AlGaAsP Distributed Bragg Reflectors for GaAsP/Si Solar Cells

We investigate (Al)GaAsP distributed Bragg reflectors (DBRs) on Si (001) to improve the quantum efficiency (QE) of 1.7 eV GaAsP solar cells in GaAsP/Si tandem devices. Samples were grown on Si (001) by molecular beam epitaxy and consisted of a 2.1 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu \mathrm{m}$</tex-math></inline-formula> GaAsP/GaP buffer followed by a ∼2 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu \mathrm{m}$</tex-math></inline-formula> DBR with 20 periods of GaAsP/Al<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i>Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-</sub><italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i>AsP alternating layers. Two different DBR designs were studied with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> = 0.4 and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> = 0.8, both targeting a peak reflectance wavelength of 700 nm. The average threading dislocation density on the DBRs was 1.4 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup>, suitable for high-performance GaAsP cells. The reflectance profiles matched well to simulations, and the GaAsP/Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.8</sub>Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.2</sub>AsP DBR had a significantly higher peak reflectance and reflectance bandwidth than the GaAsP/Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub>Ga<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.6</sub>AsP DBR due to the higher refractive index contrast. QE simulations of GaAsP cells showed an improvement of ∼1 mA/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in short-circuit current density with a DBR, which should enable a ∼5% relative efficiency boost in the GaAsP cell and superior current matching to a Si bottom cell in tandem devices.