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

Two-dimensional (2D) organic-inorganic hybrid perovskite multiple quantum wells that consist of multilayers of alternate organic and inorganic layers exhibit large exciton binding energies of order of 0.3 eV due to the dielectric confinement between the inorganic and organic layers. We have investigated the exciton characteristics of 2D butylammonium lead iodide, (CHNH)PbI using photoluminescence and UV-vis absorption in the temperature range of 10 K to 300 K, and electroabsorption spectroscopy. The evolution of an additional absorption/emission at low temperature indicates that this compound undergoes a phase transition at ≈250 K. We found that the electroabsorption spectrum of each structural phase contains contributions from both quantum confined exciton Stark effect and Franz-Keldysh oscillation of the continuum band, from which we could determine more accurately the 1s exciton, continuum band edge, and the exciton binding energy.

Several methods of harvesting singlet excitons via delayed fl uorescence have been introduced in OLED so far. These methods include up-conversion to singlet excitons by triplet-triplet annihilation (TTA) [ 3,4 ] or triplet fusion in materials that show a strong singlet fi ssion. [ 5 ] A different approach for enhancing the singlet emission that involves triplet excitons was introduced recently, whereby the triplet excitons may undergo reverse intersystem crossing (RISC) to singlet excitons and consequently give rise to thermally activated delayed fl uorescence (TADF). [6][7][8][9][10][11][12] This occurs in compounds with small electron exchange energy, and thus small singlet-triplet energy splitting, Δ E ST that enables triplet excitons to undergo thermally activated RISC to the singlet manifold. [ 13 ] A fi rm indication for TADFrelated emission in compounds that possess RISC is that the EL in these OLED is thermally activated, with activation energy E act ≈ Δ E ST Ͻ Ͻ 0.7 eV (which is Δ E ST in traditional organic semiconductors). During the last few years there has been a large interest in magnetic fi eld effect (MFE) in conjugated organic compounds, mainly because of the possibility to enhance the electroluminescence effi ciency, which was dubbed magneto-EL (MEL). [ 11,[14][15][16][17][18][19][20][21] In this effect, the magnetic fi eld changes the exchange rate between PP singlet (PP S ) and triplet (PP T ), which can be detected through the induced change in the EL emission intensity (MEL) or the current density (MC) in the device. This occurs if the PP S and PP T recombination rates ( R S , R T ) and/or dissociation rates ( d S , d T ) differ from each other. [ 17,22 ] So far the MEL maximum value, MEL max at room temperature (RT) has been less than ≈20% in OLEDs.In conventional OLEDs, spin mixing occurs within the PP states rather than at the exciton levels because the electron-hole orbitals strongly overlap in the latter species leading to large exchange energy, J , that consequently causes large energy gap, Δ E ST (=2 J ) between singlet and triplet states. In contrast, materials showing RISC may allow spin-mixing among the PP spin levels and in the exciton levels because Δ E ST is small. [ 23 ] In this case, possible spin-mixing mechanism may be the hyperfi ne interaction [ 24,25 ] and/or the Δ g mechanism, [ 26 ] where the difference, Δ g in the g -values of positive and negative carrier in the pair may promote intersystem crossing. The obtained full width Reverse intersystem crossing (RISC) from triplet to singlet states has been recently introduced to photophysics of organic chromophores. One type of RISC occurs in donor (D)-acceptor (A) composites that form an exciplex manifold in which the energy difference, Δ E ST between the lowest singlet (S 1 ) and triplet (T 1 ) levels of the exciplex is small (<100 meV) thus allowing RISC at room temperature. This adds a delayed component to the photoluminescence emission that is widely known as thermally activated delayed fl uorescence. Here, it is found t...

A new type of organic light‐emitting diode (OLED) has emerged that shows enhanced operational stability and large internal quantum efficiency approaching 100%, which is based on thermally activated delayed fluorescence (TADF) compounds doped with fluorescent emitters. Magneto‐electroluminescence (MEL) in such TADF‐based OLEDs and magneto‐photoluminescence (MPL) in thin films based on donor–acceptor (D–A) exciplexes doped with fluorescent emitters with various concentrations are investigated. It has been found that both MEL and MPL responses are thermally activated with substantially lower activation energy compared to that in the pristine undoped D–A exciplex host blend. In addition, both MPL and MEL steeply decrease with the emitter's concentration. This indicates the existence of a loss mechanism, whereby the triplet charge‐transfer state in the exciplex host blend may directly decay to the lowest, nonemissive triplet state of the fluorescent emitter molecules.

Controlling and modulating terahertz signals is of fundamental importance to allow systems level applications. We demonstrate an innovative approach for controlling the propagation properties of terahertz (THz) radiation, through use of both the excitation optical wavelength (colour) and intensity. We accomplish this using two-dimensional (2D) layered hybrid trihalide perovskites that are deposited onto silicon substrates. The absorption properties of these materials in the visible range can be tuned by changing the number of inorganic atomic layers in between the organic cation layers. Optical absorption in 2D perovskites occurs over a broad spectral range above the bandgap, resulting in free carrier generation, as well as over a narrow spectral range near the bandedge due to exciton formation. We find that only the latter contribution gives rise to photo-induced THz absorption. By patterning multiple 2D perovskites with different optical absorption properties onto a single device, we demonstrate both colour selective modulation and focusing of THz radiation. These findings open new directions for creating active THz devices.