Quantum dot integration in heterostructure solar cells

InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm² light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm², shows an open circuit voltage V_oc of 0.7 V, a short circuit current I_sc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).     

A novel direct aerodynamically assisted threading methodology for generating biologically viable microthreads encapsulating living primary cells

In a recent discovery, coaxial electrospinning was explored to encapsulate living organisms within a continuous bio‐polymeric microthread from which active biological scaffolds were fabricated (Townsend‐Nicholson and Jayasinghe, Biomacromolecules 2006, 7, 3364). The cells were demonstrated to have gone through all expected cellular activity without their viability being compromised. These biologically active threads and scaffolds have direct and tremendous applicability from regenerative to therapeutic medicine. Currently these post‐processed cells as composite threads and scaffolds are being investigated in‐depth at a cellular level to establish if the processing methodology has any affect on the cellular make‐up. We now demonstrate a competing non‐electric field driven approach for fabricating composite threads and scaffolds influenced only by a differential pressure. We refer to this novel composite thread to scaffold fabrication methodology as coaxial aerodynamically assisted bio‐threading (CAABT). Our investigations firstly, demonstrate that this technique can process handle living organisms without biologically perturbing them in anyway. Secondly the process is elucidated as possessing the ability to form composite active threads from which biologically viable scaffolds are formed. Finally our study employs florescent activated cell sorting (FACScan), a method by which the cellular dynamics and viability are quantified on control and threaded cellular samples at two prescribed time points. In parallel with FACScan, optical comparison of cellular morphology at three time points within a period of three weeks is carried out to photographically observe any changes in the post‐processed cellular phenotype. Our developmental investigations into this novel aerodynamically assisted threading methodology has unearthed a unique biomicrofabrication approach, which joins cell electrospinning in the cell threading to scaffold fabrication endeavor. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

NiCoFe/C cathode electrocatalysts for direct ethanol fuel cells

A direct ethanol fuel cell (DEFC), which is less prone to ethanol crossover, is reported. The cell consists of PtRu/C catalyst as the anode, Nafion^® 117 membrane, and Ni–Co–Fe (NCF) composite catalyst as the cathode. The NCF catalyst was synthesized by mixing Ni, Co, and Fe complexes into a polymer matrix (melamine-formaldehyde resins), followed by heating the mixture at 800 °C under inert atmosphere. TEM and EDX experiments suggest that the NCF catalyst has alloy structures of Ni, Co and Fe. The catalytic activity of the NCF catalyst for the oxygen reduction reaction (ORR) was compared with that of commercially available Pt/C (CAP) catalyst at different ethanol concentrations. The decrease in open circuit voltage (V_oc) of the DEFC equipped with the NCF catalysts was less than that of CAP catalyst at higher ethanol concentrations. The NCF catalyst was less prone to ethanol oxidation at cathode even when ethanol crossover occurred through the Nafion^®117 film, which prevents voltage drop at the cathode. However, the CAP catalyst did oxidize ethanol at the cathode and caused a decrease in voltage at higher ethanol concentrations.     

CdTe-CdS solar cells — Production in a new baseline and investigation of material properties

In this paper, we describe our new baseline for CSS-CdTe-CdS solar cells on 10 × 10 cm² substrates. The deposition of the p-n junction and all the following steps were performed at the Institut für Festkörperphysik (IFK) in Jena. Using the new baseline, we are already able to produce solar cells with similar properties as commercial ones. In the batch type process, all manufacturing steps can be investigated separately. We employ Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) and external quantum efficiency (EQE) measurements to characterise the structure of the bulk materials and interfaces. It is demonstrated that by RBS the front contact becomes accessible for thinned CdTe films. At the back contact, RBS spectra show a tellurium accumulation which is due to etching. This tellurium rich layer is confirmed by XRD with Rietveld refinement. The intermixing at the CdS-CdTe interface caused by the activation step is quantified by a bandgap determination based on EQE measurements. From the bandgap energy of the CdTe_1 − xS_x compound, we calculated the sulphur fraction x at the interface. XRD measurements imply that the activation step induces a (111) texture in CdTe. With regard to an improved manufacturing process, our cells are compared to industrial cells produced by Antec Solar Energy.     

Reversible changes of the fill factor in the ZnO/CdS/Cu(In,Ga)Se2 solar cells

The reversible persistent changes of the fill factor (FF) induced by the illumination and voltage bias along with changes in the electronic properties of the ZnO/CdS/Cu(In,Ga)Se₂ photovoltaic devices have been studied. Admittance spectroscopy and capacitance–voltage characterization reveal a correlation between the FF and the space charge distribution within the absorber. Our experiments provide evidence that a major source of FF loss in efficient devices is caused by excess negative charge close to the interface. We explain the persistent changes in the net acceptor concentration in the interface region by the relaxation effects due to compensating donors—the same mechanism, which leads to metastable changes of the doping level in the bulk of the absorber.     

Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells

We investigate thin poly(3‐hexylthiophene‐2,5‐diyl)/[6,6]‐phenyl C₆₁ butyric acid methyl ester (P3HT/PCBM) films, which are widely used as active layers in plastic solar cells. Their structural properties are studied by grazing‐incidence X‐ray diffraction (XRD). The size and the orientation of crystalline P3HT nanodomains within the films are determined. PCBM crystallites are not detected in thin films by XRD. Upon annealing, the P3HT crystallinity increases, leading to an increase in the optical absorption and spectral photocurrent in the low‐photon‐energy region. As a consequence, the efficiency of P3HT/PCBM solar cells is significantly increased. A direct relation between efficiency and P3HT crystallinity is demonstrated.     

Space-charge-limited currents for organic solar cells optimisation

Basic suppositions and microphysical origin of the occurrence of the space-charge-limited currents (SCLC) are presented in general and for the temperature-modulated space-charge-limited currents (TM-SCLC) in particular. The criteria are given for the spectroscopical method TM-SCLC to be used for localized electron states elucidation in organic semiconducting materials for organic solar cells optimization and modelling. The “visibility “of the localized states by SCLC method, i.e. the power of the SCLC method to distinguish the localized states, is tested by the modelling, varying the temperature, energy position of localized states and their concentration. Generally, it was determined that the SCLC measurements results are more reliable with the increased energy of the states, with their increased concentration and with decreased temperature. The correlation (or its absence) between the measured current and activation energy on applied voltage, expressed by the dependence of preexponential factor of conductivity on activation energy (Meyer–Neldel rule), gives the possibility to determine the energy range where the reconstruction of density of localized states function is reliable.     

Nisin expression production from Lactococcus lactis in milk whey medium

BACKGROUND: Nisin is a commercially available bacteriocin produced by Lactococcus lactis ATCC 11454 and used as a natural agent in the biopreservation of food. In the current investigation, milk whey, a byproduct from dairy industries was used as a fermentation substrate for the production of nisin. Lactococcus lactis ATCC 11454 was developed in a rotary shaker (30 °C/36 h/100 rpm) using two different media with milk whey (i) without filtration, pH 6.8, adjusted with NaOH 2 mol L^?1 and without pH adjustment, both autoclaved at 121 °C for 30 min, and (ii) filtrated (1.20 µm and 0.22 µm membrane filter). These cultures were transferred five times using 5 mL aliquots of broth culture for every new volume of the respective media. RESULTS: The results showed that culture media composed of milk whey without filtration supplied L. lactis its adaptation needs better than filtrated milk whey. Nisin titers, in milk whey without filtration (pH adjusted), was 11120.13 mg L^?1 in the second transfer, and up to 1628‐fold higher than the filtrated milk whey, 6.83 mg.L^?1 obtained in the first^t transfer. CONCLUSIONS: Biological processing of milk byproducts (milk whey) can be considered a profitable alternative, generating high‐value bioproducts and contributing to decreasing river disposals by dairy industries. Copyright © 2008 Society of Chemical Industry     

Characterization of microspherical semi-transparent solar cells and modules

Microspherical solar cells and modules have been fabricated. The spherical nature of these semi-transparent devices allows the microspherical cells to harvest both directly incident and diffuse components of sunlight thereby improving the solar energy conversion efficiency. Indoor and outdoor characterizations of these three dimensional semi-transparent cells and modules are carried out using a Lambertian reflector in order to assess the maximum efficiency of the devices. In the absence of the reflector the cell efficiency is 13.5% under standard illumination (100 mW cm⁻², A.M. 1.5, 25 °C). However, this is significantly enhanced in the presence of the reflector. Microspherical modules with the reflector are directly compared to similar semi-transparent modules comprised of traditional planar devices, in outdoor tests at low light intensity (2.5–25 mW cm⁻²) to further demonstrate the benefits of the design particularly at low angle of incident radiation.     

Phase relations in the system ln-CulnS2

By DTA and x-ray diffraction the phase relations in the pseudobinary system In-CuInS₂ have been investigated. CuInS₂ hs a melting point of 1090° C and within this system there is a broad region of liquid immiscibility. A four phase invariant reaction exists at 633° C which is of the form: L₂ = L₁+ CuInS₂ + InS.