Terahertz photonics

Abstract:

Chalcogenide-based semiconductors have recently emerged as promising candidates for optoelectronic devices, benefiting from their low-cost, solution processability, excellent stability and tunable optoelectronic properties. However, the understanding of their fundamental optoelectronic properties is far behind the success of device performance and starts to limit their further development. To fill this gap, we conduct a comparative study of chalcogenide absorbers across a wide material space, in order to assess their suitability for different types of applications. We utilize optical-pump terahertz-probe spectroscopy and time-resolved microwave conductivity techniques to fully analyze their charge-carrier dynamics. We show that antimony-based chalcogenide thin films exhibit relatively low charge-carrier mobilities and short lifetimes, compared with bismuth-based chalcogenides. In particular, AgBiS2 thin films possess the highest mobility, and Sb2S3 thin films have less energetic disorder, which are beneficial for photovoltaic devices. On the contrary, Bi2S3 showed ultralong carrier lifetime and high photoconductive gain, which is beneficial for designing photoconductors.

Abstract:

High temperature post-deposition annealing of hybrid lead halide perovskite thin films—typically lasting at least 10 min—dramatically limits the maximum roll-to-roll coating speed, which determines solar module manufacturing costs. While several approaches for “annealing-free” perovskite solar cells (PSCs) have been demonstrated, many are of limited feasibility for scalable fabrication. Here, this work has solvent-engineered a high vapor pressure solvent mixture of 2-methoxy ethanol and tetrahydrofuran to deposit highly crystalline perovskite thin-films at room temperature using gas-quenching to remove the volatile solvents. Using this approach, this work demonstrates p-i-n devices with an annealing-free MAPbI₃ perovskite layer achieving stabilized power conversion efficiencies (PCEs) of up to 18.0%, compared to 18.4% for devices containing an annealed perovskite layer. This work then explores the deposition of self-assembled molecules as the hole-transporting layer without annealing. This work finally combines the methods to create fully annealing-free devices having stabilized PCEs of up to 17.1%. This represents the state-of-the-art for annealing-free fabrication of PSCs with a process fully compatible with roll-to-roll manufacture.

Abstract:

AbstractSpintronic metal thin films excited by femtosecond laser pulses have recently emerged as excellent broadband sources of terahertz (THz) radiation. Unfortunately, these emitters transmit a significant proportion of the incident excitation laser, which causes two issues: first, the transmitted light can interfere with measurements and so must be attenuated; second, the transmitted light is effectively wasted as it does not drive further THz generation. Here, we address both issues with the inclusion of a high-reflectivity (HR) coating made from alternating layers of SiO2 and Ta2O5. Emitters with the HR coating transmit less than 0.1% of the incident excitation pulse. Additionally, we find that the HR coating increases the peak THz signal by roughly 35%, whereas alternative attenuating elements, such as cellulose nitrate films, reduce the THz signal. To further improve the emission, we study the inclusion of an anti-reflective coating to the HR-coated emitters and find the peak THz signal is enhanced by a further 4%.