Positronium scattering off a hydrogen target has been studied employing a three-state positronium model close-coupling approximation (CCA) with and without electron exchange. Elastic, excitation and quenching cross sections are reported at low and medium energies. The effect of electron exchange is found to be significant at low energies. The ratio of quenching to the total cross section (the conversion ratio) approaches the value of 0.25 with increase of energy, as expected.

Thermalization of photogenerated HCs occurs by dissipating their excess energy as heat energy through phonons which is the major intrinsic loss channel for solar cell devices. [3] Harnessing the excess energy of photoexcited HCs will allow us to achieve maximum power conversion efficiency (PCE) up to 67% for a single-junction solar cell under one sun illumination, [4] breaking the socalled Shockley-Queisser (SQ) limit of 34%. [5] Photoexcited HCs are used in photo-catalysis, photodetection, and highpower optoelectronic devices to improve efficiency. [6,7] However, rapid energy loss mechanisms of HCs in most of the conventional semiconductor nanomaterials (e.g., GaAs, PbSe, InN, and CdSe) through carrier-phonon scattering processes in sub-picosecond timescale severely restrict the utilization of non-thermalized excess energy of photo-excited HCs. [8][9][10][11] Therefore, it is essential to develop a solar absorber with retarded HC cooling rate. [12,13] Due to their extraordinary performance, metal halide perovskite nanocrystals (NCs) have recently emerged as front-runner materials in low-cost, high-performance solar cells. [14][15][16] A lot of interest has been shown to understand the HC cooling dynamics of lead halide perovskite (LHP)to find out potential applications. [17][18][19][20][21] Slow HC relaxation mechanisms in perovskite materials are reported due to the hot-phonon bottleneck effect, [22] Auger-heating effect, [23] band-filling effects, [24] dielectric screening, [25] and significant polaron screening effects. [26] However, in most cases, high pump fluence with photo-excited carrier densities of 10 18 -10 19 cm −3 was used, which is hard to accomplish in practical conditions. [22,27] It is worth noting that HC relaxation of LHP still occurs very rapidly (within hundreds of femtoseconds) under weak carrier densities (comparable to sun illumination level, ≈10 17 cm −3 ). [28,29] Thus, slowing down the HC relaxation rate of halide-based perovskite materials under low excitation power densities is a challenge for hotcarrier-based optoelectronic applications. Recently, delayed HC cooling rate has been reported in CsPbBr 3 based asymmetric multiple quantum wells (MQWs) due to sequential hot-electron transfer between CsPbBr 3 layers. [30] Tuning the HC cooling dynamics of metal halide perovskite is mainly limited to reduction of dimensionality, [31] changing cation/ halide ions, [32][33][34] and doping impurity ions. [35] Therefore, significant efforts are Metal halide perovskite nanocrystals have recently emerged as a front-runner material for high-performance solar cells. However, slowing down the hot carrier (HC) cooling of perovskites at carrier densities comparable to the sun-illumination level (≈10 17 cm −3 ) is still a thriving challenge. A new strategy is presented to retard the HC cooling via charge localization at the CsPbBr 3 / PbSe heterostructure interface. Ultrafast transient absorption study reveals two times slower HC relaxation time (from 770 fs to 1.4 ps) and much higher initial HC temper...

A deep understanding of hot carrier (HC) dynamics is important to improve the performance of optoelectronic devices by reducing the thermalization losses. Here, we investigate the hot hole cooling and transfer dynamics of CsPbBr 3 nanocrystals (NCs) using 5,10,15,20-tetra(4pyridyl) porphyrin (TpyP) molecules. Density functional theory (DFT) is used to elucidate the mechanism underlying charge extraction as well as the HC transfer process in the CsPbBr 3 −TpyP system. It is noted that the hot hole states are localized around the top surface of CsPbBr 3 , while the hot electron states are delocalized away from its top surface, indicating easy extraction of hot holes from the CsPbBr 3 by TpyP molecules, as compared to the hot electrons. The significant drop of initial hot carrier temperature from 1140 to 638 K at 400 nm excitation confirms the hot hole transfer from CsPbBr 3 NCs to TpyP molecules, which is dependent on the excitation energy, and the maximum transfer efficiency is found to be 42% (for 0.85 eV above band edge photoexcitation). In addition, the hot hole transfer rate is almost 11 times faster than the band edge hole transfer rate. Our findings are relevant for the development of next-generation perovskite-based optoelectronic devices.

Scattering of positronium atoms by hydrogen atoms has been investigated using the static exchange model and the first Born approximation (FBA) using only the exchange interaction. The FBA elastic total cross section at very low energy is very close to the estimated value of Massey and Mohr in the zero energy limit. We report total elastic, differential and quenching cross sections using the static exchange model for the energy range 0.068 - 100 eV. The present static exchange results are found to differ from the corresponding results of Fraser. The ratio of the quenching to total elastic cross section approaches the value 0.25 with increasing energy as predicted by Massey and Mohr.

Two-dimensional (2D) cesium lead halide perovskite nanoplatelets (NPLs) have received tremendous attention due to their unique properties for designing solar cell applications. Here, we investigated the crystal structure of 2D CsPbBr 3 nanoplatelets (NPLs) and their ultrafast carrier relaxation dynamics with varying the monolayer (ML) thickness using femtosecond transient absorption spectroscopy (fs-TAS). Rietveld analysis suggested that the basal planes of the NPLs are composed of ( 101) and ( 101) planes while the remaining four facets (thickness) are composed of ( 101), ( 101), (010), and (010) planes of the orthorhombic phase. The formation of the orthorhombic CsPbBr 3 NPLs by stacking the structural motifs of PbBr 6 octahedra in the crystallographic directions is evident from the atomic modeling. The change of monolayer thickness leads to a red-shift of the excitonic absorption band and PL band and enhancement of decay time. The cooling dynamics of the hot carrier to the band-edge state by phonon emission varies from 140 to 210 fs with thickness by modification of quantum confinement and dielectric screening. We observed both energy and charge transfer between 2D CsPbBr 3 NPLs with an organic chromophore, N,N′-bis(hexadecyl)perylene-3,4,9,10tetracarboxylic acid diimide (PDI), which is thickness-dependent. A deep understanding of the photoinduced carrier dynamics of the 2D CsPbBr 3 NPLs will pave the way to designing 2D perovskite-based photovoltaic devices.