Infrared: Heat Effect, Cutoff Wavelength, Wavelength Range, Thermal Imaging, Efficiency Limit, Share of Sunlight, Camera Equipment, Fault Thresholds, Coatings and Tandems, Inspection Conditions
Infrared is invisible electromagnetic radiation with wavelengths longer than visible red light, emitted by the sun and by warm objects. Key aspects include cutoff wavelength, heat effect, thermal imaging, wavelength range, share of sunlight, efficiency limit, fault thresholds, camera equipment, inspection conditions, and coatings and tandems. Sunlight reaching the ground contains a large portion of this radiation, and you feel it as warmth on your skin. Silicon cells in solar panels Washington can only convert photons with wavelengths shorter than a cutoff near 1,100 nanometers, so much of the longer-wavelength energy passes through or turns into heat. That heat raises cell temperature and slightly lowers electrical output, which is why you see temperature coefficients listed on module data sheets. The article goes on to explain the wavelength range, the share of sunlight, the cutoff wavelength, the heat effect, and how thermal imaging helps you spot faulty cells.
Key Takeaways
- Infrared (IR) makes up about 50-55% of the sunlight energy reaching the ground.
- A silicon solar cell has a bandgap cutoff wavelength of about 1,100 nm.
- Efficiency drops 0.3-0.5% per °C as infrared (IR) increases panel temperature.
- A thermal camera detects faults when a cell is above roughly 20°C hotter than others.
- The Shockley-Queisser limit keeps silicon solar cell efficiency at about 29-30%.
Do Solar Panels Use Infrared Light, and Why Do Silicon Cells Stop at About 1,100 nm?
Yes, a silicon solar cell uses infrared (IR) light to generate electricity, but only a specific portion of the spectrum because its material properties stop photon conversion at about 1,100 nm.
| Photon Spectrum Category | Wavelength Range | Interaction with Silicon | Primary Outcome |
|---|---|---|---|
| Visible Light | < 700 nm | High energy absorption | Electron excitation (Current) |
| Near-Infrared (NIR) | 700 nm – 1,100 nm | Effective absorption | Electron excitation (Current) |
| Mid/Far-Infrared | > 1,100 nm | Insufficient energy | Transmission or Thermal heating |
Solar Spectrum Energy Distribution
How the Bandgap Cutoff Wavelength Works in a Silicon Solar Cell
A silicon solar cell relies on a bandgap of about 1.1 eV, which dictates the minimum energy required to excite an electron and create a current. Photons with wavelengths longer than the bandgap cutoff wavelength of about 1,100 nm lack the energy to trigger this electron displacement. Instead of contributing to electrical flow, these longer-wavelength photons simply pass through the material or are absorbed as waste energy.
What Happens to Photons Beyond the Cutoff
- Transmission: Lower-energy photons pass through the silicon layers without interacting with the atomic structure.
- Thermal Absorption: Captured long-wavelength photons increase the molecular vibration within the cell, which raises the temperature.
- Efficiency Loss: Because this energy is not converted into electrons, it represents a fundamental limitation in energy capture.
Why Does Near-Infrared (NIR) Light Still Matter for Output?
Near-infrared (NIR) light is critical because it represents a significant portion of the solar spectrum that sits just below the 1,100 nm threshold. Since sunlight contains roughly 50-55% energy in the infrared range, the ability of silicon to capture the NIR portion ensures that a substantial amount of available irradiance is actually converted into usable power rather than lost to simple transmission.
How the Shockley-Queisser Limit Caps Single-Junction Efficiency
The Shockley-Queisser limit establishes a theoretical efficiency ceiling for single-junction silicon cells of about 29-30%. This limit is primarily driven by the fact that the fixed bandgap cannot capture photons beyond the 1,100 nm cutoff, while simultaneously losing energy as heat from photons that possess more energy than the bandgap requires. According to the American Physical Society, it is an established approximation that solar energy exceeding the 1,100 nanometer threshold is effectively lost in single-junction silicon devices because the energy of those infrared photons remains below the bandgap of the semiconductor material.
Assess your panel layout for shade to ensure that the limited spectrum captured by your silicon cells is not further obstructed by environmental blockages.
Beyond electrical generation, the remaining infrared energy impacts the operational environment of the solar panel in the following ways:- Increased cell temperatures reduce the open-circuit voltage, thereby lowering overall conversion efficiency.
- Thermal expansion stress can accelerate the degradation of module encapsulants and interconnections over time.
- Waste heat dissipation contributes to the formation of localized “hot spots” if the panel surface is partially shaded or soiled.
How Much of the Sunlight Hitting a Solar Panel Is Infrared?
