Growth of III-V solar cells on silicon by Molecular Beam Epitaxy: towards monolithic III-V/Si tandem multijunction devices

Epitaxial growth of III-V materials on silicon (Si) presents an elegant pathway in order to develop high efficiency III-V/Si multijunction solar cells. Such devices could overcome the 29.4 % efficiency limit inherent to single-junction crystalline silicon (c Si) solar cells while maintaining the comparatively low cost associated with Si substrates. The main challenge of this technology lies in the difference of lattice parameters between Si and suitable III-V materials. This lattice mismatch results in the formation of Threading Dislocations (TDs), which propagate upwards to the active regions of the devices. There, they act as recombination centres, hence reducing the minority carrier lifetime and greatly limiting the performance of the devices. A model has first been developed in order to assess the impact of the Threading Dislocation Density (TDD) on the efficiency of GaAsP/Si dual junction devices. We demonstrate that a TDD below 10^{6} cm^{-2} should be targeted in order to achieve efficiencies over 30 %. 1.7 eV Al_{0.2}Ga_{0.8}As solar cells, with an ideal bandgap for a top cell in III-V/Si dual junction architectures, have then been grown on Si substrates by Molecular Beam Epitaxy (MBE). Direct AlGaAs nucleation has been performed on Si, followed by the growth of Dislocation Filter Layers (DFLs) coupled with Thermal Cycle Annealing (TCA) steps in order to reduce the TDD. Notably, a TDD of 8(±2)×10^{6} cm^{-2} has been demonstrated. However, the performance of the cells is limited by the bulk material quality of the Al_{0.2}Ga_{0.8}As, independently of TDs. An optimisation study of the growth conditions of 1.7 eV Al_{0.22}Ga_{0.78}As solar cells on GaAs has, thus, been carried out, leading to a strong improvement in performance when increasing the growth temperature from 580 °C to 620 °C. In particular, an open-circuit voltage (V_{oc}) of 1212 mV has been demonstrated. Transfer of this improved growth recipe to Si substrates should yield devices with a V_{oc} above 1 V

Simulation study of GaAsP/Si tandem solar cells

A model, adapted from the Shockley-Queisser detailed balance model to tandem solar cells with a monolithically grown GaAsxP1-x top junction on a Si bottom junction, has been developed. Updated data have been used for the absorption spectrums. Two surface geometries, flat and ideally textured, have been investigated. As an important improvement over existing models, the effects of threading-dislocations related Shockley-Read-Hall recombinations in the GaAsxP1-x cell, due to the lattice mismatch between the GaAsxP1-x epilayers and the Si substrate, have been taken into consideration. Auger recombinations in the Si bottom cell and luminescent coupling between the cells have also been considered. For a dislocation free 2-μm thick top cell, maximal theoretical efficiencies of 41.6% and 39.1% have been calculated for a textured and a flat surface, respectively. For threading dislocation (TD) densities below 10^4 cm^-2, the impact of TDs in the GaAsxP1-x layers on the solar cell performances is very limited. With TD densities over 10^5 cm^-2, the top cell open circuit voltage is reduced, hence the overall efficiency. For TD densities over 4×10^6 cm^-2, as the diffusion length of minority carriers in the base gets smaller than the base thickness, the short circuit current in the top GaAsxP1-x cell is also reduced, resulting in a decrease in the optimal top cell bandgap. Using non ideal EQEs and surface recombination rates from published experimental data, the long-term efficiency potential of the investigated technology has been estimated to be ~35.1% for an ideally textured GaAsxP1 x/Si tandem cell with a TD density of 10^5 cm^-2 (~33.0% with a flat surface)

External Quantum Efficiency modeling of GaAs solar cells grown on Si: a method to assess the Threading Dislocation Density

A method is reported in order to determine an upper bound for the Threading Dislocation (TD) density in experimental GaAs solar cells grown lattice-mismatched on Si. The method is based on the modeling of the devices’ External Quantum Efficiency (EQE), using the classic drift-diffusion model, or Hovel model. The model is fitted to experimental EQE measurements, using the diffusion length of minority carriers as the sole fitting parameter. Assuming low surface recombination velocities at both interfaces, a lower bound for the diffusion length of minority carriers is determined. Considering non-radiative recombinations on TDs as the dominant recombination pathway, this lower bound for the diffusion length of minority carriers can be converted to an upper bound for the TD density, using the NTT model. This method is then used to assess the TD density in GaAs solar cells grown on Si by Molecular Beam Epitaxy, using Strained Layer Superlattice (SLS) Dislocation Filter Layers (DFLs) coupled with Thermal Cycle Annealing (TCA) steps in order to reduce the TD density in the active region of the devices. Upper bounds for the TD densities in the low 10^{7}cm^{-2} are thus extracted from the devices’ experimental EQE measurements

