Theory for nonequilibrium behavior of anisotype graded heterojunctions

An analysis for the terminal behavior and capacitance of anisotype graded heterojunctions is presented. A closed form expression for the I–V characteristics is derived using the depletion approximation and the rigid band model. The expression is valid for uniform nondegenerate doping and low-level injection. The transition region is modeled to extend beyond the space-charge region with a constant bandgap gradient. Variations of the dielectric constant, carrier lifetime and mobility are ignored. The nonsaturating nature of the reverse current is demonstrated. The effects of variations of bandgap and electron affinity on the I–V characteristics and junction capacitance are discussed. Numerical results for a graded nGe-pGaAs device are given.     

Carrier densities and emitter efficiency in degenerate materials with position-dependent band structure

We consider the electron and hole densities in degenerate and nondegenerate materials with a band structure that is position-dependent and/or having a nonparabolic density of states function. For nondegenerate materials it is shown that the pn-product deviates from its classical value n_i² for two reasons which generally occur conjunctively. First a modifying factor exp (ΔE_g/kT) occurs due to the real change in bandgap. Secondly, a factor exp [(Γ_n+Γ_p)/kT] is found stemming from the modified density of states or apparent change in band gap. These effects can be separated by studing the temperature dependence. Next we calculate the minority carrier current and the emitter efficiency and current gain. It is shown that β_ψ will be degraded by the increased pn-product and will be further affected by the degeneracy of the emitter material, the direction of the change depending on the doping profiles.     

Semiconductor‐Based Photoelectrochemical Water Splitting at the Limit of Very Wide Depletion Region

In semiconductor‐based photoelectrochemical (PEC) water splitting, carrier separation and delivery largely relies on the depletion region formed at the semiconductor/water interface. As a Schottky junction device, the trade‐off between photon collection and minority carrier delivery remains a persistent obstacle for maximizing the performance of a water splitting photoelectrode. Here, it is demonstrated that the PEC water splitting efficiency for an n‐SrTiO₃ (n‐STO) photoanode is improved very significantly despite its weak indirect band gap optical absorption (α < 10⁴ cm^?1), by widening the depletion region through engineering its doping density and profile. Graded doped n‐SrTiO₃ photoanodes are fabricated with their bulk heavily doped with oxygen vacancies but their surface lightly doped over a tunable depth of a few hundred nanometers, through a simple low temperature reoxidation technique. The graded doping profile widens the depletion region to over 500 nm, thus leading to very efficient charge carrier separation and high quantum efficiency (>70%) for the weak indirect transition. This simultaneous optimization of the light absorption, minority carrier (hole) delivery, and majority carrier (electron) transport by means of a graded doping architecture may be useful for other indirect band gap photocatalysts that suffer from a similar problem of weak optical absorption.     

Reverse recovery transient characteristic of PEDOT:PSS/n-Si hybrid organic-inorganic heterojunction

We study the junction behavior of poly (3,4-ethylenedioxythiophene):polystyrenesulphonate/n-Si hybrid organic/inorganic heterojunction by reverse recovery transient (RRT) characterization. RRT response for PEDOT:PSS/n-Si hybrid junction is reported for various n-Si doping concentration and forward bias current injection level. The presence of settling time of 8.3–23.5 μs in the RRT response in contradiction to Schottky junction model commonly assumed for PEDOT:PSS/n-Si hybrid structure. The decrease in the minority carrier lifetime from 126.8 μs to 39.5 μs with increased n-Si doping concentration, suggests that minority carriers are stored at n-Si side of the junction, which is consistent with a p⁺-n junction model for the hybrid structure. The minority carrier lifetime is found to depend on forward bias current injection level, attributed to trap-saturation effect of the recombination-centers at the PEDOT:PSS/n-Si junction. The DC-IV characteristics of the PEDOT:PSS/n-Si hybrid junction are also consistent with the notion of diffusion and trap assisted recombination dominated dark current. The diffusion dominated transport of PEDOT:PSS/n-Si leads to an ideal p⁺-n junction behavior that leverages on the good transport properties of Si. Our findings are important in the modeling and optimization of the characteristics of electronic devices based on the organic/Si hybrid junction.     

