Performance analysis of AlGaAs/GaAs tunnel junctions for ultra-high concentration photovoltaics

An n(++)-GaAs/p(++)-AlGaAs tunnel junction with a peak current density of 10 100Acm(-2) is developed. This device is a tunnel junction for multijunction solar cells, grown lattice-matched on standard GaAs or Ge substrates, with the highest peak current density ever reported. The voltage drop for a current density equivalent to the operation of the multijunction solar cell up to 10 000 suns is below 5 mV. Trap-assisted tunnelling is proposed to be behind this performance, which cannot be justified by simple band-to-band tunnelling. The metal-organic vapour-phase epitaxy growth conditions, which are in the limits of the transport-limited regime, and the heavy tellurium doping levels are the proposed origins of the defects enabling trap-assisted tunnelling. The hypothesis of trap-assisted tunnelling is supported by the observed annealing behaviour of the tunnel junctions, which cannot be explained in terms of dopant diffusion or passivation. For the integration of these tunnel junctions into a triple-junction solar cell, AlGaAs barrier layers are introduced to suppress the formation of parasitic junctions, but this is found to significantly degrade the performance of the tunnel junctions. However, the annealed tunnel junctions with barrier layers still exhibit a peak current density higher than 2500Acm(-2) and a voltage drop at 10 000 suns of around 20 mV, which are excellent properties for tunnel junctions and mean they can serve as low-loss interconnections in multijunction solar cells working at ultra-high concentrations

NGCPV: a new generation of concentrator photovoltaic cells, modules and systems

Starting on June 2011, NGCPV is the first project funded jointly between the European Commission (EC) and the New Energy and Industrial Technology Development Organization (NEDO) of Japan to research on new generation concentration photovoltaics (CPV). The Project, through a collaborative research between seven European and nine Japanese leading research centers in the field of CPV, aims at lowering the cost of the CPVproduced photovoltaic kWh down to 5 ?cents. The main objective of the project is to improve the present concentrator cell, module and system efficiency, as well as developing advanced characterization tools for CPV components and systems. As particular targets, the project aims at achieving a cell efficiency of at least 45% and a CPV module with an efficiency greater than 35%. This paper describes the R&D activities that are being carried out within the NGCPV project and summarizes some of the most relevant results that have already been attained, for instance: the manufacturing of a 44.4% world record efficiency triple junction solar cell (by Sharp Corp.) and the installation of a 50 kWp experimental CPV plant in Spain, which will be used to obtain accurate forecasts of the energy produced at system level

In vitro culturing of porcine tracheal mucosa as an ideal model for investigating the influence of drugs on human respiratory mucosa

It has been previously shown that fresh mucosa from different mammals could serve as raw material for in vitro culturing with the differentiation of cilia, which are the most important morphological structures for the function of the mucociliary system. Increasing legal restrictions on the removal of human tissue and changing surgical techniques have led to a lack of fresh human mucosa for culturing. Most of the animals that have been used as donors up to now are genetically not very close to human beings and must all be sacrificed for such studies. We, therefore, established a modified system of culturing mucosa cells from the trachea of pigs, which is available as a regular by-product after slaughtering. With respect to the possibility of developing “beating” cilia, it could be shown that the speed of cell proliferation until adhesion to the coated culture dishes, the formation of conjunctions of cell clusters and the proliferation of cilia were comparable for porcine and human mucosa. Moreover, it could be demonstrated that the porcine cilia beat frequency of 7.57 ± 1.39 Hz was comparable to the human mucosa cells beat frequency of 7.3 ± 1.4 Hz and that this beat frequency was absolutely constant over the investigation time of 360 min. In order to prove whether the reaction to different drugs is comparable between the porcine and human cilia, we initially tested benzalkonium chloride, which is known to be toxic for human cells, followed by naphazoline, which we found in previous studies on human mucosa to be non-toxic. The results clearly showed that the functional and morphological reactions of the porcine ciliated cells to these substances were similar to the reaction we found in the in vitro cultured human mucosa

Wafer‐bonded two‐terminal III‐V//Si triple‐junction solar cell with power conversion efficiency of 36.1% at AM1.5g

In this work, we present the fabrication and analysis of a wafer-bonded GaInP/GaInAsP//Si triple-junction solar cell with 36.1% conversion efficiency under AM1.5g spectral illumination. The new cell design presents an improvement over previous III-V//Si triple-junction cells by the implementation of a rear-heterojunction for the middle cell. Furthermore, an advanced metallodielectric rear-side grating was used for light trapping enhancement in the silicon bottom cell that increased the silicon subcell current by 1.4 mA/cm2. The external radiative efficiency was quantified to be 1.5 times higher compared to a reference device with a GaInAsP homojunction middle cell. A luminescent coupling factor of 0.46 between the middle and bottom subcell was determined. The share of recombination in the space-charge region was experimentally shown to be insignificant as intended by the rear-heterojunction design. Overall, the open-circuit voltage of the middle cell increased by 61 mV compared to the previous generation. Given the established long-term stability of III-V and silicon-based solar cells, these results are promising steps towards the future employment of III-V/Si tandem solar cells

