Ultra thin tunnel junction for use in III–V multijunction solar cells

In the ideal case, a III-V tunneljunction (TJ) used in a concentrator setup can handle high current, has a low resistance and does not absorb photons with an energy higher than the subcell(s) below it. In practice, TJs for concentrator systems are made of GaAs or AlGaAs with a low Al fraction, so absorption is present and causes an efficiency loss. Simulations show that in a standard 40 nm thick GaAs TJ the current density loss due to absorption is 1.7 mA/cm². This value can be reduced by replacing one of the GaAs layers by AlGaAs and/or using a thinner TJ. We have tested Al_0.1GaAs/GaAs TJs with a total thickness of 17.5 nm. Two n-type dopants were investigated: Te and Si. For the p-type layer C was used. Excellent TJs with peak current densities above 600 A/cm² and a series resistance of 0.5 mΩ cm² were obtained. In this ultra thin TJ current density losses were reduced to 0.6 mA/cm².