Solar Resources | Solar Photovoltaic Panels
SOLAR PHOTOVOLTAIC (PV) PANELS

1. MONO-CRYSTALLINE SOLAR CELLS
The most important material for crystalline solar cells is silicon. Next to oxygen it is the second most abundant element on earth. It is not a pure chemical element and is in the form of silicon dioxide (SiO2). At first, the unwanted oxygen has to be separated from silicon dioxide, by heating silica sand to a high temperature in a melting crucible together with coal powder. During this process, the so-called metallurgical silicon with a purity of 98 percent is created. Due to impurity of 2 percent, the raw silicon is further purified through a chemical process. The high-grade silicon can then be further processed to produce mono-crystalline cells, as well as poly-crystalline cells.

The Czochralski process (crucible drawing process) is commonly used to produce mono-crystalline silicon solar cells. During this process, a crystal nucleus with a defined orientation is immersed in a silicon-melting bath of more than 1420 deg C and taken out of the bath while slowly being turned. Cylindrical single crystals with a diameter of 0.3m and several metres in length can be produced in this way. The cylindrical single crystals are chamfered to slightly octagonal bars and then cut into thick slices of 200-300 microns (wafers). Starting from the already thick p-doped wafers, the thin n-doped layers are produced by phosphorus diffusion at temperatures of 800-1200 deg C. After the attachment of the rear contact layer, the wafers are equipped with electrical leads and with an anti-reflection (AR) layer in the front.

Layered Structure of Mono-Crystalline Cell:

Characteristics
Efficiency 15 - 18 percent module efficiency (stabilized condition)
Form round, semi-squared or squared
Thickness 300 micron thick mono-crystalline silicon layer
Structure homogeneous structure
Colour dark blue to black (with AR), grey (without AR)
2. POLY-CRYSTALLINE SOLAR CELLS
The most important material for crystalline solar cells is silicon. Next to oxygen it is the second most abundant element on earth. It is not a pure chemical element and is in the form of silicon dioxide (SiO2). At first, the unwanted oxygen has to be separated from silicon dioxide, by heating silica sand to a high temperature in a melting crucible together with coal powder. During this process, the so-called metallurgical silicon with a purity of 98 percent is created. Due to impurity of 2 percent, the raw silicon is further purified through a chemical process. The high-grade silicon can then be further processed to produce poly-crystalline cells, as well as mono-crystalline cells.

The ingot casting process is commonly used to produce poly-crystalline silicon solar cells. Raw material is heated under vacuum at up to 1500 deg C and then cooled down towards the base of the crucible, which has a temperature of about 800 deg C. Silicon blocks of 400 mm x 400 mm with a height of 300 mm are then created. The blocks are first sawn into bars and then into wafers with a thickness of 300 microns. After n-doping with phosphorus, the rear contact layer is attached. Subsequently, the electrical leads are fixed onto the front side. Subsequently, the electrical leads are fixed onto the front side with an anti-reflection (AR) layer.

Layered Structure of Poly-Crystalline Cell:

Characteristics
Efficiency 13 - 16 percent module efficiency (with AR)
Form squared
Thickness 300 micron thick poly-crystalline silicon layer
Structure crystals of various orientations are formed during block casting
Colour blue (with AR), silver grey (without AR)
3. AMORPHOUS SOLAR CELLS
Since the 1990s, development of thin-film processes (including amorphous silicon) for manufacturing solar cells has become more and more important. Here, amorphous silicon (a-Si) as photoactive semiconductor is applied as thin layers to a glass substrate. Because of the high light absorption of a-Si, total layer thickness of less than 1 micron is sufficient for converting sunlight into electricity. Compared with manufacturing temperatures of up to 1500 deg C for crystalline silicon cells, a-Si cells require disposition temperatures between 200-500 deg C. The reduced material and reduced energy consumption, and the advantage of having highly automated production, offer substantial savings when compared with crystalline silicon technology.

The electrical contact is created on the rear side with an opaque metal coating. On the front side facing the light, this function is fulfilled by a highly transparent and conductive metal oxide layer, the so-called TCO (Transparent Conductive Oxide). Typical TCO materials include zinc oxide (ZnO) and tin oxide (SnO2).

Layered Structure of Amorphous Cell:

Disadvantages:
  1. Low module efficiency, which means more solar panels are required to achieve the same nominal power (kWp).
Advantages:
  1. Lower production cost due to savings in energy, materials and time (greater automation).
  2. Better energy yield under cloudy conditions due to better utilization of diffuse and low light.
  3. Less affected by higher temperatures due to favourable (smaller) temperature coefficient.
  4. Less sensitive to shading due to their cell form (long narrow strips).
Characteristics
Efficiency 5 - 8 percent module efficiency (stabilized condition)
Form formless (freely selectable)
Thickness 1 - 3 mm substrate material (non-hardened glass) with 1 micron amorphous layers
Structure homogeneous structure
Colour red-brown to black