The portion of sunlight hitting a solar panel that is infrared (IR) accounts for about 50-55% of the total energy reaching the Earth’s surface. According to the National Renewable Energy Laboratory, infrared radiation accounts for approximately 50 percent of the total solar energy spectrum that reaches the Earth’s surface. While this radiation is invisible to the human eye, it represents the largest segment of the solar spectrum, far exceeding the 40-45% contribution from visible light and the 3-5% from ultraviolet light. At standard test conditions, where the total irradiance is approximately 1,000 W/m², roughly 500 W/m² consists of energy within the infrared (IR) range.
| Spectral Segment | Energy Contribution (%) | Irradiance at 1,000 W/m² (W/m²) | Thermal Impact Characteristics |
|---|---|---|---|
| Ultraviolet | 3% – 5% | 30 – 50 | Minimal photoelectric contribution; potential material degradation |
| Visible Light | 40% – 45% | 400 – 450 | Primary range for silicon photovoltaic energy conversion |
| Infrared (IR) | 50% – 55% | 500 – 550 | Dominant heat driver; absorbed by assembly as thermal load |
Solar Spectrum Composition
This distribution of solar energy explains why solar installations interact so significantly with thermal environments. Because a silicon solar cell is optimized primarily for visible light and a small portion of near-infrared (NIR) wavelengths, the remaining long-wavelength energy is frequently absorbed by the module assembly as heat. The composition of solar irradiance hitting your system is structured as follows:
- Infrared (IR): 50-55% of total energy
- Visible light: 40-45% of total energy
- Ultraviolet: 3-5% of total energy
Compare the spectral distribution of your specific site’s sunlight against these standard averages when assessing the expected thermal load on your installation.
Managing the thermal consequences of this solar energy distribution involves several key engineering considerations:- Employing anti-reflective coatings designed to selectively filter wavelengths to reduce excess heat absorption.
- Utilizing backside ventilation gaps to facilitate convective cooling of the module assembly.
- Selecting mounting materials with high thermal emissivity to assist in radiating absorbed heat away from the cells.
- Integrating phase-change materials into the panel backsheet to stabilize operating temperatures during peak irradiance.
Does Infrared Make Solar Panels Hot, and How Much Output Is Lost Above 25°C?
Yes, infrared (IR) makes solar panels hot because the silicon solar cell cannot convert the long-wavelength energy into electricity, forcing it to dissipate as thermal energy instead. Because solar panels typically operate 15-25°C (25-40°F) above the ambient air temperature, this internal heating directly degrades electrical performance. Per Sandia National Laboratories, solar cell efficiency is roughly inversely proportional to operating temperature because increased intrinsic carrier concentration reduces the module’s open-circuit voltage as thermal energy accumulates. You should expect your system to follow the industry-standard temperature coefficient, where the power output drops by about 0.3-0.5% for every degree Celsius the panel temperature rises above the 25°C (77°F) baseline.
Power Output Loss per Degree Celsius (°C) Increase
The following table outlines how rising temperatures influence the operational reality of your array:
| Operational Factor | Impact or Metric |
|---|---|
| Typical operating temperature rise | 15-25°C (25-40°F) above air temperature |
| Efficiency loss per degree Celsius | 0.3-0.5% drop |
| Primary cause of heating | Absorption of non-convertible infrared radiation |
Managing this temperature-dependent loss is a core challenge in system design. High ambient temperatures often lead to cumulative losses that exceed the gains from high irradiance levels alone.
Monitor your system’s average daily voltage during the hottest hours of the afternoon to determine if current ventilation or mounting configurations are successfully mitigating localized thermal accumulation.
To mitigate thermal degradation and improve long-term energy yields, consider these installation strategies:- Maintain a gap of at least 10–15 centimeters between the panels and the roof surface to promote passive convective cooling.
- Install panels in a landscape orientation or utilize mounting systems that avoid airflow blockages to prevent heat entrapment.
- Apply high-albedo, reflective roofing materials beneath the array to reduce the amount of ambient heat radiating upward toward the modules.
- Choose panels with a lower temperature coefficient (e.g., heterojunction or N-type cells) if your installation site experiences chronic high-heat conditions.
What an Infrared Thermal Camera Can Show on a Solar Panel and What It Cannot See
An infrared thermal camera shows you the operational health of a silicon solar cell by detecting long-wave infrared (LWIR) radiation emitted as heat, revealing internal malfunctions that remain invisible to the naked eye. Because every object emits energy based on its temperature, these cameras allow you to visualize subtle differences across a module’s surface, identifying variations that point to electrical failure. However, an infrared thermal camera cannot see through physical obstructions or identify non-thermal electrical issues like high-resistance wiring that has not yet reached a critical temperature threshold.
| Fault Classification | Thermal Signature Indicator | Root Cause Mechanism | Diagnostic Threshold Context |
|---|---|---|---|
| Localized Hot Spots | >10–20°C above baseline | Cracked cells or internal shunts | High heat dissipation at cell level |
| Bypass Diode Failure | Significantly elevated heat | Internal junction box short/open | Broad regional thermal anomaly |
| Contact Resistance | Localized thermal gradient | Loose or corroded electrical wiring | Localized resistance-based heating |
| Internal Cell Degradation | None (Thermal neutral) | Non-thermal material breakdown | Requires electroluminescence imaging |
Thermal Anomaly Temperature Thresholds (°C)
- Hot spots: Detects localized areas typically 10–20°C or more above surrounding cells, often indicating cracked cells or internal shunts.
