Photothermal Deflection Spectroscopy Technique Research Articles

Significant improvement of thermal conductivity and bandgap energy blue shift in annealed GaInAsSb/GaSb

This paper deals with thermal annealing time effects on thermal conductivity and optical properties of GaInAsSb grown on GaSb substrate. An increased thermal conductivity as high as 13 W/mK for GaInAsSb layers annealed during 60 min could be measured by photothermal deflection spectroscopy technique (PDS). In addition, a blue shift around 30 meV for as grown GaInAsSb layer compared to the annealed one for 60 min could be put in evidence. From an other hand, the amplitudes signal of PDS reveal a multiple reflection as function of wavelength which appear for all annealed GaInAsSb. Such multiple reflections reflect both a high crystalline quality of the layer and sharp interfaces as well as a good lattice matching between the layer and the substrate. A slight decrease in absorption coefficient is however related to the increase of the reflection coefficient with annealing time.

Full Defects Passivation Enables 21% Efficiency Perovskite Solar Cells Operating in Air

AbstractThe lattice defects in the bulk and on the surface of the halide perovskite layer serve as trap sites and recombination centers to annihilate photogenerated carriers, determining the performance and stability of perovskite optoelectronic devices. Herein, the previously reported surface defects passivation engineering is extended to a full defects passivation strategy through stereoscopically introducing the cysteamine hydrochloride (CSA‐Cl) in the bulk and on the surface of perovskites. First‐principle density functional theory (DFT) calculations are employed to theoretically verify the multiple defects passivation effect of the CAS‐Cl on the perovskite. The perovskite layer with full defects passivation exhibits superior carrier dynamics as revealed by femtosecond transient absorption due to the reduced defect density determined by a highly sensitive photothermal deflection spectroscopy technique. Consequently, a high efficiency approaching 21% is achieved for the inverted planar perovskite solar cells (PVSCs). More importantly, the CAS‐Cl passivated PVSCs exhibit operation in air, which will be beneficial for the in situ device test for understanding the photophysics involved. This work provides a promising strategy to reduce the defects in both the perovskite bulk and surface for superior optoelectronic properties, facilitating the development of highly efficient and stable PVSCs and other optoelectronic devices.

Study of ZnO nanorods grown under UV irradiation

ZnO nanorods (NRs) perpendicularly oriented to the fused silica substrates were directly grown under hydrothermal conditions at 90 °C with or without UV irradiation. The different morphological structures were visualized and analyzed by scanning electron microscopy (SEM). The crystalline configuration and the structural quality of the ZnO NRs were investigated by X-ray diffraction and Raman spectroscopy. The surface properties of ZnO NRs were examined by X-ray photoelectron spectroscopy. Photoluminescence and photothermal deflection spectroscopy techniques were employed to investigate the optical and photoelectrical characteristics of the synthesized ZnO NRs, as well as the changes induced by the absorbance of UV irradiation. Electron paramagnetic resonance spectroscopy was applied to investigate the electron spin state and the structural defects in the given structures. The results from the SEM analysis showed a significant increase in the length of the ZnO NRs grown under UV irradiation, as well as the improvement in their uniformity. An enhancement of the optical and opto-electrical quality of the UV irradiated samples was also observed.

Determination of the Density of States on N-type Ptcdi-c13 Organic Thin-film Semiconductor

ABSTRACTIn this paper we study the density of states in n-type N,N’-ditridecylperylene-3,4,9,10-tetracarboxylic diimide organic semiconductor using two different methods. The first one is based on the temperature dependence of the channel conductance in field-effect transistors. The second one is based on the subgap optical absorption coefficient measured using the Photothermal Deflection Spectroscopy technique. Both techniques allow estimating the distribution of localized states in the band gap of the semiconductor.

