Low Optical Absorption Research Articles

Linear-scaling time-dependent density-functional theory in the linear response formalism

We present an implementation of time-dependent density-functional theory (TDDFT) in the linear response formalism enabling the calculation of low energy optical absorption spectra for large molecules and nanostructures. The method avoids any explicit reference to canonical representations of either occupied or virtual Kohn-Sham states and thus achieves linear-scaling computational effort with system size. In contrast to conventional localised orbital formulations, where a single set of localised functions is used to span the occupied and unoccupied state manifold, we make use of two sets of in situ optimised localised orbitals, one for the occupied and one for the unoccupied space. This double representation approach avoids known problems of spanning the space of unoccupied Kohn-Sham states with a minimal set of localised orbitals optimised for the occupied space, while the in situ optimisation procedure allows for efficient calculations with a minimal number of functions. The method is applied to a number of medium sized organic molecules and a good agreement with traditional TDDFT methods is observed. Furthermore, linear scaling of computational cost with system size is demonstrated on (10,0) carbon nanotubes of different lengths.

Synthesis of a new chromophore containing a new electron donor

A new electro-optical chromophore with 4-(bis(2-hydroxyethyl)amino)-2,6-dipropoxybenzaldehyde as electron donor was synthesized and characterized by 1H NMR and mass spectra. Poly(methyl methacrylate) doped with this new chromophore at different loading densities was prepared in thermo-optical films by spin coating. These films showed low optical absorption at communication wavelength, good thermal stability and large thermo-optics coefficients.

Band gap and structure of single crystal BiI3: Resolving discrepancies in literature

Bismuth tri-iodide (BiI3) is an intermediate band gap semiconductor with potential for room temperature gamma-ray detection applications. Remarkably, very different band gap characteristics and values of BiI3 have been reported in literature, which may be attributed to its complicated layered structure with strongly bound BiI6 octahedra held together by weak van der Waals interactions. Here, to resolve this discrepancy, the band gap of BiI3 was characterized through optical and computational methods and differences among previously reported values are discussed. Unpolarized transmittance and reflectance spectra in the visible to near ultraviolet (UV-Vis) range at room temperature yielded an indirect band gap of 1.67 ± 0.09 eV, while spectroscopic ellipsometry detected a direct band gap at 1.96 ± 0.05 eV and higher energy critical point features. The discrepancy between the UV-Vis and ellipsometry results originates from the low optical absorption coefficients (α ∼ 102 cm−1) of BiI3 that renders reflection-based ellipsometry insensitive to the indirect gap for this material. Further, electronic-structure calculations of the band structure by density functional theory methods are also consistent with the presence of an indirect band gap of 1.55 eV in BiI3. Based on this, an indirect band gap with a value of 1.67 ± 0.09 eV is considered to best represent the band gap structure and value for single crystal BiI3.

GdN Nanoisland-Based GaN Tunnel Junctions

Tunnel junctions could have a great impact on gallium nitride and aluminum nitride-based devices such as light-emitting diodes and lasers by overcoming critical challenges related to hole injection and p-contacts. This paper demonstrates the use of GdN nanoislands to enhance interband tunneling and hole injection into GaN p-n junctions by several orders of magnitude, resulting in low tunnel junction specific resistivity (1.3 × 10(-3) Ω-cm(2)) compared to the previous results in wide band gap semiconductors. Tunnel injection of holes was confirmed by low-temperature operation of GaN p-n junction with a tunneling contact layer, and strong electroluminescence down to 20 K. The low tunnel junction resistance combined with low optical absorption loss in GdN is very promising for incorporation in GaN-based light emitters.

