In poly silicon thin film people usually use nip structure to obtain higher absorption at the bottom p-doped layer. However, why as for a-Si:H solar cell design, pin structure is preferred? In other words, what is the benefit to doing so?
Dear Chinjin,
From the conceptual point of one can use either layer stacking pin or nip. It may be that the layer stacking is constrained by the technological steps used to fabricate the structure. From the theoretical point of the thickness of the active material must be greater or equal than the penetration depth of the absorbed solar radiation. From other side, it must be smaller the diffusion length of minority carriers in this active material. If the thickness satisfies the two conditions simultaneously, then the layer stacking will not affect the performance of the solar cell.
As for the thin film amorphous pin solar cells, the major absorbing layer is the i-layer and the end n+ and p+ regions act as a collector regions for the electrons and holes respectively. The generated electrons diffuse in the i-layer to to the n+-layer and the holes diffuse to the p+-layer. The stacking is made pin from the front to the back side because the n type dopants diffuse in silicon slower than the p type dopants then preserving the pin layer structure. The other cause is that the diffusion length of the electrons is greater than that of holes. If the light is incident from the p+-side the holes will be nearer the pi junction and more holes can be collected than if the p+- layer is put at the bottom.
As for the thin film poly crystalline solar cells, it has the stacking from the side of the light incidence n+ p p+. So, it is not a pin cell. It has the layer stacking as the conventional single crystal solar cell. It is produced by plasma enhanced chemical vapor deposition CVD. The first layer to be deposited on the substrate after the anti reflective silicon nitride layer is the n+-layer following the moderately doped p layer and then the the p+ back surface field layer. It here very clear that one selected the n+-layer to start with because the n type dopants diffuse slower into the above lying layer and therefore preserve the layer structure. This means that the n type dopants has less inter diffusion during the deposition of the other layers and the subsequent annealing. at relatively high temperature.
wish you success
The reason is the gap energy of amorphous silicon are suitable for absorbing the energy of the photons of sunlight
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