Suppression of Edge Recombination in InAs/InGaAs DWELL Solar Cells

Abstract

The InAs/InGaAs DWELL solar cell grown by MBE is a standard pin diode structure with six layers of InAs QDs embedded in InGaAs quantum wells placed within a 200-nm intrinsic GaAs region. The GaAs control wafer consists of the same pin configuration but without the DWELL structure. The typical DWELL solar cell exhibits higher short current density while maintaining nearly the same open-circuit voltage for different scales, and the advantage of higher short current density is more obvious in the smaller cells. In contrast, the smaller size cells, which have a higher perimeter to area ratio, make edge recombination current dominant in the GaAs control cells, and thus their open circuit voltage and efficiency severely degrade. The open-circuit voltage and efficiency under AM1.5G of the GaAs control cell decrease from 0.914V and 8.85% to 0.834V and 7.41%, respectively, as the size shrinks from 5*5mm 2 to 2*2mm 2 , compared to the increase from 0.665V and 7.04% to 0.675V and 8.17%, respectively, in the DWELL solar cells.The lower open-circuit voltage in the smaller GaAs control cells is caused by strong Shockley-Read-Hall (SRH) recombination on the perimeter, which leads to a shoulder in the semi-logarithmic dark IV curve. However, despite the fact that the DWELL and GaAs control cells were processed simultaneously, the shoulders on the dark IV curve disappear in all the DWELL cells over the whole processed wafer. As has been discussed in previous research on transport in QDs, it is believed that the DWELL cells inhibit lateral diffusion current and thus edge recombination by collection first in the InGaAs quantum well and then trapping in the embedded InAs dots. This conclusion is further supported by the almost constant current densities of the different area DWELL devices as a function of voltage.

Miniaturization of InGaP/InGaAs/Ge solar cells for micro‐concentrator photovoltaics

“…Surface recombination along the side‐wall perimeter is known to have a greater performance impact on solar cells with a high perimeter‐to‐area ratio (i.e., very small cells), mostly due to a high density of surface states (i.e., >10 13 cm −2 ) 19 . The role of perimeter recombination in cells, which particularly affects their voltage, was studied in single junction, 20–22 in the subcells that compose a three‐junction cell, 23 and in three‐junction structures 11,12,24 . An effective reduction of GaAs surface states density can be obtained by sulfur passivation, plasma treatments, or atomic layer deposition (ALD) 25 .…”