Synthesis and Characterization of Fluorinated Poly(phenylmaleimide-co-pentafluorophenylmaleimide) for Optical Waveguides

Fluorinated polymaleimides with high thermal stability and low optical absorption loss in the optical communication wavelength of were investigated for application in low-loss waveguide materials. The fluorinated polymaleimides were prepared from two monomers phenylmaleimide (H-PMI) and pentafluorophenylmaleimide (F-PMI). All synthesized copolymers had high thermal stability (decomposition temperature ). The refractive index of the copolymers could be tuned from 1.4969 to 1.5950 in the TE mode and from 1.4993 to 1.5932 for the TM mode at 632.8 nm by copolymerizing different weight ratios of H-PMI and F-PMI. The refractive index of the copolymers decreased with increasing F-PMI content. In addition, when the amount of F-PMI was increased, optical loss and absorption loss at 632.8 nm and 1550 nm, respectively, decreased.

Thermal Characterization, Crystal Field Analysis and In-Band Pumped Laser Performance of Er Doped NaY(WO4)2 Disordered Laser Crystals

Undoped and Er-doped NaY(WO4)2 disordered single crystals have been grown by the Czochralski technique. The specific heat and thermal conductivity (κ) of these crystals have been characterized from T = 4 K to 700 K and 360 K, respectively. It is shown that κ exhibits anisotropy characteristic of single crystals as well as a κ(T) behavior observed in glasses, with a saturation mean free phonon path of 3.6 Å and 4.5 Å for propagation along a and c crystal axes, respectively. The relative energy positions and irreducible representations of Stark Er3+ levels up to 4G7/2 multiplet have been determined by the combination of experimental low (<10 K) temperature optical absorption and photoluminescence measurements and simulations with a single-electron Hamiltonian including both free-ion and crystal field interactions. Absorption, emission and gain cross sections of the 4I13/2↔4I15/2 laser related transition have been determined at 77 K. The 4I13/2 Er3+ lifetime (τ) was measured in the temperature range of 77–300 K, and was found to change from τ (77K) ≈ 4.5 ms to τ (300K) ≈ 3.5 ms. Laser operation is demonstrated at 77 K and 300 K by resonantly pumping the 4I13/2 multiplet at λ≈1500 nm with a broadband (FWHM≈20 nm) diode laser source perfectly matching the 77 K crystal 4I15/2 → 4I13/2 absorption profile. At 77 K as much as 5.5 W of output power were obtained in π-polarized configuration with a slope efficiency versus absorbed pump power of 57%, the free running laser wavelength in air was λ≈1611 nm with the laser output bandwidth of 3.5 nm. The laser emission was tunable over 30.7 nm, from 1590.7 nm to 1621.4 nm, for the same π-polarized configuration.

Electronic and Optical Properties of Chlorinated Silicon Nanoparticles

First-principles calculations are used to investigate the structure, electronic and optical properties of silicon nanocystals with chlorine-passivated surface. The nanocrystals considered were approximately spherical, with diameters between 1.5 and 3.0 nm. We show that the nanocrystals with chlorinated surface have a smaller bandgap, lower optical absorption threshold, and greater ionization energy and electron affinity than hydrogenated silicon nanocrystals of the same size.

Influence of Surface Morphology of Textured Substrate against Poly-Si Thin Film Solar Cells Performance

Since poly-Si is an indirect band gap material and has low optical absorption coefficient in the visible-infrared region, the light trapping in thin film poly-Si layer by using textured substrate is one of the important technical issue for achievement of high short current. Surface texture of a transparent conductive oxide (TCO) layer on a glass substrate as well as SnO2 with a large grain are usually utilized for the light-trapping technique, i.e., path lengths of the incident light in the poly-Si layer are effectively enhanced by the light-scattering at the textured surface. In this paper, a systematic investigation has been carried out concerning the relationship between poly-Si thin film solar cells performance and surface morphology of substrate texture as a function of root mean square roughness of substrate surface, in order to find the optimum textured substrate and realize the light trapping in the poly-Si solar cells. Furthermore, the influence of textured substrate on optical reflectance, poly-Si microstructure and photovoltaic performance are also discussed.

