Metal diffusion barriers for GaAs solar cells.

In this study accelerated ageing testing (AAT), J-V characterization and TEM imaging in combination with phase diagram data from literature are used to assess the potential of Ti, Ni, Pd and Pt as diffusion barriers for Au/Cu-based metallization of III-V solar cells. Ni barriers show the largest potential as at an AAT temperature of 250 °C both cells with 10 and 100 nm thick Ni barriers show significantly better performance compared to Au/Cu cells, with the cells with 10 nm Ni barriers even showing virtually no degradation after 7.5 days at 250 °C (equivalent to 10 years at 100 °C at an Ea of 0.70 eV). Detailed investigation shows that Ni does not act as a barrier in the classical sense, i.e. preventing diffusion of Cu and Au across the barrier. Instead Ni modifies or slows down the interactions taking place during device degradation and thus effectively acts as an 'interaction' barrier. Different interactions occur at temperatures below and above 250 °C and for thin (10 nm) and thick (100 nm) barriers. The results of this study indicate that 10-100 nm thick Ni intermediate layers in the Cu/Au based metallization of III-V solar cells may be beneficial to improve the device stability upon exposure to elevated temperatures.

Degradation mechanism(s) of GaAs solar cells with Cu contacts.

Substrate-based GaAs solar cells having a dense Au/Cu front contact grid with 45% surface coverage were exposed to accelerated life testing at temperatures between 200 and 300 °C. TEM analysis of the front contacts was used to gain a better understanding of the degradation process. During accelerated life testing at 200 °C only intermixing of the Au and Cu in the front contact occurs, without any significant influence on the J-V curve of the cells, even after 1320 h (55 days) of accelerated life testing. At temperatures ≥250 °C a recrystallization process occurs in which the metals of the contact and the GaAs front contact layer interact. Once the grainy recrystallized layer starts to approach the window, diffusion via grain boundaries to the window and into the active region of the solar cells occurs, causing a decrease in Voc due to enhanced non-radiative recombination via Cu trap levels introduced in the active region of the solar cell. To be a valid simulation of space conditions the accelerated life testing temperature should be <250 °C in future experiments, in order to avoid recrystallization of the metals with the GaAs contact layer.

Er(3+)/Yb(3+) upconverters for InGaP solar cells under concentrated broadband illumination.

The inability of solar cell materials to convert all incident photon energy into electrical current, provides a fundamental limit to the solar cell efficiency; the so called Shockley-Queisser (SQ) limit. A process termed upconversion provides a pathway to convert otherwise unabsorbed low energy photons passing through the solar cell into higher energy photons, which subsequently can be redirected back to the solar cell. The combination of a semi-transparent InGaP solar cell with lanthanide upconverters, consisting of ytterbium and erbium ions doped in three different host materials (Gd2O2S, Y2O3 and NaYF4) is investigated. Using sub-band gap light of wavelength range 890 nm to 1045 nm with a total accumulated power density of 2.7 kW m(-2), a distinct photocurrent was measured in the solar cell when the upconverters were applied whereas a zero current was measured without upconverter. Furthermore, a time delay between excitation and emission was observed for all upconverter systems which can be explained by energy transfer upconversion. Also, a quadratic dependence on the illumination intensity was observed for the NaYF4 and Y2O3 host material upconverters. The Gd2O2S host material upconverter deviated from the quadratic illumination intensity dependence towards linear behaviour, which can be attributed to saturation effects occurring at higher illumination power densities.

Photo-generated THz antennas.

Electromagnetic resonances in conducting structures give rise to the enhancement of local fields and extinction efficiencies. Conducting structures are conventionally fabricated with a fixed geometry that determines their resonant response. Here, we challenge this conventional approach by demonstrating the photo-generation of THz linear antennas on a flat semiconductor layer by the structured optical illumination through a spatial light modulator. Free charge carriers are photo-excited only on selected areas, which enables the realization of different conducting antennas on the same sample by simply changing the illumination pattern, thus without the need of physically structuring the sample. These results open a wide range of possibilities for the all-optical spatial control of resonances on surfaces and the concomitant control of THz extinction and local fields.

Nursing thesis database project: a cooperative venture for nursing faculty and computer science professionals.

The authors describe the development of an educational database project designed to promote the accessibility of nursing administration theses data for graduate students and faculty in a university nursing administration program. The roles of a nursing faculty member, a computer laboratory administrator, and a programmer are detailed as they work together from the concept phase of the project through its evaluation. The results of their combined efforts provided a reliable and efficient database, resulted in a closer interdisciplinary relationship, and allowed valuable nursing research information to be shared more widely within the university.

Beta-N-terminal glycohemoglobins in subjects with common hemoglobinopathies: relation with fructosamine and mean erythrocyte age.

