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Charge transfer processes and carrier dynamics at the pentacene - C60 interface
摘要： Heterostructures of pentacene (PEN) and Buckminsterfullerene (C60) are frequently attracting scientific interest as a well-defined small-molecule model-system for the study of internal interfaces between two organic semiconductors. They are prototypical representatives forming a donor-acceptor combination for studies of fundamental optoelectronic processes in organic photovoltaics. Despite their importance in exciton dissociation, the energetics of their interfacial charge-transfer (CT) states and their microscopic excitation dynamics are not yet clarified and still being discussed. Here, we present steady-state and time-resolved photoluminescence measurements on stacked heterostructures between these two materials. All experiments are performed in the visible and near-infrared spectral regions as CT states are expected at energies below the fundamental electronic transitions of the respective bulk materials. A characteristic, interface specific emission at around 1.13-1.17 eV is found, which we attribute to an interfacial CT state. Its excitation-energy dependence reveals the intricate relaxation dynamics of excitons formed in both constituent materials. Moreover, the analysis of the dynamics of the C60 excitons shows that the lifetime of this state is reduced in the presence of an interface with PEN. This quenching is attributed to a long-range interaction, i.e., the relaxation of excitations into the interfacial CT state.
关键词： organic heterostructures,charge-transfer exciton,light harvesting,donor-acceptor pair,Organic thin films,pentacene,fullerene
更新于2025-09-23 15:22:29
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Time-resolved photoluminescence spectral analysis of phonon-assisted DAP and e-A recombination in N+B-doped <i>n</i> -type 4H-SiC epilayers
摘要： It is crucial to clarify the roles of phonon-assisted donor-acceptor pairs (DAPs) and free-to-acceptor (e-A) emissions in n-type 4H-SiC doped with nitrogen (N) and boron (B), where N and B induce the shallow donor and the D center (deep B) acceptor levels, respectively, in order to understand the complicated carrier recombination mechanism, as well as developing fluorescent SiC with a high color rendering index by controlling the ratio of the two overlapped emissions. Here, time-resolved photoluminescence (TRPL) spectral analyses were performed, in which phonon-assisted DAPs and e-A components were individually recognized. The D center-related green luminescence (1.6–2.8 eV) shows a non-exponential decay followed by a very slow decay in TRPL measurements at room temperature (RT). It emerges that most of the DAP emission intensities decay much faster than e-A emissions and contribute to what is initially fast and non-exponential decay at low temperatures, while the slow decay at RT is mainly from e-A emissions. At a much higher temperature, such as 473 K, only the e-A emission remains and the decay transforms from non-exponential to exponential behavior. High-temperature thermal quenching of e-A emissions exhibits different behaviors for samples with differing B doping concentrations. An activation energy of 0.6 eV was estimated from the Arrhenius plot of e-A emission intensity from a B-doped sample, which matches the D center level. This indicates that the hole thermal emission rate is greatly enhanced at a high temperature, which accelerates the decay of e-A emission intensity at high temperatures.
关键词： donor-acceptor pair,time-resolved photoluminescence,silicon carbide,D center,boron doping
更新于2025-09-19 17:15:36
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Detailed analysis of radiative transitions from defects in n-type monocrystalline silicon using temperature- and light intensity-dependent spectral Photoluminescence
摘要： Sub-bandgap luminescence is characteristic of radiative transitions from defects in semiconductors. However, methods to extract defect-identifying parameters from this luminescence are lacking. Here, we present a method to extract these parameters from temperature- and intensity-dependent micro-photoluminescence (μPL) spectra. The initial “coarse” analysis determines the relevant radiative recombination mechanism by fitting the integrated defect PL spectra with phenomenological models for the temperature- and intensity-dependence. The subsequent “detailed” analysis fits the integrated defect PL spectra using rigorous physical models for the defect radiative recombination and spectral line-shape. Finally, defect parameters are extracted, including the defect energy level(s). As we obtain these values directly from the defect luminescence, our method provides higher confidence than more traditional indirect methods, such as those involving band-to-band PL and photoconductance. We demonstrate our method on spatially non-uniform defects with radiative transitions in n-type monocrystalline silicon samples. It is shown that the defect PL originates from the donor-acceptor pair recombination mechanism, involving a shallow acceptor and deeper donor energy level. The acceptor level is extracted from the temperature-dependent spectra, whilst the intensity-dependent spectra give the sum of acceptor and donor energies.
关键词： Radiative transitions,Silicon,Photoluminescence,Defects,Donor-acceptor pair recombination
更新于2025-09-19 17:13:59
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Investigation of Photophysical Properties of Ternary Zn–Ga–S Quantum Dots: Band Gap versus Sub-Band-Gap Excitations and Emissions
摘要： Highly luminescent ternary Zn?Ga?S quantum dots (QDs) were synthesized via a noninjection method by varying Zn/Ga ratios. X-ray diffraction and Raman investigations demonstrate composition-dependent changes with multiple phases including ZnGa2S4, ZnS, and Ga2S3 in all samples. Two distinct excitation pathways were identified from absorption and photoluminescence excitation spectra; among them, one is due to the band-gap transition appearing at around 375 and 395 nm, whereas another one observed nearby 505 nm originates from sub-band-gap defect states. Photoluminescence (PL) spectra of these QDs depict multiple emission noticeable at around 410, 435, 461, and 477 nm arising from crystallographic point defects formed within the band gap. The origin of these defects including zinc interstitials (IZn), zinc vacancies (VZn), sulfur interstitials (IS), sulfur vacancies (VS), and gallium vacancies (VGa) has been discussed in detail by proposing an energy-level diagram. Further, the time-dependent PL decay curve strongly suggests that the tail emission (appear around 477 nm) in these ternary QDs arises due to donor?acceptor pair recombination. This study enables us to understand the PL mechanism in new series of Zn?Ga?S ternary QDs and can be useful for the future utilization of these QDs in photovoltaic and display devices.
关键词： photophysical properties,ternary Zn?Ga?S quantum dots,sub-band-gap defect states,donor?acceptor pair recombination,photoluminescence,band-gap transition
更新于2025-09-16 10:30:52