Photovoltaic Cell (Solar Cell)

A photovoltaic cell is a semiconductor-based energy conversion device that directly transforms the energy of photons in sunlight into electrical energy. Its fundamental operating principle is based on the photovoltaic effect. This effect occurs when photons generate electron–hole pairs in a semiconductor material, and these charge carriers are separated by an electric field, resulting in an electric current. The term “photovoltaic” is derived from the Greek word photo, meaning light, and voltaic, named after Alessandro Volta, who designed the first device to produce electric current.

History

The photovoltaic effect was first observed in 1839 by Alexandre Edmond Becquerel using platinum electrodes. In 1873, Willoughby Smith discovered the photoconductive property of selenium. In 1883, Charles Fritts developed the first photovoltaic cell based on selenium. In 1954, silicon-based cells produced at Bell Laboratories achieved an efficiency of 6%. Between 1957 and 1960, Hoffman Electronics increased efficiency to 14%. The energy crises of the 1970s accelerated research efforts. In 1985, cells with 20% efficiency were produced. In 2006, Spectrolab achieved 40% efficiency using multi-junction cells. By 2009, this figure reached 41.6%.

Structure and Operating Principle

Photovoltaic cells typically feature a p–n junction structure formed by the combination of p-type and n-type semiconductor layers. Semiconductors such as silicon, gallium arsenide (GaAs), cadmium telluride (CdTe), and copper indium gallium selenide (CIGS) are commonly used. P-type semiconductors are created by doping with elements having three valence electrons, while n-type semiconductors are produced by doping with elements having five valence electrons. At the interface between the p and n layers, a natural electric field is formed. When photon energy from sunlight exceeds the bandgap energy of the semiconductor, electrons in the valence band are excited to the conduction band, generating electron–hole pairs. The electric field separates these pairs, causing a current to flow through an external circuit.

Types

First-generation cells, known as crystalline silicon cells, are divided into monocrystalline and polycrystalline types. Monocrystalline cells have high purity and a regular crystalline structure, resulting in high efficiency, but their production cost is relatively high. Polycrystalline cells have irregularities in their crystalline structure, which leads to lower efficiency compared to monocrystalline cells; however, they are more widely used due to simpler manufacturing processes and lower costs.

Second-generation cells are thin-film cells made from materials such as amorphous silicon, cadmium telluride (CdTe), and copper indium gallium selenide (CIGS). They offer advantages such as flexibility, light weight, low cost, and portability; however, their efficiency is generally lower than that of crystalline silicon-based cells.

Third-generation cells, representing new technologies, incorporate advanced designs such as organic photovoltaics, dye-sensitized cells, and perovskite cells. These types aim to achieve high efficiency through multi-junction structures.

A ground-mounted photovoltaic (PV) panel array (generated by artificial intelligence.)

Electrical Characteristics

The performance of photovoltaic cells is evaluated using current–voltage (I–V) and power–voltage (P–V) curves. The open-circuit voltage (Voc) is the voltage measured when the cell current is zero, while the short-circuit current (Isc) is the current measured when the cell voltage is zero. The fill factor (FF) is the ratio of the maximum power output to the product of the open-circuit voltage and short-circuit current, indicating the cell’s ideality. The maximum power point (MPP) is the operating point at which the product of current and voltage is highest. Different cell types achieve efficiencies between 10% and 30% under laboratory conditions and between 5% and 20% under real-world conditions.

Efficiency and Influencing Factors

Efficiency is the ratio of the electrical power generated to the solar energy incident on the cell. An increase in cell temperature negatively affects the electrical properties of the semiconductor material, reducing efficiency; therefore, cooling is of great importance. Material properties, quality of optical coatings, surface contamination (dust, dirt), angle of incidence of radiation, and spectral composition directly influence efficiency.

Experimental Findings (Air Gap and Temperature Effects)

In a study involving 150 W monocrystalline PV panels positioned at different heights (0, 10, 20, and 30 cm), the panel at 20 cm height achieved the highest efficiency of 19.66%. The air gap behind the panel reduces cell temperature through natural convection, thereby improving efficiency; however, an excessive increase in the air gap size can reduce the effectiveness of natural convection and negatively impact efficiency.

Mathematical Models and I–V Characteristics

The electrical behavior of PV cells is commonly described using equivalent circuit models. The simplest model consists of a current source in parallel with a diode and series and shunt resistances. The four-parameter model uses light current (IL), reverse saturation current (I0), series resistance (Rs), and diode ideality factor (α). The five-parameter model adds shunt resistance (Rsh). Improved four-parameter and two-diode models provide higher accuracy. Comparisons with experimental measurements show that the improved four-parameter model exhibits high correlation, with R² values ranging from 95.86% to 99.86%. For parameter estimation, manufacturer data, derivative-based methods, and artificial intelligence techniques such as genetic algorithms and particle swarm optimization can be employed.