ABSTRACT In fabricating quantum dot light-emitting diodes (QDLEDs) based on manganese-doped zinc ... more ABSTRACT In fabricating quantum dot light-emitting diodes (QDLEDs) based on manganese-doped zinc sulfide (Mn-doped ZnS) nanocrystals, we use a range of dithiol-containing ligands in forming thin films of nanocrystals to study the role of interdot spacing on the performance of the QDLEDs. Use of dithiols as ligands to the doped nanocrystals is crucial since the redox energy levels of such ligands lie below both the d-states of Mn ions, and since a transition between the d-states yields photoluminescence (PL) emission in the doped system, the ligands could not quench the PL emission of the nanocrystals. In thin films of nanocrystals, the length of the ligands controls the intensity of the PL emission due to several factors. When the length of the ligands is long, the ligands on one hand increase the PL intensity, and they also reduce the conductivity of the emitting layer due to an increase in the interdot spacing. We observe that the performance of the QDLEDs is optimum at an intermediate length of the ligands. The spectral range of electroluminescence (EL) emission does not depend on the length of the ligands since the PL emission of Mn-doped ZnS nanocrystals, and hence also the EL of the devices, appears to be due to a radiative transition between the d-states of manganese ions.
ABSTRACT We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS ... more ABSTRACT We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS shell layer and characterize the devices under dc and ac voltages. By applying an ac voltage, we achieve pulsed electroluminescence (EL) from these quantum dot LEDs (QDLEDs). As frequency of the applied ac voltage increases, we observe a phase-lag between the pulsed EL and applied sinusoidal voltage. From frequency response of the pulsed emission, we determine −3 dB frequency of the ac-QDLEDs. The frequency response of EL also provides a route to study mechanism of the LEDs. The EL spectra under dc and ac voltages provide information on carrier transport and exciton generation in these devices. From the resemblance between EL and photoluminescence spectra, which appear due to transitions between d-states of Mn-ions in doped nanocrystals, we infer that excitons form directly in the quantum dots in these QDLEDs.
Physica E: Low-dimensional Systems and Nanostructures, 2014
ABSTRACT We report solution-processed growth of zinc-alloyed silver–indium-sulfide (AIZS) nanocry... more ABSTRACT We report solution-processed growth of zinc-alloyed silver–indium-sulfide (AIZS) nanocrystals followed by fabrication and characterization of light-emitting diodes (LEDs) based on such nanostructures. While growing the low dimensional crystals, we vary the ratio between the silver and zinc contents that in turn tunes the bandgap and correspondingly their photoluminescence (PL) emission. We also dope the AIZS nanocrystals with manganese, so that their PL emission, which appears due to a radiative transition between the d-states of the dopants, becomes invariant in energy when the diameter of the quantum dots or the dopant concentration in the nanostructures varies. The LEDs fabricated with such undoped and manganese-doped AIZS nanocrystals emit electroluminescence (EL) that matches the PL spectrum of the respective nanomaterial. The results demonstrate examples of quantum dot LEDs (QDLEDs) based on nontoxic AIZS nanocrystals.
Double-coated CsPbBr3@SiO2@PVP perovskite NCs exhibit higher luminous intensity and better struct... more Double-coated CsPbBr3@SiO2@PVP perovskite NCs exhibit higher luminous intensity and better structural stability than those without PVP, enabling indium ion detection in water.
The emergence of the new class of organic–inorganic hybrid perovskite materials has found numerou... more The emergence of the new class of organic–inorganic hybrid perovskite materials has found numerous applications in a plethora of next-generation optoelectronic devices like solar cells, light-emitting diodes, photodetectors, and lasers. Synthetic controls of perovskite materials through composition engineering, solvent chemistry, morphology and surface controlling, surface passivation, and band engineering have made the perovskite solar cells the fasted growing technology in the history of solar cells evolution. The perovskite semiconductors possess a number of unique functionalities like easily tunable band-gap energy, solution processability, form long-range crystals at low temperatures (<150 °C), excellent charge transport properties, and self-resistance toward electronic impurity. However, ionic nature and relaxed structural arrangement of the perovskite crystals makes them vulnerable to degradation by moisture and temperature. The stability of perovskite-based solar cells is...
