Fully Evaporated Solar Cells Exploiting High-Bandgap CsPbI2Br: Characterisation and device engineering

To push the Shockley-Queisser limit for photovoltaic cell PCE, tunable perovskite-based top cells for triple-junctions have been investigated. The tunability of the bandgap of CsPbI2Br, in combination with its photo-physical properties and thermal stability, makes the material ideal for this application, but it needs to be engineered and deeply characterised. Furthermore, being fully evaporated, the cell can avoid the annealing step usually needed, preventing changes in the underlying tandem induced by high temperatures. The objective of this thesis is to investigate the air- and photo-stability of co-evaporated, high-bandgap CsPbI2Br-based solar cells, and to analyse and engineer this cell for triple-junction applicability. Dual source thermal co-evaporation of CsBr and PbI2 was used to form a photoactive CsPbI2Br layer, with a bandgap of 1.92 eV. TEM, SEM, XRD, PL, UV-Vis-NIR, PDS, EQE, and J-V measurements were employed to analyse its air- and photo-stability, and its photovoltaic parameters in a single junction. The effects of various low-temperature annealing steps in a controlled atmosphere were compared. The air-stability mainly depends on phase transition under humidity. Stoichiometric and CsBr-rich perovskite exhibit a distorted perovskite γ-phase, making them relatively air-stable. Moreover, the influence of the thin film thickness has been investigated, revealing that CsPbI2Br layers over 185 nm are significantly more air-stable than thinner layers. The choice of HTL material strongly influences the phase transition rate of the perovskite. Concerning photo-stability, it depends on phase segregation, influenced by layer thickness, laser intensity, surrounding atmosphere, and crystal structure. Thin CsPbI2Br layers exhibit a quicker phase segregation than thicker layers. Annealed samples show a changed crystal structure, notably the absence of a δ-phase XRD peak. The optimal layer thicknesses for the investigated configurations are 190 nm for CsPbI2Br, 10 nm for SpiroTTB, 30 nm for C60. A record PCE for this composition is obtained of 9.33% (Jsc=12.88 mA/cm2 , Voc=1.01 V, FF=71.7%, stabilised PCE=8.82%), with those thicknesses, using a device structure of glass/ITO/Spiro-TTB/CsPbI2Br/C60/TmPyPb/Ag. The air- and photo-stability of the perovskite are increased by engineering of the crystal structure and adjacent layers. The effects of phase segregation distinguished in this thesis can be explained through a polaronic model, and are significantly reduced using a CsPbI2Br thickness above 185 nm, an inert atmosphere, and an annealing at 80°. However, the annealing step lowers the Jsc due to an introduction of defects.