InGaN BASED SOLAR CELLS
InGaN BASED SOLAR CELLS
Indium gallium nitride (InGaN) has the potential of forming optoelectronic devices – including solar cells – covering a range from 0.7 eV to 3.4 eV. This energy range matches closely the usable emission present in the solar spectrum, as seen from space (air mass zero; AM0). Beside the inherent thermal ruggedness of the III-Nitrides, which make them fit for high temperature applications such as in solar cells used in high terrestrial concentrator systems, it has recently been determined that these Nitride materials can offer exceptional radiation tolerance that is well beyond the level that can be achieved with conventional solar cell materials that are currently flown into space.
Although an InGaN based solar cell would be a less mature technology when it is first commercialized compared to other III-V semiconductors, and hence will not likely initially have as high efficiency as its III-V counterparts , the important factor is that it will degrade far less over the life of the mission. Therefore, a 32% efficient Nitride based solar cell with no degradation will be equivalent, if not superior to, a conventional III-V cell, which would start at 36% efficiency, but degrade to 30% at end of life (EOL). Likewise, a 36% efficient Nitride based cell can offer an EOL comparable to that of a 42% conventional III-V cell.
High Efficiency InGaN Solar Cells
The objective of this project was to develop InGaN-containing solar cell with a cell level conversion efficiency which exceeds 40% under 500 AM1.5 suns, that is traceable to an efficiency greater than 30% under 1 sun AM0 illumination. In order to meet this objective, the following has been accomplished:
- Growth of InGaN films with high phase purity across a wide compositional range
- Investigation of various substrates and buffer layers for InGaN
- Investigation of InGaN material properties
- Modeling of InGaN photovoltaic devices
- Fabrication and evaluation of InxGa1-xN test devices
Through the course of this project, InGaN films across a wide compositional range have been grown and characterized, various strategies for improvement of InGaN film quality have been investigated, InGaN photovoltaic devices were modeled, and InxGa1-xN test devices were fabricated and evaluated. Short circuit currents of 1.8 mA/cm2 with optical response at energies 0.3 eV lower than previous results, were achieved . Although the short circuit current was improved, the open circuit voltages exhibited for all of the devices was low. The efficiency of the devices peaked at values of 0.07%. As a result, the objectives of this project, which were the demonstration of a hybrid InGaN-Si tandem cell and a tandem two junction InGaN cell, were not achieved. It remains a challenge to produce material of sufficient quality to achieve high efficiencies, although it is believed that InGaN will ultimately be effective as a high efficiency solar cell material.
Our approach in this project was to use Radio Frequency Molecular Beam Epitaxy (RFMBE) in
order to fabricate p-type and n-type InxGa1-xN films and test device structures. The III-Nitride layers in our investigation were grown in a custom-made MBE chamber equipped with standard effusion cells for Group III and dopant flux delivery, which includes Ga, Al, In, Si, and Mg. Typical partial pressures in Torr for those elements used in this work were: (1.0-4.5)x10-7 for Ga, (1.0-7.5)x10-7 for In, less than 1.0×10-9 for Si, and less than 1.0×10-8 for Mg. Active nitrogen species were generated by a Veeco Uni- Bulb RF plasma source operated at 400 – 450 W forward power and N2 flow of 1.0 – 1.5 sccm. The growth rate resulting from the parameters listed was from 0.5 μm/hr to 0.75 μm/hr. The film growth conditions were specially developed to have a non-constant V/III ratio, although the time-averaged ratio was near 1:1. The sample manipulator is compatible with 2” in diameter substrates and operating temperatures of 900°C. The chamber is pumped with a 2200 L/s turbomolecular pump and reaches a base pressure of <5×10-10 Torr.
The optical properties of the films were evaluated by photoluminescence (PL), time resolved photoluminescence (TRPL), and by spectroscopic ellipsometry (SE). The PL system consisted of at 17 mW HeCd laser (325nm), a closed-cycle compressed He cryostat, a Jobin Yvon (ISA) Triax 320 monochrometer with 1200 lines/mm gratings, and a Hammamatsu R928 photomultiplier tube. The TRPL system included a mode-locked Ti:Sapphire laser with a 76 MHz repetition rate and pulse duration of 150 fs, a 0.3 m focal length monochromator, and silicon streak camera. The effective time resolution of the system was ~100 ps. The ellipsometer is a J.A. Woollam M2000D with a wavelength range of 192 nm to 1699 nm and angular range of 40º to 90º from normal.
