Internationally, the standard strip width is approximately 1 cm, referred to as a sub-cell. These sub-cells are connected in series internally. Therefore, the output current of an integrated solar cell is the current of each sub-cell, and the total output voltage is the series voltage of all sub-cells. In practical applications, the structure and area of the cell can be selected according to the required current and voltage to fabricate amorphous silicon solar cells.
The first layer is ordinary glass, which serves as the base for the battery.
The second layer is TCO, or transparent oxide conductive film.
On the one hand, light passes through it and is absorbed by the battery, so it needs high light transmittance; on the other hand, as an electrode of the battery, it needs to be conductive. TCO is generally fabricated with a textured surface, mainly to reduce reflected light and thus increase light absorption. Solar cells are formed by depositing these two layers as substrates.
The first layer of a solar cell is the P-layer, also known as the window layer; the second is the i-layer, the intrinsic layer of the solar cell, where photogenerated carriers are mainly generated; then comes the n-layer, which connects the i-electrode and the back electrode. Finally, there are the back electrode and the Al/Ag electrode.
Due to the numerous defects in a-Si (amorphous silicon), the pn junction of a-Si is unstable, and its photoconductivity is negligible under illumination, resulting in almost no effective charge collection. Therefore, the basic structure of a-Si solar cells is not a pn junction but a pin junction. Boron doping forms the P-region, phosphorus doping forms the n-region, and the i-region is the intrinsic layer, which is undoped or lightly doped (because undoped a-Si is weakly n-type). The heavily doped p and n regions form a built-in potential inside the cell to collect charge. Simultaneously, both can form ohmic contacts with the conductive electrodes to provide power to the external circuitry. The i-region is the photosensitive region, where photogenerated electrons and holes are the source of photovoltaic power. The goal is to maximize the absorption of incident light into the i-region and effectively convert it into electrical energy. Therefore, the i-region must both maximize the absorption of incident light and maximize the transport of photogenerated carriers to the external circuitry.
The long-range disorder of amorphous silicon structures disrupts the momentum conservation selection rule for electronic transitions in crystalline silicon, effectively transforming it from an indirect bandgap material into a direct bandgap material. It exhibits a very high absorption coefficient for photons; typically, a layer of a-Si with a thickness of around 0.5 μm can absorb almost all light in the sensitive spectral region. Therefore, the thickness of a-Si cells with pin structures is approximately 0.5 μm, while the thickness of the p and n layers, which constitute the dead light absorption region, is on the order of 10 nm.
