Growth and Characterization of Tandem-Junction Photovoltaic Nanowires

In order to satisfy the growing energy needs of our planet’s population, and at the same time mitigate global warming, sustainable energy sources such as solar energy are indispensable. In addition to conventional silicon-based solar cells, nanotechnology offers interesting approaches for complementary applications. Multi-junction solar cells based on III–V semiconductors hold today’s world-record efficiencies—twice as efficient as solar cells found on rooftops nowadays—but their high cost is limiting their terrestrial use.
In this thesis, nanowires for photovoltaic applications are studied. Nanowire solar cells have the potential to reach the same efficiencies as the world-record III–V solar cells while only using a fraction of the material. First, InP single-junction nanowires were investigated. For solar energy harvesting, large-area nanowire solar cells have to be processed but so far only devices with less than one mm2 have been fabricated. To lay the foundation of large-area nanowire solar cells, the wafer-scale synthesis of InP nanowire arrays was systematically studied. Then the effect of embedding InP nanowires in different oxides was investigated. Due to their inherent large surface-to-volume ratio, nanowires require surface passivation. However, fixed charge carriers in the passivating layer can alter the electrostatic potential of nanowires, which was directly imaged by measuring the electron-beam-induced current. Furthermore, the current-voltage characteristics of single nanowires under in situ illumination was measured and correlated with electron-beam-induced current measurements, by using a setup that combines a nanoprobe system with an optical fiber coupled to a multi-LED setup inside a scanning electron microscope.
Guided by the multi-LED and electron-beam-induced current setup, tandem-junction nanowires were developed. After identifying and subsequently preventing the occurrence of a parasitic junction when combining an InP n–i–p junction with a tunnel diode, GaInP/InP tandem-junction nanowires were synthesized. An optical and electrical bias was applied to individually measure the electron-beam-induced current of both sub-cells. Finally, axially defined, GaInP/InP/InAsP triple-junction photovoltaic nanowires optimized for light absorption exhibiting an open-circuit voltage of up to 2.37 V were synthesized. The open-circuit voltage amounts to 94 % of the sum of the respective single-junction nanowires. These results pave the way for realizing the next-generation of scalable, high-performance, and ultra-high power-to-weight ratio multi-junction, nanowire-based solar cells.