Christopher S. Murray

MOCVD growth of lattice-matched and mismatched InGaAs materials for thermophotovoltaic energy conversion

The details of MOCVD growth of lattice-matched (0.74 eV) and lattice-mismatched (0.55 eV and 0.6 eV) InGaAs-based thermophotovoltaic (TPV) devices on InP substrates are discussed. The optimization of growth conditions, structural parameters and run-to-run consistency have played a key role in the development of high quality TPV devices, particularly in the development of lattice-mismatched materials.

Monolithic Interconnected Modules (MIMs) for Thermophotovoltaic Energy Conversion

Monolithic interconnected modules (MIMs) are under development for thermophotovoltaic (TPV) energy conversion applications. MIM devices are typified by series-interconnected photovoltaic cells on a common, semi-insulating substrate and generally include rear-surface infrared (IR) reflectors. The MIM architecture is being implemented in InGaAsSb materials without semi-insulating substrates through the development of alternative isolation methodologies. Motivations for developing the MIM structure include: reduced resistive losses, higher output power density than for systems utilizing front surface spectral control, improved thermal coupling and ultimately higher system efficiency. Numerous design and material changes have been investigated since the introduction of the MIM concept in 1994. These developments as well as the current design strategies are addressed.

InGaAs monolithic interconnected modules (MIMs)

A monolithic interconnected module (MIM) structure has been developed for thermophotovoltaic (TPV) applications. The MIM device consists of many individual InGaAs cells series-connected on a single semi-insulating InP substrate. An infrared (IR) back surface reflector (BSR), placed on the rear surface of the substrate, returns the unused portion of the TPV radiator output spectrum back to the radiator for recuperation, thereby providing for high system efficiencies. Also, the use of a BSR reduces the requirements imposed on a front surface interference filter and may lead to using only an anti-reflection coating. As a result, MIMs are exposed to the entire radiator output, and with increasing output power density. MIMs were fabricated with an active area of 0.9/spl times/1 cm, and with 15 cells monolithically connected in series. Both lattice-matched and lattice-mismatched InGaAs/InP devices were fabricated, with bandgaps of 0.74 and 0.55 eV, respectively. The 0.74 eV MIMs demonstrated an open-circuit voltage (Voc) of 6.16 V and a fill factor of 74.2% at a short-circuit current (Jsc) of 0.84 A/cm/sup 2/, under flashlamp testing. The 0.55 eV modules demonstrated a Voc of 4.85 V and a fill factor of 57.8% at a Jsc of 3.87 A/cm/sup 2/. The near IR reflectance (2-4 /spl mu/m) for both lattice-matched and lattice-mismatched structures was measured to be in the range of 80-85%. Latest electrical and optical performance results for these MIMs is presented.

Electrical and Optical Performance Characteristics of 0.74-eV p/n InGaAs Monolithic Interconnected Modules

There has been a traditional trade-off in thermophotovoltaic (TPV) energy conversion development between system efficiency and power density. This trade-off originates from the use of front surface spectral controls such as selective emitters and various types of filters. A monolithic interconnected module (MIM) structure has been developed which allows for both high power densities and high system efficiencies. The MIM device consists of many individual indium gallium arsenide (InGaAs) cells series-connected on a single semi-insulating indium phosphide (InP) substrate. The MIM is exposed to the entire emitter output, thereby maximizing output power density. An infrared (IR) reflector placed on the rear surface of the substrate returns the unused portion of the emitter output spectrum back to the emitter for recycling, thereby providing for high system efficiencies. Initial MIM development has focused on a 1 cm2 device consisting of eight series interconnected cells. MIM devices, produced from 0.74 eV InG...

