Sub-bandgap Region Research Articles

Diameter and gap controlled fabrication of periodic SiMW/SiNW arrays: A broadband absorber for high efficiency silicon solar cell

Vertically aligned and periodic silicon micro/nano-wire (SiMW/SiNW) arrays of different diameters were synthesized by combining lithography-free silica particle patterning, reactive ion etching (RIE), and modified metal-assisted chemical etching (MACE) methods. All required silica particles were synthesized, and process parameters were optimized to fabricate sub 50 nm SiNW to large size SiMW with the controlled inter-wire gap. Fabricated SiMW/SiNW array samples show a significant reduction of reflectance (7–9 %) in the silicon absorption band (300 nm–1000 nm). Additional absorption of 25–33 % compared to the planar silicon is observed in the sub-bandgap region of silicon (1100–2000 nm) for these samples, which signifies the effect of field confinement within these structures. The estimated short circuit current density (JSC) using the experimental absorption spectrum of SiMW is 49.96 mA/cm2, which is 16 % higher than the Lambertian limit of ∼43 mA/cm2. This result can lead to the theoretical efficiency of the solar cell up to 28.73 % without the inclusion of the surface passivation effect.

Transport mechanisms in hyperdoped silicon solar cells

According to intermediate band (IB) theory, it is possible to increase the efficiency of a solar cell by boosting its ability to absorb low-energy photons. In this study, we used a hyperdoped semiconductor approach for this theory to create a proof of concept of different silicon-based IB solar cells. Preliminary results show an increase in the external quantum efficiency (EQE) in the silicon sub-bandgap region. This result points to sub-bandgap absorption in silicon having not only a direct application in solar cells but also in other areas such as infrared photodetectors. To establish the transport mechanisms in the hyperdoped semiconductors within a solar cell, we measured the J–V characteristic at different temperatures. We carried out the measurements in both dark and illuminated conditions. To explain the behavior of the measurements, we proposed a new model with three elements for the IB solar cell. This model is similar to the classic two-diodes solar cell model but it is necessary to include a new limiting current element in series with one of the diodes. The proposed model is also compatible with an impurity band formation within silicon bandgap. At high temperatures, the distance between the IB and the n-type amorphous silicon conduction band is close enough and both bands are contacted. As the temperature decreases, the distance between the bands increases and therefore this process becomes more limiting.

Direct Observation of the Charge Transfer States from a Non-Fullerene Organic Solar Cell with a Small Driving Force.

For organic solar cells (OSCs), the charge generation mechanism and the recombination loss are heavily linked with charge transfer states (CTS). Measuring the energy of CTS (ECT) by the most widely used technique, however, has become challenging for the non-fullerene-based OSCs with a small driving force, resulting in difficulty in the understanding of OSC physics. Herein, we present a study of the PM6:Y6 bulk heterojunction. It is demonstrated that electro-absorption can not only reveal the dipolar nature of Y6 but also resolve the morphology-dependent absorption signal of CTS in the sub-bandgap region. The device with the optimum blending weight ratio shows an ECT of 1.27 eV, which is confirmed by independent measurements. Because of the charge transfer characteristics of Y6, the charge generation at PM6:Y6 interfaces occurs efficiently under a small but non-negligible driving force of 0.14 eV, and the total recombination loss is as low as 0.43 eV.

Photoconduction of Polar and Nonpolar Cuts of Undoped Sr0.61Ba0.39Nb2O6 Single Crystals

In the last two decades, variably doped strontium barium niobate (SBN) has attracted a lot of scientific interest mainly due to its specific non-linear optical response. Comparably, the parental compound, i.e., undoped SBN, appears to be less studied so far. Here, two different cuts of single-crystalline nominally pure strontium barium niobate in the composition Sr0.61Ba0.39Nb2O6 (SBN61) are comprehensively studied and analyzed with regard to their photoconductive responses. We present conductance measurements under systematically varied illumination conditions along either the polar z-axis or perpendicular to it (x-cut). Apart from a pronounced photoconductance (PC) already under daylight and a large effect upon super-bandgap illumination in general, we observe (i) distinct spectral features when sweeping the excitation wavelength over the sub-bandgap region as then discussed in the context of deep and shallow trap states, (ii) extremely slow long-term relaxation for both light-on and light-off transients in the range of hours and days, (iii) a critical dependence of the photoresponse on the pre-illumination history of the sample, and (iv) a current–voltage hysteresis depending on both the illumination and the electrical-measurement conditions in a complex manner.

