This topic is part of the research and development activities carried out within the GeePs laboratory and the IPVF in the field of PV materials and devices. In particular, these are studies conducted at the micro/nano scale using atomic force microscopy (AFM) metrology. In this field we use the C-AFM (Conductive AFM) and KPFM (Kelvin probe microscopy) electrical extensions to study and explore the local properties of different technological building blocks involved in PV devices.
The student will first have to carry out a bibliography work on C-AFM and KPFM instrumentation, and in particular applied to the study of PV materials and devices. In parallel, he/she will be trained in the use of AFM, with a particular focus on data acquisition and processing. He/she will also participate in the implementation of the KPFM measurement in the time domain where the contact potential difference (CPD) will be measured under pulsed and/or frequency modulated illumination. Several signal processing approaches applied to the surface photovoltage (SPV) measurement will be studied in order to access the characteristic times or phase shifts with the idea to evaluate and map the minority carrier lifetime .
In a second phase, the student will apply its new expertise to study different technologies developed at the IPVF:
– Perovskite materials have grown at an unprecedented rate making possible to produce PV cells with conversion efficiencies of more than 20%. The IPVF is interested in different compositions of these materials, which may show fluctuations in composition due to their polycrystalline structure. These fluctuations can be analyzed by KPFM microscopy.
– the interfaces between perovskite materials and silicon, with a view to the subsequent fabrication of tandem cells. The KPFM analysis can be carried out on different types of interfaces, notably in order to study the band bending between these two materials.
– so-called “passivating contacts“ for crystalline silicon (c-Si) absorbers. These structures consist of a very thin layer of silicon oxide (<2 nm) covered by a layer of highly doped polysilicon (10-20 nm). This particular technological block has the advantage of minimizing surface recombination losses by avoiding direct contact between the metal and the c-Si. The electronic transport properties through these passivating layers are nowadays very poorly known and are the subject of advanced characterization work by several research groups around the world. The results of this study should provide additional information on transport mechanisms, and in particular tunnelling transport through ultrathin oxides including the presence of “pinholes”.
– CIGS material used in PV devices is a polycrystalline thin film where the grain boundaries do not always act as recombinant centers, as can be expected in conventional semiconductor devices. The presence of impurities seems to cause a passivation effect. KPFM under illumination can be an interesting approach that can be implemented to study local variations in the open circuit voltage (VOC) and lifetime. Similarly, the C-AFM can be used to analyze the distribution of saturation current in the dark.
The thesis will take place at the GeePs laboratory (Génie Electrique et Electronique de Paris) and at the IPVF (Institut Photovoltaïque d’Ile-de-France).
CV and cover letter can be sent to :
Planned start: January 2020, located at IPVF and GeePs