Student Seminar
Name: Ms. Rittika Dey
Title: Energy Funnelling and Charge Extraction in Layered 2D Perovskites: Insights from TA and 2D Action Spectroscopies
Date & Time: Thursday, 16th October 2025 at 4.00 p.m.
Venue: Rajarshi Bhattacharyya Memorial Lecture Hall, Chemical Sciences Building
Abstract:
In this seminar, I will discuss the application of time-resolved optical spectroscopic techniques to elucidate the photoinduced transport and relaxation in layered 2D lead–halide perovskites, where films comprise of a distribution of quantum-well thicknesses that establish bandgap gradients across the depth of the stack. Using steady-state optical femtosecond transient absorption (TA), and two-dimensional TA (2D-TA), confirm rapid excitonic funnelling from thinner to thicker wells (decreasing bandgap with increasing n), with sequential transfer predominantly between wells whose indices differ by one. Global analysis of TA yields inter-well rate constants on the 100 picosecond scale and assigns the principal early-time features to single-exciton manifolds of the constituent wells.1
To isolate device-relevant channels, complementary action spectroscopies are implemented on working photovoltaic stacks: nonlinear photocurrent (NLPC) and nonlinear fluorescence (NLFL). NLPC weighs only those pathways that culminate in charge separation and collection. Comparing 2D NLPC and 2D NLFL over 1–2000 picosecond cleanly distinguishes productive electron transfer from lossy radiative decay: sub-nanosecond dynamics are exciton/energy-transfer dominated, while electron transfer becomes prominent at later times, producing delayed growth of cross-peaks in NLPC for small →large well excitation.2,3
NLPC reveal that continuum states contribute more effectively to photocurrent than sharp excitonic resonances; thus, energy-funnelling cascades seen by optical probes need not yield superior device current. Photocurrent efficiency increases with well thickness (larger n), and exciton dissociation competes successfully only at longer delays(> 1 nanosecond)—explaining the weak excitonic imprint in action spectra despite their prominence in linear/transient absorption spectra.
References:
(1) Williams, O. F.; Guo, Z.; Hu, J.; Yan, L.; You, W.; Moran, A. M. Energy Transfer Mechanisms in Layered 2D Perovskites. J. Chem. Phys. 2018, 148 (13). https://doi.org/10.1063/1.5009663.
(2) Zhou, N.; Ouyang, Z.; Yan, L.; Mcnamee, M. G.; You, W.; Moran, A. M. Elucidation of Quantum-Well-Specific Carrier Mobilities in Layered Perovskites. J. Phys. Chem. Lett. 2021, 12 (4), 1116–1123. https://doi.org/10.1021/acs.jpclett.0c03596.
(3) Zhou, N.; Hu, J.; Ouyang, Z.; Williams, O. F.; Yan, L.; You, W.; Moran, A. M. Nonlinear Photocurrent Spectroscopy of Layered 2D Perovskite Quantum Wells. J. Phys. Chem. Lett. 2019, 10 (23), 7362–7367. https://doi.org/10.1021/acs.jpclett.9b02959.