We investigate the interaction of light with matter, using light absorption or photoluminescence (PL) techniques to access key information about optoelectronic properties in materials and devices. We employ absorption and photo-thermal deflection spectroscopy (PDS) measurements to assess bandgap and semiconductor quality. We perform a number of techniques including time-resolved photoluminescence and transient absorption (TAS) across a range of time scales relevant to carrier recombination and device operation. We also employ state of the art time-resolved microscopy methods, allowing high resolution imaging in space, time or energy. For example, spatially resolved electroluminescence or photoluminescence intensity maps (Fig. 1) enable us to assess film homogeneity and correlate luminescence intensity with carrier lifetime, identifying poorly luminescent regions corresponding to non-radiative power losses in devices.[1] We are also developing methods to perform these measurements on full operating devices including bias and current collection, as well as new techniques to enable 3D tomographic images to probe buried recombination pathways by employing two-photon excitation.[2] Our setups are adapted to access temperature ranges of 4 K - 500 K and different environmental conditions.
a)Schematic of the confocal photoluminescence microscope. b) Helium cold finger cryostat to cool down the sample down to 3.8 K for temperature dependent measurements. Examples of photoluminescence maps, displaying how the intensity (c), the lifetime (d) and the median wavelength (e) change across a perovskite grain structure. f) Transient absorption spectroscopy: Pump-probe measurement of a lead halide perovskite film shows the change in light transmission due to photobleaching at the band edge.
[1] deQuilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; *Ginger, D. S., Impact of Microstructure on Local Carrier Lifetime in Perovskite Solar Cells. Science, 2015, 348, 683-686
[2] Stavrakas, C.; Zhumekenov, A. A.; Brenes, R.; Abdi-Jalebi, M.; Bulovic, V.; Bakr, O. M.; Barnard, E. S.; *Stranks, S. D. Probing Buried Recombination Pathways in Perovskite Structures using 3D Photoluminescence Tomography, Energy Environ. Sci., 2018, 11, 2846-2852
[3] Stavrakas, G.; Delport, G.; Zhumekenov, A. A.; Anaya, M.; Chahbazian, R.; Bakr, O. M.; Barnard, E. S.; *Stranks, S. D. Visualizing Buried Local Carrier Diffusion in Halide Perovskite Crystals via Two-Photon Microscopy ACS Energy Lett. 2019, 5, 117-123