Novel III-V Heterostructures for High Efficiency Solar Cells: Studies of Electrical and Optical Properties

The thesis deals with the investigation of optical, electrical and structural properties of III-V semiconductor materials and nanostructures with applications in the development of next generation solar cells. In particular, the focus is on the study of quantum well (QW) and quantum dot (QD) nanostructures, and dilute nitride materials. Nanostructures can improve solar cell performance by, e.g., extending the absorption edge, providing an intermediate band, or suppressing reflection at the solar cell surface. Dilute nitrides, on the other hand, can provide better utilization of the solar spectrum, thus increasing the conversion efficiency of multijunction solar cells. The interplay between fabrication parameters as well as post growth treatments of the investigated structures, and their optical and electrical properties, are assessed by several characterization methods, including photoluminescence and capacitance spectroscopy. The results show that small changes in these parameters can have a significant influence on the defect populations and overall properties of the heterostructure, eventually defining the solar cell performance.Starting from the outer layer of the solar cell device, the surface structure was found to play an important role in connection with thermal annealing. Short chemical treatments modifying the GaAs surface had a huge influence on optical and structural properties of the studied QWs upon annealing. Furthermore, ammonium sulfide treatment of the solar cell AlInP window layer was found to modify the surface structure and improve the solar cell performance. Optimization of the amount of deposited InAs and use of the so-called “flushing technique” was found to remove unwanted defects in QD layers. For the GaSb QD heterostructures, the influence of material fluxes during the growth, thermal annealing, and stacking of QD layers on optical and solar cell properties was studied. Dilute nitride QWs, acting as strain compensation and mediation layers for QD layers, were found to extend the absorption edge in the solar cell structure, and provide steps for charge carriers to thermally escape from the QD layer. Stacked strain free GaAs QD nanostructures, fabricated by refilling of self-assembled nanoholes, were found to emit photoluminescence related to several quantum dot states with narrow linewidths. The investigated GaAs QDs were also extremely temperature stable upon high temperature thermal annealing, indicating low defect densities. The formation of defects in bulk dilute nitride solar cells and their relation to process parameters on the one hand, and solar cell properties on the other hand, was also studied; optimal fabrication conditions were then devised. Incorporation of Sb was found to decrease the background doping density but at the same time broaden the deep level transient spectroscopy spectra. Furthermore, the As flux used during the fabrication of the dilute nitride solar cell was found to have a remarkable influence on solar cell performance

Nanostructures for light management in thin-film GaAs quantum dot solar cells

We have investigated structures for thin-film GaAs quantum dot solar cells. Light trapping at quantum dot bands is realized by a triangular grating reflector whose aspect ratio is identified as the main design parameter

Performace of Dilute Nitride Triple Junction Space Solar Cell Grown by MBE

Dilute nitride arsenide antimonide compounds offer widely tailorable band-gaps, ranging from 0.8 eV to 1.4 eV, for the development of lattice-matched multijunction solar cells with three or more junctions. Here we report on the performance of GaInP/GaAs/GaInNAsSb solar cell grown by molecular beam epitaxy. An efficiency of 27% under AM0 conditions is demonstrated. In addition, the cell was measured at different temperatures. The short circuit current density exhibited a temperature coefficient of 0.006 mA/cm2/°C while the corresponding slope for the open circuit voltage was −6.8 mV/°C. Further efficiency improvement, up to 32%, is projected by better current balancing and structural optimization

Epitaxial lift-off process for GaAs solar cells controlled by InGaAs internal sacrificial stressor layers and a PMMA surface stressor

Epitaxial lift-off (ELO) techniques enable the development of thin-film III–V solar cell devices that are flexible and lightweight. To this end, we report an ELO process employing an internal sacrificial stressor layer (ISSL) and a surface polymer (PMMA) stressor layer. The combined action enhances the lateral etching rate and promotes a more controllable release process. The ISSL consists of quantum well-like GaInAs heterostructures that enable an accurate control of the stress required to enhance the lateral etching of the sacrificial layer, and hence the release of the thin film. More specifically, the use of the ISSL results in about 5-fold faster etch rate of the AlAs sacrificial layer. The ISSL layers can be etched away after the lift-off. Likewise, the PMMA surface stressor, which serves also as a sacrificial intermediate transfer layer, can be easily removed. The proof-of-concept device demonstration of the enhanced ELO technique was made by fabricating single-junction GaAs solar cells. The solar cell performance was evaluated under AM1.5d illumination and by external quantum efficiency measurements. Modelling based analysis shows that although the GaAs solar cell would require improvement of the front contact, yet the novel release process was successfully validated.publishedVersionPeer reviewe

Novel III-V Heterostructures for High Efficiency Solar Cells: Studies of Electrical and Optical Properties

