Ultrafast energy and charge transfer in optoelectronic materials
Metal halide perovskites and quantum dots are highly promising solution-processed semiconductors for optoelectronic applications. Their solution processability enables cost-effective and scalable manufacturing, as they can be deposited onto various substrates using simple techniques. These materials possess exceptional optoelectronic properties: metal halide perovskites exhibit high light absorption coefficients across a broad spectrum, while quantum dots offer tunable emission wavelengths. Their impressive device performance is demonstrated in solar cells, LEDs, and other optoelectronic devices. Metal halide perovskite solar cells have achieved competitive power conversion efficiencies, while quantum dot LEDs exhibit high color purity and efficiency. The versatility of these materials allows for precise tuning of their properties through compositional and structural design.
The ultrafast photophysics and dimensionality of these materials underpins their device performance. Confining these materials to nanoscale dimensions - 3D-confinement in the case of quantum dots, and 2D-confinement in the case of quantum wells - results in the formation of strongly bound excitons. For 2D perovskite quantum wells, the layered structure of these materials provides an avenue for exciton formation and migration, which affects their light absorption and emission properties. Understanding the exciton dynamics and the role of defects and surface interactions is crucial for optimizing the performance of 2D perovskite devices. This area of my research makes use of ultrafast nonlinear optical spectroscopy (pump-probe and two-dimensional electronic spectroscopy). I use a suite of other characterization techniques (X-ray diffraction, photoelectron spectroscopy, photoluminescence spectroscopy) to understand the interplay between a material's properties and its ultrafast dynamics; and use these photophysical insights to inform on materials design for optoelectronic devices.
Resolving energy and transfer on femtosecond timescales in solution-processed semiconductors. My doctoral studies at Princeton University and the University of Toronto made use of ultrafast transient absorption spectroscopies, photoelectron spectroscopy, and synchrotron X-ray diffraction experiments to gain insight into the carrier dynamics of metal halide 2D perovskite quantum wells. I probed the timescales over which excitons and charge carriers migrated within and between these structures, and how these carrier transfer mechanisms were influenced by surface ligands and dimensionality. Using a combination of 2DES and broadband pump-probe spectroscopy, I demonstrated that that interwell exciton transfer occurs on timescales of 100s of femtoseconds, whereas charge transfer occurred on slower timescales of 100s of picoseconds (Fig. 1). These observations further led to the discovery that 2D perovskites exhibit an intrinsic type-I band alignment, but their valence band maxima can be shifted into a type-II alignment, which facilitates interwell hole transfer. This result reconciled conflicting hypotheses throughout the field, where both types of band alignment had been separately observed.
Fig. 1. Band diagram of mixed 2D/3D metal halide perovskite quantum wells. Excitons (bound electron-hole pairs) undergo energy transfer in 100s of femtoseconds, whereas carriers hop between different layers in 100s of picoseconds. Refs: A. H. Proppe et al., SPIE, 2019 ; A. H. Proppe et al., JPCL, 2019; Bermudeuz,† A. H. Proppe† et al., JACS, 2019
Fig. 2. Transient absorption spectra of a hybrid organic-inorganic perovskite quantum well thin film, using a dication NDI ligand. The rapid decay of the perovskite bandedge bleaching signal (red traces) are due to ~700 fs electron transfer to the NDI ligand. Ref: A. H. Proppe et al., J. Phys. Chem. C., 2020
Ultrafast charge transfer from organic ligands to inorganic perovskites. 2D perovskite quantum wells (PQWs) are highly versatile optoelectronic materials owing to their large oscillator strengths, bandgaps tunable via the quantum size effect, and higher stability relative to 3D counterparts. The majority of examples of PQWs make use of small aryl- and alkylammonium A’-site cations to tune dimensionality and stability, with fewer examples of larger molecules that exhibit frontier orbital energies near those of the inorganic component of the perovskite. In this work, I studied two new lead-iodide-based systems that incorporated a naphthalene diimide (NDI) dye molecule as a dication ligand. Given the band alignment between the perovskite quantum well bands and the HOMO/LUMO of the ligand, charges should be able to migrate between the inorganic and organic layers.
Using ultrafast transient absorption spectroscopy, we indeed observed a rapid (~700 fs) decay of the photoexcited perovskite carrier population in the presence of the NDI ligand and fully quenched photoluminescence: this is consistent with ultrafast perovskite-to-NDI electron transfer. We also found that films synthesized with NDI, PbI2 and methylammonium inhibited the growth of PQWs, and instead result in a mixture of weakly confined perovskite and 1D perovskitoid structures. When both formamidinium and methylammonium are used as A-site cations, we observe spectroscopic signatures of quantum-confined 2D structures similar to PQWs with a polydisperse well width distribution. These charge-accpeting ligand molecules offer a route to structures with charges separately localized on inorganic and organic components.