Because amorphous silicon is a random network structure with long-range disorder, it exhibits strong scattering of charge carriers, resulting in ineffective carrier collection. To improve the conversion efficiency and stability of amorphous silicon solar cells, the pn structure of monocrystalline silicon solar cells is generally not adopted. This is because lightly doped amorphous silicon has a small Fermi level shift. If both sides are lightly doped, or one side is lightly doped while the other side is heavily doped, the band bending is small, limiting the cell's open-circuit voltage. If heavily doped p+ and n+ materials are directly used to form a p+-n+ junction, the cell performance will be poor due to the high defect state density and low minority carrier lifetime in the heavily doped amorphous silicon material. Therefore, an undoped amorphous silicon layer (i-layer) is typically deposited between the two heavily doped layers as the active collector region, i.e., the pin structure.
In amorphous silicon solar cells, photogenerated carriers are primarily generated in the undoped i-layer. Unlike crystalline silicon solar cells, where carrier movement is mainly due to diffusion, in amorphous silicon solar cells, photogenerated carriers drift primarily due to the influence of the internal electric field because of their short diffusion length. When amorphous silicon cells adopt a pin structure, they can operate under illumination. However, due to light-induced degradation, the cell performance is unstable, and the conversion efficiency gradually decreases with prolonged illumination. Therefore, the cell structure and manufacturing process require further optimization.
Factors affecting the performance of amorphous silicon solar cells
The main factors affecting the conversion efficiency and stability of amorphous silicon solar cells include: the transparent conductive film, the properties of the window layer (including the optical bandgap width, conductivity, doping concentration, activation energy, and light transmittance), the interface state (interface defect state density) and bandgap matching between layers, the thickness of each layer (especially the thickness of the i-layer), and the solar cell structure. Amorphous silicon thin-film solar cells are generally structured in a stacked manner, integrated, or heterojunction form.
Amorphous silicon solar cells have a simple manufacturing process, operate at low temperatures, and consume little energy, leading to a year-on-year increase in their market share. Currently, more than half of thin-film solar cell companies use amorphous silicon thin-film technology, and it is expected that amorphous silicon thin films will occupy a major share in future thin-film solar cells within a few years. However, low photoelectric conversion efficiency and light-induced degradation are the two major problems currently existing in amorphous silicon thin-film solar cells. To improve efficiency and stability, further exploration is needed in new device structures, new materials, new processes, and new technologies.
For example, in terms of battery structure, layered and integrated designs can be adopted; in terms of transparent conductive films, transparent conductive films that not only have low resistivity but also block ion contamination, increase incident light absorption, and have anti-radiation effects can be used to replace current conductive films such as ITO, ZnO, and ZnO#Al; in terms of window layer materials, new window layer materials with wide optical band gaps and low resistance can be explored, such as amorphous silicon carbon, amorphous silicon oxide, microcrystalline silicon, and microcrystalline silicon carbon; in terms of amorphous silicon thin film preparation technology, RF-PECVD, ultra-high vacuum PECVD, very high frequency (VHF) PECVD, and microwave PECVD technologies can be improved to extend the photon lifetime of the thin film, improve the carrier transport capacity, and enhance the electronic properties and stability of the thin film; in terms of interface treatment, technologies such as hydrogen passivation and the insertion of buffer layers can be adopted to reduce interface recombination losses and improve the battery's short-circuit current and open-circuit voltage.
Although low efficiency and unstable performance are currently the main obstacles to the large-scale industrial production of amorphous silicon thin-film solar cells, various technologies for optimizing amorphous silicon thin-film cells are still feasible. With further technological development, amorphous silicon thin-film solar cells will be widely used.
