Solar Power » Technologies

The Principles of Photovoltaics

The layers of a solar module

Photo Cell Cross SectionAll pv- modules contain a number of layers from the light-facing side to the back:

  1. Protection Layer: Usually made from glass, though in thin-film modules this can also be transparent plastic.
  2. Front Contact: The electric contact at the front, has to be transparent, as otherwise, light would not get into the cell.
  3. Absorption Material: The heart of the module is the layer where the light is absorbed and converted into electric current. All materials used are semiconductors. In many cells, this is just one material, in most instances, silicon. However, in order to improve performance, there could be multiple layers of different materials. In addition, all layers are be doped. I.e. each layer is split further into an n-doped and a p-doped zone. See below for more details on doping.
  4. Metal back contact: A conductor at the back completes the electric circuitry.
  5. Laminate Film: A laminate ensures that the structure is water-proof and insolated from heat.
  6. Back glass: This layer gives protection on the back side of the module. It may be glass, it may also be made of aluminium or plastic.
  7. Connectors: Finally, the module is fitted with connectors and cables, so it can be wired.

 

The Photo-effect

In simple terms, the photo effect describes the conversion of light into an electric current. To describe this mechanism more formally, it is best to think of light in terms of a stream of photons where each photon carries one quantum of energy. Each photon is associated with just one wavelength or frequency. High-frequency photons have more energy than the ones with low frequency.

Photo Effect

 

Intrinsic Semiconductor

In a pure semi-conductor the outermost electron of the underlying molecule is not heavily bound. An incoming photon with enough energy can promote the electron from the valence band to become a free electron in the conduction band. See figure above. This in turn leaves a positive hole in the valence band. The minimum energy that is necessary for this to happen is called the band gap. The band gap varies from material to material and also varies with temperature, which is why performance of solar modules deteriorates with higher temperatures. However, in an intrinsic semiconductor, no resulting electric current is observed, since the promoted electrons re-combine again with the holes.

Doped Semiconductors

Doping means the addition of a small percentage of foreign atoms in regular crystal lattice of the semiconductor.
  • p-type: Adding atoms with one electron less creates a layer with fewer negative electrons in the valence band, pushing the overall energy level up. For instance: In Silicon, add Boron, Aluminium or Gallium.
  • n-type: Adding atoms with one electron more creates a layer with more electrons in the valence band, pushing the overall energy elvel down. In Silicon, n-type dopants are Antimony, Arsenic or Phosphorous.

Semiconductor with p-n Junction

Where p-type and n-type layers join at the p-n junction, electrons and holes diffuse to create the charge-free depletion zone. Moreover, the junction creates a slope in the resulting energy bands. Now, when a photon promotes an electron to the conduction band, it can subsequently "roll down" through the depletion zone into a lower energy band rather than instantly re-combine with a hole. This is what generates the photo current.


Band Gaps

Band GapsThe band gap, which is the minimum energy required to catapult an electron out of the valence band varies by material. It is usually expressed in units of [eV], read: "electron Volts" where "one electron" is the elementary charge, e.

To convert electron-Volts into wavelenghts of incoming light, we use the forumla:

electron volts converted into wavelength

  • Small band gap: Long waves (e.g. infrared) can still cause the photo effect.
  • Wider band gap: Long waves can not be converted into an electric current and will just pass through the material unabsorbed.

 

Spectral Sensitivity

Spectral Sensitivity of semiconductors In reality, the photo effect is not like an on- off- switch depending on the energy of the photon. At longer wavelengths, electrons may still flow due to energy from ambient temperature. On the other hand, short wave photons may not be able to be absorbed, as they have too much energy.

The Response Rate measures a material's ability to convert light into an electric current:

Response Rate

Due to the spectral sensitivities, this rate is highly dependant on the wavelength of the incoming light. Most of the energy of the sun light is in the visible spectrum.

Although silicon captures a wider spectrum than the human eye, it is more sensitive to infrared than to visible light, thus less efficient in converting the all-important visible spectrum..

Other materials (III-V compounds) such as GaAs, GaInP or GaAsAl cover wider ranges where the spectral sensitivity is better matched to the incident sun light, thus increasing efficiency of the cell. By applying two materials in a tandem or multi-junction cell, an even wider spectrum can be captured.

 

Crystalline Silicon

Thin-Film

Multi-Junction

Mono-crystalline: The atoms form a regular lattice. Due to the regular structure, mono-crystalline has a better response rate. Purity is > 99.99999% (7N)

Poly-crystalline: This is in effect a series of crystals rather than one crystal. Purity is 99.999% (5N).- 99.9999% (6N)

Thin film modules use compounds with very stron light absorption characteristics, requiring only a thin layer. The absorption materials are deposited on glass or foil. They include

Amorphous Silicon (a-Si): Atoms do not form diamonds, but are randomly put together. Not surprisingly, the response rate is much lower than in mono-crystalline structure. However, it captures more of the highly-intensive light than crystalline silicon and can be changed by alloying it with, among others, Germanium or Carbon.

CdTe: Cadmium-Telluroid: Inexpensive technology with medium efficiency.

CIS/CIGS (cadmium-indium-gallium-selenide and copper indium gallium diselenide): Have efficiencies as high as crystalline silicon.
Differences in spectral sensitivities between materials can be exploited by adding multiple absorption layers on top of each other, resulting in multiple p-n junctions.

The material with the largest band gap is positioned closest to the incoming light, absorbing all the high- energy photons. For low- energy (long wavelength) photons, this first layer will be transparent until they hit the second layer with a lower band gap. This way, overall efficiency can be increased to 40%, compared to 20% for monocrystalline silicon.

Downloads: Download here spreadsheet containing band gap conversions and other pv calculations.

 

References:- Hyperphysics: Semiconductor Band Gaps
- Siemens Datasheet for a Si-Diode with spectral sensitivity

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