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Register a new userRecombination and localization: unfolding the pathways behind conductivity losses in Cs2AgBiBr6 thin films
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Cs2AgBiBr6 (CABB) has been proposed as a promising non-toxic alternative to lead halide perovskites. However, low charge carrier collection efficiencies remain an obstacle for the incorporation of this material in optoelectronic applications. In this work, we study the optoelectronic properties of CABB thin films using steady state and transient absorption and reflectance spectroscopy. We find that optical measurements on such thin films are distorted as a consequence of multiple reflections within the film. Moreover, we discuss the pathways behind conductivity loss in these thin films, using a combination of microsecond transient absorption and time-resolved microwave conductivity spectroscopy. We demonstrate that a combined effect of carrier loss and localization results in the conductivity loss in CABB thin films. Moreover, we find that the charge carrier diffusion length and sample thickness are of the same order. This suggests that the materials surface is an important contributor to charge carrier loss.
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In this work, we studied the pathways for formation of stoichiometric tcn~thin films. Polycrystalline and epitaxial tcn~films were prepared using reactive direct current magnetron (dcMS) sputtering technique. A systematic variation in the substrate temperature (Ts) during the dcMS process reveals that the lattice parameter (LP) decreases as Ts~increases. We found that nearly stoichiometric tcn~films can be obtained when Ts~= 300,K. However, they emerge from the transient state of Co target ($phi$3,inch). By reducing the target size to $phi$1,inch, now the tcn~phase formation takes place from the metallic state of Co target. In this case, LP of tcn~film comes out to be $sim$99p~of the value expected for tcn. This is the largest value of LP found so far for tcn. The pathways achieved for formation of polycrystalline tcn~were adopted to grow an epitaxial tcn~film, which shows four fold magnetic anisotropy in magneto-optic Kerr effect measurements. Detailed characterization using secondary ion mass spectroscopy indicates that N diffuses out when Ts~is raised even to 400,K. Measurement of electronic structure using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy further confirms it. Magnetization measurements using bulk magnetization and polarized neutron reflectivity show that the saturation magnetization of stoichiometric tcn~film is even larger than pure Co. Since all our measurements indicated that N could be diffusing out, when tcn~films are grown at high Ts, we did actual N self-diffusion measurements in a CoN sample and found that N self-diffusion was indeed substantially higher. The outcome of this work clearly shows that the tcn~films grown prior to this work were always N deficient and the pathways for formation of a stoichiometric tcn~have been achieved.
We report on the influence of the chemical composition on the electronic properties of molybdenum oxynitrides thin films grown by reactive sputtering on Si (100) substrates at room temperature. The partial pressure of Ar was fixed at 90 %, and the remaining 10 % was adjusted with mixtures N$_2$:O$_2$ (varying from pure N$_2$ to pure O$_2$). The crystalline and electronic structures and the electrical transport of the films depend on the chemical composition. Thin films grown using oxygen mixtures up 2 % have gamma-Mo$_2$N phase and display superconductivity. The superconducting critical temperature T$_c$ reduces from ~ 6.8 K to below 3.0 K as the oxygen increases. On the other hand, films grown using oxygen mixtures richer than 2 % are mostly amorphous. The electrical transport shows a semiconductor-like behavior with variable-range hopping conduction at low temperatures. The analysis of the optical properties reveals that the samples have not a defined semiconductor band gap, which can be related to the high structural disorder and the excitation of electrons in a wide range of energies
Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the $10^{17}$ to $10^{19}$ cm$^{-3}$ range, by infrared spectroscopy, first principle calculations, pump-probe spectroscopy and dc transport measurements to determine the relation between plasma edge and carrier density and to quantify mid-infrared plasmon losses. We demonstrate that the unscreened plasma frequency can be tuned in the 400 - 4800 cm$^{-1}$ range and that the average electron scattering rate, dominated by scattering with optical phonons and charged impurities, increases almost linearly with frequency. We also found weak dependence of losses and tunability on the crystal defect density, on the inactivated dopant density and on the temperature down to 10 K. In films where the plasma was optically activated by pumping in the near-infrared, we found weak but significant dependence of relaxation times on the static doping level of the film. Our results suggest that plasmon decay times in the several-picosecond range can be obtained in n-type germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature.
Solid oxide oxygen ion and proton conductors are a highly important class of materials for renewable energy conversion devices like solid oxide fuel cells. Ba2In2O5 (BIO) exhibits both oxygen ion and proton conduction, in dry and humid environment, respectively. In dry environment, the brownmillerite crystal structure of BIO exhibits an ordered oxygen ion sublattice, which has been speculated to result in anisotropic oxygen ion conduction. The hydrated structure of BIO, however, resembles a perovskite and the protons in it were predicted to be ordered in layers. To complement the significant theoretical and experimental efforts recently reported on the potentially anisotropic conductive properties in BIO, we measure here the proton and oxygen ion conductivity along different crystallographic directions. Using epitaxial thin films with different crystallographic orientations the charge transport for both charge carriers is shown to be anisotropic. The anisotropy of the oxygen ion conduction can indeed be explained through the layered structure of the oxygen sublattice in brownmillerite BIO. The anisotropic proton conduction however, further supports the suggested ordering of the protonic defects in the material. The differences in proton conduction along different crystallographic directions attributed to proton ordering in BIO are of a similar extent as those observed along different crystallographic directions in materials where proton ordering is not present but where protons find preferential conduction pathways through chain-like or layered structures.
In developing photovoltaic devices with high efficiencies, quantitative determination of the carrier loss is crucial. In conventional solar-cell characterization techniques, however, photocurrent reduction originating from parasitic light absorption and carrier recombination within the light absorber cannot be assessed easily. Here, we develop a general analysis scheme in which the optical and recombination losses in submicron-textured solar cells are evaluated systematically from external quantum efficiency (EQE) spectra. In this method, the optical absorption in solar cells is first deduced by imposing the anti-reflection condition in the calculation of the absorptance spectrum, and the carrier extraction from the light absorber layer is then modeled by considering a carrier collection length from the absorber interface. Our analysis method is appropriate for a wide variety of photovoltaic devices, including kesterite solar cells [Cu2ZnSnSe4, Cu2ZnSnS4, and Cu2ZnSn(S,Se)4], zincblende CdTe solar cells, and hybrid perovskite (CH3NH3PbI3) solar cells, and provides excellent fitting to numerous EQE spectra reported earlier. Based on the results obtained from our EQE analyses, we discuss the effects of parasitic absorption and carrier recombination in different types of solar cells.
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