Dilute Nitride Solar Cells Research Articles

Design and analysis of GaAsN based solar cell for harvesting visible to near-infrared light

In the present study, the performance parameters of GaAsN dilute nitride-based semiconductor solar cell with and without AlGaAs blocking layers have been investigated in detail by Solar Cell Capacitance Simulator in one dimensional software program (SCAPS-1D). The thickness of absorber, buffer, and blocking layers are varied to achieve the improvement of open circuit voltage, short circuit current, fill factor, efficiency and also to optimize the device structure. The impact of doping and defect densities on the solar cell performance parameters have been analyzed minutely inside the absorber, buffer, and blocking layers. The solar cell thermal stability parameters are also investigated in the temperature region from 273K to 373K. The efficiency of 43.90% and 40.05% are obtained from the proposed solar cells with and without AlGaAs blocking layer, respectively. The present findings may provide insightful approach for fabricating feasible, cost effective, and efficient dilute nitride solar cell.

High performance low-bandgap (0.8 eV) single junction GaInNAsSb solar cells incorporating Au-based back surface reflectors

Low-bandgap GaInNAsSb single junction solar cells incorporating a planar Au back surface reflector for enhancing the photocurrent generation are reported. In particular, a 700 nm thick GaInNAsSb junction with a bandgap of 0.78 eV (corresponding to 6.2% N) incorporating the back reflector exhibited a short-circuit current density of 15.2 mA/cm2 for AM1.5D (1000 W/m2) illumination, when placed underneath a GaAs-filter mimicking the operation of a multijunction architecture. The corresponding external quantum efficiency represents the highest external quantum efficiency reported so far for dilute nitride solar cells with bandgaps below 0.8 eV. Electrical simulations reveal that the relatively high value of the quantum efficiency is attributed to a low p-type carrier concentration and partially to the varying doping level in the GaInNAsSb region. Moreover, the absorption coefficients for low-bandgap GaInNAsSb materials are estimated and used in a comprehensive optical analysis to understand the photon harvesting performance of the solar cells with reflector. The analysis reveals incomplete absorption in the GaInNAsSb layers for the double-pass configuration enabled by the planar back reflector pointing to the need for deploying more advanced structured reflectors. However, the structures exhibited high collection efficiencies laying the foundation for further improvements of the quantum efficiency values. Moreover, the low-bandgap dilute nitride solar cells show potential to meet the current-matching requirements in next-generation multijunction devices with five or more junctions, thus being a prospective alternative for replacing Ge as the bottom junction.

Correlations between electrical and optical properties in lattice-matched GaAsPN/GaP solar cells

In this work, we investigate correlations between optical and electro-optical properties of GaAsPN/GaP p–i–n solar cells grown by MBE on GaP(001) substrates. A photoluminescence model is first proposed to extract the long range compositional fluctuation energy scale from low temperature photoluminescence spectra of the GaAsPN dilute-nitride material. Solar cells grown with the same nitrogen content at different temperature reveal very different electrical performances. A 4.08mA/cm2 short-circuit current density has been obtained, that is an excellent value given the small absorber thickness (300nm) and high material bandgap. Comparisons between solar cells photoluminescence, I–V (under AM1.5G illumination) and IQE parameters reveal correlations between their optical and electrical parameters. These results reinforce PL measurements interest for dilute nitride solar cells growth optimization.

P- to n-type conductivity transition in 1.0 eV GaInNAs solar cells controlled by the V/III ratio

In this work, we report a p- to n-type conductivity transition of GaInNAs (1.0 eV bandgap) layers in p-i-n dilute nitride solar cells continuously controlled by the V/III ratio during growth. Near the transition region, we were able to produce GaInNAs layers with very low effective electrically active doping concentrations resulting in wide depleted areas. We obtained internal quantum efficiencies (IQEs) up to 85% at 0.2 eV above the bandgap. However, the high IQE comes along with an increased dark current density resulting in a decreased open circuit voltage of about 0.2 V. This indicates the formation of non-radiant defect centers related to the p-type to n-type transition. Rapid-thermal annealing of the solar cells on the one hand helps to anneal some of these defects but on the other hand increases the effective doping concentrations.

