基于激子阻挡层的高效率绿光钙钛矿电致发光二极管

金属卤化物钙钛矿材料由于具有高的光致发光量子产率、高色纯度、带隙可调等杰出的光学性能,被作为发光材料广泛地用于制备钙钛矿电致发光二极管(perovskite light-emitting diodes,PeLEDs).虽然取得了较好的研究进展,但是其效率和稳定性还未达到商业化的要求,还需要进一步提高.为了提高PeLEDs的效率和稳定性,本文使用旋涂法,引入了一种具有宽带隙和较好空穴传输能力的有机小分子材料4,4′-cyclohexylidenebis[N,N-bis(p-tolyl)aniline](TAPC)作为激子阻挡层,获得了效率和寿命都得到提高的全无机PeLEDs.研究表明,PeLEDs效率和寿命得到提高的物理机制主要源于两方面:1)TAPC具有恰当的最高占有分子轨道能级,与PEDOT:PSS的最高占有分子轨道能级和CsPbBr3的价带边形成了阶梯式能级分布,有利于空穴注入和传输;同时TAPC具有较高的最低未占分子轨道能级,能够有效地阻止电子泄漏到阳极端,并能很好地将电子和激子限制在发光层内;2)TAPC层的引入可以避免钙钛矿发光层与强酸性的空穴注入材料Poly(3,4-ethylenedioxythiophene):poly(p-styrene sulfonate)(PEDOT:PSS)的直接接触,进而免除钙钛矿发光层由于与PEDOT:PSS的直接接触所导致的激子淬灭,从而提高了激子的发光辐射复合率.     

Influence of the exciton blocking layer on the stability of layered organic solar cells

The life-time of multi-layer organic solar cells based on the couple donor acceptor copper phthalocyanine/fullerene is studied as a function of the nature of the exciton blocking layer (EBL). It is shown that organic EBL are more efficient than are the inorganic In₂S₃ EBLs. Moreover among the organic EBL, Alq₃ is the most efficient EBL protecting layer. An organic solar cell’s lifetime depends on oxygen- and water-contamination of the organic materials. The solar cell’s degradation may correspond to bulk or interface phenomena. Using equivalent electrical schemes of solar cell diodes, we show that the structure degradation is mainly related to bulk modification. It is proposed that oxygen- and water-diffusion into the C₆₀ induce a large increase in its resistivity and, therefore an increase in the series resistance, which decreases the solar cell efficiency. In the case of In₂S₃ EBLs, the degradation law predicts that with time two different phenomena will be present. The classical oxygen- and water-diffusion into the organic material, during the first hour of air exposure, leads to a modification in the In₂S₃ EBL/organic interface properties.     

Efficiency improvement of red organic light-emitting diodes using a blue phosphorescent exciton blocking layer

Quantum efficiency of red organic light-emitting diodes was improved using a blue phosphorescent emitting layer as an exciton blocking layer. Compared with 8.1% quantum efficiency of standard devices without an exciton blocking layer, high quantum efficiency of 14.1% was obtained using a blue phosphorescent emitting layer between the hole transport layer and the red emitting layer.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.     

Measurement of the dead layer thickness in a p-type point contact germanium detector

A 994 g mass p-type PCGe detector has been deployed during the first phase of the China Dark matter EXperiment, aiming at direct searches for light weakly interacting massive particles. Measuring the thickness of the dead layer of a p-type germanium detector is an issue of major importance since it determines the fiducial mass of the detector. This work reports a method using an uncollimated ~(133)Ba source to determine the dead layer thickness. The experimental design, data analysis and Monte Carlo simulation processes, as well as the statistical and systematic uncertainties are described. A dead layer thickness of 1.02 mm was obtained based on a comparison between the experimental data and the simulated results.     

