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Due to the increasing need for the use of safer nanomaterials in the electronic and optoelectronic sectors, there is a growing emphasis on the development of less-toxic quantum dots (QD) as replacements for traditional cadmium or lead-based QD technologies. Although the ternary Cu-In-S (CIS) QDs have gained prominence due to their advantageous optical properties and unique tunability, their intrinsically defective structures present some drawbacks for device applications. A one-pot, high-throughput process was developed to synthesize quaternary Zn-Cu-In-S (ZCIS) QDs, which show higher structural crystallinity and lattice stability. Stoichiometric variation was explored to determine its impact on QD optical and structural properties. It was found that the trial-cationic structure of the quaternary ZCIS QDs allowed further variation and flexibility of QD properties. Induction of Cu deficiency via In- or Zn-excess precursor ratios improved photoluminescence (PL) intensity and structural stability. ZnS overcoating was investigated, but it was found that the Zn-containing ZCIS cores, which are more stable than core CIS QDs, benefitted only minimally from Zn-passivation. The highly emissive quaternary QDs produced through this scalable synthesis procedure are widely tunable and functionalizable, capable of emission from 564-650 nm, and with a high lifetime up to 159 ns, making them more suitable for a variety of device applications.

Details of a large-area-compatible sub-600 °C process for the fabrication of high mobility field-effect transistors on single crystalline silicon are presented. It is shown that lift-off and solid-phase-epitaxy (SPE) can be used in conjunction with plasma enhanced chemical vapor deposition (PECVD) to create MOSFET transistors with mobilities comparable to other high-performance techniques. Furthermore, use of an amorphous silicon/silicon oxynitride (SiO x N y ) sacrificial bilayer is shown to make the process capable of self-aligned silicidation. The topography of the deposition is studied using cross-sectional scanning electron microscopy (SEM). It is shown that a successful lift-off can be achieved through the proper design of the sacrificial bilayer despite the high degree of step coverage generally exhibited by PECVD films. Cross-sectional transmission electron microscopy (TEM) is used to reveal the microstructure of the epitaxy and silicidation. Current-voltage characteristics of the fabricated transistors are presented and the field effect mobility of electrons on the fabricated devices is reported. The developed process is particularly useful in applications where high mobility transistors need to be built on silicon/nonsilicon hybrid platforms that exclude high processing temperatures.

Printable electrodes will play an important role in future flexible electronics in photovoltaic, displays, and sensor devices. Novel materials such as nanocomposites can be attractive as electrode materials in such devices. Work function is an important parameter in the design of device structures for optimal performance. However, definition of work function of nanocomposite materials is complicated due to their non-homogeneity compared to single phase materials. In this study, an effective work function of a novel screen-printable graphene-decorated nanocomposite is presented. Two methods were used to determine the effective work function. The first method relies on the tunneling current in a conductor–insulator–conductor structure with the nanocomposite functioning as one of the conductors, while the second method measures the work function of the nanocomposite at the interface with vacuum using ultraviolet photoemission spectroscopy. Values measured using the two methods are in a fairly good agreement. The difference is believed to be due to the heterogeneous nature of this nanocomposite which invites for the definition of an effective value for the work function, which will be a key parameter in the design of practical devices with nanocomposite electrodes.

Quantum dots (QD) are rapidly making their way into several application sectors including optoelectronics, and there is a strong need to focus on non-toxic QDs. In this work, we have synthesized graphene QDs in the size range of 1.4–4.2 nm from inexpensive graphite by oxidative cleavage using a sulphuric and nitric acid mixture. A subsequent hydrogen peroxide oxidation step, investigated using two thermal budgets, has resulted in QDs with excellent photoluminescence (PL) intensity. Prolonged, higher temperature oxidation results in smaller size GQDs. X-ray photoelectron spectroscopy analysis confirmed the role of ·OH radicals in the oxidation process and Raman analysis revealed that the higher thermal budget oxidation results in lower defect density. To overcome the challenges in device adaptability due to the inherent acidity in the QDs, a post-synthesis neutralization process was devised. The neutralized GQDs were formed into a film to be used as the active layer in a photodetector device. Fluorescence decay analysis showed there is no significant change in lifetime because of the film formation process. The fabricated GQD photodetector device exhibited high photocurrent under ultraviolet illumination with an ON/OFF ratio of 400% at an applied bias of ±1 V. The device performance underlines the high potential for the non-toxic, top-down synthesized GQDs for application in optoelectronic devices.

