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Organic–inorganic perovskites are highly promising solar cell materials with laboratory-based power conversion efficiencies already matching those of established thin film technologies. Their exceptional photovoltaic performance is in part attributed to the presence of efficient radiative recombination pathways, thereby opening up the possibility of efficient light-emitting devices. Here, we demonstrate optically pumped amplified spontaneous emission (ASE) at 780 nm from a 50 nm-thick film of CH3NH3PbI3 perovskite that is sandwiched within a cavity composed of a thin-film (~7 µm) cholesteric liquid crystal (CLC) reflector and a metal back-reflector. The threshold fluence for ASE in the perovskite film is reduced by at least two orders of magnitude in the presence of the CLC reflector, which results in a factor of two reduction in threshold fluence compared to previous reports. We consider this to be due to improved coupling of the oblique and out-of-plane modes that are reflected into the bulk in addition to any contributions from cavity modes. Furthermore, we also demonstrate enhanced ASE on flexible reflectors and discuss how improvements in the quality factor and reflectivity of the CLC layers could lead to single-mode lasing using CLC reflectors. Our work opens up the possibility of fabricating widely wavelength-tunable "mirror-less" single-mode lasers on flexible substrates, which could find use in applications such as flexible displays and friend or foe identification.

We have investigated the extent to which delocalization of the ground-state and excited-state wave functions of a p-conjugated molecule affects the excitation energy transfer (EET) between such molecules. Using femtosecond photoluminescence spectroscopy, we experimentally monitored the EET along well-defined supramolecular chains of extended conjugated molecules. Comparison with Monte Carlo simulations reveals that only a model incorporating a localized emitter and delocalized absorber wave function accurately reproduces these data. Our findings demonstrate that self-localization of the initially excited state, following fast relaxation of the nuclear lattice, has a significant impact on the EET dynamics in molecular assemblies.

We present time-resolved photoluminescence measurements on two series of oligo-p-phenylenevinylene materials that self-assemble into supramolecular nanostructures with thermotropic reversibility in dodecane. One set of derivatives form chiral helical stacks, while the second set form less organized "frustrated" stacks. Here we study the effects of supramolecular organization on the resonance energy transfer rates. We measure these rates in nanoassemblies formed with mixed blends of oligomers and compare them with the rates predicted by Förster theory. Our results and analysis show that control of supramolecular order in the nanometer length scale has a dominant effect on the efficiency and dimensionality of resonance energy transfer.

We present time-resolved photoluminescence measurements on two series of oligo-p-phenylenevinylene (OPV) materials that are functionalized with quadruple hydrogen-bonding groups. These form supramolecular assemblies with thermotropic reversibility. The morphology of the assemblies depends on the way that the oligomers are functionalized; monofunctionalized OPVs (MOPVs) form chiral, helical stacks while bifunctionalized OPVs (BOPVs) form less organized structures. These are therefore model systems to investigate the effects of supramolecular assembly, the effects of morphology, and the dependence of oligomer length on the radiative and nonradiative rates of -conjugated materials. The purpose of this work is to use MOPV and BOPV derivatives as model systems to study the effect of intermolecular interactions on the molecular photophysics by comparing optical properties in the dissolved phase and the supramolecular assemblies. A simple photophysical analysis allows us to extract the intrinsic radiative and nonradiative decay rates and to unravel the consequences of interchromophore coupling with unprecedented detail. We find that interchromophore coupling strongly reduces both radiative and intrinsic nonradiative rates and that the effect is more pronounced in short oligomers.

Exciton dynamics in chiral helixes of an oligo-p-phenylenevinylene deriv. monofunctionalized with a ureido-s-triazine (MOPV) were studied. The latter functional group leads to oligomer dimerization by hydrogen bonding in dodecane soln., resulting in reversible supramol. assembly by solvophobic and p-p interactions. MOPV is a model system to study exciton transfer dynamics as it is possible to study the effect of intermol. interactions by comparing optical properties in the dissolved phase (above the transition temp.) and the supramol. assemblies. The model allows to ext. microscopic informations such as the av. exciton hopping annihilation rates. It is believed that this model also allows the quant. description of the exciton dynamics on supramol. architectures from subpicosecond to nanosecond timescales.

Femtosecond transient absorption measurements were carried out on p-conjugated supramol. assemblies in a high-pump-fluence regime. Oligo(p-phenylenevinylene) monofunctionalized with ureido-s-triazine (MOPV) self-assembles into chiral stacks in dodecane soln. below 75 Deg at a concn. of 4 * 10-4 M. Exciton bimol. annihilation was obsd. in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of 11Bu-exciton spectral signatures and by the sublinear fluence dependence of time- and wavelength-integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV soln. where the phase equil. is shifted significantly away from supramol. assembly, slightly below the transition temp. A mesoscopic rate-equation model is applied to ext. the bimol. annihilation rate const. from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimol. annihilation rate is high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramol. nanostructures have electronic properties that are intermediate between mol. aggregates and polymeric semiconductors

The ultrafast dynamics of photoexcitations were studied in chiral stacks of conjugated p-phenylene vinylene mols. functionalized with hydrogen-bonding groups. In soln., p-p interactions between the mols. give rise to fast exciton diffusion along the stacking axis of the assemblies. The chiral nature of the assemblies is found to cause a rotation of the dipole moment of excitons propagating along the stacks as indicated by time-resolved measurements of the photoluminescence polarization anisotropy. The obsd. exciton diffusion and energy relaxation dynamics in the mol. stacks are similar to those found in conjugated polymer films. Moreover, through changes in the temp. of the surrounding solvent, the stacks can be dissocd. reversibly as shown by a marked redn. in the diffusivity of excitons. [on SciFinder (R)]