Design flexibility of ultra-high efficiency 4-junction inverted metamorphic solar cells

We present recent improvements to the 4-junction inverted metamorphic solar cell. The device now includes a (Ga)InAsP buffer that transitions to lattice constants greater than InP, which allows access to GaInAs subcells with bandgaps < 0.74 eV and an additional 2 mA/cm2 of bottom junction photocurrent at AM1.5D. However, the optimal design depends on the spectrum and operating temperature. We show how the device flexibility can be used to fine-tune the design for various spectra in order to maximize energy yield for a given operating condition. 1-sun devices achieve 35.3% efficiency under the AM0 spectrum and 37.8% efficiency under the global spectrum at 25°C. Concentrator devices achieve 45.7% peak efficiency under 234x the direct spectrum and maintain over 45% efficiency at 700x at 25°C. Other device improvements include a 4-layer anti-reflection coating with low power loss, and reduced series resistance.