Ultrafast transient sub-bandgap absorption of monolayer MoS2

The light–matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications, which is intrinsically determined by the bandgap of the materials involved. To extend the applications beyond the bandgap limit, it is of great significance to study the light–matter interaction below the material bandgap. Here, we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from ~0.86 µm to 1.4 µm. Even though this spectral range is below the bandgap, we observe a significant absorbance enhancement up to ~4.2% in the monolayer molybdenum disulfide (comparable to its absorption within the bandgap region) due to pump-induced absorption by the excited carrier states. The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states (i.e., real carrier states in the short-wavelength region of ~<1 µm, and exciton states in the long wavelength region of ~>1 µm). Our results elucidate the fundamental understanding regarding the optical properties, excited carrier states, and carrier dynamics in the technologically important near-infrared region, which potentially leads to various photonic and optoelectronic applications (e.g., excited-state-based photodetectors and modulators) of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.

Variable-period oscillations in optical spectra in sub-bandgap long wavelength region: signatures of new dispersion of refractive index?

Variable-period oscillations (VPOs) in various optical spectra in sub-bandgap long wavelength region, including reflection, transmission, absorption, and even luminescence spectra, are a frequently observed phenomenon in various semiconducting and dielectric films. These functional films include nitrides, oxides, silicides, sulfides, and perovskites. Although the phenomenon is widely known to be caused by optical interference, a generalized analytical model for it has not yet been established, probably due to both varying oscillation period and amplitude. In this article, we attempt to develop such a model by introducing a new concise dispersion of the sub-bandgap refractive index, i.e. containing a frictional component. In particular, we show that the VPOs in the reflectance and corresponding transmission shall have reverse maxima and minima in intensity, giving a self-consistent explanation to the existing experimental results in literature. Furthermore, the present analytical method is proven to be accurate for finding optical constants and thicknesses of films. Finally, the rationality of the new dispersion is further argued in terms of a generalized refractive index of crystal with lattice anomalies (e.g. defected and strained lattice), which was derived by Gonçalves et al from the fractional Drude–Lorentz model. In addition, the equivalency between the VPOs and Newton’s rings in energy space is discussed based on the present model.

Understanding the origin of broad-band emission in CH3NH3PbBr3

Broad-band emissions related to self-trapped excitons in the sub-bandgap region (600–800 nm) in organic–inorganic hybrid perovskites can be controlled using suitable synthesis procedure.

Simple method to extract extinction coefficients of films with the resolution of 10-5 using just transmission data and application to intermolecular charge-transfer absorption in an exciplex-forming organic film.

A simple method is presented to determine the thickness, refractive index and extinction coefficient of a film with the resolutions of ±1 nm, ±2 × 10-3, and 6 × 10-5, respectively. The method requires only ultraviolet-visible-near-infrared spectroscopy measurement of the transmittance of films with thicknesses of a few micrometers. A very small extinction coefficient of intermolecular charge transfer (CT) absorption of an exciplex-forming organic film is measured in the sub-bandgap wavelength region. This is to demonstrate that the CT absorption with extinction coefficient in the range of 10-3 to 10-5 can be measured indeed using the method, which is in the same resolution as photothermal deflection spectroscopy and Fourier transform photocurrent spectroscopy. The simplicity and feasibility of the proposed approach is expected to promote active study of intermolecular CT absorption in exciplex-forming films.

Temperature-Dependent Optical Band Gap in CsPbBr3, MAPbBr3, and FAPbBr3 Single Crystals.

Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr3, CsPbBr3 and for the first time FAPbBr3) by spectroscopic ellipsometry in the range of 1–5 eV while varying the temperature from 183 to 440 K. An extremely low absorption coefficient in the sub-band gap region was found, indicating the high optical quality of all three crystals. We extracted the band gap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions, i.e., orthorhombic–tetragonal and tetragonal–cubic, show distinct optical behaviors, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting in tailoring the value of the band gap at a given temperature, whereas differences in the thermal expansion affect the slope of the band gap trend as a function of temperature.

Effect of impact ionization on the performance of quantum ratchet embedded intermediate band solar cell: An extensive simulation study

An investigation has been carried out on the performance of quantum ratchet embedded intermediate band solar cell by considering impact ionization in the sub-bandgap region. The architecture under investigation is modelled using Silvaco ATLAS TCAD. More emphasis has been provided in the analysis of important paradigms like quantum efficiency, spectral response, generated electric field and recombination involved in the cell, which are the sole reason behind the low efficiency (Eff), fill factor (FF), short circuit current density (Jsc) and open circuit voltage (Voc). The use of impact ionization solves the lower Jsc issue by generating more than one electron-hole pair by the use of single photon, while the use of ratchet band solves the voltage preservation problem by enhancing the carrier lifetime of the minority generated photocarriers. With this advanced feature, this architecture provides a higher efficiency of > 80%.

Effects of helium annealing in low-temperature and solution-processed amorphous indium-gallium-zinc-oxide thin-film transistors

In this paper, low-temperature, solution-processed amorphous indium-gallium-zinc-oxide thin-film transistors (a-IGZO TFTs) with annealing temperatures as low as 300°C were fabricated using different annealing gases of He, N2, and O2 and their electrical characteristics and long-term stability were analyzed. The field-effect mobility was improved by nearly two times for the a-IGZO TFTs annealed in helium ambient (He sample) in comparison with those annealed in oxygen and nitrogen ambient environments. The subthreshold slope and threshold voltage were also improved for the a-IGZO TFTs annealed in the helium ambient environment due to the low defect density of states in the sub-bandgap region. However, X-ray photoelectron spectroscopy measurements indicate that the electrical characteristics of the low-temperature solution-processed a-IGZO TFTs show severe channel-length dependencies due to the oxygen vacancy variations in the active region. In addition, long-term stability measurements up to seven days reveal that due to the inherently low electron density and high defect density of states in the active region, an increase in the carrier density in the active region induces a large negative shift of the threshold voltage without changing the field-effect mobility or subthreshold slope. It is understood that low-temperature solution-processed a-IGZO TFTs strongly depend on the formation of oxygen vacancies, which can be resolved by controlling the indium mole fraction under a helium annealing condition.

Ultrafast light-induced softening of chalcogenide thin films above the rigidity percolation transition

Little is known about the role of network rigidity in light-induced structural rearrangements in network glasses due to a lack of supporting experiments and theories. In this article, we demonstrate for the first time the ultrafast structural rearrangements manifested as induced absorption (IA) over a broad spectral range in a-GexAs35-xSe65 thin films above the mean-field rigidity percolation transition, quantified by the mean coordination number ⟨r⟩ = 2.40. The IA spectrum arising from self-trapped excitons induced structural rearrangements by softening the glass network that strikingly reveals two relaxation mechanisms which differ by one order of magnitude. The fast kinetics of electron-lattice interaction occurs within 1 ps, exhibits a weak dependence on rigidity, and dominates in the sub-bandgap region. In a stark contrast, the slow kinetics is associated with the structural changes in the bandgap region and depends strongly on network rigidity. Our results further demonstrate that amplitude of IA scales a linear relationship with excitation fluence which provides a unique way to induce structural rearrangements in an over-coordinated network to exploit it for practical purposes. Our results modify the conventional concept of rigidity dependence of light-induced effects in network glasses, when excited with an ultrafast laser.

Annealing induced photosensitivity modulation of zinc selenide thin film in the sub-band gap optical absorption region

The photosensitivity of the chemical bath deposited zinc selenide (ZnSe) thin film of bandgap 2.8 eV air annealed at different temperatures has been studied under sub-band gap optical illuminations of photon energies 1.77, 1.91, and 2.07 eV, respectively. The study reveals significant and systematic changes in the photocurrent even when the photon energy is restricted well below the band gap energy of the material. In addition, the photocurrent decreases more or less exponentially with the annealing temperature, although the absorption coefficients of the annealed samples in the sub-band gap region are seen to be enhanced compared to the as-deposited sample. The XRD measurements show the cubic phase of ZnSe, accompanying additionally the orthorhombic ZnSeO3 crystallites in the deposited film. The study further reveals comparatively stronger crystalline improvement of the ZnSeO3 crystals than the cubic ZnSe crystals upon annealing. The observed photosensitivity of the film is compared to that of the cadmium sulphide film deposited by a chemical bath deposition technique and found to be sharply contrasting in behavior. We attribute the unusual photosensitivity of the ZnSe film to ZnSeO3 crystallites which act simultaneously as self generators and absorbers of photoelectrons in the ZnSe crystals, without effectively contributing to the overall photoconductivity of the material.

A simple method to measure intermolecular charge-transfer absorption of organic films

Abstract Intermolecular charge transfer (CT) complexes of organic molecules are actively researched due to their wide use and strong influence on organic optoelectronic device performance. In previous studies, exciplexes, excited-state intermolecular CT complexes, failed to show absorption in ultraviolet-visible-near-infrared absorption spectroscopy measurements. Recently, however, direct exciplex CT absorption from the ground state of donor-acceptor mixed films was detected in the sub-bandgap region by Fourier transform photocurrent spectroscopy (FTPS) and photothermal deflection spectroscopy (PDS) measurements. Here, we report a simple method to measure intermolecular CT absorption that combines simple reflection–transmission and ellipsometry measurements of thick films to determine the extinction coefficient for the small-molecular CT state. The extinction coefficients extracted from different film thicknesses were consistent, up to 10−4 in the sub-bandgap wavelength region, comparable to that obtained with PDS executed on thin films.

Sputtered boron indium oxide thin-film transistors

Boron indium oxide (BIO) is studied for thin-film transistor (TFT) channel layer applications. Sputtered BIO thin films exhibit an amorphous phase over a wide range of B2O3/In2O3 ratios and remain amorphous up to 500°C. The band gap decreases linearly with decreasing boron content, whereas device performance generally improves with decreasing boron content. The best amorphous BIO TFT exhibits a field-effect mobility of 10cm2V−1s−1, turn-on voltage of 2.5V, and sub-threshold swing of 0.72V/dec. Decreasing the boron content to 12.5% leads to a polycrystalline phase, but further increases the mobility up to 20–40cm2V−1s−1. TCAD simulation results suggest that the reason for higher performance after increasing the anneal temperature from 200 to 400°C is due to a lower defect density in the sub-bandgap region of the BIO channel layer.

Ultrafast photoinduced anisotropy in GeSe2thin film.

In this Letter, we demonstrate for the first time that anisotropy can be induced at ultrafast time scales in an otherwise isotropic a-GeSe2 thin film using polarized femtosecond light. This photoinduced anisotropy (PA) spans the bandgap to the sub-bandgap region and self-annihilates over picosecond time scales. The ultrafast decay rate of PA is a clear indication that the observed effect is due to photoinduced transient defects in the sub-bandgap region and associated structural rearrangement in the near-bandgap region.

Sn-doped CdTe as promising intermediate-band photovoltaic material

The formation energies, charge transition levels and quasiparticle defect states of several tin-related impurities are investigated within the DFT + GW formalism. The optical spectrum obtained from the solution of the Bethe–Salpeter equation shows that the absorption strongly increases in the sub-bandgap region after doping, suggesting a two-step photoexcitation process that facilitates transitions from photons with insufficient energy to cause direct transitions from the valence to the conduction band via an intermediate-band. We propose Sn-doped CdTe as a promising candidate for the development of high-efficiency solar cells, which could potentially overcome the Shockley–Queisser limit.

Yb- and Er-doped fiber laser Q-switched with an optically uniform, broadband WS2 saturable absorber.

We demonstrate a ytterbium (Yb) and an erbium (Er)-doped fiber laser Q-switched by a solution processed, optically uniform, few-layer tungsten disulfide saturable absorber (WS2-SA). Nonlinear optical absorption of the WS2-SA in the sub-bandgap region, attributed to the edge-induced states, is characterized by 3.1% and 4.9% modulation depths with 1.38 and 3.83 MW/cm2 saturation intensities at 1030 and 1558 nm, respectively. By integrating the optically uniform WS2-SA in the Yb- and Er-doped laser cavities, we obtain self-starting Q-switched pulses with microsecond duration and kilohertz repetition rates at 1030 and 1558 nm. Our work demonstrates broadband sub-bandgap saturable absorption of a single, solution processed WS2-SA, providing new potential efficacy for WS2 in ultrafast photonic applications.

Ultrafast defect dynamics: A new approach to all optical broadband switching employing amorphous selenium thin films

Optical switches offer higher switching speeds than electronics, however, in most cases utilizing the interband transitions of the active medium for switching. As a result, the signal suffers heavy losses. In this article, we demonstrate a simple and yet efficient ultrafast broadband all-optical switching on ps timescale in the sub-bandgap region of the a-Se thin film, where the intrinsic absorption is very weak. The optical switching is attributed to short-lived transient defects that form localized states in the bandgap and possess a large electron-phonon coupling. We model these processes through first principles simulation that are in agreement with the experiments.

Enhanced Performance of Polymeric Bulk Heterojunction Solar Cells via Molecular Doping with TFSA.

Organic solar cells based on bis(trifluoromethanesulfonyl)amide (TFSA, [CF3SO2]2NH) bulk doped poly[N-9''-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT):C71-butyric acid methyl ester (PC71BM) were fabricated to study the effect of molecular doping. By adding TFSA (0.2-0.8 wt %, TFSA to PCDTBT) in the conventional PCDTBT:PC71BM blends, we found that the hole mobility was increased with the reduced series resistance in photovoltaic devices. The p-doping effect of TFSA was confirmed by photoemission spectroscopy that the Fermi level of doped PCDTBT shifts downward to the HOMO level and it results in a larger internal electrical field at the donor/acceptor interface for more efficient charge transfer. Moreover, the doping effect was also confirmed by charge modulated electroabsorption spectroscopy (CMEAS), showing that there are additional polaron signals in the sub-bandgap region in the doped thin films. With decreased series resistance, the open-circuit voltage (Voc) was increased from 0.85 to 0.91 V and the fill factor (FF) was improved from 60.7% to 67.3%, resulting in a largely enhanced power conversion efficiency (PCE) from 5.39% to 6.46%. Our finding suggests the molecular doping by TFSA can be a facile approach to improve the electrical properties of organic materials for future development of organic photovoltaic devices (OPVs).