Context

In the past years, photovoltaics (PV) became one of the cheapest sources of energy. 95% of commercial solar cells are made of silicon, and their lab-scale record efficiencies of 27.3 % are now close to the theoretical limit (29.4 %). Yet, expectations of both the society and the PV industry are still high, and most of the research efforts are now dedicated to pushing forward the efficiency of solar cells. Silicon-based tandem devices are the most-regarded solutions for next-generation photovoltaics. To keep low costs and preserve the silicon bottom cell, low-temperature deposition is mandatory for the top cell. Current options are polycrystalline, high-bandgap semiconductors like hybrid perovskites and inorganic Cu(In,Ga)(S,Se)2 -CIGS- or CdTe thin films, but they are still limited by both efficiency and/or stability issues that are hardly explained by current models. Further developments require a better understanding of the properties and limitations of low-cost thin-film materials. In particular, it is necessary to differentiate the properties of grain interiors and grain boundaries, and the macroscopic fluctuations that may occur in semiconductor alloys.

Scientific project:

The goal of this project is twofold. From the one side combining CathodoLuminescence (CL) and PhotoLuminescence (PL) techniques would provide a multi-scale analysis tool for elementary processes and properties of bulk materials (doping levels, diffusion length, carrier lifetime, defect levels and densities...) and surfaces (surface recombination velocity, density of defects, surface charges…). The host team has been using CL extensively to study - down to the nanometer scale - some of the aforementioned systems such as Cd(Se)Te cells (Selenium diffusion/passivation¹, lifetime increasing²), GaAs thin-films and nanowires (doping assessment^3,4), or hybrid perovskite (degradation⁵). On the other side, given this experimental background, we aim at pushing our analysis a step further by calibrating our tool in absolute terms, i.e. extracting the number of CL photons emitted by the sample.  Another long-term objective is to use advanced computational methods for correlative data analysis, and simulation to build a realistic model of thin-film solar cells using the measured quantities.

This internship will first focus on the unique CL tool available at C2N. Its basic principle is the following: a material is excited with an electron beam in a scanning electron microscope (SEM), providing a spatial resolution of 10 nm. Secondary electrons are collected to form an SEM image, and emitted photons (CL) are collected simultaneously to acquire an hyperspectral image (luminescence spectrum at each point of the map). Time-resolved CL (TRCL) is also available to measure the luminescence decay after a pulsed excitation.

The candidate will be first trained on the CL/TRCL tool. Then, she/he will use this technique to perform and analyze multiscale CL/TRCL mapping, with the goal to develop new methods to reveal the dynamics of carriers and correlate these properties to the functional parameters of solar cells.

The institute:

The candidate will work with several members of the sunlit team (C2N) and in close collaboration with the Institut photovoltaïque d’Ile-de-France (IPVF).

Websites: https://sunlit-team.eu , https://www.c2n.universite-paris-saclay.fr/en , https://www.ipvf.fr

Description of the CL setup and recent publications: https://sunlit-team.eu/resources/cl-and-trcl-tool/

Profile:

Student in M2 with a solid knowledge in semiconductor physics and optics.

Possibility to continue with a PhD grant in 2026.

Send CV and motivation letter to stephane.collin@cnrs.fr and  stefano.pirotta@c2n.upsaclay.fr.

Starting date: 02/2026 (adjustable).

Scientific project:

The photovoltaic technology is largely dominated by silicon devices (≈ 90% of the market), which present very limited progress margins today, with an efficiency intrinsically limited below 30%. It is largely agreed that the next device generation will combine several materials, beyond silicon alone. We are developing an innovating technology, to produce solar cells based on III-V materials, already presenting high efficiencies (up to 46% under concentration), with significant cost reductions.

Our strategy is to recycle the III-V substrates, which represent the largest device cost share, for several epitaxial growths. To do so, we are developing innovative processes to modify the substrate surface, so that the fabricated layers can be easily detached, leaving a surface compatible with subsequent growths. A promising route recently suggested consist in transferring a graphene layer before performing the epitaxy, as displayed in the attached file. I was shown that the graphene permits the fabrication of a monocrystalline material, while allowing its exfoliation. Developing this method requires exploring fundamental physical phenomena as well as defining practical methodologies.

The intern will work on the development of the process as well as on the characterization of the obtained structures and intermediate products. He / she will propose further developments of the techniques already existing at the laboratory, as well as suggest the exploration of new methods. He / she will propose models to explain the observed phenomena, and design experiments for their validation. To complete those tasks, the intern will use his own knowledge as well as the scientific literature. The intern will take advantage of a unique collection of fabrication and characterization methods (XPS, TEM, SEM, luminescence) available in partner laboratories. This environment will provide various opportunities to tackle this project challenge and gain experience.

Profile:

The candidate must possess solid knowledges in material physics, characterization, and fabrication processes in a clean room environment. He / she must show good project management skills, for the development of technological procedures involving numerous parameters. He / she will be able to work independently and suggest innovative solutions to reach the project objectives. Collaborative work being at the core of the program, communication skills are required for team working as well as regular presentation of work progress in internal meetings.

Starting date: from 01/02/2026 (adjustable)

Duration: 6 months

This internship can be followed by a PhD.

Websites: https://sunlit-team.eu

poursuite en thèse envisageable

Internship proposal for a second-year master student Synthesis of quantum dots in III-V nanowires
Duration : 6 months starting from March
Salary ~ 600 EUR/month
Supervisor : Federico PANCIERA
Laboratory : Centre for Nanoscience and Nanotechnology (C2N), U. Paris-Saclay/CNRS
Web: https://elphyse.c2n.universite-paris-saclay.fr/en/

Contacts:

federico.panciera@c2n.upsaclay.fr

Context of the project : 

Semiconductor nanowires (NWs) exhibit unique properties that make them potential building blocks for a variety of next generation devices such as biosensors, solar cells, transistors, quantum light sources and lasers. In order to take advantage of the physical properties of NWs, it is crucial to control their geometry, crystal structure and doping. This goal will ultimately be achieved by a deep understanding of the growth mechanisms. The most common growth technique is the vapor-liquid-solid (VLS) method, where a liquid metal droplet catalyzes the growth of a solid NW from gas phase precursors. In this growth mode, the droplet plays a fundamental role in determining the structure of the nanowire, and the remarkable range of structures enabled by VLS can be thought of as the result of engineered changes to the droplet.
For example, growth of III-V semiconductor NWs using the VLS method can result in crystal structures different from their bulk phase. In GaAs NWs stable zincblende (ZB) phase coexists with metastable wurtzite (WZ) structure resulting in NWs having a mixed-phase structure. Remarkably, the valence and conduction bands of the two phases are misaligned so that small sections of one phase within the other effectively confine charge carriers.
Controlled switching between the two phases enables the synthesis of novel heterostructures, crystal-phase quantum dots (CPQD), with exceptional properties and potential applications in photonics [1] and quantum computing [2,3]. In contrast to compositional heterojunctions, CPQDs have intrinsically abrupt interfaces and
hence do not suffer from alloy intermixing at the interface, which hampers precise control of the electronic properties in compositional heterostructures.