Theoretical calculations of Debye length,built-in potential and depletion layer width versus dopant density in a heavily doped p-n junction diode

Theoretical calculations of Debye length, built-in potential, depletion layer width and capacitance as a function of dopant density in a heavily doped p-n junction diode are described in this paper. The heavy doping effects such as carrier degeneracy, dopant density-dependent dielectric constant and bandgap narrowing are accounted for by using the empirical approximation for the reduced Fermi-energy given by[1] and the dopant density dependent dielectric constant given by[2], as well as the bandgap narrowing model proposed by[3]. The results show that: (1) bandgap narrowing and carrier degeneracy have important effects on the junction built-in potential; (2) carrier degeneracy and dopant density-dependent dielectric constant are important to Debye length for the abrupt junction case, and (3) the dopant density-dependent dielectric constant is a key parameter which strongly affects the values of depletion layer width and depletion capacitance. These findings are important for modeling of heavily doped p-n junction devices in the VLSI applications.     

Calculation of charge distributions and minority-carrier injection ratio for high-barrier schottky diodes

Numerical calculations of charge distributions and injection ratios for high-barrier Schottky diodes are performed to extend the understanding of this type of phenomena under various conditions. The calculations are performed for two doping concentrations, 10¹⁴ and 10¹³ cm^?3, in n-type silicon and for several barrier heights in the range 0.92-0.79 eV. Two sets of carrier lifetimes are used to give nominal diffusion lengths that are much larger than, or comparable with, the dimension of the structure. The boundary conditions at the barrier were those of the combined diffusion-emission model. The back contact was modeled as a perfect ohmic contact, or as a low-high junction. The results are compared with experiments involving the use of injecting Schottky rectifiers, capable of giving low forward-voltage drop and sustaining moderately-high reverse voltages.     

Band structure adjustment of solar cells by gradient doping

This paper presents a study of the influence of gradient doping on solar cell performance. The gradient doping of the emitter layer of a a-Si:H(n)/a-Si:H(i)/c-Si(P) solar cell was simulated using the AFORS-Het software simulation package. Band structure adjustment due to gradient doping was studied. The adjustment manifests itself in two separate ways: the gradient quasi-Fermi level splitting (Δε_F) and the gradient band slope. The relationship between Δε_F and doping concentrations has been theoretically deduced and simulated using the AFORS-Het software package. The study shows that the Δε_F caused by gradient doping can improve the spectral response of the cell at long wavelengths and enhance the open circuit voltage. The steeper energy band caused by the higher doping gradient, G, promotes the effective separation of the carriers and reduces their recombination by 2–3 orders of magnitude compared to that of the uniform band. The photovoltaic conversion efficiency of the silicon heterojunction solar cell increases from 13.65% (uniform doping) to 20.86% for the gradient doping cell with the highest G and steepest energy band. This numerical simulation shows that the band structure adjustment caused by gradient doping can achieve higher utilization of solar energy and suppress the recombination of the carriers at the heterojunction interface. The study will guide practical preparation of future solar cells.     

A solar cell with enhanced photocurrent

Calculations have been performed on an n⁺p-solar cell structure with an added smaller bandgap extra layer. Continuity in the valence band is assumed between the two different semiconductor materials. In this way the usual requirement in a tandem cell of equal photo carrier generation in the different materials is relaxed.     

Drift-diffusion theory of symmetrical double-junction diodes

Using numerical methods, we have calculated the current-voltage characteristics, energy contours and carrier distributions of a symmetrical double junction diode (n⁺nn⁺ and n⁺pn⁺). It is found that the I-V characteristics at low currents and voltages depend greatly on the doping concentration of the base region; at hihg currents, they do not. In that regime, the characteristics bunch together, and can be approximated with remarkable fidelity by the Mott-Gurney law for space-charge controlled conduction in solids. Characteristics are presented for different impurity densities and base widths.     

Minority carrier MIS tunnel diodes and their application to electron- and photo-voltaic energy conversion—II. Experiment

The theory of metal-insulator-semiconductor (MIS) tunnel diodes has been described in a companion paper for the case where the insulating layer is so thin that large tunnel currents can flow between the metal and the semiconductor. Of particular interest was the case where the dominant component of tunnel current is between the metal and the minority carrier energy band in the semiconductor (minority carrier diodes). In the present paper, these diodes are investigated experimentally. The differences between minority and majority carrier diodes are demonstrated. Minority carrier diodes are shown to possess properties similar to p-n junction diodes as predicted theoretically. The effectsof different metal contacts, insulator thicknesses, and substrate resistivities are investigated and confirm previous theory. The application of the minority carrier MIS tunnel diodes to energy conversion employing the electron-voltaic effect is investigated experimentally. The diodes were found to be more efficient in this application than p-n junction diodes. Other possible applications of the diodes are as photo-voltaic energy converters, as injecting contacts, and as photo-diodes or elements of photo-diode arrays.     

