Terahertz photonics

Abstract:

The copper-silver-bismuth-iodide compound Cu₂AgBiI₆ has emerged as a promising lead-free and environmentally friendly alternative to wide-bandgap lead-halide perovskites for applications in multijunction solar cells. Despite its promising optoelectronic properties, the efficiency of Cu₂AgBiI₆ is still severely limited by poor charge collection. Here, we investigate the impact of commonly used charge transport layers (CTLs), including poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), CuI, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and SnO₂, on the structural and optoelectronic properties of coevaporated Cu₂AgBiI₆ thin films. We reveal that while organic transport layers, such as PTAA and PCBM, form a relatively benign interface, inorganic transport layers, such as CuI and SnO₂, induce the formation of unintended impurity phases within the CuI-AgI-BiI₃ solid solution space, significantly influencing structural and optoelectronic properties. We demonstrate that identification of these impurity phases requires careful cross-validation combining absorption, X-ray diffraction and THz photoconductivity spectroscopy because their structural and optoelectronic properties are very similar to those of Cu₂AgBiI₆. Our findings highlight the critical role of CTLs in determining the structural and optoelectronic properties of coevaporated copper-silver-bismuth-iodide thin films and underscore the need for advanced interface engineering to optimize device efficiency and reproducibility.

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:

FAPbI₃ has emerged as a promising semiconductor for photovoltaic applications offering a suitable bandgap for single-junction cells and high chemical stability. However, device efficiency is negatively affected by intrinsic quantum confinement (QC) effects that manifest as additional peaks in the absorption spectra. Here, we show that aerosol-assisted crystallization is an effective method to improve crystallinity and suppresses regions exhibiting QC in FAPbI₃. We demonstrate that films with minimized QC effects exhibit markedly enhanced optoelectronic properties, such as higher charge-carrier mobilities and recombination lifetimes. Films crystallized under an aerosol solvent flow of either a mixture of <i>N</i>,<i>N</i>-dimethylformamide and dimethyl sulfoxide or methylammonium thiocyanate vapor displayed reduced charge-carrier recombination losses and improved diffusion lengths compared to those of thermally annealed control films. Our study indicates clear correlations between suppression of QC features in absorption spectra with optimization of crystallinity and mitigation of internal strain, highlighting pathways toward high-performance solar cells.