How Do Solar Panels Work?A Complete Guide to Their Principles & Efficiency Factors
The core principle behind solar panels (also known as photovoltaic panels) is the photoelectric effect, which directly converts the energy of photons in sunlight into usable electrical energy. This entire process requires no combustion or mechanical movement, making it a clean and sustainable energy conversion technology. To understand how it works, we need to break down the process into three core stages: “material foundation → energy conversion → circuit output”. Below is a detailed explanation:
I. Core Foundation: Semiconductor Materials and PN Junctions (Why Silicon?)
The key component of a solar panel is the photovoltaic cell unit (silicon-based cells are currently the mainstream). Silicon has become the core material due to its unique “semiconductor properties”—it neither conducts electricity freely like metals (which have a large number of free electrons) nor completely blocks charge movement like insulators. Its conductivity can be precisely adjusted through “doping” (adding trace amounts of other elements), laying the groundwork for subsequent energy conversion.
The silicon wafers in photovoltaic cells are specifically engineered into a PN junction structure (the physical core of power generation), which involves two key steps:
- P-type semiconductor: A small amount of boron (with only 3 outer-shell electrons) is doped into high-purity silicon. Silicon atoms normally form stable covalent bonds using their 4 outer-shell electrons; when boron is added, each boron atom “lacks 1 electron”, creating a large number of “holes” (can be understood as “positively charged vacancies”) in the silicon crystal. It is important to note that holes are not actual “positively charged particles” but rather “charge vacancies” left behind when electrons depart. When surrounding electrons fill these holes, the holes appear to “move in the opposite direction of electron flow”, which is equivalent to the directional movement of positive charges on a macroscopic scale—this is the key to conductivity in P-type semiconductors.
- N-type semiconductor: A small amount of phosphorus (with 5 outer-shell electrons) is doped into high-purity silicon. When phosphorus atoms bond with silicon atoms, one extra electron is left unparticipated in covalent bonding. These electrons become freely movable “free electrons” (negatively charged), giving the N-type semiconductor an overall negative charge characteristic.
When P-type and N-type semiconductors are tightly bonded at high temperatures, free electrons in the N-region naturally diffuse toward the holes in the P-region due to concentration differences. As diffusion proceeds, the N-side of the PN junction interface becomes positively charged (having lost electrons), while the P-side becomes negatively charged (having gained electrons). Eventually, a stable built-in electric field (directed from the N-region to the P-region) is formed. This electric field acts like a “one-way barrier”: it prevents remaining electrons in the N-region from continuing to diffuse into the P-region and also stops holes in the P-region from moving into the N-region, preparing for the subsequent “capturing of photon energy and charge separation”.
II. Core Process: 4-Step Conversion from Photons to Electrical Energy
When sunlight hits the surface of a photovoltaic cell, a precise sequence of energy conversion reactions is triggered, consisting of 4 consecutive and indispensable steps:
1. Photon Absorption: Screening and Capturing Solar Energy
Sunlight contains photons of different wavelengths, including ultraviolet, visible light, and infrared (shorter wavelengths correspond to higher photon energy). The silicon material in photovoltaic cells selectively absorbs photons with sufficient energy:
- For intrinsic silicon (high-purity pure silicon), the photon energy must exceed silicon’s “band gap” (approximately 1.12 electron volts, eV) to break the covalent bonds of silicon atoms, releasing “free electrons” and corresponding “holes” (collectively called “electron-hole pairs”).
- In mass-produced silicon cells, “impurity energy levels” are formed due to doping. A small number of photons with energy slightly lower than 1.12 eV (e.g., those at the edge of near-infrared light) can also indirectly excite a small number of electron-hole pairs through “impurity level transitions”. However, this contribution is extremely low (usually less than 5%) and has a limited impact on core power generation efficiency.
Note: Photons with too little energy (e.g., far-infrared light) cannot break chemical bonds and either pass directly through the cell or are converted into heat. Photons with too much energy (e.g., ultraviolet light) can excite electrons but dissipate the excess energy as heat—this is one reason why solar cells generate slight heat during operation.
2. Charge Separation: “Directional Screening” by the Built-in Electric Field
If the photon-excited “electron-hole pairs” are not separated promptly, they will recombine within 10⁻⁶ seconds (releasing heat that cannot be converted into electrical energy). At this point, the built-in electric field of the PN junction plays a critical role:
- Negatively charged free electrons: Subject to an electric field force opposite to the direction of the built-in electric field, they are pushed toward the N-region.
- Positively charged holes (equivalent to positive charges): Subject to an electric field force in the same direction as the built-in electric field, they are pushed toward the P-region.
Through this process, a large number of free electrons (negative charge) accumulate in the N-region, and a large number of holes (equivalent positive charge) accumulate in the P-region, forming a stable “potential difference” across the PN junction (similar to the voltage difference between the positive and negative terminals of a dry cell).
