Factors Affecting the Conversion Efficiency of Solar Panels
The conversion efficiency of solar panels is a core indicator for measuring their ability to convert sunlight into electrical energy. Its value is jointly influenced by four major categories of factors: material properties, external environment, structural design, and usage wear. The specific mechanism and extent of impact of each category are as follows:
I. Core Influencing Factor: Material Properties (Determining the Efficiency Ceiling)
The core of a solar cell relies on the “photovoltaic effect”, and the physical and chemical properties of materials directly determine the upper limit of this effect’s efficiency—making them the fundamental factor affecting conversion efficiency.
| Material Type | Key Characteristics | Impact on Conversion Efficiency | Typical Efficiency Range (Lab/Mass Production) |
|---|---|---|---|
| Monocrystalline Silicon | Complete crystal structure, long carrier lifetime | It has strong light absorption capacity and low electron transmission loss, resulting in a high efficiency ceiling. However, it requires high material purity, which leads to higher costs. | 26%-28% / 23%-25% |
| Polycrystalline Silicon | Multiple crystal grains, presence of grain boundary defects | Grain boundaries cause electron recombination loss, so its efficiency is lower than that of monocrystalline silicon. Nevertheless, it has low costs and mature mass production processes. | 23%-25% / 20%-22% |
| Thin-Film Materials (e.g., Cadmium Telluride (CdTe), Perovskite) | Tunable bandgap, thin and lightweight | – Cadmium Telluride (CdTe): Its bandgap matches the solar spectrum, and it has a high light absorption coefficient (a 1μm thickness can absorb 90% of sunlight). Its mass production efficiency is close to that of polycrystalline silicon. – Perovskite: It has a tunable bandgap and extremely low preparation costs. Its laboratory efficiency has caught up with that of monocrystalline silicon, but it has poor stability (vulnerable to water and heat). |
Perovskite: 31%+ (tandem)/No large-scale mass production; CdTe: 22% / 18%-20% |
| Other New Materials (e.g., Gallium Arsenide (GaAs)) | Direct bandgap, high-temperature resistance | It has extremely high light absorption efficiency and strong high-temperature resistance, making it suitable for concentrated photovoltaic (CPV) systems. However, its high cost limits its application to special scenarios (e.g., aerospace). | 35%-40% (concentrated)/Minimal mass production |
Key Principle: The “bandgap width” of a material must match the solar spectrum. If the bandgap is too wide, it will filter out low-energy red and infrared light (which cannot excite electrons). If the bandgap is too narrow, high-energy blue and ultraviolet light will convert excess energy into heat (instead of electrical energy). Both scenarios lead to efficiency loss.
II. External Environmental Factors (Determining Practical Usage Efficiency)
The efficiency measured in laboratories is a value under “ideal conditions” (Standard Test Conditions: 25°C, 1000W/㎡ solar irradiance, AM1.5 spectrum). In practical use, the external environment significantly reduces efficiency, making it the main factor affecting actual power generation capacity.
1. Light Conditions: Irradiance and Spectrum
- Solar Irradiance: Conversion efficiency has a “positive but non-linear” relationship with irradiance. At low irradiance levels (e.g., on cloudy days, in the early morning, or late evening), efficiency increases as irradiance rises. However, at excessively high irradiance levels (e.g., during direct noon sunlight), if heat dissipation is insufficient, efficiency growth will slow down or even decline due to “heat loss”.
- Spectral Characteristics: The solar spectrum varies with altitude, latitude, and weather conditions (e.g., high-altitude areas have a higher proportion of ultraviolet light, while low-latitude areas have a higher proportion of visible light). If the bandgap of the battery material does not match the actual spectrum (e.g., monocrystalline silicon has low utilization of infrared light), some photons cannot be absorbed, which reduces efficiency.
2. Temperature: A Critical Negatively Correlated Variable
Temperature is one of the most significant factors in the external environment, and it has a negative correlation with efficiency—for every 1°C increase in temperature, the conversion efficiency of mainstream crystalline silicon batteries decreases by 0.3%-0.5%.
- Principle: High temperatures accelerate the “recombination” of electron-hole pairs (i.e., electrons combine with holes before reaching the electrodes, failing to form a current). At the same time, high temperatures increase the internal resistance of the battery, further causing electrical energy loss.
- Typical Scenario: On a summer noon, the surface temperature of solar panels can rise from 25°C to over 60°C, and their efficiency may drop from 24% to below 20%.
3. Shading and Dust: Physical Losses
- Shading: Obstacles such as trees, buildings, and bird droppings can cause the “shadow effect” on local areas of solar panels. The shaded area not only fails to generate electricity but also acts as a “load” (current generated by other areas flows through the shaded area, causing local heating and even burning the battery cells—a phenomenon known as the “hot spot effect”).
- Dust/Dirt: Dust, sand, and leaves that accumulate over time reflect or absorb part of the sunlight, reducing the amount of light that reaches the battery cells. Studies have shown that a mere 1mm-thick layer of dust can reduce efficiency by 10%-20%. In dusty areas, long-term lack of cleaning can lead to an efficiency loss of over 30%.
