Probing the momentum relaxation time of charge carriers in ultrathin semiconductor layers

We report on a terahertz time-domain technique for measuring the momentum relaxation time of charge carriers in ultrathin semiconductor layers. The phase sensitive modulation technique directly provides the relaxation time. Time-resolved THz experiments were performed on n-doped GaAs and show precise agreement with data obtained by electrical characterization. The technique is well suited for studying novel materials where parameters such as the charge carriers' effective mass or the carrier density are not known a priori

Contactless Visualization of Fast Charge Carrier Diffusion in Hybrid Halide Perovskite Thin Films

Organic–inorganic metal halide perovskite solar cells have recently attracted considerable attention with reported device efficiencies approaching those achieved in polycrystalline silicon. Key for an efficient extraction of photogenerated carriers is the combination of low nonradiative relaxation rates leading to long carrier lifetimes and rapid diffusive transport. The latter, however, is difficult to assess directly with reported values varying widely. Here, we present an experimental approach for a contactless visualization of the charge carrier diffusion length and velocity in thin films based on time-resolved confocal detection of photoluminescence at varying distances from the excitation position. Our measurements on chloride-treated methylammonium lead iodide thin films, the material for which the highest solar cell efficiencies have been reported, reveal a charge carrier diffusion length of 5.5–7.7 μm and a transport time of 100 ps for the first micrometer corresponding to a diffusion constant of about 5–10 cm² s^–1, similar to GaAs thin films

Controlling crystal growth by chloride-assisted synthesis: Towards optimized charge transport in hybrid halide perovskites

Understanding the charge carrier dynamics and charge transport in metal halide perovskites and their correlation with the synthesis procedure is crucial for the fabrication of competitive thin film photovoltaic devices and their further improvement. In this work we investigate two CH3NH3PbI3 (MAPI) films both deposited by a two-step protocol and only differing in the optional addition of a chloride containing salt to the precursor solution. Although being highly akin from a chemical and structural point of view, these films show substantial differences in their optoelectronic characteristics. For chloride-treated perovskite films we find an enhanced average power conversion efficiency of 10.3% and charge carrier mobility of μ=2.16 cm2/Vs in comparison to 5.3% and μ=1.62 cm2/Vs for untreated MAPI films, respectively. Moreover, we observe different light-soaking behavior and increased photoluminescence lifetimes of 70 ns in the case of chloride-treated MAPI and 100 ns for pure MAPI. From in-situ photoluminescence lifetime measurements during the crystallization process we conclude that the chloride addition during film deposition results in slower crystal growth, leading to fewer defects and higher crystalline order

Influence of metallic and dielectric nanowire arrays on the photoluminescence properties of P3HT thin films

The optical properties of organic semiconductor thin films deposited on nanostructured surfaces are investigated using time-resolved two-photon photoluminescence (PL) microscopy. The surfaces consist of parallel aligned metallic or dielectric nanowires forming well-defined arrays on glass substrates. Keeping the nanowire dimensions constant and varying only their spacing from 40 to 400 nm, we study the range of different types of nanowire–semiconductor interactions. For silver nanowires and spacings below 100 nm, the PL intensity and lifetime of P3HT and MDMO-PPV decrease rapidly due to the short-ranged metal-induced quenching that dominates the PL response with respect to a possible plasmonic enhancement of optical transition rates. In the case of P3HT however, we observe an additional longer-ranged reduction of non-radiative losses for both metallic and dielectric nanowires that is not observed for MDMO-PPV. Excitation polarization dependent measurements indicate that this reduction is due to self-assembly of the P3HT polymer chains along the nanowires. In conclusion, nanostructured surfaces, when fabricated across large areas, could be used to control film morphologies and to improve energy transport and collection efficiencies in P3HT-based solar cells

Titanium Doping and Its Effect on the Morphology of Three-Dimensional Hierarchical Nb₃O₇(OH) Nanostructures for Enhanced Light-Induced Water Splitting

This study presents a simple method that allows us to modify the composition, morphological, and surface properties of three-dimensional hierarchical Nb₃O₇(OH) superstructures, resulting in strongly enhanced photocatalytic H₂ production. The superstructures consist of highly ordered nanowire networks and self-assemble under hydrothermal conditions. The presence of titanium affects the morphology of the superstructures, resulting in increased surface areas for higher doping levels. Up to 12 at. % titanium is incorporated into the Nb₃O₇(OH) crystal lattice via substitution of niobium at its octahedral lattice sites. Further titanium excess results in the formation of niobium-doped TiO₂ plates, which overgrow the surface of the Nb₃O₇(OH) superstructures. Photoluminescence spectroscopy indicates fewer charge recombination processes near the surface of the nanostructures with an increasing titanium concentration in the crystal lattice. The combination of larger surface areas with fewer quenching sites at the crystal surface yields higher H₂ evolution rates for the doped samples, with the rate being doubled by incorporation of 5.5 ± 0.7 at. % Ti

Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene

We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. On the basis of a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: besides the well-known incoherent contribution driven by nonequilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence

Time-Resolved Microscopic Study of Charge Carriers in Newly Developed Perovskite Thin Films

Organic-inorganic metal halide perovskite solar cells (PSCs) have recently gained enormous attention with reported device efficiencies already approaching results achieved for multicrystalline silicon devices. However, the fundamental properties of this material system need to be further elucidated and some essential questions need to be addressed [1]