Time-Resolved Photoluminescence Mapping of CIGS Devices Using a Combination of a Superconducting Nanowire Detector and a Confocal Microscope

Over the years, luminescence spectroscopy has become one of the fundamental methods for analyzing the photophysical properties of a variety of samples, ranging from organic molecules to semiconductor materials and photovoltaic (PV) devices. It is worth emphasizing that detection sensitivity is a key parameter to meet today's demands for handling weak luminescent samples and forshort measurement times in the optical evaluation of PV devices. The introduction of single-photon counting based data acquisition has proven to yield a major sensitivity increase and very high dynamic range – it is the ideal method for measuring weak photoluminescence (PL).<br/>The commonly used steady-state luminescence spectroscopy methods provide valuable insights into the photophysics of a sample. However, such results give only a partial view of the sample’s behavior after photoexcitation. A further piece of the puzzle is often revealed by performing time-resolved luminescence spectroscopy, as it provides deeper insights into the photophysical processes occurring in the sample under investigation. An even more comprehensive picture is gained by including spatial information. Acquiring time-resolved spectroscopic data at regions of interest (ROI) in the sample can help in inferring structural-to-photophysical relationships in PV materials. Gathering such information is an important step toward the optimization of structure as well as preparation process of such materials in order to increase the performance of PV devices.<br/>In terms of detector performance, superconducting nanowire single-photon detectors (SNSPDs) stand out due to achievable close-to-unity detection efficiency in NIR spectral range, picosecond time resolution (down to &lt; 20 ps (FWHM)), and low dark noise (&lt; 100 counts/s)^1,2. SNSPDs are a rather new technology that is being used in multiple applications in the field of quantum optics, luminescence lifetime measurements, and singlet oxygen detection. The high quantum efficiency is especially important for material science applications in the NIR-range beyond 1000 nm, where other available single photon detectors have low sensitivity, high dark noise and slow time response.<br/>In this report we demonstrate the combination of the MicroTime 100 upright confocal fluorescence lifetime microscope with the Single Quantum EOS SNSPDs as a powerful tool for photophysical research on CIGS (Cu(InGa)Se₂) devices, yielding spatial and temporal information on semiconductor samples studied through PL emission.<br/>While one of the used SNSPDs had a classical single-mode fiber coupling to guide the light onto the sensor, the other detector used an internal multi-mode fiber instead². We detected a significant increase in photoluminescence sensitivity of both designs compared to a standard NIR-PMT (H10330-45, Hamamatsu) as well as a several times higher sensitivity of the multi-mode fiber coupled nanowire compared to the singe-mode fiber one, in spite of comparable photon quantum efficiencies in this wavelength range for the sensor only. The increased sensitivity combined with the lower dark count rate resulted in an increase of the signal-to-noise ratio by more than 3 orders of magnitude compared to the NIR-PMT. Moreover, the very high sensitivity of the SNSPD as well as high temporal resolution (instrument response function of the overall system was below 100 ps) reveals clearly the differences in the PL decay profiles of the defect and unquenched areas of the weakly luminescent thin-film CIGS sample³even at low illumination levels, which is extemely difficult to resolve when using the NIR-PMT detector.<br/><br/>¹https://doi.org/10.1063/5.0045990<br/>²https://doi.org/10.1364/AO.58.009803<br/>³https://doi.org/10.1021/acsaem.9b01370