The relation between grain size, grain shape and grain distribution and the transmission pattern of solar spectrum of the films can be established by comparing the spectral profiles with the photomicrographs of the films [2, 11-16]. Snl2 and MnBr2 Films: Visual observations of plates 1 and 2 show that Snl2 (plate 1) films are a bit larger but more widely spaces while MnBr2 (plate 2) films are a bit smaller, more compacted and almost continuous throughout.

Wavelength (nm) Fig 1. Spectral Transmittance of VTF (Sn^ and MnBr, )

Fig 3. Spectral Transmittance of IRTSSF (Ag 15 and PbS )

A comparism between the surface microstructure and spectral transmittance of the films show that these films have high UV absorption and NIR reflection with high visible transmittance. The MnBr2 films, which are more closely packed, show higher UV absorption, IR reflection and improved visible transmission than the Snl2 films. This behaviour could also be due to the fact that films that are more oblong and form a closer network tend to absorb UV and reflect IR. On the other hand, the Snl2 films, which are more roundish and seem more, separated from each other, give room for improved transmittance of UV and NIR radiations [2]. In general, Snl2 and MnBr2 Films make transmission from highly absorbing to highly transmitting in the spectral range 320 to 440mm and from highly transmitting to highly reflecting in the range of 700 to 820mm. The spectral pattern therefore shows a gradual rise from zero to maximum absorption/minimum transmission in the ultraviolet and a gradual fall from minimum absorption/rise to maximum transmission in the visible region. This is followed by a gradual fall in transmission/rise in reflectance in the near infrared. Figure 1 [2]

PbBr2 and Pbl2 Films: PbBr2 (plate 3) have more roundish black grains, which are wider apart than Pbl2 (plate 4). Signs of agglomerating particles are shown by the somewhat more visible films of Pbl2 indicating closer and more extended films dotted in between the tiny grains. The generally wide and more uniform spacing of the grains indicates higher reflection of near infrared radiation. Figure 2.

Ag2S and PbS Films: Plates 5 and 6 show that the films have wider shiny (almost whitish) films with smaller tiny darker grains embedded in the shiny background. The grains are not roundish at all but have extensive oblong shapes, which form almost complete continuous network. This collaborated the spectral transmittance pattern (fig. 3), which show high UV and Visible absorption with an improved NIR transmission due to the large grains.

The comparisms between surface structure and spectral properties show that:-

Small oblong extended network-like grains allow high UV absorption, NIR reflection and substantial visible transmission through the tinny spaces in between the grains. This is the case with Snl2 and MnBr2 films.

Medium, darker, more roundish and widely spread gains promote high UV — VIS transmission and high NIR reflection at a wavelength of about 520 to 550mm as in PbBr2 and Pbl2 films.

Larger shinny grains that form almost a network of film favour high UV — VIS absorption and high NIR transmission at a cut of about 850mm as shown for Ag2S and PbS films.