REALIZATION OF SOLAR CELLS BASED ON SILICON / OXIDE JUNCTIONS

Transparent and conductive films of SrTiO3 , ITO, and Tl2O3 have been deposited by R.F. cathodic sputtering and by anodic oxidation onto Si substrates in order to realize SIS cells. A photoconversion efficiency of 8.8% has been obtained for Si/SiOx/Tl2O3 cells. On the other hand for Si/SiOx/SrTiO3(ITO) the photoconversion efficiency is lower than 1% because of the too large thickness of the SiOx interfacial layer.

Transparent and conductive films of SrTiO3, ITO, and T1203 have been deposited by R.F. cathodic sputtering and by anodic oxidation onto Si substrates in order to realize SIS cells.A photoconversion efficiency of 8.8% has been obtained for Si/SiOx/Tl203 cells.On the other hand for Si/SiOx/SrTiOa(ITO) the photoconversion efficiency is lower than 1% because of the too large thickness of the SiOx interfacial layer.
Transparent and conductive films of SrTiO3, Indium Tin Oxide (ITO), and T1203 have been deposited by R.E cathodic sputtering and by anodic oxidation (tables I and II).
SrTiO3 deposited at room temperature is amorphous and is relatively transparent to visible light (Eg 3.0 eV, ce 104.cm-).The Fermi level in this film is pinned at about 0.6 eV under the conduction band edge (EF 4.0 eV) by the energy states Ti 3/ "3d (tEg) originating from Ti-O dangling bonds1.These energy levels are also responsible for the electronic transport in this film (/e(25C) 10-.cm2/V.s, Ea 0.07 eV).SrTiO3 deposited at 250C under a controlled reductive atmosphere (60% Ar + 40% HE) is partially crystallized (noted as SrTiO3 (p.c.)) and its composition is, in fact, SrTiO2.8.The band gap of the film is reduced to 2.5 eV by the interaction between the Ti 3/ "3d (tEg) non-bonding states.The Fermi level is just near the bottom of the conduction band.The electronic transport is assured by the free electrons in the conduction band (/Ze(25C) 60 cm/V.s,Ea 0.03 eV)2.
The energy band diagrams of the films are reported on the Fig. 1.ITO is amorphous when deposited at room temperature and becomes crystallized when deposited at 250C.T1203 crystallizes in the same structure as ITO.Both ITO and T1203 are degenerated n+-type semiconductors.T1203 is trans- parent to visible light (like ITO) in spite of its small band energy gap value (Eg 1.4 eV).The conduction band edge of T1203 (E.A. 5.0 eV) is 0.8 eV lower than that of ITO (E.A. 4.2 eV).All these phenomena can be explained on the basis of the band energy diagram common for ITO and T1203 (Fig. 2).The proposed energy diagram model is in good agreement with that proposed by Switzer3.
Because of their transparency, electrical conductivity, and electron affinity, thin films of SrTiO3 (p.c) and ITO have been deposited onto p-Si (100) face, and that of T1203, onto n-Si (100) face, to realize the SIS (Semiconductor-Insulator-Semi- conductor) tunnel solar cells (Fig. 3).E c ox /__e_.__:The ohmic back contact is assured by silver paste on p-Si, and by an In-Ga eutectic alloy on n-Si.Silver forms the ohmic contact on p-Si because of the alloying effects between Ag and SiOx4.
Different etching processes of silicon surfaces have been used in order to min- imize the influence of the interface states.Thus there is no Fermi level pinning effects observed (Table III).
The efficiency of the Si-p/SiOx/SrTiO3 (500/) and the Si-p/SiOx/ITO (100/) cells are small (rl < 1%) because of the existence of a too thick insulating interfacial layer of SiOx (d > 20A) which is inherent to the experimental conditions (Fig. 4  and 5).
In the case of the Si-n/SiOx/Tl203 (1000A) cells, the thickness of the SiOx layer is appropriate (10/ < d < 20/); an efficiency of 8.8% has thus been obtained (Fig. 6).A semi-theoretical study demonstrates that an efficiency of 12.3%, close to that obtained by A. Switzer3, can be achieved by optimizing the etching process of the silicon surface2.

FIGURE 2
FIGURE 2 Energy band diagram of ITO (a) and T1203 (b) films.

TABLE R .
F. sputtering deposition condition.
Energy band diagram of SIS solar cells.

TABLE III The
Schottky-barrier heights (b) of the solar cells.