Thin-Window Cells

Aqueous cathodic electrodeposition has been shown to offer a low-cost route for the fabrication of large surface n-CdS/p-CdTe solar cells. In a typical procedure, CdTe films, 1-2 xm thick, are electrodeposited from common acidic tellurite bath over a thin window layer of a CdS-coated substrate under potential-controlled conditions. The as-deposited CdTe films are stoichiometric, exhibit strong preferential (111) orientation, and have n-type conductivity (doping density typically... 

Fig. 1.6. Thin layer cell for in situ IR absorption-desorption measurements based in a model proposed by Seki et al. [32]. WE = working electrode, W = window, L=lead, M = micrometer screw, S = steel spring, SM = simmer gasket.

External reflectance. The most commonly applied in situ IR techniques involve the external reflectance approach. These methods seek to minimise the strong solvent absorption by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The result is a thin layer of electrolyte trapped between electrode and window usually 1 to 50 pm. A typical thin layer cell is shown in Figure 2.40. 

To minimize absorption from the solution, optical thin layer cells have been designed. The working electrode has the shape of a disc, and is mounted closely behind an IR-transparent window. For experiments in aqueous solutions the intervening layer is about 0.2 to 2 ftm thick. Since the solution layer in front of the working electrode is thin, its resistance is high this increases the time required for double-layer charging - time constants of the order of a few milliseconds or longer are common - and may create problems with a nonuniform potential distribution. 

M.J. Souza, Super Thin Windows for High Density 3He Target Cells, Fusion, 42,... 

The thin layer cell was composed of two quartz plates as the windows and a polymer film as the spacer of 4 pm to 20 pm in thickness. Once molten 1 is introduced into the thin layer cell and cooled to room temperature, the amorphous phase becomes stable enough for practical use. The transparency was maintained for more than one year in the thin layer cell. X-ray diffraction study showed no diffraction peak, and polarized microscopy observation also indicated the amorphous state. 

The basic structure of polycrystalline cadmium telluride (CdTe) thin-film cells has a glass superstrate and a layer of TCO as front contact, a near-transparent n-type cadmium sulfide (CdS) window layer, p-type CdTe, and a metallic rear contact. The CdTe is usually deposited by three families of techniques. In the first group (vapor transport deposition, close space sublimation, physical vapor deposition, and sputtering) elemental vapors of Cd and Te condense and react on the substrate. In the second (electrodeposition), Cd + and HTe02" ions in acidic electrolyte are galvanically reduced at the surface ... 

The thin-layer cell used in SNIFTIRS experiments is essentially the same as that used in EMIRS. However, Pons and co-workers [92] (SNIFTIRS), Bewick et al. [53] (EMIRS), and Kunimatsu and co-workers [94] (using the IRRAS technique) have all employed prismatic CaF2 windows with 65° bevelled edges [see Fig. 39(b)]. There are two main advantages of this arrangement. 

The ferrous-exchanged zeoHte was transferred as a slurry in distilled water into a Pyrex glass Mossbauer sample cell under oxygen-free, nitrogen gas. The ceU had two very thin Pyrex windows separated by a 1 mm gap. The sample filled this gap after the excess water had been removed under vacuum. It is essential to keep the dry zeoHte free from contact with air or oxygen as some immediate oxidation to the Fe + state can occur. The thin windows of the cell aUowed some 40% transmission of the 14.4 keV y-rays to occur. The Mbssbauer spectrometer used and the complete experimental details are described in detail elsewhere [7]. 

The precise size of the gap between the optical windows is determined from the number of interference fringes in the spectrum recorded when the thin-layer cell is empty [43]. Panel 13 B shows the transmittance of an empty cell (Wqo = 1) equipped with ZnSe windows and a lO-pm thick Teflon spacer. An interference pattern arises because of the multiple reflections from the internal walls of the cell. The number of interference fringes AN in the frequency range Vi i>2 is related to the cell thickness d in the following fashion [43] ... 

A thin-layer cell is fabricated based on two optically transparent windows. The thickness of the electrolyte layer is dictated by the thickness of the optically transparent working electrode, usually a gold minigrid, indium-doped tin-oxide-coated glass slide or a reticulated vitreous carbon slice. This approach is not surface-sensitive, and it is effectively restricted to nonaqueous solvents, because the minimum electrolyte thicknesses that can be achieved cause severe attenuation in the IR beam. 

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