The excellent photovoltaic performance of halide perovskites goes along with a high photoluminescence (PL) yield that makes them suitable for a wide range of photonic devices and various optoelectronic applications, such as photodetectors, lasers and light emitting diodes.

One of the main advantages of hybrid halide perovskite is the tunability of the crystal size and the ensuing band gap. However, due to the complexity of the system, often traditional macroscopic characterisation tools are not able to unveil the physical processes underlying the working principles of the solar cells. New approaches in the characterisation of the materials are thus required to overcome these hurdles, leading to a deep understanding of the carrier transport and recombination processes.

Here, we present an innovative characterisation method, based on the use of a combination of multidimensional photoluminescence analysis, to probe transport properties and image material inhomogeneities. In particular, the use of a fast camera to record spatial information enables the study of the temporal diffusion of carriers, providing access to transport properties such as mobility, lifetimes and recombination rate [1]. One of the main advantages of our method over traditional cartographies techniques such as confocal systems, lies in the elimination of artefacts due to the combination of diffusion and recombination phenomena in the luminescence map intensities.

In particular, we developed two experimental set-ups: a hyperspectral imaging system (HI) yielding spectrally-resolved PL images [2] and a time-resolved fluorescence imaging (TR-FLIM) set-up [3]. We employed these optical techniques to study lateral transport of charge carriers in last generation multi-cation hybrid perovskites. By monitoring the PL signal following a pulsed local illumination, we highlight the presence of three contributions: pure electronic propagation, photon recycling and photon propagation. The use of time-resolved PL profiles allowed us first to decorrelate electronic and photonic regimes and then to quantify their impact on charge carrier transport. In addition, a photon propagation regime was observed, confirming the wave-guided propagation of PL photons inside the halide perovskite. Finally, we fit the experimental data by rigorously solving the continuity equation for electron and holes to determine values for bulk-related parameters, namely the charge carrier diffusion coefficient (Dn =0.022 ± 0.006 cm²/s) and lifetime ( τn =110 ± 30 ns), the external radiative coefficient (Reh* = 5.5*10-11 ± 2*10-11 cm³/s), as well as for device-related light management parameters regarding photon recycling and light in-coupling. Thanks to the proportionality between local emission and recycling, we also derived the internal radiative coefficient R_eh= 1.5*10-10 ± 5*10-11 cm³/s. An equivalent electronic diffusion length can be determined around 450 nm.

While the photoluminescence signal represents a direct signature of radiative and non-radiative recombination, measuring the local variations of the luminescence signal under electric bias allows a direct access to charge carrier collection and transport properties for each kind of charge carrier. We thus apply an electrical voltage on a bare perovskite thin film absorber while using the same multi-dimensional luminescence imaging methods. Thanks to this novel experimental approach, we have been able to directly image and measure the drift mechanisms occurring within the material, providing new insights on these complex phenomena.