Exploring a Stable and Dense 3D Lead Chloride Hybrid with Emission of Self-Trapped Excitons toward X-Ray Scintillation

The exceptional photophysical properties of 3D organic–inorganic lead halide hybrids (OILHs) endow their significant potential for usage in optoelectronics, which has sparked intense research on novel 3D OILHs and associated applications. However, constructing new 3D OILHs based on large organic cations suffers from tough challenges due to the limitation of the Goldschmidt tolerance factor rule, let alone further explorations of their practical applications. Herein, a brand-new 3D lead chloride hybrid, (1MPZ)Pb₄Cl₁₀·H₂O ( 1 , 1MPZ = 1-methylpiperazine) is reported, featuring a dense 3D lead chloride framework made of the corner-, edge-, and face-shared lead chloride polyhedra. 1 presents a broadband white light emission with a large Stokes shift and a nanosecond photoluminescence lifetime, which originates from radiative recombination of self-trapped excitons (STEs) induced by the highly distorted structure. Such a reabsorption-free and fast-decayed STEs emission coupling with the dense 3D architecture further enables 1 with effective X-ray scintillation with good sensitivity. Impressively, 1 also shows superior environmental and radiation stability. This study provides a new 3D OILH with appealing luminescence, not only expanding the 3D OILH family but also inspiring the exploitation of their optoelectronic applications.     

Upconverting Nanoparticle Relays for Resonance Energy Transfer Networks

Networks of fluorophores arranged at the nanoscale can perform basic computation using resonance energy transfer (RET) to transport and manipulate information in the form of excitons. As excitons travel through RET circuits, they are red‐shifted due to vibrational energy loss at each transfer event. This loss prohibits RET circuits from being cascaded to form larger, more computationally complex systems. To address this issue, a nanoassembly capable of converting three or more low energy excitons into a single high energy exciton is designed and fabricated. Deemed the RET relay, this device uses upconverting nanoparticles to achieve anti‐Stokes energy transfer from near‐infrared excited fluorophores to visibly excited fluorophores. In this work, the relay is explored by first breaking it into its halves. Each fluorophore's ability to donate energy to or from the nanoparticle is characterized by a series of photoluminescence experiments. The adsorption of these fluorophores to the particle is modeled as a Langmuir process, revealing the fractional occupancy of each dye that optimizes energy transfer. A fully functional relay is then demonstrated by exciting the near‐infrared dye and extracting the visible dye's fluorescence. Lastly, the performance of the entire construct is optimized over a small sampling of assembly reaction coordinates.     

Hanle Effect of Charged and Neutral Excitons in Quantum Wells

We measured magnetic field depolarization of charged and neutral exciton cw photoluminescence of a system consisting of two coupled quantum wells with a residual concentration of holes. Using the Hanle expression, we obtained exciton lifetime and electron spin relaxation time, which are in agreement with results of time-resolved experiments. We suppose that the tunneling takes place via an emission of an LO phonon and we find this process spin conserving.     

Charge‐recombination processes in oligomer‐ and polymer‐based light‐emitting diodes: A molecular picture

Abstract— An overview of our recent work on the mechanisms of singlet and triplet exciton formation in electroluminescent π‐conjugated materials will be presented. According to simple spin statistics, only one‐fourth of the excitons are formed as singlets. However, deviations from that statistics can occur if the initially formed triplet charge‐transfer (CT) excited states are amenable to intersystem crossing or dissociation. Although the electronic couplings between the CT states and the neutral exciton states are expected to be largest for the lowest singlet and triplet excitons (S¹ and T¹, respectively), the possibility for direct recombination into T¹ is always very small due to the large exchange energy. In small molecules, spin statistics is expected to be observed because both singlet and triplet exciton formations proceed via higher‐lying S_n/T_n states with similar electronic couplings and fast formation rates. In extended conjugated chains, however, that the ¹CT → S¹ pathway is faster while the ³CT → ^Tn channels become much slower, opening the route to intersystem crossing or dissociation among the ³CT states.     

Triplet-induced degradation: An important consideration in the design of solution-processed hole injection materials for organic light-emitting devices

Solution-processed organic light-emitting devices (OLEDs) still require improvements in their operational lifetime in order for them to become commercially viable. One factor that limits the lifetime of these devices is the instability of the hole injection layer (HIL). Therefore, understanding its degradation mechanism is crucial for the development of more stable solution-processed OLEDs. In this work, we use an archetypal fluorescent OLED in conjunction with an experimental solution-processed HIL in order to elucidate the degradation mechanism in these HILs. Our studies show that degradation is caused by triplet excitons. This new triplet-induced hole injection degradation is expected to be a common phenomenon in OLEDs, and therefore should have important implications for the design of stable HILs.     

The Kondo Effect of Surface Excitons

A recombination radiation line of real excitons in dense two-dimensional electron gas at the [100] silicon surface is observed in luminescence spectra of metal-oxide-semiconductor (MOS) structures. A new effect of anisotropic paramagnetic reduction of the luminescence line indicates a strong influence of the Kondo correlations on electron paramagnetism of the excitons.     

Sharp line emission spectra from GaAs FET like structures

Magneto-optical analysis of prominent photoluminescence lines from GaAs FET structures has been performed. Fifteen samples were investigated. Each consisted of a sulfur doped active layer on a high resistivity buffer layer (both epitaxially grown films) on a chromium doped GaAs substrate. The active layers were generally 2μm thick or less, except for two thicker layers (4 and 5μm) grown especially for this study. Buffer layer thicknesses ranged from 1.5 to 26μm. A model based on carrier diffusion through the active layer has been used to interpret the spectra as originating from the active-buffer interface region. All spectra contain strong-evidence of two donorbound exciton complexes associated with sulfur (1.51417eV) and silicon (1.51412eV). Other sharp spectral features included up to six lines associated with more complicated complexes. Linear Zeeman and quadratic diamagnetic behavior of the lines in applied magnetic fields are discussed. Supported under AF Contract F33615-77-C-5003 Supported under AF Contract F33615-76-C-1207     

Limitations and Perspectives on Triplet‐Material‐Based Organic Photovoltaic Devices

Organic photovoltaic cells (OPVs) have attracted broad attention and become a very energetic field after the emergence of nonfullerene acceptors. Long‐lifetime triplet excitons are expected to be good candidates for efficiently harvesting a photocurrent. Parallel with the development of OPVs based on singlet materials (S‐OPVs), the potential of triplet materials as photoactive layers has been explored. However, so far, OPVs employing triplet materials in a bulk heterojunction have not exhibited better performance than S‐OPVs. Here, the recent progress of representative OPVs based on triplet materials (T‐OPVs) is briefly summarized. Based on that, the performance limitations of T‐OPVs are analyzed. The shortage of desired triplet materials with favorable optoelectronic properties for OPVs, the tradeoff between long lifetime and high binding energy of triplet excitons, as well as the low charge mobility in most triplet materials are crucial issues restraining the efficiencies of T‐OPVs. To overcome these limitations, first, novel materials with desired optoelectronic properties are urgently demanded; second, systematic investigation on the contribution and dynamics of triplet excitons in T‐OPVs is necessary; third, close multidisciplinary collaboration is required, as proved by the development of S‐OPVs.