In comparison to crystalline solar cells, thin-film solar cells use significantly less material. The development of thin-film solar cells was in part driven by high material costs of crystalline silicon. Thin-film solar cells were much more efficient in terms of material usage and had potentially lower fabrication costs. Thin-film solar cells are fabricated from layers of doped semiconducting materials. These layers produce a charge separating junction often in the form of a p-i-n junction (McEvoy et al. 2003). As they use significantly less material and are inherently thin, thin-film solar cells can be semitransparent to visible light. When a transparent substrate (such as glass) is used, this can allow some irradiance to pass through the device (Shah et al. 2004). Although a single-junction thin-film solar cells can be relatively inefficient in comparison with their crystalline counterparts, several junctions can be stacked so as to produce a more efficient device (Shah et al. 2004). These multi-junction devices can be made from identical junctions or the junctions can be tuned to different wavelengths and parts of the solar spectrum so as to absorb as much of the spectrum as possible. There are a range of examples of thin-film solar cells, and one of the most well-known examples is amorphous silicon.

Hydrogenated amorphous silicon solar cells have been in development since the late 1970s (Wilson 1980). Although thought to be a good alternative to crystalline silicon, amorphous silicon solar cells had one significant drawback in the form of a light-induced degradation, photodegradation, known as the Staebler-Wronski Effect. This is the process whereby the performance and efficiency of the amor­phous silicon solar cell degrade upon extended exposure to light (Staebler and Wronski 1977). The degradation occurs over a period of time as the solar cell is exposed to light. The efficiency and performance of the cell degrade asymptotically to a stabilized minimum, upon which point the stabilized cell does not degrade any further. Any additional exposure to light after this point has minimal effect on the solar cell’s performance. This light-induced degradation can be revered by annealing the cell above 150 °C for a period of time (Staebler and Wronski 1977). Despite these drawbacks to amorphous silicon, a thin-film amorphous silicon solar cell has been produced on a antireflection-coated glass substrate with a reported stabilized efficiency of 9.47 % (Meier et al. 2004). The spectral response from this solar cell is shown in Fig. 15.3. As can be seen, the device had lower quantum efficiency in the blue and the infrared portions of the spectrum compared to the crystalline silicon PERL solar cell. However, it does have a comparable peak in efficiency in other parts of the solar spectrum.