Barrier height enhancement in p-silicon MIS solar cells

Calculations are presented which indicate that the barrier height of metal-thin insulator-p-silicon diodes can be greatly enhanced by the presence of positive charge in the interfacial layer. Application of the model to recent MIS silicon solar cell data suggests that oxide charge densities of 3-4 × 10¹²charges.cm^-2could be responsible for the high performance of the reported cells.     

Schottky solar cells on thin epitaxial silicon

Schottky solar cells fabricated on 10, 20 and 30 μm epitaxial silicon produce a current density ranging from about 10–22 mA/cm², depending on Si thickness and orientation, in close agreement with theoretically predicted data. These results are also in close agreement with recent data on p-n solar cells, using thin epitaxial silicon. Data reported herein predict that 10% efficient Schottky solar cells could be produced using about 20 μ of silicon on a suitable substrate. A 7.6% efficient Schottky solar cell on epitaxial silicon has been recently fabricated and tested using AM1 sunlight (100 mW/cm²).     

Theory of Schottky barrier heights of amorphous MIS solar cells

The Schottky barrier height φ_m of amorphous MIS solar cells depends greatly on the density of surface states D_s, and on the density of localized states near the Fermi level. At low density of gap states, the φ_m is maximum for D_s being around 10¹⁸ m^?2 eV^?1. With an illumination of light, the barrier height decreases as the collection of minority carriers near the interface increases.     

The barrier height change and current transport phenomena with the presence of interfacial layer in MIS Schottky barrier solar cells

In this comprehensive study, several interesting results which are different from those previous are reported. We find the barrier height decreases for n-type and increases for p-type when positive ions are introduced into the insulating layer. The increase of open circuit voltage can be traced to the suppression of the dark saturation current by the depletion field induced by the positive charge, and to the diminution of the majority tunneling current by the oxide potential barrier. The tunneling probabilities for majority and minority carriers are different; there are only a finite amount of majority carriers with thermionic energy greater than q(V_bi ? V_s) which can surmount the depletion potential and tunnel into the metal, whereas the photogenerated minority carriers derive kinetic energy in the depletion layer making tunneling easier. Transport coefficients for electrons to transmit from metal to semiconductor and from semiconductor to metal are different for the departure of built in potentials during illumination.     

A comparison of majority- and minority-carrier silicon MIS solar cells

A systematic experimental investigation is reported of metal-SiO2-silicon (MIS) solar cells, as a function of SiO2thicknessd, in the useful range 8 Å <d< 20 Å. Both majority-carder (Au-SiO2- nSi) and minority-carrier (Al-SiO2-pSi) structures are studied and their performance compared for SiO2layers prepared under identical oxidation conditions and with identical silicon surface treatments. The short-circuit current densities are observed to be suppressed by tunneling through the SiO2layers ford gsim 17Å, whereas fill factors begin to decrease at even smaller values of d. The optimum effective AM1 conversion efficiencies for the majority-carrier cells are 9-10 percent for 10 Å ≲d≲ 14 Å, and for the minority-carrier cells are 11-12 percent ford simeq 10-11Å. These results are in agreement with theoretical calculations, also presented here, which take account of both electrostatic and dynamic effects of interface states, and of their dependence on bias voltage and illumination.     

Barrier height diminution in Schottky diodes due to electrostatic screening

Electrostatic screening in the metal contact of a Schottky (metal-semiconductor) diode is shown to influence the calculated electrical characteristics of the diode. A thin space-charge layer is formed at the surface of the metal contact by capacitively induced free charges, This results in a voltage dependent diminution of the barrier height of the diode that increases in magnitude with increasing semiconductor dielectric constant and carrier concentration. Predicted values of the barrier height diminution exceed those attributed to image forces or tunneling effects for materials with dielectric constants greater than about 20. In diodes using semiconducting ferroelectric or piezoelectric materials, an additional diminution of the barrier height results from free charges induced in the metal contact by a remanent polarization field or an externally applied mechanical stress. Current-voltage characteristics of a metal-semiconductor diode are shown to be significantly influenced by the electrostatic screening effect. A soft breakdown current as opposed to saturation current is predicted for reverse biases while an exponential forward current with an η coefficient exceeding unity is predicted for forward biases. Photoemission characteristics are also affected. A voltage-dependent diminution of the threshold energy for photoresponse is predicted. Capacitance-voltage characteristics, on the other hand, differ only slightly from those of an ideal Schottky diode except in the case of a ferroelectric diode where excessively large screening effects are possible.     

