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See detailEffects of Annealing and Light on Co-evaporated Methylammonium Lead Iodide Perovskites using Kelvin Probe Force Microscopy in Ultra-High Vacuum
Gallet, Thibaut UL; Martin Lanzoni, Evandro UL; Redinger, Alex UL

in 2019 IEEE 46th Photovoltaic Specialists Conference (PVSC) (2020, February 06)

Careful surface analysis is essential to understand the electronic and ionic behaviors in perovskite absorbers. In this contribution we discuss Kelvin probe force microscopy performed in ultra-high vacuum ... [more ▼]

Careful surface analysis is essential to understand the electronic and ionic behaviors in perovskite absorbers. In this contribution we discuss Kelvin probe force microscopy performed in ultra-high vacuum on as-grown and annealed co-evaporated methylammonium lead iodide perovskite thin films. By comparing the contact potential difference upon annealing and illumination, we find that annealing increases the average workfunction, indicating a change either in doping or in surface composition. Illumination also increases the average workfunction, indicating a p-type absorber, by reducing band bending as the photo-generated carriers screen the surface states. The effect of light shows a two-step process, with a first fast trend, linked to the surface photovoltage and a second slower trend indicating a possible redistribution of mobile charges. [less ▲]

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See detailFermi-level pinning in methylammonium lead iodide perovskites
Gallet, Thibaut UL; Grabowski, David; Kirchartz, Thomas et al

in Nanoscale (2019)

Hybrid organic inorganic perovskites are ideal candidates for absorber layers in next generation thin film photovoltaics. The polycrystalline nature of these layers imposes substantial complications for ... [more ▼]

Hybrid organic inorganic perovskites are ideal candidates for absorber layers in next generation thin film photovoltaics. The polycrystalline nature of these layers imposes substantial complications for the design of high efficiency devices since the optoelectronic properties can vary on the nanometre scale. Here we show via scanning tunnelling microscopy and spectroscopy that different grains and grain facets exhibit variations in the local density of states. Modeling of the tunneling spectroscopy curves allows us to quantify the density and fluctuations of surface states and estimate the variations in workfunction on the nanometre scale. The simulations corroborate that the high number of surface states leads to Fermi-level pinning of the methylammonium lead iodide surfaces. We do not observe a variation of the local density of states at the grain boundaries compared to the grain interior. These results are in contrast to other reported SPM measurements in literature. Our results show that most of the fluctuations of the electrical properties in these polycrystalline materials arise due to grain to grain variations and not due to distinct electronic properties of the grain boundaries. The measured workfunction changes at the different grains result in local variations of the band alignment with the carrier selective top contact and the varying number of surface states influence the recombination activity in the devices. [less ▲]

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See detailFabrication of Si tunnel diodes for c-Si based tandem solar cells using proximity rapid thermal diffusion
Fave, Alain; Lelièvre, Jean-François; Gallet, Thibaut UL et al

in Energy Procedia (2017), 124

Increasing competitiveness of photovoltaic (PV) devices is currently an important objective in technological research, especially with the development of tandem solar cells based on c-Si as the bottom ... [more ▼]

Increasing competitiveness of photovoltaic (PV) devices is currently an important objective in technological research, especially with the development of tandem solar cells based on c-Si as the bottom cell. For a monolithical structure, a tunnel diode in between the top and bottom cells is necessary. In this work we report on the development of the fabrication of Si tunnel junction using a combination of spin-on doping and proximity rapid thermal diffusion. A desirable attribute of this process is simplicity. Two different structures p++/n++ or n++/p++ were fabricated on (100) Si substrates. Carrier density profiles were measured by ECV to characterize the shallow doping profiles. Vertical tunnel diodes were fabricated and I(V) characteristics are presented. It is shown that device peak current densities up to 270 A/cm² are achieved using this technique, which is the best value reported with such simple technique. [less ▲]

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