Mechanisms of Spontaneous and Amplified Spontaneous Emission in CH3 NH3 Pb I3 Perovskite Thin Films Integrated in an Optical Waveguide

In this paper, the physical mechanisms responsible for optical gain in CH3NH3PbI3 (MAPI) polycrystalline thin films are investigated experimentally and theoretically. Waveguide structures composed by a MAPI film embedded in between PMMA and silica layers are used as an efficient geometry to confine emitted light in MAPI films and minimize the energy threshold for amplified spontaneous emission (ASE). We show that photogenerated exciton density at the ASE threshold is as low as (2.4-12)×1016cm-3, which is below the Mott transition density reported for this material and the threshold transparency condition deduced with the free-carrier model. Such a low threshold indicates that the formation of excitons plays an important role in the generation of optical gain in MAPI films. The rate-equation model including gain is incorporated into a beam-propagation algorithm to describe waveguided spontaneous emission and ASE in MAPI films, while using the optical parameters experimentally determined in this work. This model is a useful tool to design active photonic devices based on MAPI and other metal-halide semiconductors

Comparative study of the photophysical properties of nonplanar tetraphenylporphyrin and octaethylporphyrin diacids

The photophysical properties of the lowest excited singlet states, S1(,*), of two porphyrin diacids have been investigated. The diacids are H4TPP2 and H4OEP2 , the diprotonated forms of free base tetraphenylporphyrin (H2TPP) and octaethylporphyrin (H2OEP), respectively. Both diacids exhibit perturbed static and dynamic characteristics relative to the parent neutral complexes in solution at room temperature. These properties include enhanced yields of S1 S0 radiationless deactivation (internal conversion), which increase from ~0.1 for H2TPP and H2OEP to 0.4 for H4OEP2 and 0.6 for H4TPP2 . The fluorescence lifetimes of both diacids are strongly temperature dependent, with an activation enthalpy of ~1400 cm-1 for S1-state deactivation. The enhanced nonradiative decays and many other photophysical consequences of diacid formation are attributed primarily to nonplanar macrocycle distortions. Both H4TPP2 and H4OEP2 have been shown previously by X-ray crystallography to adopt saddle-shaped conformations, and the magnitudes of the perturbed properties for the two diacids in solution correlate with the extent of the deviations from planarity in the crystals. A model is proposed to explain the nonradiative decay behavior of the porphyrin diacids that is relevant to nonplanar porphyrins in general. The model includes the existence of decay funnels on the S1(,*)-state energy surface that are separated from the equilibrium conformation and other minima by activation barriers. It is suggested that these funnels involve configurations at which the potential-energy surfaces of the ground and excited states approach more closely than at the equilibrium excited-state structure(s) from which steady-state fluorescence occurs. Possible contributions to the relevant nuclear coordinates are discussed

The photophysical and metal coordination properties of the N-CH3 substituted porphyrins: H(N-CH3)TPP vs H(CH3)OEP

The effect of N-methyl substitution on photophysical and metal coordination properties of the respective derivatives of octaethylporphyrin (H2OEP) and tetraphenylporphyrin (H2TPP) was studied by means of steady-state and time-resolved optical spectroscopies combined with semi-empirical quantum-chemical calculations and coordination chemistry methods. In case of H2TPP, the insertion of the methyl substituent into the center of the porphyrin macrocycle leads to noticeable nonplanar distortions of the molecule and is accompanied by changes of its photophysical and physicochemical properties towards those manifested by "classical" nonplanar porphyrins. Contrasting to that, N-methyl substituted H2OEP does not undergo significant nonplanar distortions and possesses photophysical characteristics mainly similar to unsubstituted H2OEP, except for the long-wavelength shift of the absorption and emission bands. The Zn coordination/Zn complex dissociation and macrocycle thermal stability parameters were also determined for both N-methyl substituted and parent unsubstituted macrocycles, which correlate well with a higher degree of nonplanarity of the N-methyl substituted H2TPP as compared to H2OEP. Basing on the results of this study the conclusion postulated is that N-methyl substitution has a different effect on the photophysical and coordination properties of H2TPP vs. H2OER Copyright (c) 2005 Society of Porphyrins & Phthalocyanines