Colloidal quantum dots (QD) are rapidly making their way into several optoelectronic applications. While Cd- and Pb-based QDs are promising for high-performance devices, toxicity remains a concern. In this regard, Cu-In-S (CIS) QDs provide an alternative option for scalable, commercial production. A low-temperature, high-throughput process was used to synthesize CIS QDs with 1-dodecanethiol (DDT) ligands. While QD synthesis with a DDT ligand is facile, there are drawbacks when it comes to device implementation and interfacing with electron transport films such as ZnO. A ligand exchange process was employed to replace the DDT ligands in the as-synthesized QDs with 3-mercaptopropionic acid (MPA); owing to the short and bifunctional nature of the MPA molecules, this process improved the surface adhesion and carrier transport to ZnO. I-V measurements on planar structures showed the significance of ligand exchange in CIS QDs for heterojunction device implementation.

Printable electrodes that induce less stress and require lower curing temperatures compared to traditional screen-printed metal pastes are needed in thin wafer devices such as future solar cells, and in flexible electronics. The synthesis of nanocomposites by incorporating graphene nanopowders as well as silver nanowires into epoxy-based electrically conductive adhesives (ECA) is examined to improve electrical conductivity and to develop alternate printable electrode materials that induce less stress on the wafer. For the synthesized graphene and Ag nanowire-decorated ECA nanocomposites, the curing kinetics were studied by dynamic and isothermal differential scanning calorimetry measurements. Thermogravimetric analysis on ECA, ECA-AG and ECA/graphene nanopowder nanocomposites showed that the temperatures for onset of decomposition are higher than their corresponding glass transition temperature (T g) indicating an excellent thermal resistance. Printed ECA/Ag nanowire nanocomposites showed 90% higher electrical conductivity than ECA films, whereas the ECA/graphene nanocomposites increased the conductivity by over two orders of magnitude. Scanning electron microscopy results also revealed the effect of fillers morphology on the conductivity improvement and current transfer mechanisms in nanocomposites. Residual stress analysis performed on Si wafers showed that the ECA and nanocomposite printed wafers are subjected to much lower stress compared to those printed with metallic pastes. The observed parameters of low curing temperature, good thermal resistance, reasonably high conductivity, and low residual stress in the ECA/graphene nanocomposite makes this material a promising alternative in screen-printed electrode formation in thin substrates.

Al-ZnO/Si heterojunction-based large area, self-powered, low reverse leakage current, and broadband photodetectors with high responsivity are fabricated using the simple and low temperature process. Different from widely applied (p)Si-based heterojunctions, these devices are (n)Si-based and offer superior performance. With optimized Al–ZnO thickness, excellent light confinement is achieved. An interfacial passivation layer that improves the hetero-interface helps achieved a reverse current density as low as $4.5 \times 10^{-7}$ A/cm $^{-2}$ at −0.5 V for a 4.84 cm2 device, with rectification ratio of $2.7 \times 10^{3}$ at ±0.5 V. In self-powered mode, the photodetector achieved responsivities of 0.292, 0.426, and 0.486 A/W (external quantum efficiency, EQE of 78.7%, 96.1%, and 98.8%) for primary blue (460 nm), green (550 nm), and red (610 nm) light, respectively. The operation of self-powered Al–ZnO/(n)Si photodetectors is demonstrated for the first time with modulated optical signal at circuit level.

