A microwave plasma technique has been used to oxidize hydrogenated amorphous silicon at low temperatures. By a water vapour treatment of the surface, very large open circuit voltages, up to 905 mV, have been obtained in metal insulator semiconductor (MIS) type solar cell structures. The grown oxides were analysed by Auger electron spectroscopy (AES), ellipsometry and other methods. The plasma oxide thickness lies in the range 11-17 angstroms. Although dry oxidation led to similar oxide thicknesses, wet oxidation provided better results. As expected, the plasma oxide reduces the MIS forward and reverse dark current and leads to an increase in open circuit voltage and photocurrent. The photocurrent enhancement is due to an increased blue response of the cell. Significant differences exist between native and plasma oxides. This has been shown in an impressive way in surface wetting experiments with a water droplet on oxidized surfaces. The open circuit voltage of cells stored for 3 years remained nearly constant, which indicates a good chemical stability of the MIS cell.
IN CRYSTALLINE SILICON IT IS WELL KNOWN, THAT A THIN TUNNELING OXIDE AT THE INTERFACE BETWEEN THE ABSORBING SEMICONDUCTOR AND THE BARRIER FORMING METAL LEADS TO AN INCREASE OF THE OPEN CIRCUIT VOLTAGE AND THUS TO THE EFFICIENCY OF SOLAR CELLS. OUR STUDY DEALS WITH THE PASSIVATION OF AMORPHOUS SILICON. THE INFLUENCE OF THE PASSIVATED A-SI:H SURFACE IS EXAMINED BY A MIS (METAL-INSULATOR-SEMICONDUCTOR) SOLAR CELL. IN ORDER TO REDUCE THE ACTIVATION ENERGY OF THE OXIDE FORMING PROCESS, WE DEVELOPED A PLASMA REACTOR POWERED BY A 2.45 GHZ MICROWAVE GENERATOR. THIS MULTIPLE GAS SYSTEM ALLOWS TO STUDY DIFFERENT PLASMA REACTIONS. AFTER A WATER-VAPOUR PLASMA TREATMENT WE OBSERVED A CONSIDERABLE INCREASE OF THE OPEN CIRCUIT VOLTAGE FROM 610 MV UP TO 900 MV WITH PT AND UP TO 870 MV WITH IR AS BARRIER METAL. THE MIS SOLAR CELLS PROVED TO BE VERY STABLE. THE CELL WITH 900 MV OPEN CIRCUIT VOLTAGE STABILIZED AFTER A YEAR AROUND 870 MV.
BECAUSE OF THE HIGH REFLECTION FROM THE BARRIER METAL AND THE A-SI:H SURFACE ONE GETS LOW PHOTOCURRENTS. BY COATING THE CELL WITH A ZRO2 ANTIREFLECTIVE LAYER THE PHOTOCURRENT COULD BE IMPROVED FROM 7 MA/CM2 TO ABOUT 11 MA/CM2 FOR POLISHED SAMPLES. A FURTHER IMPROVEMENT OF THE PHOTOCURRENT COULD BE ACHIEVED BY ROUGHENING THE STAINLESS STEEL SUBSTRATE. ASTONISHINGLY THE A-SI:H ON THE SAND-BLASTED SUBSTRATE DOES NOT EXHIBIT DESTRUCTIVE INFLUENCE ON THE MIS-INTERFACE. THE ROUGHENING PROCEDURE IS STILL NOT OPTIMIZED, BECAUSE WE ONLY USED A 50 MICROMETER GRAIN SIZE FOR SAND-BLASTING. OUR MIS SOLAR CELLS WITH A-SI:H AS BASE MATERIAL AND THE PLASMA PASSIVATION EXHIBIT EFFICIENCIES OF ABOUT 5%.