Vibrational Relaxation Dynamics of a Semiconductor Copper(I) Thiocyanate (CuSCN) Film as a Hole-Transporting Layer

Abstract

The semiconductor CuSCN film, which is typically used as the hole-transporting layer (HTL) in solar cell studies, has been investigated by Fourier transform infrared (FTIR) spectroscopy and ultrafast transient infrared (IR) spectroscopy. A sharp peak at 2175 cm −1 corresponding to the CN vibrational stretching mode in CuSCN was observed, and the peak frequency remained unchanged by varying the thickness of the CuSCN thin film. Vibrational relaxation measurements showed that the 0−1 and 1−2 transitions of CN stretching can be observed at 2175 and 2140 cm −1 , respectively. The heat-induced absorption and bleaching peaks (2167 and 2175 cm −1 ) can be clearly seen at a waiting time of 40 ps. The vibrational relaxation of the CN stretching mode determined from the 1−2 transition exhibited a biexponential decay with time constants of 7.4 ± 0.5 (90%) and 158 ± 50 ps (10%). Importantly, the abnormal anisotropy decay of the CN stretching mode in the CuSCN thin film was also observed for the first time. A detailed analysis showed that the distinct anisotropy decay curve could be described using a triexponential decay function, which was explained by three different processes: resonance energy transfer (∼8 ps), a thermalization process (∼40 ps), and molecular rotation (∼150 ps). The time scale of the thermalization process caused by the vibrational relaxation in CuSCN is at a time scale of 40 ps, which is important for us to understand the thermally activated charge-transport property of the CuSCN film employed as the HTL. Further UV pump−IR probe measurement revealed that the carrier scattering and relaxation processes in the CuSCN film are strongly associated with the vibrational excitation and relaxation dynamics of the CN stretching mode. It is expected that the fundamental understanding of the vibrational relaxation dynamics of the CuSCN thin film should provide helpful insight to elucidate its role as the HTL in solar cell studies at the molecular level.

Specific Host–Guest Interactions in the Crown Ether Complexes with K+ and NH4+ Revealed from the Vibrational Relaxation Dynamics of the Counteranion

“…The specific interaction between the cations and the crown ethers can be understood from the viewpoint of the vibrational relaxation dynamics of the SCN – anions. As previous studies demonstrated, the vibrational population decay of the SCN – probe is sensitive to the local environments in the condensed phases. ,, Also, the orientational dynamics of the SCN – would be expected to be different, which would provide a further molecular-level understanding of the specific host–guest interactions in the complexes of crown ethers.…”

“…The ultrafast IR spectroscopy setup has been described in our previous studies. − Here, we only give the simplest description. The ultrafast IR spectroscopic measurement was based on the pump–probe scheme.…”

Understanding the Microscopic Structure of a “Water-in-Salt” Lithium Ion Battery Electrolyte Probed with Ultrafast IR Spectroscopy

“…The ultrafast 2D IR spectroscopy experimental setup had been described in our previous reports. , The scheme of the 2D IR spectroscopy was realized by using the pump–probe design. A picosecond (ps) amplifier (Spectra Physics) with output frequency at 800 nm and a femtosecond (fs) amplifier (Spectra Physics) with output frequency at 800 nm were independently operated and synchronized with the same seed pulse generated from a 78 MHz Ti-sapphire oscillator (Spectra Physics).…”

Time-resolved infrared absorption spectroscopy applied to photoinduced reactions: how and why

“…The nature of primary photo-excitations in organic semiconductors, particularly in conjugated polymers, has been the subject of an active debate in the scientific community for more than 3 decades [176][177][178][179]. The mechanisms underlying photogeneration of charge carriers as well as thermalization promptly after photoexcitation are indicative of the strength of electron-electron interactions relative to the bandwidth (as manifested by the exciton binding energy), while carriers transport, trapping, and recombination processes are intimately related to the degree of order, purity, and morphology of the materials.…”