The electrical properties of the films were characterized by electrochemical capacitance-voltage measurements. The measurements were performed in a Biorad Polaror ECV Profiler in a 0.01M KOH solution using an electrolyte contact area of 1 mm2 and at frequencies ranging from 300 Hz to 10 kHz.
The structural properties of the films were evaluated by x-ray diffraction ω/2Θ scans and, in some cases, by reciprocal space mapping (RMS) about the (10-15) diffraction peak. The majority of the ω/2Θ scans were performed on a Bede 200 HRSRD High Resolution x-ray diffraction system. Reciprocal space maps were collected in a custom four-circle diffractometer using Cu Kα1 radiation from a Rigaku 12 kW rotating anode source and a Xe proportional counter detector.
Devices were evaluated by spectral photoresponse measurements using a 120 W xenon lampdispersed by a 0.3 m monochromator and calibrated against a standard UV-enhanced Si photodiode. Illuminated J-V measurements to determine Jsc, Voc, fill factor, and efficiency, were performed using an Oriel AM0 simulator.
The crystalline nitride films in this project were grown primarily on commercially available HVPE GaN-on-sapphire template wafers that are equivalent to those used in the GaN optoelectronics industry. The commercial GaN templates were specified to have defect densities lower than those of typical MBE-grown GaN on sapphire, but were not independently measured. Some nitride growth experiments were performed on ZnO wafers and on silicon substrates.
The possible advantages of InxGa1-xN alloys for use in solar cell devices include:
- Widely tunable direct bandgap that can be tailored to specific solar spectral regions
- Excellent radiation tolerance of the InGaN material
- Tandem cells with multiple junctions would now be possible within the same material system instead of mixing Group IV and Group III-V system materials. This would help to reduce material incompatibilities, such as thermal expansion mismatch
- Good thermal, mechanical, and chemical resistance
The possible disadvantages of InxGa1-xN alloys for use in solar cell devices include:
- Lack of native substrate for low defect density growth
- Strong band bending at the InGaN surface for indium compositions ≥ 40%
A variety of InGaN single junction devices based on uniform composition homojunction structures and also so InGaN-GaN superlattice  structures were tested for spectral response and dark J-V. AM0 measurements were performed on the uniform composition devices. A maximum short circuit current of 1.8 mA/cm2 with optical response near 2.0 eV was achieved. Although the short circuit current was improved, the open circuit voltages exhibited for all of the devices was low. The efficiency of the devices peaked at values that were just below 0.1%. Ultimately, we were unable to achieve the high efficiency InGaN single junction cells that had been our objective in the project.
Table below represents a summary of spectral response and AM0 measurements of uniform composition InGaN homojunction devices:
High Bandgap InGaN Solar Cells
The objective of this project was the development of InGaN photovoltaic cells  to support high efficiency multi-junction solar energy technologies. Through the course of this project growth conditions for InGaN films of 0.65μm thickness with indium fractions up to 19% that have smooth morphologies, no indium phase segregation, and background n-type carrier levels in the low 1017cm-3, have been develpoped. For Mg-doping of InGaN, growth conditions were devised for free hole concentrations suitable for solar cell test device fabrication. Models for specific device bandgaps were also developed. Multiple InGaN-based homojunction device structures were fabricated and tested. A short circuit current of 1.0 mA/cm2 is reported for an In0.14Ga0.86N-based device under AM0 conditions. Dark and illuminated I-V results indicated the existence of electrical non-idealities whose causes are known and ultimately resolvable through improved device designs, which have been simulated.
A processed wafer containing test devices with areas from 0.2 mm2 to 3 mm2 is shown in the photograph below:
The graphs below show (a) J-V scan for the 2.85 eV device under a UV-poor AM0 simulator, (b) J-V scan under concentrated UV illumination:
In order to gain additional insight into the performance of the cell, the results were compared to the theoretically modeled device. The comparison highlights both the positive aspects of the InGaN device and also those areas for which improvement can be sought in the future work. On a very positive note, the Jsc value near 1 mA/cm2 compares quite favorably to the expected value of 1.6 mA/cm2. The measured results for the Voc, fill factor, and efficiency were impacted by the series resistance of the device.