3. Current Formation: “Conduction and Flow” in the External Circuit
Charge separation by the built-in electric field alone can only create a “potential difference”—it cannot generate a continuous current. This is because electrons in the N-region and holes in the P-region cannot cross the “barrier” of the built-in electric field on their own.
At this stage, if metal electrodes are soldered to the N-region and P-region of the cell (forming an external circuit that can connect to loads such as light bulbs, batteries, or power grids), free electrons in the N-region will flow directionally toward the P-region through the external circuit (combining with holes in the P-region). This directional flow of electrons forms a direct current (DC), and the load thus receives electrical energy (e.g., a light bulb illuminates).
4. Electrical Energy Output: “Large-Scale Integration” of Cell Arrays
A single photovoltaic cell has very weak power generation capacity: a standard silicon-based cell (with an area of approximately 156mm × 156mm) has an open-circuit voltage of about 0.5V and a short-circuit current of 8-10A, which cannot directly meet the electricity needs of households or industries. Therefore, in practical applications, “series-parallel connection + packaging” is used to achieve large-scale power supply:
- Series connection: Multiple cells are connected in the order of “positive → negative → positive → negative” to increase the total voltage (e.g., 20 cells in series can produce 10V, and 40 cells in series can produce 20V).
- Parallel connection: Multiple series-connected cell groups are connected in the order of “positive → positive, negative → negative” to increase the total current (e.g., two 20V series cell groups connected in parallel maintain a total voltage of 20V while doubling the current).
- Packaging and system matching: The series-parallel connected cell array is encapsulated into a “solar panel” using tempered glass, EVA film, and a backsheet. It is then paired with an inverter (converts DC to 220V/380V alternating current (AC) to meet household and industrial electricity standards) and energy storage devices (such as lithium-ion batteries or lead-acid batteries, which store excess electricity generated during the day for use at night or on cloudy days) to form a complete photovoltaic power supply system.
III. Key Supplement: Core Factors Affecting Power Generation Efficiency
The power generation efficiency of solar panels is not 100% (currently, the mass production efficiency of mainstream silicon-based cells is approximately 20%-24%, and the maximum laboratory efficiency is about 26%-30%). It is mainly affected by three categories of factors:
1. External Environmental Factors
- Light conditions: Stronger light intensity (e.g., midday on sunny days) and longer light duration (e.g., summer in high-latitude regions) result in more excited electron-hole pairs and higher power generation efficiency. Cloudy weather, nighttime, or obstructions (e.g., dust, leaves, snow) on the panel surface will significantly reduce photon absorption, potentially decreasing efficiency by more than 50%.
- Ambient temperature: The conductivity of silicon increases with temperature, but the built-in electric field of the PN junction weakens accordingly, leading to reduced charge separation efficiency. Generally, for every 1℃ increase in temperature, the efficiency of silicon-based cells decreases by 0.3%-0.5%. Therefore, photovoltaic systems in high-temperature areas need to pay attention to panel heat dissipation (e.g., using elevated brackets and reserving ventilation gaps).
2. Battery Material Types
Batteries made of different materials exhibit significant performance differences. The following is a comparison of current mainstream types:
| Battery Type | Silicon Purity | Mass Production Efficiency | Lifespan | Cost | Application Scenarios |
|---|---|---|---|---|---|
| Monocrystalline Silicon Cell | Over 99.999% | 22%-24% | 25-30 years | High | Rooftop photovoltaics, large-scale ground power stations |
| Polycrystalline Silicon Cell | Approximately 99.99% | 20%-22% | 25-30 years | Medium (mainstream) | Large-scale ground power stations, residential photovoltaics |
| Thin-Film Cell | Varies by material | 12%-18% | 15-20 years | Low | Building-integrated photovoltaics (e.g., photovoltaic curtain walls), portable devices |
Note: Among thin-film cells, cadmium telluride (CdTe) cells are a mature mass-produced technology (e.g., the main product of First Solar in the United States). Perovskite cells are currently mostly developed in the form of “perovskite-silicon tandem cells” (with laboratory efficiency exceeding 30%). Pure perovskite thin-film cells have not yet been mass-produced due to “poor long-term stability (vulnerable to moisture and heat)”.
3. Manufacturing Processes and Packaging Technology
- Manufacturing processes: Technologies such as “passivation layer technology” (reducing electron-hole pair recombination) and “grid line design” (reducing electron transmission losses) can improve efficiency.
- Packaging quality: High-quality EVA film and weather-resistant backsheets can reduce the erosion of cells by moisture and ultraviolet rays, extend service life, and indirectly ensure long-term power generation efficiency.
Summary
The essence of how solar panels work is: using the built-in electric field of a semiconductor PN junction to convert the energy of photons in sunlight into the kinetic energy of directional electron movement, then forming a continuous current through an external circuit, and finally outputting usable electrical energy. This process achieves direct, emission-free conversion of “light energy to electrical energy”, making it one of the core clean energy technologies to address the global energy crisis and climate change, as well as a key direction for future energy structure transformation.