Performance comparison of AlGaAs, GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Abstract

Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10−4 Ω cm2 followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10−4 Ω cm2 within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.

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

“…TJ requirements of low resistance and high transparency oppose each other, as per the theoretical dependence of the peak tunneling current [9]: where a higher bandgap increases transparency but also reduces the peak tunneling current. 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.…”

“…These simulations are similar to those described in [9], but used a dynamic nonlocal-path direct band-to-band tunneling model [18], in combination with a nonlocal-path trap-assisted tunneling model [18]. In each of the simulations, the density-of-states effective mass is used for the conduction band and the valence band.…”

“…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. A clear divergence is observed for the lowest doped and highest doped AlGaAs/GaAs TJs.…”

“…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].…”