A photovoltaic (PV) cell or solar cell is a non-mechanical device that directly converts sunlight into electricity via the photovoltaic effect, utilizing semiconductors to generate electric current from incoming photons.1
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The global energy demand continues to rise due to population growth, heightening climate change concerns as non-renewable fossil fuels still dominate energy production. A substantial shift to renewable energy is needed, with photovoltaic technology playing a critical role due to its ease of installation and cost-effectiveness.2
Working Principle
A PV cell is a p-n junction device where the p-type region contains positively charged holes created by acceptor impurity atoms, and the n-type region contains negatively charged electrons from donor impurity atoms. The photovoltaic effect involves three main stages: photon absorption, charge carrier separation, and charge carrier utilization.3
Initially, photons are absorbed within the p-n junction semiconductor, creating electron-hole pairs. If a photon’s energy (E = hυ) exceeds the semiconductor's band gap, it excites an electron from the valence to the conduction band, leaving a hole in the valence band.3
Additionally, the excess photon energy (hυ–hυ0) imparts extra kinetic energy to the electron and hole, where hυ0 represents the minimum energy (or work function) required by the semiconductor to generate an electron-hole pair. This energy gap corresponds to the work function, and any surplus energy is dissipated as heat within the semiconductor.3
The charge carriers generated by light are then separated. In an external solar circuit, holes flow away from the junction through the p-region while electrons move across the n-region and pass through the circuit, recombining with holes upon completing the circuit. This movement of electrons drives the electric current.3
The p-type layer is typically designed to be thicker than the n-type layer to ensure that electrons can travel through the circuit quickly, generating current before recombining with holes. An anti-reflective coating is also applied to the n-layer to increase light transmission to the semiconductor material and reduce surface reflection.3
Silicon is a commonly used semiconductor in solar cells. Crystalline silicon (c-Si) cells consist of silicon atoms arranged in a crystal lattice, providing an organized structure that enhances the efficiency of converting light into electricity.4 Solar cells made from silicon are known for their long lifespan, low cost, and high efficiency, with modules lasting 25 years or more while still producing over 80 % of their original power.4
Among c-Si cells, monocrystalline solar cells are often preferred due to their higher efficiency and smaller space requirements compared to multicrystalline cells. Other materials used in solar cells include gallium arsenide (GaAs), copper indium selenide/copper indium gallium selenide (CIS/CIGS), cadmium telluride/cadmium sulfide (CdTe/CdS), and amorphous silicon (a-Si).3
Components of a Photovoltaic Cell
The structure of a PV cell is divided into three main sections: the top layers (emitter and coatings), the middle layer (base), and the bottom layer (rear contact). The top layers include tempered glass, an anti-reflective coating, framing, and texturization. Depending on the cell’s specific application and manufacturing process, additional layers may be present or omitted.5
A layer of high-transmission, toughened glass serves as the outermost, transparent shield that protects the silicon, which cannot be directly exposed to air. Ethylene-vinyl acetate (EVA) resin layers are applied on both sides of the crystalline silicon (c-Si), with an additional toughened glass sheet on top to ensure durability.5 The contact fingers and busbars also collect electrons from photon-induced electron-hole pairs, completing the circuit to generate electrical output.
Optical reflection loss is a significant factor limiting solar cell efficiency.5 To minimize this, an anti-reflective coating is applied, made from a thin dielectric material layer with a specific thickness. This layer is designed to create interference effects, causing the reflected wave from the top surface to be out of phase with the wave reflected from the semiconductor.5 This out-of-phase alignment leads to destructive interference, resulting in zero net reflected energy and thereby reducing energy loss.
