Schottky barrier junction solar cells

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

If the thickness of the insulator is reduced below about 10 A the concept of a tunnel MIS diode apparently becomes invalid, based at least on experimental evidence, and these thin structures perform as basic Schottky barriers. Above 28-30 A the diodes behave as equilibrium tunnel diodes. From Fig 7a it can be observed that even in the minority carrier regime under forward bias the region over which ideal p-n junction diode behaviour is predicted is insulator thickness dependent. Since in the case of p-n junctions in silicon under normal AMI illumination about 0.5 - 0.7 V is developed across the junction this means that for significant conversion efficiencies in these mi MIS devices insulator thickness should not exceed about 20 A. At greater thickness there will be some suppression of the photo-current due to the shape of the I-V characteristic rather similar to that observed in p-n junction solar cells with large series resistance. 

If the Schottky barrier cells are by far the most extensively studied, the limited absorption spectra of single-layer films, combined with the narrow widths of their depletion layer, restrict the ultimate sunlight conversion to about 4% [55]. An organic p-n junction, in contrast, should have a higher efficiency because of improved matching of absorption and solar spectra by the use of more than one absorber in the depletion layer. The p-n junction, in fact, is composed of two layers, constituted of a p-type and an n-type organic conductor. 

Rectification and photovoltaic effects in organic p-n junctions were first reported by Kearns and Calvin [101] and by Meier [3]. The combination of rhodamines or triphenylmethane dyes (both n-type) with merocyanines or phthalocyanines (both p-type) generated photovoltages up to 200 mV and photocurrents of about 10 8 A at low light intensity, with power conversion efficiency much less than 1%. More recent studies have been performed on merocyanine and malachite green [89,90] and on phthalocyanines and TPyP (a porphyrin derivative) [102,103]. These devices showed stronger spectral sensitization and better spectral match to a solar spectrum than those of Schottky barrier cells using only one component. 

Light sensors made from a-Si H are either p-i-n or Schottky barrier structures. Unlike crystalline silicon, a p-n jimction is ineffective without the undoped layer, because of the high defect density in doped a-Si H. Illumination creates photoexcited carriers which move to the junction by diffusion or drift in the built-in potential of the depletion layer and are collected by the junction. A photovoltaic sensor (solar cell) operates without an externally applied voltage and collection of the carriers results from the internal field of the junction. When the sensor is operated with a reverse bias, the charge collection generally increases and the main role of the doped layers is to suppress the dark current. A Schottky device replaces the p-type layer with a metal which provides the built-in potential. 

Currently, much work is devoted to the synthesis of conducting polymers for use in a variety of applications. Polyacetylene, the prototype conducting polymer, has been successfully demonstrated to be useful in constructing p-n heterojunctions, (1) Schottky barrier diodes, (2,3) liquid junction photoelectro-chemical solar cells, (4) and more recently as the active electrode in polymeric batteries. (5) Research on poly (p-phenylene) has demonstrated that this polymer can also be utilized in polymeric batteries. (6)... 

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. 

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... 

The effect of substrate thickness upon the short circuit current of p-n junction cells under AM4 is shown in figure 13. In practice the extent to which the thickness of single crystal solar cells may be reduced is normally limited by the fragility of the substrate - typically v 300 y thick. A similar limitation exists for Schottky Barrier cells, however, if thin film substrates are to be used it should be noted that the properties of the films will be widely different from the bulk material. This is particularly the case for a-silicon where absorption coefficients are more than 10 times greater and 5% efficient cells have been achieved using active regions 1, y thick. ... 

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