Optical properties of Ge–Sb–Te chalcogenides

We have used RF sputtering to grow amorphous films of GeTe, Sb2Te3, Ge2Sb2Te5, and Ge2Sb2Te7, and employed photothermal deflection spectroscopy (PDS) to characterize the optical absorption in these films. The PDS technique can measure the optical band gap, the exponential ‘band tails’, and the absorption due to deep defects in these materials. The gross features of the absorption of amorphous Ge2Sb2Te7 and Ge2Sb2Te5 are independent of the substrate temperature from about 290K to 300K. The optical band gaps (as measured at α=104cm−1) of amorphous Ge2Sb2Te5, and Ge2Sb2Te7 are all about 1.0eV, although the absorption does depend slightly on the growth rate at room temperature. The band gaps of amorphous Sb2Te3 and GeTe are about 0.8 and 1.0eV, respectively, consistent with previously published results.

Absorption phenomena in organic thin films for solar cell applications investigated by photothermal deflection spectroscopy

A high sensitive approach is presented to detect in particular the low level absorption features in pure and blended organic semiconductor films, revealing a.o. defect induced sub gap absorption and new interactions between the materials. Because sub bandgap absorption features are typically characterized by very low absorption coefficients, it is not possible to resolve them using common transmission and reflection measurements. Therefore the very sensitive and ground state spectroscopic technique of Photothermal Deflection Spectroscopy (PDS) has been developed, and introduced to characterize thin films of MDMO-PPV and PCBM, as well as films of MDMO-PPV containing an increasing amount of PCBM ranging from 5 to 90% weight fraction. The measured spectra of MDMO-PPV are interpreted in terms of defect induced absorption phenomena. The spectral position of the observed transitions in PCBM have been determined and verified. The PDS-study on MDMO-PPV/PCBM blended films revealed for the first time interaction between the two materials in the ground state. To get more insight in the interaction mechanism between the constituting materials a systematic Transmission Electron Microscopy (TEM) study has been carried out to reveal the morphology of the films. The obtained TEM-results on nanomorphology of the blended films show clear correlations with the PDS-results.

Measurement of absolute minority species concentration and temperature in a flame by the photothermal deflection spectroscopy technique.

It is experimentally demonstrated that absolute concentrations of minority species in flames can be measured by the photothermal deflection spectroscopy (PTDS) technique. In addition, the PTDS signal simultaneously yields the flame temperature the measurement point. Absolute concentration profiles of OH have been measured in a flat-flame burner with methane as fuel. The PTDS measurements agree well with those obtained independently by the absorption technique. The flame temperature measurements by PTDS are also in good agreement with those obtained by the Boltzmann distribution among the rotational levels of OH.

An investigation of the photothermal deflection spectroscopy technique for temperature measurements in a flame

The technique of photothermal deflection spectroscopy (PTDS) is investigated for temperature measurements in a flame. The spatial distribution of temperature is measured in two different types of burners operating with methane and oxygen, and methane and air. The spatial distributions of the temperature are in qualitative agreement with our expectations. At two points, the temperatures measured by PTDS have been compared with those measured by Boltzmann distributions. The measurements agree within their uncertainties. All the effects that affect the reliability of this technique are investigated and discussed. Finally, the pros and cons of the PTDS technique for temperature measurement are discussed.

Absorption coefficient spectra of μc-Si in the low-energy region 0.4–1.2 eV

Optical absorption spectra in the low-energy region 0.4–1.2 eV is reported for μc-Si:H using a photothermal deflection spectroscopy technique. Absorption coefficient spectra in the low-energy region contain important information related to defects and hydrogen. It is demonstrated that there is a good correlation between electron spin densities and integrated absorption coefficient spectra from 0.7 to 1.2 eV. The amount of the hydrogen molecules in microvoids is much larger in μc-Si:H than that in a-Si:H. Light illumination effects in PDS spectra has also been studied from a view point of photo degradation of the μc-Si:H.

Infrared Photothermal Deflection Spectroscopy: A New Probe for the Investigation of Electrochemical Interfaces

An infrared photothermal deflection spectroscopy system has been developed using infrared tunable diode lasers in the wavelength region from 8 to 16 μm. This system allows the collection of in situ optical absorption spectra of species at electrode/electrolyte interfaces in an electrochemical cell. The infrared spectra of LiOH and formed on the surface of a lithium foil were recorded. The infrared photothermal deflection spectroscopy technique has been used to monitor the lithium electrode surface in an operating cell, . The in situ spectra (in the region) show that the film formation at Li/polymer electrolyte interfaces is dominated by LiOH, although there also appears to be organic species in the surface film.