Copper Indium Sulfide (CIS) based quantum dots (QDs) have strong potential for future large-scale applications in photovoltaic, display, and optoelectronic sectors, partly due to their minimal toxicity. CIS QDs are usually grown at temperatures over 200 °C. In this work, we present a gram-scale, noninjection synthesis of CIS-based QDs in a low temperature regime and report, through systematic analyses, on the influence of temperature ramping profiles and precursor compositions on the properties of core CIS and core/shell CIS/ZnS QDs. It is established that the temperature ramp method is as important as the peak temperature itself. Nonabrupt temperature ramping results in core CIS QDs with more ligand coverage and fewer defects, and makes ZnS overgrowth possible even with lower Zn:S molar ratios. Precursor level molar ratios of 1:2 for Cu:In and 8:1 for Zn:S resulted in improved efficiency in CIS and CIS/ZnS QDs with strong and long-lived emissions in the 85–245 ns range. Implementing a dropwise addition of the ZnS precursor leads to prolonged QD extraction and shorter emission wavelengths. The different processes showed a wide range of tunability in the visible-to-IR range along with intense photoluminescence. The ZnS shell growth dependence on temperature ramping mode is explained through a mechanism for sulfur consumption, from the core or from the ligands’ thiol groups. The low temperature regime processes, tunability for wide range of emissions, and identified pathways for long-lived emissions make these less-toxic, CIS-based QDs amenable to large area, scaled-up processing for device applications.

Al–ZnO/(n)Si and Al–ZnO/(p)Si heterojunction structures were fabricated by RF-sputtering of Al–ZnO thin film on Si substrates and their electrical and photoresponse properties were comparatively analyzed. The Al–ZnO/(n)Si heterojunction exhibited excellent diode behavior with an equivalent barrier height of 0.72 eV and an rectification ratio of $6.7 \times 10^{3}$ at ±0.5 V, while no rectifying behavior was observed for Al–ZnO/(p)Si device. Illuminated I–V analysis performed with different light intensities revealed excellent photoresponse properties and the strong dependence of the photocurrent on external bias for both Al–ZnO/(n)Si and Al–ZnO/(p)Si structures. The Al–ZnO/(n)Si structure requires smaller reverse bias than Al–ZnO/(p)Si for similar photoresponse. Based on dark I–V, illuminated I–V, and the energy band diagrams, photoresponse mechanisms were explained. In order to further understand the photoresponse mechanisms, quantum efficiency measurements were carried out, with applied external voltage bias. A general descriptive model was presented for the first time for the photogenerated current density in both Al–ZnO/(n)Si and Al–ZnO/(p)Si heterojunctions at different voltage bias.

Aerosolized quantum dots (QDs) and other nanoparticles (NPs) represent a significant safety hazard in the development of large-scale manufacturing processes that utilize nanomaterials. The safety concerns posed by nanoparticle exposure limit work even at the research and prototyping stage. QDs, which can be composed of a variety of semiconductor materials, are typically in the 1-10 nm size range. Material selection is an important aspect of safety risks, as many common QDs have highly toxic Cd- or Pb- based structures that magnify safety risks. In this work, the integration of QDs into photovoltaic (PV) devices is demonstrated during module lamination, in which QD film is deposited onto the solar glass and laminated in contact with ethylene-vinyl acetate (EVA). We present safe handling methods and considerations for QD EVA lamination in a research and prototyping environment. Before implementation in large- scale, large-area manufacturing environments, it is significant to establish safety procedures in handling of nanomaterials at the research phase. This work presents development and integration of nanomaterials in the R&D phase of PV, and provides some guidelines on implementation of safety measures.

Quantum dots (QDs) are investigated for photovoltaic (PV) device applications; due to safety concerns regarding the use of Cd, Pb-based materials, less-toxic copper indium sulfide (CIS) and zinc copper indium sulfide (ZCIS) QDs have gained prominence. In advanced PV structures, the formation and morphology of QD films is significant; the construction of uniform, defect-free QD films are desirable. Several strategies are employed in QD film formation, most notably spin-casting, drop-casting, and dip-coating. We studied the film deposition and self-assembly QDs in formation of device-quality films. Using dip-coating, we developed a method to deposit uniform, ordered less-toxic CIS and ZCIS QD films for large-area, scaled-up PV applications.

We have synthesized reduced graphene oxide (rGO) in a green solution-processable method to be used as a transparent conductive electrode (TCE) in photovoltaic devices. Ascorbic acid (L-AA) reductant was used as an environmentfriendly alternative for hydrazine. rGO produced by L-AA can display promising electrical and optical properties. rGO can be dissolved in water and other organic solvents, then solution-deposited to form thin films on the substrate to form TCE. FTIR measurements determined the chemical composition of the synthesized rGO to be mainly carbon. The conductivity of rGO was measured; the resultant value indicated the structure to be deoxygenated graphene. The synthesized rGO is applicable to large-scale TCE in photovoltaic devices.

