Ultrafast charge carrier dynamics in quantum confined 2D perovskite

Folpini, Giulia; Gatto, Lorenzo; Cortecchia, Daniele; Devetta, M.; Crippa, Gabriele; Vozzi, C.; Stagira, S.; Petrozza, Annamaria; Cinquanta, Eugenio

doi:10.1063/5.0008608

Cited by 18 publications

(19 citation statements)

References 49 publications

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“…The tandem spectroscopy results demonstrate therefore that excitons are always a minority species and are immersed in a plasma of unbound charge carriers. Clearly excitons are still formed, since PL is emitted from excitons, consistently with recent reports, [38][39][40][41][42] but they are not the majority species.…”

Section: Ultrafast Tandem Spectroscopysupporting

confidence: 88%

“…[11,17,[66][67][68] The most interesting comparison is perhaps with ultrafast measurements in the THz spectral region, where bound excitons and unbound carriers could be discriminated. THz measurements have provided, together with evidence for free carriers in 3D HPs and ultrafast exciton formation in 2D materials, some interesting signatures of significant presence of free carriers even in 2D perovskites and at low temperatures, [38][39][40][41][42]66,67] where only excitons would have been expected. The overall scenario from THz measurements appears therefore compatible with our interpretation of contemporaneous presence of bright excitons and a polaron plasma.…”

Section: (9 Of 13)mentioning

confidence: 99%

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Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

Simbula¹,

Pau²,

Wang³

et al. 2021

Advanced Optical Materials

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spectrum featuring a marked excitonic resonance, the majority optical excitation in prototypical HP materials for photovoltaics are not bound excitons, but unbound charge carriers. Therefore, electrons and holes excited by solar light can be directed to the electrodes at a negligible energy cost, without the need to split tightly bound excitons as in organics. Taking advantage of the flexibility of the materials class, layered 2D HPs are obtained by inserting bulky organic cations into the formulation, leading to materials inherently more stable than their 3D counterparts against degradation. [8][9][10] However, in 2D HPs the exciton binding energy can be as large as 400 meV, [8] so that it is commonly assumed that their excited states are mostly excitons.A second peculiar characteristic of the excited states in perovskites is the formation of large polarons, that is, charge carriers coupled to lattice deformations and delocalized over many crystal lattice sites. [11][12][13][14][15][16][17][18][19][20] Unlike small polarons in organics, localized in a single molecule, large polarons are compatible with band transport, but are also able to screen the excited states from scattering with defects and reduce non-radiative recombination through trap states, resulting in large mobilities and long lifetimes. Large polarons are also believed to reduce scattering with phonons and have been proposed as an explanation for hot carriers persisting for several nanoseconds at temperatures significantly higher than the lattice one. [12,16,17,[21][22][23][24][25][26] Large polarons may therefore be the enabling microscopic mechanism for efficient solar cells, including innovative architectures that exploit photoconversion with hot carriers. [27,28] Theoretical estimations forecast that the energy associated with polaron formation is comparable with the binding energy gained by forming an exciton, maybe even larger in some materials. [12,14,24,[29][30][31][32] When do polarons form and whether excitons or polarons are the lowest-energy optical excitations is still an open question. The issue is particularly relevant for layered 2D HPs, where polaronic effects have been demonstrated, although it is not clear if small or large polarons are formed. [33][34][35][36][37] In spite of the large exciton binding energy, unbound charge carriers have been reported, so that it is not clear yet how much energy needs to be spent in solar cells to split bound excitons.

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Section: Ultrafast Tandem Spectroscopysupporting

confidence: 88%

Section: (9 Of 13)mentioning

confidence: 99%

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

Simbula¹,

Pau²,

Wang³

et al. 2021

Advanced Optical Materials

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“…Similar timescales for exciton formation have been reported with more ambiguous THz probes by Folpini el al in (BA) 2 PbI 4 . 53 (a) (b)…”

Section: Discussionmentioning

confidence: 99%

The Effect of Tin Substitution on the Excitonic Properties of Two Dimensional Metal Halide Perovskites

