Solar Cells and Modules by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9783030464875
Publisher: Springer International Publishing

To enable textured surfaces, reducing, thus, reflection—and allowing more light to enter into the solar cell.

To provide high mobility of the charge carriers leading to high conductivity, and, thus, to higher electric currents.

To provide absorber layers with negligible degradation.

Heterojunction cells combine the positive aspects of amorphous thin-film technology with those of crystalline technology. In a nutshell, it can be said that the amorphous layer serves as a passivation layer, leading to the reduction of recombination centres at the surface, and that the crystalline bulk acts as an «energy conversion machine».

Due to the excellent passivation quality of amorphous silicon, the recombination of charge carriers on the surface of the crystalline silicon wafer is no longer the main limiting factor for the efficiency η of the solar cell (see also Chap. 3). Rather, the quality of the wafer material comes to the fore. The easiest way to improve wafer quality is to use n-doped wafer material instead of the commonly used p-doped material. n-doped wafers are doped with phosphorus. In these n-doped wafers, the lifetime of the minority charge carriers (holes) is roughly 5 ms (ms). As a comparison: in p-doped wafers the lifetime of the minority charge carriers (electrons) is approximately 1 ms. As a result, the diffusion length1 is about double as high in n-doped material than in p-doped wafers.

The structure of a typical heterojunction cell can be seen in Fig. 7.3. We can identify a total of six layers at the top and bottom. On each side there are two amorphous layers: an intrinsic one a-Si: H(i) and a doped layer a-Si:H(n or p); the latter are covered by TCO layers. The following section describes this simple and symmetrical structure and presents its outstanding advantages over other cell concepts.

Fig. 7.3Cross-section of a bifacial heterojunction cell. The p-n junction is in this case on the front side

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