Environment and energy
EQE Testing Beyond the Band Edge by Bentham
November 9, 2018
The extraction of band gap, Urbach energy and the investigation of sub-band gap defect states in photovoltaic devices necessitates the measurement of the EQE into and beyond the band edge. This powerful approach calls for a high sensitivity EQE tool.
The EQE of photovoltaic devices is best performed by illuminating the device with a wavelength tuneable light source assembled from a broad band light source and a single monochromator. Recording the photocurrent generated by the device under test in response to the monochromatic beam of known optical power enables direct determination of EQE and offers high quality data in the response range of the device.
At and beyond the band gap, stray light transmitted by the monochromator – light in addition to the selected wavelength – gives rise to a photocurrent response in excess of that due to sub -band gap absorption. This leads to an artificially high EQE result which distorts the profile of the band edge and ensures that electronically active defect states residing deep in the band gap are rarely observed.
Schemes to suppress stray light using long-pass filters is limiting. The filters must be selected according to the band gap under study and may have transmission windows at longer wavelengths which will reduce their suppression efficacy.
The ultimate solution is to replace the single monochromator with a double.
Measurement of a HeNe laser measured by single and double monochromator enables evaluation of the monochromator slit function.
Origin of Stray light
The essence of a monochromator is to act as a tuneable filter, to select the desired wavelength from a broadband input source. Light reaching the exit port at other wavelengths is termed stray light.
The design of high quality monochromators minimises stray light by ensuring under-filling of optics, low-reflectivity painted surfaces, baffles and in some cases the selection of holographic over ruled diffraction gratings. Despite this, transmission of stray light by a single monochromator – including light scattered from the mirrors and grating defects – is unavoidable.
Stray light can be further suppressed by passing the selected wavelength through a second monochromator stage.
The target wavelength is re-selected and transmitted to the exit port whilst the stray light component is significantly attenuated.
This results in a squaring of the stray light performance and a spectrally pure tuneable light source.
Monochromator slit function showing superior suppression of out-of-band stray light, of the double configuration.
The photocurrent generated by sub-band gap absorption is several orders of magnitude lower than the band gap absorption. The optical power of the double monochromator tuneable light source should be optimised to maximise the generated photocurrent,whilst a low-noise current amplifier is used to enable measurement down to the picoampere level. Finally, working at the band gap requires good sample temperature control. There can be nothing worse than measuring a moving target!
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