Nonradiative Carrier Recombination Centers Research Articles

Core/Shell Quantum Dots Solar Cells

AbstractSemiconductor nanocrystals, the so‐called quantum dots (QDs), exhibit versatile optical and electrical properties. However, QDs possess high density of surface defects/traps due to the high surface‐to‐volume ratio, which act as nonradiative carrier recombination centers within the QDs, thereby deteriorating the overall solar cell performance. The surface passivation of QDs through the growth of an outer shell of different materials/compositions called “core/shell QDs” has proven to be an effective approach to reduce the surface defects and confinement potential, which can enable the broadening of the absorption spectrum, accelerate the carrier transfer, and reduce exciton recombination loss. Here, the recent research developments in the tailoring of the structure of core/shell QDs to tune exciton dynamics so as to improve solar cell performance are summarized. The role of band alignment of core and shell materials, core size, shell thickness/compositions, and interface engineering of core/thick shell called “giant” QDs on electron–hole spatial separation, carrier transport, and confinement potential, before and after grafting on the carrier scavengers (semiconductor/electrolyte), is described. Then, the solar cell performance based on core/shell QDs is introduced. Finally, an outlook for the rational design of core/shell QDs is provided, which can further promote the development of high‐efficiency and stable QD sensitized solar cells.

Degradation Mechanisms of Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence Molecules

Degradation of organic light-emitting diodes (OLEDs) operated continuously at a constant current density is investigated using photoluminescence techniques. The OLEDs contained the thermally activated delayed fluorescence emitting dopant (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). OLED degradation proceeds mainly on the basis of excited-state instability of host molecules rather than processes related to 4CzIPN. Additionally, the electrochemical instability of radical cations and anions influences long-term OLED degradation. The formation of exciton quenchers and nonradiative carrier recombination centers acts to reduce OLED luminance. These findings highlight the need for new host material development to fabricate more stable TADF-OLEDs.

In Situ Photochemical Surface Passivation of CdSe/ZnS Quantum Dots for Quantitative Light Emission and Enhanced Photocurrent Response in Solar Cells

Suppression of energy-wasting nonradiative carrier relaxation in photoactivated semiconductor quantum dots (QDs) is an essential requirement for applications such as QD-sensitized solar cell (QDSSC). Here we report that in situ surface passivation of CdSe/ZnS QDs using dithiothreitol (DTT) results in the 4-fold enhancement of photoluminescence (PL) quantum efficiency of QDs. By the analyses of PL intensities and blinking statistics of single QDs, the PL enhancement is assigned to a combination of processes such as unexpected appearance of bright QDs, gradual increase in the PL intensity of QDs, and substantial suppression of blinking. These processes not only suggest the suppression of nonradiative carrier recombination in QDs passivated with DTT molecules but also enable us to classify the nonradiative carrier recombination centers in QDs into low-density defects in which the rates of nonradiative recombination largely exceed the rates of radiative relaxation and high-density defects in which the rates of nonradiative carrier recombination are comparable to the rates of radiative relaxations. In the former case, passivation of the defects by the spontaneous addition of one or a few DTT molecules to QDs with PL intensity below the detection limit results in the formation of highly luminescent QD-(DTT)n adducts. In the latter case, highly luminescent QD-(DTT)n adducts are formed from already luminescent QDs by the relatively slow addition of DTT molecules. These two types of surface passivation reactions are governed by both chemical and photochemical processes. Overall, the passivation of defects in QD by DTT results in the suppression of energy-wasting nonradiative relaxations, which enables us to drive ca. 33% excess photocurrent in a photoelectrochemical cell constructed using QD-(DTT)n as the sensitizer.

Nonradiative Carrier Recombination Centers of Cl-Doped ZnSe Epitaxial Layers

The photothermal and photoluminescence spectroscopy for nondoped and chlorine (Cl)-doped ZnSe epitaxial layers were carried out at liquid nitrogen and room temperatures. We found two kinds of nonradiative carrier recombination centers which are Cl-related defects with activation energies of about 30 and 120 meV for the first time.

Photoluminescence and photoacoustic spectra of N-doped ZnSe epitaxial layers grown by molecular beam epitaxy

Photoluminescence (PL) and piezoelectric photoacoustic (PPA) spectra have been studied at low temperatures for nondoped and N-doped ZnSe epitaxial layers grown by molecular beam epitaxy. Deep emission bands observed in nondoped ZnSe are disappeared with nitrogen doping in the PL spectra. On the other hand, new nonradiative signals appear with nitrogen doping in the PPA spectra. This indicated that nonradiative carrier recombination centers are introduced by the nitrogen doping process in molecular beam epitaxy growth.

Temperature characteristics of threshold currents of columnar-shaped self-assembled InGaAs/GaAs quantum-dot lasers: Influence of nonradiative recombination centers

We examined the temperature characteristics of threshold currents in columnar-shaped self-assembled InGaAs quantum-dot lasers. The threshold currents started to increase above 200 K, leading to a low characteristic temperature of 67 K above room temperature. The relationship between the spontaneous emission intensity, P, and the current, I, varied from P∝I below 160 K to P∝I2 above 298 K. Nonradiative carrier recombination centers are identified as the origin of the low characteristic temperature. A strong correlation between the increase in the threshold current and the shape of power-current curves was found, giving a hint about the origin of the homogeneous broadening of the single-dot optical gain.

Study of growth temperature in gas-source molecular-beam epitaxy growth of InGaAs/GaAs quantum well lasers

Strained-layer InGaAs/GaAs single quantum well lasers were grown by gas-source molecular-beam epitaxy at temperatures between 430 and 610 °C. The optimum growth temperature for the quantum wells of the lasers was found to be about 520 °C, as deduced from the measurement of threshold current density of the lasers and room-temperature photoluminescence (PL) of the quantum wells. If the growth temperature is lower than 520 °C, the threshold current density dramatically increases due to the formation of nonradiative carrier recombination centers in the quantum well. These recombination centers originate from lattice mismatch defects and alloy disordering. If the growth temperature is higher than 520 °C, indium desorption and segregation become increasingly severe. Thus, at high growth temperature, the InGaAs/GaAs interfaces become rough and the threshold current density increases. In addition, rapid thermal annealing was observed to remove most of the nonradiative recombination centers from the quantum wells and to improve the threshold current density of the lasers and PL intensity of the quantum wells.

Role of growth temperature in GSMBE growth of strained-layer InGaAs/GaAs quantum well lasers

Strained-layer InGaAs/GaAs single quantum well lasers were grown by gas-source molecular-beam epitaxy at growth temperatures in a range of 430 to 610°C. The optimum growth temperature for the quantum well of the lasers was found to be about 520°C. If the growth temperature is lower than this value, the threshold current density dramatically increases due to the presence of a high density of non-radiative carrier recombination centers in the quantum well, which give rise to alloy disordering and lattice mismatch defects in the quantum well. If the growth temperature is higher than 520°C, indium evaporation and segregation become severe during the growth of the quantum well. Thus, the InGaAs/GaAs interfaces become rough and the threshold current density degrades. In addition, room-temperature photoluminescence was investigated for the samples in detail to clarify the quality of the quantum well.