committee member) / Zhang, Yong-Hang (Committee member) / Arizona State University (Publisher)
Created2017
Description
III-V-bismide semiconductor alloys are a class of materials with applications in the mid and long wave infrared spectrum. The quaternary alloy InAsSbBi is attractive because it can be grown lattice-matched to commercially available GaSb substrates, and the adjustment of the Bi and Sb mole fractions enables both lattice constant and bandgap to be tuned independently. This dissertation provides a comprehensive study of the surface morphology and the structural and chemical properties of InAsSbBi alloys grown by molecular beam epitaxy.
210 nm thick InAsSbBi layers grown at temperatures from 280 °C to 430 °C on (100) on-axis, (100) offcut 1° to (011), and (100) offcut 4° to (111)A GaSb substrates are investigated using Rutherford back scattering, X-ray diffraction, transmission electron microscopy, Nomarski optical microscopy, atomic force microscopy, and photoluminescence spectroscopy. The results indicate that the layers are coherently strained and contain dilute Bi mole fractions.
Large surface droplets with diameters and densities on the order of 3 µm and 106 cm-2 are observed when the growth is performed with As overpressures around 1%. Preferential orientation of the droplets occurs along the [011 ̅] step edges offcut (100) 1° to (011) substrate. The surface droplets are not observed when the As overpressure is increased to 4%. Small crystalline droplets with diameters and densities on the order of 70 nm and 1010 cm-2 are observed between the large droplets for the growth at 430°C. Analysis of one of the small droplets indicates a misoriented zinc blende structure composed of In, Sb, and Bi, with a 6.543 ± 0.038 Å lattice constant.
Lateral variation in the Bi mole fraction is observed in InAsSbBi grown at high temperature (400 °C, 420 °C) on (100) on-axis and (100) offcut 4° to (111)A substrates, but is not observed for growth at 280 °C or on (100) substrates that are offcut 1° to (011). Improved crystal and optical quality is observed in the high temperature grown InAsSbBi and CuPtB type atomic ordering on the {111}B planes is observed in the low temperature grown InAsSbBi. Strain induced tilt is observed in coherently strained InAsSbBi grown on offcut substrates.
210 nm thick InAsSbBi layers grown at temperatures from 280 °C to 430 °C on (100) on-axis, (100) offcut 1° to (011), and (100) offcut 4° to (111)A GaSb substrates are investigated using Rutherford back scattering, X-ray diffraction, transmission electron microscopy, Nomarski optical microscopy, atomic force microscopy, and photoluminescence spectroscopy. The results indicate that the layers are coherently strained and contain dilute Bi mole fractions.
Large surface droplets with diameters and densities on the order of 3 µm and 106 cm-2 are observed when the growth is performed with As overpressures around 1%. Preferential orientation of the droplets occurs along the [011 ̅] step edges offcut (100) 1° to (011) substrate. The surface droplets are not observed when the As overpressure is increased to 4%. Small crystalline droplets with diameters and densities on the order of 70 nm and 1010 cm-2 are observed between the large droplets for the growth at 430°C. Analysis of one of the small droplets indicates a misoriented zinc blende structure composed of In, Sb, and Bi, with a 6.543 ± 0.038 Å lattice constant.
Lateral variation in the Bi mole fraction is observed in InAsSbBi grown at high temperature (400 °C, 420 °C) on (100) on-axis and (100) offcut 4° to (111)A substrates, but is not observed for growth at 280 °C or on (100) substrates that are offcut 1° to (011). Improved crystal and optical quality is observed in the high temperature grown InAsSbBi and CuPtB type atomic ordering on the {111}B planes is observed in the low temperature grown InAsSbBi. Strain induced tilt is observed in coherently strained InAsSbBi grown on offcut substrates.
ContributorsKosireddy, Rajeev Reddy (Author) / Johnson, Shane R (Thesis advisor) / Smith, David J. (Committee member) / Alford, Terry L. (Committee member) / Soignard, Emmanuel (Committee member) / Arizona State University (Publisher)
Created2020