Graded Band Gap Research Articles

Calculations of the reflectance of porous silicon and other antireflection coating to silicon solar cells

Calculations of the reflectance for antireflection coating (ARC) made of porous silicon (PSi) were carried out. Although PSi has been the focus of interest in the last few years, a satisfactory model for calculation of its reflectance features is still absent. A combination of the optical matrix method and a graded-band gap model of PSi, which takes into account the gradient in porosity, has recently been suggested. Calculations based on the optical matrix method were carried out also for well-known ARC like MgF 2, ZnS and Ta 2O 5 in order to prove the feasibility of the method. Good agreement with experimental data was found for all ARC types.

Graded-Band-Gap Semiconductors: the Possibilities for Improvement of p-n Junction Performance

ABSTRACTThe paper theoretically analyzes properties of graded-band-gap (GBG) p-n structures in which band gap linearly enlarges with increasing the distance from the junction metallurgical edge. It is shown that by means of band-gap grading one can significantly reduce the diffusion reverse current caused by carrier thermal generation in the structure base regions and Ohmic contacts. The numerical estimations have been made for the case of p-n junction on the basis of CdHgTe solid solution.

Three cascade solar cells with graded band-gap layer on the base of GaAs–AlGaAs heterosystem

A new structure of three cascade solar cells with graded band-gap layer on the base of GaAs–AlGaAs heterosystem is designed to decrease the thermal losses, arising as a result of absorption of short-wavelength radiation. These solar cells were created by combining liquid-phase epitaxy with gas-phase zinc diffusion technologies. In these structures multilayer cascade elements reduce losses, caused by generation of “hot” carriers and the upper graded band-gap layer improves the conversion of short-wavelength radiation. The cell structure exhibited high short-wavelength sensitivity and the following parameters (under 1-sun conditions): open-circuit voltage (Voc=1.03 V); short-circuit current (Isc=24.9 mA/cm2); fill factor (FF=0.74) and efficiency (η=22.3%). Therefore it can be successfully utilized as power supplies in high-located areas, and as sensors of ultra-violet radiation.

Optimal optical generation profiles in a-Si:H p–i–n solar cells

The dependence of the maximum power output P M and short-circuit current J SC on the form (relative variation with position) of the optical generation rate profile in an a-Si:H p–i–n solar cell has been investigated computationally. It was found that there was an optimal form for the generation profile, and that P M increased from 4.64 to 5.29 mW cm −2 , an increase of about 14%, when this optimal generation profile was used in the simulation. Optimal doping of the i-layer of the cell with phosphorous led to a P M of 5.60 mW cm −2 , and when the optimal generation profile for this P-profiled cell was found, it yielded a P M of 7.86 mW cm −2 , an increase of about 40%. This suggests that the combination of P-profiling and optimal generation could lead to significant improvements in cell performance. Moreover, it was found that for both cells the form of the optimal generation profile could be associated with the position of the peak in the external quantum efficiency, obtained from the spectral response. The possibility of using band-gap grading to achieve an optimal generation rate profile has been suggested.

Back surface band gap gradings in Cu(In,Ga)Se 2 solar cells

We fabricate and analyse Cu(In,Ga)Se 2-based solar cells which have a graded band gap by an increased Ga content towards the Mo back contact. The open circuit voltage and the short circuit current strongly improve with the band gap grading. Electronic device analysis reveals that the open circuit voltage increase is directly related to the increased band gap energy at the back surface. We interpret the obtained results to a large extent as reduced back contact recombination by the introduction of an increased band gap close to the back contact.

Optical and photoelectrical properties of Hg 1− xCd xTe/CdTe epitaxial films with graded band gap

The infrared transmission and photoconductivity spectra of HgCdTe epitaxial films with graded band gap were investigated both theoretically and experimentally. Theoretical calculations were performed in the framework of the WKB approximation. The composition profile has been obtained from a fitting procedure. In order to reduce the total number of fitting parameters as well as to improve accuracy of this procedure the differential of the transmission versus photon energy curves was used. The best fit was obtained for an exponential composition gradient.

Integrated Optical and Electrical Modeling of a-Si:H Based Solar Cells

ABSTRACTNew features of the ASA software package are described that allow the analyses of the performance of hydrogenated amorphous silicon (a-Si:H) based solar cells with rough interfaces and buffer and graded layers. Using the updated version of ASA improved understanding of i) the effect of the front and back contact roughness on the external quantum efficiency and the absorption of light in the individual layers of a-Si:H solar cells and ii) the role of band gap grading of the intrinsic a-SiGe:H layer in a solar cell is obtained. The simulated results using the ASA package agree well with measured data on solar cells and are used to give insight into the behavior of the internal parameters of the solar cells.

