Can solar panels work with artificial light

Solar panels can utilize artificial light, but under 500 lux of indoor light, their output is less than 1% of their rated value.

It is recommended to use amorphous silicon panels placed very close to an incandescent lamp (within 5 cm) to utilize its infrared spectrum to generate microampere-level currents, which can only power miniature loads such as calculators.

Efficiency

Conversion Rate is Extremely Low

Standard monocrystalline silicon solar panels are designed for the ground-level solar spectrum (AM1.5G).

Their bandgap energy is fixed at around 1.12 electron volts (eV), making them most sensitive to the near-infrared and red light regions with wavelengths between 700 nanometers and 1100 nanometers, where their quantum efficiency can reach over 90%.

However, currently, mainstream commercial white LED lights usually adopt the "blue chip plus yellow phosphor" technology route.

Their spectral energy peak is concentrated in the 450-nanometer blue light band, while the response of silicon batteries in this band is only 40% to 50% of their peak.

When the photon energy is too high (for example, the energy of a 450 nm blue light photon is about 2.75 eV) and hits silicon atoms, the part of the energy exceeding the 1.12 eV bandgap (about 1.63 eV) cannot be converted into electrical energy.

Instead, it is instantly converted into lattice thermal vibration through the thermal photon relaxation process, causing the panel temperature to rise rather than increasing the current.

This heat loss is caused by the mismatch between photon energy and the semiconductor bandgap, which means that when a monocrystalline silicon cell is illuminated by an LED light source, the conversion efficiency, which is originally 18% to 22% under standard test conditions, is directly cut in half.

The actual spectral match factor is usually lower than 0.4.

From the spectral perspective alone, the efficiency has already been lost by more than 60%.

Light Intensity is Insufficient

The Standard Test Conditions (STC) define a light intensity of 1,000 Watts per square meter. At this time, the photocurrent is far greater than the internal leakage current of the cell (determined by shunt resistance).

But in an indoor lighting environment, even in a bright office, the illuminance is usually maintained between 300 Lux and 500 Lux.

For white LED light sources, 1 Lux corresponds to a radiation power density of approximately 0.003 Watts per square meter.

Therefore, an illuminance of 500 Lux is equivalent to a radiation intensity of only 1.5 Watts per square meter, which is only 0.15% of standard sunlight.

Under such weak light, the photocurrent plummets from the level of 8 Amps or 10 Amps to milliamperes or even microamperes.

At this time, the impact caused by the internal shunt resistance of the cell is multiplied.

Leakage current, which can be ignored under strong light, diverts a large part of the photocurrent under weak light, causing the Fill Factor to drop sharply from above 0.75 to below 0.5.

When the incident light power is lower than 20 Watts per square meter, the conversion efficiency of monocrystalline silicon batteries is no longer constant but decays rapidly in a logarithmic curve.

For a module with originally 20% efficiency, the actual output efficiency under 500 Lux illuminance is usually less than 3%, or even lower.

Not All-Powerful

A 100-watt incandescent lamp converts only about 2% to 5% of electrical energy into visible light, and the remaining 95% is all thermal radiation (far infrared).

Although silicon batteries can utilize part of the near-infrared rays (700 nm to 1100 nm), for long-wave infrared rays with wavelengths exceeding 1200 nm, silicon materials are almost transparent or can only absorb them as heat, unable to stimulate electron transitions.

According to the Stefan-Boltzmann law, the filament temperature of an incandescent lamp is usually between 2700 Kelvin and 3000 Kelvin, and its radiation peak wavelength is around 1000 nm.

Although it covers the response zone of silicon, most of the energy is still located in the invalid zone above 1100 nm.

If you try to shorten the distance to increase the light intensity, such as placing the solar panel 10 cm away from the bulb, the light intensity does increase according to the inverse square law of distance, but the accompanying thermal radiation will cause the junction temperature of the cell to rise rapidly.

The open-circuit voltage temperature coefficient of silicon batteries is about negative 0.35% per degree Celsius.

When the cell temperature rises from 25°C to 65°C, the open-circuit voltage will be permanently lost by more than 14%, further pulling down the already meager output power.

The More You Calculate, The More You Lose

Assuming that the efficiency of the grid generating electricity and transmitting it to your home is 35% (considering fossil fuel heat engine efficiency and line loss), you use 1,000 Watt-hours of electrical energy to light up an LED lamp.

Currently, the luminous efficacy of high-quality LEDs is about 100 lumens per watt to 120 lumens per watt. Even for top-tier fixtures, their wall-plug efficiency is hard to exceed 40%, meaning 600 Watt-hours of energy directly turn into heat.

The remaining 400 Watt-hours of light energy dissipate into space. Since it is not focused, only a very small part (probably less than 5%) can fall vertically on the surface of the solar panel.

