ELECTRICAL CHARACTERIZATIONS AND CIRCUIT MODELS OF HYBRID QUANTUM-DOT SOLAR CELLS AND MODIFIED HOLLOW OPTICAL MICROCAVITIES

Abstract

This thesis studies two photonic structures and aims to apply theirs in enhancing the solar cell performance. They are quantum-dot (QD) structure and optical microcavity. The first interesting photonic structure is studied for enhancing the efficiency of the hybrid QD solar cells by the increase of stacked hybrid QD layers and the differently stacking QD sequence. We investigate both DC and AC circuit parameters. The DC circuit parameters are extracted from the fitting of current-voltage characteristics. The parameters are the reverse saturation current, the diode ideality factor, the photocurrent, the series resistance and the shunt resistance. From the analysis, the short-circuit current is always approximate to the photocurrent. The ideality factor of 2 indicates the dominant recombination current within solar cells.

The first experimentation is studied for the increase of stacking hybrid GaSb/GaAs and InAs/GaAs layers (type-A). The increase of photocurrent is observed when the stacked hybrid QD layers increases. In addition, the open-circuit voltage of 3-pairs hybrid QD solar cell also increases. The series resistance increases and the shunt resistance decreases. The solar cell efficiency and the fill factor of the 1-pair of type-A QD sample are higher than those of the 3-pairs sample because increase of stacking layers increase the total layer thickness. The reverse saturation current and the photocurrent of 3-pairs hybrid QD sample has the less values than those of 1-pair sample. The second experimentation is the study of the different stacking hybrid QD sequence. The DC circuit parameters of two types (type-A and type-B) hybrid QD solar cells are investigated. The reverse saturation current and the photocurrent of the sample with correct stacking sequence have higher values. The correct stacking sequence of hybrid InAs/GaAs and GaSb/GaAs QD (type-B) can result in a better solar cell performance.

AC circuit parameters of hybrid QD solar cells are also studied. They are extracted by fitting the frequency response under the controlled bias condition at maximum power point. The diode capacitance, the diode resistance, the AC series resistance and the AC shunt resistance are obtained. The increase of stacking type-A QD layers (3-pairs) results in higher values of the capacitance and the diode resistance because of the high number density of the buried QDs. Moreover, the increase of stacking layers affects other parameters and the increase of stacking layers is more sensitive with the AC signal.

The second investigated photonic structure is the hollow optical whispering-gallery-mode (WGM) type microcavities. This work focuses on the modified characteristics of hollow optical microcavities by using finite-difference time-domain simulation and photoluminescence (PL) measurement. Designed structure of periodic hole array (hollow cylindrical microcavities) and hollow hyperboloidal microcavities are presented. Structural parameters of the hollow cylinder microcavity are varied. Electric and magnetic field profiles and enhanced resonant modes are presented. The quality factor variation as a function of mode index relates with spatial field mode pattern and the hole array. The resonant frequency depends on both hole radius and waveguide width. The resonant frequency blueshifts with the increasing radius and the reducing width. For the hyperboloidal microcavities, the new shape, which is also called anti-bottle resonator, is designed and fabricated. We present the experimental observation of the resonant modes by PL measurement. Range of effective refractive index approximates 1.15-1.35. Rather low Q factors of about 250-445 of the modes at wavelength range of 530-570 nm are achieved.

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