Disorder and absorption edges in ion-irradiated hydrogenated amorphous carbon films.

The optical properties of ion-irradiated hydrogenated amorphous carbon films in the low and medium absorption coefficient region (1--${10}^{5}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) were investigated as functions of the hydrogen atomic fraction (5--50 H at. %) by using a photothermal deflection spectroscopy technique. The data are interpreted using Urbach and Tauc theories for the different spectral regions. It was found that an increase in hydrogen content induces an overall increase of order in the film, leading to a sharpening of the band edge slope, in agreement with the known tendency of trigonal carbon to cluster in ordered graphitic domains. Correspondingly a decrease of the Tauc optical energy gap (from 1.6 to 0.2 eV) was observed due to the formation of weakly bonded \ensuremath{\pi} electron states in these materials. The data are compared with those reported by several authors on a-Si and a-Si:H samples.

Photothermal deflection and photoluminescence studies of CdS and CdSe quantum dots

We report on the observation of the quantum confinement effect by using photothermal deflection spectroscopy (PDS) experiment and the time dependence of optically induced degradation in CdS and CdSe semiconductor-doped glasses. The observed absorption peaks in the PDS experiment, together with a simple model, were used to evaluate the average radius of semiconductor microcrystals. It is found that the estimated average radii of quantum dots are consistent with that obtained from other methods. This result demonstrates that the PDS technique provides an alternative tool for the study of the optical properties of semiconductor microscrystals. The time dependence of the luminescence degradation of the impurity band, which is attributed to the process of Auger ionization, follows a stretched-exponential function. The inconsistency with the previously proposed exponential relaxation may be due to the size distribution of CdS and CdSe microcrystals.

First photothermal deflection spectroscopy set-up in Egypt

We have succeeded in carrying out some ultra-sensitive photothermal experiments, using photothermal deflection spectroscopy (PDS) (or mirage effect). We have also employed the PDS technique to study the optical and thermal properties of solid/air interfaces. The experimental set-up of the transverse PDS has been described.The experimental results show that there is an exponential decay in PD signals with increase in the chopper frequency as well as with an increase of the phase difference. The PD signal decreases with increasing the probe beam normal offset to allow us to calculate the thermal diffusion length of the deflection medium (air in our case). We determined the thermal diffusion of air to be = 0.43 mm at the chopper frequency 50 Hz, which is in agreement with published values. Further, we employed the PD technique to obtain the spectra for two dyes [Rhodamine 6G (Rh6G) and Crystal Violet (CV)] of different optical absorption structures. The PDS shows a good resolution of the peaks of CV which are hardly separated in optical absorption measurements.Finally, a comparison of the photoacoustic spectra (PAS) of the two dyes was carried out, which showed that PDS has better spatial spectral resolution.

Highly Sensitive Absorption Measurements in Organic Thin Films: Charge Transfer Complexes and Light-Induced Changes

ABSTRACTUsing the sensitive absorption technique of Photothermal Deflection Spectroscopy (PI1S) it is possible to detect subtle spectral features in the absorption spectra of polymeric thin films that cannot be detected by other means. This paper describes observations of weak charge-transfer complex (CIC) formation and also light-induced changes in various organic films. The filns studied were those commonly used as charge transport materials in organic photoconductors, comprised of a small transport molecule and a polymer binder. The optically measured CTC bands can be used to establish single electron energy levels of the constituents. Reproducible photo-induced changes are manifest both in the strong UV transitions as well as in the “sub-gap” absorption and appear to be correlated with the photo-chemical stability of the constituents. The light-fatigue has both a reversible as well as irreversible component.

