Hot carriers perspective on the nature of traps in perovskites

Abstract

Amongst the many spectacular properties of hybrid lead halide perovskites, their defect tolerance is regarded as the key enabler for a spectrum of high-performance optoelectronic devices that propel perovskites to prominence. However, the plateauing efficiency enhancement of perovskite devices calls into question the extent of this defect tolerance in perovskite systems; an opportunity for perovskite nanocrystals to fill. Through optical spectroscopy and phenomenological modeling based on the Marcus theory of charge transfer, we uncover the detrimental effect of hot carriers trapping in methylammonium lead iodide and bromide nanocrystals. Higher excess energies induce faster carrier trapping rates, ascribed to interactions with shallow traps and ligands, turning these into potent defects. Passivating these traps with the introduction of phosphine oxide ligands can help mitigate hot carrier trapping. Importantly, our findings extend beyond photovoltaics and are relevant for low threshold lasers, light-emitting devices and multi-exciton generation devices.

Guidelines for the characterization of metal halide nanocrystals

“…The study of the physical properties of MH NCs is a vivid field of research relying on various continuous-wave and time-resolved optical spectroscopies performed under controlled temperature [55][56][57][58][59][60] and environmental conditions [61][62][63] at both the ensemble and the single-particle level (Figure 4) [64][65][66][67]. This enables us to build a comprehensive photophysical picture including the bandgap energy [20,27], the emission spectrum and its excitonic versus defect/dopantbased contributions [6,[68][69][70][71][72], the exciton [73] and biexciton binding energies [74], and the rates of radiative and nonradiative processes in single NC and ensembles [75][76][77] and in hybrid architectures [78][79][80][81][82], as well as the extent of blinking processes and single-photon emission properties [64][65][66][67] (Figure 4).…”

Evolving Stark Effect During Growth of Perovskite Nanocrystals Measured Using Transient Absorption

“…Select SSTA spectra for NCs at various stages of growth are shown in Figure 4. For each sample, the transient spectrum redshifts approximately 10 nm during the first 500 fs as a result of carrier cooling (Righetto et al, 2020). The spectra are quite stable for the remainder of the measured 60 ps time window.…”

Non-toxic near-infrared light-emitting diodes

“…As demonstrated by Gamelin et al, LHP hosts doped with ytterbium ions can reach staggering 170% PLQYs in the NIR (Milstein et al, 2018). The underlying quantum cutting has been applied to lead-containing NIR LEDs by Tom Miyasaka, reaching an outstanding 5.9% EQE at 984 nm (Ishii and Miyasaka, 2020;Righetto et al, 2020a). Future work will certainly focus on reproducing similar results with lead-free perovskites.…”

“…Among other mitigation strategies, coupling LHPs with polymers has also been widely explored to diminish their lead weight content. However, in these devices, the toxicity of lead is exacerbated by its extreme bioavailability Privitera et al, 2017;Zhao et al, 2018a;Righetto et al, 2020a). As recently demonstrated by Abate's group for LHPs solar cells, setting a threshold for toxic metal content is dangerous because used lead halides salts present a ten-fold higher bioavailability, with respect to other common lead contaminants .…”

“…Such excellent performance parameters mainly originate from a highly dimensional (i.e., with excellent connectivity of frontier atomic orbitals in all three dimensions) (Xiao et al, 2017) and defect-tolerant electronic structure (Figure 7A). Furthermore, their easy solution processing allows inducing partial quantum confinement by means of additives and ligands, which increases exciton binding energies and hence boosts radiative rates (Zhao et al, 2018b;Sum et al, 2020;Park et al, 2019;Righetto et al, 2020a).…”