Engineering the reciprocal space for ultrathin GaAs solar cells

III-V solar cells dominate the high efficiency charts, but with significantly higher cost than other solar cells. Ultrathin III-V solar cells can exhibit lower production costs and immunity to short carrier diffusion lengths caused by radiation damage, dislocations, or native defects. Nevertheless, solving the incomplete optical absorption of sub-micron layers presents a challenge for light-trapping structures. Simple photonic crystals have high diffractive efficiencies, which are excellent for narrow-band applications. Random structures a broadband response instead but suffer from low diffraction efficiencies. Quasirandom (hyperuniform) structures lie in between providing high diffractive efficiency over a target wavelength range, broader than simple photonic crystals, but narrower than a random structure. In this work, we present a design method to evolve a simple photonic crystal into a quasirandom structure by modifying the spatial-Fourier space in a controlled manner. We apply these structures to an ultrathin GaAs solar cell of only 100 nm. We predict a photocurrent for the tested quasirandom structure of 25.3 mA/cm$^2$, while a planar structure would be limited to 16.1 mA/cm$^2$. The modified spatial-Fourier space in the quasirandom structure increases the amount of resonances, with a progression from discrete number of peaks to a continuum in the absorption. The enhancement in photocurrent is stable under angle variations because of this continuum. We also explore the robustness against changes in the real-space distribution of the quasirandom structures using different numerical seeds, simulating variations in a self-assembly method

Characterization of multiterminal tandem photovoltaic devices and their subcell coupling

Three-terminal (3T) and four-terminal (4T) tandem photovoltaic (PV) devices using various materials have been increasingly reported in the literature, but measurement standards are lacking. Here, multiterminal devices measured as functions of two load variables are characterized unambiguously as functions of three device voltages or currents on hexagonal plots. We demonstrate these measurement techniques using two GaInP/GaAs tandem solar cells, with a middle contact between the two subcells, as example 3T devices with both series-connected and reverse-connected subcells. Coupling mechanisms between the subcells are quantified within the context of a simple equivalent optoelectronic circuit. Electrical and optical coupling mechanisms are most clearly revealed using coupled dark measurements. These measurements are sensitive enough to observe very small luminescent coupling from the bottom subcell to the top subcell in the prototype 3T device. Quick simplified measurement techniques are also discussed within the context of the complete characterization