Metal-insulator-semiconductor Contacts Research Articles

High-efficiency OECO Czochralski-silicon solar cells for mass production

Abstract To improve the economy of photovoltaics, efficiencies of solar cells have to be drastically increased without using complex technologies. This work demonstrates that with the obliquely evaporated contact metal-insulator-semiconductor (MIS)-n+p solar cell structure recently developed at ISFH efficiencies exceeding 21% can be obtained using only four simple fabrication steps: (i) mechanical surface grooving, (ii) P-diffusion, (iii) oblique vacuum evaporation of Al, and (iv) plasma silicon nitride deposition. Cell design and processing sequences are outlined together with the importance of MIS contacts as both low-cost and high efficiency features. The custom-made pilot line equipment for mass production of 20% efficient 10×10 cm2 Cz silicon solar cells including Ga doped wafers is described.

Simultaneous analysis of current–voltage and capacitance–voltage characteristics of metal–insulator–semiconductor diodes with a high mid-gap trap density

We present an improved method to analyze simultaneously the current–voltage and capacitance–voltage characteristics of metal–insulator–semiconductor (MIS) diodes. We use the method to study the effect of Zn doping concentration on the current transport in Au MIS contacts fabricated on In0.21Ga0.79As layers grown by metalorganic vapor phase epitaxy on highly doped GaAs substrates. At room temperature and for low reverse bias voltage, the generation/recombination process via mid-gap traps is the only dominant mechanism in these MIS diodes. For high reverse bias, both this mechanism and thermionic-field emission control current transport. The generation/recombination current observed is due to donor type mid-gap traps whose density shows an almost linear dependence with Zn concentration. The value of the barrier height at zero bias and at room temperature (φb0=0.73 V±12%) is independent of the Zn concentration. For the procedure used to prepare the In0.21Ga0.79As:Zn surfaces, the thickness of the oxide layer and the transmission coefficient of holes across this layer depend on the Zn doping concentration in the range 7×1014⩽NA⩽5×1018 cm−3. Zn doping seems to inhibit the formation of the unintentional native oxide on the surface of In0.21Ga0.79As epilayers.

New experimental evidence for minority-carrier reflection at negative-barrier MIS contacts

Recently, measurements of the open-circuit voltage of solar cells with negative-barrier metal-insulator-semiconductor (MIS) back contacts have been used to demonstrate that such contacts can function as the electrical analogues of metallurgical high-low junctions. In this brief, further experimental evidence for the minority-carrier reflecting properties of the negative-barrier MIS junction is presented. First, it is shown that a negative-barrier Mg-SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> -nSi back contact can be used to enhance the long-wavelength photoresponse of p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -n solar cells in the same manner as a diffused n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> back-surface field. Secondly, measurements of the effective surface-recombination velocity for an Mg-SiO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> -nSi contact and for a diffused n-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> high-low junction formed on an identical substrate are reported. Both junctions gave very low values of recombination velocity, on the order of 50 cm/s.

Induced back-surface field solar cells on p-type silicon substrates

Recent research has shown that negative-barrier metal insulator semiconductor (MIS) contacts can exhibit the same minority-carrier reflecting properties as diffused back-surface field regions. This letter describes the use of negative-barrier platinum-MIS contacts to form the first induced back-surface field solar cells on p-type silicon substrates.Pt-SiOx -pSi back contacts have been applied both to cells with diffused front junctions and to cells with minority-carrier injecting MIS front junctions. In both cases an enhancement in open-circuit voltage comparable to that obtained with diffused back-surface fields has been observed. The negative-barrier MIS contact has also been found to increase the infrared photocurrent.

An induced back surface field solar cell employing a negative barrier metal-insulator-semiconductor contact

A simple theory of current flow in negative barrier metal-insulator-semiconductor (MIS) junctions is developed. The properties of negative barrier MIS contacts are found to be closely related to those of positive barrier MIS junctions in which the semiconductor surface is strongly inverted. It is shown that by an appropriate choice of barrier metal work function and insulator thickness it should be possible to produce a negative barrier MIS contact which does not impede the flow of majority carriers, but which acts as a surface of low effective recombination velocity for minority carriers. It is demonstrated experimentally that such minority-carrier reflecting negative barrier contacts can provide an enhancement in solar cell open circuit voltage equal to that obtained with a diffused back surface field. This is the first report of an induced back surface field solar cell.