Improved exciton dissociation efficiency by a carbon-quantum-dot doped workfunction modifying layer in polymer solar cells

In addition to the phase-separated morphology of donor and acceptor, the internal field created by work-function difference between cathode and anode can also influence the exciton dissociation probability. In this study, we have demonstrated enhanced photovoltaic performance by increasing exciton dissociation efficiency. To improve both work-function modification effect and charge transport properties, we have incorporated novel carbon quantum dots (CQD) having NH₂ ligands into the polyethyleneimine (PEI) work-function modifying layer as a dopant. A study of net photocurrent density as a function of effective voltage showed that devices with a CQD-doped PEI layer had a much higher charge separation probability compared to devices with a pristine PEI layer. A Kelvin-probe force microscopy study demonstrated that a CQD-doped PEI layer induced lower work-function of ITO than that of ITO with a pristine PEI, which induced a stronger internal field. This strengthened internal field induced better exciton dissociation efficiency, thereby improving solar cell performance.     

Metal-electrode-free inverted organic photovoltaics using electrospray-deposited PEDOT:PSS and spin-coated HAT-CN exciton blocking layer

Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) films were fabricated using an electrospray deposition (ESD) method. The ESD PEDOT:PSS films exhibited higher PSS content on the surface than spin-coated PEDOT:PSS films, which results in a higher work function. Based on this result, metal-electrode-free inverted organic photovoltaics (OPVs) were fabricated. The ESD PEDOT:PSS was used as the top electrode on the poly(3-hexythiophene-2,5-diyl) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) light-absorbing layer. The power conversion efficiency (PCE) of OPVs was significantly increased with the 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile layer. The improved PCE would be attributed to the suppression of exciton quenching at the P3HT:PCBM and PEDOT:PSS interface.     

The free exciton in KI and RbI: Emissions related to the triplet B exciton

We put in evidence the existence of the triplet exciton B by its emissions as associated with LO phonons in KI and RbI. We explain the possibility of such emissions by the process of interband transition coupling the B exciton with the C polariton (low branch). The exchange interaction and spin-orbit interaction values are calculated.     

The excited states of an intersubband exciton

The energy of intersubband collective spin-and charge-density excitations is calculated for a system of quasi-two-dimensional electrons at certain values of the angular-momentum component perpendicular to the plane of the electron free motion. The calculation is carried out within the Hartree-Fock approximation. It is shown that the excited states correspond to the electron transitions in the vicinity of the Fermi momentum. The dispersion of the collective excitations is considered.     

Resonant Raman scattering at higher M0 exciton edge in layer compound InSe

Resonant Raman spectra of γ-rich InSe have been measured at the absorption peak, E'₁, which lies 1.2 eV above the lowest direct gap. The resonance behaviors observed indicate that the E'₁ peak is due to the formation of damped M₀ excitons.     

Two-component exciton model

We have measured the broadening, and in one case also the shift, of alkalinS andnD Rydberg levels by alkali perturbers (Cs by Cs, Rb by Cs and K, and K by Rb) applying the Doppler-free two-photon technique and thermionic detection. Taking into account recent experimental data for the self-broadening of K and Rb Rydberg levels we found that for low principal quantum numbersn the broadening data of the levels are in agreement within the limits of experimental uncertainty in the pure and mixed cases. For highn we could establish that the broadening of the levels is governed by the perturber-valence electron and the perturber-ion core interaction. With increasingn the broadening rates are only dependent on the sort of perturber. As reported recently for Rb-Rb and K-K, oscillations in the broadening data as a function ofn were also observed for Cs-Cs and in the mixed vapours. Our results do not support recent theories which describe the oscillations in terms of resonances in the scattering of the quasifree valence electron on the perturber atom. In a classical absorption experiment oscillations in the self-broadening and shift of the Cs principal series lines discovered by Mazing and Serapinas were found to be due to satellite structures which are merged in non-Lorentzian line wings.     

Modulated exciton spectroscopy

In this paper, the principal results obtained on the modulated excitonic spectra of Cu₂O and CuCl in the presence of an electric field, are reviewed. The behaviour of excitons in the electric field is discussed and compared with the actual theories. A symmetry analysis of the experimental results is performed which gives information about the origin of the optical transitions as well as on the nature of the field effects on the excitons.     