4. Tilt Angle and Orientation: Optimization of Light Absorption
The installation tilt angle and orientation of solar panels determine the “effective area” for receiving sunlight:
- Orientation: The optimal orientation in the Northern Hemisphere is due south (due north in the Southern Hemisphere). For every 15° deviation from due south, annual power generation decreases by 5%-10% (because the incident angle of sunlight becomes oblique, increasing light reflection).
- Tilt Angle: The optimal tilt angle is close to the local latitude (e.g., Beijing has a latitude of 39°, so the optimal tilt angle is approximately 35°-40°). At this angle, sunlight can “vertically incident” on the panels, maximizing light absorption. Excessive deviation from the optimal tilt angle (e.g., horizontal installation) will lead to a significant reduction in light absorption in winter (the sun’s altitude angle is low in winter, resulting in high reflectivity during oblique incidence).
III. Structural Design Factors (Optimizing the Energy Conversion Process)
The structural design of solar panels affects the entire process of “light absorption – electron transmission – electrical energy output”. A reasonable design can reduce losses and improve efficiency.
1. Battery Cell Structure: Reducing Electron Recombination
- Passivation Layer: Coating a passivation layer (e.g., silicon oxide, silicon nitride) on the surface of battery cells can reduce electron recombination loss on the surface. For example, the “PERC (Passivated Emitter and Rear Cell)” technology increases the mass production efficiency of monocrystalline silicon from 22% to over 24% by adding a passivation layer on the back of the cell.
- Heterojunction (HJT): Adopting a heterojunction structure of “amorphous silicon + monocrystalline silicon” further reduces the electron recombination rate while improving high-temperature resistance. Its laboratory efficiency has exceeded 27%, and its mass production efficiency reaches 25%-26%.
2. Anti-Reflective Coating: Enhancing Light Absorption
The “anti-reflective coating” (usually a silicon nitride film) on the surface of solar panels can reduce sunlight reflection loss. An uncoated silicon wafer has a reflectivity of approximately 35% for visible light (a large amount of light is reflected away), while the reflectivity can be reduced to below 5% after coating—significantly increasing light absorption.
3. Grid Line Design: Reducing Series Resistance
The metal grid lines (“positive and negative electrodes”) on the surface of battery cells are responsible for collecting generated electrons. However, the grid lines themselves block part of the sunlight and cause resistance loss:
- Too Thick Grid Lines: They result in a large shading area, reducing light absorption;
- Too Thin Grid Lines: They have high resistance, increasing loss during electron transmission.
The current mainstream design is “thin grid lines + multiple busbars” (e.g., 9 busbars, 12 busbars), which reduces shading while lowering resistance and can improve efficiency by 1%-2%.
4. Packaging Process: Protection and Light Transmission
- Cover Glass: High-transmittance (>93%) ultra-white tempered glass must be used. If the glass has low transmittance (e.g., ordinary glass), it will directly reduce the amount of incident light;
- Encapsulation Film (EVA/PVB): It is used to bond the glass, battery cells, and backsheet, and it needs to have high light transmission and low yellowing properties. If the film ages and yellows (after long-term exposure to sunlight), its transmittance will decrease, and efficiency will decline accordingly.
IV. Usage Wear Factors (Long-Term Efficiency Degradation)
The conversion efficiency of solar panels will slowly degrade over time—a process that is an irreversible long-term loss mainly caused by material aging and structural degradation.
1. Light-Induced Degradation (LID): Initial Rapid Degradation
- Occurrence Stage: Within the first 1000 hours (approximately 1-3 months) after a new solar panel is first exposed to sunlight;
- Principle: Impurities (e.g., boron) in the silicon combine with oxygen to form “boron-oxygen complexes”, which increase the electron recombination rate;
- Degradation Magnitude: Traditional monocrystalline silicon has a degradation rate of approximately 2%-3%, while batteries using the “boron-free doping” technology can reduce this rate to below 1%.
2. Aging Degradation: Long-Term Slow Loss
- Causes: Long-term exposure to sunlight (ultraviolet rays), high temperatures, and humidity changes lead to material aging—such as yellowing of the encapsulation film, cracking of the backsheet, and corrosion of grid lines;
- Degradation Magnitude: Industry standards require a 25-year degradation rate of ≤20% (i.e., an annual degradation of approximately 0.8%), while high-quality products can achieve a 25-year degradation rate of ≤15% (approximately 0.6% per year).
Summary: Priority of Core Influencing Factors
- Fundamental Factor: Battery materials (monocrystalline silicon > polycrystalline silicon > thin-film; perovskite has great potential but needs to overcome stability issues);
- Key to Practical Use: Temperature (negative correlation), solar irradiance (positive correlation), shading/dust (significant losses);
- Optimizable Factors: Installation tilt angle/orientation, anti-reflective coating, passivation layer design, and regular cleaning.
To maximize the conversion efficiency of solar panels, a comprehensive optimization approach is required, covering “material selection (prioritizing monocrystalline silicon) + installation (due south orientation with a tilt angle matching the local latitude) + maintenance (regular cleaning and avoiding shading)”.