Le mardi 14 novembre prochain, Azmat Ali soutiendra son travail de thèse intitulé :

Elucidating interplay, stability and charge transfer dynamics at lead halide perovskite nanocrystal / 2D transition metaldichalcogenide interface for solar cell applications.

Les travaux d'Azmat Ali ont été effectués au sein de l’Institut des Nanosciences de Paris sous la direction de Nadine Witkowski.

Membres du jury :

• M. Arnaud Etcheberry (Directeur de recherche émérite, ILV, Rapporteur,Versailles, France))
• Mme. Selina Olthof (Professeure associée, IPC-UOC, Rapporteure, Cologne,Germany)
• M. Philip Schulz (Directeur de recherche, IPVF, Examinateur, Palaiseau,France)
• Mme Nadine Witkowski (Professeure, Directrice de thèse, INSP, Paris,France)
• M. Fredrik O. L. Johanssson (Docteur, Uppsala Université, Invité, Uppsala, Sweden)

Abstract

Lead halide perovskites are considered strong contenders for the next generation of photovoltaics, nonetheless the intrinsic instability of halide perovskite remains a bottleneck for the commercialization of this technology. The striking properties of halide perovskites, fully inorganic CsPbBr3, used in this thesis, when combined with the exceptional properties of nanocrystals (NCs), can result in a material that possesses the attributes of both. Yet, the fundamental properties of perovskite NCs are strongly modified at the interface with charge transport layers and exposure to light. In this thesis, using photoelectron spectroscopy (PES), the dynamical processes that occur at the surface and in the interfaces of CsPbBr3 NCs on metal, semiconductor and oxides, when exposed to infrared (IR) and ultraviolet (UV) light are investigated. The decomposition of perovskite on all the substrates under UV illumination gives a common degradation product of metallic lead (Pb0) and bromide gas (Br2(g)). However, for the CsPbBr3 NCs on gold (Au), both UV light and intense x-rays irradiation not only leads to the anticipated degradation product, Pb0 and Br2(g), but also gives rise to a new chemical specie, which is associated with the underpotential deposition of lead (PbUPD) on the Au surface. Furthermore, UPD of Pb is shown to occur only when the perovskite structure breaks and a direct contact of perovskite and Au is made. Moreover, UV light degradation to the perovskite NCs with regard to metallic lead formation occurs to a lesser extent on ITO and MoOx than on MoS2 and Au, revealing that substrates with wider bandgaps prevent the decomposition of the perovskite. Similarly, different effects of NCs to IR light is observed on Au, and MoS2 and MoOx. IR illumination affects the perovskite NCs on Au in the same way as the UV light and intense x-rays do, but no bromide gas is formed. IR illumination of the CsPbBr3 NCs on MoS2 and MoOx results in intraband surface photovoltage. This photovoltage stems from the deep defects-states located in the middle of the bandgap. However, no decomposition of the perovskite is observed. The findings of this thesis emphasize the substrate dependent properties of the perovskite and their influence when exposed to IR and UV light.