Vacancy Manipulation Induced Optimal Carrier Concentration,Band Convergence and Low Lattice Thermal Conductivity in Nano-Crystalline SnTe Yielding Superior Thermoelectric Performance

Synergetic optimization of electrical and thermal transport properties is achieved for SnTe-based nano-crystalline materials. Gd doping is able to suppress the Sn vacancy, which is confirmed by positron annihilation measurements and corresponding theoretical calculations. Hence, the optimal hole carrier concentration is obtained, leading to the improvement of electrical transport performance and simultaneous decrease of electronic thermal conductivity. In addition, the incremental density of states effective mass m* in SnTe is realized by the promotion of the band convergence via Gd doping, which is further confirmed by the band structure calculation. Hence, the enhancement of the Seebeck coefficient is also achieved, leading to a high power factor of 2922 µW m⁻¹ K⁻² for Sn_0.96Gd_0.04Te at 900 K. Meanwhile, substantial suppression of the lattice thermal conductivity is observed in Gd-doped SnTe, which is originated from enhanced phonon scattering by multiple processes including mass and strain fluctuations due to the Gd doping, scattering of grain boundaries, nano-pores, and secondary phases induced by Gd doping. With the decreased phonon mean free path and reduced average phonon group velocity, a rather low lattice thermal conductivity is achieved. As a result, the synergetic optimization of the electric and thermal transport properties contributes to a rather high ZT value of ≈1.5 at 900 K, leading to the superior thermoelectric performance of SnTe-based nanoscale polycrystalline materials.     

An extension of the ideal diode analysis for the heavily doped p-n diode

In this paper, an extension of the ideal-diode analysis for the heavily-doped p-n junction diode is proposed. The heavy doping effects such as carrier degeneracy and band gap narrowing are accounted for by using a tractable empirical approximation for the reduced Fermi-energy given by[12] and employing effective intrinsic density. Under the assumption of low-level injection, it is found that the injected minority-carrier current, and the charge storage in the quasi-neutral regions should depend exponentially on values of F(Y), where F(Y) is a function of dopant dentisy at the depletion edge of the quasi-neutral emitter (or) base region of the p-n junction. Results of our calculations of excess hole current for the short base and the long-base diode show significant change from the values predictged by the conventional diode theory.     

p‐Doping of Copper(I) Thiocyanate (CuSCN) Hole‐Transport Layers for High‐Performance Transistors and Organic Solar Cells

The ability to tune the electronic properties of soluble wide bandgap semiconductors is crucial for their successful implementation as carrier‐selective interlayers in large area opto/electronics. Herein the simple, economical, and effective p‐doping of one of the most promising transparent semiconductors, copper(I) thiocyanate (CuSCN), using C₆₀F₄₈ is reported. Theoretical calculations combined with experimental measurements are used to elucidate the electronic band structure and density of states of the constituent materials and their blends. Obtained results reveal that although the bandgap (3.85 eV) and valence band maximum (?5.4 eV) of CuSCN remain unaffected, its Fermi energy shifts toward the valence band edge upon C₆₀F₄₈ addition—an observation consistent with p‐type doping. Transistor measurements confirm the p‐doping effect while revealing a tenfold increase in the channel's hole mobility (up to 0.18 cm² V^?1 s^?1), accompanied by a dramatic improvement in the transistor's bias‐stress stability. Application of CuSCN:C₆₀F₄₈ as the hole‐transport layer (HTL) in organic photovoltaics yields devices with higher power conversion efficiency, improved fill factor, higher shunt resistance, and lower series resistance and dark current, as compared to control devices based on pristine CuSCN or commercially available HTLs.     

Vertical-type 3D/Quasi-2D n-p Heterojunction Perovskite Photodetector

This research demonstrates a state-of-the-art vertical-transport photodetector with an n-type 3D MAPbI₃/p-type quasi-2D (Q-2D) BA₂MA₂Pb₃I₁₀ perovskite heterojunction. This structure introduces a ≈0.6 V built-in electric field at the n-p junction that greatly improves the characteristics of the perovskite photodetector, and the presence of Q-2D perovskite on the surface improves the life. The electrical polarities of the 3D and the Q-2D perovskite layers are simply controlled by self-constituent doping, making clearly defined n-p characteristics. Doctor-blade coating is used to fabricate the photodetector with a large area. The Q-2D materials with highly oriented (040) Q-2D (n = 2,3) planes are near the surface, and the (111) preferred planes mixed with high index Q-2D materials (n = 4,5) are found near the 3D/Q-2D interface. The stacking and interface are beneficial for carrier extraction and transport, yielding an external quantum efficiency of 77.9%, a carrier lifetime long as 295.7 ns, and a responsibility of 0.41 A W⁻¹. A low dark current density of 6.2 × 10⁻⁷ mA cm⁻² and a high detectivity of 2.82 × 10¹³ Jones are obtained. Rise time and fall time are fast as 1.33 and 10.1 µs, respectively. The results show the application potential of 3D/Q-2D n-p junction perovskite photodetectors.     