Gap density of states in amorphous silicon-germanium alloy: Influence on photothermal deflection spectroscopy and steady-state conductivity measurements

A series of amorphous hydrogenated silicon-germanium films has been produced by the glow-discharge dissociation of a GeH4 +SiH4 +H2 gas mixture. The subgap optical absorption has been measured with the photothermal deflection spectroscopy (PDS) technique, and the results are discussed and referred to a model density of states. The density of defects has been found meaningfully correlated to the bulk material, despite a number of possible complications intrinsic of the PDS technique and/or of the material itself, which can potentially prevent the correct determination of the density of defects from the subgap optical absorption. Namely, the defect density ranges from 1016 cm−3 in a-Si:H to 5×1017 cm−3 in a Ge:H, whereas the Urbach tail width is only slightly increased from 50 to 70 meV. The amount of disorder and the defect density of the material are compared to the predictions of an available thermodynamical model: the increasing number of defects brought about by germanium results in being an intrinsic feature of the material. Furthermore, the impact of the variable concentration of silicon and germanium dangling-bond states on the position of the Fermi level has been studied by means of the same model density of states: a noticeable correlation was found between the activation energy of the steady-state dark conductivity (EA ) and the volume density of silicon and germanium dangling bonds (Nd ). In other words, the three-step trend of EA vs x can be considered to be a fingerprint of the separate dangling-bond densities in germanium and silicon. These can be deconvolved, provided that the dangling-bond states in the forbidden energy gap are known as a function of the alloy composition.

Band Edge and Neutral Soliton Absorption in Polyacetylene: The Role of Coulomb Correlation

Abstract The technique of Photothermal Deflection Spectroscopy (PDS) has provided the clearest measurement of the absorption edge in polyacetylene. Compensated trans-(CH)x has an Urbach edge which covers orders of magnitude change in absorption in the range 0.9-1.7 eV. Parallel measurements of PDS and spin resonance on compensated trans-(CH)x fails to reveal the predicted neutral soliton (So) absorption near midgap. The So absorption has been measured in a Photoinduced Absorption (PA) experiment in which the PA spectrum is dominated by a change in the number of So centers. These results provide new insight into such fundamental issues as the importance of Coulomb correlations and the role of disorder in polyacetylene.

A Study of Co and Mn in ZnO Varistors

AbstractZnO varistors are polycrystalline materials which have highly non—linear current/voltage characteristics controlled by depletion regions around the grain boundaries. Dissolved transition metals at concentrations of about 1 at.% are required to achieve this large nonlinearity, but their role is unclear. To address this issue we have usedthe technique of photothermal deflection spectroscopy to measure the optical absorption of varistors containing Co or Mn, both common dopants. Prominent peaks and edges appear in the spectra, and are associated with substitutional Co2+and Mn2+ ions whose ground states are deep within the ZnO bandgap. Electro— and photoluminescence spectra show Stokes shifted peaks for some of these transitions. The spectral dependence of the photoconductivity shows that some of the absorption features correspond to ionization of the impurities. This result is used to show how the dark impact ionization of these impurities due to a high applied voltage can contribute to the current/voltage nonlinearity.

Optical absorption in ion-implanted lead lanthanum zirconate titanate ceramics

Optical absorption measurements have been performed on unmodified and on ion-implanted lead lanthanum zirconate titanate ceramics using the photothermal deflection spectroscopy technique. Bulk absorption coefficients depend on the average grain size of the material while the absorption associated with the ion-damaged layers does not. The damage-induced surface absorptance correlates well with the photosensitivity observed in implanted PLZT devices, supporting earlier models for the enhanced imaging efficiency of the materials.

Optical Absorption in Polyacetylene: A Direct Measurement Using Photothermal Deflection Spectroscopy

We present direct measurements of the optical absorption in undoped and ammoniatreated polyacetylene utilizing the technique of photothermal deflection spectroscopy. Our spectra show an Urbach-type edge and a broad, N${\mathrm{H}}_{3}$-sensitive sub-band-gap absorption. Quantitative comparisons of absorption and spin-resonance results do not show the predicted midgap absorption from neutral solitons.

Direct measurement of gap-state absorption in hydrogenated amorphous silicon by photothermal deflection spectroscopy

We have measured the subgap otpical absorption of undoped, singly doped, and compensated hydrogenated amorphous silicon down to 0.6 eV using the sensitive technique of photothermal deflection spectroscopy. We show that this absorption is due to silicon dangling-bond defects located approx.1.4 eV below the conduction band. While doping also creates defects approx.1.4 eV below the conduction band, compensation removes them. The results suggest that for the undoped material the density-of-states maximum found in field-effect measurements is due to silicon dangling bonds.