Amorphous silicon (a-Si) is disordered thin-film PV material, unlike crystalline silicon with its uniform lattice structure or poly-crystalline silicon with grains of crystal structure. Amorphous silicon is a material where some atoms in the structure remain unbonded, lacks long-range order, but can be produced more cheaply. Such a lack of long-range order in the structural arrangement of the atoms is the result of unsatisfied or “dangling” bonds. The lack of long-range order has a severe impact on the material properties of a-Si, and the “passivation” of these dangling bonds is required before a-Si can be used as solar cell material. Passivation incorporates atomic hydrogen with amorphous silicon to a level of 5-10%, saturating the dangling bonds, thereby improving the quality of the material.

Nevertheless, the material properties of a-Si are significantly different from those of crystalline silicon. For example, the band gap increases from 1.1 eV in crystalline silicon to 1.7 eV in amorphous silicon and the absorption coefficient of a-Si is much higher than that of crystalline silicon. In addition, the presence of the large number of dangling bonds cause a high defect density and low diffusion lengths.

The complete a-Si:H cell structure is shown on figure below. The standard amorphous silicon cell is made of multiple a-Si layers. Each layer is hydrogenated. The a-Si:H is sandwiched between a transparent conductor (tin oxide) and the back metallization (Al or Al/ZnO or ZnO), all layers being deposited on a glass or other substrate. The normal p- and n-type layers, plus an intrinsic (i-type) layer establish an electric field to separate the electron-hole pairs. These layers also determine the voltage of the device. The intrinsic, or undoped, layer is the active layer of the device, where most of the solar cell’s current originates. The polarity of the voltage is positive in the tin-oxide, and negative on the aluminum. The a-Si:H could consist of one or more p, i and n layers creating single, tandem or triple junction cells. For example tandem cells have better performance (then single junction) by stacking two cells on above the other to form a p-i-n/p-i-n structure.

a-Si tandem module cross section

Cross-Section of Tandem Junction a-Si Device

Structure of Monolithic a-Si Plate

Structure of Glass-Glass Laminated Module

Following are some of the positive features of amorphous silicon (a-Si) PV devices:

• The electrical performance of PV cells has traditionally been reported under Standard Test Conditions (STC). In bright sunlight, however, the operating temperature of a free-standing modules and cells can operate way above ambient temperature. This brings the issue of temperature coefficients for various PV technologies, so that modules lose output as their temperature is raised. The coefficient is -0.17 %/°C for a-Si and -0.5 %/°C for c-Si. Thus, under real conditions the performance of the amorphous module (despite its lower STC efficiency) approaches more closely that of the crystalline module.
• There is another effect that in practice improves the performance of a-Si relative to some other technologies, and that is low light performance. A deployed module spends only a small fraction of its time receiving full 1000W/m2. For ideal solar cells the output power should be roughly proportional to light intensity. a-Si obeys this rule, but crystalline silicon does not. In some applications this makes a big difference. a-Si perform superior under low light conditions.
• Another consideration is the softer I-V curve of a-Si relative to crystalline Si. The rounder curve makes it easier to draw the maximum power from an amorphous array, whereas for c-Si the power falls rapidly if the operating voltage is allowed to rise much beyond that of the maximum power point.
• Amorphous silicon modules have several other positive features, such as the softer reverse I-V characteristic, which don’t allow hot spots to build up if an individual cell is shaded. Another advantage is that for glass-based modules, the films are deposited on the rear side of the front glass, so that the encapsulating polymer (e.g. EVA) lies behind the solar cells and cannot reduce the transmission of light to the cells. Thus the loss in output observed in some wafer-based modules simply cannot occur.
• One of the known problems with amorphous silicon is the fact that it suffers from less-than-adequate efficiency. This problem is aggravated by the well known Staebler-Wronski effect (SWE), which degrades initial efficiencies by about 20% in natural sunlight. Initially the SWE intrigued the PV industry’s faith in a-Si as a PV technology until it became clear that after one to two months, a-Si modules become remarkably stable. As an example, a-Si array that has been tested at NREL’s outdoor test facility for 10 years shows less than 1% degradation per year, which is about the same rate at which crystalline silicon loses power over time. Today, a-Si manufacturers sell modules with power ratings that correspond to their estimate of degraded, stabilized performance.

Amorphous silicon very low manufacturing cost per watt, as well as module area, creates the space for manufacturing perfect building integrated PV (BIPV) elements (figure below), where large building envelope areas could be covered with the PV technology available at the lowest prices per square meter. Using a-Si as building skin elements could easily be cost comparable with standard insulating low-e coated double glazed units, except that thin-film a-Si units generate power and their pay-back could be calculated. Another great advantage of a-Si thin-films is a possibility to utilize them instead of the visual components by using transparent back conductors. Moreover, so far no other thin-film technology can be used for manufacturing semitransparent modules, which can be integrated into buildings in attractive designs.

STF is a global leader in development of manufacturing equipment for the thin-film BIPV module production Beside capabilities for providing manufacturing equipment for creating transparent PV modules (using ZnO as back contact), we also create machines for producing colored PV modules by depositing dichroic reflective interference filtering films, as well as utilizing some other coating techniques.