The Bentham PVE300 has proven extremely popular in both research institutes and industry for the spectral characterization of PV devices. As PV technologies progress, so does the capability of the PVE300. In this article, the measurement of the spectral response/ EQE and IQE of multi-junction solar cells is introduced.
Measuring the spectral responsivity/ EQE of the component junctions of a monolithic multi-junction solar cell individually is not possible as 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 applicable light bias to produce a surplus photocurrent in all other junctions.
In this manner, the photocurrent produced by the multi-junction device is defined by the response of the junction under test to the monochromatic probe of the PVE300 Photovoltaic EQE (IPCE) and IQE solution.
Where the junction under test displays a low reverse breakdown voltage or a low shunt resistance – as is common with low band gap materials such as the germanium bottom cell – complex interactions between junctions can result in erroneous results.
The exact measurement of multi-junction PV devices requires the usage of enhanced light biasing for all junctions and voltage biasing for the bottom junction.
Having established the spectral response of all component junctions, one can straightaway compute the EQE, and, with the incorporation 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
Monolithic Multiple Junction (MJ) Solar Cells
Although standards1 exist for the assessment of the spectral response of monolithic multiple junction (MJ) solar cells, the measurement method is far from standard.
Without such guidance, however, it is probable that in the case of subcells having low reverse breakdown voltage or low shunt resistance, major measurement errors will result.2
In looking to measure the spectral response/EQE of monolithic MJ solar cells, such as the III-V monolithic GaInP/GaInAs/Ge triple junction solar cell shown here, the measurement of component subcells on an individual basis is not possible as they are epitaxially grown on one substrate and interconnected by tunnel diodes.
Subcell spectral response must thus be established by making use of the effect of current limitation, accomplished through light biasing.
Besides biasing the subcell under test at a level of one sun to mimic use conditions, it is essential to light bias the non-tested subcells at a higher intensity such that the former produces the smallest photocurrent and is thus current limiting. It follows that good control of the light bias source spectrum is required.
Since the current of the MJ cell is restricted by the subcell under test, it follows that the non-tested cells with surplus photocurrent function close to their Voc. If the MJ cell is tested under short circuit conditions, then a negative voltage approximately equal to the sum of the Voc of the other cells is positioned across the tested subcell.
Although the photocurrent produced by light biasing may be thought to be as a constant, the presence of the monochromatic probe gives rise to variations in subcell operating voltage, reliant on the probe wavelength and the response range of the current limiting subcell under test.
Where the subcell under test has non-ideal properties such as a low reverse breakdown voltage or a low shunt resistance, frequently displayed in low bandgap materials such as germanium, this can result in changes in the measured photocurrent, and the incorrect reporting of spectral response/EQE: the result depends mainly on the true nature of the I-V curves of both the subcell under test and that of the non-tested subcells.
Outside the response range of the subcell under test, the monochromatic probe gives rise to an increase in working voltage of the non-tested cells, compensated for by a reduction in the operating voltage of the subcell under test. This may cause an increase in Jsc, showing a response where one is not expected.
Within the subcell response range, the presence of the monochromatic probe will directly result in an increase in Jsc, shifting the non-tested cells to lower operating voltage, and compensated by an increase in the voltage of the tested subcell. The resulting current may be less than that expected in response to the monochromatic probe, resulting in a lower reported spectral response/EQE.
Both effects are commonly observed phenomenon in the measuring of the bottom cell of the GaInP/GaInAs/Ge triple junction solar cell.
Shifting the external voltage of the cell will tend to lessen both of the above effects: in moving to higher gradient regions of the I-V curve of the non-tested cells, lower shifts in subcell operating voltage are met, causing less variation in the current of the cell under test.
Optimization of Light Biasing
Increasing the photocurrent produced by the non-tested subcells results in increased gradient of the I-V curve in the proximity of their operating voltage, while decreasing the photocurrent produced by the subcell under test will cause a reduction of operating voltage closer to Voc where the gradient of the I-V curve is steepest.
Both will give rise to less variation in the current of the cell under test, and is achieved through appropriate filtering.
MJ Cell Example
For the correct measurement of the GaInP/GaInAs/Ge triple junction solar cell, the following procedure is suggested.
Current limiting may be proved by spectral response measurement at various levels of non-tested cell light bias intensity.
GaInP Top Junction
The GaInP junction responds ~300-700 nm. The device under test should be illuminated simultaneously by a solar simulator at one sun bias, and a second simulator filtered with a red long-pass filter. The spectral response may then be measured directly over the extended range 300-800 nm.
GaInAs Middle Junction
The GaInAs junction responds ~500-900 nm. The device being tested should be illuminated simultaneously by a solar simulator at one sun bias, and a second simulator filtered with a blue band pass filter, transmitting also in the infrared. The spectral response can then be measured directly over the extended range 300-1100 nm.
Ge Bottom Junction
The Ge junction responds ~900-1800 nm. The device under test should be illuminated simultaneously by a solar simulator at one sun bias, and a second simulator filtered using an IR rejection filter.
To take this subcell to short circuit condition, an external voltage must be applied. Under dual solar simulator irradiation, the operating voltage, Vop of the top two junctions is recorded (typically ~2-2.5 V). The voltage across the device should then be fixed to -Vop to take the third junction into short circuit.
The spectral response can then be measured directly over the extended range 800-1800 nm. Where the precise bias voltage is used, a maximum spectral response can be recorded. This may be confirmed by repeating the measurement at other levels of voltage bias.
- 1 ASTM- E2236-05 “Standard Test Methods for Measurement of Electrical Performance and Spectral Response of Nonconcentrator Multijunction Photovoltaic Cells and Modules”
- 2 Meusel et al, “Spectral Response Measurements of Monolithic GaInP/Ga(In)As/Ge Triple-Junction Solar Cells: Measurement Artifacts and their Explanation”, Prog. Photovolt: Res. Appl. 2003; 11:499–514
- Meusel, M., Baur, C., Letay, G., Bett, A.W., Warta, W. and Fernandez, E., 2003. Spectral response measurements of monolithic GaInP/Ga (In) As/Ge triple‐junction solar cells: Measurement artifacts and their explanation. Progress in Photovoltaics: Research and Applications, 11(8), pp.499-514.
- IEC 60904-8- 1:2017 - Photovoltaic devices - Part 8-1: Measurement of spectral responsivity of multi-junction photovoltaic (PV) devices
This information has been sourced, reviewed and adapted from materials provided by Bentham Instruments Limited.
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