[量子效率、模擬器IV量測入門]一看就懂的太陽能電池基礎原理

Solar cell is a device that, when being exposed to sunlight, generates voltage or electric current through photovoltaic effect. Solar cells are made with two types of semiconductors - p-type and n-type-to form a p-n junction. In the junction, electrons move toward the p-type side and holes move towards n-type side. This is where electric field is formed. When sunlight shines on solar cells, the energy of photons excites electrons to its conduction band. At this state, electrons can move freely within the material to create electric current.

pn.jpg

Appendix I: P-N Junction Energy State


Photovoltaic Principles
1.Absorption(Absorption)
When incident photon energy is bigger than the band gap, energy is absorbed through either intrinsic absorption, extrinsic absorption or free carrier absorption.

2. Photocarrier Generation(Photocarrier Generation)

After absorbing photons, semiconductors produce electron-hole pair.

3.Transportation (Transport)

If the electron-hole pair is formed in depletion region, it will be separated by the electric field and drift to each side. If the electron-hole pair is formed in intrinsic region, it will diffuse to depletion region. Then be separated and drift to each side.

4.Collection(Collection)

The photocarriers are extracted with electrical contacts to work in external circuit. When the electrons have passed through the circuit, they will recombine with holes at a metal-absorber interface.


Appendix II: Electron-Hole Separation


Solar cells absorb photons with different wavelengths differently. Shorter wavelength photons such as ultraviolet can immediately excite electrons and generate photocarriers. Longer wavelength photons, such as infrared light, can penetrate deeper into the material and be absorbed to generate phototcarries. Medium wavelength photons tend be absorbed in the depletion region. Because electric field is very strong in the depletion region, electron-hole pair will be separated into free electron and hole immediately. Charges will be moved by electromotive force to metal-semiconductor interface and results in higher efficiency.


▲Appendix III : Different wavelengths penetrate and absorbed differently


Let’s understand what short circuit and open circuit are before we get into I-V curve.

Let’s understand what short circuit and open circuit are before we get into I-V curve. 

Short circuit condition: After sunlight shines on the device, connect positive and negative electrodes to make RL =0 to be in a short circuit condition. Meanwhile, the voltage between two electrodes Va=I RL. To wit, short-circuit current, ISC, is the current through the solar cell when the voltage across the solar cell is zero. 

Open circuit condition: After sunlight shines on the device, draw maximum voltage by connecting the electrodes to nothing. Let it open. Therefore, RL =∞. Meanwhile, the current through the cell Iout=0. To wit, open-circuit voltage, Voc, is the maximum voltage available from a solar cell, and it occurs at zero current.

rl.jpg

▲Appendix IV : Short Circuit (Left) and Open Circuit (Right)

I-V Curve

What we can learn from short circuit and open circuit is that: we can manipulate the output current and voltage by changing external resistance. Imagine we put a variable resistor on each electrode. By changing the variable resistor, we can control the output current. When we let RL =0, we maximize output current ISC. When we let RL=∞, there is no output current. We maximize the voltage Voc. We can get a corresponding voltage Va and a circuit current Iout=Va/RL when RL keeps changing between 0~∞. We can now draw an I-V curve as it shows in appendix 5. We multiply any I and corresponding V on this I-V curve to calculate the output power, Pmax=I∙V. That is being said, by manipulating resistance, we can control output power. In which, we call the maximum power point Pmax and its corresponding current Imax and voltage Vmax.

piv.jpg

▲Appendix V : I-V Curve


Other Parameters of Solar Cells

Power Conversion Efficiency η : PCE is the ratio of output electrical power and input electromagnetic energy. 

Pmax is the maximum power. E is incident photon energy. From the PCE formula, we know that different E will result in inverse proportion of PCE. Light intensity at the equator is very different from light intensity at the North Pole. So, based on what standard should we be calculating? Luckily, scientists around the world reached agreements to set global standards. For instance, IEC 60904 series is a globally recognized standard. On Earth surface, PCE is calculated with light intensity at AM1.5G 1000 W/m2 .

FF (Fill-Factor)
To create solar cells, the purpose is to convert light into electrical energy. By the definition of Power (P), the output power is zero in both short circuit (Vout = 0) and open circuit (Iout = 0) condition. Practically, solar cells cannot operate when the output power is zero. Therefore, we need to find the maximum power point to reach maximum efficiency. Theoretically, solar cells can operate at Imax=Isc, Vmax=Voc, Pmax=Isc•Voc to generate maximum power. In reality, because of structure design and technical obstacles…etc, it’s hard to reach the maximum power. Therefore, in conjunction with Voc and ISC, we use fill- factor to determine maximum power of solar cells. FF is defined as the ratio of the maximum power from the solar cell to the product of Voc and ISC.


When Imax=Isc, Vmax=Voc, FF=100% Through FF, we can quantify the efficiency character of solar cells and use it as an evidence for adjusting, comparing and making improvement.


Related links:AM1.5GAM1.5D

Related Products:

Reference Cell QE-R SS-F5-3A