Solar cells single-junction

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. 

As it has been described in various other review articles before, the conversion efficiencies of photovoltaic cells depend on the band gap of the semiconductor used in these systems The maximum efficiency is expected for a bandgap around Eg = 1.3eV. Theoretically, efficiencies up to 30% seem to be possible . Experimental values of 20% as obtained with single crystal solid state devices have been reported " . Since the basic properties are identical for solid/solid junctions and for solid/liquid junctions the same conditions for high efficiencies are valid. Before discussing special problems of electrochemical solar cells the limiting factors in solid photovoltaic cells will be described first. 

FIG. 72. Schematic cross-section of (a) a single junction p-i-n o-Si H superstrata solar cell and (b) a tandem solar cell structure. (From R. E. I, Schropp and M. Zeman. "Amorphous and Microcrystalline Silicon Solar Cells—Modeling, Materials and Device Technology," Kluwer Academic Publishers, Boston, 1998, with permission.)... 

Ideally, all photons with a wavelength of about 900 nm or shorter should be harvested and converted to electric current. This limit is derived from thermodynamic considerations showing that the conversion efficiency of any single-junction photovoltaic solar converter peaks at approximately 33% near the threshold energy of lAeV.1 2 There are numerous ways to convert the solar radiation directly into electrical power or chemical fuel. The silicon solar cell is the most efficient in this respect. Nevertheless, the capital cost of such devices is not attractive for large-scale applications. 

Water is involved in most of the photodecomposition reactions. Hence, nonaqueous electrolytes such as methanol, ethanol, N,N-d i methyl forma mide, acetonitrile, propylene carbonate, ethylene glycol, tetrahydrofuran, nitromethane, benzonitrile, and molten salts such as A1C13-butyl pyridium chloride are chosen. The efficiency of early cells prepared with nonaqueous solvents such as methanol and acetonitrile were low because of the high resistivity of the electrolyte, limited solubility of the redox species, and poor bulk and surface properties of the semiconductor. Recently, reasonably efficient and fairly stable cells have been prepared with nonaqueous electrolytes with a proper design of the electrolyte redox couple and by careful control of the material and surface properties [7], Results with single-crystal semiconductor electrodes can be obtained from table 2 in Ref. 15. Unfortunately, the efficiencies and stabilities achieved cannot justify the use of singlecrystal materials. Table 2 in Ref. 15 summarizes the results of liquid junction solar cells prepared with polycrystalline and thin-film semiconductors [15]. As can be seen the efficiencies are fair. Thin films provide several advantages over bulk materials. Despite these possibilities, the actual efficiencies of solid-state polycrystalline thin-film PV solar cells exceed those obtained with electrochemical PV cells [22,23]. 

Single-jet crystal growth method, 19 179 Single-junction crystalline solar cells, 23 41-42... 

Fig. 8.2 Operation mechanism of single-crystal silicon p/n junction solar cell.

Photovoltaic devices made of selenium have been known since the 19th Century. Pioneering research in semiconductors, which led to the invention of the transistor in 1947, formed the basis of the modem theory of photovoltaic performance. From this research, die silicon solar cell was the first known photovoltaic device that could convert a sufficient amount of the sun s energy to power complex electronic circuits. The conventional silicon cell is a solid-state device in which a junction is formed between single crystals of silicon separately doped with impurity atoms in order to create n (negative) regions and p (positive) regions which respectively are receptors to electrons and to holes (absence of electrons). See also Semiconductors. The first solar cell to be demonstrated occurred at Bell Laboratories (now AT T Bell Laboratories) in Murray Hill, New Jersey in 1954. 

Fig. 6.54. NREL AMI.5 I-V characteristics of an amorphous p-i-n single junction solar cell deposited on LP-CVD ZnO coated glass after light-soaking of 800 h. The front of the glass substrate is covered by a broadband AR-coating. Reprinted with permission from [76]...

Here we describe the layer structure for single junction as well as for tandem solar cells consisting of a-Si H and pc-Si l I. Further, this section will deal with the stability of silicon thin film solar cells and the possibility to reduce degradation by special design. 

Fig. 8.4. Layer structure of single junction n-i-p (substrate) and p-i-n (superstrate) solar cells. Also included is an amorphous/microcrystalline tandem solar cell structure...

---