Directional Negative Thermal Expansion and Large Poisson Ratio in CH₃NH₃PbI₃ Perovskite Revealed by Strong Coherent Shear Phonon Generation

Despite the enormous amount of attention CH₃NH₃PbI₃ has received, we are still lacking an in-depth understanding of its basic properties. In particular, the directional mechanical and structural characteristics of this material have remained elusive. Here, we investigate these properties by monitoring the propagation of longitudinal and shear phonons following the absorption of a femtosecond pulse along various crystalline directions of a CH₃NH₃PbI₃ single crystal. We first extract the sound velocities of longitudinal and transverse phonons along these directions of the crystal. Our study then reveals the negative directional thermal expansion of CH₃NH₃PbI₃, which is responsible for strong coherent shear phonon generation. Finally, from these observations, we perform elastic characterization of this material, revealing a large directional Poisson’s ratio, which reaches 0.7 and that we associate with the weak mechanical stability of this material. Our results also provide guidelines to fabricate a transducer of high-frequency transverse phonons

Ultra Long-Lived Radiative Trap States in CdSe Quantum Dots

Surface states and traps play an important role in the photophysics of colloidal quantum dots. These states typically lead to large red-shifted photoluminescence. We have used steady-state and time-resolved spectroscopic techniques to investigate the nature of the traps and their lifetimes in colloidal CdSe quantum dots. We conclude that at least two different types of traps contribute to the photoluminescence. The trapping is more pronounced at higher excitation energies compared to the band edge excitation. Hole trapping is dominant in CdSe quantum dots. The time-resolved photoluminescence and pump–probe measurements show that the trapped holes live for longer than tens of microseconds at room temperature

Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements

Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump–probe transient absorption and charge extraction of photogenerated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 × 10^–12 cm^–3 s^–1. This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed

Carrier Recombination Processes in Gallium Indium Phosphide Nanowires

Understanding of recombination and photoconductivity dynamics of photogenerated charge carriers in Ga_<i>x</i>In_1–xP NWs is essential for their optoelectronic applications. In this letter, we have studied a series of Ga_<i>x</i>In_1–xP NWs with varied Ga composition. Time-resolved photoinduced luminescence, femtosecond transient absorption, and time-resolved THz transmission measurements were performed to assess radiative and nonradiative recombination and photoconductivity dynamics of photogenerated charges in the NWs. We conclude that radiative recombination dynamics is limited by hole trapping, whereas electrons are highly mobile until they recombine nonradiatively. We also resolve gradual decrease of mobility of photogenerated electrons assigned to electron trapping and detrapping in a distribution of trap states. We identify that the nonradiative recombination of charges is much slower than the decay of the photoluminescence signal. Further, we conclude that trapping of both electrons and holes as well as nonradiative recombination become faster with increasing Ga composition in Ga_<i>x</i>In_1–xP NWs. We have estimated early time electron mobility in Ga_<i>x</i>In_1–xP NWs and found it to be strongly dependent on Ga composition due to the contribution of electrons in the X-valley

Ultrafast Charge Transfer from CdSe Quantum Dots to p‑Type NiO: Hole Injection vs Hole Trapping

Semiconductor quantum dot (QD) to metal oxide electron injection dynamics is well documented in the scientific literature. In contrast to that, not much is known so far about hole injection time scales in such systems. The current study fills this gap. We investigate photocathodes consisting of CdSe QDs and p-type NiO to study hole injection dynamics from the valence band of the QDs to NiO. The combination of two complementary techniques, ultrafast time-resolved absorption and fluorescence spectroscopies, enabled us to distinguish between hole trapping and injection. A kinetic component on the time scale of a few hundreds of picoseconds was identified as hole injection. By changing the size of the QDs, the driving force of the hole injection was tuned and we demonstrated that the hole injection rates are well described by the Marcus theory of charge transfer. In order to enhance the overall hole injection efficiency, we have passivated the CdSe QDs by a gradient ZnS shell. The core–shell QDs show significantly slower hole injection; still, since trapping was almost eliminated, the overall hole injection efficiency was greatly enhanced

Multifaceted Deactivation Dynamics of Fe(II) <i>N</i>‑Heterocyclic Carbene Photosensitizers