Promotoras as Mental Health Practitioners in Primary Care: A Multi-Method Study of an Intervention to Address Contextual Sources of Depression

We assessed the role of promotoras—briefly trained community health workers—in depression care at community health centers. The intervention focused on four contextual sources of depression in underserved, low-income communities: underemployment, inadequate housing, food insecurity, and violence. A multi-method design included quantitative and ethnographic techniques to study predictors of depression and the intervention’s impact. After a structured training program, primary care practitioners (PCPs) and promotoras collaboratively followed a clinical algorithm in which PCPs prescribed medications and/or arranged consultations by mental health professionals and promotoras addressed the contextual sources of depression. Based on an intake interview with 464 randomly recruited patients, 120 patients with depression were randomized to enhanced care plus the promotora contextual intervention, or to enhanced care alone. All four contextual problems emerged as strong predictors of depression (chi square, p < .05); logistic regression revealed housing and food insecurity as the most important predictors (odds ratios both 2.40, p < .05). Unexpected challenges arose in the intervention’s implementation, involving infrastructure at the health centers, boundaries of the promotoras’ roles, and “turf” issues with medical assistants. In the quantitative assessment, the intervention did not lead to statistically significant improvements in depression (odds ratio 4.33, confidence interval overlapping 1). Ethnographic research demonstrated a predominantly positive response to the intervention among stakeholders, including patients, promotoras, PCPs, non-professional staff workers, administrators, and community advisory board members. Due to continuing unmet mental health needs, we favor further assessment of innovative roles for community health workers

Uncertainty estimation of spectral matching ratio based power rating of CPV modules

The concentrator standard conditions are defined in the standard IEC 62670-1. Those conditions demand for spectral conditions equivalent to AM1.5d as described in IEC 60904-3. The power output of CPV modules has to be rated at the concentrator standard conditions and thus at AM1.5d spectral irradiance. According to IEC standard 62670-3 the prevailing spectral conditions have to be characterized using spectral matching ratios (SMR). The SMR values have to be within three percent of unity to allow for standardized power ratings. The SMR values are calculated from component cell sensor readings. The most commonly used component cells are based on lattice-matched triple-junction cell structures with bandgaps of 1.9, 1.4 and 0.7 eV. In this work, the usage of these component cells for power ratings on CPV modules equipped with other types of multi-junction cells is investigated. This investigation is based on representative power outputs of CPV modules. These power outputs were calculated using i) the spectral irradiance modeling software SMARTS2, ii) measured external quantum efficiencies and iii) the two-diode model. The outcome of this investigation is an estimation for the measurement uncertainty of rated CPV module power output when using the SMR filtering approach recommended in IEC 62670-3

Translating outdoor CPV I-V measurements to a CSTC power rating and the associated uncertainty

A complete procedure is presented for translating outdoor concentrator photovoltaic (CPV) I-V measurements to the Concentrator Standard Test Conditions (CTSC) (1000W/m2 and 25°C cell temperature). Methods are demonstrated for measuring all the necessary input parameters for the translation, including outdoor thermal transient measurements and indoor dark I-V curves. Four modules are subjected to the translation method based on multiple months of outdoor data, one module measured at National Renewable Energy Laboratory and three at Fraunhofer ISE. The modules are also characterized under a sun simulator to provide a comparison to the translation approach. The results show that translated CSTC efficiencies are in good agreement with the efficiencies from the solar simulator. Two of the modules agreed within 1%, whereas the other two modules agree within approximately 4%. An uncertainty analysis of the input parameters is discussed in the context of the total uncertainty associated with the translation to CSTC. The reference voltage and efficiency temperature coefficient are the key parameters impacting the translation uncertainty, whereas uncertainty in the outdoor data is driven by spectral and meteorological parameters

Stepwise measurement procedure for the characterization of large-area photovoltaic modules

In this paper, we present an indoor measurement procedure for characterizing the electrical performance of large aperture photovoltaic modules. Because of the fact that sun simulators, especially for concentrator photovoltaic applications, are strongly limited in the size of the uniformly illuminated area, we developed a measurement procedure that allows characterizing modules with a larger aperture area than the aperture provided by the sun simulator. The procedure is based on the concept of stepwise illumination of the module area and measurement of the corresponding I-V curves-without the need to contact the subunits directly. Using the additionally measured dark I-V curve of the module, the characteristic I-V curve of the full module can be calculated