Despite their emergence as promising materials for low-cost and efficient energy power generation... more Despite their emergence as promising materials for low-cost and efficient energy power generation technology, hybrid or-ganic-inorganic lead-halide perovskites' instability towards moisture and heat stress remains a serious obstacle that needs to be tackled for commercialization. Here, we show improved moisture and thermal stability through the use of cesium oleate to modify the perovskite/hole transporting material (HTM) interface. Passivation using cesium oleate does not induce the formation of any low dimensional perovskites, suggesting that the organic species only passivates the perovskite's surface and grain boundaries. As a result, enhanced hydrophobic character of perovskite film is realized upon passivation, evi-denced by high water contact angle of 107.4 degree and improved stability at ambient condition (relative humidity of ~70%, room temperature). Concomitantly, the proposed passivation strategy leads to increase amount of cesium concentration within the films, resulting in beneficial enhanced thermal stability of the film at 85oC. By maintaining the solar absorber's three-dimensional (3D) structure while concurrently passivating the interfacial defects and vacancies, improved open-circuit voltage (Voc) and unsacrificed short-circuit current density (Jsc) were obtained from the treated devices, leading to power conversion efficiencies of over 18%. When stored in a humid environment (relative humidity of ~55%), devices with cesium oleate passivation maintain 88% of its initial PCEs after 720 hours, degrading two times slower than those of the control. This work offers a strategy of coating 3D perovskites with unique combination of inorganic cation and long chain organics to pro-vide hydrophobicity and moisture stability to the solar absorber layer, while maintaining good device performances.
In studying the role of GO in LEDs and PVs, a single pair of active materials has been used in bo... more In studying the role of GO in LEDs and PVs, a single pair of active materials has been used in both the devices.
We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS shell lay... more We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS shell layer and characterize the devices under dc and ac voltages. By applying an ac voltage, we achieve pulsed electroluminescence (EL) from these quantum dot LEDs (QDLEDs). As frequency of the applied ac voltage increases, we observe a phase-lag between the pulsed EL and applied sinusoidal voltage. From frequency response of the pulsed emission, we determine −3 dB frequency of the ac-QDLEDs. The frequency response of EL also provides a route to study mechanism of the LEDs. The EL spectra under dc and ac voltages provide information on carrier transport and exciton generation in these devices. From the resemblance between EL and photoluminescence spectra, which appear due to transitions between d-states of Mn-ions in doped nanocrystals, we infer that excitons form directly in the quantum dots in these QDLEDs.
We fabricate and characterize light-emitting diodes (LEDs) based on thin-films of NiO, Mn-doped Z... more We fabricate and characterize light-emitting diodes (LEDs) based on thin-films of NiO, Mn-doped ZnS, and n-ZnO nanoparticles acting as hole-transporting, emitting, and electron-transporting layers, respectively. We vary the thickness of the individual layers and record current-voltage and luminance-current characteristics. By examining the fitting of the current-voltage characteristics of these all-inorganic LEDs, we find that the hole-and electron-transporting layers lower barrier heights for hole-and electron-injection from the respective electrodes that correspondingly improve electro-luminescence (EL) output. The photoluminescence emission of Mn-doped ZnS nanoparticles and EL emission of the LEDs resembled implying that excitons formed in the doped nanostructures followed by a radiative transition between d-states of Mn-ions.
We fabricate light-emitting diodes ͑LEDs͒ based on Mn-doped ZnS nanocrystals along with hole-tran... more We fabricate light-emitting diodes ͑LEDs͒ based on Mn-doped ZnS nanocrystals along with hole-transporting N , NЈ bis͑3-methylphenyl͒-N , NЈ-diphenyl-benzidine ͑TPD͒. With Mn-doping, ZnS nanostructures exhibit a strong photoluminescence. The LEDs exhibit electroluminescence ͑EL͒ from Mn-doped ZnS quantum dots and TPD. In order to open up channels for energy-transfer from TPD to quantum dots and to achieve EL from only the nanoparticles, we grow core-shell nanoparticles with Mn-doped ZnS in the core and CdS as the shell layer. Excitons formed in TPD can now transfer their energy directly to the shell-layer to yield EL from only the nanoparticles.
We have introduced copper-doped semiconducting nanocrystals in light-emitting diodes (LEDs). Char... more We have introduced copper-doped semiconducting nanocrystals in light-emitting diodes (LEDs). Characteristics of the devices show that electroluminescence (EL) emission in these LEDs is color tunable. In copper-doped ZnS nanocrystals in the core and Zn 1Àx Cd x S host as a shell-layer, photoluminescence (PL) arises from a transition from conduction band-edge of the host to 3d-levels of copper-ions. The PL of the nanocrystals and hence the EL of LEDs based on such nanostructures become tunable by varying the Cd-content in Zn-Cd-S alloys, that is, Zn 1Àx Cd x S with different values of x, which changes the conduction band-edge of the host.
