Temperature Dependence Of Band Gap Research Articles

Field Performance versus Standard Test Condition Efficiency of Tandem Solar Cells and the Singular Case of Perovskites/Silicon Devices.

Multijunction cells may offer a cost-effective route to boost the efficiency of industrial photovoltaics. For any technology to be deployed in the field, its performance under actual operating conditions is extremely important. In this perspective, we evaluate the impact of spectrum, light intensity, and module temperature variations on the efficiency of tandem devices with crystalline silicon bottom cells with a particular focus on perovskite top cells. We consider devices with different efficiencies and calculate their energy yields using field data from Denver. We find that annual losses due to differences between operating conditions and standard test conditions are similar for single-junction and four-terminal tandem devices. The additional loss for the two-terminal tandem configuration caused by current mismatch reduces its performance ratio by only 1.7% when an optimal top cell bandgap is used. Additionally, the unusual bandgap temperature dependence of perovskites is shown to have a positive, compensating effect on current mismatch.

Synthesis and physical properties of low cost, non toxic, earth abundant p-type CuInS2 thin film as absorber layer for photovoltaic application

Copper Indium disulfide (CIS) thin films were synthesized by chemical solution deposition method on glass substrates. Low processing temperatures of 420 °C under nitrogen annealing is key feature of this study.CIS thin films on glass were prepared by dissolving copper (II) chloride, Indium (III) chloride and Thiourea in 2-methoxy ethanol. Surface structures and morphology of the synthesized CIS thin films studied by grazing incidence X-ray diffraction and atomic force microscopy, respectively, revealed a phase pure growth and smooth morphology with average crystallite sizes of the order of 2.97 nm. Hall Effect study demonstrates that synthesized CIS thin film is p-type with holes as majority charge carriers. Temperature dependence of optical band gap was studied in the wavelength range of 200-1400 nm and was found to decrease with increasing temperature. Detailed results on synthesis and physical properties will be presented

Temperature dependence of direct and indirect band gaps of Bi13I2S18 hexagonal rod crystals

Single crystals of Bi13I2S18 have been grown by chemical vapor transport method. The band-edge properties of Bi13I2S18 are evaluated using temperature-dependent optical absorption and thermal-modulated reflectance (TR) measurements in the temperature range between 30 and 300 K. Single-crystal X-ray diffraction of different orientations showed structural anisotropy and identified the crystalline phase of the as-grown Bi13I2S18 hexagonal micro pillars. According to transmission and TR measurements, the Bi13I2S18 hexagonal rod is confirmed to be an indirect semiconductor that simultaneously possesses indirect (Egind at ∼0.73 eV) and direct (Egd at ∼1.06 eV) optical gaps. The averaged value of direct and indirect gaps is close to 1 eV that may efficiently absorb the sunlight spectrum for solar energy application. Temperature-energy variation and line-width broadening behavior (TR) of the band-edge transition of Bi13I2S18 are also analyzed and discussed herein.

Long-wavelength sensitivity limit in MBE-grown PbSnTe:In films: Correlation with the film structure and composition

PbSnTe:In films with a long-wavelength sensitivity limit of over 20 μm and low “dark” conductivity are studied. The spectral dependences of the photoconductivity obtained at different temperatures using a Fourier spectrometer are compared with data on the film composition determined by X-ray microanalysis. The established nonmonotonic temperature dependence of the long-wavelength sensitivity limit is attributed to the combination of the temperature dependence of the PbSnTe band gap and the Burstein–Moss effect making the largest contribution to low-temperature measurements due to the long lifetime of excess charge carriers. It is shown that the difference between the band-gap values determined from the measured composition and temperature dependences of the long-wavelength sensitivity limit can be related to the inhomogeneity of the composition of the films grown by molecular beam epitaxy.

Temperature dependence of band gaps and conformational disorder in PEDOT and its selenium and tellurium derivatives: Density functional calculations.

The conducting polymer poly(3,4-ethylenedioxythiophene), or PEDOT, is an attractive material for flexible electronics. We present combined molecular dynamics and quantum chemical calculations, based on density functional theory, of EDOT oligomers and isoelectronic selenium and tellurium derivatives (EDOS and EDOTe) to address the effect of temperature on the geometrical and electronic properties of these systems. With finite size scaling, we also extrapolate our results to the infinite polymers, i.e., PEDOT, PEDOS, and PEDOTe. Our computations indicate that the most favourable oligomer conformations at finite temperature are conformations around the flat trans-conformation and a non-flat conformation around 45° from the cis-conformation. Also, the dihedral stiffness increases with the atomic number of the heteroatom. We find excellent agreement with experimentally measured gaps for PEDOT and PEDOS. For PEDOT, the gap does not increase with temperature, whereas this is the case for its derivatives. The conformational disorder and the choice of the basis set both significantly affect the calculated gaps.