Superimposing the remaining 3% conversion rate of the photovoltaic panel under weak light and spectral mismatch, the final recovered electrical energy may be only 0.5 Watt-hours to 1 Watt-hour.

The end-to-end energy recovery rate of the entire system is less than one-thousandth. With the cost of 1 kWh of electricity input, the recovered value is less than 0.001 kWh.

This energy dissipation of up to 99.9% is regarded as a completely failed design in any engineering application.

Dedicated Weak-Light Panels

The atomic arrangement of amorphous silicon is disordered, leading to a wider bandgap of about 1.7 eV.

This happens to correspond to the visible light range, and its absorption coefficient for the 400 nm to 700 nm band emitted by fluorescent lamps or LEDs is an order of magnitude higher than that of crystalline silicon.

Under a low illuminance of 200 Lux, high-quality indoor-specific photovoltaic cells can maintain a conversion efficiency of 8% to 10% and are less affected by temperature.

But even so, limited by the total input energy density (only a few watts per square meter), their output power density remains at the level of microwatts per square centimeter.

For a 50-square-centimeter indoor photovoltaic module, under 500 Lux illuminance, the maximum output power is hard to exceed 0.005 Watts, which can only maintain the operation of low-power sensors or LCD screens and is completely unable to undertake the task of charging batteries.

Intensity Gap

The Gap is Too Wide

In the photovoltaic industry, the rated power of all solar panels is measured based on "Standard Test Conditions (STC)," where the standard defined light intensity is 1,000 Watts per square meter.

What is this concept? This is equivalent to the energy density of the sun shining vertically on the ground at noon on a clear, cloudless day.

And your home or office lighting environment, even if you feel it is brightly lit, typically has a light intensity of only 300 Lux to 500 Lux.

There is a huge unit conversion trap here: Lux is a unit of brightness perceived by the human eye, while Watts per square meter is a physical energy unit.

For common white LED light sources, their luminous efficacy determines that 1 Lux corresponds to only about 0.003 Watts per square meter of radiant power.

A simple calculation shows that for 500 Lux office lighting, the actual physical energy radiated onto the solar panel is only about 1.5 Watts per square meter.

"This is not just a little weaker; the energy density has dropped by three orders of magnitude. 1,000 Watts per square meter versus 1.5 Watts per square meter means indoor artificial light intensity is less than 0.15% of noon sunlight. Expecting to use this 0.15% input energy to generate meaningful electricity is as unrealistic as trying to drive a hydroelectric turbine with water from a dropper."

Useless at a Distance

The sun is 150 million kilometers away from the earth. For ground applications, it can be regarded as a parallel light source. Whether you put the panel on the roof or on the ground, the light intensity is almost the same.

But an artificial light source is a point source, and the light diverges in all directions.

Suppose you have a bulb with a luminous flux of 1600 lumens.

When you test it at a distance of 1 meter from the bulb, the light has spread to a spherical surface with a radius of 1 meter, with a surface area of about 12.56 square meters.

At this time, the illuminance is only about 127 Lux, which translates to an energy density of less than 0.4 Watts per square meter.

If you want to obtain an intensity close to sunlight, you must extremely shorten the distance.

According to calculations, to simulate an intensity of 1,000 Watts per square meter, you need to place the bulb closer than 4 cm to the cell panel.

At this point, a contradiction arises: every time the distance doubles, the light intensity is only one-quarter of the original.

When you place the solar panel at a "normal use distance" of 30 cm from a desk lamp, the light intensity has decayed to a few tenths of that at the light source surface.

This geometric energy dilution is destined to ensure that unless you wrap the solar panel tightly around the bulb, you can never collect enough energy flux in physical space.

Current Won't Rise

Many users use a multimeter to measure solar panels under indoor light and find that the open-circuit voltage (Voc) is surprisingly 80% or even 90% of the rated value, so they mistakenly believe it can charge.

This is a classic physics trap. The voltage of a silicon cell is proportional to the logarithm of light intensity. Even if the light intensity drops by 100 times, the voltage may only drop by 0.2 Volts to 0.3 Volts, looking "sufficient in voltage."

However, the photocurrent (Isc) has a strictly linear relationship with light intensity.

When the light intensity drops from 1000 Watts per square meter to 1 Watt per square meter, the current is literally divided by 1000.

"It's like you have a huge reservoir dam (high voltage), but the water flowing out of the pipe is only drop by drop (current is negligible). Power is voltage multiplied by current. When the current value is multiplied by a seemingly good voltage, the result is still close to zero. A 100W module that can output 5 Amps of current in sunlight usually outputs only 2 mA to 5 mA under indoor lighting. This meager current is not even enough to offset the resistance loss of the long wires themselves, let alone drive the chemical reaction inside the cell."

