Description
Date/Time: 04-23-2019 - Tuesday - 05:00 PM - 07:00 PM
Jonatan Holmér1 Lunjie Zeng1 Thomas Nordqvist2 Peter Krogstrup2 Jesper Nygård2 Ludvig de Knoop1 Eva Olsson1

1, Chalmers University of Technology, Gothenburg, , Sweden
2, University of Copenhagen, Copenhagen, , Denmark

Semiconductor nanowires are promising building-blocks for the next generation of solar cells [1]. With diameters of a few hundred nanometers, vertical nanowires can act as waveguides for the incoming sunlight and thereby enhance the light absorption. The nanoscale geometry further enables the growth of defect free heterostructure interfaces including also defect free growth on lattice mismatched substrates, not possible for bulk structures due to the magnitude of interfacial strain. Because of these and other unique properties, nanowire-based solar cells have the potential to provide higher efficiencies and lower material costs than their thin-film counterparts. One of the main challenges for realizing these high-performance nanowire solar cells lies in the optimization of the electrical and photovoltaic properties at the individual nanowire level. Here characterization techniques play a vital role. Standard solar cell characterization methods do not reveal the contribution of individual nanowires. Techniques involving single nanowire lift-off alter the working conditions, do not include the effect of the nanowire/substrate interface and generally demand a great deal of sample preparation. Hence, techniques where single nanowires can be probed directly on the growth substrate are advantageous.
In this work, we have developed a technique where a scanning tunneling microscope (STM)-probe is operated inside a scanning electron microscope (SEM) for characterization of the photovoltaic and electrical properties of single as-grown semiconductor nanowires. The SEM provides live imaging with nanoscale resolution while the piezo-controlled STM-probe contacts individual nanowires forming a closed electrical circuit. Reproducible ohmic contacts between the STM-probe and the nanowires were established through electron beam induced Pt deposition and Joule heating. Photocurrent measurements were enabled by mounting a light emitting diode (LED) in the SEM chamber. Current – voltage (I-V) measurements in both dark and illuminated conditions were performed on single GaAs nanowires with built-in radial p-i-n junctions. The best performing nanowire had an apparent power conversion efficiency of around 11 % under white LED illumination. Electron beam induced current (EBIC) maps were recorded to show the local charge separation ability within a nanowire with a spatial resolution of around 250 nm. Furthermore, the STM-probe was used to bend single nanowires, which affected the I-V characteristics. In conclusion, the in situ STM-SEM technique offers the possibility to investigate and compare the functional properties of individual as-grown nanowires that are part of a nanowire solar cell. The properties can be directly linked to the structure of the individual nanowires. This may play a key role in the further improvement of nanowire solar cells.

References.
[1] G. Otnes and M. T. Borgström, “Towards high efficiency nanowire solar cells,” Nano Today, vol. 12, pp. 31–45, 2017.
[2] J. Holmér, L. Zeng, T. Kanne, P. Krogstrup, J. Nygård, L. de Knoop, E. Olsson, "An STM – SEM setup for characterizing photon and electron induced effects in single photovoltaic nanowires", Nano Energy, vol. 53, pp. 175-181, 2018.

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