Growth of Co-evaporated Cu(In,Ga)Se2 – The Influence of Rate Profiles on Film Morphology

ABSTRACTThe influence of the evaporation rate profiles on the microstructure of co-evaporated Cu(In,Ga)Se2, (CIGS), is discussed. The influence of Cu excess in the beginning of the CIGS growth has been investigated. In addition, the Ga rate has been varied in order to create bandgap grading in the CIGS film. By studying CIS and CGS films separately and as CGS/CIS stacks results on interdiffusion of In and Ga interdiffusion have been obtained. The resulting thin films are investigated mainly using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). Solar cell devices were prepared and IV measurements performed on samples with varying CIGS deposition parameters.

Herb's bipolar transistors

The profound influence of Herbert Kroemer's ideas on the development of high-performance bipolar transistors is described. The historical context and subsequent development of innovations such as the drift base, achieved through concentration gradients and later with semiconductor bandgap grading, the use of wide bandgap emitters, concepts of collector-up transistors, and the introduction of new heterojunction materials, are reviewed.

A new approach to high-efficiency solar cells by band gap grading in Cu(In,Ga)Se 2 chalcopyrite semiconductors

High efficiencies in Cu(In,Ga)(S,Se) 2 solar cells result from alloying CuInSe 2 base material with the corresponding Ga- or S-containing compound. Compositional grading is one important issue in these devices. To obtain high efficiencies a reconstructed Cu-depleted absorber surface is essential. We consider this Cu/In grading non-intentional, process related and present a model which explains its importance. Another approach to improve performance is controlled intentional band gap grading via Ga/In and S/Se grading during the deposition. We show that appropriate grading can improve current and voltage of the device simultaneously. The key objective is to design a larger band gap for recombination and a lower band gap for absorption to energetically separate the mechanisms of carrier recombination and current generation.

Low gap Amorphous (Si,Ge) Solar Cells

ABSTRACTWe report on the growth and fabrication of low-gap a-(Si,Ge) solar cells, including cells made from a-Ge:H. The cells were deposited from silane and germane, or germane alone, using a high dilution of hydrogen in a remote ECR plasma reactor. We have been able to achieve a Tauc gap of as low as 1.15 eV with good device properties. The fill factors were in the 50%-60% range, and the Urbach energies measured in devices were in the range of 45 meV. The devices were of the p/i/n type with light entering the p layer. The devices were fabricated on stainless steel substrates. We also report on experiments to improve graded gap solar cells in a- (Si,Ge) using a novel combination of ppm-boron dopant and bandgap grading. It is shown that this grading significantly improves the device performance, both voltage and fill factor.

Structural Aspects of Band Gap Grading in Cu(In,Ga)(S,Se)2-Based Solar Cells

Structural analysis of multinary Cu(In,Ga)(S,Se)2 films demonstrates the feasibility of compositional grading in the depth of the absorber with respect to the S/(S+Se) ratio. Solar cells prepared from films with moderate S/Se gradients exhibit high open circuit voltages and efficiencies up to 13.9%. In this contribution we investigate strong S/Se gradients. Of particular interest are the thickness of a nearly homogeneous top layer as well as the impact of the crystallinity on the current voltage characteristics of solar cells. We show that the crystallinity of a homogeneous top layer strongly correlates with the performance of the corresponding devices.

The impact of bandgap offset distribution between conduction and valence bands in Si-based graded bandgap HBT's

This work examines the impact of bandgap offset distribution between conduction and valence bands in Si-based graded bandgap HBT's using dc and ac simulation. For a fixed total bandgap offset, a conduction band pushed up by the total offset, together with a valence band pushed up by 2× the total offset gives the best ac performance, and allows the highest operational current for high frequency applications in an n–p–n HBT. A retrograded mole fraction profile, when properly optimized, can produce nearly identical ac performance for different bandgap offset distributions. These suggest that contrary to popular belief, applying careful optimization can yield excellent transistor performance for any arbitrary band alignment for both n–p–n and p–n–p graded bandgap HBT's.

