SINGLE-CRYSTAL SILICON PHOTO VOLTAIC (PV) CELLS

What Are Single Crystal PV Cells?

While single crystal silicon cells are still the most common cells, the fabrication process of these cells is relatively energy intensive, resulting in limits to cost reduction for these cells. Since single-crystal silicon is an indirect bandgap semiconductor (Eg = 1.1 eV), its absorption constant is smaller than that of direct bandgap materials.

This means that single-crystal silicon cells need to be thicker than other cells in order to absorb a sufficient percentage of incident radiation.

This results in the need for more material and correspondingly more energy involved in cell processing, especially since the cells are still produced mostly by sawing of single-crystal silicon ingots into wafers that are about 200 mm thick.

To achieve maximum fill of the module, round ingots are first sawed to achieve closer to a square cross-section prior to wafering.

After chemical etching to repair surface damage from sawing, the junction is diffused into the wafers. Improved cell efficiency can then be achieved by using a preferential etch on the cell surfaces to produce textured surfaces.

The textured surfaces reflect photons back toward the junction at an angle, thus increasing the path length and increasing the probability of the photon being absorbed within a minority carrier diffusion length of the junction.

Following the chemical etch, contacts, usually aluminum, are evaporated and annealed and the front surface is covered with an anti reflective coating. The cells are then assembled into modules, consisting of approximately 33 to 36 individual cells connected in series.

Since the open-circuit output voltage of an individual silicon cell typically ranges from 0.5 to 0.6 V, depending upon irradiance level and cell temperature, this results in a module open circuit voltage between 18 and 21.6 V.

The cell current is directly proportional to the irradiance and the cell area. A 4-ft2 (0.372-m2) module (active cell area) under full sun will typically produce a maximum power close to 55 Wat approximately 17 V and 3.2 A.