Simulation study of GaAsP/Si tandem cells including the impact of threading dislocations on the luminescent coupling between the cells

A model, derived from the detailed balance model from Shockley and Queisser, has been adapted to monolithically grown GaAsP/Si tandem dual junction solar cells. In this architecture, due to the difference of lattice parameters between the silicon bottom cell – acting as the substrate – and the GaAsP top cell, threading dislocations (TDs) arise at the III-V/Si interface and propagate in the top cell. These TDs act as non-radiative recombination centers, degrading the performances of the tandem cell. Our model takes into account the impact of TDs by integrating the NTT model developed by Yamaguchi et. al.. Two surface geometries have been investigated: flat and ideally textured. Finally the model considers the luminescent coupling (LC) between the cells due to reemitted photons from the top cell cascading to the bottom cell. Without dislocations, LC allows a greater flexibility in the cell design by rebalancing the currents between the two cells when the top cell presents a higher short-circuit current. However we show that, as the TD density (TDD) increases, non-radiative recombinations take over radiative recombinations in the top cell and the LC is quenched. As a result, non-optimized tandem cells with higher short-circuit current in the top cell experience a very fast degradation of efficiency for TDDs over 10^4cm^-2. On the other hand optimized cells with matching currents only experience a small efficiency drop for TDDs up to 10^5cm^-2. High TDD cells therefore need to be current-matched for optimal performances as the flexibility due to LC is lost

1.7eV Al0.2Ga0.8As solar cells epitaxially grown on silicon by SSMBE using a superlattice and dislocation filters

Lattice-mismatched 1.7eV Al0.2Ga0.8As photovoltaic solar cells have been monolithically grown on Si substrates using Solid Source Molecular Beam Epitaxy (SSMBE). As a consequence of the 4%-lattice-mismatch, threading dislocations (TDs) nucleate at the interface between the Si substrate and III-V epilayers and propagate to the active regions of the cell. There they act as recombination centers and degrade the performances of the cell. In our case, direct AlAs/GaAs superlattice growth coupled with InAlAs/AlAs strained layer superlattice (SLS) dislocation filter layers (DFLSs) have been used to reduce the TD density from 1×10^9cm^-2 to 1(±0.2)×10^7cm^-2. Lattice-matched Al0.2Ga0.8As cells have also been grown on GaAs as a reference. The best cell grown on silicon exhibits a Voc of 964mV, compared with a Voc of 1128mV on GaAs. Fill factors of respectively 77.6% and 80.2% have been calculated. Due to the lack of an anti-reflection coating and the non-optimized architecture of the devices, relatively low Jsc have been measured: 7.30mA.cm^-2 on Si and 6.74mA.cm^-2 on GaAs. The difference in short-circuit currents is believed to be caused by a difference of thickness between the samples due to discrepancies in the calibration of the MBE prior to each growth. The bandgap-voltage offset of the cells, defined as Eg/q-Voc, is relatively high on both substrates with 736mV measured on Si versus 572mV on GaAs. The non-negligible TD density partly explains this result on Si. On GaAs, non-ideal growth conditions are possibly responsible for these suboptimal performances

Corticosteroid therapy for the management of paradoxical inflammatory reaction in patients with pulmonary tuberculosis

Background Paradoxical reaction after the initiation of tuberculosis treatment is defined as increased inflammation following effective antimycobacterial treatment. This is a phenomenon that can severely complicate a patient's recovery, potentially leading to further morbidity and residual deficits. Paradoxical reaction remains poorly understood regarding its pathophysiology and management. Only a limited number of reports look critically at the available therapeutic options, with evidence of the efficacy of prednisolone therapy being primarily limited to extrapulmonary PR only. Case We describe two HIV negative patients who were admitted to our department with pulmonary tuberculosis, presenting with inflammatory patterns attributable to PR and their response to adjunctive steroid therapy. Discussion and Conclusions The presented cases further highlight the need for immunological studies and randomized trials for corticosteroid therapy are needed to better understand this phenomenon as well as provide an evidence-base for anti-inflammatory treatment. Furthermore, by means of this case series, we are also able to highlight the potential variability in the symptomatology of the lesser known PR phenomenon, in which we observed a hypotensive shock-like syndrome not previously described in literature