A summary of the measured and theoretical results for this device is given below:
In summary, through the course of this project growth conditions for InGaN films of 0.65μm thickness with indium fractions up to 19% that have smooth morphologies, no indium phase segregation, and background n-type carrier levels in the low 1017cm-3, have been developed. Free hole concentrations suitable for solar cell test device fabrication were obtained through Mg doping. Models for specific device bandgaps were developed, and structures were fabricated and tested. A short circuit current of 1.0 mA/cm2 was obtained for an In0.14Ga0.86N-based device under AM0 conditions. Dark and illuminated I-V results indicated the existence of electrical non-idealities, which impacted the fill factor and short circuit current values.
InGaN High Temperature Photovoltaic Cells
The ultimate goal of this project was to develop InGaN photovoltaic cells  for high temperature and/or high radiation environments to TRL 4 and define the development path for the technology to TRL 5 and beyond. The following objectives have been considered in this project:
- Refine Single Junction Device Structures
- Optimize the Processing of Single Junction Devices
- Demonstrate InGaN based solar cells for high temperature and high radiation applications.
The project was divided into the following tasks:
- Fabrication of InGaN and measurement of the optical, electrical, and structural properties;
- Modeling of InGaN junctions
- Modeling of cell layouts
- Device Fabrication
- Device Testing
Several material optimization methods were employed, including the use of InGaN-containing superlattice structures and the use of intermediate layers for strain management. The superlattice approach was successful in achieving nominally unrelaxed thick layers with absorption down to 2.9 eV, but further lowering of the film bandgap introduced relaxation of the layers. The use of intermediate layers achieved somewhat thinner layers that were also nominally unrelaxed, and with absorption down to 2.6 eV.
The investigated materials were used to fabricate p-n junction devices which were tested for spectral response at temperatures up to 250 ºC. Both types of structures exhibited less than ideal electrical properties, and had quantum efficiencies up to 0.15 at room temperature in there absorbing regions. Some of the devices were operable at temperatures of 250 ºC with performance degradation of 44% at 175 ºC and 59% at 250 ºC, thus validating the hypothesis of high temperature stability for InGaN.
Combinations of superlattices were chosen with the constraints that the individual InGaN and AlGaN layers’ thicknesses would, in most cases, not exceed their respective critical thickness on GaN, with the intention of not forming dislocations though layer relaxation. A second consideration limiting the well and barrier thickness values is carrier transport. In order for vertical transport through the SL structure to readily occur, the layers must remain thin so that quantum tunneling is probable. In the case of n-type AlGaN-GaN SLs, periods of 10 nm or less were required inorder not to substantially degrade conductivity.
Table below summarizes some of the possible InGaN-AlGaN combinations that would be lattice-matched or slightly mismatched (< 0.25 %) to GaN:
The temperature behavior of p-type InGaN-GaN superlattices was analyzed. For two highly doped layers, the hole-mobility product (μ*p) was (0.5-1) x1019 cm-1V-1s-1. Under the assumption of a mobility of 2.5 cm2/Vs, this results in ~(4-2)x1018 cm-3 free holes at room temperature. Due to the high Mg doping needed to get the free hole density > 1018, the value of 2.5 cm2/Vs could be an overestimation. The μ*p product for the lower indium-fraction SL is quite high, nearly six times higher than our highly doped p-GaN (μ*p ~ 0.5-1 x 1019 cm-1V-1s-1). The interpretation is that a strong increase in both Mg incorporation and activation occurs in the SL structure.
Figure below shows the relative resistivity with temperature for the two SL samples:
Overall, despite the disappointing overall efficiency, the temperature-based behavior of thedevices produced in this project indicates that InGaN could perform well as a high temperaturesolar cell. Even with the sub-optimal I-V behavior of the devices, substantial photoresponse was measured for 2.9 eV cells up to temperatures of 250 ºC, and the cells exhibited no noticeable degradation in room temperature performance once cooled down from 250 ºC. Until appropriate lattice-matching substrates and/or buffer schemes can be achieved, InGaN-based photovoltaics with improved I-V and photoresponse will continue to be a challenging endeavor.
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