ABSTRACT In fabricating quantum dot light-emitting diodes (QDLEDs) based on manganese-doped zinc ... more ABSTRACT In fabricating quantum dot light-emitting diodes (QDLEDs) based on manganese-doped zinc sulfide (Mn-doped ZnS) nanocrystals, we use a range of dithiol-containing ligands in forming thin films of nanocrystals to study the role of interdot spacing on the performance of the QDLEDs. Use of dithiols as ligands to the doped nanocrystals is crucial since the redox energy levels of such ligands lie below both the d-states of Mn ions, and since a transition between the d-states yields photoluminescence (PL) emission in the doped system, the ligands could not quench the PL emission of the nanocrystals. In thin films of nanocrystals, the length of the ligands controls the intensity of the PL emission due to several factors. When the length of the ligands is long, the ligands on one hand increase the PL intensity, and they also reduce the conductivity of the emitting layer due to an increase in the interdot spacing. We observe that the performance of the QDLEDs is optimum at an intermediate length of the ligands. The spectral range of electroluminescence (EL) emission does not depend on the length of the ligands since the PL emission of Mn-doped ZnS nanocrystals, and hence also the EL of the devices, appears to be due to a radiative transition between the d-states of manganese ions.
ABSTRACT We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS ... more ABSTRACT We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS shell layer and characterize the devices under dc and ac voltages. By applying an ac voltage, we achieve pulsed electroluminescence (EL) from these quantum dot LEDs (QDLEDs). As frequency of the applied ac voltage increases, we observe a phase-lag between the pulsed EL and applied sinusoidal voltage. From frequency response of the pulsed emission, we determine −3 dB frequency of the ac-QDLEDs. The frequency response of EL also provides a route to study mechanism of the LEDs. The EL spectra under dc and ac voltages provide information on carrier transport and exciton generation in these devices. From the resemblance between EL and photoluminescence spectra, which appear due to transitions between d-states of Mn-ions in doped nanocrystals, we infer that excitons form directly in the quantum dots in these QDLEDs.
Physica E: Low-dimensional Systems and Nanostructures, 2014
ABSTRACT We report solution-processed growth of zinc-alloyed silver–indium-sulfide (AIZS) nanocry... more ABSTRACT We report solution-processed growth of zinc-alloyed silver–indium-sulfide (AIZS) nanocrystals followed by fabrication and characterization of light-emitting diodes (LEDs) based on such nanostructures. While growing the low dimensional crystals, we vary the ratio between the silver and zinc contents that in turn tunes the bandgap and correspondingly their photoluminescence (PL) emission. We also dope the AIZS nanocrystals with manganese, so that their PL emission, which appears due to a radiative transition between the d-states of the dopants, becomes invariant in energy when the diameter of the quantum dots or the dopant concentration in the nanostructures varies. The LEDs fabricated with such undoped and manganese-doped AIZS nanocrystals emit electroluminescence (EL) that matches the PL spectrum of the respective nanomaterial. The results demonstrate examples of quantum dot LEDs (QDLEDs) based on nontoxic AIZS nanocrystals.
Double-coated CsPbBr3@SiO2@PVP perovskite NCs exhibit higher luminous intensity and better struct... more Double-coated CsPbBr3@SiO2@PVP perovskite NCs exhibit higher luminous intensity and better structural stability than those without PVP, enabling indium ion detection in water.
The emergence of the new class of organic–inorganic hybrid perovskite materials has found numerou... more The emergence of the new class of organic–inorganic hybrid perovskite materials has found numerous applications in a plethora of next-generation optoelectronic devices like solar cells, light-emitting diodes, photodetectors, and lasers. Synthetic controls of perovskite materials through composition engineering, solvent chemistry, morphology and surface controlling, surface passivation, and band engineering have made the perovskite solar cells the fasted growing technology in the history of solar cells evolution. The perovskite semiconductors possess a number of unique functionalities like easily tunable band-gap energy, solution processability, form long-range crystals at low temperatures (<150 °C), excellent charge transport properties, and self-resistance toward electronic impurity. However, ionic nature and relaxed structural arrangement of the perovskite crystals makes them vulnerable to degradation by moisture and temperature. The stability of perovskite-based solar cells is...
