The ongoing development of new-generation optoelectronic structures requires a continuous improvement of experimental methods used to characterise the physical properties of constituent materials in multi-layer devices. The traditional approach to obtaining absorption-related features in the solid state, intended to reveal the band structure in semiconductors, often suffers from a limited sensitivity imposed by a strong background signal and light scattering. Subtle changes in the sub-band-gap spectral region of semiconductors contain crucial information about defect activity and temperature-activated effects in the tails of the density of states. Extracting parameters such as the Urbach energy, representing the energetic disorder in thin-film materials, calls for alternative tools overcoming the optical detection constraints.

This talk will showcase the recent advancements in photothermal spectroscopy applied to investigate optical absorbance in a wide range of organic and hybrid semiconductors. Detection schemes based on converting the modulated light beam energy into heat oscillations overcome the sensitivity limits, enabling accurate probing of optical transitions characterised by low oscillator strength, preserving high dynamic range in layers as thin as 1 – 10 nm. Specific benefits of two selected experimental methods, photoacoustic spectroscopy (PAS) and photothermal deflection spectroscopy (PDS), will be presented in relation to the physical parameters of thin films crucial for the fabrication of device stacks [1,2]. The absorption edge sharpness, quantified by the Urbach energy, is compared against external quantum efficiency spectra on fully assembled solar cell and light-emitting diode structures utilising lead-halide perovskites and their low-dimensional counterparts as the active materials. Photothermal spectroscopy, due to its insensitivity to light scattering, also overcomes limitations commonly found in the characterisation of solution-processed materials at low concentrations. The obtained results provide a deeper insight into the fundamental properties of emerging material platforms, serving as a predictor of energy conversion efficiency in thin-film optoelectronic devices [3]. The talk concludes by presenting prospects for non-contact studies of thermal transport in solid state by ultrafast optical thermometry.