The influence of exciton motion on spin-resonance absorption

We use a stochastic model for the exciton motion which comprises both the coherent and the incoherent motion. The incoherent part is taken care of by a stochastic process which allows the local excitation energy and the transition matrix element to fluctuate by means of a Markovian process. The interaction between the spins and their surroundings is described by the usual spin-Hamiltonian which is, however, simplified to a spin 1/2 particle (instead of the triplet state). In the present paper we solve exactly the two limiting cases of completely coherent and incoherent motion (for two molecules). In the incoherent case the influence of the exchange interaction integral is taken into account by perturbation theory. We find expressions which are immediately comparable with ESR-experimental data. This comparison and additional information derived from optical absorption measurements allow us to determine all free parameters of our model uniquely. In particular, the fluctuations of the exchange interaction integral (with strength γ₁) play an important role. From these parameters we may furthermore calculate the correlation time of the proton spin resonance in agreement with experimental data. The results show clearly that at room temperature in anthracene crystals the exciton undergoes a hopping process.     

The 2s exciton in intermediate dimensionality structures

We report the observation of the 1s and 2s ground state excitons in small-offset superlattices with alternating layers of non-magnetic (ZnSe) and diluted magnetic (Zn_xMn_1 − xSe) semiconductors. Due to the large Zeeman splitting of the electron states in ZnMnSe, valence and conduction band offsets in these structures can be tuned by external magnetic field over the scale of tens of meV. Two series of superlattices were investigated, with Mn concentrations close to 10 and 15 atomic percent, respectively. Each series consisted of five superlattices with different layer thicknesses. The 2s exciton of the ground state is observed in all samples, and exhibits a diamagnetic shift that is consistent with earlier studies. The ability to determine the energy separation between the 1s and 2s exciton states allows us to analyze our results in the framework of thefractional dimension analysis , and to extract the values of exciton binding energies for the superlattices from the analysis.     

The intensity-dependent dielectric function of an exciton system

The nonlinear dielectric function of a direct band gap semiconductor is calculated using non-equilibrium Green's function technique. The interaction between the semiconductor and the intense radiation field is assumed to occur via excitons. As an example we calculate the polariton dispersion relation and the intensity-dependent changes of refraction index for GaAs.The author would like to thank Prof. Nguyen Van Hieu for projecting the problem and helpful discussions.     

The negatively charged exciton in double-layer quantum dots

The hyperangular equation for charged semiconductor complexes in a double-layer harmonic quantum dot was solved numerically by using the correlated hyperspherical harmonics as basis functions. By using this method, we have calculated the energy spectra of the low-lying states of a charged exciton as a function of the radius of the quantum dot and the binding energy spectra of the ground state as a function of the radius of the quantum dot for a few values of the distance between the vertically coupled dots and the electron-to-hole mass ratio. Received 3 December 1999     

The kinetics of exciton photoluminescence in mercuric iodide

Time-resolved photoluminescence (TRPL) of red mercuric iodide single crystal is measured at low temperatures and its two-photon luminescence is measured at room temperature. Sharp near band-gap luminescence is observed around 530 nm and was ascribed to radiative annihilation of free and bound excitons; the phonon replica of exciton luminescence are found between 533 and 540 nm at low temperatures. TRPL experiment reveals that near band-gap luminescence comprises fast and slow decay components and shows the different relaxation processes between free and bound exciton annihilation. Luminescence of bound excitons steeply lowers with increasing temperature and disappears about 40 K. A luminescence tail band is observed around 540 nm that is ascribed to defects in the anion sublattice. The temporal behavior of the tail band is described by rate equations very well. A broad luminescent band appears at 630 nm. The decay curves suggest that the luminescence is ascribed to the radiative recombination of donor-acceptor pairs and there are two kinds of mechanisms to control the decay. At room temperature, a luminescent band appears at the band-gap region, which shows the band-gap at room temperature is about 2.125 eV.     

The exciton states and the hypochromism of polypeptides

The exciton states of the α and β-helical conformations of polypeptides are studied by a new molecular exciton theory. It is found that the α-helical structure shows hypochromism while the parallel and anti-parallel β forms exhibit no remarkable changes. In the α-helical structure the band splitting and the optical rotational strength are reasonably well calculated and the migration velocities for the two exciton states are estimated.