The thesis deals with the investigation of optical, electrical and structural properties of III-V semiconductor materials and nanostructures with applications in the development of next generation solar cells. In particular, the focus is on the study of quantum well (QW) and quantum dot (QD) nanostructures, and dilute nitride materials. Nanostructures can improve solar cell performance by, e.g., extending the absorption edge, providing an intermediate band, or suppressing reflection at the solar cell surface. Dilute nitrides, on the other hand, can provide better utilization of the solar spectrum, thus increasing the conversion efficiency of multijunction solar cells. The interplay between fabrication parameters as well as post growth treatments of the investigated structures, and their optical and electrical properties, are assessed by several characterization methods, including photoluminescence and capacitance spectroscopy. The results show that small changes in these parameters can have a significant influence on the defect populations and overall properties of the heterostructure, eventually defining the solar cell performance.Starting from the outer layer of the solar cell device, the surface structure was found to play an important role in connection with thermal annealing. Short chemical treatments modifying the GaAs surface had a huge influence on optical and structural properties of the studied QWs upon annealing. Furthermore, ammonium sulfide treatment of the solar cell AlInP window layer was found to modify the surface structure and improve the solar cell performance. Optimization of the amount of deposited InAs and use of the so-called “flushing technique” was found to remove unwanted defects in QD layers. For the GaSb QD heterostructures, the influence of material fluxes during the growth, thermal annealing, and stacking of QD layers on optical and solar cell properties was studied. Dilute nitride QWs, acting as strain compensation and mediation layers for QD layers, were found to extend the absorption edge in the solar cell structure, and provide steps for charge carriers to thermally escape from the QD layer. Stacked strain free GaAs QD nanostructures, fabricated by refilling of self-assembled nanoholes, were found to emit photoluminescence related to several quantum dot states with narrow linewidths. The investigated GaAs QDs were also extremely temperature stable upon high temperature thermal annealing, indicating low defect densities. The formation of defects in bulk dilute nitride solar cells and their relation to process parameters on the one hand, and solar cell properties on the other hand, was also studied; optimal fabrication conditions were then devised. Incorporation of Sb was found to decrease the background doping density but at the same time broaden the deep level transient spectroscopy spectra. Furthermore, the As flux used during the fabrication of the dilute nitride solar cell was found to have a remarkable influence on solar cell performance

Improving the current output of GaInNAs solar cells using distributed Bragg reflectors

The influence of AlGaAs-based distributed Bragg reflector (DBR) on the performance of a GaInNAs n-i-p solar cells is reported. The DBR increased the short circuit current density by ~1 mA/cm2, owing to increased external quantum efficiency in the wavelength range from 1120 nm to 1240 nm. As a result of the incorporation of the DBR structure, the series resistance of the cell was increased by 4 mOhm-cm2.acceptedVersionPeer reviewe

Effective Tensile Strength of an Ice Sheet Using a Three-dimensional FEM-DEM Approach

A sea ice sheet may fail through several mechanisms when interacting with an obstacle. Susceptibility and mode of failure behave as functions of the physical size of a sheet. In this paper we compute the effective uniaxial tensile strength of an ice sheet for several different specimen sizes by applying a three-dimensional combined finite-discrete element approach. We evaluate the effect of the loading rate by using two displacement rates. Although a rate-independent cohesive formulation is used and the displacement rates applied are low, a significant rate effect emerges.Peer reviewe

Finite-discrete element modelling of sea ice sheet fracture

A rate-independent, de-cohesive damage model for the fracture modelling of large, cellular, plate-like, quasi-brittle structures is proposed. A hybrid, three-dimensional finite-discrete element method to investigate sea ice sheet fracture is then introduced, followed by three applications. The uniaxial tensile fracture of an ice sheet of varying physical sizes is examined first. The effects of both the size of an ice sheet and the loading rate applied on the effective tensile strength are investigated. The vertical penetration fracture of an ice sheet loaded by a rigid, flat-ended, cylindrical indenter is examined next. The breakthrough loads and strengths of an ice sheet of varying physical sizes are computed, applicable scaling rules as regards to the vertical breakthrough strength searched for. To conclude, the breaking of an ice sheet containing a circular hole by a surfacing, rigid, truncated cone is studied (an axisymmetric contact problem). The loads on the cone are computed and then compared with loads that can be obtained analytically for a case in which a structure is stationary, a sheet moves, and the contact is unilateral. While computing the tensile and the breakthrough strengths, a set of self-similar sheet samples with an in-plane size range of 1:16 is examined. The samples are square; have a side length of either L=10, 20, 40, 80, or 160 m; and a thickness of either h=0.5, 1.0, or 1.5 m. With the sheets containing holes, only the largest samples (L=160 m) are investigated. The results indicate that i) both the tensile and the breakthrough strengths are strong functions of both L and h; ii) the tensile strength is a strong function of the applied loading rate; iii) the failure mode as regards to the vertical penetration fracture changes drastically as a function of L; iv) the model is able to demonstrate both radial and circumferential cracking; and that v) the proposed (in-direct) approach to compute ice loads on a conical offshore structure provides realistic results.Peer reviewe