Solar Schottky junction

Four different types of junctions can be used to separate the charge carriers in solar cebs (/) a homojunction joins semiconductor materials of the same substance, eg, the homojunction of a p—n sibcon solar ceb separates two oppositely doped layers of sibcon 2) a heterojunction is formed between two dissimbar semiconductor substances, eg, copper sulfide, Cu S, and cadmium sulfide, CdS, in Cu S—CdS solar cebs (J) a Schottky junction is formed when a metal and semiconductor material are joined and (4) in a metal—insulator—semiconductor junction (MIS), a thin insulator layer, generaby less than 0.003-p.m thick, is sandwiched between a metal and semiconductor material. 

Schottky junction - [SILICON AND SILICON ALLOYS - PURE SILICON] (Vol 21) -m solar cells [PHOTOVOLTAIC CELLS] (Vol 18)... 

If neither of these goals can be realized, layered semiconductors may not become useful electrode material in either semiconductor liquid junction or Schottky junction devices. Fortunately, evidence is already being obtained that the negative effects due to steps can be at least temporarily and partially alleviated (35, 36). Future development of chemical methods to inhibit deflection of minority carriers to the edges of steps and to reduce the high recombination rates at steps may open the way for the use of polycrystalline layered chalcogenide semiconductors in solar cell devices. 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

Photocurrents due to the electrochemical reduction and oxidation of H2O (H2 and O2 formation) usually occur at considerable overvoltages. Since this is an important problem for the solar production of a chemical fuel many researchers have tried to reduce the overvoltage by using a catalyst. In this case, it has to be realized again that the deposition of a metal monolayer on a semiconductor surface leads to the formation of a Schottky junction (see Section 2.2). Accordingly, the question arises whether there... 

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 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. 

In principle, photoelectrochemical cells can be used for the conversion of solar energy into electrical energy or for the production of a storable fuel. The first type (regenerative cells) consists of a semiconductor and inert counter electrode and a redox system in the electrolyte. The current-voltage behaviour is described by the diode equation, which is also valid for pure solid state devices (pn-junction, Schottky diode) i.e. 

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)... 

In its action, the regenerative type PEC cell is a full analog of the solid-state solar cell based on the semiconductor/metal junction called Schottky diode (see, e.g., [4]). Band diagram of the Schottky diode both in the dark and in the tight is presented in Fig. 4, a and b respectively. In the dark, Fermi levels of the semiconductor and the metal are equal (cf Fig. 2, a), F = F gt Upon illumination of the semiconductor, a photopotential emerges in it, (ppj,. 

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. 

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