Materials and process development for the monolithic interconnected module (MIM) InGaAs/InP TPV devices

The selection, development, and testing of materials and processes for MIM fabrication are described. Topics covered include isolation trenches, contact and interconnect metallization, dielectric isolation barriers, back surface reflectors, and antireflection coatings. (AIP)

0.55 eV n/p/n MIM TPV cell development

A monolithic interconnected module (MIM) has been described which serves as the photovoltaic converter and the spectral control mechanism in thermophotovoltaic (TPV) devices. Current MIM development has been directed toward reducing the bandgap of the device to optimize the performance for low temperature radiators (/spl sim/1200 K). Low bandgap (0.55 eV) MIMs have been developed from InGaAs grown lattice mismatched to InP. A variety of buffer layer schemes have been investigated and the impact of buffer layer design on device performance has been examined.

n/p/n tunnel junction InGaAs Monolithic Interconnected Module (MIM)

The Monolithic Interconnected Module (MIM), originally introduced at the First NREL thermophotovoltaic (TPV) conference, consists of low-bandgap indium gallium arsenide (InGaAs) photovoltaic devices, series interconnected on a common semi-insulating indium phosphide (InP) substrate. An infrared reflector is deposited on the back surface of the substrate to reflect photons, which were not absorbed in the first pass through the structure. The single largest optical loss in the current device occurs in the heavily doped p-type emitter. A new MIM design (pat pend.) has been developed which flips the polarity of the conventional MIM cell (i.e., n/p rather than p/n), eliminating the need for the high conductivity p-type emitter. The p-type base of the cell is connected to the n-type lateral conduction layer through a thin InGaAs tunnel junction. 0.58 eV and 0.74 eV InGaAs devices have demonstrated reflectances above 90% for wavelengths beyond the bandgap (>95% for unprocessed structures). Electrical measurement...

Growth, processing and characterization of 0.55-eV n/p/n monolithic interconnected modules

This paper describes recent efforts on the growth, processing and characterization of 0.55 eV InGaAs n/p/n monolithic interconnected modules (MIMs) for thermophotovoltaic energy conversion. Novel compositional grading schemes, improved cell architectures and a variety of interconnect schemes have been investigated in order to maximize device power density and efficiency. Recent work has focused on the investigation of various cell interconnect strategies to optimize efficiency and recuperation without sacrificing electrical performance. The electrical and optical results of this study are presented herein.

0.74/0.55-eV Ga/sub x/In/sub 1-x/As/InAsP/sub 1-y/ monolithic, tandem, MIM TPV converters: design, growth, processing and performance

Thermophotovoltaic (TPV) tandem converter technology is being explored in an effort to improve both the efficiency and power density of TPV systems. Inverted, tandem structures incorporate epitaxially grown 0.74-eV lattice-matched Ga/sub 0.47/In/sub 0.53/As and 0.55-eV lattice-mismatched Ga/sub 0.28/In/sub 0.72/As diodes. Additionally, a strategy has been developed to allow voltage matching between these two subcells. Performance modeling calculations show that, under typical operating conditions, the 0.74/0.55-eV tandem converter should outperform a 0.5-eV single junction converter by 15% on an efficiency basis and by 15% on a power-density basis. This paper will present details regarding the design, growth, fabrication, and electrical, optical, and structural characterization of voltage-matched tandem TPV devices.

Multijunction InGaAs thermophotovoltaic devices

A monolithic interconnected module (MIM) structure has been developed for thermophotovoltaic (TPV) applications. The MIM consists of many individual InGaAs cells series-connected on a single semi-insulating (S.I.) InP substrate. An infrared (IR) back surface reflector (BSR), placed on the rear surface of the substrate, returns the unused portion of the TPV radiator output spectrum back to the radiator for recuperation, thereby providing for high system efficiencies. MIMs were fabricated with an active area of 0.9 {times} 1 cm, and with 15 cells monolithically connected in series. Both lattice-matched and lattice-mismatched InGaAs/InP devices were fabricated, with bandgaps of 0.74 and 0.55 eV, respectively. The 0.74 eV MIMs demonstrated an open-circuit voltage (Voc) of 6.16 V and a fill factor of 74.2% at a short-circuit current (Jsc) of 0.84 A/cm{sup 2}, under flashlamp testing. The 0.55 eV MIMs demonstrated a Voc of 4.85 V and a fill factor of 57.8% at a Jsc of 3.87 A/cm{sup 2}. Electrical performance results for these MIMs are presented.