Cascade-type excitation energy relay in organic thin-film solar cells

We investigated small-molecule-based organic photovoltaic (PV) cells with three different electron-donating material layers, two thiophene/phenylene co-oligomers [α,ω-bis(biphenyl-4-yl)terthiophene (BP3T) and α,ω-diphenyl sexithiophene (P6T)] and copper phthalocyanine (CuPc), and one electron-accepting material layer (C60). A cascade-type energy relay between the three donor layers occurred, and the incident photon-current conversion efficiency improved in the blue light region, where CuPc has very low optical absorption. An increase in P6T photoluminescence intensity in a two-layer sample (BP3T/P6T) on quartz confirmed interlayer excitation transfer (ET) from BP3T and P6T. The bulk heterojunction architecture in our interlayer-ET-based organic PV cell was effective. Moreover, P6T appeared to have a relatively long exciton diffusion length of several tens of nanometers.

Core Structures of Dislocations within CdTe Grains

ABSTRACTCdTe is well known as an excellent photovoltaic material for high efficiency solar cell applications because it has a direct band-gap, low fabrication cost and high optical absorption coefficient. However, the nonradiative recombination and low average minority carrier lifetime caused by the defects in CdTe solar cells limit its efficiency. So far, grain boundaries (GB) have been considered to be the major origin of the nonradiative recombination. However, we show that CdTe grains contain many dislocations that could limit device efficiency. Scanning transmission electron microscopy (STEM) was used to determine the atomic structure of intrinsic and extrinsic stacking faults and their terminating partial dislocation cores. Z-contrast images are sensitive to atomic number and are able to distinguish Cd and Te atomic columns. Unpaired Cd and Te atomic columns were found to form the partial dislocation cores, suggesting the presence of dangling bonds. These defects are likely to be electrically active, and may be the origin of the low minority carrier lifetime.

Oxide p‐n Heterojunction of Cu2O/ZnO Nanowires and Their Photovoltaic Performance

Oxide p‐n heterojunction devices consisting of p‐Cu2O/n‐ZnO nanowires were fabricated on ITO/glass substrates and their photovoltaic performances were investigated. The vertically arrayed ZnO nanowires were grown by metal organic chemical vapor deposition, which was followed by the electrodeposition of the p‐type Cu2O layer. Prior to the fabrication of solar cells, the effect of bath pH on properties of the absorber layers was studied to determine the optimal condition of the Cu2O electrodeposition process. With the constant pH 11 solution, the Cu2O layer preferred the (111) orientation, which gave low electrical resistivity and high optical absorption. The Cu2O (pH 11)/ZnO nanowire‐based solar cell exhibited a higher conversion efficiency of 0.27% than the planar structure solar cell (0.13%), because of the effective charge collection in the long wavelength region and because of the enhanced junction area.

Plasmonic nanoparticle enhanced and extended performance of Light-sensitive nanocrystal skins

ABSTRACTWe report on light-sensitive nanocrystal skin (LS-NS) platforms composed of monolayer visible nanocrystals (NCs) on top of bilayers of polyelectrolyte polymers. These LS-NS devices are operated on the principle of photogenerated potential buildup, unlike common photodetectors that operate on the basis of charge collection. The resulting devices are as highly sensitive as common photosensors, despite utilizing a monolayer of NCs and requiring no applied external bias. In this device architecture, using only a single NC monolayer also allows to reduce noise current generation. This LS-NS platform is highly stable under ambient conditions with fully sealed NC monolayer, promising for low-cost large-area UV/visible sensing applications. However, such visible NC based LS-NS devices exhibit limited performance in the long wavelength range due to the low optical absorption of these NCs (e.g., CdTe NCs) in this spectral range. Here, to enhance the device sensitivity, incorporating silver nanoparticles into LS-NS is proposed and demonstrated. For that, the optical absorption of CdTe monolayer NCs in the LS-NS devices is increased using the embedded silver nanostructures. With plasmon coupling, we observe a 2.6-fold enhancement factor in the photosensitivity around the localized surface plasmonic resonance peak of the nanostructures. Higher sensitivity improvement is also obtained at longer wavelengths. To predict the enhancement in the sensitivity of the LS-NS, numerical simulations are performed and the simulation results are found to agree well with the experimental data. Plasmonically enhanced LS-NS hold great promise for large-area photosensing applications extending from UV to IR including windows and facades of smart buildings.

