Transient and CW spectroscopies unravel giant Rashba splitting in 2D hybrid perovskite multilayers.

We studied the femtosecond dynamics of photoexcitations in films containing semiconducting and metallic single-walled carbon nanotubes (SWNTs), using various pump-probe wavelengths and intensities. We found that confined excitons and charge carriers with subpicosecond dynamics dominate the ultrafast response in semiconducting and metallic SWNTs, respectively. Surprisingly, we also found from the exciton excited state absorption bands and multiphoton absorption resonances in the semiconducting nanotubes that transitions between subbands are allowed; this unravels the important role of electron-electron interaction in SWNT optics.

We used a variety of optoelectronic techniques such as broadband fs transient and cw photomodulation spectroscopies, electroabsorption, and short-circuit photocurrent in bulk heterojunctions organic solar cells for studying the photophysics in pi-conjugated polymer-fullerene blends with below-gap excitation. In contrast to the traditional view, we found that below-gap excitation, which is incapable of generating intrachain excitons, nevertheless efficiently generates polarons on the polymer chains and fullerene molecules. The polaron action spectrum extends deep inside the gap as a result of a charge-transfer complex state formed between the polymer chain and fullerene molecule. With appropriate design engineering the long-lived polarons might be harvested in solar cell devices.

1617 wileyonlinelibrary.com Studying the room temperature broadband ultrafast transient response of photoexcitations in three perovskite fi lms, namely MAPbI 3 , MAPbI 1.1 Br 1.9, and MAPbI 3− x Cl x (MA = CH 3 NH 3 ), allowed unravelling the branching ratio between photogenerated carriers and excitons, a key factor for optoelectronic applications of perovskites. An instantaneously generated mid-IR photoinduced absorption (PA) band, PA 1 is observed in all three perovskites, as well as a strong derivative-like band of photoinduced bleaching (PB) and PA (PA 2 ) close to the corresponding absorption band edge. From the distinguished different decay dynamics of the PA bands in MAPbI 3 , PA 1 is interpreted as due to the exciton transition, whereas PA 2 and PB are due to band-fi lling effect caused by the photocarriers. In contrast, all bands in MAPbI 1.1 Br 1.9 and MAPbI 3− x Cl x share the same dynamics and are therefore due to the same species, namely photogenerated excitons. The transient photoinduced polarization memory (POM) for both excitons and photocarriers as well as the steady-state photoluminescence (PL) emission are observed in MAPbI 3 , but not in MAPbI 1.1 Br 1.9 and MAPbI 3− x Cl x because they possess cubic symmetry at room temperature. The estimated long excitons diffusion length (≈150 nm) in MAPbI 3 opens up the possibility of photocarriers generation at interfaces and grain boundaries even when the exciton binding energy is large compare to k B T .