Hot-carrier extraction in nanowires

Sammanfattning: A hot-carrier solar cell aims to generate power from energetic, photoexcited, charge carriers, so called hot carriers, in order to reach higher conversion efficiencies than current solar cell technology.Creating a hot-carrier solar cell has proven challenging for two main reasons: hot carriers lose their energy very quickly, and they need to be extracted over distances of a few hundrednanometers via energy selective filters.Semiconducting III-V nanowires offer high flexibility and control in heterostructure growth, enabling the realisation of numerous types of energy filters, in combination with promising properties such as reduced thermal conductivity, increased hot-carrier temperatures, and various possibilities to tune optical absorption.This thesis aims to expand current knowledge of how to optimally design devices for hot-carrier extraction in practice.Specifically, three experimental papers (I-III) study the generation of electrical power by extracting charge carriers across energy selective filters within single semiconducting nanowires. The fourth paper (IV) reviews current literature relating to hot carriers in nanowires.The experiments are based on InAs nanowires with epitaxially defined heterostructures of InP or InAsP that form energy filters. Charge carrier extraction is studied by three different means: excitation of a non-equilibrium distribution by optical or electron-beam exposure, or the generation of an equilibrium distribution by heat. In Papers I and II, hot-carrier extraction is spatially resolved over a rectangular InP barrier. Paper I uses the high spatial resolution of an electron beam, while Paper II studies the operation of a similar devices under highly focused optical excitation. Both papers observe hot-carrier extraction around the barrier. The mechanism for extraction is better understood and valuable input for the future design of hot-carrier photovoltaic devices is extracted, such as hot-electron diffusion lengths on the order of a few hundred nanometers. Paper III studies thermoelectric power generation in a nonlinear transport regime of a ramp-shaped potential barrier, realised by gradually changing x in InAs_xP_(1-x). It is observed that fill factor, and thus maximum output power, can be tuned beyond the linear response limits. This opens up a new door of possibility for tuning the performance of both thermoelectric and hot-carrier photovoltaic systems.