Amounts of beta-N-terminal glycohemoglobins (HbX1c), serum fructosamine, and erythrocyte polyamines were determined in nondiabetic adults with HbAA, HbAC, HbAS, HbCC, HbSC, HbSS, and HbS/hereditary persistent HbF (HPFH). The groups did not differ in fructosamine concentrations. Mean (95% confidence limits) HbX1c percentages were: 4.4 (4.1-4.8) for HbA1c in HbAA, 4.3 (3.9-4.8) for HbA1c in HbAC, 4.1 (3.6-4.6) for HbC1c in HbAC, 4.4 (4.0-4.7) for HbA1c in HbAS, 2.6 (range: 2.3-3.8) for HbC1c in HbCC, 2.0 (1.5-2.4) for HbS1c in HbSC, 0.9 (0.6-1.3) for HbS1c in HbSS, and 1.3 (range: 0.8-2.4) for HbS1c in HbS/HPFH. There was a nonlinear inverse relation between HbX1c and erythrocyte polyamines, indicating that HbX1c percentage decreases with decreasing mean erythrocyte age. We conclude that amounts of HbX1c in subjects with heterozygous hemoglobinopathies should be expressed as a percentage of HbX0 + HbX1c, not total hemoglobin. Interpretation of HbX1c in subjects with a decreased erythrocyte half-life is difficult; measurement of fructosamine seems a suitable alternative.

Screening cord blood for hemoglobinopathies and thalassemia by HPLC.

We evaluated the use of an HPLC method for screening hemoglobins in cord blood. We studied the genotype frequencies of the structural hemoglobin variants HbS and HbC and the synthesis variants alpha- and beta(+)-thalassemia in babies born on Curaçao. During three months, 67.2% of all (748) newborns were screened: 122 (24.3%) had an abnormal hemoglobin pattern, of which 53 (43.4%) had a hemoglobinopathy (HbS or HbC), 64 (52.2%) had alpha-thalassemia (HbBarts greater than 0.5%, corresponding to heterozygous or homozygous alpha-thalassemia-2), and 5 (4.1%) had a hemoglobinopathy plus alpha-thalassemia. None of the newborns with heterozygous HbS and HbC had concomitant beta(+)-thalassemia. The population genotype frequency of heterozygous alpha-thalassemia-2 was calculated to be 30.7%. The data are in excellent agreement with those previously established for the adult population and those available from the black population in the United States and Jamaica. Based on the HPLC results, we estimate that 67.1% of newborns with heterozygous alpha-thalassemia-2 remain undetected. A coincidental finding was a relation between demonstrable alpha-thalassemia and short gestation. Because of its superior separating power and high sensitivity for quantifying relatively low percentages of hemoglobins in the presence of HbF0, the HPLC method was preeminently suitable for screening cord-blood samples.

Supplementation of patients with homozygous sickle cell disease with zinc, alpha-tocopherol, vitamin C, soybean oil, and fish oil.

Thirteen patients (aged 0.7-17.9 y) with homozygous sickle cell disease were supplemented with alpha-tocopherol, vitamin C, zinc, and soybean oil (suppl 1; for 8 mo) and alpha-tocopherol, vitamin C, and fish oil (suppl 2; for 7 mo). Urinary zinc (suppl 1), plasma vitamin C, plasma cholesterol ester and erythrocyte (RBC) omega 3 fatty acids (suppl 2), and plasma and RBC alpha-tocopherol (suppl 1 and 2) increased. Suppl 1 decreased irreversibly sickled cells by 37.5%, decreased RBC protoporphyrin and urinary porphyrins, and increased the RBC total fatty acid-cholesterol ratio. Suppl 2 decreased plasma triglycerides, further increased the RBC alpha-tocopherol, moderately increased the RBC double-bond index, but decreased the RBC total fatty acid-cholesterol ratio. Zinc, copper, and porphyrins showed prolonged changes. The supplements did not change hemoglobin concentrations, RBC age (reticulocytes, polyamines), or number of aplastic and vasoocclusive crises. Zinc reduces irreversibly sickled cells. Augmentation of RBC antioxidant status by alpha-tocopherol and vitamin C and incorporation of omega 3 fatty acids into RBCs do not affect hemolytic component. Effects on vasoocclusive component are unclear.

Tolerance maintenance depends on persistence of the tolerizing antigen: evidence from transplantation studies on Xenopus laevis.

In order to assess the role of antigen persistence in the tolerant state, tolerance was induced in Xenopus laevis by the embryonic transplantation of whole eyes or tail tissue. Both types of transplants were seen to heal in and persist, with no signs of immunological incompatibility. At metamorphosis, tail resorption occurred and grafted tail tissue was lost. Eye transplants were maintained through metamorphosis in most eye grafted animals. Eye graft recipients which had maintained the transplant were observed to accept challenge skin allografts from donors of the same genotype as the eye donor in all but one case, while recipients which had lost the eye transplant at metamorphosis or had the eye transplant experimentally removed sometimes did not accept the challenge skin graft. Animals tail grafted as embryos did not accept post metamorphic skin grafts from donors of the same genotype as the tail tissue donor, but rejection was not accelerated. It is proposed that tolerance induction is dependent on the presence of appropriately presented antigen at a time when precursor thymocyte cells are migrating to the thymus, prior to their processing into alloreactive cells, and that tolerance maintenance is dependent upon the persistence of the tolerizing antigen.