- Bypass diode failure: Identifies modules with faulty diodes by showing significantly elevated temperatures in the junction box area.
- Connection issues: Highlights loose or corroded electrical contacts that create resistance and localized heat.
- Imaging limitations: Cannot reveal internal cell degradation that does not manifest as a temperature deviation; for those non-thermal faults, you would need electroluminescence imaging instead.
Per International Electrotechnical Commission, standard 62446-3 recommends that infrared inspections are conducted under specific irradiance conditions, typically exceeding 600 W/m², to ensure that electrical faults produce a temperature signature distinct enough to be captured by your equipment.
Follow these essential best practices when conducting an infrared thermography inspection on solar modules:- Perform inspections under clear, sunny conditions with a minimum solar irradiance level of 600 W/m² to ensure sufficient thermal contrast.
- Maintain a consistent camera viewing angle of 45 to 60 degrees to minimize reflection interference from the glass surface.
- Verify that the inverter is actively tracking the Maximum Power Point (MPP) during the survey to ensure electrical stress is present for heat detection.
- Document the serial number and precise coordinates for every identified anomaly to streamline post-inspection maintenance and warranty claims.
- Cross-reference thermal findings with string-level DC current measurements to distinguish between module-level defects and array-level wiring issues.
Infrared Wavelength Range From Visible Red Light to Microwaves in Sunlight Reaching Panels
The infrared (IR) wavelength range from visible red light to microwaves in sunlight reaching panels spans a broad portion of the electromagnetic spectrum, categorized by how these waves interact with energy absorption and atmospheric conditions. This region begins at the edge of the visible spectrum and extends deep into the longer wavelengths that silicon solar cells cannot convert into electrical current.
Upper Wavelength Limits of Infrared Sub-Regions (µm)
| Spectrum Division | Wavelength Range | Interaction with Solar Panels |
|---|---|---|
| Visible Red Light | ~700 nm | High conversion into electricity |
| Near-Infrared (NIR) | 0.75 – 1.4 µm | Partial conversion near bandgap cutoff |
| Short-Wave (SWIR) | 1.4 – 3 µm | Absorbed primarily as heat |
| Mid-Wave (MWIR) | 3 – 8 µm | Absorbed as heat |
| Long-Wave (LWIR) | 8 – 15 µm | Thermal radiation emission |
| Far-Infrared | 15 µm – 1 mm | Negligible impact/absorbed as heat |
While the silicon solar cell remains effective through the visible spectrum and into the near-infrared (NIR) range, the transition toward longer wavelengths—specifically mid-wave and long-wave infrared (LWIR)—marks a shift from potential power generation to thermal accumulation. Per the American Institute of Physics, photovoltaic cells mainly transform visible and near-infrared light into electrical energy, while infrared wavelengths that exceed 1,200 nanometers largely serve to heat the panel instead of exciting electrons. Understanding this distribution helps you differentiate between useful energy and the portion of the solar spectrum that contributes to the temperature rise of your installation. By mapping the transition from photon conversion to heat absorption, you can better anticipate how your equipment manages energy flow across these varying spectral bands.
Monitor your panel array during the peak of the day when irradiance levels are high to determine if the thermal signature reflects typical energy absorption or localized cell defects.
Frequently Asked Questions
Is unusable infrared part of why silicon solar cells can’t go past about 30% efficiency?
Yes, unusable infrared is a primary reason why silicon solar cells face a theoretical efficiency ceiling of about 29-30%. Because silicon has a bandgap of about 1.1 eV, it only converts light up to the bandgap cutoff wavelength of about 1,100 nm, causing longer infrared wavelengths to be lost as heat.
How many degrees hotter than its neighbors does a solar cell need to be before an infrared scan counts as a fault?
A solar cell needs to be more than roughly 20°C hotter than its neighbors to signal a fault. A typical hot spot will show 10-20°C or more above surrounding cells, but once the temperature difference exceeds that 20°C threshold, it indicates a likely defect that requires professional inspection.
What resolution do drone infrared cameras need for solar inspection, and how much does a handheld thermal camera cost?
Drone thermal cameras typically require resolutions between 320×240 and 640×512 pixels for effective solar inspection. For handheld units, you can expect a price range in the US of roughly $300 to over $10,000, depending on the specific features and precision required for detecting faults in your array.
What weather and sunlight conditions give the most reliable infrared solar panel inspection results?
The most reliable results occur under clear skies with low wind and irradiance above about 600-700 W/m². These conditions ensure that faults are clearly visible during your inspection. You should avoid overcast days or high winds, as these factors can mask the thermal signatures of underperforming components or damaged cells.
How do anti-reflective glass coatings and perovskite-silicon tandem cells deal with infrared compared with standard solar panels?
Anti-reflective glass coatings mainly target visible light to improve transmission, whereas a perovskite-silicon tandem cell is designed to capture more of the spectrum. While standard silicon solar cells waste much of the infrared as heat, new multi-layer materials aim to improve conversion efficiency by better utilizing these wavelengths of infrared.