Folpini¹,

Palummo²,

Cortecchia³

et al. 2021

Preprint

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<div>With growing interest in the lead-free derivatives of the metal-halide perovskites (MHP), it is imperative to fully understand the contribution of the metal cation to their desirable excitonic characteristics. Here, we explore this question by performing an in-depth spectroscopic and theoretical analysis of phenethylammonium tin iodide ((PEA)<sub>2</sub>SnI<sub>4</sub>), a prototypical tin based MHP, and rigorously compare it with its lead counterpart. We elaborate on the origin of multiple excitonic resonances uniquely observed in the linear absorption spectrum of (PEA)<sub>2</sub>SnI<sub>4</sub> at energies about 200-300meV above the primary exciton. By performing calculations based on density functional theory and many- body perturbation theory, we suggest that the excitonic series at these higher energies are composed of electronic transitions from a lower lying valence band. Importantly, the valence band splitting is driven by the octahedral conformations that follow subtle variations in the organic-inorganic interactions within the crystal lattice. We experimentally show that the presence of the higher energy excitonic resonance results in a relatively slow nanosecond component in the formation dynamics of the primary exciton, in addition to the ultrafast phonon-driven hot carrier thermalization. While the presence of such slow relaxation channel for the excitons might be beneficial to many optoelectronic applications, our work suggests its possible control via systematic design of the organic cation. Moreover, our observations indicate that spin-orbit coupling does not play a primary role in the intricate yet crucial changes in the excitonic characteristics imparted by the tin substitution.</div>

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“…Such studies would therefore be a good area for further development, for example, in investigating the potential for spacer cations to influence these LO phonon modes. [165] OPTP is a time-resolved technique where the terahertz pulse acts as the probe, and an optical pulse serves as the pump. As with transient absorption spectroscopy, changes in the absorption of THz radiation can be mapped over time (Figure 8a).…”

Section: Terahertz Spectroscopymentioning

confidence: 99%

Layered Perovskites in Solar Cells: Structure, Optoelectronic Properties, and Device Design

Sirbu

Balogun

Milot

et al. 2021

Advanced Energy Materials

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optoelectronic properties akin to industrial juggernauts gallium arsenide and silicon. [1,2] Yet, the materials are compatible with solution-processing techniques such as inkjet printing and spray coating. [3] This unprecedented combination has led to the development of solar cells which already show power conversion efficiencies of over 25%, similar to state-of-the-art poly-Si and other thin-film technologies, in just over 10 years of development. [4] Hybrid perovskites are particularly interesting for indoor applications, with impressive efficiencies of over 35% demonstrated under indoor illumination which may prove a viable option to power the ever expanding Internet of Things. [5] Rather than competing with the current technology giants, perovskites can work together with other solar cell technologies in a tandem configuration. Record efficiencies of over 29% for silicon-perovskite tandem solar cells have already been reported and further progress is expected in the near future. [6] Although perovskite solar cells (PSCs) are on the verge of mass production, two main challenges remain: stability [7] and toxicity. [8] Multiple reviews already address the toxicity concern, and it will not be discussed here. [9][10][11] Layered hybrid perovskites (LPKs) have emerged as an answer to battle stability concerns. Although known for decades, LPKs have only recently been incorporated in photovoltaic (PV) applications, resulting in modest device power conversion efficiencies due to poorer optoelectronic properties. [12][13][14] However, their key advantage is the stabilizing effects of the organic interlayers, which prevents perovskite degradation. [15,16] The organic spacer inhibits ion migration which prevents the formation of irreversible degradation products. [17] Alternatively, LPKs can be deposited over the state-of-the-art perovskite materials to combine the high performance of 3D perovskites with the stabilizing properties of LPKs. [15] Here, the limiting factor is inefficient charge transport across the organic interlayer, currently preventing the full exploitation of the layered structure in PSCs.This can be approached in two ways, either by keeping the LPK layer very thin, currently the most popular approach, or by increasing the electrical conductivity of the material. [18,19] The conductivity can be enhanced through increasing the number of inorganic layers n, but this comes at the price of reduced stability. [20][21][22] The better approach is to enhance the charge transport in LPKs through the molecular engineering of the organic Layered hybrid perovskites (LPKs) have emerged as a viable solution to address perovskite stability concerns and enable their implementation in wide-scale energy harvesting. Yet, although more stable, the performance of devices incorporating LPKs still lags behind that of state-of-the-art, multication perovskite materials. This is typically assigned to their poor charge transport, currently caused by the choice of cations used within the organic layer. On balance, a compromise bet...

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Ultrafast charge carrier dynamics in quantum confined 2D perovskite

Cited by 18 publications

References 49 publications

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

Polaron Plasma in Equilibrium with Bright Excitons in 2D and 3D Hybrid Perovskites

The Effect of Tin Substitution on the Excitonic Properties of Two Dimensional Metal Halide Perovskites

Layered Perovskites in Solar Cells: Structure, Optoelectronic Properties, and Device Design

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