Bhavya Rakheja: Atomic Layer Deposition of Thin Films for Perovskite Solar Cells: Transport and Passivation Layers

Abstract

Over the past two decades, lead halide perovskite solar cells (PSCs) have matured from laboratory-scale technology to industrial application. State-of-the-art PSCs employ a p-i-n architecture comprising C60 and SnOx electron transport layers (ETLs) atop perovskite. This thesis focuses on the investigation of SnOx and AlOx by Atomic layer deposition (ALD) for PSC applications and on understanding the ETL interfaces.

SnOx ALD using tetrakis(dimethylamido)tin (TDMASn) and H2O reactants, is investigated with respect to film properties, device performance, and interfacial chemistry. X-ray photoelectron spectroscopy (XPS) studies reveal that TDMASn renders the direct deposition of SnOx on perovskite unsuitable. On FAPbI3 and FAPbBr3 surfaces, TDMASn induces FA+ deprotonation, and further degradation. FAPbI3 partially decomposes into PbI2 and subsequently undergoes phase degradation, whereas TDMASn reacts with FAPbBr3, causing Br- migration into the SnOx layer. These reactions preclude the SnOx ALD on perovskites, making C60 a necessary yet imperfect solution due to its mechanical limitations and potential to induce non-radiative recombination at the perovskite/C60 interface.

For PSCs incorporating C60 between the perovskite and SnOx, the H2O dose used during SnOx ALD plays a crucial role. High H2O dose enables barrier-free transport and efficient PSC performance, whereas a low H2O dose results in SnOx with a slightly lower electron affinity value and oxide trap charges, which limits device performance. Furthermore, SnOx growth on perovskite/C60 stacks can still introduce degradation pathways. Operando ambient-pressure XPS studies indicate the occurrence of halide migration through defects or the bulk of C60 during the SnOx ALD, particularly in mixed iodide-bromide and pure-bromide compositions.

ALD AlOx interfacial treatment between the perovskite and C60 results in a similar VOC with a slightly lower FF compared to reference PSCs, while providing a more stable power conversion efficiency. AlOx ALD on perovskite causes surface alterations similar to those observed for SnOx, including FA+ deprotonation and interaction between the Al-precursor (trimethylaluminum) and Br- ions, which may contribute to the reduced FF. Overall, this thesis reveals challenges arising from ALD precursor reactivity and highlights the need for C60-free compact layers that protect perovskites during SnOx growth and eliminate the need for interfacial treatments at the perovskite/C60 interface.