Author: mkepenek

Title | Impact of self-assembled monolayers as hole transport layers in a mixed MA free Pb/Sn perovskite solar cell

Date | January 8, 2024 @ 09:30

Abstract | Mixed lead (Pb) – tin (Sn) perovskite (PK) solar cells increase rapidly in efficiency lately notably because of improvement of bulk defects and interface passivation. Those devices commonly use poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) as hole transport layer due to its ease of process and good energy level alignment but it also impedes the thermal stability of the device [1]. Here, we use self-assembled monolayer (SAM), [2-(9H-Carbazol-9-yl)ethyl]phosphonic acid (2-PACz) and methylphosphonic acid (MPA) to replace PEDOT:PSS layer. Devices with SAM instead of PEDOT:PSS show poor performances which is opposite to Kapil et al. results obtained with mixed methylammonium (MA) based Pb-Sn perovskite [2]. By using photoluminescence (PL) and ultraviolet photoelectron spectroscopy (UPS), we show that this behaviour is due to a potential barrier created at the interface SAM/MA free PK. But, SAM layers also show less interface non radiative recombination which highlight our interest for those layers. Those results identify 2PACz and MPA effectiveness limits in mixed MA free Pb-Sn perovskite solar cells and give insight for other SAM tests.

[1] R. Prasanna et al., Nature Energy 4, 939 (2019), https://doi.org/10.1038/s41560-019-0471-6.

[2] G. Kapil et al., ACS Energy Lett. 7, 966 (2022), https://doi.org/10.1021/acsenergylett.1c02718

Title | Chiral Halide Perovskite Materials for Optoelectronic and Spintronic Devices

Date | December 4, 2023

Abstract | Chiral Halide Perovskites (HPs) bring new perspectives for chiroptical applications, such as circularly polarized luminescence (CPL), and spintronic applications. For example, the emergent field of chiro-spintronics proposes to use chiral molecules as a substitute for ferromagnetic materials thanks to the spin-specific interaction between electrons and chiral molecules, a phenomenon called CISS, “Chirality-Induced Spin Selectivity”. By combining the optoelectronic properties of HP networks with chirality transfer from inserted organic cations, I aim to reveal the potential of chiral HP materials for CPL and chiro-spintronics. After an introduction about the objectives of my CNRS project, I will present the main results obtained during the first year. Starting from small chiral alkyl cations, we obtained a large series of 2D and 1D lead-halide networks. The 2D HPs have been investigated for the CISS effect, which allowed us to highlight the impact of crystal symmetry in the spin polarization ability of this class of materials [1]. With the 1D lead-halide networks, we could reveal the intrinsic and extrinsic chiroptical responses by circular dichroism measurements on thin films (unpublished results).

[1] A. Abhervé, N. Mercier, A. Kumar, T. K. Das, J. Even, C. Katan, M. Kepenekian, Adv. Mater. 2023, in press. https://doi.org/10.1002/adma.202305784

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