Schottky Barrier Solar Cells

Two papers on the theory of SBSCs have appeared. The magnitude of the voltage attainable across metal-semiconductor interfaces in thermodynamic disequilibrium has been formulated in terms of electrochemical affinities, using the method of irreversible thermodynamics.70 The role of the interfacial layer, which may act to increase the open-circuit voltage and the fill factor, has also been discussed.71 

The fabrication of the first efficient large area (ca. 1 cm8) SBSC has been reported.72 73 It consists of a Schottky barrier of 5 nm Cr overlaid with 5—7 nm Cu on 2 fi cm p-type silicon, with a top (ohmic) contact of A1 fingers covered with 69 nm of antireflective silicon oxide coating. The sunlight conversion efficiency of the cell is 9.5%. 

SBSCs containing organic semiconductors are extremely inefficient, mainly because of high resistivity and low charge-carrier mobility. Fang74 has analysed 

Blocking contact Ohmic for electrons in contact conduction band 

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The promise of photoelectrochemical devices of both the photovoltaic and chemical producing variety has been discussed and reviewed extensively.Cl,, 3,4) The criteria that these cells must meet with respect to stability, band gap and flatband potential have been modeled effectively and in a systematic fashion. However, it is becomirg clear that though such models accurately describe the general features of the device, as in the case of solid state Schottky barrier solar cells, the detailed nature of the interfacial properties can play an overriding role in determining the device properties. Some of these interface properties and processes and their potential deleterious or beneficial effects on electrode performance will be discussed. 

D. R. Lilliongton and W. G. Townsend, Effects of interfacial oxide layers on the performance of sihcon Schottky-barrier solar cells, Appl. Phys. Lett. 28(2), 97, 1976. 

Fig. 12. Illuminated /(V) curves for—3- tm-thick a-Si H/Pt Schottky-barrier solar cell. The curves taken with blue light are shown on X100-expanded current scale. The light exposure was...

P. Panayotatos and H. C. Card, Recombination in the space-charge region of Schottky barrier solar cells, Solid State Electron. 23 (1980) 41-47. 

E. J. Soukup and D. R. Slocum, A model for the collection of minority carriers generated in the depletion region of a Schottky barrier solar cell, Solar Cells 7 (1982-1983) 297-310. 

J. F. McCann, S. Hinckley, and D. Haneman, An analysis of the current-voltage characteristics of thin-film front wall illuminated and back wall illuminated liquid junction and Schottky barrier solar cells, J. Electrochem. Chem. 137 (1982) 17-37. 

R. 0. Loutfy, and J. H. Sharp, Photovoltaic properties of metal-free phthalocyanines. I. AI/H2PC Schottky barrier solar cells, J. Chem. Phys. 71 1211 (1979). 

Band-bending and hence open-circuit voltage in Schottky barrier solar cells are favoured by an insulating layer with acceptor/ donor surface states which are in better communication with the n-type/p-type semiconductor than they are with the metal. 

We must revert to our theme of recombination process in solar cells, but now with special reference to Schottky barrier solar cells. For simplicity attention will be confined to a p-type semiconductor. We know from Fig. 3.3(a) that the electrostatic field acts to the right, and from Fig. 3.1 that the light-induced current flows to the right. The photogenerated holes move therefore into the semiconductor, while the photogenerated electrons tend to accumulate near the plane x = 0. This causes a rise of the electron quasi-Fermi level F above the equilibrium Fermi level, which is... 

The above is a description of the theory of the Schottky barrier solar cell, rather than a detailed exposition of it. Before we discuss typical results, we note the approximations still involved. They are ... 

Fig. 13.3 The current density - voltage characteristics of the p-type Schottky barrier solar cell for various interfacial layer thicknesses 6. The parameters of Table 13.1 have been used.

Consider again a p-type Schottky barrier solar cell with reasonably low density of interfacial surface states... 

Indeed the degradation of cells by corrosion due to water vapour has been attributed to the growth of the insulating layer or the development of an additional layer Fig. 13.5 gives experimental points marked by crosses on current-voltage curves for p-type Schottky barrier solar cells. They are compared with the theory described in these lectures, using a series resistance of 3.2 (in an equivalent circuit) for a cell area of Icm, and adopting = 6 lO m eV One sees the severe depression in... 

Fig. 13.5 P-type Schottky barrier solar cell (a) before (b) after degradations. — Experimental curve. Curves are theoretical ...

M.H. Klimpke and P.T. Landsberg. An improved analysis of n-type Schottky barrier solar cells, 2nd European Photovoltaic Solar Energy Conference, Berlin, April, 1979 (Dordrecht Eeidel 1979), p.678. 

Schottky Barrier solar cells, submitted to Solid-State Electronics. 

In practice it has been found that intimate contacts formed between metal films and crystalline semiconductors exhibit poor photovoltaic response. This is caused by the fact that the thermionic emission dark current at the Schottky barrier leads to significantly higher dark currents than is normally encountered in a homojunction or heterojunction structure. This problem, however, can be got round while still preserving the potential advantages of the Schottky barrier by allowing a very thin oxide or insulating layer to be formed between the semiconductor and the metal contact. The introduction of this layer leads to the ccxnmon form of Metal-Insulator-Semiconductor Schottky Barrier Solar Cell (MIS SBSC) with hich we are primarily concerned here. 

In the Schottky barrier solar cell light energy is transmitted into a semiconductor substrate through an extremely thin semitransparent metal layer . The metal and semiconductor are chosen so that the difference in their work functions causes a depletion region and hence an electric field to be produced below the semiconductor surface. Those photons absorbed within the semiconductor produce electron holes pairs which are separated under the action of the electric field and flow to opposite sides of the junction. The separated charge tends to forward bias the junction causing a... 

On the assumption that it will be possible to get around the long term stability problem discussed in the previous section the future for Schottky Barrier solar cells must depend on our ability to develop cells on cheaper substrates while maintaining cell conversion efficiencies 10% or better. There are a number of alternative substrates that are currently under investigation among which the following are the most promising. 

Takahashi, K., Tsuji, K., Inote, K., Yamaguchi, T., Kanura, T., Murata, K. Enhanced photocurrent by Schottky-barrier solar cell composed of regioregular polythiophene with merocyanine dye. Synth. Met. 130, 177-183 (2004)... 

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