Measurement of 4 Junction PV Cells
The Bentham PVE300 has proven exceptionally popular in research institutes and industry for the spectral characterisation of PV devices. As PV technologies evolve, so does the capability of the PVE300. Here we introduce the measurement of the spectral response/ EQE and IQE of multi-junction solar cells.
It is not possible to measure the spectral responsivity/ EQE of the component junctions of a monolithic multi-junction solar cell separately, since they are epitaxially grown on one substrate and interconnected by tunnel diodes.
The spectral response of a junction is measured by putting it into current limitation by applying an appropriate light bias to generate an excess photocurrent in all other junctions.
In this manner, the photocurrent generated by the multi-junction device is defined by the response of the junction under test to the monochromatic probe of the PVE300.
Where the junction under test exhibits a low shunt resistance or low reverse breakdown voltage– as is common with low band gap materials such as the germanium bottom cell– complex interactions between junctions can lead to erroneous results.
The correct measurement of multi-junction PV devices requires the use of optimised light biasing for all junctions and voltage biasing for the bottom junction.
Having determined the spectral response of all component junctions, one can directly compute the EQE, and, with the addition of a reflectance measurement- in the PVE300 using the DTR6 integrating sphere accessory, compute the IQE.
Comparison of EQE and IQE
EQE = Bold lines
IQE = Pale lines