Zinc oxide (ZnO) nanowires (NWs) are gaining importance in diverse applications because of their excellent electrical and optical properties. The modulation of the ZnO NW bandgap by doping further enhances their potential. In this work, we have analyzed this bandgap tuning and have hydrothermally synthesized Aluminum (Al) doped and undoped ZnO NWs on Al doped ZnO (AZO) seed layer. The photoluminescence (PL) properties of the NWs have been investigated and it is found that doping increases the optical bandgap of the NWs and also enhances the near-band-edge (NBE) emission while suppressing defect emissions in the visible region. Al doping also increases the free carrier concentration and subsequent electrical conductivity of the NWs. The improvement in the optical and electrical properties of the Al doped ZnO NWs makes them a promising candidate for applications in photovoltaic (PV) devices.

Operation on-site for Internet of Things (IoT) applications and (to a lesser extend) driven by the needs of Industry 4.0 and the requirement for “bringing part of the lab to the sample” (for on-site chemical analysis applications), are the main driving forces behind development of fieldable micro- or nano-sensors, and of micro- or nano-instruments. Such approaches typically require battery-operation (thus requiring regular battery-replacement). Would it not be ideal if field-operated systems were powered from nanoenergy harvested from ambient sources or even if they were self-powered (i.e., without needing an external power supply)? In this paper, two approaches are explored: One approach involves use of Tribo Electric Nanogenerators (TENGs) and the other a self-powered detector.

Dye-sensitized solar cells (DSSCs) are still attracting great attention as a potential alternative to conventional silicon solar cells, and a lot of current research is focusing on enhancing their efficiency through the individual improvement of the DSSCs components, as well as their integration. Among all components, mesoporous TiO 2 plays a major role, because it is the charge collector for the dye molecules or perovskite in liquid and solid DSSCs, respectively. Specificities of TiO 2 are now well-known: an anatase crystalline structure for an optimized bandgap, small particle size to shorten the charge diffusion path, reduced grain boundaries to limit current resistivity, significant porosity to enhance integration and interaction between the photon absorbers and the charge-collecting anode. We reported previously how well-crystallized small nanoparticles were successfully synthesized via a room temperature synthesis by Hegazy and Prouzet (2012) [ 10 ], leading to very small crystallites (4–10 nm) of pure TiO 2 anatase. Since as-synthesized materials still contain hydration water, a major drawback in DSSCs, we report here how dehydration and crystallinity can be improved by annealing at a moderate temperature, without any sintering or significant particle coarsening. The resulting impact on properties is confirmed by testing the resulting annealed TiO 2 nanopowder in photovoltaic liquid DSSCs. In addition, we describe a comprehensive study of the preparation and test of TiO 2 based photoanodes, including UV exposure and HCl treatment. Photovoltaic tests were conducted on a large active surfaces (5 cm 2 ) cells to give a more realistic picture of the influence of each parameter involved in the cell performance, characterized under AM 1.5 illumination with 100 mWcm −2 light intensity. The specific contribution of each parameter on the cell performance is discussed. We demonstrate why the TiO 2 nanopowder developed in our previous synthesis, in connection with the suitable treatments during the photoanode preparation, can replace commercial powders, with a greater success in cell photoefficiency.

Copper Indium Sulfide (CIS)-based quantum dots (QDs) are considered as a safer alternative compared to carcinogenic cadmium- and lead-based QDs. Here, we present a facile, high throughput, and non-injection method of synthesizing CIS-based QDs. The structure, shape, size, and crystalline structure of the synthesized QDs were studied using high resolution transmission electron microscopy (HRTEM). The effects of temperature and compositional dependency on the structure and optical properties of the resulting QDs were investigated using elemental, absorption, photoluminescence (PL), and time-resolved spectroscopic analyses. We observed that a gradient increase of temperature during the core growth, as well as addition of excess indium (In) and zinc (Zn) precursors during core and core/shell synthesis, at low growth temperatures, resulted in QDs with improved PL and lifetime. The large Stokes shift, broad emission spectra, and long-lived emission of the synthesized QDs reveal their potential applicability to third generation photovoltaic and optoelectronic devices.

