Fluctuations of the peak current of tunnel diodes in multi-junction solar cells

Abstract

Interband tunnel diodes are widely used to electrically interconnect the individual subcells in multi-junction solar cells. Tunnel diodes have to operate at high current densities and low voltages, especially when used in concentrator solar cells. They represent one of the most critical elements of multi-junction solar cells and the fluctuations of the peak current in the diodes have an essential impact on the performance and reliability of the devices. Recently we have found that GaAs tunnel diodes exhibit extremely high peak currents that can be explained by resonant tunnelling through defects homogeneously distributed in the junction. Experiments evidence rather large fluctuations of the peak current in the diodes fabricated from the same wafer. It is a challenging task to clarify the reason for such large fluctuations in order to improve the performance of the multi-junction solar cells. In this work we show that the large fluctuations of the peak current in tunnel diodes can be caused by relatively small fluctuations of the dopant concentration. We also show that the fluctuations of the peak current become smaller for deeper energy levels of the defects responsible for the resonant tunnelling.

Effect of low and staggered gap quantum wells inserted in GaAs tunnel junctions

“…Thus, the classic way to fabricate TJs with high J peak suitable for MJSCs is to increase the n and p doping levels, which can be very challenging with some materials. In particular, the standard n type dopant for GaAs is Si, whose solubility limits in GaAs is around 10 19 cm -3 [3] for usual epitaxial growth conditions, indeed limiting the maximum achievable J peak with Si-doped GaAs TJs around 25 A/cm² [4] [5] [6]. The use of Te dopant instead of Si for GaAs based TJs grown by MOVPE allows to get higher n-doping levels as high as 3x10 19 cm -3 , leading to record J peak of 10 kA/cm² [7].…”

Mechanically stacked solar cells for concentrator photovoltaics

“…Monolithically-grown triple-junction III-V/Ge PV solar cells currently represent the state-of-the-art in terms of the highest sunlight to electricity conversion efficiencies achieved to date. Nevertheless, drawbacks associated with current matching, crystal lattice mismatch between optimum cell materials and cell interconnect (tunnel junction) reliability, particularly in the case of solar concentrator systems, are of some concern [2]. Mechanically stacked solar cells (MSSC) can overcome these issues [3], however, they present alternative challenges primarily due to the complexity of integration of the various sub-cells with minimal optical loss and at minimal cost.…”

Time-dependent analysis of AlGaAs/AlGaAs tunnel junctions for high efficiency multi-junction solar cells

“…While in this work AlGaAs/AlGaAs TJ are studied, devices fabricated using GaAs/GaAs were studied experimentally and numerically in the past [4][5][6] . Previous research has presented the effects of temperature on the current density-voltage (J-V) characteristics of AlGaAs/AlGaAs 7 and variations of doping concentration on the resistance of three TJ types 8 .…”