Photovoltaic processes in metal-semicrystalline silicon Schottky barriers and implications for grating solar cells

Experimental investigations of the photovoltaic properties of metal-silicon Schottky barriers are reported, in which edge collection of the photocurrent is dominant as in the majority-carrier grating solar cells proposed by Green. Both elemental metals and alloy Schottky electrodes, and both crystalline and semicrystalline cast silicon have been studied. The superposition principle for dark currents and photocurrents is shown unambiguously to be violated, and the effects of grain-boundary recombination and shunt resistance are identified. Limitations to the operation of these devices above the semiempirical limit of continuous Schottky barrier solar cells is seriously compromised by enhanced space-charge recombination current. This results from the large photocurrent densities for low contact area/active area ratios. The treatment of the space-charge recombination mechanism under optical illumination follows the normal Sah-Noyce-Shockley approach, but we introduce here a built-in “recombination potential” to encompass the non-zero recombination for short-circuit conditions.     

Schottky barrier height of sputtered TiN contacts on silicon

The Schottky barrier height of sputtered TiN on both p- and n-type silicon was determined by I-V and C-V measurements. The barrier height is found to increase on n-Si and to decrease on p-Si, upon thermal annealing. The experimental results are explained in terms of sputtering damage. This damage is modeled by donor-like traps whose concentration decays exponentially from the silicon surface. A characteristic length equal to 45 Å accounts for the observed characteristics. The trap-free values of the barrier height were obtained by I-V measurements after sequential thermal annealing up to 600°C. These values are φ_Bn = 0.55 V on n-type and φ_Bp = 0.57 V on p-type silicon.     

The dependence of the photocurrent of MIS solar cells on thickness of Schottky barrier metals

The thickness of the barrier metals on the Schottky solar cells is very critical to the conversion efficiency. A theoretical calculation of the short circuit current of the Schottky barrier solar cells on applying Fuchs-Sondheimer's theory to calculate the electrical resistivity in thin metal films, and Handy's approach to calculate the series resistance on a given configuration of the contact grids shows that the optimum thickness which gives maximum short circuit current closely depends onthe intensity of the illuminating light and on the series resistance of the device. The optimum thickness shifts toward thicker film as the illuminating light or the series resistance increases. Fabrication of these devices on MIS solar cells indicates that the monitored optimum thickness satisfactorily agrees with the theoretical values.     

Recent progress in MIS solar cells

Metal–insulator–semiconductor (MIS)-type solar cells have an inherent cost advantage compared to p-n junction solar cells. First-generation MIS–inversion layer (MIS–IL) solar cells, already successfully produced in an industrial pilot line, are restricted to efficiencies of 15–16%. With the second-generation MIS–IL silicon solar cells, based on drastically improved surface passivation by plasma-enhanced chemical vapour-deposited silicon nitride, simple technology can be combined with very high efficiencies. The novel inversion layer emitters have the potential to outperform conventional phosphorus-diffused emitters of Si solar cells. A 17.1% efficiency could already be achieved with the novel point-contacted ‘truncated pyramid’ MIS–IL cell. A new surface-grooved line-contact MIS–IL device presently under development using unconventional processing steps applicable for large-scale fabrication is discussed. By the mechanical grooving technique, contact widths down to 2 μm can be achieved homogeneously over large wafer areas. Bifacial sensitivity is included in most of the MIS-type solar cells. For a bifacial 98 cm² Czochralski (Cz) Si MIS-contacted p-n junction solar cell with a random pyramid surface texture and Al as grid metal, efficiencies of 16.5% for front and 13.8% for rear side illumination are reported. A 19.5% efficiency has been obtained with a mechanically grooved MIS n⁺p solar cell. The MIS-type silicon solar cells are able to significantly lower the costs for solar electricity due to the simple technology and the potential for efficiencies well above 20%.©1997 John Wiley & Sons, Ltd.     

MIS and SIS solar cells

In this review paper we show that MIS (metal-insulator-semiconductor) and SIS (semiconductor-insulator-semiconductor) solar cells are basically one and the same type of device, even though they are usually regarded as being separate and are reported as such. Experimental results on the two most common systems, Al-SiOx-pSi and ITO-SiOx-pSi (ITO designates indium-tin-oxide) are presented to support a model where tunnel current through the insulator or interface is the transport mechanism between the metal or oxide semiconductor (acting as a collecting grid) and the base converting semiconductor. However, theI-Vcharacteristics of the devices are dominated by diffusion current flow in the bulk of the base converting substrate and display the usual Shockley diode equation behavior, in the absence of additional defect current mechanisms.     

Barrier height temperature coefficient in ideal Ti/n-GaAs Schottky contacts

We have measured the I-V characteristics of Ti/n-GaAs Schottky barrier diodes (SBDs) in the temperature range of 60-320 K by the steps of 20 K. The SBDs have been prepared by magnetron DC sputtering. The ideality factor n of the device has remained almost unchanged between 1.02 and 1.04 from 120 to 320 K, and 1.10 at 100 K. Therefore, it has been said that the experimental I-V data are almost independent of the sample temperature and quite well obey the thermionic emission (TE) model at temperatures above 100 K. Furthermore, the barrier height (BH) Φ_b0 slightly increased with a decrease in temperature, 320-120 K. The Φ_b0 versus temperature plot from intercepts of the forward-bias ln I versus V curves has given a BH temperature coefficient of α = −0.090 meV/K. The Norde’s function has been easily carried out to determine the temperature-dependent series resistance values because the TE current dominates in the I-V characteristics. Therefore, the Φ_b0 versus temperature plot from the Norde’s function has also given a BH temperature coefficient value of α = −0.089 meV/K. Thus, the negligible temperature dependence or BH temperature coefficient close to zero has been attributed to interface defects responsible for the pinning of the Fermi level because their ionization entropy is only weakly dependent on the temperature.     

