Amorphous ( protocrystalline) and microcrystalline silicon solar cells

The use of thin-film silicon for SCs is one of the most promising approaches to realize both high performance and low cost due to its low material cost, ease of manufacturing and high efficiency. Microcrystalline silicon (pc) SCs as a family of thin film SCs formed by plasma CVD at low temperature are assumed to have a shorter carrier lifetime than single-crystal cells, and it is common to employ a p-i-n structure including an internal electric field in the same way as an amorphous SC. These cells can be divided into p-i – n and n-i-p types according to the film deposition order, although the window layer of the SC is the p-type layer in both cases. A large difference is that the underlying layer of a p-i-n cell is the transparent p-type electrode, whereas the underlying layer of an n-i-p cell is the n-type back electrode. Light-trapping techniques are a way of increasing the performance of mc – SCs. This is a core technique for cells made from pc-silicon because — unlike a-silicon — it is essentially an indirect absorber with a low absorption coefficient. That is, the thickness of the Si film that forms the active layer in a mc-silicon SC is just a few pm, so it is not able to absorb enough incident light compared with SCs using ordinary crystalline substrates. As a result, it is difficult to obtain a high photoelectric current. Light trapping technology provides a means of extending the optical path of the incident light inside the SC by causing multiple reflections, thereby improving the light absorption in the active layer (Yamamoto et al., 2004). Light trapping in this method of categorizing the SCs according to p-i-n or n-i-p types, can be achieved in two ways: (1) by introducing a highly reflective layer at the back surface to reflect the incident light without absorption loss, and (2) by introducing a textured structure at the back surface of the thin-film Si SC (see Fig.7.) (Komatsu et al.,2002; Yamamoto et al.,2004).

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Fig. 7. Cross-sections through light-trapping pc-silicon SC devices: (a) first generation (flat back reflector); (b) second generation (textured back reflector, thinner polycrystalline silicon layer). Reprinted with permission from Solar Energy Vol. 77, Kenji Yamamoto et al., A high efficiency thin film silicon solar cell and module, pp. 939-949 © 2004, Elsevier.

Although the pc-silicon cells formed at low temperature have a potential for high efficiently, their efficiency in single-cell structures is currently only about 10%, which is much lower than that of bulk poly crystalline cells. In order to achieve high efficiencies, Yamamoto et al.(2004) investigated the use of two-and three-stacked (hybrid) structures in which multiple

cells with different light absorption characteristics are stacked together. This approach allows better characteristics to be obtained with existing materials and processes. The advantages of using a layered structure include the following: (1) it is possible to receive light by partitioning it over a wider spectral region, thereby using the light more effectively; (2) it is possible to obtain a higher open-circuit voltage; and (3) it is possible to suppress to some extent the rate of reduction in cell performances caused by photo-degradation phenomena that are observed when using a-silicon based materials. Therefore, they have engaged in thin film amorphous and microcrystalline (a-Si/pc-Si) stacked solar cell (Yamamoto et al., 2004).

The advantage of a high Jsc for our pc-Si Sc as mentioned before was applied to the stacked cell with the combination of a-Si cell to gain stabilized efficiency as the study done by K. Yamamoto et al. (2001) since a-Si has a photo-degradation while a pc-Si cell is stable. They have also prepared three stacked cell of a-Si:H/pc-Si/c-Si (triple), which will be less sensitive to degradation by using the thinner a-Si and they have investigated the stability of a-Si:H/pc-Si/pc-Si (triple) cell, too (Yamamoto et al., 2001, 2004). Some other three stacked Si-based SCs can be named such as a-Si/a-SiGe/a-SiGe(tandem) and a-Si/nc-Si/nc-Si (tandem) SCs. The last efficiency reported for a-Si/a-SiGe/a-SiGe(tandem) is about 10.4% and for a-Si/nc-Si/nc-Si (tandem) is approximately 12.5% (Green et al., 2011).

As a next generation of further high efficiency of SC, the new stacked thin film Si SC is proposed where the transparent inter-layer was inserted between a-Si and pc-Si layer to enhance a partial reflection of light back into the a-Si top cell (see fig.8.). This structure is called as internal light trapping enabling the increase of current of top cell without increasing the thickness of top cell, which leads less photo-degradation of stacked cell (Yamamoto et al., 2004).

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thin film poly-Si Bottom Cell

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Sun-Light

Fig. 8. Schematic view of a-Si/poly-Si (pc-Si) stacked cells with an interlayer. Reprinted with permission from Solar Energy Vol. 77, Kenji Yamamoto et al., A high efficiency thin film silicon solar cell and module, pp. 939-949 © 2004, Elsevier.

By introduction of this interlayer, a partial reflection of light back into the a-Si top cell can be achieved. The reflection effect results from the difference in index of refraction between the interlayer and the surrounding silicon layers. If n is the refractive index and d is the thickness of inter-layer, the product of An*d determine the ability of partial reflection. Namely, the light trapping is occurred between the front and back electrode without inter­layer, while with inter-layer, it is also occurred between inter-layer and back electrode. This could reduce the absorption loss of TCO and a-Si:H (see Fig.9.) (Yamamoto et al., 2004).

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Fig. 9. Spectral-response of the cell with and without interlayer. Bold and normal line shows the spectral-response of the cell with and without inter-layer. Reprinted with permission from Solar Energy Vol. 77, Kenji Yamamoto et al., A high efficiency thin film silicon solar cell and module, pp. 939-949 © 2004, Elsevier.

Some of the advantages of thin film SCs are being characterized to low temperature coefficient, the design flexibility with a variety of voltage and cost potential. Therefore, the thin film Si SCs can be used for the PV systems on the roof of private houses as seen in Fig.10. (Yamamoto et al., 2004).

Another similar work was done by F. Meillaud et al.(2010). They investigated the high efficiency (amorphous/microcrystalline) "micromorph" tandem silicon SCs on glass and plastic substrates. High conversion efficiency for micromorph tandem SCs as mentioned before requires both a dedicated light management, to keep the absorber layers as thin as possible, and optimized growth conditions of the pc-silicon(pc-Si:H) material. Efficient light trapping is achieved in their work by use of textured front and back contacts as well as by implementing an intermediate reflecting layer (IRL)between the individual cells of the tandem. The latest developments of IRLs at IMT Neuch’atel are: SiOx based for micromorphs on glass and ZnO based IRLs for micromorphs on flexible substrates successfully incorporated in micromorph tandem cells leading to high, matched, current above 13.8mA/cm2 for p-i-n tandems. In n-i-p configuration, asymmetric intermediate reflectors (AIRs) were employed to achieve currents of up to 12.5mA/cm2 (see fig. 11.) (Meillaud et al., 2011).

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Fig. 10. KW system at the top of the building Osaka (HYBRID modules). Note that modules are installed at low angle (5 degree off from horizontal).

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Fig. 11. External quantum efficiency of the top cell in an n-i-p/n-i-p micromorph on flexible substrate (i-layer thickness:200nm). Reprinted with permission from Solar Energy Vol. 95, Meillaud et al., Realization of high efficiency micromorph tandem silicon solar cells on glass and plastic substrates: Issues and potential, pp. 127-130 © 2010, Elsevier.

The last efficiency reported by Oerlikon Solar Lab, Neuchatel for a-Si/pc-Si (thin film cell) is about 11.9% (Green et al., 2011, as cited in Bailat et al., 2010). More precisely talk about tandem cells will be done in following sections.