Terahertz photonics

Abstract:

Thermal evaporation is an industrially compatible technique for fabricating metal halide perovskite thin films, without the requirement for hazardous solvents. It offers precise control over film thickness and is a good candidate for large-scale production of commercial optoelectronic metal halide perovskite devices, such as solar cells. The use of additives to passivate electronic defects in solution-processed metal halide perovskite has led to dramatic increases in device performance. However, there are a few reports of vapor-deposited films with coevaporated passivating agents. Triphenylphosphine oxide (TPPO) has been used as an effective surface passivating agent in solution-processed metal halide perovskite films. It is a promising candidate passivating agent for coevaporation, where it is beginning to be used with encouraging results. However, here we report that triphenylphosphine oxide is incompatible with thermal deposition in the same deposition chamber. Such TPPO remnants are found to result in severe suppression of the perovskite phase, long-range crystalline ordering, and optical absorption of lead halide perovskite films subsequently deposited in the same chamber. TPPO contamination persists even through repeated baking cycles, with the reduction of the contaminant to acceptable levels requiring vacuum chamber dismantling and manual cleaning. We conclude that TPPO should not be coevaporated in order to prevent the contamination of future batches.

Abstract:

Impurities in formamidinium iodide affect perovskite thin films differently depending on fabrication route. By comparing solution processing with thermal vapour deposition, this study reveals distinct mechanisms in which impurities influence nucleation, growth, and final film quality and stability. While metal halide perovskites have yielded remarkable power conversion efficiencies in photovoltaic applications, uncertainty concerning their long-term stability remains a significant barrier to widespread deployment. Previous studies have demonstrated that trace impurities present in perovskite precursor materials can influence the crystallisation dynamics of perovskite thin-films and hence, affect crystal structure, film morphology and optoelectronic properties. However, the nature of the impurities in formamidinium iodide (FAI) and their effect(s) on film quality and device performance remain underexplored. In this work, we carry out a detailed analysis of the impurities present in commonly used commercial FAI sources, and probe their impact on the composition, structure, and optoelectronic quality of the resulting perovskite thin-films and devices. We find that while some FAI impurities can improve the optoelectronic properties of solution-processed perovskite thin-films, in vapour-processed films, their presence alters the sublimation behaviour of FAI, favouring irreversible degradation pathways which lead to the formation of sym -triazine. While sym -triazine does not directly incorporate into the perovskite films, the impurity-driven variation in sublimation behaviour results in films which can deviate from the target stoichiometry, even under otherwise optimised conditions; and thus, do not fully convert into the desired photoactive phase, eventually causing poor material stability. Our results highlight the importance of understanding and controlling impurity concentrations in perovskite precursor materials as a route to enhancing both performance and process reproducibility in perovskite solar cells.

Abstract:

Inorganic cesium‐based metal halide perovskite (MHP) semiconductors have great potential as active layers in optoelectronic devices, such as perovskite light‐emitting diodes (PeLEDs) and perovskite lasers. However, precise control of crystal type, quality, and thickness is required to create high‐performance and reproducible devices. Vapor‐phase vacuum deposition enables fabrication of MHP thin films and devices with excellent uniformity and control over layer thickness, although a full understanding of crystal growth mechanisms and products has proved elusive. Here, conditions of vapor co‐deposition of CsBr and PbBr are related with the optical performance and atomic microstructure of resulting CsPbBr3 thin films. It is found that the structure is predominantly photoactive γ‐CsPbBr3 over a wide range of conditions, but the presence of impurity phases and Ruddlesden–Popper (RP) planar defects both degrade optical performance as quantified through measured amplified spontaneous emission (ASE) thresholds. Furthermore, the atomic structure of the dominant impurity phases is resolved: CsPb2Br5 and Cs4PbBr6. It is revealed that a small nominal excess of CsBr‐precursor flux during co‐evaporation can significantly enhance the nucleation of thin films, resulting in well‐defined grains greater than 500 nm in size and the relative suppression of RP planar defects. Such films exhibit intensified photoluminescence (PL) emission and a reduced ASE threshold of 30.9 µJ cm−2.

Abstract:

Tin-halide perovskites currently offer the best photovoltaic performance of lead-free metal-halide semiconductors. However, their transport properties are mostly dominated by holes, owing to ubiquitous self-doping. Here we demonstrate a noncontact, optical spectroscopic method to determine the effective mass of the dominant hole species in FASnI3, by investigating a series of thin films with hole densities finely tuned through either SnF2 additive concentration or controlled exposure to air. We accurately determine the plasma frequency from mid-infrared reflectance spectra by modeling changes in the vibrational response of the FA cation as the plasma edge shifts through the molecular resonance. Our approach yields a hole effective mass of 0.28m e for FASnI3 and demonstrates parabolicity within ∼100 meV of the valence band edge. An absence of Fano contributions further highlights insignificant coupling between the hole plasma and FA cation. Overall, this approach enables noncontact screening of thin-film materials for optimized charge-carrier transport properties.