Date of Degree

5-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor(s)

Yuhang Ren

Committee Members

Sultan Catto

Ying Chih Chen

Patrick Folkes

Igor Kuskovsky

Yuhang Ren

Subject Categories

Condensed Matter Physics

Keywords

photovoltaic device, solar cell, copper indium gallium diselenide, fabrication, efficiency, thin film

Abstract

The chalcopyrite semiconductor CuInxGa1-xSe2 is considered as the most promising material for high efficiency thin film solar cells due to its exceptional radiation stability, tunable direct bandgap, high light absorption coefficient and low cost preparation methods. In this thesis, we present the systematic investigation of the deposition conditions to optimize the CuInxGa1-xSe2 device performance using the two-step deposition method. Further, we utilized nonlinear optical methods to investigate the deposition parameters to optimize the bulk and interface properties of photovoltaic devices.

First, we investigated the deposition parameters to optimize the structural, electrical, optical and adhesion properties of molybdenum, Mo, and aluminum doped zinc oxide, ZnO:Al, electrode layers. Our results show that electrical and adhesion properties of Mo films can be optimized by modifying the intrinsic mechanical compressive and tensile stress. Mo films deposited under tensile stress exhibit good adhesional strength and high resistivity, whereas films deposited under compressive stress exhibit poor adhesional stress and low resistivity. In order to obtain Mo films with both good adhesion and low sheet resistance, we deposited the films in a bilayer structure. Similarly, increased Ar flux is found to improve the crystalline quality of the ZnO:Al films due to Ar-ZnO:Al collisions and heating effect which lead to increased grains size and shape, reducing intergranular voids in ZnO:Al films. Moreover, lower sputter pressure is found to increase transmittance of light due to reduced grain boundary scattering of light. Post-annealing treatment in hydrogen atmosphere is found to enhance the conductance of the ZnO:Al film which is attributed to desorption of negatively charged species, mainly oxygen from the grain boundaries.

Second, we investigated the uniformity, morphology and homogeneity of CuInGa precursor and CuInxGa1-xSe2 films. In order to obtain uniform chemical homogeneity, and smooth surface, we deposited the precursor in the Mo/In/CuGa/In structure. Reducing the deposition time for In resulted in a precursor film with smoother morphology, and depositing a stacked structure enabled uniform chemical homogeneity. Next, we designed our chemical vapor deposition system to fabricate uniform CuInxGa1-xSe2 films with reproducible results. In our chemical vapor deposition system, we incorporated a flow controller to control the selenium diffusion rate, hence the thickness of the MoxSey layer, and a pressure regulator to control the selenium vapor pressure in order to fabricate CuInxGa1-xSe2 films with the right selenium content and the stoichiometry. A sophisticated exhaust gas collection system is incorporated to trap the unreacted residual Se vapor, and prevent the CVD system from contamination and to obtain reproducible results. A graphite CuInGa precursor fixture was utilized and oriented normal to the direction of selenium flux. Our selenization results show that the CuInxGa1-xSe2 device performance can be optimized by tuning the Cu/In composition. An increase in Se flux facilitates In2Se3 and Ga2Se3 phases, resulting in CuInxGa1-xSe2 films with larger grains, and better device performances. We also show selenium cracking helps to reduce the defect density in the bulk and interface of the CuInGaSe film by filling in the copper vacancy defects.

Third, we utilized Raman spectroscopy and time-resolved photoluminescence spectroscopy to investigate the deposition parameters to optimize the crystalline quality and therefore the interval radiative quantum efficiency of MBE-grown GaAs/AlGaAs double heterostructures. Our results reveal an improvement in lattice disorder in both the GaAs and AlGaAs layers at elevated temperatures as the As/Ga flux ratio is reduced, which is consistent with the obtained longest minority carrier lifetime. Moreover, we reveal that incorporation of a distributed Bragg reflector layer significantly reduces the defect density in the subsequent layers. Our results show that the combined analysis of Raman and TRPL spectra provide a powerful tool for understanding defect mechanism and carrier dynamics in GaAs/AlGaAs DH structures.

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