Bahareh Sadeghimakki

Very High Brightness Quantum Dot Light-Emitting Devices via Enhanced Energy Transfer from a Phosphorescent Sensitizer

We demonstrate very efficient and bright quantum dot light-emitting devices (QDLEDs) with the use of a phosphorescent sensitizer and a thermal annealing step. Utilizing CdSe/CdS core/shell quantum dots with 560 nm emission peak, bis(4,6-difluorophenylpyridinatoN,C2) picolinatoiridium as a sensitizer, and thermal annealing at 50 °C for 30 min, green-emitting QDLEDs with a maximum current efficiency of 23.9 cd/A, a power efficiency of 31 lm/W, and a brightness of 65,000 cd/m(2) are demonstrated. The high efficiency and brightness are attributed to annealing-induced enhancements in both the Forster resonance energy transfer (FRET) process from the phosphorescent energy donor to the QD acceptor and hole transport across the device. The FRET enhancement is attributed to annealing-induced diffusion of the phosphorescent material molecules from the sensitizer layer into the QD layer, which results in a shorter donor-acceptor distance. We also find, quite interestingly, that FRET to a QD acceptor is strongly influenced by the QD size, and is generally less efficient to QDs with larger sizes despite their narrower bandgaps.

Spectrally Resolved Dynamics of Synthesized CdSe/ZnS Quantum Dot/Silica Nanocrystals for Photonic Down-Shifting Applications

Photonic structures capable of luminescence down-shifting (LDS) have strong application potential in several areas of optoelectronics. Such structures can be formed by overcoating quantum dots (QDs) with integrable, transparent layers. In this paper, silica was grown on CdSe/ZnS QDs to form QD/silica nanocrystals (NCs) in a microemulsion synthesis process. The synthesized structures were structurally and optically characterized to understand the growth mechanism, luminescence properties, and the influence of process parameters on excitonic decay and lifetime. Process conditions were established to have single QDs at the centers of the silica particles. The effects of temperature, excitation duration, size of QDs, and type of ligands on decay dynamics were established. Temperature- and time-resolved excitonic decay study of QD/silica NCs suggested carrier-trapping at the QD/silica interface and the exciton-phonon coupling to be the two main nonradiative processes limiting the luminescence efficiency. The synthesized NCs displayed intense photoluminescence (PL) with slight decrease in lifetime. The PL efficiency of the NCs improved for longer illumination. The NC structures that safely embed QDs in transparent medium are good candidates for LDS applications in photovoltaic, imaging, and detection devices.

ZnO nanowire arrays synthesized on ZnO and GaN films for photovoltaic and light-emitting devices

Abstract. The wide bandgap, one-dimensional zinc oxide (ZnO) nanowires (NWs) and their heterostructures with other materials provide excellent pathways for efficient photovoltaic (PV) and light-emitting devices. ZnO NWs sensitized with quantum dots (QDs) provide high-surface area and tunable bandgap absorbers with a directional path for carriers in advanced PV devices, while ZnO heterojunctions with other p-type wide bandgap materials lead to light-emitting diodes (LEDs) with better emission and waveguiding properties compared with the homojunction counterparts. Synthesis of the structures with the desired morphology is a key to device applications. In this work, ZnO NW arrays were synthesized using hydrothermal method on ZnO and GaN thin films. Highly crystalline, upright, and ordered arrays of ZnO NWs in the 50 to 250-nm diameter range and 1 μm in length were obtained. The morphology and optical properties of the NWs were studied. Energy dispersive x-ray spectroscopy (EDX) analysis revealed nonstoichiometric oxygen content in the grown ZnO NWs. Photoluminescence (PL) studies depicted the presence of oxygen vacancy and interstitial zinc defects in the grown ZnO NWs, underlining the potential for LEDs. Further, hydrophobically ligated CdSe/ZnS QDs were successfully incorporated to the NW arrays. PL analysis indicated the injection of electrons from photoexcited QDs to the NWs, showing the potential for quantum dot-sensitized solar cells.

Zinc oxide nanowire arrays for photovoltaic and light-emitting devices

Knowledge of carrier transfer, in quantum dot sensitized solar cells, is the key to engineering the device structure and architecture optimization. In this work, Zinc oxide (ZnO) nanowire (NW) arrays were synthesized on glass wafers and on GaN thin films for application in photovoltaic and light-emitting devices. The nanowires grown on glass wafers were incorporated with CdSe/ZnS quantum dots (QD) and their steady state and lifetime photoluminescence (PL) were studied to investigate the feasibility of electron transfer from excited QDs to ZnO NWs. The results provide an indication that the injected electrons, from excited high quantum efficiency QDs, live longer and hence facilitate electron transport without undergoing non-radiative recombination at surface trap states. Morphology and optical properties of the ZnO nanowires on GaN film were also studied for application in light-emitting devices.

Uniform embedment of CdSe/ZnS quantum dot arrays in thin oxide layers for luminescence down shifting in PV devices

A transparent layer consisting of ordered arrays of core/shell cadmium selenide(CdSe)/zinc sulfide(ZnS) quantum dots(QDs) sandwiched in thin silicon dioxide (SiO2) layers has been prepared by embedment of films of QDs in oxide layer deposited by electron beam (e-beam) to use as spectral down shifter. Well ordered nanostructures are revealed by TEM and excitonic photoluminescence (PL) spectra. Low reflection, high transparency and intense PL peaks are the main characteristics of the arrays of QDs introduced into the oxide layer. EQE and IV measurements show that multilayered film of QDs and a near-optimum oxide thickness increase the quantum yield and decrease reflection in the UV-NIR range of the layer, while causing some scattering that tends to reduce the effectiveness of the layer.