Low temperature dynamics of surface and bulk electronic structure of quantum dots

Absolute energies of band edges have proven to be very important for various applications like hydrogen generation, solar water splitting and solar cell optimization. Energy differences as small as 50–100 meV have been shown to largely affect device efficiencies. Device operational temperature can vary largely and temperature dependence of band gap is well known in bulk semiconductor literature. However, there are only a few studies on variation of band gap in quantum dots and none of them characterize the relative energy variation of band edges in spite of their importance in various applications. This is mainly due to the absence of an internal standard that can be used to study the variation of band edges. Here, in this paper, we introduce a technique wherein we utilize Cu dopant emission as an internal probe. Using this technique, we report the variation of band gap, conduction band and valence band edges of CdS and CdSe quantum dots as a function of temperature and size. We found that band gap variation is similar to that of bulk but with a higher average phonon energy. The band edge variation is characterized by a dominant conduction band shift for larger sizes with decreasing temperature while the smaller size QDs show the variation in both conduction band and valence band. Further, we have also utilized this method to study the binding energy of the trap states as a function of temperature using Cu photoluminescence quantum yield and average lifetime of Cu photoluminescence.

The role of electron-phonon interactions on the coherence lifetime of monolayer transition metal dichalcogenides

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS2, MoSe2 and WSe2 atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ‘ab initio’ calculations of the phonon energies and the phonon density of state reveal strong interaction with the E′ and E″ phonon modes.

Effects of Temperature Dependence of Energy Bandgap on I–V Characteristics in CNTFETs Models

In this paper, we analyze the effects of temperature dependence of energy bandgap on [Formula: see text] characteristics in some carbon nanotube field effect transistors (CNTFETs) models proposed in literature in order to identify the one more suitable for computer aided design (CAD) applications. At first we consider a compact, semi-empirical model, already proposed by us, performing [Formula: see text] characteristic simulations at different temperatures. Our results are compared with those obtained with the Stanford-Source virtual carbon nanotube field-effect transistor model (VS-CNFET), obtaining [Formula: see text] characteristics comparable, but with lower CPU calculation time.

Temperature effects on the structural and optical properties of the TlInSe2xS2(1−x) mixed crystals ([formula omitted

In this work, we have studied the temperature effects on the recrystallization process and on the energy band gap of the TlInSe2xS2(1−x) mixed crystals at the critical composition (x=0.3) where structural phase transition from tetragonal to monoclinic takes place. Remarkable effect which included permanent recrystallization process, enlargements in the monoclinic crystallite size, decreases in the compressing strain and in the dislocation density as well as in the stacking faults and in the energy band gap with increasing temperature were observed. In addition, the temperature dependent energy band gap of the crystal which is investigated in the same temperature range that was used for the recrystallization process revealed that the recrystallization process is associated with energy band gap narrowing.

The role of electron-phonon interactions on the coherence lifetime of monolayer transition metal dichalcogenides

We investigate the excitonic dephasing of transition metal dichalcogenides, namely MoS2, MoSe2 and WSe2 atomic monolayer thick and bulk crystals, in order to understand the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with theoretical calculations of the phonon spectra, reveal the important role electron-phonon interactions plat in dephasing process. The temperature dependence of the electronic band gap and the excitonic linewidth combined with ‘ab initio’ calculations of the phonon energies and the phonon density of state reveal strong interaction with the E’ and E” phonon modes.

Spectrally selective solar absorber with sharp and temperature dependent cut-off based on semiconductor nanowire arrays

Spectrally selective absorbers (SSA) with high selectivity of absorption and sharp cut-off between high absorptivity and low emissivity are critical for efficient solar energy conversion. Here, we report the semiconductor nanowire enabled SSA with not only high absorption selectivity but also temperature dependent sharp absorption cut-off. By taking advantage of the temperature dependent bandgap of semiconductors, we systematically demonstrate that the absorption cut-off profile of the semiconductor-nanowire-based SSA can be flexibly tuned, which is quite different from most of the other SSA reported so far. As an example, silicon nanowire based selective absorbers are fabricated, with the measured absorption efficiency above (below) bandgap ∼97% (15%) combined with an extremely sharp absorption cut-off (transition region ∼200 nm), the sharpest SSA demonstrated so far. The demonstrated semiconductor-nanowire-based SSA can enable a high solar thermal efficiency of ≳86% under a wide range of operating conditions, which would be competitive candidates for the concentrated solar energy utilizations.