A-Si:H/c-Si Heterojunction Solar Cell Performances Using 50 μm Thin Wafer Substrate

이종접합태양전지는 단결정 실리콘 기판 표면에 고품질 비정질 실리콘층을 적층함으로써 전기의 근원인 전하의 재결합 손실을 줄여 높은 개방전압을 얻을 수 있다는 특징이 있다. 초박형 태양전지는 기존 태양전지보다 뛰어난 광전변환 특성(Photovoltaic characteristic)을 가지고 두께가 얇아 제품 형상 시 자유도가 높아진다. 본 논문에서는 n-type Bare wafer(160{sim}180{mu}m)를 이용하여 50{mu}m의 웨이퍼를 제작하였다. a-Si:H(p)_a-Si:H(i)_c-Si(n)의 광흡수층 구조를 성막하여 cell을 제작하였다. 그 결과 Voc(Open Circuit Voltage)가 0.666, Jsc(Short-Circuit Current)가 34.77, FF(Fill Factor) 69.413, Efficency 16.07%를 달성했다.

Long-Period Gratings Based on Surface Plasmon Polariton Waveguides in Fluorinated Polymer

This letter presents devices composed of long-period gratings and long-range surface plasmon polariton waveguides. The waveguide is formed by embedding a thin Au stripe in a low optical absorption perfluorocyclobutane polymer. The grating structure is created on a corrugated silica substrate and then transferred to the polymer layer, producing a periodic modulation of refractive index along the waveguide. Strong grating resonances are observed. The coupling mechanism and the device thermal dependence are examined.

Electronic properties, doping, and defects in chlorinated silicon nanocrystals

Silicon nanocrystals with diameters between 1 and 3 nm and surfaces passivated by chlorine or a mixture of chlorine and hydrogen were modeled using density functional theory, and their properties compared with those of fully hydrogenated nanocrystals. It is found that fully and partially chlorinated nanocrystals are stable, and have higher electron affinity, higher ionization energy and lower optical absorption energy threshold. As the hydrogenated silicon nanocrystals, chlorinated silicon nanocrystals doped with phosphorus or boron require a high activation energy to transfer an electron or hole, respectively, to undoped silicon nanocrystals. The electronic levels of surface dangling bonds are similar for both types of surface passivation, although in the chlorinated silicon nanocrystals some fall outside the narrower energy gap.

Antireflection Coating Design for Triple-Junction III–V/Ge High-Efficiency Solar Cells Using Low Absorption PECVD Silicon Nitride

The design of antireflection coating (ARC) for multijunction solar cells is challenging due to the broadband absorption and the need for current matching of each subcell. Silicon nitride, which is deposited by plasma-enhanced chemical vapor deposition (PECVD) using standard conditions, is widely used in the silicon wafer solar cell industry but typically suffers from absorption in the short-wavelength range. We propose the use of silicon nitride deposited by low-frequency PECVD (LFSiN) optimized for high refractive index and low optical absorption as a part of the ARC design for III–V/Ge triple-junction solar cells. This material can also act as a passivation/encapsulation coating. Simulations show that the SiO <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{{\rm 2}}$</tex> </formula> /LFSiN double-layer ARC can be very effective in reducing the reflection losses over the wavelength range of the limiting subcell for top subcell-limited, as well as middle subcell-limited, triple-junction solar cells. We also demonstrate that the structure’s performance is stable over expected variations in the layer parameters (thickness and refractive index) in the vicinity of the optimal values.