The effect of rapid thermal annealing (RTA) on n-type hydrogenated amorphous silicon (a-Si:H) films deposited on single-crystal silicon (c-Si) wafers was studied by electrical and optical methods. Deposition of a-Si:H films by plasma-enhanced chemical vapor deposition (PECVD) was optimized for high deposition rate and maximum film uniformity. RTA processed films were characterized by spreading resistance profiling (SRP), Hall effect, spectroscopic ellipsometry, defect etching, and transmission electron microscopy (TEM). It was found that the films processed between 600 °C and 1000 °C were highly crystalline and that the defect density in the films diminished with increasing thermal budget. Junctions formed by the RTA processed n-type a-Si:H films on p-type c-Si wafers were tested for device applicability. It was established that these films can be used as the emitter layer in n + p photovoltaic (PV) devices with over 14% conversion efficiency.

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.

Two new physical designs for sensors that can be used for strain measurement in large structures, such as bridges, wind turbines, or airplanes, are presented. While the proposed sensor designs focus on high sensitivity, they are based on simple operating principle of comb-drive differential variable capacitances and chevron displacement amplification. The chevron beams convert small amount of applied strains to measurable changes in capacitance of comb fingers. The design of the structures enables simple fabrication methods for the realization of the sensors. Two designs are proposed with the first design can also be used as a sensitive resonant strain sensor. Device performances are validated both by analytical solutions and also by finite-element method simulations. The obtained nominal capacitance is 25 fF, with sensitivities of 13 and 2.7 aF per microstrain ( $\mu \varepsilon$ ) while demonstrating a maximum strain range of ±1000 and $\pm 1800~\mu \varepsilon $ , respectively, for the first and second designs. As a resonant strain sensor, the first design exhibits a sensitivity of $\sim 8.6$ Hz/ $\mu \varepsilon $ .

Maintenance and repair of wind turbine structures have become more challenging and at the same time essential as they evolve into larger dimensions or located in places with limited access. Even small structural damages may invoke catastrophic detriment to the integrity of the system. So, cost-effective, predictive, and reliable structural health monitoring (SHM) system has been always desirable for wind turbines. A real-time nondestructive SHM technique based on multisensor data fusion is proposed in this paper. The objective is to critically analyze and evaluate the feasibility of the proposed technique to identify and localize damages in wind turbine blades. The structural properties of the turbine blade before and after damage are investigated through different sets of finite-element method simulations. Based on the obtained results, it is shown that information from smart sensors, measuring strains, and vibrations data, distributed over the turbine blades can be used to assist in more accurate damage detection and overall understanding of the health condition of blades. Data fusion technique is proposed to combine these two diagnostic tools to improve the detection system that provides a more robust reading with reduced false alarms.

Lowering production costs for crystalline silicon solar cells is critical. Fabricating large volumes of higher efficiency industrial cells is key to lower costs. Also critical are improving the efficiency of metal–insulator–semiconductor inversion layer (MIS-IL) solar cells, improving substrate materials, and further developing commercial PV modules.

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.

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.

We demonstrate a scalable process for manufacturing electronic backplanes suitable for large area OLED displays utilizing small isolated regions of Single-Crystal Silicon (SCS) on arbitrary substrates. Planarized SCS particles form the active regions. Mobility (p-type) of ∼400 cm2/V·s was measured.

The transmission of strain fields in adhesively bonded MEMS strain sensors is analyzed. In strain sensors that are attached to host structures using adhesive layers such as epoxy, complete strain transfer to the sensor is hindered due to the influence of the adhesive layer on the transfer. This paper presents an analytical model, validated by finite element method simulation, to provide insight and accurate formulation for strain transfer mechanism for bonded sensors. A shear-lag parameter has been introduced to account for the component geometry and properties. The model is capable of predicting the strain transmission ratio through a sensor gauge factor, and it clearly establishes the effects of the flexibility, length, and thickness of the adhesive layer and MEMS sensor substrate. Finally, modifications to sensor substrates, by the implementation of micromachined tapered edges and trench etching, are proposed in order to increase the strain transmission ratio. It has been found, for the selected case study, that the sensor sensitivity could be enhanced by up to 30%. This work on bonding analysis is applicable for performance prediction and calibration in sensor systems design.

