The objective of this PhD project is to achieve an in-depth understanding of fundamental physical phenomena governing the carrier generation, extraction and collection in nanowires (NWs) at different scales. The PhD candidate will make use of the recently developed multi-scale technique combining electrical (Electron Beam Induced Current) and optical (Laser Beam Induced Current and cathodoluminescence) characterization tools to tackle the conversion mechanism from the single NW level up to the whole device level. These advanced characterization tools will provide access to the key parameters governing the photovoltaic conversion (minority carrier diffusion length, carrier density, surface recombination velocity, etc). Combining the advanced modeling with the experimental investigation, the PhD fellow will establish novel device architectures for high-efficiency PV converters and will participate in the nanowire device fabrication using the dedicated clean-room facilities of CTU-Minerve

Description

Among the alternative sustainable energy resources, the most promising is the solar energy conversion, which can deliver a power of tens of milliWatts per cm2. Today, a record conversion efficiency above 40% is achieved for a 3-junction tandem cell based on III-V semiconductors. However, the cost per kilowatt-hour of electricity generated with this photovoltaic (PV) technology is far too high for widespread applications. The problem of creating cost-effective high-performance solar cells is still open.
One alternative is to exploit the advantages of III-V semiconductor nanowires (NWs) to develop novel PV devices with high conversion efficiency and moderate production cost. Thanks to the strain accommodation by the free lateral surface and the small footprint, NWs can be grown on low-cost misfitting substrates and highly mismatched materials can be stacked within the NW without formation of dislocations. Moreover, NW arrays have very attractive optical properties such as a small optical reflectance and strong capability for efficient light trapping leading to an increased absorption in comparison to thin films.
To enhance the performance of NW PV devices, bottlenecks related to the design, material growth and processing have to be removed. In particular, for NW solar cells standard macroscopic PV characterization averaged over millions of nano-objects does not provide all the information necessary to understand the device physics and to optimize the performance. It is essential to push the comprehensive analyses down to the nanometric scale and to probe individual nanowire p-n junctions in order to analyze the material quality, to determine the surface recombination rate, to assess the wire-to-wire homogeneity and to detect eventual failures.

The PhD candidate will make use of the recently developed multi-scale technique combining electrical (Electron Beam Induced Current) and optical (Laser Beam Induced Current and cathodoluminescence) characterization tools to tackle the conversion mechanism from the single NW level up to the whole device level. These advanced characterization tools will provide access to the key parameters governing the PV conversion (minority carrier diffusion length, carrier density, surface recombination velocity, etc).

Related papers of the team :
1. M. Tchernycheva, et al, Nanoscale 7, 11692 (2015).
2. M. Tchernycheva, et al., Nanotechnology 23 (26), 265402 (2012).
3. L Yu, et al., Nanotechnology 24 (27), 275401 (2013).
4. P. Lavenus, et al., Nanotechnology 25, 255201 (2014).

Person requirements
We are searching for a candidate with a good academic record in physics, material sciences or related areas, who is motivated to pursue research in nanotechnologies, and in particular in the science and technology of nanowires. A good command of English language, with oral and written skills is required.