Inverted metamorphic AlGaInAs/GaInAs tandem thermophotovoltaic cell designed for thermal energy grid storage application

Abstract

We demonstrate an inverted metamorphic multijunction (IMM) photovoltaic cell comprising lattice-mismatched 1.2 eV AlGaInAs and 1.0 eV GaInAs junctions optimized for high-temperature thermophotovoltaic (TPV) applications. This device differs from traditional IMM solar cells because the mismatched junctions are grown at a single lattice constant. This architecture enables removal of the compositionally graded buffer which otherwise filters light from the junctions below and absorbs subbandgap light via free-carrier absorption. Sub-bandgap absorption dramatically reduces the efficiency of TPV systems using high reflectivity cells to enable bandedge spectrum filtering. Three components required development to enable this device: 1) a latticemismatched 1.2 eV AlGaInAs junction, 2) a metamorphic contact layer grown after the graded buffer, and 3) a transparent tunnel junction that sits in front of the 1.0 eV GaInAs junction. Growth conditions that minimize oxygen defect incorporation maximize AlGaInAs cell quality, enabling a 0.41 V bandgap open circuit voltage offset at 22 mA/cm 2 under AM1.5D. A mismatched GaInAs:Se layer is developed as a low resistance contact. Lastly we develop a GaAsSb:C/GaInP:Se tunnel junction suitable for high-power densities with more transparency than the GaAsSb:C/GaInP:Se structure used in past IMM cells. We characterize the tandem device under a high-intensity spectrum that approximates the emission from a 2150 °C blackbody radiator and deduce a projected ideal TPV efficiency of 39.9% at ~30% of the blackbody irradiance, and 36% ideal TPV efficiency under the full 118 W/cm 2 irradiance. Improvements to the back-surface reflectivity and series resistance are expected to increase the ideal TPV efficiency well above 40%.

Thermophotovoltaic efficiency of 40%

“…The results for the 1.2/1.0 eV tandem showed greater efficiency than for the 1.4/1.2 eV tandem at lower emitter temperatures because of its lower bandgaps. The efficiency of the 1.2/1.0 eV tandem reached a maximum of 39.3 ± 1% at 2,127 °C, quite close to 2,150 °C, which is the temperature at which our device model predicted this bandgap combination would be optimal 27 . The average efficiency between 1,900 and 2,300 °C was 38.2% and the efficiency remained high across a 400 °C range of emitter temperatures.…”

“…For the 1.2/1.0 eV design 27 , a 0.2 µm GaAs buffer layer was grown first, then a GaInP CGB consisting of 0.25 µm GaInP steps, spanning the range Ga 0.51 In 0.49 P to Ga 0.19 In 0.81 P, with the final layers being a 1.0 µm Ga 0.19 In 0.81 P strain overshoot layer and a 0.9 µm Ga 0.22 In 0.78 P step back layer lattice matched to the in-plane lattice constant of the Ga 0.19 In 0.81 P. A 0.3 µm Ga 0.70 In 0.30 As:Se front contact layer was grown next, followed by the top cell, starting with a 0.02 µm Ga 0.22 In 0.78 P:Se window, a 1.0 µm Al 0.15 Ga 0.55 In 0.30 As:Se emitter, an undoped 0.1 µm Al 0.15 Ga 0.55 In 0.30 As i-layer, a 2.1 µm Al 0.15 Ga 0.55 In 0.30 As:Zn base and a 0.07 µm Ga 0.22 In 0.78 P:Zn BSF. Then the tunnel junction, comprising a 0.2 µm Al 0.15 Ga 0.55 In 0.30 As:Zn layer, a 0.05 µm GaAs 0.72 Sb 0.28 :C++ layer and a 0.1 µm Ga 0.22 In 0.78 P:Se++ layer, was grown.…”

“…This is key, because the spectrum of light redshifts towards longer wavelengths as the radiator temperature is lowered, which is why traditional TPV cells that are paired with emitters of less than 1,300 °C are typically based on 0.74 eV InGaAs or 0.73 eV GaSb. Considerable work on low-bandgap semiconductors has been undertaken with the envisioned application of converting heat from natural gas combustion 3 – 9 , concentrated solar power 24 , space power applications 25 , 26 and, more recently, energy storage 1 , 2 , 27 . This pioneering body of work has led to the identification of three key features that now enable TPVs to become a competitive option for converting heat to electricity commercially: high-bandgap materials paired with high emitter temperatures, high-performance multi-junction architectures with bandgap tunability enabled by high-quality metamorphic epitaxy 16 and the integration of a high-reflectivity BSR for band-edge filtering 11 , 13 .…”

“…2 ). The lower bandgap 1.2/1.0 eV tandem offers the potential for higher power density than the 1.4/1.2 eV tandem because it converts a broader band of the incident spectrum, and consequently the requirements on the BSR are less stringent to obtain high efficiency 27 . Higher power density can also be a practical engineering advantage.…”

Review of Carnot Battery Technology Commercial Development

“…The system works with molten silicon, which cycles between 1900 • C (discharged tank) and 2400 • C (charged tank). When discharging the system, specialized solar cells, known as multi-junction photovoltaic cells with a conversion efficiency of 29%, are used to generate electricity [145,146]. It is also considered that the thermal energy storage technology developed in these projects could be used in concentrated solar power plants (CSP plants) [141].…”