An organic liquid polymer based on methyltriethoxysilane has been synthesized. The material;when spin-coated onto wafers and thermally treated;lead to dielectric films of low permittivity;which can be used in hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) based large area imaging arrays as an inter-level dielectric between the TFT and pixel levels. Fourier Transform Infrared Spectroscopy (FTIR) shows prominent peaks of Si-CH"3 stretch and Si-O stretch modes. The dielectric constant (k) of the film was found to decrease with increasing curing temperature (T"c"u"r"e). However;k values were

The calibration and comparison to experimental data of a parallel computing multi-zone combustion model for simulating operational characteristics of an n-butanol/n-heptane fuelled Homogeneous Charge Compression Ignition (HCCI) engine utilizing the negative valve overlap (NVO) technology is described. The model is calibrated using one experimentally characterized operating point and by taking into account the major features of NVO. The model simulations at other four operating points closely match the cylinder pressure trace and heat release rates of the experiments. The unburned hydrocarbon emission is predicted to a reasonable level while NOx (nitric oxide NO and nitrogen dioxide NO2) formation is under-predicted. The difficulty in mapping the NOx emission is attributed to the fact that the combustion model operates within the closed cycle period of engines and therefore does not capture the complexity of the charge stratification within the NVO equipped engine. Nevertheless, the trend of increasing NOx levels with the increasing fraction of butanol in the fuel mixture is captured and the model is able to predict the pressure, heat release rates, and combustion phasing for the three fuel blends tested.

The growth process for spherical silicon crystals using melting and recrystallization of polysilicon powder and polysilicon granules is investigated and the structural properties of the fabricated silicon spheres are characterized. The recrystallization conditions that influence the crystallinity of the spheres are studied. It is shown by means of defect delineation that crystalline structures are obtained when the melt is solidified at temperatures above 1400°C. The results of electron backscattering confirm the formation of single-grain structures. When solidification takes place at temperatures below 1400°C, double- and multiple-grain structures start to form. The concentration of impurities during the recrystallization of the spheres, monitored by secondary-ion mass spectroscopy, show that the rate of impurity removal from the crystalline phase is much lower than predicted by segregation at the melt–solid interface, which is attributed to the nonequilibrium conditions in the solidification process.

The photoconductivity decay (PCD) in silicon (Si) spheres after excess carrier generation by a laser impulse is analyzed. The effect of the non-uniformity of minority carrier lifetime in spherical Si is modelled. It is shown that PCD in Si spheres is determined by the subsurface lifetime and depth of the denuded zone, where oxygen and other impurity content is low as a result of the sphere fabrication process. Combining the model with experimental PCD data, subsurface lifetime and the denuded zone depth of the spheres are extracted. The method is useful in monitoring the purification steps in Si sphere fabrication and in the design of electronic devices.

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.

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.

Heterojunction solar cells formed by nanocrystalline silicon films on fine-grained multicrystalline silicon substrates are simulated in the presence of grain boundaries. The effects of grain boundaries on the dark and illuminated current–voltage (I–V) characteristics and spectral response (SR) of heterojunction (HJ) solar cells are assessed using 1D and 2D device simulations. The grain boundary in fine-grained multicrystalline silicon is modelled in two ways: as a defective surface with continuous defect distribution throughout the bandgap, and as a hypothetical sheet with a certain recombination velocity for electrons and holes. The SR and I–V characteristics of HJs are exploited to characterize grain boundary effects on the photovoltaic properties of the solar cells and photodetectors. Simulation results show noticeable differences on the dark I–V and SR of on- and off-grain boundary HJs. Grain boundary effects become important when fine-grained multicrystalline substrates are used. Measurement results of tiny test structures fabricated on the grain boundary show consistently inferior dark I–V and SR characteristics compared to those fabricated away from the grain and allow us to quantify the recombination at the grain boundary.

