Testing Solar Cells with the ModuLab XM PhotoEchem
The ModuLab XM PhotoEchem is an optical and electrical measurement system, originally designed for studying dye sensitised solar cells (DSSC). It can also be used to analyse other types of solar cells, including the new generation of hybrid organic-inorganic lead halide based perovskite photovoltaics, which have high solar power conversion efficiency.
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Testing Solar Cells
A range of measurements can be performed on solar and photo-electrochemical cells using the PhotoEchem:
- Impedance spectroscopy (in the dark and under illumination) (IS)
- Intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS)
- Open circuit voltage decay measurements (OCVD)
- Charge extraction measurements (CE)
This post summarises each technique. Full details, with example experiments and results are available in an application note from Solartron Analytical.
Impedance Spectroscopy of Solar Cells
Impedance spectroscopy is a small amplitude measurement technique, in which a sinusoidal voltage or current is applied to the system under study. Generally, the response of systems such as solar cells is non-linear. This means that the amplitude of the perturbing signal has to be kept small enough so the output signal can be described using linear terms, in a series expansion of the system response. In this case, the system can be represented using linear R, C circuit elements.
The impedance of solar cells can be measured either in the dark or under illumination. The dynamic response of solar cells depends on these key processes, which can all be reflected in the impedance response:
- Charge transport
- Charge storage
- Electron-hole recombination
- Interfacial charge transfer
Intensity Modulated Photovoltage and Photocurrent Methods
IMVS (Intensity Modulated Photovoltage Spectroscopy) and IMPS (Intensity Modulated Photocurrent Spectroscopy) are closely related techniques. The intensity of a light source (usually an LED) is modulated by a few percent and the response (current or voltage) is measured as a function of modulation frequency.
IMPS and IMVS are used widely in the characterisation of electron transport and back reaction in mesoscopic dye sensitised solar cells. IMPS was originally developed to study photo-electrochemical reactions at the semiconductor / electrolyte interface. It has also been used to research the kinetics of light-driven water splitting reactions at semiconductor photoelectrodes. Both methods can also be used for other types of solar cells, and they both complement conventional impedance spectroscopy.
- Normally used at open circuit
- Provides information about recombination
- Normally used at short circuit
- Provides information about carrier transport
Open Circuit Voltage Decay (OCVD)
The previous methods all use small amplitude perturbations to linearise system response in the frequency domain. Linearisation is obviously convenient when you are modelling a system in terms of linear circuit elements (resistors and capacitors). However, to fully characterise a non-linear system such as a solar cell, measurements need to be repeated at various dc potentials or dc illumination intensities.
An alternative technique, also possible with the Modulab XM PhotoEchem, is to apply a large amplitude perturbation (voltage, current or illumination) and measure the system response in the time domain.
The most useful large-amplitude technique for studying DSSCs and perovskite solar cells is the open circuit decay method. This involves illuminating the solar cell at open circuit to establish a steady state open circuit voltage, and then interrupting the illumination. The subsequent decay of open circuit voltage is then measured using an ultra-high impedance amplifier.
This method was originally developed for conventional silicon solar cells, where under certain conditions the voltage decay gives information about the minority carrier lifetime, typically in the microsecond region. It has also been found to provide valuable information about DSSCs and hybrid perovskite solar cells.
Charge Extraction Measurements with DSSCs
The high concentrations of trapped electrons in DSSCs (up to 1019 cm-3) can be measured using an analytical method called charge extraction. This involves illuminating the cell at open circuit to fill the traps below the quasi-Fermi level, which corresponds to the open circuit voltage. The illumination is then switched off and the voltage decays for a set time. The cell is then short circuited and electrons flow into the external circuit. The current ‘spike’ caused by the release of the trapped electrons is integrated over a suitable period (usually tens of seconds) to determine the trapped charge:
The experiment is then repeated with progressively longer decay times, to obtain the trapped electron concentration as a function of open circuit voltage, and hence of the quasi Fermi level position. The extracted charge is then plotted as a function of the open circuit voltage at which the cell was short circuited. The charge can then be converted into an electron concentration, using the known thickness and porosity of the mesoporous titania layer.
More information about these techniques, including detailed examples and results, is available in an application note from Solartron Analytical. If you have any questions about how they could work for your research, of if you would like a quote for the Modulab XM PhotoEchem, please contact us: