Uniform Band Gap Research Articles

Effect of defect density, bandgap profile, material composition, thickness, and doping density of the absorber layer on the performance of thin film solar cell based on antimony selenosulfide Sb2(Se1-ySy)3

This article deals with the optimization by simulation of a graded bandgap thin film solar cell based on antimony selenosulfide Sb2(Se1-ySy)3 having the following structure: Front contact/n-ZnO/i-ZnO/p-SbSSe/n-CdS/Back contact. The simulation is performed using SCAPS-1D software. The optimization process includes optimizing the bulk defect density, bandgap profile, material composition, thickness, and doping density of the absorber layer of thin film solar cell based on antimony selenosulfide Sb2(Se1-ySy)3. We found that for a bulk defect density below 1013 cm-3 , using an absorber material with a graded bandgap profile leads to an efficiency of 25.33 % (For a bulk defect density of 1010 cm-3 ) higher than that with a uniform bandgap profile. However, for a bulk defect density of 1013 cm-3 , both profiles provide almost the same maximum solar cell conversion efficiencies of about 13.6 %. Ultimately, for a bulk defect density above 1013 cm-3 , the graded bandgap profile is not useful, and a maximum solar cell conversion efficiency of 10.5 % (For a bulk defect density of 1014 cm-3 ) is achieved with a uniform bandgap profile. These optimization results help to improve the efficiency of low-cost fabricated thin-film solar cells.

A facile approach for sulphur and nitrogen co-doped carbon nanodots to improve photothermal eradication of drug-resistant bacteria

In this study, we produced S, N co-doped CNDs (SN@CNDs) by using dimethyl sulfoxide (DMSO) and formamide (FA) as single sources of S and N, respectively. We varied the S/N ratios by adjusting the volume ratios of DMSO and FA and investigated their effect on the red-shift of the CNDs' absorption peak. Our findings demonstrate that SN@CNDs synthesized using a volume ratio of 5:6 between DMSO and FA exhibit the most significant absorption peak redshift and enhanced near-infrared absorption performance. Based on comparative analysis of the particle size, surface charge, and fluorescence spectrum of the S@CNDs, N@CNDs, and SN@CNDs, we propose a possible mechanism to explain the change of optical properties of CNDs due to S, N doping. Co-doping creates a more uniform and smaller band gap, resulting in a shift of the Fermi level and a change in energy dissipation from radioactive to non-radiative decay. Importantly, the as-prepared SN@CNDs exhibited a photothermal conversion efficiency of 51.36% at 808 nm and demonstrated exceptional photokilling effects against drug-resistant bacteria in both in vitro and in vivo experiments. Our facile method for synthesizing S and N co-doped CNDs can be extended to the preparation of other S and N co-doped nanomaterials, potentially improving their performance.

Simulation of new thin film Zn(O,S)/CIGS solar cell with bandgap grading

Copper-Indium-Gallium-diSelenide (CIGS) thin film solar cell is a promising candidate for energy harvesting because of its high absorption coefficient and low cost compared to silicon-based solar cells. Absorber layer bandgap grading is a suitable method to improve CIGS thin film solar cell performance. Bandgap grading leads to a decrease in the recombination rate at the rear surface, which increases the open circuit voltage. Furthermore, bandgap grading improves the short circuit current due to the enhancement of collection probability. This paper introduces various routes for improving the performance of thin film CIGS solar cells by using bandgap grading. As a first step, both the bandgap energy and the thickness of the CIGS absorber layer of a uniform bandgap profile are optimized to get the best performance. Simulation is performed using SCAPS software and optimization results show that CIGS absorber layer with a bandgap of 1.2 eV and a thickness of 0.7 μm achieves a 22.48% efficiency. Then, bandgap grading with a parabolic distribution of various profiles is investigated and compared. It is found that with a parabolic double bandgap grading profile, which is a combination of front and back grading, an efficiency of up to 24.16% is achieved. This improvement is obtained using a gallium composition ratio of 0.1 for the minimal bandgap at 0.1 μm and 0.13 μm from the back contact and front contact, respectively. This result represents a 7.47% improvement compared to the baseline structure of a CIGS solar cell.

Corrigendum to “Using whole cottonseed to replace dried distillers grains plus solubles and prairie hay in finishing beef cattle rations balanced for physically effective neutral detergent fiber” (Appl. Anim. Sci. 38:417–432)

The modulation transfer function (MTF) of graded band-gap AlGaAs/GaAs reflection-mode photocathodes was determined using two-dimensional Poisson and continuity equations through numerical method. Based on the MTF model, we calculated the theoretical MTF of graded and uniform band-gap reflection-mode photocathodes. We then analyzed the effects of Al composition, wavelength of incident photon, and thicknesses of AlGaAs and GaAs layer on the resolution. Calculation results show that graded band-gap structures can increase the resolution of reflection-mode photocathodes. When the spatial frequency is 800 lp/mm and wavelength is 600 nm, the resolution of graded band-gap photocathodes generally increases by 15.4–29.6%. The resolution improvement of graded band-gap photocathodes is attributed to the fact that the built-in electric field in graded band-gap photocathodes reduces the lateral diffusion distance of photoelectrons.

The impact of Ga and S concentration and gradient in Cu(In,Ga)(Se,S)2 solar cells

As one of the leading photovoltaic materials, Cu(In,Ga)(Se,S)2 has two distinct advantages over other candidates. First, its conduction band and valence band could be independently tuned by adjusting the Ga/(Ga + In) and S/(S + Se), respectively. Next, its composition distribution could be feasibly tuned by controlling the thin film deposition process. These merits have been utilized in fabricating the Cu(In,Ga)(Se,S)2 solar cells with world record efficiency. However, these features have not yet been satisfactorily simulated. In order to understand the mechanism of high-performance Cu(In,Ga)(Se,S)2 solar cells, we obtained the tunable bandgap of absorber layer and suitable conduction band offset between absorber layer and buffer layer via Ga and S concentration and gradient, by combining both first-principle calculations and numerical simulations with Poisson equation and the continuity equation. In particular, compared with other photovoltaic materials, the bandgap of Cu(In,Ga)(Se,S)2 could be tuned flexibly across the layer thickness to realize spatial energy band optimization with double-concentration-grading absorber. The efficiency of 23.29% was obtained based on theoretical calculations and numerical simulations, which is almost the same as the reported result, in the considerations of (i) optimum bandgap of absorber, (ii) band energy offset, (iii) double (Ga- and S-) concentration grading, and (iv) low defect density in the absorber layer. In addition, we obtained a Cu(In,Ga)(Se,S)2 solar cell with a high S content uniform band gap efficiency of 23.53%. Therefore, we provided an understanding for tunable engineered band energy research in high-efficient Cu(In,Ga)(Se,S)2 solar cells.

Bottom‐Up Synthesized Nanoporous Graphene Transistors

AbstractNanoporous graphene (NPG) can exhibit a uniform electronic band gap and rationally‐engineered emergent electronic properties, promising for electronic devices such as field‐effect transistors (FETs), when synthesized with atomic precision. Bottom‐up, on‐surface synthetic approaches developed for graphene nanoribbons (GNRs) now provide the necessary atomic precision in NPG formation to access these desirable properties. However, the potential of bottom‐up synthesized NPG for electronic devices has remained largely unexplored to date. Here, FETs based on bottom‐up synthesized chevron‐type NPG (C‐NPG), consisting of ordered arrays of nanopores defined by laterally connected chevron GNRs, are demonstrated. C‐NPG FETs show excellent switching performance with on–off ratios exceeding 104, which are tightly linked to the structural quality of C‐NPG. The devices operate as p‐type transistors in the air, while n‐type transport is observed when measured under vacuum, which is associated with reversible adsorption of gases or moisture. Theoretical analysis of charge transport in C‐NPG is also performed through electronic structure and transport calculations, which reveal strong conductance anisotropy effects in C‐NPG. The present study provides important insights into the design of high‐performance graphene‐based electronic devices where ballistic conductance and conduction anisotropy are achieved, which could be used in logic applications, and ultra‐sensitive sensors for chemical or biological detection.

Improvement of power conversion efficiency by a stepwise band-gap structure for silicon quantum dot solar cells

As a promising next-generation solar cell, the power conversion efficiency of a silicon quantum dot (Si-QD) solar cell is still low. In this work, the band-gap structure of a Si-QD layer was modified to improve the power conversion efficiency of a Si-QD solar cell. A stepwise band-gap Si-QD (SB Si-QD) layer with a high bandgap top layer (about 2.22 eV) and a low band-gap bottom layer (about 1.98 eV) was grown on a Si (100) substrate. The open circuit voltage and short circuit current were improved by band-gap engineering of the Si-QD absorption layer. As a result, the power conversion efficiency of the SB Si-QD solar cell increased from 16.50% to 17.50%, compared to that of a Si-QD solar cell with a uniform band gap. This results will provide a guide to design advanced Si-QD solar cells by considering the band-gap structure in the Si-QD absorption layer.

Bandgap tuning of two-dimensional materials by sphere diameter engineering.

Developing a precise and reproducible bandgap tuning method that enables tailored design of materials is of crucial importance for optoelectronic devices. Towards this end, we report a sphere diameter engineering (SDE) technique to manipulate the bandgap of two-dimensional (2D) materials. A one-to-one correspondence with an ideal linear working curve is established between the bandgap of MoS2 and the sphere diameter in a continuous range as large as 360 meV. Fully uniform bandgap tuning of all the as-grown MoS2 crystals is realized due to the isotropic characteristic of the sphere. More intriguingly, both a decrease and an increase of the bandgap can be achieved by constructing a positive or negative curvature. By fusing individual spheres in the melted state, post-synthesis bandgap adjustment of the supported 2D materials can be realized. This SDE technique, showing good precision, uniformity and reproducibility with high efficiency, may further accelerate the potential applications of 2D materials.

In Situ Observation of Emission Behavior during Anion-Exchange Reaction of a Cesium Lead Halide Perovskite Nanocrystal at the Single-Nanocrystal Level.

Postsynthesis anion-exchange reaction of cesium lead halide (CsPbX3; X = Cl, Br, and I) perovskite nanocrystals (NCs) has emerged as a unique strategy to control band gap. Recently, the partially anion-exchanged CsPb(Br/I)3 NC was reported to form an inhomogeneously alloyed heterostructure, which could possibly form some emission sites depending on the halide composition in the single NC. In this work, we observed the in situ emission behavior of single CsPb(Br/I)3 NCs during the anion-exchange reaction. Photon-correlation measurements of the single NCs revealed that the mixed halide CsPb(Br/I)3 NC exhibited single-photon emission. Even when irradiated with an intense excitation laser, the single NC exhibited single-photon emission with a photoluminescence spectrum of a single peak. These results suggested that the heterohalide compositions of the CsPb(Br/I)3 NC do not form any emission sites with different band gap energies; instead, the NC forms emission sites with uniform band gap energy as a whole NC via quantum confinement.

Optimization bandgap gradation structure simulation of Cu2Sn1−xGexS3 solar cells by SCAPS

The bandgap of the ternary chalcogenide Cu2SnS3 (CTS) can be tuned by alloying with Ge. The performance of CTS based solar cell devices with varies band gap grading profiles have been simulated with respect to the solar cells with a uniform band gap absorbing layer. It was revealed that band gap engineering geared to controlling the grading profile in the absorber layer lead to performance enhancement comparing to that of a device without band gap grading. Moreover, bandgap profiles with various back metal working function (φm) were simulated. An over 4–5% efficiency improvement was obtained due to the increasing φm with different band gap grading profile. The optimum PCE of 15.65%, 19.03% and 19.9% have been obtained with uniform, single and double band gap structures respectively. Moreover, the effects of various defects density on solar cell properties were investigated and the results indicate that there is a threshold of 1 × 1016 cm−3 for both acceptor/donor and neutral defects. The depth profile of the carrier recombination rate was calculated to understand of the fundamental device physics. The result show that a great improvement of both Voc and Jsc was obtained in the back surface grading structure cell comparing to that of the uniform bandgap cell due to the additional quasi-electric field associated to the affinity (conduction band) variation with position benefitting the carrier collection and reducing the back surface recombination and bulk recombination typically characterized by the diffusion length. A slight enhancement of short-circuit current density without significantly sacrificing the open-circuit voltage was obtained in the double band gap grading structure comparing to the single back grading owing to an increasement of front grading within the SCR.

Numerical reproduction of a perovskite solar cell by device simulation considering band gap grading

Recent progress in halide perovskite solar cells exceeding 20% efficiency is remarkable. However, the understanding of device operation based on reasonable device model and physical parameters is not sufficient. In this study, experimental current density-voltage (J-V) characteristic and external quantum efficiency (EQE) of a 20%-efficient perovskite solar cell with mixed halide of I and Br in a literature were numerically reproduced by a device simulator called Solar Cell Capacitance Simulator (SCAPS), where device parameters extracted by fitting experimental J-V and EQE curves were used. It is demonstrated that the simulated EQE curve was not matched with the experimental one under the assumption of uniform band gap energy (Eg) resulting from a constant Br content throughout the perovskite absorber. On the other hand, the experimental J-V and EQE curves were reasonably reproduced by the simulation under the assumption of single-graded Eg of the absorber, resulting from the graded Br content. Moreover, the perovskite solar cell with Eg-grading absorber has theoretically showed higher open-circuit voltage and power conversion efficiency (PCE) than those with uniform Eg. The results are useful to not only improve the understanding of device operation but also indicate possibility of the further improvement of PCE using Eg-grading perovskite absorber.

CIGS thin film solar cells with graded-band gap fabricated by CIS/CGS bilayer and CGS/CIS/CGS trilayer systems

High efficiency CuIn1-xGaxSe2 (CIGS) thin film solar cells are usually deposited on Mo-coated soda-lime glass (SLG) substrates by the three-stage co-evaporation process which is intended to create double-graded band gap. It requires short total deposition time and rapid changing of substrate temperature in order to obtain the double-graded band gap which is quite difficult for high source-temperatures. When the total deposition time cannot be made short enough, uniform band gap is rather achieved. In this work, the depositions of CIS/CGS bilayer are employed in order to create back grading while CGS/CIS/CGS trilayers are for the double-grading, owing to non-uniformity and different diffusivity of In and Ga constituents in the films. The bilayers with back grading show the increasing short-circuit current density (Jsc) due to the assisting back surface field, but the open-circuit voltage (Voc) is relatively low due to the reduction of Ga content at the front surface. For the CGS/CIS/CGS trilayers, double grading and the increasing of Voc are observed when compared with the bilayers due to the increasing Ga content near the junction. The efficiencies of the devices fabricated from the CIS/CGS bilayer and CGS/CIS/CGS trilayer absorbers show the maximum value of 12.5% and 15.5%, respectively. The external quantum efficiency (EQE) of the bilayer and trilayer absorbers exhibit the enhancement in the long wavelengths compared to that of the three-stage process absorbers.

