Determination of transport properties in optoelectronic devices by time-resolved fluorescence imaging

This talk aims at introducing time-resolved fluorescence imaging (TR-FLIM) as an optical characterization method for optoelectronic devices. It allows to obtain time-resolved photoluminescence maps with a temporal resolution of 500 ps and a micrometric spatial resolution. A first experiment under wide-field illumination on a GaAs-based solar cell is presented as a proof of concept. Thanks to a model including diffusion and recombination of minority charge carriers, we could fit the experimental photoluminescence (PL) transients and decorrelate key optoelectronic properties for the considered device. For various fluence levels, we could determine a constant bulk lifetime τ_n = 75 ns, a constant effective diffusion length L_eff = 190 μm, and an injection-dependent contact recombination velocity S_n, which is explained by the saturation of interface states. The wide-field illumination notably avoids lateral diffusion artefacts leading to a significant underestimation of τ_n. TR-FLIM also has a noteworthy interest for optoelectronic materials showing heterogeneous properties, as organic-inorganic halide perovskite. With the same set-up, we could investigate various chemical compositions for this semi-conductor, and highlight the need for another self-consistent model linking TRPL transients with physical properties, as no clear definition of a lifetime appears. However, the crucial role of the perovskite/TiO₂ can be underlined, in particular for the sample containing Cesium, as well as the probable role of charge carrier in-depth diffusion following a pulsed excitation. As a conclusion, TR-FLIM appears as a versatile characterization method and we open the gate to further studies of other optoelectronic devices with this set-up.