Navid M. S. Jahed

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.

Deposition and parametric analysis of RF sputtered ZnO:Al thin films with very low resistivity

RF sputtered, aluminum-doped zinc oxide (ZnO:Al or AZO) is an attractive candidate material as transparent conductive oxides in the fabrication of opto-electronic devices. High electrical conductivity and optical transparency are two key requirements in such applications. This paper reports on the formation of AZO films on glass substrates in an RF-sputtering chamber modified to facilitate in situ heating during deposition. The influence of chamber pressure, RF power, and deposition temperature has been systematically studied and the electrical parameters such as film resistivity, carrier concentration, carrier mobility as well as optical transmission have been analyzed. Film deposition at 250 °C and a low chamber pressure of 0.5 mT resulted in a very low resistivity of 2.94 × 10−4 ohm cm. The structural properties of the films with the lowest resistivity have been further analyzed by x-ray diffraction (XRD) and PL measurements and are compared with the film deposited at room temperature. The XRD results show dominant peaks along (103) orientation for the AZO films with slightly improved crystal quality at higher temperature. Evolution of near band edge and deep level emission photoluminescence peaks also indicate improvement in crystal structure with increased deposition temperature.

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.

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.

Highly conductive TCO by RF sputtering of Al:ZnO for thin film photovoltaics

Al-doped ZnO is a promising transparent conductive oxides for fabrication of opto-electronic devices. In this paper we shed lights on controlling carrier concentration and mobility of Al:ZnO by controlling deposition parameters (RF power, pressure and substrate temperature). Al:ZnO thin film with the lowest recorded resistivity of ρ=2.94E-04 Ω.cm at deposition temperature of 250°C has been achieved. Light transmission of Al:ZnO and ZnO samples deposited on glass at different substrate temperature has been studied. Additionally, investigation were made to assess the effect of deposition temperature on the photoluminescence spectra (PL) of Al:ZnO sputtered on silicon and glass substrate.

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.