Simulation of optimum band structure of HTM-free perovskite solar cells based on ZnO electron transporting layer

Expensive gold electrode and hole transporting material (HTM) currently used in perovskite solar cells are the major economic constraints to the scaling-up of this promising technology. Designing high-efficiency perovskite solar cell with graded band-gap structure to replace HTM and with low-cost electrode is urgently needed for real cell production. In our work numerical simulation of perovskite solar cell with the configuration of FTO/ZnO/CH3NH3Pb(I1-xBrx)3/Carbon is performed using SCAPS-1D program. The band gap of CH3NH3Pb(I1-xBrx)3 absorber is tuned in the range of 1.5 eV to 2.3 eV by variation of the Br doping content. Both the uniform band gap and graded band gap absorber were examined. Based upon the simulated results, a promising efficiency of 17.89% can be realized with a back grading profile that consists of a graded thickness of 50 nm as well as 1.9 eV band gap at back surface. This work will provide guidelines for further efficiency enhancement of low cost perovskite solar cells.

Extending absorption of near-infrared wavelength range for high efficiency CIGS solar cell via adjusting energy band

The efficient photon harvesting in near infrared wavelength range is still a challenging problem for high performance Cu(In1-x, Gax)Se2 (CIGS) solar cell. Herein, adjusting the energy band distribution of CIGS solar cell could provide significant academic guidance for devices with superior output electric power. To understand the role of each functional layer, the optimal 3000 nm CIGS absorber layer with 1.3 eV bandgap and 30 nm CdS buffer layer were firstly obtained via simulating the uniform band-gap structures. By introducing CIGS absorber layer with a double grading Ga/(Ga+In) profile, the power conversion efficiency of the double gradient band gap cell is superior to that of uniform band-gap cell through extending absorption of near-infrared wavelength range. Upon optimization, the best power conversion efficiency of CIGS with a double gradient band gap solar cell is improved significantly to 24.90%, among the best values reported in literatures, which is an 8.17% relative increase compared with that of the uniform band-gap cell. Our findings provide a theoretical guide toward the design of high performance solar cells and enrich the understandings of the energy band engineering for developing of novel semiconductor devices.

Optimization and improvement of a front graded bandgap CuInGaSe2 solar cell

This paper reports simulations of gradual bandgap CIGS absorber and its impact on the characteristics of a solar cell. The bandgap of the CIGS absorber varies linearly and drops from Egmax (at the junction limit) to Egmin (in the vicinity of the rear contact).We introduce an effective absorption coefficient based on this variation. We will demonstrate that this gradual profile contributes to an improvement up to 171mV of the open circuit voltage Voc of the cell that is linked to the modification of the internal electrical field distribution within the absorber. However, a joint reduction of 1.50mA/cm2 of short circuit current density, Jsc, is observed. Overall, the conversion efficiency increases from 19.2%, for a uniform bandgap absorber structure, to 24.9% in that case of gradual bandgap. Additionally, we investigate the impact of absorber thickness and temperature on cell characteristics.

Modeling and simulation of band-gap profiling with planar heterojunction of hole-transporting layer-free perovskite solar cells

This study entailed modeling a perovskite absorber involving band-gap grading at the back of the absorber and double-grading profiles of hole-transporting layer-free perovskite solar cells. Device simulation based on continuity equations and Poisson’s equation was carried out by using AMPS-1D software. The optimum grading profile consisted of a band gap of 1.7 eV at the interface between the TiO2 and absorber with a graded thickness of 300 nm, uniform 1.5 eV of 50 nm, and back surface 2.1 eV with a graded thickness of 50 nm. The attained simulated efficiency was 22.68% (open-circuit voltage, Voc = 1.34 V; short-circuit current density, Jsc = 19.98 mA cm−2; fill factor, FF = 0.84), which is close to the uniform band gap of 1.5 eV of the whole absorber with a hole-transporting layer (Spiro-OMeTAD). This was mainly because of back grading forming a conduction band energy barrier to suppress the transportation of photo-generated electrons from the absorber to the back electrode, thereby improving carrier collection. The results indicate that the hole-transporting layer could be replaced by optimal band-gap profiling of the absorber, with near to no decayed performance of the perovskite solar cells.