Quantum Yield of GaN and (Ga, Al)N Band-Gap Graded Ultraviolet p–n Detectors

Calculation of the quantum yield has been carried out for an AlxGa1—xN(n)–GaN(p) photodiode as an ultraviolet detector with energy band-gap graded (EBGG) AlxGa1—xN(n) layer. The analytical solution of the one-dimensional continuity equation was used in the calculation. The spatial dependence of the material properties, such as the absorption coefficient and the energy band-gap of the n-type layer of the photodiode is included in the solution. The band-gap grading due to a reduced absorption coefficient at the surface region (where recombination occurs) and the built in electric field results in an increase of the generated minority carrier collection efficiency. Within the range of small values of EBGG there is a substantial rise in the detector quantum yield with increasing grading. In the case of surface recombinationless AlxGa1—xN(n) front side layer, the further increase of the grading does not result a significant rise of quantum yield, especially in the spectral region near to the cutoff. Much more pronounced increase in quantum yield with a grow of the grading has neem found when the surface recombination was represent.

Energy transport simulation for graded HBT's: Importance of setting adequate values for transport parameters

An energy transport simulation method for graded AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is presented in which Al composition-, doping density-, and energy-dependences of transport parameters are considered. For several representative Al composition and doping densities, parameters such as electron mobility, energy relaxation time, and upper-valley fraction are evaluated as a function of electron energy by a Monte Carlo method. For the other Al composition, these are determined by a linear interpolation method. Calculated cutoff frequency characteristics and electron velocity profiles are compared with those obtained by using more simplified approaches, demonstrating the importance of giving adequate transport parameters, particularly in analyzing graded band-gap base HBTs.

Preparation of amorphous hydrogenated silicon-germanium material and solar cells using the thermocatalytic chemical vapor deposition

Hydrogenated amorphous silicon-germanium (a-SiGe:H) films were produced by the “thermocatalytic chemical vapor deposition” [(TC-CVD) also called “hot-wire” CVD] method. With respect to proposed superstrate solar cell application of the material, the substrate temperature was kept around 200 °C. The band gap Eg of the films could be varied between 1.1 and 1.77 eV by changing the ratio of silane to germane flow. The photoelectronical properties of the alloy films have been found sufficient for solar cell application when Eg⩾1.5 eV and deteriorate for further reduction of the band gap. Using the concept of band gap grading, p-i-n solar cells containing a TC-CVD-a-SiGe:H i layer with initial conversion efficiencies up to η=6.42% (Eg=1.5 eV) have been fabricated. The influence of moderate hydrogen dilution of the process gas on the conversion efficiency and stability of the solar cells has also been investigated.

Simulation of Hydrogenated Amorphous Silicon Germanium Alloys for Bandgap Grading

AbstractComputer simulations are reported of hydrogenated amorphous silicon germanium (a-SiGe:H) layers that make up the graded part of the intrinsic layer near the interfaces of a-SiGe:H solar cells. Therefore the graded part is approached with a ‘staircase’ bandgap profile, consisting of three layers within which the material properties are constant. Calibrated model parameters are obtained by matching simulation results of material properties of intrinsic a-SiGe:H single layers to measurements. Using the obtained model parameter sets subsequent simulations of p-i-n devices with intrinsic material similar to the single layers are matched to measured current-voltage characteristics. The changes in parameter values are evaluated as a function of optical gap.

Role of Bandgap Grading for the Performance of Microcrystalline Silicon Germanium Solar Cells

AbstractThe optical absorption of microcrystalline silicon germanium alloys (μc-Si1-xGex:H) increases in the near infrared region with increasing germanium content. Therefore, these alloys are promising candidates for the application as intrinsic absorber material in thin film solar cells with tandem and triple structure or IR-detectors. The material properties for a germanium content (x) up to 0.6 and the performance of solar cells based on this material were investigated. Graded bandgap structures are used to optimize the cell performance. For cells with x > 0.3 a continuously graded bandgap in the rear 20 nm of the i-layer (at the i/n interface) results in an enhancement of the open circuit voltage by 40-80 mV compared to an abrupt bandgap discontinuity. When the design of the p/i interface region is changed in a similar way no effect on Voc is observed.

Distribution of components in epitaxial graded band gap heterostructures Cd(Mn,Zn)Te – Cd(Mn,Zn)HgTe and their photoelectrical properties

Distribution of components in epitaxial graded band gap heterostructures Cd(Mn,Zn)Te – Cd(Mn,Zn)HgTe and their photoelectrical properties

The limiting efficiency of band gap graded solar cells

Two fundamental mechanisms limit the maximum attainable efficiency of solar cells, namely the radiative recombination and Auger recombination. We show in this paper that proper band gap grading of the solar cell localizes the Auger recombination around the metallurgical junction. Two beneficial effects result from this Auger recombination localization; first the cell is less sensitive to the surface conditions, and second, the previous estimates for the limiting efficiency of solar cells by Shockley, Tiedje, and Green are revised upwardly. We calculate the optimum bandgap grading profile for several real material systems, including GaInAsP lattice matched to InP, and a-SiGe on a-Si substrate.