How Solar Cells Work: Principles and Key Components

Why Silicon?

Silicon is the primary material used in the production of solar cells. It is the second most abundant element on Earth, though it is rarely found in its pure form in nature.
It is most commonly found as sand and silicate minerals. Silicon belongs to Group 14 (IV) of the periodic table, alongside carbon.
While carbon is the fundamental element of organic chemistry, silicon plays a similar role in the inorganic world. Just like carbon, silicon has four valence electrons in its outer shell and forms four covalent bonds.
Unlike conductors or pure insulators, silicon is a semiconductor, making it the ideal base material for photovoltaic (PV) cells.

Energy Gap: Eg

Materials in nature are classified as conductors, semiconductors, or insulators based on their electronic structure and energy band configuration.
Silicon is an intrinsic semiconductor whose electrical resistance decreases as temperature increases. This behavior is determined by its band gap ($E_g$)—the energy separation between the valence band (highest occupied level) and the conduction band (lowest unoccupied level).
The term “Fermi energy” describes the energy level in a quantum system of non-interacting fermions at absolute zero ($0\text{ K}$).
In intrinsic silicon, the Fermi level lies approximately in the middle of the band gap, representing the energy state with a 50% probability of being occupied by an electron.
At absolute zero, pure silicon behaves as a perfect insulator, but at room temperature, thermal excitation allows a small number of electrons to jump into the conduction band.

p-n Junction Solved

P-type semiconductors are created by introducing Group 13 elements into the crystal lattice (a process called doping). When a boron atom replaces a silicon atom, its three valence electrons form covalent bonds with three adjacent silicon atoms. The missing fourth bond leaves an unoccupied electron state known as a hole (or cavity). Because boron accepts an electron from the lattice, it is called an acceptor impurity.

N-type semiconductors are formed using Group 15 elements, such as phosphorus. When a phosphorus atom replaces silicon, four of its five valence electrons form covalent bonds with surrounding silicon atoms. The fifth electron is weakly bound and easily emitted into the conduction band as a free charge carrier. Because phosphorus donates a free electron, it is called a donor impurity.

Combining these two regions forms a p-n junction. Due to the concentration gradient, free electrons from the n-region diffuse into the p-region, while holes diffuse in the opposite direction. This movement creates a narrow region depleted of mobile charge carriers, leaving behind fixed ionized donor and acceptor ions. This establishes an internal electric field and a potential barrier (space charge region) that halts further net diffusion, putting the p-n junction into dynamic equilibrium.

How Current is Generated

When light shines on the solar cell, photons penetrate the thin n-type layer and reach the depletion region.
If an incoming photon has sufficient energy, it excites an electron from the valence band to the conduction band, generating an electron-hole pair. The internal electric field sweeps the free electron toward the n-side and the hole toward the p-side.
This charge separation creates a potential difference (voltage) across the terminals of the cell.
The fundamental condition for photo-generation is that the photon energy ($h\nu$) must be equal to or greater than the band gap energy ($E_g$):

Where:

• $\lambda$ – Wavelength of the incident photon
• $\nu$ – Frequency of the incident photon
• $c$ – Speed of light ($3 \times 10^8\text{ m/s}$)
• $h$ – Planck’s constant ($6.626 \times 10^{-34}\text{ J}\cdot\text{s}$)

Photons with wavelengths longer than the threshold cutoff ($\lambda_g = \frac{hc}{E_g}$) do not have enough energy to excite electrons and pass through or are reflected.
Conversely, photons with energy greater than $E_g$ will generate electron-hole pairs, but any excess energy ($h\nu - E_g$) is lost as heat, thermalizing into the crystal lattice.
This trade-off creates an optimal band gap value: a smaller band gap absorbs a wider portion of the solar spectrum, but results in a lower output voltage (due to higher reverse saturation current). A larger band gap yields higher output voltage but absorbs fewer photons.

Under illumination, the generated charge flow is equivalent to a current source in parallel with a diode. Key operational parameters include:

• $I_{sc}$ (Short-Circuit Current): The maximum current when output voltage is zero. It is directly proportional to solar irradiance (e.g., at $1000\text{ W/m}^2$, $I_{sc} \approx 4\text{ A}$; at $500\text{ W/m}^2$, $I_{sc}$ is halved).
• $V_{oc}$ (Open-Circuit Voltage): The maximum voltage available when no current is flowing through the cell.

Individual solar cells are connected in series to increase voltage and in parallel to increase current, forming photovoltaic modules and arrays.

Conclusion

This article provides a foundational overview of photovoltaic cell operations. For deeper mathematical models and advanced semiconductor physics, reference literature in solar energy engineering is recommended.