Performance analysis of AlGaAs/GaAs tunnel junctions for ultra-high concentration photovoltaics

Abstract

An n++-GaAs/p++-AlGaAs tunnel junction with a peak current density of 10 100 A cm−2 is developed. This device is a tunnel junction for multijunction solar cells, grown lattice-matched on standard GaAs or Ge substrates, with the highest peak current density ever reported. The voltage drop for a current density equivalent to the operation of the multijunction solar cell up to 10 000 suns is below 5 mV. Trap-assisted tunnelling is proposed to be behind this performance, which cannot be justified by simple band-to-band tunnelling. The metal-organic vapour-phase epitaxy growth conditions, which are in the limits of the transport-limited regime, and the heavy tellurium doping levels are the proposed origins of the defects enabling trap-assisted tunnelling. The hypothesis of trap-assisted tunnelling is supported by the observed annealing behaviour of the tunnel junctions, which cannot be explained in terms of dopant diffusion or passivation. For the integration of these tunnel junctions into a triple-junction solar cell, AlGaAs barrier layers are introduced to suppress the formation of parasitic junctions, but this is found to significantly degrade the performance of the tunnel junctions. However, the annealed tunnel junctions with barrier layers still exhibit a peak current density higher than 2500 A cm−2 and a voltage drop at 10 000 suns of around 20 mV, which are excellent properties for tunnel junctions and mean they can serve as low-loss interconnections in multijunction solar cells working at ultra-high concentrations.

Chemical beam epitaxy growth of AlGaAs/GaAs tunnel junctions using trimethyl aluminium for multijunction solar cells

“…It has been suggested that a GaAs/AlGaAs TJ with Al content of either 10% [10] or 30% [9,11], or AlGaAs/AlGaAs TJs with Al content <20% [9] can provide an acceptable tradeoff between low resistance and transparency. However, equation 1 applies for a homojunction only.…”

“…Also included are the best literature results for a GaAs/GaAs (8630 A/cm 2 ) and Al 0.3 Ga 0.7 As/GaAs (10100 A/cm 2 ) TJ from [11]. A GaAs/GaAs model from [9] was scaled to fit either our GaAs/GaAs result or the best literature result.…”

“…Presence of defects at the GaAs/AlGaAs interface could slightly extend the depletion width [19], thus reducing the peak tunneling current. ,black), the best literature results from [11](triangle, black), a GaAs/GaAs model taken from [9] and scaled to fit experimental results (lines), and the Sentaurus simulations for our TJ designs (red).…”

“…TJs suitable for MJSC used in CPV must present both a low resistance (equivalently, a high peak tunneling current) and a high transparency to minimize losses. Due to higher optical transparency, an AlGaAs/GaAs TJ seems better suited for multijunction solar cells [9][10][11].…”

Sputtered metal oxide broken gap junctions for tandem solar cells

“…A metal layer is the simplest and would allow recombination, but metals block light transmission to the lower cell. Epitaxial tunnel junctions have been deposited by molecular beam epitaxy (MBE) [8] and metal organic chemical vapor deposition (MOCVD) [9], and laser ablation have produced Broken Gap Junction (BGJ) devices [10]. The first two approaches deposit epitaxial crystalline films with heavily doped p-and n-type layers which improve intraband tunneling by minimizing the depletion region [11], while laser ablation processing is not widely adopted in industry.…”