The effects of temperature on optical properties of InGaN/GaN multiple quantum well light-emitting diodes

The effects of temperature on the optical properties of InGaN/GaN quantum well (QW) light-emitting diodes have been investigated by using the six-by-six K-P method taking into account the temperature dependence of band gaps, lattice constants, and elastic constants. The numerical results indicate that the increase of temperature leads to the decrease of the spontaneous emission rate at the same injection current density due to the redistribution of carrier density and the increase of the non-radiative recombination rate. The product of Fermi-Dirac distribution functions of electron fcn and hole (1−fvUm) for the transitions between the three lowest conduction subbands (c1–c3) and the top six valence subbands (v1–v6) is larger at the lower temperature, which indicates that there are more electron-hole pairs distributed on the energy levels. It should be noted that the optical matrix elements of the inter-band transitions slightly increase at the higher temperature. In addition, the internal quantum efficiency of the InGaN/GaN QW structure is evidently decreased with increasing temperature.

Composition and temperature dependent optical properties of AlxGa1-xN alloy by spectroscopic ellipsometry

A series of AlxGa1-xN/AlN/Sapphire films with x=0.35–0.75 and different thickness of epi-layer were prepared by metalorganic chemical vapor deposition (MOCVD). Spectroscopic ellipsometry (SE) was used to study the temperature-dependent refractive indices and optical bandgaps of the AlxGa1-xN films ranging from 300 to 823K. Parametric semiconductor (PSEMI) models were used to describe the dielectric functions of AlGaN/AlN layers. The fitting results of refractive index, energy bandgap, thickness and surface roughness at 300K are in good agreement with photoluminescence (PL), scanning electron microscopy (SEM) measurements and the existing literature. Our finding indicates that the crystal quality of the samples with x=0.47 and 0.60 are better than those with x=0.35 and 0.75. As the temperature rises, the increasing of refractive index for the low Al content AlxGa1-xN layers is stronger than that of high Al content in the transparent region, and the reduction of bandgap with high Al content is larger than that of low Al content. For all the samples (x=0.35–0.75), an analytical expression for temperature-dependent refractive index in the wavelength range of 195–1650nm was obtained using the Sellmeier law, and the quantitative analysis of the SE-derived temperature-dependent bandgap was conducted by using the Bose-Einstein equation.

Sub-micron-sized delafossite CuCrO2 with different morphologies synthesized by nitrate–citric acid sol–gel route

Currently, copper chromium oxide crystallizing in delafossite structure attracts huge research interest due to its versatile applications arising from its layered structure. In this work, delafossite CuCrO2 was synthesized by sol–gel method from their respective hydrated nitrate salts with citric acid as a chelating agent. The phase formation temperature was found to be between 750 and 775∘C. At 750∘C, the partial formation of delafossite CuCrO2 spheres with particle size in nano-regime was observed in the midst of platelets of spinel CuCr2O4. A green coloured powder with particle size 125–350 nm exhibiting distorted spheres was obtained at 775∘C. The increase in temperature has a profound impact on the particle size, morphology and the optical properties of CuCrO2. The X-ray powder diffraction studies revealed the formation of 3R-CuCrO2 phase (rhombohedral, space group R-3m) as a major product in the temperature range 775–1000∘C. The unit cell parameters were found to be a = b = 2.9711 Å and c = 17.0723 Å at 1000∘C. Scanning electron micrographs illustrated the different morphologies from spheres to hexagonal form via distorted spheres and cubes. The UV–Vis diffuse reflectance spectra measured for the powders exhibited semiconductor characteristics with an interesting size-related and temperature-dependent bandgap.

Temperature evolution of structural and electrical property for Sr7Mn4O15 at high temperature

Temperature dependence of structural evolution for single phase Sr7Mn4O15 is determined using the in-situ X-ray diffraction with Rietveld refinements in the temperature range of 300–1200 K. It's found that thermal expansion of Sr7Mn4O15 lattice parameters increase linearly with temperature. The structure of face-sharing clusters prefers expansion in the transverse, and corner-sharing clusters prefers the stretch in the longitudinal. Resistance decreases sharply with increasing temperature at high temperature, which means that the Sr7Mn4O15 is a well candidate in applications as negative temperature coefficient thermistor. Based on the fitted atomic parameters given from Rietveld refinements, temperature dependent band gap of Sr7Mn4O15 is estimated with first principle calculation, which exhibits a dramatic reduction from 1.141 eV to 0.349 eV with increasing the temperature.