Texturizing is achieved by etching the top surface to create a "rough" silicon texture, increasing the likelihood that reflected light will bounce back onto the surface rather than escaping into the air, as it would with a flat surface.5
The middle layers mainly consist of crystalline material and encapsulations. An encapsulation, typically a polymer resin like EVA, is used to enhance adherence to the top glass and protect the cells. To remove air and make the EVA transparent, the layered structure—comprising glass, EVA, cabled cells, EVA, and plastic film or glass—is processed through heating and pressurization.5
The bottom layers are primarily made up of the rear metallic electrode, which may also include back covers and encapsulation. This rear electrode, or back contact, is positioned after the crystalline p-n junction silicon region and collects minority carriers, generating an output current that corresponds to the intensity of incident light.5
Efficiency Factors
PV cell efficiency is influenced by several factors, including cell materials, the angle of incident sunlight, temperature effects on the photovoltaic system, and shading on solar panels.6,7 For instance, the maximum power conversion efficiencies (PCE) of GaAs, CIS/CIGS, CdTe/CdS, a-Si, and monocrystalline silicon are over 30 %, 22.8%, 18.3 %, 11.3 %, and 20 %, respectively.3
The PCE of silicon solar cells remains nearly constant with an incident angle between 0 ° and 45 °, but decreases at angles approaching 60°, with optimal performance occurring between -45 ° and +45 °.7 The efficiency also reduces significantly when the temperature of the cell is increased on a hot day.6
PV cell efficiency can be enhanced by stacking multiple semiconductors to create multijunction solar cells, where each layer absorbs a distinct portion of the solar spectrum due to its unique bandgap. This design optimizes sunlight utilization and results in efficiencies exceeding 45 %, a significant improvement over single-junction cells.4
Bifacial PV modules can also increase PCE as they are designed to capture sunlight on both the front and rear surfaces, utilizing direct sunlight on the front and ground-reflected light on the back. Although bifacial modules use silicon cells with similar spectral responses to monofacial modules, their dual-sided design enhances energy yield by capturing diffused and reflected light from the environment.8
Conclusion
PV cells play a vital role in harnessing solar energy. They utilize advanced semiconductor technologies to convert sunlight into electricity while addressing global energy demands and climate concerns.
Advances in PV technology, including bifacial and multijunction solar cells, offer significant potential to enhance energy efficiency and sustainability. As the demand for renewable energy rises and efforts to mitigate climate change intensify, PV technology is positioned as a key contributor in the shift toward a cleaner energy future.
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References and Further Reading
- U.S. Energy Information Administration. (2024). Photovoltaics and electricity. [Online] U.S. Energy Information Administration. Available at https://www.eia.gov/energyexplained/solar/photovoltaics-and-electricity.php (Accessed on 27 October 2024)
- Marques Lameirinhas, RA., et al. (2022). A Photovoltaic Technology Review: History, Fundamentals and Applications. Energies. DOI: 10.3390/en15051823, https://www.mdpi.com/1996-1073/15/5/1823
- Al-Ezzi, AS., Ansari, MNM (2022). Photovoltaic Solar Cells: A Review. Applied System Innovation. DOI: 10.3390/asi5040067, https://www.mdpi.com/2571-5577/5/4/67
- Solar Energy Technologies Office. (n.d.). Solar Photovoltaic Cell Basics. [Online] Solar Energy Technologies Office. Available at https://www.energy.gov/eere/solar/solar-photovoltaic-cell-basics (Accessed on 27 October 2024)
- LibreTexts. (2014). Solar Cells. [Online] LibreTexts. Available at https://eng.libretexts.org/Bookshelves/Materials_Science/Supplemental_Modules_(Materials_Science)/Materials_and_Devices/Solar_Cells (Accessed on 27 October 2024)
- Bhukya, MN., Kumar, M., Vipin, Chandervanshi. (2021). Factors affecting the efficiency of solar cell and technical possible solutions to improve the performance. Modeling, Simulation and Optimization: Proceedings of CoMSO. DOI: 10.1007/978-981-15-9829-6_49, https://link.springer.com/chapter/10.1007/978-981-15-9829-6_49
- Sharma, R. (2019). Effect of obliquity of incident light on the performance of silicon solar cells. Heliyon, https://www.cell.com/heliyon/fulltext/S2405-8440(19)35576-8
- Badran, G., Dhimish, M. (2024). Comprehensive study on the efficiency of vertical bifacial photovoltaic systems: A UK case study. Scientific Reports. DOI: 10.1038/s41598-024-68018-1, https://www.nature.com/articles/s41598-024-68018-1
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