Schottky 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. 

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

Four different types of junctions can be used to separate the charge earners in solar cells (1) a homojunction joins semiconductor materials of the same substance, e.g., the homojunction of a p — n silicon solar cell separates two oppositely doped layers of silicon (2) a heterojunction is formed between two dissimilar semiconductor substances, e.g., copper sulfide, Cu S, and cadmium sulfide, CdS, in CuxS—CdS solar cells (3) 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, generally less than 0.003-pim thick, is sandwiched between a metal and semiconductor material. 

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. 

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. 

The issue of Schottky barrier formation to ZnO is not treated in this chapter as such contacts are not of big importance in thin-film solar cells. This is related to the fact that in thin film solar cells metals are only used to contact highly-doped films. For degenerately doped semiconductors, the barrier heights become very small because of the large space charge associated with depletion layers in such materials. 

There are other structures that have been used to make a-Si H solar cells such as Schottky-barrier and MIS configurations, but since the conversion efficiencies are generally less than 6%, we will not discuss these structures here (for more information on these structures, see Carlson, 1982b). 

A single layer solar cell is the simplest solar cell. It is essentially a Schottky diode with a polymer layer sandwiched between two metal electrodes. Several groups have... 

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. 

Photovoltaic solar power conversion was the first major application proposed for a-Si H and to date is the largest in production. The first devices were reported by Carlson and Wronski in 1976 and had an efficiency of only 2-3%. Some of the early devices were Schottky barrier cells, but were quickly discarded in favour of p-i-n cells. Since the first report, there has been a remarkable increase in the efficiency of the cells, increasing by roughly 1 % conversion efficiency per year, to a present value of 14%, as is shown in Fig. 10.17. The increase has resulted from a variety of innovations in the design, materials, and structure of the cells. The electronic properties of the solar cell are described next and then these innovations are outlined more or less in the order in which they occurred. 

Physical incorporatiem of phthalocyanines and porphyrins in polymers was mentioned in Chap. 2.1.1 and 2.1.2. Moreover, photovoltaic properties of Schottky bavier solar cells were checked by dispersing metal free Pc in a polymer binder At peak solar power (135 mW/cm ) a power conversion efficiency of 1,2% has been obtained. 

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. 

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)... 

Other Semiconductors. Gerischer,41 in a very helpful paper on electrochemical photo and solar cells, has explained the mode of action of the semiconductor-electrolyte interface (when the semiconductor is in its depletion mode) as a Schottky barrier, and how this can lead to separation of hole-electron pairs... 

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,. 

Fig. 4. Energy diagram of a solid-state solar cell of the Schottky diode type a - in the dark, b - in the light (upon closing external circuit with the load).

Since protection of electrodes against corrosion in the photoelectrolysis cells is a question of vital importance, many attempts have been made to use protective films of different nature (metals, conductive polymers, or stable semiconductors, eg., oxides). Of these, semiconductive films are less effective since they often cause deterioration in the characteristics of the electrode to be protected (laying aside heterojunction photoelectrodes specially formed with semiconducting layers of different nature [42]). When metals are used as continuous protecting film (and not catalytical "islands" discussed above), a Schottky barrier is formed at the metal/semiconductor interface. The other interface, i.e., metal/electrolyte solution is as if connected in series to the former and is feeded with photocurrent produced in the Schottky diode upon illuminating the semiconductor (through the metal film). So, the structure under discussion is but a combination of the "solar cell" and "electrolyzer" within the photoelectrode Unfortunately, light is partly lost due to absorption by the metal film. 

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