A three-dimensional numerical model is presented to simulate spherical p-n junction silicon solar cells, which is a promising new technology for photovoltaic (PV) energy conversion for terrestrial applications. Material properties imposed by the sphere formation method, geometry of the device, and the specific device structure stemming from the fabrication technology are taken into account in the optical and electrical models of the device. The spherical device is numerically simulated based on these models using finite-difference method in a spherical system of coordinates, generating the internal quantum efficiency and current-voltage (I-V) characteristics of the device. It has been shown that the efficiency of a spherical solar cell is slightly lower than a conventional device; however, the slightly inferior performance does not outweigh the cost advantage. It has been also found that subsurface diffusion length from effective impurity segregation and the depth of the denuded zone in spherical devices are parameters that mainly affect the device efficiency. Based on the simulation and analysis, design guidelines have been presented for spherical PV devices.

Affordability is the key to any renewable energy technology in becoming a viable alternative to conventional energy technology. Solar photovoltaics (PV) is a promising and environment-friendly energy conversion technique, experiencing steady market growth. Cost reduction of PV cells relies largely on the use of low-cost silicon materials which tend to compromise the device performance. This underscores the need for new, material-specific process technologies to be used in order to achieve an acceptable $/Wp. In this paper, first an overview of different techniques for the fabrication of low-cost silicon will be presented. We highlight the need to modify the processing technology while using defective, cost-effective Si substrates. This is followed by the description of research efforts on the development of a cost-effective heterojunction cell fabrication technology, as well as the results on the simplified process.

Amorphous silicon (a-Si:H) deposited at 150 °C by plasma-enhanced chemical vapor deposition (PECVD) is investigated as a structural layer for low temperature microelectromechanical system (MEMS) fabrication. The process development of depositing thick a-Si:H films and the material characterization of the film stress and hydrogen content is presented. To demonstrate a MEMS application, bimorph thermal actuators incorporating a-Si:H and aluminum were designed and fabricated resulting in tip deflection of hundreds of microns. The PECVD film coverage of various sidewall structures was also studied by scanning electron microscopy. Our a-Si:H films formed a sidewall coverage angle of 80° with the substrate. Mechanical simulations relate this angle within the range of minimum stress and maximum deflection of the actuator. Our results indicate that amorphous silicon is as an attractive structural material offering a low thermal budget for post-processing and integration of MEMS devices with complementary metal-oxide–semiconductor technology.

The quality factor (Q) of inductors on silicon (Si) is limited by the series resistance of the metal at low frequency and by the substrate resistivity at high frequency. Oxide is generally used to isolate the useful signal of the inductor from the lossy Si substrate. However, stoichiometric silica (SiO2) is processed at high temperature which restricts the possibility of post-complementary metal-oxide semiconductor integration, and due to the large thermal mismatch with Si, thick oxide films introduce considerable stress. In this work, we apply electrochemically formed porous Si (PS) and intrinsic amorphous Si (i-a-Si:H) deposited at low temperature (250°C) in an approach asan isolation bilayer for planar inductors on Si for rf integrated circuits. An improvement of morethan 45% in Q was measured at a frequency of 6.4GHz for 1.6nH inductor incorporating PS andi-a-Si:H films. The experimental results presented indicate that these low-temperature materials are promising for the isolation of rf devices on lo...

We describe a maskless reactive ion etching process that employs CF4 gas plasma to create nanoscale structures in silicon. Process conditions are controlled to produce pillars of up to 2μm tall and less than 50nm wide. The contributing mechanisms are discussed based on the trends observed for varying plasma conditions. Higher pressures or lower self-bias voltages result in pyramidal structures. Lower pressure and higher voltage result in needlelike structures that resemble silicon wires. By carefully controlling the automasking process mechanism, columnar silicon structures were reproducibly formed with good uniformity all over the wafer. The regularity of the fabricated structures, process controllability, and process compatibility of dry etching are promising for potential photonic and optoelectronic applications.

