Photovoltaic Cells: Types, Performance, and Applications
Incorporation of a Solar Cell
A solar cell is formed from a wafer of semiconductor material, such as silicon, on which boron (a trivalent impurity) is spread to create a P-region and a thin layer of phosphorus (a pentavalent impurity) is spread to create an N-region, forming a PN junction. To improve performance, the sun-facing side undergoes surface treatment to reduce reflection. A metal mesh on this surface provides electrical connection while maximizing sun exposure. This mesh consists of fine metal plates with widths of 20-150mm. These grids form the negative terminal of the cell, while the positive terminal is achieved through metallization on the back. An anti-reflective layer is deposited on the front to further enhance photon absorption.
Types of Photovoltaic Cells
A. A Brief History of the Solar Cell
The first solar cell was developed in 1883 by Charles Fritts, who coated a semiconductor (selenium) with gold. In 1946, the first practical photovoltaic cell was produced. In 1954, Bell Labs discovered the light sensitivity of semiconductors doped with impurities. The space race of the late 1950s significantly influenced solar cell development for powering satellites. In 1970, the first gallium arsenide (GaAs) cell was made, which dominated manufacturing until the 1980s when silicon cells became more prevalent.
B. Types of Solar Cells
Various types of solar cells and modules are available in the market and research labs. The most common are:
- Monocrystalline silicon
- Polycrystalline silicon
- Amorphous silicon
Monocrystalline Silicon Cells
These cells have an efficiency of 15 to 18%.
Polycrystalline Silicon Cells
These cells have an efficiency between 13 and 16%, with values increasing annually.
Amorphous Silicon Cells
These cells have an efficiency of 6-9%.
Gallium Arsenide Cells
These cells offer a yield of 25%.
Loss and Performance of Photovoltaic Solar Cells
Losses in photovoltaic cells occur due to various factors:
- Photon energy too low to generate electron-hole pairs (22% loss)
- Photon energy too high, breaking the silicon bond (30% loss)
- Recombination of electrons and holes (8.5% loss)
- Reflection of solar radiation and shading from electrical connections (3% loss)
- Cell voltage loss due to resistance (20% loss)
Parameters of a Photovoltaic Cell
The current-voltage curve defines the behavior of a photovoltaic cell. Key parameters include:
- ICC: Short circuit current
- VCA: Open circuit voltage
- Peak Power (WP): The maximum electrical power a cell can provide
Two factors influence solar cell behavior:
- Voltage across a PN junction varies with temperature.
- Current supplied by a solar cell is proportional to radiation intensity and cell surface area.
Photovoltaic Modules
Photovoltaic modules consist of multiple solar cells (typically 36-96) connected in series or parallel and protected from the weather.
Structure of a Photovoltaic Module
A photovoltaic module comprises:
- Front Cover: Usually 4mm thick tempered glass
- Encapsulation: Typically ethyl vinyl acetate (EVA)
- Back Cover: Often a layer of polyvinyl fluoride (PVF)
- Frame: Usually anodized aluminum for rigidity and strength
- Access Box: Located at the rear, with dust and water protection
- Cells: Connected with welded metal strips or embedded grids
Photovoltaic Module Parameters
Key operational characteristics of a PV module include:
- Operating Point: Determined by the connected load resistance and solar radiation
- Current: Proportional to the irradiation site and limited by the short circuit current
- Temperature Effects:
a. Open circuit voltage decreases with increasing temperature.
b. Short circuit current increases with increasing temperature.
c. Power output decreases with increasing temperature.
Power and Production Tolerance
Manufacturers classify photovoltaic modules by their peak power, which is the maximum power they can generate at the maximum power point of their VI characteristic curve. Tolerances exist for each module’s parameters. Over a module’s lifetime (estimated around 25 years), aging leads to a decrease in generated power. Manufacturers typically guarantee 90% power for the first 10 years and 80% power for 25 years.
Hot Spot Effect
Shading of a cell in a series-connected PV module can cause it to act as a power dissipater, leading to overheating, known as the hot spot effect. To prevent damage from localized heating, bypass diodes are used with series-connected cells.
Connecting Photovoltaic Modules
When a single module’s current and voltage are insufficient, multiple modules can be grouped to achieve the desired values. Connecting modules in series increases voltage, while connecting them in parallel increases current. A photovoltaic generator refers to the entire set of modules in a system. Three connection types are common:
Connecting Modules in Series
The positive terminal of one module is connected to the negative terminal of the next. This configuration results in the generator’s current being equal to that of a single module, while the generator voltage is the sum of the individual module voltages.
IG (Generator Intensity) = IM (Module Intensity)
VG (Generator Voltage) = Ns (Number of Modules in Series) x VM (Module Voltage)
Connecting Modules in Parallel
The positive terminals of all modules are connected together, as are the negative terminals. This configuration results in the generator’s current being the sum of the individual module currents, while the generator voltage remains the same as that of a single module.
IG = Np (Number of Modules in Parallel) x IM
VG = VM