Thesis presented March 06, 2015

Abstract:
In this work we report on epitaxial growth and characterization of InGaN/GaN multi quantum wells (MQWs) heterostructures for application in photovoltaic devices. The samples were grown by metal-organic vapor phase epitaxy (MOVPE) on (0001) sapphire substrate. The structural and optical characterization is performed by X-ray diffraction, transmission electron microscopy, photoluminescence spectroscopy and transmission measurements. To investigate the presence of photovoltaic effect and estimate the electrical performance of the samples, they were processed into solar cells by means of the photolithography, inductively coupled plasma reactive-ion etching and metallization to manage n and p contacts. We studied the influence of different InGaN/GaN active region designs on the structural, optical and electrical properties of the samples, i.e. number of InGaN quantum wells, QW and quantum barrier thicknesses and indium composition in the wells. Two main mechanisms have to be taken into account for an efficient optimization of photovoltaic devices: photon absorption and carrier collection. We showed that an increase of the MQWs number, their thickness and the In-content allows absorption improvement, but causes losses in the carrier collection efficiency due to: the increase of the active region thickness (lower electric field), the difficulty of the carrier to escape from deeper QWs and the strain relaxation (structural defect creation). The decrease of the barrier thickness can solve the first two issues, but the problem with strain relaxation remains. In the best design, we report the value of 2.00% of conversion efficiency for 15×In_0.18Ga_0.82N/GaN samples with spectral response extending to 465 nm.

Keywords:
Solar cell, Quantum well, MOVPE, InGaN/GaN

On-line thesis.