Hybrid Solar Cells and Hybrid LEDs utilising Photonic Quasi Crystals and Colloidal Quantum Dots

Abstract

This thesis concerns itself with the application of Photonic Quasi Crystal (PQC) surface nanostructuring and colloidal Quantum Dots (QD) to GaN based Light Emitting Diode (LED) and crystalline Si based solar cells. For LEDs, PQC structuring can beutilised to modify the angular emission profile of the device, a property commonly referred to as directionality. Here, it was found that the angular emission profile could be tuned to favour the forward direction with 12-fold symmetric PQC patterning yielding an intensity concentration of over 42 % within the first 20 degrees from the normal.
The lattice pitch emerged as a critical performance factor. In addition, spectroscopic angular reflectometry was used to analyse the photonic band structure and the cavity dynamics of the PhC/PQC structuring in the GaN LEDs. For the Si Photovoltaic (PV) devices, the main aspects investigated were the reduction of top surface reflection as well as the interaction of the structuring with additional absorber layers. For Si, it was found that well-defined structures with feature sizes of 150 nm and depths of 900 nm could be reliably fabricated using e-beam lithography and a two step dry etching procedure. Through experimentally verified optical simulation, it was shown that low solar corrected reflectance values of 9.2 % are achievable for densely packed 1000 nm deep holes arranged in a 12-fold lattice. Here, the air Fill Fraction (FF) emerged as the determining performance factor for a given depth. To achieve sufficient surface passivation for the Si structuring, the deposition of the Al₂O₃ passivation layer had to be preceded by a surface damage removal wet etching step. The shape of the holes prior to the wet etching step was revealed to be critical. For LEDs the tunable emission and absorption properties of QDs can be exploited to efficiently convert high energy light emitted by the underlying device, such as blue, to lower energy light, such as green or red, thereby achieving colour conversion. To ensure that only the converted light is detectable, selective band pass filters were modelled and fabricated with reflected blue light getting another chance to be converted. This resulted in an overall colour conversion efficiency of 20.8 %. An important aspect of colour conversion layers is their photostability under constant illumination. Encapsulation via sputter-deposited Si₃N₄ gave the best performance with over 95 % of the initial intensity after 12 hours of low brightness illumination. For PV devices, QDs offer the possibility of converting part of the incoming solar spectrum to photons that the base solar cell can utilise more efficiently. Incorporation of QD layers into planar Al₂O₃ passivated PV devices resulted in 38 % increase in power conversion efficiency when compared to the device without the QD layer. Here, the layer thickness emerged as the most important performance factor. Nanosecond scale Photoluminescence (PL) measurements were used to reveal that non-radiative energy transfer from QDs to Si can occur with an efficiency of 15 %. The application of a QD layer to a PQC patterned cell resulted in a modest boost of 18%.

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