Window layers

This confinement yields a higher carrier density of elections and holes in the active layer and fast ladiative lecombination. Thus LEDs used in switching apphcations tend to possess thin DH active layers. The increased carrier density also may result in more efficient recombination because many nonradiative processes tend to saturate. The increased carrier confinement and injection efficiency faciUtated by heterojunctions yields increasing internal quantum efficiencies for SH and DH active layers. Similar to a SH, the DH also faciUtates the employment of a window layer to minimise absorption. In a stmcture grown on an absorbing substrate, the lower transparent window layer may be made thick (>100 /tm), and the absorbing substrate subsequendy removed to yield a transparent substrate device. 

Eig. 3. Depiction of the light extraction, ie, escape cones of light emission, for various LED chip stmctures consisting of absorbing substrate devices having (a) thin window layers (top cone) (b) thick window layers (top cone and four one-half side cones) (c) thin window plus the implementation of a distributed Bragg reflector between the active layer and the substrate (top and bottom cone). Also shown is (d), the optimal stmcture for light extraction, a... 

Emission L, nm Active layer material Stmcture Window layer material Substrate Lattice matched Growth technique Other... 

During epitaxial growth, the semiconductor layers must be doped to form thep—n junction and conductive current spreading window layers. Eor III—V materials, zinc, Zn beryUium, Be carbon, C magnesium. Mg and siUcon, Si are commonly employed as -type dopants, whereas tellurium, Te ... 

Two different high efficiency types of GaAs cell stmctures ate being developed. These are GaAs homojunctions with a top window layer of Ga ... 

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... 

OPTICAL PROPERTIES. The most commonly reported optical properties are optical transmission, with some smdies also on photoluminescence. The importance of the optical transmission for CdS in particular lies in its use in photovoltaic cells, where it acts as a window layer. The CdS should be as transparent as possible to the incoming radiation. The transmission is a function of thickness, bandgap, and fihn structure (is the fihn transparent or scattering ). The bandgap in most studies is constant (ca. 2.45 eV at room temperature), although somewhat larger values have been obtained due to size quantization in very small crystalhtes. 

It should be pointed out that this deposition was carried out for films ca. 50 nm thick the study was carried out with CdS window layers for solar cells in mind, which are usually thin. It is possible that much longer depositions result in different impurities. Thus the sparingly soluble cadmium carbonate and cyanamide will be converted to CdS if enough sulphide ion is formed with time (or, for the complex-decomposition mechanism, if enough adsorbed thiourea decomposes on the surface of the solid phases). Of course, longer time also means more thiourea decomposition products. 

A number of mixed sulphide/hydroxides have been deposited, mainly in the search for improved window layers for photovoltaic cells (Chap. 9). These are mostly probably mixed-phase films, although in one case, ln(OH)S, experimental evidence suggests true compound formation [46]. Most of these films have been dealt with in previous chapters (see Chap. 4 under ZnS and Chap. 6 under In and Sn sulphides). One study (described from the viewpoint of its properties in photovoltaic cells in Chap. 9) has not been described previously and will be mentioned briefly here. This deals with Zn(0,0H) and Zn(0,0H,S) deposited from Zn-ammine solutions, the latter film from solutions also containing thiourea [47]. It is of interest to note that the Zn(0,0H) films did not deposit on glass but did on both ZnO- and CulnSe2-type substrates. Even after annealing at 300°C, hydroxide groups were still present in those films. 

Fig. 4.2. Structure and energy band diagram of a Cu(In,Ga)Se2 (CIGS) thin-film solar cell. The ZnO window layer typically consists of a combination of a nominally undoped ZnO and a highly doped ZnO layer...

Thin ZnO films can be used either as a transparent and conductive window layer, or as a buffer layer, within CuInS2 (CIS) and Cu(In,Ga)Se2 (CIGS) thin film solar cell devices (see Chaps. 4 and 9). In both cases, the ZnO layers... 

Fig. 6.47. A CIS solar cell that uses ZnO layer as window layer (front TCO) and/or buffer layer...

ZnO can be advantageously used as TCO layer in various kinds of thin film silicon solar cells, either as back or as front contact, or even as an intermediate reflector between the amorphous and the microcrystalline p-i-n junctions, within the micromorph tandem solar cell [58] (see also Chap. 8). Figure 6.48 illustrates the various possibilities for using a ZnO layer within a thin film silicon solar cell. In the present paragraph, we will comment about the use of CVD ZnO both as front contact (or window layer) and also as back contact (or part of the back reflector). 

Front ZnO layers Somewhat similarly to the case of ZnO films that are used as window layers in CIS/CIGS solar cells, the front ZnO in a thin film silicon solar cell has to fulfill the following criteria ... 

Fig. 6.48. A micromorph tandem cell, for which a ZnO layer can be used as front contact (i.e., window layer), back contact, and/or intermediate reflector [58]...

In 1981 an efficiency of 7.5% was obtained for a p-i-n structure (3.3 mm2) in which the p layer was a boron-doped silicon-carbon-hydrogen alloy (a-Si C H) (Tawada et al., 1981). A further improvement in conversion efficiency to 8.5% was obtained in 1982 with a stacked junction structure (9 mm2) that utilized an amorphous silicon-germanium-hydrogen alloy (a-Si Ge H) in the back junction of three stacked p-i-n junctions (Nakamura et al., 1982). More recently, an efficiency of 10.1% has been achieved in a p-i-n structure (1.2 cm2) utilizing p-type a-Si C H as a window layer (Catalano et al., 1982). 

Abstract ZnO layers are very frequently used as window layers in photovoltaic solar cell... 

Key words Polycrystalline films, heterointerface window layers, electrical transport, doping, light trapping... 

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