Dr Qimu Yuan

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:

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:

The perovskite semiconductor, CsPbI3, holds excellent promise for solar cell applications due to its suitable bandgap. However, achieving phase‐stable CsPbI3 solar cells with high power conversion efficiency remains a major challenge. Ruddlesden–Popper (RP) defects have been identified in a range of perovskite semiconductors, including CsPbI3. However, there is limited understanding as to why they form or their impact on stability and photophysical properties. Here, the prevalence of RP defects is increased with increased Cs‐excess in vapor‐deposited CsPbI3 thin films while superior structural stability but inferior photophysical properties are observed. Significantly, using electron microscopy, it is found that the atomic positions at the planar defect are comparable to those of a free surface, revealing their role in phase stabilization. Density functional theory (DFT) calculations reveal the RP planes are electronically benign, however, antisites observed at RP turning points are likely to be malign. Therefore it is proposed that increasing RP planes while reducing RP turning points offers a breakthrough for improving both phase stability and photophysical performance. The formation mechanism revealed here can apply more generally to RP structures in other perovskite systems.

Abstract:

Designing next-generation light-harvesting devices requires a detailed understanding of the transport of photoexcited charge carriers. The record-breaking efficiencies of metal halide perovskite solar cells have been linked to effective charge-carrier diffusion, yet the exact nature of charge-carrier out-of-plane transport remains notoriously difficult to explain. The characteristic spatial inhomogeneity of perovskite films with nanograins and crystallographic disorder calls for the simultaneous and hitherto elusive in situ resolution of the chemical composition, the structural phase and the ultrafast dynamics of the local out-of-plane transport. Here we simultaneously probe the intrinsic out-of-plane charge-carrier diffusion and the nanoscale morphology by pushing depth-sensitive terahertz near-field nanospectroscopy to extreme subcycle timescales. In films of the organic–inorganic metal halide perovskite FA_0.83Cs_0.17Pb(I_1−xCl_x)₃ (where FA is formamidinium), domains of the cubic α-phase are clearly distinguished from the trigonal δ-phase and PbI₂ nano-islands. By analysing deep-subcycle time shifts of the scattered terahertz waveform after photoexcitation, we access the vertical charge-carrier dynamics within single grains. At all of the measured locations, despite topographic irregularities, diffusion is surprisingly homogeneous on the 100 nm scale, although it varies between mesoscopic regions. Linking in situ carrier transport with nanoscale morphology and chemical composition could introduce a paradigm shift for the analysis and optimization of next-generation optoelectronics that are based on nanocrystalline materials.

Abstract:

The efficiency and longevity of metal-halide perovskite solar cells are typically dictated by nonradiative defect-mediated charge recombination. In this work, we demonstrate a vapor-based amino-silane passivation that reduces photovoltage deficits to around 100 millivolts (>90% of the thermodynamic limit) in perovskite solar cells of bandgaps between 1.6 and 1.8 electron volts, which is crucial for tandem applications. A primary-, secondary-, or tertiary-amino–silane alone negatively or barely affected perovskite crystallinity and charge transport, but amino-silanes that incorporate primary and secondary amines yield up to a 60-fold increase in photoluminescence quantum yield and preserve long-range conduction. Amino-silane–treated devices retained 95% power conversion efficiency for more than 1500 hours under full-spectrum sunlight at 85°C and open-circuit conditions in ambient air with a relative humidity of 50 to 60%.