Since a solar cell is the only generator in a solar PV system, it is one of the most important parts in a solar PV system. In the following paragraphs, a simple introduction of a solar cell and how it operates is discussed, with reference links for better understanding.
A solar cell: A solar cell is a solid-state electrical device (p-n junction) that converts the energy of light directly into electricity (DC) using the photovoltaic effect. The process of conversion first requires a material which absorbs the solar energy (photon), and then raises an electron to a higher energy state, and then the flow of this high-energy electron to an external circuit. Silicon is one such material that uses such process. A solar cell structure is shown in figure 1 and a solar panel configuration in figure 2.
Figure 1. A Solar Cell Structure
Figure 2: Solar Panel Configuration
PV cells: PV cells are most commonly made of silicon, and come in two common varieties, crystalline and thin-film cells, as detailed in table 1.
A p-n junction: It is formed by joining p-type (high concentration of hole or deficiency of electron) and n-type (high concentration of electron) semiconductor material. Due to this joining, excess electrons from n-type try to diffuse with the holes of p-type whereas excess hole from p-type try to diffuse with the electrons of n-type. Movement of electrons to the p-type side exposes positive ion cores in the n-type side, while movement of holes to the n-type side exposes negative ion cores in the p-type side, resulting in an electron field at the junction and forming the depletion region. An animated visual explanation is shown in this link: http://www.pveducation.org/pvcdrom/pn-junction/formation-pn-junction.
A light-generated current: Generation of current in a solar cell, known as the “light-generated current,” involves two important processes. An animated ideal flow at short circuit is shown at this link: http://www.pveducation.org/pvcdrom/solar-cell-operation/light-generated-current.
- Absorption of incident photons to create electron-hole pairs. Electron-hole pairs will generate in the solar cell provided that the incident photon has an energy greater than that of the band gap. However, electrons (in the p-type material), and holes (in the n-type material) are meta-stable and will only exist, on average, for a length of time equal to the minority carrier lifetime before they recombine. If the carrier recombines, then the light-generated electron-hole pair is lost and no current or power can be generated.
- Collection of these carriers by the p-n junction prevents this recombination by using a p-n junction to spatially separate the electron and the hole. The carriers are separated by the action of the electric field existing at the p-n junction. If the light-generated minority carrier reaches the p-n junction, it is swept across the junction by the electric field at the junction, where it is now a majority carrier. If the emitter and base of the solar cell are connected together (i.e., if the solar cell is short-circuited), then the light-generated carriers flow through the external circuit.
Photovoltaic effect: The collection of light-generated carriers does not by itself give rise to power generation. In order to generate power, a voltage must be generated as well as a current. Voltage is generated in a solar cell by a process known as the “photovoltaic effect.” The collection of light-generated carriers by the p-n junction causes a movement of electrons to the n-type side and holes to the p-type side of the junction. Under short circuit conditions, the carriers exit the device as light-generated current. An animated explaination of the photovoltaic effect is shown at this link: http://www.pveducation.org/pvcdrom/solar-cell-operation/photovoltaic-effect.
With this basic idea of the operation of a solar cell, a thorough explanation of modeling of a solar cell by using a diode with the diode ideality factor and the operation temperature as well as the parasitic resistance (due to manufacturing defects) will be discussed later.
For more basics on how solar photovoltaics work, check out: Understanding The Technology Behind Solar PV Systems