Characterisation of high-efficiency multi-junction solar cells and tunnel junctions

Abstract

Tunnel junctions for use in solar cells and monolithic multi junction solar cells are studied experimentally. The current density-voltage characteristic of an AlGaAs/AlGaAs tunnel junction having a mesa resistance of 0.11 mO·cm2 is determined using time-averaged measurements. A tunneling peak higher than the operating point of a solar cell is recorded by this method, with a value of ∼950 A/cm2. Due to the unstable nature of the negative differential resistance region of the current density-voltage curve, measurements of the tunneling peak and valley current densities are obscured. A time-dependent analysis is performed on this sample, from which a tunneling peak of a value larger than 1100 A/cm 2 is determined. An A1GaAs/InGaP tunnel junction having a tunneling peak of 80 A/cm2 is presented. Multi junction solar cells fabricated using indium tin-oxide as transparent top electrodes are measured. These cells have a maximal efficiency of 25.1% at 3 suns illumination and 26.1% at 20 suns, ∼40% lower efficiency than the standard multi junction solar cell.