Solar Spectrum Splitting Parallel Junction High Efficiency Concentrating Photovoltaics

Abstract

A compact, single element concentrator comprising a near linear array of prisms has been designed to simultaneously split and concentrate the solar spectrum. Laterally aligned solar cells with different bandgaps are devised to be fabricated on a common Si substrate, with each cell absorbing a different spectral band optimized for highest overall power conversion efficiency. Epitaxial Ge on Si is used as a low cost virtual substrate for III-V materials growth. Assuming no optical loss for the prism concentrator, no shadowing and perfect carrier collection for the solar cells, simulations show that 39% efficiency can be achieved for a parallel four-junction (4PJ) InGaP-GaAs-Si-Ge cell under 200X concentration, and higher efficiency is possible with more junctions.

Solar spectral variations and their influence on concentrator solar cell performance

“…The subcells in our parallel triple junction InGaP-GaAs-Ge are considered to be grown on inexpensive Si substrates, either monolithically integrated on the same Si substrate 5 or each having its own substrate. Due to the well-matched lattice constants between the III-V materials (In 0.51 Ga 0.49 P and GaAs) and Ge, the subcells can be epitaxially grown on a single crystalline Ge buffer layer on top of a Si substrate 16 .…”

“…Realistic design of the subscells on Si substrates is made, optimized to SOC mentioned previously. The absorber thicknesses are chosen to be 2.0, 2.0, 2.5 µm for InGaP, GaAs, and Ge subcells, respectively 5 . A typical antireflection coating (ARC) is applied for each subcell (MgF 2 /ZnS double layer ARC for the two III-V subcells and Si 3 N 4 for Ge).…”

“…However, this difference may not be significant given that the parallel junction may encounter extra losses from the optical spectrum splitter. To make parallel junctions really excel, it is important to take full advantage of the flexibility in materials choice to optimize the combination of bandgaps and to incorporate subcells with favorable bandgaps, such as replacing the Ge junction with a material that has a bandgap near 1 eV, for example, Si 5 or dilute nitride III-V compounds 23 , or by adding other materials with bandgaps higher than that of InGaP.…”

“…Traditional concentrator photovoltaics (CPV) uses a vertically stacked tandem junction configuration. In recent years, parallel junctions, where laterally aligned solar cells with differing bandgaps are used along with spectrum splitting optical elements have stimulated renewed interest [1][2][3][4][5][6] . Compared to tandem junctions, , using parallel junctions allows subcells to be optimized independently and eliminates the current matching constraint, thereby rendering higher efficiency.…”