Compositionally Graded Indium Gallium Nitride Solar Cells

For the past several decades, methods to harvest solar energy have been investigated intensively. A majority of the work done in this field has been on solar cells made with silicon – the most mature semiconductor material. Recent developments in material fabrication and processing techniques have enabled other semiconductor materials to attract practical interest and research effort as well. Indium gallium nitride (InGaN) is one such material. The material properties of InGaN indicate that solar cells made with it have the potential to achieve much higher power density than a standard silicon solar cell. High power density InGaN solar cells could replace silicon cells in applications where size and weight are critical, or in environments where silicon devices cannot survive. This is especially true of space, and most InGaN development has been done with that in mind. However, at high enough power densities, InGaN solar cells could begin to compete with silicon devices in commercial applications. The goal of this research is to investigate the effect a novel growth technique for InGaN – graded layer deposition – has on the power density of an InGaN solar cell. In this research, first a baseline InGaN solar cell was grown, fabricated, and characterized. A standard PiN (P: p-type, i: intrinsic, N: n-type) structure was used for this baseline device. The reference alloy composition was chosen to be 20% indium and 80% gallium (In­0.2Ga0.8N). This sample was grown using molecular beam epitaxy (MBE) under standard conditions for the material. Once the reference crystal was fabricated it was optically and electrically characterized. The material composition was verified through a combination of x-ray diffraction (XRD), photoluminescence (PL), and transmittance/reflectance measurements. The quality of the surface of the crystal was examined using atomic force microscopy (AFM). Once the optical characterization of the material was complete, the crystal was processed for electrical characterization. Individual devices were constructed by etching away much of the p-type and intrinsic layer, leaving behind circular mesas. Each mesa was then given a top and bottom contact, so that it could be connected to test equipment electrically. After the crystal was processed into a solar cell in this way, each device was connected to a test source electrically, and the current-voltage (I-V) curves were taken. This information was used to find the current and power densities of each device. The second step in this work was fabricating and characterizing a graded layer device that was similar to the reference cell. To this end, the graded layer device was chosen to have a starting composition of 25% indium and 75% gallium, with an ending composition of 15% indium and 85% gallium in place of the intrinsic layer. This new crystal was grown under identical conditions as the baseline cell, except for the graded layer, which required a slightly different approach. The graded layer crystal was then characterized and processed consistent with the reference in an attempt to get as accurate of a comparison between the two as possible. The results of this research could significantly affect the field of III-nitride solar cells

Numerical simulation of InGaN Schottky solar cell

The Indium Gallium Nitride (InGaN) III-Nitride ternary alloy has the potentiality to allow achieving high efficiency solar cells through the tuning of its band gap by changing the Indium composition. It also counts among its advantages a relatively low effective mass, high carriers\^a mobility, a high absorption coefficient along with good radiation tolerance.However, the main drawback of InGaN is linked to its p-type doping, which is difficult to grow in good quality and on which ohmic contacts are difficult to realize. The Schottky solar cell is a good alternative to avoid the p-type doping of InGaN. In this report, a comprehensive numerical simulation, using mathematically rigorous optimization approach based on state-of-the-art optimization algorithms, is used to find the optimum geometrical and physical parameters that yield the best efficiency of a Schottky solar cell within the achievable device fabrication range. A 18.2% efficiency is predicted for this new InGaN solar cell design

