Defect detection in III-V multijunction solar cells using reverse-bias stress tests

Reverse biasing triple-junction GaInP/Ga(In)As/Ge solar cells may affect their performance by the formation of permanent shunts even if the reverse breakdown voltage is not reached. In previous works, it was observed that, amid the three components, GaInP subcells are more prone to degrade when reverse biased suffering permanent damage, although they present an initial good performance. The aim of this work is, firstly, to study the characteristics of the defects that cause the catastrophic failure of the devices. For this, GaInP isotype solar cells were analysed by visual inspection and electroluminescence maps and submitted to reverse bias stress test. We find that specific growth defects (i.e. hillocks), when covered with metal, cause the degradation in the cells. SEM cross-section imaging and EDX compositional analysis of these defects reveal their complex structures, which in essence consist of material abnormally grown on and around particles present on the wafer surface before growth. The reverse bias stress test is proposed as a screening method to spot defects hidden under the metal that may not be detected by conventional screening methods. By applying a quick reverse bias stress test, we can detect those defects that cause the degradation of devices at voltages below the breakdown voltage and that may also affect their long-term reliability.