Development of Perovskite Absorber Layers synthesised by Techniques Compatible with Non-flat Substrates

Development of Perovskite Absorber Layers synthesised by Techniques Compatible with Non-flat Substrates

Nespoli, Jasmeen (TU Delft Mechanical, Maritime and Materials Engineering; TU Delft Materials Science and Engineering)

Mazzarella, L. (mentor)
Taheri, P. (graduation committee)
Houtepen, A.J. (graduation committee)

Delft University of Technology

Materials Science and Engineering

2021-11-29

Among the emerging photovoltaic (PV) technologies under development, perovskite solar cells (PSCs) stood out in the last decade as one the fastest advancing solar technology to date, reaching a power conversion efficiency of 25.5% in 2020. Perovskite (PVK) absorbers show great versatility thanks to compositional and bandgap engineering. In this MSc Thesis Project, multications mixed-halides PVK thin-films of composition CsxFA1-xPbI3-xBrx are developed. Multiple-source layer-by-layer thermal evaporation and sequential thermal evaporation/spin coating hybrid synthesis techniques are investigated because of their compatibility with non-flat substrates. As they would enable the conformal growth of PVK on the microsized pyramidal textured surface of a c-Si- based solar cell, the long-term goal is the application in a monolithic (2-T) PVK/Si tandem. PVK engineering is carried out through tuning the main processing parameters with the aim of obtaining high-quality absorber layers. The optimisation of the PVK thin-films is based on criteria such as composition, phase, bandgap, crystallinity, homogeneity, uniformity and charge carrier transport properties. After optimising the annealing thermal treatment, the homogeneous and uniform thin-films resulting from the thermal evaporation of CsBr, PbI2 and FAI show a highly crystalline photoactive α phase of PVK, as well as a sharp-edge absorption onset corresponding to a bandgap (Eg) of Eg=1.60 eV and high absorption coefficient (α) in the order of α=10^4-10^5 cm^-1. Then, the two-step spin coating technique is explored as a preliminary study to develop the hybrid method, with the aim of analysing the surface wettability and reactivity of an inorganic layer of PVK inorganic precursors (CsBr and PbI2) when an organic solution (FAI in IPA) is spun on it. In spite of the optimisation of the FAI solution spun volume and concentration and the annealing parameters, the spin-coated thin-films present a mixture of photoactive and non-photoactive phases, i.e. PbI2-xBrx and α-, γ- and δ-PVK phases. The ineffective interdiffusion reaction leading to inhomogeneous and non-uniform PVK thin-films shows that the processing parameters need further fine-tuning. However, as spin coating does not allow to deposit PVK on top of non-flat substrates, the thermal evaporation/spin coating hybrid method is developed. The solution of FAI in IPA has been spun on a thermally evaporated inorganic bilayer of CsBr/PbI2. Albeit incomplete conversion to PVK is under suspicion, the resulting α-PVK thin-film shows good crystallinity and homogeneity and acceptable uniformity. It also presents high absorption (A=70%) for λ<500 nm (blue light) and low absorption for λ>500 nm (from green to red light), although the bandgap of Eg=1.53 eV needs to be further optimised by adjusting the content of Cs+ and most importantly Br–. On top of that, for all the synthesis techniques under research some unsuccessfully deposited PVK thin-films show signs of thermal and atmospheric degradation, leading to the decomposition of the α phase of PVK and formation of PbI2 and other non-photoactive phases, along with poor optical properties. Such degradation phenomena, which might be ascribed to the ineffective incorporation of Cs+ leading to structural instabilities, highlights the difficulties and limitations of PVK engineering.

Solar Energy
Perovskite
Thermal evaporation
Hybrid Method
Texture

http://resolver.tudelft.nl/uuid:1f149537-c99d-41ff-ad2b-b249e6d8d334

Student theses

master thesis

© 2021 Jasmeen Nespoli