Blog

Title | Investigation of interfaces in perovskite-based optoelectronic devices

Date | November 6, 2023

Abstract | Metal halide perovskites (MHP) form a novel class of materials that recently found application in solar cells (SC). Being in an early stage of study and understanding, MHP-based SCs still suffer from low operational lifetime, exacerbated by using chemically reactive selective carrier transport layers (SCTLs), such as TiO₂ and NiO. During this PhD, the optoelectronic properties of MHPs deposited on SCTLs have been studied by combining advanced characterization techniques, such as X-ray photoemission spectroscopy (XPS), and surface photovoltage (SPV), with 2D drift-diffusion simulations. Our analysis of XPS data acquired in near thermal equilibrium conditions suggests that the MHP layer employed in SCs is very lowly doped (N_dop<1011 cm^-3), indicating high electronic quality. Moreover, charge photogeneration and redistribution in MHP/TiO₂ stacks, probed by SPV, indicate the presence of deep traps inside TiO₂ that store a large number of electrons under illumination. Furthermore, a signature of shallow traps in the MHP layer was found. These traps temporarily capture photogenerated electrons, slowing down charge extraction, and causing carrier loss.

Title | Driving Lattice Dynamics in 2D Perovskites via THz Kerr Effect

Date | October 2, 2023

Abstract | Charge carrier-phonon interaction governs lead halide perovskite’s optoelectronic properties. Understanding the complex vibrational dynamics and microscopic coupling mechanisms is crucial for material design. The THz-induced Kerr effect spectroscopy (TKE) technique allows us to drive the crystal lattice using sub-picosecond THz electric field pulses and observe the ensuing dynamics by probing the transient changes of birefringence. Recently, in 3D MAPbBr₃ at low temperature, we demonstrated coherent THz control of the octahedral twist modes, which potentially play an important role in dynamic charge carrier screening [1]. In this talk, I will introduce the technique and discuss our most recent studies on layered Ruddlesden-Popper HOIPs, which combine the intriguing properties originating from the soft, polar, and anharmonic lattice with confinement effects due to lowered dimensionality.  In quasi-2D compounds, already at room temperature we observe long-lived coherent phonon oscillations in the 0.5–3 THz range [2].  We analyze the mode symmetry and possible driving mechanisms and compare samples of different dimensionality to infer the microscopic origin of the vibrations.

References

[1]    M. Frenzel, M. Cherasse et al., Sci. Adv.9, eadg3856(2023) DOI:10.1126/sciadv.adg3856

[2]    Z. Zhang et al., Sci. Adv.9, eadg4417(2023) DOI:10.1126/sciadv.adg4417