Crystalline Solar Cells

Introduction

Most solar cells are made of a single crystal or multi-crystalline silicon material. Silicon ingots are made by the process of crystal growth, or by casting in specially designed furnaces. The ingots are then sliced into thin wafers. Single crystal wafers are usually of 125x125 mm or larger sizes with 'pseudo-square' shape; multi-crystalline wafers are typically square-shaped with a dimension of 100 X 100 mm or larger. Using high-temperature diffusion furnaces, 'impurities' like boron or phosphorous are introduced into the silicon wafers to form a p-n junction. The silicon wafers are thus converted into solar cells. When exposed to sunlight, a current is generated in each cell. Contacts are attached to the top and bottom of each solar cell to enable inter-connections and drawing of the current.

Thin-Film solar cells

Thin-Film solar cells are made from amorphous silicon (a-Si), cooper indium selenide/cadmium sulphide (CuInSe2/CdS) or cadmium telluride/cadmium sulphide (CdTe/CdS), by using thin-film deposition techniques. These technologies are at various stages of development and have not yet reached the maturity of crystalline silicon. Production of thin-film PV modules is also limited.

PV module

PV modules are usually made from strings of crystalline silicon solar cells. These cells are made of extremely thin silicon wafers (about 300 Um) and hence are extremely fragile. To protect the cells from damage, a string of cells is hermetically sealed between a layer of toughened glass and layers of ethyl vinyl acetate (EVA). An insulating tedlar sheet is placed beneath the EVA layers to give further protection to the cell string. An outer frame is attached to give strength to the module and to enable easy mounting on structures. A terminal box is attached to the back of a module; here, the two ends (positive and negative) of the solar string are welded or soldered to the terminals. This entire assembly constitutes a PV module. When the PV module is in use, the terminals are connected either directly to a load, or to another module to form an array. Single PV modules of capacities ranging from 10 Wp to 120 Wp can provide power for different loads. For large power applications, a PV array consisting of a number of modules connected in parallel and/or series is used.

Standard Capacity/ Ratings and Specifications

The wattage output of a PV module is rated in terms of peak watt (Wp) units. The peak watt output power from a module is defined as the maximum power output that the module could deliver under standard test conditions (STC). The STC conditions used in a laboratory are

• 1000 watts per square metre solar radiation intensity
• Air-mass 1.5 reference spectral distribution
• 25oC ambient temperature.

SPV modules of various capacities are available, and are being used for a variety of applications. Theoretically, a PV module of any capacity (voltage and current) rating can be fabricated. However, the standard capacities available in the country range from 5 Wp to 120 Wp. The voltage output of a PV module depends on the number of solar cells connected in series inside the module. In India, a crystalline silicon module generally contains 36 solar cells connected in series. The module provides a usable direct current (DC) voltage of about 16.5 V, which is normally used to charge a 12-V battery.

In an SPV system, the components other than the PV module are collectively known as 'balance of system' (BoS), which includes batteries for storage of electricity, electronic charge controller, inverter, etc. These batteries are charged during the daytime using the DC power generated by the SPV module. The battery/battery bank supplies power to loads during the night or non-sunny hours. An inverter is required to convert the DC power from the PV module or battery to AC power for operating the load. Some loads such as DC pumps do not require an inverter or even a battery bank.

Main Application

Solar Energy

For further information please contact

Ministry of Non-Conventonal Energy Sources,
Govt. of India
Block-14, CGO Complex, Lodi Road
New Delhi – 110 003
Tel: 91 11 2436 0707/ 2436 0404
Fax: 91 11 2436 1298