Characterization of the post-thermal annealing effect for p-GaAs/i-InGaAsN/n-GaAs hetero-junction solar cells

In this study, we demonstrated the fabrication and characterization of p-GaAs/i-InGaAsN/n-GaAs double hetero-junction solar cells (DHJSCs). The intrinsic InGaAsN absorption layer which is lattice-matched with GaAs substrates was grown by the metal-organic vapor phase epitaxy (MOVPE) system. The metal-organic sources used in the MOVPE growth induced unintentionally-doped carbon adatoms, and this in turn caused some carbon-related defects. This carbon pollution subsequently deteriorated the optical and electrical characteristics of the dilute nitride material. With ex-situ post-annealing being carried out in the furnace, it could not only modify the crystallinity of the InGaAsN material, but the deterioration in the performance of the dilute nitride solar cell due to the nitrogen incorporation could also be mitigated. After conducting the thermal annealing at 550°C for 1h in nitrogen ambience, the conversion efficiency of the DHJSCs was increased from 0.61% to 4.46% under Air Mass 1.5 Global (1000W/m2, AM1.5G) irradiance. The external quantum efficiency (EQE) spectrum further revealed that the efficiency of DHJSCs had been enhanced significantly owing to the crystalline improvement of the intrinsic InGaAsN absorption layer after the ex-situ post-thermal annealing was implemented. This improvement also led to a significant increase in the short-circuit current. The X-ray photoelectron spectroscopy (XPS) measurement result also showed that the ex-situ thermal annealing contributed to the improvement of crystal quality and the reduction of carbon related defects and clusters in the as-grown InGaAsN absorption layer.

Moth‐eye antireflection coating fabricated by nanoimprint lithography on 1 eV dilute nitride solar cell

ABSTRACTWe report on the performance of biomimicked antireflection coating applied to dilute nitride solar cell. The coating consists of nanostructures replicating the moth‐eye geometry and has been fabricated by nanoimprint lithography directly within the window layer covering the dilute nitride absorbing junction. The mean reflectivity within the spectral range of 320–1800 nm remains under 5% for incident angles up to 45°. The effect of the coating on the cell performance was assessed by measuring the current–voltage characteristics under simulated solar illumination. A clear performance increase was identified when comparing a solar cell with the moth‐eye coating with a solar cell having a standard SiNx/SiO2 coating. Copyright © 2012 John Wiley & Sons, Ltd.

Dilute nitride GaInNAs and GaInNAsSb solar cells by molecular beam epitaxy

Dilute nitride films with a roughly 1 eV band gap can be lattice-matched to gallium arsenide and germanium, and therefore could become a critical component in next-generation multijunction solar cells. To date most dilute nitride solar cells have been plagued with poor efficiency, due in large part to short diffusion lengths. This study focuses on two techniques aimed at improving the quality of dilute nitride films grown by molecular beam epitaxy: the utilization of biased deflection plates installed in front of the nitrogen plasma source, and the introduction of antimony during growth. Results from GaInNAs cells grown with and without deflection plates, and GaInNAsSb solar cells are reported. The use of biased deflection plates during GaInNAs growth improved every aspect of solar cell performance. For the GaInNAs devices grown with deflection plates, the dark current density, open-circuit voltage, and fill factor were the best of the devices studied. The GaInNAsSb cells had the highest quantum efficiency, almost 80% at maximum, mainly due to low background doping densities providing these devices with wide depletion widths. The GaInNAsSb materials also had quite narrow band gaps of 0.92 eV. Because of the high collection efficiency coupled with the narrow band gap, the sub-GaAs short-circuit current density produced by the GaInNAsSb cells is 14.8 mA∕cm2, which was the highest of the devices studied. This current is nearly 50% greater than the best dilute nitride solar cells in the literature, and is the first dilute nitride cell to produce enough current to current match the upper two subcells in a triple-junction device, composed of GaInP/InGaAs/GaInNAsSb.