Effects of graded band-gap structures on spectral response of AlGaAs/GaAs photocathodes.

The effects of AlGaAs/GaAs layer thickness and Al composition range on the spectral response and integral sensitivities of reflection-mode graded band-gap AlGaAs/GaAs photocathodes have been investigated and simulated. The experimental results demonstrate that the spectral response over the wavelength region of interest for graded band-gap photocathodes is greater than that for uniform band-gap cathodes, and the increase in long-wavelength response is more pronounced. These results can be attributed to the built-in electric field in the graded band-gap AlGaAs layer. We established a spectral response model of graded band-gap photocathodes based on the numerical solution of coupled Poisson and continuity equations. According to the model, we calculated the theoretical spectral response and sensitivities of graded band-gap cathodes, and found the optimum Al(x)Ga(1-x)As layer thicknesses are 6, 10, 16, and 22 μm for the reflection-mode cathodes with linearly graded Al composition x ranges of 0 to 0.1, 0.2, 0.3, and 0.4, respectively.

Resolution characteristics of graded band-gap reflection-mode AlGaAs/GaAs photocathodes

The modulation transfer function (MTF) of graded band-gap AlGaAs/GaAs reflection-mode photocathodes was determined using two-dimensional Poisson and continuity equations through numerical method. Based on the MTF model, we calculated the theoretical MTF of graded and uniform band-gap reflection-mode photocathodes. We then analyzed the effects of Al composition, wavelength of incident photon, and thicknesses of AlGaAs and GaAs layer on the resolution. Calculation results show that graded band-gap structures can increase the resolution of reflection-mode photocathodes. When the spatial frequency is 800lp/mm and wavelength is 600 nm, the resolution of graded band-gap photocathodes generally increases by 15.4–29.6%. The resolution improvement of graded band-gap photocathodes is attributed to the fact that the built-in electric field in graded band-gap photocathodes reduces the lateral diffusion distance of photoelectrons.

Numerical simulation of front graded and fully graded AlGaAs/GaAs solar cell

One of the promising methods to enhance the performance of third generation solar cells is to use compositionally graded layers. The aim of this paper is to study the effect of fully graded AlGaAs solar cells using numerical simulation with SCAPS 1D. The gradient is simultaneously in the composition and in doping concentration. To attain this goal, we have optimized the standard p–i–n device in first step. Then, we have optimized the usual front graded device simulation of the fully graded device was done and the most important observation is that the open circuit-voltage (Voc) is much higher than Voc of device with uniform band gap where the open circuit voltage density decrease inconsiderately. We showed that a grading strategy with fully graded device can indeed lead to a more favorable trade-off between Jsc and Voc than can be obtained with uniform AlGaAs device.

Photovoltaic light absorber with spatial energy band gradient using PbS quantum dot layers

Using a combination of quantum dots (QDs) of different sizes, and thus different bandgaps, to extend the light-harvesting spectrum of a photovoltaic device in tandem architecture is a promising strategy for increasing the solar cell efficiency. In this study, we propose a new architecture for a solar cell device consisting of a graded-bandgap active layer made of lead sulfide QDs of different sizes and based on the structure of the Schottky junction. Colloidal PbS QDs with bandgaps of 1.55, 1.44, and 1.36eV were synthesized and used to construct a series of Schottky junction solar cells. Cells with a graded-bandgap active layer exhibited an increase in short-circuit current density (Jsc) but yielded lower open-circuit voltage (Voc) compared with cells having a uniform bandgap. We found that adding a thin electron energy-boosting layer (EEB) made from 1.55-eV-bandgap QDs can partially compensate for the thermalization loss, and thus enhance Jsc and recover Voc. Consequently, this study provides a conceptual basis for further improvement in colloidal QD-based solar cells.