Design of multijunction GaPNAs/Si heterostructure solar cells by computer simulation

The designs of two- and three-junction solar cells based on GaPNAs/Si lattice-matched hetero-structures are calculated. It is shown that the efficiency of two-junction solar cells constituted by a junction based on a GaPNAs solid solution with a band gap E _g of 1.78 eV and a junction based on Si may reach a value of 30.3% under AM1.5 D, 100 mW/cm², and 35.4% under AM1.5D, 20 W/cm². The maximum values of the efficiency of the three-junction solar cell constituted by top and middle junctions based on GaPNAs with E _g of 2 and 1.5 eV, respectively, and a Si-based bottom junction are 39.2% under AM1.5 D, 100 mW/cm², and 44.5% under AM1.5D, 20 W/cm². It is shown that the thickness and minority carrier lifetime of the photoactive layers affect the efficiency of solar-light conversion by the heterostructures being developed.     

Space-charge-limited (SCL) effects in thin,Cu compensated CdS structures

A theory which is presented for handling one carrier space-charge-limited effects in thin, compensated high-resistivity layers, includes both the voltage drop across a high resistivity region and a π junction. Good agreement is obtained between the theory and the measured I–V characteristics of Cu-compensated CdS. A trapping energy state 0·37 eV below the CdS conduction band and of density 1～2×10¹⁵ cm^?3 is identified.     

Ordering and disordering of doped Ga0.5In0.5P

The band gap of Ga_0.5In_0.5P is reported as a function of doping level and growth rate. The lowest band gaps are obtained for hole concentrations of about 2 × 10¹⁷ cm⁻³. For samples doped p-type above 1 × 10¹⁸ cm⁻³, the band gap increases dramatically, regardless of growth rate. This effect is shown to be the result of disordering during growth rather than a change in the equilibrium surface structure with doping. The doping level dependence of the band gap of Ga_0.5In_0.5P samples grown at higher and lower growth rates differs for selenium and zinc doping even though the effects of high doping are the same for both dopants.     

Computer analysis of induced-inversion layer MOS solar cells

A computer analysis of induced inversion layer MOS solar cells is described. The analysis simultaneously solves Poisson's equation and the continuity equation in one dimension and provides a very effective method for solar cell evaluation. Numerical solutions of the carrier continuity equation in the inversion layer illustrate how cell designs may be improved in order to obtain higher short wavelength spectral response. Very shallow junctions (on the order of 0.07-0.1 μm) are shown to be optimum with higher electric fields in a direction to aid the collection of carriers generated by very high energy photons. The results also indicate that induceed inversion layer cells are less sensitive to surface recombination velocity variations than diffused p-n junction cells and have higher minority carrier lifetime. Furthermore, the effect of a p-p⁺ low-high junction on the back surface is examined and the results indicate that it is insignificant when the substrate doping concentration is optimized. High inversion layer sheet resistance values are evaluated and minimized with the contact diffusion used in the analysis designed to reduce the high inversion layer sheet resistance. Design improvements in cell performance are evaluated and identified with further improvement possible here. Conversion efficiency for silicon of 17.3% at AMO in the inversion layer solar cell is predicted assuming 95% transmission through the transparent conductor.     

Design of AlxGa1−xAs/GaAs/InyGa1−yAs triple junction solar cells with anti-reflective coating

Multi-junction solar cells (SC) made from III–V compound semiconductors are still in the development phase. Here, we perform calculations for multi-junction cells: Al_xGa_1−xAs top junction, GaAs middle junction and In_yGa_1−yAs bottom junction (all of these materials with band-gaps between 2.1 and 0.8 eV) in order to obtain the optimal band gap and thickness for each junction under the AM1.5 solar radiation spectrum. The ideal photo-current density is around 15.5 mA/cm². In order to reduce the natural reflectivity, an anti-reflective coating (ARC) was chosen, based on a MgF₂/ZnS double layer, allowing for a significant increase of the current density with respect to a cell without it. Calculations of external quantum efficiency (QE) were also performed for the three cases mentioned above: ideal one, taking into account the total reflection and with the ARC double layer. Finally, when more realistic calculations are done, taking into account the carrier recombination at each sub-cell, and the light reflection for a tandem cell with the designed ARC on top, the expected conversion efficiency (η), under the AM 1.5 spectrum (without concentration), was determined to be around 38.5%, making this an attractive III–V compound tandem cell to be investigated in the near future.