Uncovering Photoinduced Charge Transfer Dynamics in Thin Films and Devices of the Extremely-Thin Absorbing Material Sb₂S₃

Widespread energy conversion from sunlight requires solar cells fabricated using stable, sustainable, non-toxic semiconductors based on earth-abundant elements. The continually shrinking feature sizes in nanoelectronics and the drive for inexpensive solar energy conversion systems make fundamental understanding of charge transfer processes at interfaces and within actual devices essential to next generation photo and photoelectrochemical systems. Such devices are comprised of stacks of light-absorbing layers and hole and electron transporting materials forming thin film p-i-n structures. Necessarily, they must operate under both electrochemical applied potentials (in situ) and constant steady-state irradiation (operando) conditions. However, when measuring the photophysics and photo-induced chemical processes of these materials, experiments have thus far primarily been carried under <i>ex situ</i> conditions – with either the material alone or in partly formed semi-conductor stacks. More real-life conditions are needed to fully understand how charge carriers are generated, how they flow through materials and how material interfaces mediate charge transfer across multiple layers in these photophysical mechanisms. I will discuss our initial efforts at designing and developing experimental probes and data analysis tools capable of characterizing photochemical reactions under more realistic operating conditions. Using transient absorption spectroscopy (TAS), we <i>directly </i>observe carrier diffusion, electron transfer, hole transfer and charge recombination through uniform ultra-thin (&lt; 3 nm) layers of insulating or transport materials deposited by atomic layer deposition (ALD) that are coupled to photoactive materials. Our work focuses on stibnite (Sb₂S₃) as the photo-active layer, which is of particular interest due to the suitable band gap of 1.7 eV and high absorption coefficient (1.8 × 10⁵ cm^–1 at 450 nm). We quantify the dynamics that can be observed in thin transparent films through the use of <i>in situ</i> and <i>operando </i>TAS. Use of <i>in situ</i> and <i>operando</i> TAS affords the opportunity to study these materials in operating conditions and gain a quantitative understanding of the electron transfer dynamics at play in thin films.