Barrier height inhomogeneities in a Ni/SiC-6H Schottky n-type diode

Two models have been used in order to explain the anomalies observed in a Ni/SiC-6H Schottky n-type diode I(V) characteristic. Both, parallel conduction and potential fluctuation models showed that the barrier's height is around a mean value of 1.86 V, corresponding to a factor of ideality of n=1. Another conclusion was that  V.It has been, also, explained why the Arrhénius or Richardson plot (ln(I_s/T²) versus 1/T) is not linear and why the area of the low barrier height A_l, representing a defective zone, is approximately about 0.12% of the total area contact.     

Schottky Barrier Impedance Measurements at UHF (Short Papers)

The observed frequency dependence of the real part of the small signal impedance of Schottky barrier varactor diodes has previously been explained via physical phenomena. A detailed experimental investigation shows that the frequency dependence is due to inevitable systematic errors in the measurement procedure used.     

MIS solar cells: A review

The metal-thin-film insulator-semiconductor (MIS) structure is currently receiving much attention in solar-cell studies. Both theoretical and practical investigations indicate that this structure offers a means of overcoming the principal deficiency of Schottky barrier solar cells, namely low open-circuit photovoltage, while maintaining the attractive features that have led the metal-semiconductor junction to be considered as a possible alternative to the p-n junction for large-area, terrestrial, solar-cell applications. The thin insulating layer allows control over not only the magnitude of the dark current flowing through the diode, but also the dominant type (majority or minority carrier) of this current. Desirably low values of dark current have been postulated for majority carrier devices incorporating suitable charge-trapping centres, located either within the insulator or at the semiconductor-insulator interface, and for minority carrier devices employing suitable insulator thicknesses, metal work functions, and semiconductor resistivities. Theories based on these models are reviewed in this paper and their relevance to explaining photovoltage enhancement in practical Si and GaAs MIS cells is examined. The factors affecting other salient solar-cell properties (photocurrent, fill factor, conversion efficiency) are also considered and suggestions as to the parameters limiting present device performance are given.     

An analytical model for optimizing the performance of graphene based silicon Schottky barrier solar cells

In this paper, a model taking into account the effects of carrier loss mechanisms has been developed. The model simulates the photovoltaic properties of the graphene/n-type silicon Schottky barrier solar cells (G/n-Si_SBSC), and it can reproduce the experimentally determined parameters of the G/n-Si_SBSC. To overcome the low efficiencies of G/n-Si_SBSC, their performances have been optimized by modifying the work function of graphene and Si properties, accounted for variation of its thickness and doping level. The obtained results show that the work function of graphene has the major impact on the device performance. Also, the temperature dependence of the G/n-Si_SBSC performance is investigated.     

Impurities in silicon solar cells

The effects of various metallic impurities, both singly and in combinations, on the performance of silicon solar cells have been studied. Czochralski crystals were grown with controlled additions of secondary impurities. The primary dopants were boron and phosphorus while the secondaires were: A1, B, C, Ca, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, P, Pd, Ta, Ti, V, W, Zn, and Zr. Impurity concentrations ranged from 10¹⁰to 10¹⁷/cm³. Solar cells were made using a conventional diffusion process and were characterized by computer reduction ofI-Vdata. The collected data indicated that impurity-induced performance loss was primarily due to reduction of the base diffusion length. Based on this observation, an analytic model was developed which predicts cell performance as a function of the secondary impurity concentrations. The calculated performance parameters are in good agreement with measured values except for Cu, Ni, and Fe, which at higher concentrations, degrade the cell substantially by means of junction mechanisms. This behavior can be distinguished from base diffusion length effects by careful analysis of theI-Vdata. The effects of impurities in n-base and p-base devices differ in degree but submit to the same modeling analysis. A comparison of calculated and measured performance for multiple impurities indicates a limited interaction between impurities, e.g., copper appears to improve titanium-doped cells.     

Noise Reduction in GaAs Schottky Barrier Mixer Diodes (Short Paper)

The sensitivity of heterodyne receivers operating at millimeter and submillimeter wavelengths is limited by the noise produced in the mixer element. In this paper we investigate the presence of excess noise in GaAs Schottky barrier mixer diodes. Comparison of the measured noise data with that predicted from noise models indicates that these devices typically exhibit excess noise. An additional fabrication step, which removes several hundred angstroms from the GaAs surface before the anode contact is formed, greatly reduces this excess noise. This additional step is outlined, and experimental evidence is presented.