High-quality heterojunction diodes with near-unity ideality factor were fabricated by direct deposition of a (n+) nanocrystalline silicon film on top of fine-grained (p) multicrystalline silicon substrates. A very good ideality factor of 1.08 was achieved using a single-diode model in the medium forward-bias regime. Current-voltage (IV) characteristics of the diodes measured in the dark show that the recombination at the heterointerface is much less than the recombination at the space-charge region in the low forward-bias regime confirming the fact that the junction quality is good and is suitable for photovoltaic applications. Internal quantum efficiency measurements performed on these cells show a high (>70%) blue response partly due to a high transparency of the n+ nanocrystalline material. Illuminated IV of the heterojunction solar cells show a high fill factor of 78%–79% and an acceptable open circuit voltage of 550mV for a simple structure without a rear-surface passivation or transparent conductive...

Deposition conditions and some structural and electrical properties of amorphous silicon nitride (SixNy:H) films deposited on Si substrates have been studied for photovoltaic applications. A plasma enhanced chemical vapor deposition (PECVD) system has been used for the study. Experiments have been performed varying the flow ratios and dilution of the reactant gases. Increased hydrogen (H2) dilution leads to reduced deposition rate and a better controllability in the growth process. The hydrogen content in the film also decreases with increasing H2 dilution of the reactant gases. Flow ratio of the reactant gases (SiH4/NH3) also influences the growth rate. There is an optimal reactant gas mix to maximize the film growth rate. However, the film stoichiometry is also modified by changing the gas mix, with higher flow ratios resulting in Si-rich films. The level of interfacial recombination of minority carriers has been studied by capacitance-voltage and effective lifetime measurements. Bombardment by the energetic species in the plasma leads to plasma damage at the interface. These interfacial defects can be annealed by a post-deposition, low temperature treatment.

The quality factor (Q) of inductors on silicon (Si) is limited by the series resistance of the metal at low frequency and by the substrate resistivity at high frequency. Oxide is generally used to isolate the useful signal of the inductor from the lossy substrate. However, stoichiometric silica (SiO2) is processed at a high temperature, which eliminates the possibility of post-CMOS integration. By contrast, plasma enhanced chemical vapour deposition (PECVD) amorphous Si can be deposited at a low temperature and is easily integrated with most Si-based processes. Intrinsic amorphous hydrogenated silicon (i-a-Si:H) also displays low conductivity. In this work, i-a-Si:H deposited at a low temperature (250?) is used in a novel approach as the isolation material for planar inductors on Si for RF integrated circuits. An improvementof more than 50% in Q is measured when 1.5 µm i-a-Si:H film is deposited on the Si substrate prior to fabrication of the inductor. This result demonstrates the influence of i-a-Si:H film on the RF performance of an inductor. Intrinsic a-Si:H is shown to be a promising material for the isolation of RF devices on low-resistivity Si.

Spin-coated low-k dielectrics are now widely used in integrated circuit processing due to their low permittivity and planarization properties. Another area of potential application is in large area digital imaging using amorphous silicon (a-Si:H) technology where low-k dielectrics enable new integration schemes for thin film transistors (TFT) and sensors. In this work, the properties of spin-coated, polymeric, low-k dielectric materials, BCB (benzocyclobutene) and HSQ (hydrogen silsesquioxane), are studied after treating them with low temperature anneals. Fourier transform infrared spectroscopy (FTIR), high frequency capacitance–voltage, topographic planarization, and wafer deflection stress measurements have been used to characterize the films so as to correlate with processing conditions. Lower k values are obtained for lower process temperatures and are correlated by capacitance and FTIR measurements. Annealing in the presence of O2 appears to increase the permittivity. The BCB films yield low stress a...

Rapid Thermal Processing (RTP), originally developed for processing microelectronic devices has been investigated in the recent decade for its potential in the production of Si solar cells. This paper will discuss the use of RTP for industrial Si solar cells with screen-printed contacts. Printed metal contacts require adapted emitters when good fill factors should be achieved. Multi-crystalline Si substrates require to adapt the temperature ramps of RTP to avoid minority carrier lifetime degradation from activated defect centres. Finally, industrial processing requires high throughput that cannot be achieved with conventional RTP equipment. This paper will present an advanced selective emitter process and a recently developed continuous RTP system that meet for the first time the requirements to make RTP compatible with industrial solar cell processing. The limits of industrial RTP solar cell processing will be discussed.