Despite their emergence as promising materials for low-cost and efficient energy power generation... more Despite their emergence as promising materials for low-cost and efficient energy power generation technology, hybrid or-ganic-inorganic lead-halide perovskites' instability towards moisture and heat stress remains a serious obstacle that needs to be tackled for commercialization. Here, we show improved moisture and thermal stability through the use of cesium oleate to modify the perovskite/hole transporting material (HTM) interface. Passivation using cesium oleate does not induce the formation of any low dimensional perovskites, suggesting that the organic species only passivates the perovskite's surface and grain boundaries. As a result, enhanced hydrophobic character of perovskite film is realized upon passivation, evi-denced by high water contact angle of 107.4 degree and improved stability at ambient condition (relative humidity of ~70%, room temperature). Concomitantly, the proposed passivation strategy leads to increase amount of cesium concentration within the films, resulting in beneficial enhanced thermal stability of the film at 85oC. By maintaining the solar absorber's three-dimensional (3D) structure while concurrently passivating the interfacial defects and vacancies, improved open-circuit voltage (Voc) and unsacrificed short-circuit current density (Jsc) were obtained from the treated devices, leading to power conversion efficiencies of over 18%. When stored in a humid environment (relative humidity of ~55%), devices with cesium oleate passivation maintain 88% of its initial PCEs after 720 hours, degrading two times slower than those of the control. This work offers a strategy of coating 3D perovskites with unique combination of inorganic cation and long chain organics to pro-vide hydrophobicity and moisture stability to the solar absorber layer, while maintaining good device performances.
In studying the role of GO in LEDs and PVs, a single pair of active materials has been used in bo... more In studying the role of GO in LEDs and PVs, a single pair of active materials has been used in both the devices.
We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS shell lay... more We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS quantum dots with a CdS shell layer and characterize the devices under dc and ac voltages. By applying an ac voltage, we achieve pulsed electroluminescence (EL) from these quantum dot LEDs (QDLEDs). As frequency of the applied ac voltage increases, we observe a phase-lag between the pulsed EL and applied sinusoidal voltage. From frequency response of the pulsed emission, we determine −3 dB frequency of the ac-QDLEDs. The frequency response of EL also provides a route to study mechanism of the LEDs. The EL spectra under dc and ac voltages provide information on carrier transport and exciton generation in these devices. From the resemblance between EL and photoluminescence spectra, which appear due to transitions between d-states of Mn-ions in doped nanocrystals, we infer that excitons form directly in the quantum dots in these QDLEDs.
We fabricate and characterize light-emitting diodes (LEDs) based on thin-films of NiO, Mn-doped Z... more We fabricate and characterize light-emitting diodes (LEDs) based on thin-films of NiO, Mn-doped ZnS, and n-ZnO nanoparticles acting as hole-transporting, emitting, and electron-transporting layers, respectively. We vary the thickness of the individual layers and record current-voltage and luminance-current characteristics. By examining the fitting of the current-voltage characteristics of these all-inorganic LEDs, we find that the hole-and electron-transporting layers lower barrier heights for hole-and electron-injection from the respective electrodes that correspondingly improve electro-luminescence (EL) output. The photoluminescence emission of Mn-doped ZnS nanoparticles and EL emission of the LEDs resembled implying that excitons formed in the doped nanostructures followed by a radiative transition between d-states of Mn-ions.
We fabricate light-emitting diodes ͑LEDs͒ based on Mn-doped ZnS nanocrystals along with hole-tran... more We fabricate light-emitting diodes ͑LEDs͒ based on Mn-doped ZnS nanocrystals along with hole-transporting N , NЈ bis͑3-methylphenyl͒-N , NЈ-diphenyl-benzidine ͑TPD͒. With Mn-doping, ZnS nanostructures exhibit a strong photoluminescence. The LEDs exhibit electroluminescence ͑EL͒ from Mn-doped ZnS quantum dots and TPD. In order to open up channels for energy-transfer from TPD to quantum dots and to achieve EL from only the nanoparticles, we grow core-shell nanoparticles with Mn-doped ZnS in the core and CdS as the shell layer. Excitons formed in TPD can now transfer their energy directly to the shell-layer to yield EL from only the nanoparticles.
We have introduced copper-doped semiconducting nanocrystals in light-emitting diodes (LEDs). Char... more We have introduced copper-doped semiconducting nanocrystals in light-emitting diodes (LEDs). Characteristics of the devices show that electroluminescence (EL) emission in these LEDs is color tunable. In copper-doped ZnS nanocrystals in the core and Zn 1Àx Cd x S host as a shell-layer, photoluminescence (PL) arises from a transition from conduction band-edge of the host to 3d-levels of copper-ions. The PL of the nanocrystals and hence the EL of LEDs based on such nanostructures become tunable by varying the Cd-content in Zn-Cd-S alloys, that is, Zn 1Àx Cd x S with different values of x, which changes the conduction band-edge of the host.
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