Simulation study of a new InGaN p-layer free Schottky based solar cell

On the road towards next generation high efficiency solar cells, the ternary Indium Gallium Nitride (InGaN) alloy is a good passenger since it allows to cover the whole solar spectrum through the change in its Indium composition. The choice of the main structure of the InGaN solar cell is however crucial. Obtaining a high efficiency requires to improve the light absorption and the photogenerated carriers collection that depend on the layers parameters, including the Indium composition, p-and n-doping, device geometry.. . Unfortunately, one of the main drawbacks of InGaN is linked to its p-type doping, which is very difficult to realize since it involves complex technological processes that are difficult to master and that highly impact the layer quality. In this paper, the InGaN p-n junction (PN) and p-in junction (PIN) based solar cells are numerically studied using the most realistic models, and optimized through mathematically rigorous multivariate optimization approaches. This analysis evidences optimal efficiencies of 17.8% and 19.0% for the PN and PIN structures. It also leads to propose, analyze and optimize player free InGaN Schottky-Based Solar Cells (SBSC): the Schottky structure and a new MIN structure for which the optimal efficiencies are shown to be a little higher than for the conventional structures: respectively 18.2% and 19.8%. The tolerance that is allowed on each parameter for each of the proposed cells has been studied. The new MIN structure is shown to exhibit the widest tolerances on the layers thicknesses and dopings. In addition to its being player free, this is another advantage of the MIN structure since it implies its better reliability. Therefore, these new InGaN SBSC are shown to be alternatives to the conventional structures that allow removing the p-type doping of InGaN while giving photovoltaic (PV) performances at least comparable to the standard multilayers PN or PIN structures.Comment: Superlattices and Microstructures, Elsevier, 201

Plasmonic nanoparticle enhanced photocurrent in GaN/InGaN/GaN quantum well solar cells

We demonstrate enhanced external quantum efficiency and current-voltage characteristics due to scattering by 100 nm silver nanoparticles in a single 2.5 nm thick InGaN quantum well photovoltaic device. Nanoparticle arrays were fabricated on the surface of the device using an anodic alumina template masking process. The Ag nanoparticles increase light scattering, light trapping, and carrier collection in the III-N semiconductor layers leading to enhancement of the external quantum efficiency by up to 54%. Additionally, the short-circuit current in cells with 200 nm p-GaN emitter regions is increased by 6% under AM 1.5 illumination. AFORS-Het simulation software results were used to predict cell performance and optimize emitter layer thickness

Demonstration of Forward Inter-band Tunneling in GaN by Polarization Engineering

We report on the design, fabrication, and characterization of GaN interband tunnel junction showing forward tunneling characteristics. We have achieved very high forward tunneling currents (153 mA/cm2 at 10 mV, and 17.7 A/cm2 peak current) in polarization-engineered GaN/InGaN/GaN heterojunction diodes grown by plasma assisted molecular beam epitaxy. We also report the observation of repeatable negative differential resistance in interband III-Nitride tunnel junctions, with peak-valley current ratio (PVCR) of 4 at room temperature. The forward current density achieved in this work meets the typical current drive requirements of a multi-junction solar cell.Comment: 3 figure

Bandgap engineering in semiconductor alloy nanomaterials with widely tunable compositions

Over the past decade, tremendous progress has been achieved in the development of nanoscale semiconductor materials with a wide range of bandgaps by alloying different individual semiconductors. These materials include traditional II-VI and III-V semiconductors and their alloys, inorganic and hybrid perovskites, and the newly emerging 2D materials. One important common feature of these materials is that their nanoscale dimensions result in a large tolerance to lattice mismatches within a monolithic structure of varying composition or between the substrate and target material, which enables us to achieve almost arbitrary control of the variation of the alloy composition. As a result, the bandgaps of these alloys can be widely tuned without the detrimental defects that are often unavoidable in bulk materials, which have a much more limited tolerance to lattice mismatches. This class of nanomaterials could have a far-reaching impact on a wide range of photonic applications, including tunable lasers, solid-state lighting, artificial photosynthesis and new solar cells

Methods For Growing A Non-phase Separated Group-iii Nitride Semiconductor Alloy

Systems and methods for MBE growing of group-III Nitride alloys, comprising establishing an average reaction temperature range from about 250 C to about 850 C; introducing a nitrogen flux at a nitrogen flow rate; introducing a first metal flux at a first metal flow rate; and periodically stopping and restarting the first metal flux according to a first flow duty cycle. According to another embodiment, the system comprises a nitrogen source that provides nitrogen at a nitrogen flow rate, and, a first metal source comprising a first metal effusion cell that provides a first metal at a first metal flow rate, and a first metal shutter that periodically opens and closes according to a first flow duty cycle to abate and recommence the flow of the first metal from the first metal source. Produced alloys include AlN, InN, GaN, InGaN, and AlInGaN.Georgia Tech Research Corporatio

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar (112¯2) oriented In0.12Ga0.88N quantum well