The discovery of slow hot carrier cooling in hybrid organic–inorganic lead halide perovskites (HOIPs) has provided exciting prospects for efficient solar cells that can overcome the Shockley–Queisser limit. Questions still loom over how electron‐phonon interactions differ from traditional polar semiconductors. Herein, the electron‐phonon coupling (EPC) strength of common perovskite films (MAPbBr3, MAPbI3, CsPbI3, and FAPbBr3) is obtained using transient absorption spectroscopy by analyzing the hot carrier cooling thermodynamics via a simplified two‐temperature model. Density function theory calculations are numerically performed at relevant electron‐temperatures to confirm experiments. Further, the variation of carrier‐temperature over a large range of carrier‐densities in HOIPs is analyzed, and an “S‐shaped” dependence of the initial carrier‐temperature to carrier‐density is reported. The phenomenon is attributed to the dominance of the large polaron screening and the destabilization effect which causes an increasing‐decreasing fluctuation in temperature at low excitation powers; and a hot‐phonon bottleneck which effectively increases the carrier temperature at higher carrier‐densities. The turning point in the relationship is indicative of the critical Mott density related to the nonmetal‐metal transition. The EPC analysis provides a novel perspective to quantify the energy transfer in HOIPs, electron‐lattice subsystem, and the complicated screening‐bottleneck interplay is comprehensively described, resolving the existing experimental contradictions.
Harnessing
hot-charge relaxation in lead halide perovskites (LHPs)
is the key to developing next-generation high-performance concentrator
solar cells that break the Shockley–Queisser limit. Though
the physical origins of the slow hot-carrier cooling and their interplays
have been unveiled, consensus is still lacking concerning the mechanisms
of many-body interactions during hot-charge relaxation. Here, we propose
a unified theory to explain the spectral and temporal evolution of
the band edge in LHPs at the early time-scale following femtosecond
laser excitation. We demonstrate that at early times, the hot-biexciton
effect imposes a transient bandgap shrinkage decaying rapidly with
exciton dissociation. Subsequently, bandgap renormalization (BGR)
effect dominates the bandgap change, with a partial compensation by
the free-carrier Stark (FCS) effect. Additionally, we confirmed that
the shift in the photo-bleaching (PB) peak in the transient absorption
(TA) spectra is modulated by carrier temperature rather than the bandgap
change, which has negligible influence on the bleaching position,
contrary to previous studies. Importantly, this work demonstrates
the significant role played by the hot-biexciton interaction to the
exciton generation-dissociation and carrier relaxation dynamics in
perovskite solar cell materials at early times. Our insights resolve
the existing contradictions on the nature of early-time photo-induced
absorption and PB shift via reliable quantifications. By unraveling
the role of hot-charge cooling and the intricate many-body interactions
among the hot-biexciton interplay, BGR and FCS effects, our study
contributes to a deeper comprehension of the fundamental photo-physics
in LHPs.
We investigate the charge carrier dynamics in HgTe quantum dots emitting in the second near-infrared window (1000−2500 nm). To provide a link between fundamental physics and practical application, we made consistent studies of the charge carrier dynamics evolution for quantum dots in different states: colloidal solutions of quantum dots capped with a long-chain ligand; thin films made from them; and finally, exchanged to short-chain ligand films suitable for field effect transistor based devices. Ultrafast transient absorption spectroscopy reveals an ultralow Auger-related nonradiative relaxation threshold at 0.1 exciton per quantum dot, both in colloidal solutions and solid films, with a rate of 30 ns −1 . The exchange from long-to short-chain ligands causing closer packing of the HgTe quantum dots leads to a strong increase of the Auger recombination rate of up to 100 ns −1 . The competition between the Auger process and excitonic recombination significantly affects the performance of HgTe-based thin film photodetectors operating at room temperature, resulting in a 2 orders of magnitude drop in responsivity when the excitation flux was increased from 0.01 to 5 W•cm −2 .
The performance of the blue perovskite light-emitting
diodes (PeLEDs)
is limited by the low photoluminescence quantum yields (PLQYs) and
the unstable emission centers. In this work, we incorporate sodium
bromide and acesulfame potassium into a quasi-2D perovskite to control
the dimension distribution and promote the PLQYs. Benefiting from
the efficient energy cascade channel and passivation, the sky-blue
PeLED has an external quantum efficiency of 9.7% and no shift of the
electroluminescence center under operation voltages from 4 to 8 V.
Moreover, the half lifetime of the devices reaches 325 s, 3.3 times
that of control devices without additives. This work provides new
insights into enhancing the performance of blue PeLEDs.
In article number 2003071, Kam Sing Wong and co‐workers quantify electron‐phonon coupling strength using a simplified two‐temperature model in several hybrid lead halide perovskites. Also, the intricate competition of hot‐carrier cooling mechanisms between large polaron screening and hot‐phonon bottleneck effect are described by unveiling an S‐shaped temperature versus carrier density relationship. The findings are expected to help shed light on hot‐carrier harvesting in high‐performance photovoltaic applications.
Although the electroluminescent performances of perovskite light‐emitting diodes (PeLEDs) are continuously improved through defect management strategies, the complicated design of passivation ligands brings great challenges to the rational defect‐annihilation process. Herein, considering the bonding strength with uncoordinated Pb2+, the methoxy group with strong electron donating ability is introduced to commonly used phenethylammonium ligand as an efficient additive, namely 4‐methoxy‐phenethylammonium iodide (4‐MeO‐PEAI), to facilitate passivation process in perovskite light‐emitting materials. It is demonstrated that the 4‐MeO‐PEAI agent substantially increases the crystal orientation, enlarges the crystalline grain size, and mitigates the deep‐level trap centers through strong bonding between the methoxy group with unpaired Pb2+ ions. The external quantum efficiencyvalue of the PeLEDs with optimized passivation reaches a maximum of 21.6%, with an emission peak of 790 nm. In addition, a nearly threefold increase of the operational half‐lifetime T50 of the 4‐MeO‐PEAI‐mediated devices is observed as compared to the reference sample. Further theoretical calculation results suggest that the adhesion of the ligands on perovskite surface via vacancies leads to an increased dissociation barrier of perovskite; thus, ameliorating the degradation of the PeLEDs under electric field. The findings provide an effective design strategy of the passivation agents to produce high‐performance perovskite‐based optoelectronics devices.
The hot-charge relaxation mechanisms remain contentious in lead halide perovskites, though regarded as frontrunners for future photovoltaics. A model uncovering the spectral and temporal band-edge evolution provides novel insights into many-body interplay and photo-bleaching shift.
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