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April

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TechBlog3: Why are PV efficiencies so low?

by Sindhura

After a bit of a delay, we bring forth the third technical blog for you to learn the science behind solar power. This blog talks about PV solar modules efficiencies and addresses why nature limits them to such low numbers and how scientists are trying to overcome these limitations.

 

It would probably not have escaped your notice that the efficiencies of the solar PV modules are quite low. You usually see transformers and inverters, other electrical equipments used in solar plants have efficiencies above 95%. And yet, the PV panels have efficiencies < 20%.The most commonly used PV technology, crystalline-Si, has best commercial efficiencies 16-19%. The emerging thin film technologies give at best 12% (CdTe thinfilms). So, almost 80-90% of the energy in the sunlight is not being utilized by the solar cells. Why? And where is this wasted energy going? Read further to find answers to these questions.

 

Efficiencies are limited by the very nature of energy conversion in solar cells. What does a solar cell do? It converts the energy carried by photons (mass-less particles of pure energy) in light into moving electrical charges i.e. current and thus generate electrical power [Read our previous article "how solar cell works" to understand how this conversion happens].

 

SOLAR RADIATION:

So let us start by looking at what is in the light.

Light is made of pure energy particles called photons. The curious thing, is these particles also are waves. Imagine a photon particle going through space. As it goes, it causes vibrations in electro-magnetic field (electric field and magnetic field are perpendicular to the wave propagation direction).  So, it is also a wave and that is why light is called “Electromagnetic Radiation”.

The energy carried by the wave is inversely related to the wavelength  - the distance between the crests of a wave. So, smaller wavelengths bring higher energies. You can imagine it physically like this – when the wavelength is short, there will be many wave crests reaching you in a given time (if the wave speed is fixed), so you would receive more energy.

In such a wide electromagnetic spectrum, you can see that visible light is but a very thin band.  In the universe, stars are the major source of electromagnetic radiation and the kind of radiation emitted by a star depend on its temperature. The hotter the star, the smaller the wavelengths emitted.

Our closest star, the Sun, is about 5000 oC hot and emits light composed of photons of many different wavelengths. Although it is only the visible light that we can see (the VIBGYOR  has wavelengths 400- 700 nms), Sun emits other radiations as well. Of all this, the earth’s atmosphere filters some of it and we receive only a portion of it on the surface in 300 nm – 2500 nm range.

The majority of the photons have energies between 400-700 nms (you can see in the above picture that the intensity is highest in this region). In terms of energies, on earth we are receiving 0.5-4 eV (an electron volt, “eV”, is electron charge times one Volt, so 1.67×10-19 Joules. It is a unit of energy not voltage).

So now that we know what is in the light given to us by our Sun, let us proceed to the conversion in the Photovoltaic (PV) cell.

 

SQ LIMIT:

Let us say our PV cell with has a band gap energy, “EG”. Let us also assume that the cell surface is perfectly non-reflective and all the light goes in. Let us see what happens to photons of different energies.

  • Photons with energy < EG:  Don’t have sufficient energy to excite the electrons from valence to conduction band. These will go straight through the cell. We cannot use their energies.
  • Photons with E > EG: These will be absorbed. Of their total energy “E” -
    • “EG” portion of their total energy is used to generate e-hole pairs and electricity is produced.
    • (E- EG), the excess energy is converted into heat.

What happens physically is – high energy photons excite electrons from valence band to an elevated level in the conduction band. The larger E-Eg difference, the higher up in conduction band the electrons jump. And then they promptly, within a few nano seconds, come down to bottom level of conduction band while losing energy as heat. 

So of all the incoming energy, we use only “EG” portion of those fraction of photons with higher energies than bang gap. So, as the band gap increases, the efficiency at first increases – because the portion of energy utilized is higher. But after a point the fraction of utilizable photons falls such bringing down the efficiency.  

 

Let us try to understand this with a simple example -

Suppose we had three materials with bandgaps 1,2 and 3 eVs. And the solar radiation spectrum is roughly an inverter bell – like this

 So there are 50 photons with energy in the range 0-1eV; 100 photons with 1-2 eV; 100 with 2-3 eV and another 50 with energies 3-4 eV.

  •  For the band gap of 1eV:  We can use all those photons with > 1eV, i.e. 250 photons. Each photon gives 1 eV of its energy to create charges, the rest is lost as heat. So when this sort of light falls on the 1eV band gap material, 250 eV of electricity comes out.
  • If the band gap was 2eV: we can use only those with > 2eV, i.e. 150 photons. Each of the photon gives 2 eV to create charges. So the energy generated is 300 eV.
  • If the band gap was 3 eV: Only 50 photons have higher energy. and each gives 3 eV. So the total energy output is 150 eV.
 In 1961 Shockley and Queisser did a similar exercise, but with exact solar spectrum and more number of bandgaps. They published a paper on calculation of this conversion efficiency as a function of the band gap. And this came to be known as Shockley-Queisser limit, commonly called SQ Limit. This Efficiency vs. EG (band gap energy) plot is shown below:

 

So when the band gap of the material is too small, many photons can be absorbed but the energy portion utilized will also be very small. When the band gap is too high, most of the absorbed photon energy could be used, but the number of absorbed photons themselves will be small. The maximum utilization of energy occurs ~1.3 eV. And the corresponding peak efficiency possible (single junction) is 33%.

Of course this is the perfect solar cell (single junction) limit. The real materials have defects, grain boundaries, contact resistances, reflection at the surface etc that will bring down the overall efficiency. Currently, solar cells made from different materials are not yet close to the Shockley limit. The Department of Energy, USA published a graph of currently achieved efficiencies of different materials.

 

HOW TO OVERCOME SQ LIMIT?

Now you might have a question popped in your head – “Then how are certain new technologies in R&D labs reporting efficiencies as high as 40%?

These technologies very innovatively alter the generic solar cell design itself! They do this by increasing the number of p-n junctions or by adding new energy stages within the band gap or by altering the energy of photons themselves.

  • Multijunction/ tandem: They stack multiple p-n junctions on top of one another. The largest bandgap material is at the top with decreasing gaps towards the bottom. When sunlight falls on the top layer, the highest energy carrying photons are absorbed and the rest go through to the next junction where slightly lower energy photons are absorbed and so forth. So many more photons are absorbed than in a single junction.
  • Band gap manipulation: Impurities are introduced into the material to create additional energy levels in the band gap. So the incoming photons have many different energy steps available. This helps increase the number of photons that get absorbed as well as reduce the energy wastage in the form of heat.
  • Conversion of photon energies: There are two ways to do this.
    • Upconversion: here multiple photons of lower than EG energy are absorbed into a material that will emit a single photon with energy larger than EG.
    • Downconversion: the energy of photons with larger than EG are converted into multiple photons of energy equal to EG.

There are many other innovative techniques are being developed by scientists to overcome the SQ limit but most of them are still in laboratories.

Only the multi-junction solar cells have reached commercialization with available efficiencies as high as 30%. But they are more expensive and have increased complexity. Because of this, their use is limited to concentrated solar cells (CPV) where the amount of PV material used is much smaller and applications in aerospace where their high power-to-weight ratio is desirable.

 

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