The process of extracting band gap, Urbach energy and investigating sub-band gap defect states in photovoltaic devices requires the measurement of the EQE into and beyond the band edge. Such a powerful approach needs an EQE tool that is highly sensitive.
Performing an EQE measurement of a photovoltaic device is best done by illuminating the sample with a light source that has a tunable wavelength, assembled from a broad band light source and a single monochromator. Direct determination of EQE is enabled by recording the photocurrent generated by the device under test in response to the monochromatic beam of known optical power, offering high quality data within 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 results in an artificially high EQE, distorting the profile of the band edge and obscuring the electronically active defect states residing deep in the band gap.
Schemes to suppress stray light using long-pass filters are limiting. The selection of the filters must be made in accordance to the band gap under study, with the possibility of transmission windows at longer wavelengths that reduce their suppression efficacy.
The ultimate solution, therefore, is to use a double monochromator rather than a single.
Measuring into and beyond the band gap of c:Si
Origin of Stray Light
A monochromator is required to essentially act as a tunable filter, to select the desired wavelength from a broad band input source. Stray light is the light that reaches the exit port at other wavelengths.
High quality monochromators are designed in such a way so as to minimize 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 such precautions, stray light transmission by a single monochromator - including light scattered from the mirrors and grating defects - is unavoidable.
It is possible to further suppress stray light by passing the selected wavelength through a second monochromator stage.
Thus, re-selection and transmission of the target wavelength to the exit port occurs whilst the stray light component is significantly attenuated. This leads to a squaring of the stray light performance and a tuneable light source that is spectrally pure.
Monochromator slit function showing superior suppression of out-of-band stray light, of the double configuration.
The sub-band gap absorption leads to photocurrent generation that is several orders of magnitude lower than the band gap absorption. Optimization of the optical power of the double monochromator tuneable light source is essential to maximize the generated photocurrent, whilst a low-noise current amplifier is used to enable measurement down to the picoampere level. Finally, operating at the band gap requires good sample temperature control. After all, there can be nothing worse than measuring a moving target!
This information has been sourced, reviewed and adapted from materials provided by Bentham Instruments Limited.
For more information on this source, please visit Bentham Instruments Limited.