Excited state dynamics of three iron(II) carbene complexes that serve as prototype Earth-abundant photosensitizers were investigated by ultrafast optical spectroscopy. Significant differences in the dynamics between the investigated complexes down to femtosecond time scales are used to characterize fundamental differences in the depopulation of triplet metal-to-ligand charge-transfer (3MLCT) excited states in the presence of energetically accessible triplet metal-centered (3MC) states. Novel insights into the full deactivation cascades of the investigated complexes include evidence of the need to revise the deactivation model for a prominent iron carbene prototype complex, a refined understanding of complex 3MC dynamics, and a quantitative discrimination between activated and barrierless deactivation steps along the 3MLCT → 3MC → 1GS path. Overall, the study provides an improved understanding of photophysical limitations and opportunities for the use of iron(II)-based photosensitizers in photochemical applications

Multiexciton Absorption Cross Sections of CdSe Quantum Dots Determined by Ultrafast Spectroscopy

Multiexciton absorption cross sections are important for analysis of a number of experiments, including multiple exciton generation and stimulated emisson. We present a rigorous method to determine these cross sections using transient absorption (TA) measurements. We apply the method to CdSe quantum dots (QDs) and core–shell (CdSe)­ZnS QDs. The method involves measuring TA dynamics for various excitation intensities over a broad time range and analyzing the experiments in terms of a kinetic multiexciton model taking into account all contributions to the signal. In this way, we were able to quantify exciton and multiexciton absorption cross sections at different spectral positions. The absorption cross sections decrease with increasing number of excitations, qualitatively in agreement with the state-filling effective mass model but showing a slower decrease. The cross sections for single-exciton to biexciton absorption range between 57 and 99% of the ground to single-exciton cross section

Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals Single-Trap Control of Luminescence

Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH₃NH₃PbI₃) nanocrystals of size ∼200 × 30 × 30 nm³. The photoluminescence blinking amplitude caused by a single quencher was a hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to nonradiative recombination. These trapping sites can be activated and deactivated by light

GaAsP Nanowires Grown by Aerotaxy

We have grown GaAsP nanowires with high optical and structural quality by Aerotaxy, a new continuous gas phase mass production process to grow III–V semiconductor based nanowires. By varying the PH₃/AsH₃ ratio and growth temperature, size selected GaAs_1–<i>x</i>P_<i>x</i> nanowires (80 nm diameter) with pure zinc-blende structure and with direct band gap energies ranging from 1.42 to 1.90 eV (at 300 K), (i.e., 0 ≤ <i>x</i> ≤ 0.43) were grown, which is the energy range needed for creating tandem III–V solar cells on silicon. The phosphorus content in the NWs is shown to be controlled by both growth temperature and input gas phase ratio. The distribution of P in the wires is uniform over the length of the wires and among the wires. This proves the feasibility of growing GaAsP nanowires by Aerotaxy and results indicate that it is a generic process that can be applied to the growth of other III–V semiconductor based ternary nanowires

Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite

Solar cells based on organometal halide perovskites have seen rapidly increasing efficiencies, now exceeding 15%. Despite this progress, there is still limited knowledge on the fundamental photophysics. Here we use microwave photoconductance and photoluminescence measurements to investigate the temperature dependence of the carrier generation, mobility, and recombination in (CH₃NH₃)­PbI₃. At temperatures maintaining the tetragonal crystal phase of the perovskite, we find an exciton binding energy of about 32 meV, leading to a temperature-dependent yield of highly mobile (6.2 cm²/(V s) at 300 K) charge carriers. At higher laser intensities, second-order recombination with a rate constant of γ = 13 × 10^–10 cm³ s^–1 becomes apparent. Reducing the temperature results in increasing charge carrier mobilities following a T^–1.6 dependence, which we attribute to a reduction in phonon scattering (Σμ = 16 cm²/(V s) at 165 K). Despite the fact that Σμ increases, γ diminishes with a factor six, implying that charge recombination in (CH₃NH₃)­PbI₃ is temperature activated. The results underline the importance of the perovskite crystal structure, the exciton binding energy, and the activation energy for recombination as key factors in optimizing new perovskite materials