Analysis of Temperature Dependent Effects on <italic>I–V</italic> Characteristics of Heterostructure Tunnel Field Effect Transistors

This paper provides an analysis of the intrinsic factors influencing the temperature dependence of the $I_{d}-V_{ds}-V_{gs}$ characteristics of heterostructure Tunnel FETs based on GaSb/InAs tunneling junctions. The temperature dependence of energy bandgap, quantum confinement energy-shifts, and fermi-level position are quantified. There is significant cancellation among the various effects, such that the overall $I_{d}-V_{ds}- V_{gs} $ characteristics are expected to have remarkably small temperature dependence, of the order of 10 – 20 mV shift in $V_{gs} $ over the temperature range of 0 – 125 °C. Considerations are also discussed for representative extrinsic effects such as trap-assisted tunneling, which affect many experimental devices to a variable extent.

The importance of temperature dependent energy gap in the understanding of high temperature thermoelectric properties

In this work, we show the importance of temperature dependent energy band gap, Eg(T), in understanding the high temperature thermoelectric (TE) properties of material by considering LaCoO3 (LCO) and ZnV2O4 (ZVO) compounds as a case study. For the fix value of band gap, Eg, deviation in the values of α has been observed above 360 K and 400 K for LCO and ZVO compounds, respectively. These deviation can be overcomed by consideration of temperature dependent band gap. The change in used value of Eg with respect to temperature is ∼4 times larger than that of In As. This large temperature dependence variation in Eg can be attributed to decrement in the effective on-site Coulomb interaction due to lattice expansion. At 600 K, the value of ZT for n and p-doped, LCO is ∼0.35 which suggest that it can be used as a potential material for TE device. This work clearly suggest that one should consider the temperature dependent band gap in predicting the high temperature TE properties of insulating materials.

Dimensional Effects on the Charge Density Waves in Ultrathin Films of TiSe2.

Charge density wave (CDW) formation in solids is a critical phenomenon involving the collective reorganization of the electrons and atoms in the system into a wave structure, and it is expected to be sensitive to the geometric constraint of the system at the nanoscale. Here, we study the CDW transition in TiSe2, a quasi-two-dimensional layered material, to determine the effects of quantum confinement and changing dimensions in films ranging from a single layer to multilayers. Of key interest is the characteristic length scale for the transformation from a two-dimensional case to the three-dimensional limit. Angle-resolved photoemission spectroscopy (ARPES) measurements of films with thicknesses up to six layers reveal substantial variations in the energy structure of discrete quantum well states; however, the temperature-dependent band gap renormalization converges at just three layers. The results indicate a layer-dependent mixture of two transition temperatures and a very-short-range CDW interaction within a three-dimensional framework.

One-shot calculation of temperature-dependent optical spectra and phonon-induced band-gap renormalization

Recently, Zacharias et al [Phys. Rev. Lett. 115, 177401 (2015)] developed a new ab initio theory of temperature-dependent optical absorption spectra and band gaps in semiconductors and insulators. In that work the zero-point renormalization and the temperature dependence were obtained by sampling the nuclear wavefunctions using a stochastic approach. In the present work, we show that the stochastic sampling can be replaced by fully deterministic supercell calculations based on a single optimal configuration of the atomic positions. We demonstrate that a single calculation is able to capture the temperature-dependent band gap renormalization including quantum nuclear effects in direct and indirect-gap semiconductors, as well as phonon-assisted optical absorption in indirect-gap semiconductors. In order to demonstrate this methodology we calculate from first principles the temperature-dependent optical absorption spectra and the renormalization of direct and indirect band gaps in Si, C, and GaAs, and we obtain good agreement with experiment and with previous calculations. In this work we also establish the formal connection between the Williams-Lax theory of optical transitions and the related theories of indirect absorption by Hall, Bardeen, and Blatt, and of temperature-dependent band structures by Allen and Heine. Furthermore, we identify an additional band gap renormalization that arises in the case of degenerate band extrema, and which has not been considered in previous work. The present methodology enables systematic ab initio calculations of optical absorption spectra at finite temperature, including both direct and indirect transitions. This feature will be useful for high-throughput calculations of optical properties at finite temperature, and for calculating temperature-dependent optical properties using high-level theories such as GW and Bethe-Salpeter approaches.

Temperature dependence of the photocurrent in Ge1−x Sn x layers

The temperature dependence of the photocurrent in Ge1−x Sn x layers was measured for different Sn compositions (0.00 ≤ x ≤ 0.83%). As the Sn composition in the Ge1−x Sn x layer increased, the photocurrent peak was clearly red shifted in comparison with that of Ge. As the measurement temperature was increased, the photocurrent peak was also clearly red shifted. The measured temperature dependence of the energy band gap was nicely fitted by using Varshni’s empirical expression. Temperature-dependent bowing parameters were fitted for the equation relationship.