Novel Energy Materials and Advanced Characterisation

Abstract:

Mixed-halide perovskites offer bandgap tunability essential for multijunction solar cells; however, a detrimental halide segregation under light is often observed. Here we combine simultaneous in situ photoluminescence and X-ray diffraction measurements to demonstrate clear differences in compositional and optoelectronic changes associated with halide segregation in MAPb(Br0.5I0.5)3 and FA0.83Cs0.17Pb(Br0.4I0.6)3 films. We report evidence for low-barrier ionic pathways in MAPb(Br0.5I0.5)3, which allow for the rearrangement of halide ions in localized volumes of perovskite without significant compositional changes to the bulk material. In contrast, FA0.83Cs0.17Pb(Br0.4I0.6)3 lacks such low-barrier ionic pathways and is, consequently, more stable against halide segregation. However, under prolonged illumination, it exhibits a considerable ionic rearrangement throughout the bulk material, which may be triggered by an initial demixing of A-site cations, altering the composition of the bulk perovskite and reducing its stability against halide segregation. Our work elucidates links between composition, ionic pathways, and halide segregation, and it facilitates the future engineering of phase-stable mixed-halide perovskites.

Abstract:

Organic semiconductors are commonly used as charge-extraction layers in metal-halide perovskite solar cells. However, parasitic light absorption in the sun-facing front molecular layer, through which sun light must propagate before reaching the perovskite layer, may lower the power conversion efficiency of such devices. Here, we show that such losses may be eliminated through efficient excitation energy transfer from a photoexcited polymer layer to the underlying perovskite. Experimentally observed energy transfer between a range of different polymer films and a methylammonium lead iodide perovskite layer was used as basis for modelling the efficacy of the mechanism as a function of layer thickness, photoluminescence quantum efficiency and absorption coefficient of the organic polymer film. Our findings reveal that efficient energy transfer can be achieved for thin (≤10 nm) organic charge-extraction layers exhibiting high photoluminescence quantum efficiency. We further explore how the morphology of such thin polymer layers may be affected by interface formation with the perovskite.

Abstract:

Metal halide perovskites have generated interest across many fields for the impressive optoelectronic properties achievable in films produced using facile solution-processing techniques. Previous research has revealed the colloidal nature of perovskite precursor inks and established a relationship between the colloid distribution and the film optoelectronic quality. Yet, the identity of colloids remains unknown, hindering our understanding of their role in perovskite crystallization. Here, we investigate precursor inks of the prototypical methylammonium lead triiodide perovskite using cryo-electron microscopy (cryo-EM) and show, for the first time, that the colloids are neither amorphous nor undissolved lead iodide, as previously speculated, but are a crystalline, non-perovskite material. We identify this as a perovskite precursor phase and discuss this as a potential means to understanding the role of chloride in processing. This work establishes cryo-EM as a viable technique to elucidate the nature of colloids in perovskite inks, a vital step toward a fundamental understanding of thin-film crystallization.