Leaking Electricity Everywhere

In addition to the scarcity of input energy, the parasitic resistance effect inside the panel is infinitely magnified under weak light, further widening the negative impact of the intensity gap.

A solar cell can be equivalent to an ideal current source connected in parallel with a diode and a shunt resistor.

Under strong light, the photocurrent is very large (several Amps), and the few milliamperes of current leaking through the shunt resistor can be ignored, just like a little water seeping from the bank when a big river rushes by.

However, under the faint intensity of artificial light, the photocurrent itself is only a few milliamperes.

At this time, the proportion of "leakage" caused by the shunt resistor becomes shockingly high.

It is possible that 50% or even more of the generated photocurrent is directly short-circuited through defect channels inside the cell and never flows to the external circuit.

A cell that originally had a 20% conversion efficiency under strong light will see its effective conversion efficiency plummet to below 1% under an intensity of 500 Lux due to this surge in the internal leakage current ratio.

Best Match

Finding the Right Path

The bandgap of monocrystalline silicon and polycrystalline silicon batteries is fixed at 1.12 eV, which means that they are "heavy dependents" of infrared light and red light, with the highest absorption rate for photons with wavelengths from 700 nm to 1100 nm.

However, common cool white LEDs (color temperature 6000 K) or fluorescent lamps in offices concentrate their energy mainly in the visible light region of 400 nm to 600 nm.

For crystalline silicon, although these high-energy photons can be absorbed, the excess energy exceeding the bandgap will be directly dissipated through thermal relaxation, causing the photoelectric conversion efficiency to lose more than 50% right at the start.

In contrast, amorphous silicon (a-Si), due to the disorder of its atomic arrangement, has a wider bandgap of about 1.7 eV to 1.8 eV, which happens to correspond to the edge of visible light at a wavelength of about 700 nm.

The range that amorphous silicon is "picky" about is exactly the visible light band that is most abundant in indoor lighting.

Its absorption coefficient for blue and green light is two orders of magnitude higher than that of crystalline silicon.

This explains why amorphous silicon thin films with a thickness of less than 1 micron perform far better than monocrystalline silicon slices with a thickness of 160 microns under indoor weak light.

· Spectral Overlap: The spectral overlap between monocrystalline silicon and incandescent lamps can reach more than 30% because incandescent lamps emit a large amount of infrared rays; while the overlap between monocrystalline silicon and white LEDs is usually lower than 15%, as most emission spectra fall outside the cell's high-efficiency response zone.

· Weak Light Response Threshold: Crystalline silicon batteries usually need an irradiance of at least 50 Watts per square meter (about 5000 Lux) to produce a stable voltage output, while amorphous silicon batteries can still maintain more than 85% of open-circuit voltage under extremely low illuminance of 20 Lux (0.2 Watts per square meter).

· Temperature Coefficient Impact: Under thermal light sources like halogen lamps, for every 1 degree Celsius rise in cell temperature, the power output of crystalline silicon drops by 0.45%, while amorphous silicon drops by only 0.25%.

Who is the Perfect Match?

A halogen lamp is essentially a blackbody radiation source. When the filament temperature reaches 3000 Kelvin, its radiation peak is located around 1000 nm, which hits the "bullseye" of monocrystalline silicon batteries.

If you place a monocrystalline panel with a rated power of 100 Watts at a distance of 50 cm from a 500-Watt halogen work light, you may measure a current of 0.5 Amps to 1 Amp, but this is equivalent to exchanging 500 Watts of input for less than 15 Watts of output, and the panel will heat up to 70 degrees Celsius within 10 minutes.

For the power supply of indoor electronic products, the real "perfect match" is Dye-Sensitized Solar Cells (DSSC) or Organic Photovoltaics (OPV) paired with LED light sources.

These new materials can adjust molecular energy levels through chemical means, making their absorption peaks precisely align with the 450nm blue light peak or 550nm phosphor peak emitted by LEDs.

Specialized Application

Under standard sunlight (AM1.5G), the efficiency of DSSC may be only 11%, far lower than the 22% of monocrystalline silicon.

But under 1000 Lux warm white LED (color temperature 2700K) illumination, the situation is reversed.

Due to the high degree of spectral matching, the photoelectric conversion efficiency of optimized DSSC or organic photovoltaic cells indoors can soar to 20% or even 30%.

This is because they abandon the capture of infrared light and focus on collecting the little bit of visible light energy available in the indoor environment.

In contrast, under the same environment, monocrystalline silicon's actual efficiency will plummet to 1% to 4% due to the drop in fill factor and shunt resistor leakage.

So, if you are powering an IoT sensor, requiring only 50 microwatts to 100 microwatts of power, then a DSSC or amorphous silicon module with an area of only 10 square centimeters is the only feasible engineering choice;

If you try to use a monocrystalline silicon panel to do this, you need at least 10 times the area to make up for its "blind" state under the LED spectrum.