Investigation of structural and optical properties of CdSe quantum dots in glass matrices for photovoltaic application

In this work preparation methods for spin-cast uniform layers of cadmium selenide (CdSe) quantum dots (QDs) with specific thickness and subsequent film treatment methodologies are presented. Dimensional and lattice structures as well as the homogeneity of the nanocrystals distribution over the film thickness are studied through high resolution transmission electron microscopy (HRTEM). Ultra violet (UV) spectrometric and spectrofluorometric measurements are performed to obtain absorption-emission spectrum of the provided films. The results show that the absorption is sensitive to size of the nanocrystals and the refractive index of the medium in which they are embedded. Refractive index of the thin films is extracted using spectroscopic Ellipsometery. Results are presented on the incorporation of the CdSe in glass matrices and a graded index structure is optimized for embedment of nanocrystals. The photon conversion ability of the fabricated layer has been verified. Effect of size and glass matrix contraction on the Raman shifts of CdSe quantum dots has been also investigated. The results from such characterization methods are vital in knowing the properties of the nanocrystals, as well as in optimizing a converter layer for solar cell applications.

Toxicity and safety aspects of nanoparticle spread in third generation photovoltaic device processing environments

Detection strategies for analysis of the nanomaterials toxicity, although challenging, will be in much demand as nanotechnology becomes more common-place in third generation photovoltaics (PV). Experimentally feasible approaches must be designed and engineered to detect quantum dots (QDs) and nanoparticles (NPs) in PV device processing environment. Identifying the level of risk to human body upon exposure to nanomaterials is another important factor that needs consideration. In this work evidence on the detection of aerosolized nanoparticles was experimentally verified using gold NP adsorbent, followed by spectroscopic measurements. Results from in-vitro cytotoxicity study with HeLa cell cultures and fluorescent plate reading also showed that core/shell CdSe/ZnS QDs are responsible for cell death following exposure.

Toxicity and safety study of Cd-based and Cd-free quantum dots in third-gen PV and scaled-up processing platforms

Quantum dots (QDs) are being incorporated at an accelerated rate into Third-Gen photovoltaic (PV) and scaled-up PV processing platforms for production of high efficiency devices. As a result, studies are needed to examine QD toxicity in workplace environment. Herein, we report on a rapid and sensitive detection methods to examine risk of QD exposure in PV processing. QD-associated toxic elements were detected in slight amounts using gold nanoparticles (Au NPs) probe, followed by photoluminescence and Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) analyses, which indicated the possibility of QD aerosolization during deposition, transferring and testing of the QD film. Cytotoxicity effects of different type QDs were also studied using cell culture viability. The results indicate that QD material and their coating are important factors in producing cytotoxicity effects. It was also demonstrated that CIS QDs have less cytotoxic effects on HeLa and CHSE cells than CdSe QDs, and may be considered non-toxic in comparison.

Proof of down-conversion by CdSe/ZnS quantum dots on silicon solar cells

The performance of the crystalline silicon solar cells can be improved with spectrally engineered layers. Core/shell CdSe/ZnS quantum dots (QDs) with tunable absorption-emission wavelengths in UV-Vis range can be applied as luminescence centers in luminescence down-conversion layers (LDC). In this work, CdSe/ZnS QDs with an emission wavelength of 620nm and luminescence quantum efficiency (LQE) of 85% were deployed on planar and textured crystalline silicon solar cells for down-conversion. The structural and optical characteristics of the QDs and the employed layer were studied. The cell performances before and after LDC layer deployment were investigated. Experimental verification was obtained on luminescence down-conversion of the incident photons in the cells containing LDC layers.

Liquid junction quantum dot solar cells based on ZnO nanowires arrays

Liquid junction solar cells sensitized with quantum dots are promising structures as low cost and high efficiency photovoltaic devices. However, the reported short circuit current and open circuit voltage is lower than the theoretical values. The performance of these cells can be improved by the use of defect-free photoelectrodes and absorbing materials. In this work, hydrothermally grown zinc oxide (ZnO) nanowires (NWs), CdSe/ZnS quantum dots (QDs), a polysulfide solution and a copper film were used as the photoelectrode, light absorber, hole scavenger, and counter electrode respectively to form a NW-based QD sensitized solar cell. The structural characteristics of the grown NWs and QD sensitized NW architectures were studied. Current-voltage (J-V) characteristic of the fabricated device was also investigated. Photoluminescence (PLLiquid junction solar cells sensitized with quantum dots are promising structures as low cost and high efficiency photovoltaic devices. However, the reported short circuit current and open circuit voltage is lower than the theoretical values. The performance of these cells can be improved by the use of defect-free photoelectrodes and absorbing materials. In this work, hydrothermally grown zinc oxide (ZnO) nanowires (NWs), CdSe/ZnS quantum dots (QDs), a polysulfide solution and a cupper film were used as the photoelectrode, light absorber, hole scavenger, and counter electrode respectively to form a NW-based QD sensitized solar cell. The structural characteristics of the grown NWs and QD sensitized NW architectures were studied. Current-voltage (J-V) characteristic of the fabricated device was also investigated. Photoluminescence (PL) study on NW/QD and QD/electrolyte architectures provided evidence on carrier transfer from the light absorbing centers to the electron and hole conducting media.) study on NW/QD and QD/electrolyte architectures provided evidence on carrier transfer from the light absorbing centers to the electron and hole conducting media.