Table of Contents
Introduction
The basic photovoltaic effect is the physical process that allows solar cells to convert sunlight into electricity. It links light, materials, and electric charge. In this chapter we focus on what happens inside a solar cell at the microscopic level, without going into specific technologies or system design, which appear in later chapters.
Light, Photons, And Semiconductors
Sunlight can be viewed as a stream of tiny packets of energy called photons. Each photon carries an energy that depends on its color. Blue light photons have more energy than red light photons. In photovoltaic materials this photon energy can be used to move electric charges.
Most solar cells are made from semiconductors, such as silicon. A semiconductor is a material whose ability to conduct electricity is between that of a conductor like copper and an insulator like glass. In a simple picture, electrons in a semiconductor can occupy allowed energy bands. The most important are the valence band, where electrons are normally bound, and the conduction band, where electrons are free to move and contribute to electric current.
Between the valence band and the conduction band there is an energy gap, called the band gap. An electron must gain at least the band gap energy to move from the valence band to the conduction band. If a photon has energy $E_{\text{photon}}$ greater than or equal to the band gap energy $E_g$, it can lift an electron across this gap.
Key relation for photon energy:
$$E_{\text{photon}} = h \nu = \frac{h c}{\lambda}$$
where $h$ is Planck's constant, $\nu$ is the frequency, $c$ is the speed of light, and $\lambda$ is the wavelength.
A photon can create an electron in the conduction band only if
$$E_{\text{photon}} \geq E_g.$$
Generation Of Electron–Hole Pairs
When a photon with sufficient energy is absorbed by the semiconductor, it excites an electron from the valence band to the conduction band. This excitation creates two charge carriers at once. The electron that has moved to the conduction band carries negative charge, and the vacancy it leaves behind in the valence band behaves like a positive charge, called a hole.
The pair of charges, one electron and one hole, is called an electron–hole pair. Without any built in field or external circuit, these carriers move randomly and often recombine, which means the electron falls back into a hole, releasing energy as heat or light. For a solar cell to produce useful electricity, many of these generated carriers must be separated and collected before they recombine.
The rate at which electron–hole pairs are generated depends on the intensity of light and the ability of the material to absorb photons of different wavelengths. Only photons with energy above the band gap are effective for generating charge carriers in a given semiconductor.
The p–n Junction And Built In Electric Field
The central structure that enables carrier separation in a typical solar cell is the p–n junction. This is formed by joining two regions of the same semiconductor that have been treated, or doped, differently.
In the p type region, there is an excess of holes as majority carriers. In the n type region, there is an excess of electrons as majority carriers. When these two regions are brought into contact, electrons from the n side diffuse into the p side, and holes from the p side diffuse into the n side. As they cross the junction, many electrons and holes recombine.
This movement leaves behind charged ions that cannot move. On the n side near the junction, positively charged donor ions remain, while on the p side, negatively charged acceptor ions remain. This region, which is depleted of free carriers and contains fixed charges, is called the depletion region.
The separated positive and negative ions in the depletion region create an internal electric field that points from the positive charges toward the negative charges. This electric field is crucial for the photovoltaic effect, because it acts on newly generated electrons and holes and drives them in opposite directions.
In a simple picture, the built in electric field $E_{\text{bi}}$ across the p–n junction arises from charge separation in the depletion region and provides the driving force that separates photo generated carriers without any external voltage.
Separation And Collection Of Photogenerated Charges
When light enters a solar cell, photons are absorbed mainly near or within the depletion region and its surroundings. Each absorbed photon with sufficient energy creates an electron–hole pair. The built in electric field in the depletion region acts immediately on these carriers. It pushes electrons toward the n side and holes toward the p side.
Once the carriers have been separated by this internal field, they can move through their respective regions. Electrons travel through the n type material toward the front contact, while holes travel through the p type material toward the back contact, or vice versa, depending on the cell design. Metal contacts on the surfaces of the semiconductor collect these charges and allow them to flow into an external circuit.
If the external terminals of the solar cell are not connected, charges accumulate, and an electric potential difference, or voltage, builds up across the cell. This is called the open circuit voltage. If the terminals are connected through a load, such as a lamp, electrons flow through the external circuit, do electrical work in the load, and then return to recombine with holes on the other side of the junction. This continuous flow of charge is the electric current generated by the photovoltaic effect.
From Light To Electric Power
The power output of a solar cell is the product of the current it delivers and the voltage across its terminals. The intensity of light and the properties of the semiconductor determine how many electron–hole pairs are generated and how many are collected as useful current. For a given illumination, there is a characteristic current–voltage curve that describes how the output current changes with the applied voltage.
One important concept is that there is a specific operating point at which the product of current and voltage is maximized. This is the maximum power point. In actual photovoltaic systems, special electronic devices are used to keep solar modules operating close to this point, but the underlying reason such a point exists is rooted in the basic photovoltaic effect and the behavior of the p–n junction under illumination.
Electric power from a solar cell:
$$P = I \times V$$
where $P$ is power, $I$ is current, and $V$ is voltage. At the maximum power point:
$$P_{\text{max}} = I_{\text{mpp}} \times V_{\text{mpp}}.$$
Limits To The Photovoltaic Conversion Process
The basic photovoltaic effect also has inherent limitations. Not all sunlight can be converted into electrical energy. Photons with energy less than the band gap pass through the material without being absorbed. Photons with energy much greater than the band gap lose the excess energy as heat when the excited electron relaxes toward the conduction band edge.
Recombination of electrons and holes reduces the number of carriers that can be collected. Some recombination occurs in the bulk of the semiconductor, and some at surfaces or defects. Reflection of light at the surface and resistive losses in contacts and material also reduce the usable output.
These microscopic loss mechanisms set a theoretical upper limit on the efficiency of a single junction solar cell, which expresses the fraction of incident solar power that can be converted into electrical power through the basic photovoltaic effect. Understanding these limits helps explain why real solar cells have efficiencies below 100 percent, even under ideal conditions.
Summary
The basic photovoltaic effect is the sequence of physical events by which light energy becomes electrical energy in a solar cell. Photons with sufficient energy excite electrons across the band gap of a semiconductor, creating electron–hole pairs. A built in electric field in a p–n junction separates these charges and directs electrons and holes toward opposite contacts. When the cell is connected to a circuit, the separated charges flow as an electric current and deliver power to an external load. The details of materials, structures, and technologies all build on this fundamental process.