MBE growth of 1.7eV Al0.2Ga0.8As and 1.42eV GaAs solar cells on Si using dislocations filters: an alternative pathway toward III-V/ Si solar cells architectures

Metamorphic epitaxial growth of III-V solar cells on Si has attracted significant interest for the development of III V/Si photovoltaic architectures. In this work, we present an alternative pathway – using MBE growth techniques – based on the direct nucleation of Al_{x}Ga_{1-x}As materials on Si, followed by the growth of a 1.7eV Al_{0.2}Ga_{0.8}As or a 1.42eV GaAs solar cell. Dislocation Filter Layers (DFLs), in conjunction with Thermal Cycle Annealing (TCA), have been used to reduce the Threading Dislocation Density (TDD) below 10^{7}cm^{-2} in the base of the cell; close to the best results demonstrated with metamorphic buffers

Rheumatoid arthritis - treatment: 180. Utility of Body Weight Classified Low-Dose Leflunomide in Japanese Rheumatoid Arthritis

Background: In Japan, more than 20 rheumatoid arthritis (RA) patients died of interstitial pneumonia (IP) caused by leflunomide (LEF) were reported, but many of them were considered as the victims of opportunistic infection currently. In this paper, efficacy and safety of low-dose LEF classified by body weight (BW) were studied. Methods: Fifty-nine RA patients were started to administrate LEF from July 2007 to July 2009. Among them, 25 patients were excluded because of the combination with tacrolimus, and medication modification within 3 months before LEF. Remaining 34 RA patients administered 20 to 50 mg/week of LEF were followed up for 1 year and enrolled in this study. Dose of LEF was classified by BW (50 mg/week for over 50 kg, 40 mg/week for 40 to 50 kg and 20 to 30 mg/week for under 40 kg). The average age and RA duration of enrolled patients were 55.5 years old and 10.2 years. Prednisolone (PSL), methotrexate (MTX) and etanercept were used in 23, 28 and 2 patients, respectively. In case of insufficient response or adverse effect, dosage change or discontinuance of LEF were considered. Failure was defined as dosages up of PSL and MTX, or dosages down or discontinuance of LEF. Last observation carried forward method was used for the evaluation of failed patients at 1 year. Results: At 1 year after LEF start, good/ moderate/ no response assessed by the European League Against Rheumatism (EULAR) response criteria using Disease Activity Score, including a 28-joint count (DAS28)-C reactive protein (CRP) were showed in 14/ 10/ 10 patients, respectively. The dosage changes of LEF at 1 year were dosage up: 10, same dosage: 5, dosage down: 8 and discontinuance: 11 patients. The survival rate of patients in this study was 23.5% (24 patients failed) but actual LEF continuous rate was 67.6% (11 patients discontinued) at 1 year. The major reason of failure was liver dysfunction, and pneumocystis pneumonia was occurred in 1 patient resulted in full recovery. One patient died of sepsis caused by decubitus ulcer infection. DAS28-CRP score was decreased from 3.9 to 2.7 significantly. Although CRP was decreased from 1.50 to 0.93 mg/dl, it wasn't significant. Matrix metalloproteinase (MMP)-3 was decreased from 220.0 to 174.2 ng/ml significantly. Glutamate pyruvate transaminase (GPT) was increased from 19 to 35 U/l and number of leukocyte was decreased from 7832 to 6271 significantly. DAS28-CRP, CRP, and MMP-3 were improved significantly with MTX, although they weren't without MTX. Increase of GPT and leukopenia were seen significantly with MTX, although they weren't without MTX. Conclusions: It was reported that the risks of IP caused by LEF in Japanese RA patients were past IP history, loading dose administration and low BW. Addition of low-dose LEF is a potent safe alternative for the patients showing unsatisfactory response to current medicines, but need to pay attention for liver function and infection caused by leukopenia, especially with MTX. Disclosure statement: The authors have declared no conflicts of interes