Growth and optical properties of Sn–Si nanocomposite thin films

The growth and optical properties of nanocomposite thin films comprising of nanocrystalline Sn and Si are reported. The nanocomposite films are produced by thermal annealing of bilayers of Sn and Si deposited on borosilicate glass substrates at various temperatures from 300 to 500 °C for 1 h in air. X-ray diffraction reveals that the as-deposited bilayers consist of nanocrystalline Sn films with a crystallite size of 30 nm, while the Si thin films are amorphous. There is onset of crystallinity in Si on annealing to 300 °C with the appearance of the (111) peak of the diamond cubic structure. The crystallite size of Si increases from 5 to 18 nm, whereas the Sn crystallite size decreases with increase in annealing temperature. Significantly, there is no evidence for any Sn–Si compound, and therefore it is concluded that the films are nanocomposites of Sn and Si. Measured spectral transmittance curves show that the films have high optical absorption in the as-deposited form which decreases on annealing to 300 °C. The films show almost 80 % transmission in the visible-near infrared region when the annealing temperature is increased to 500 °C. There is concomitant decrease in refractive index from 4.0, at 1750 nm, for the as-deposited film, to 1.88 for the film annealed at 500 °C. The optical band gap of the films increases on annealing (from 1.8 to ~2.9 eV at 500 °C). The Sn-Si nanocomposites have high refractive index, large band gap, and low optical absorption, and can therefore be used in many optical applications.

A photoacoustic imager with light illumination through an infrared-transparent silicon CMUT array

A novel hardware design and preliminary experimental results for photoacoustic imaging are reported in this paper. This imaging system makes use of an infrared-transparent capacitive micromachined ultrasonic transducer (CMUT) chip for ultrasound reception and illuminates the image target through the CMUT array. The cascaded arrangement between the light source and transducer array allows for a more compact imager head and results in more uniform illumination. Taking advantage of the low optical absorption coefficient of silicon in the near infrared spectrum as well as the broad acoustic bandwidth that CMUTs provide, an infrared-transparent CMUT array has been developed for ultrasound reception. The center frequency of the polysilicon-membrane CMUT devices used in this photoacoustic system is 3.5 MHz, with a fractional bandwidth of 118% in reception mode. The silicon substrate of the CMUT array has been thinned to 100 μm and an antireflection dielectric layer is coated on the back side to improve the infrared-transmission rate. Initial results show that the transmission rate of a 1.06-μm Nd:Yag laser through this CMUT chip is 12%. This transmission rate can be improved if the thickness of silicon substrate and the thin-film dielectrics in the CMUT structure are properly tailored. Imaging of a metal wire phantom using this cascaded photoacoustic imager is demonstrated.

Facile and Effective Post-Production Separation of Single-Walled Carbon Nanotubes with Paired Aromatic Molecules: A Molecular Tweezers Approach

As-produced single-walled carbon nanotubes (SWNTs) are mostly metallic and semiconducting mixtures. The population of semiconducting SWNTs in the as-produced mixtures is generally much higher than that of their metallic counterparts. Therefore, postproduction separation of metallic and semiconducting SWNTs is important to many potential uses of the nanotubes, especially those demanding higher electrical conductivity yet lower optical absorption. Because planar aromatic species are known to have somewhat different interactions with metallic and semiconducting SWNTs, in this work a molecule with a pair of planar aromatic moieties was specifically designed and synthesized to exaggerate the difference in its noncovalent functionalization and solubilization of the two types of nanotubes. As demonstrated, the molecule exhibited significant selectivity toward semiconducting SWNTs in the solubilization to allow the convenient harvesting of bulk metallic and semiconducting fractions in high purities. The results suggest that molecules with paired aromatic species or essentially molecular tweezers may represent a new class of agents for more effective and also relatively simple postproduction bulk separation of metallic and semiconducting SWNTs.

Fabrication of High-Quality Boron-Doped Microcrystalline Silicon Thin Films on Several Types of Substrates

Boron-doped (p-type) μc-Si:H thin film window layers are widely used in silicon thin film solar cell fabrication due to their low optical absorption and high doping efficiency compared with their amorphous counterparts. In this paper, the layer-by-layer method is investigated to fabricate high-quality thin p layers (< 30nm). To investigate the influence of the type of substrate, p layers are deposited onto planar glass and textured glass (bare or coated with ZnO:Al thin film). The crystalline volume fraction (crystallinity) and conductivity of the deposited p layers are measured to characterise the films’ properties. We find that by increasing the number of hydrogen plasma treatment steps and their duration, the films’ crystallinity and electrical properties improve strongly. We also find that the substrate surface roughness strongly influences the film properties. This may be due to the different nucleation conditions resulting from the various substrate topographies.