We’ve covered the workings of a solar cell in the first two parts of this series – Basics of solar power Part I and Basics of solar power Part II. In part three of this series and as mentioned in the previous post, we’ll now find out what new materials have been discovered/invented in order to boost the solar cell efficiency. It is important to know that it is not only the characteristics of the materials that boost efficiency of the solar cells but other factors too and will be discussed in this post.
History of solar cell materials:
The history of solar cell materials can be traced back to 1873, when Willoughby Smith discovered the photoconductivity of selenium. Later in 1953, a mere 4.5% efficiency was achieved by Bell Laboratory when they fabricated the first crystalline silicon solar cells.
Factors affecting solar cell efficiency:
After 140 years since the discovery of photoconductivity of selenium, the world record for solar cell efficiency has rose to 44.7%. The reason why we haven’t reached a 100% are plenty. One apparent reason is dust. As dust accumulates on the cell’s surface, it blocks the sunlight that is available to it. Rovers in the space that study planets run on solar power. These too require frequent cleaning in order to work efficiently day in and day out. Rovers thus must dust themselves off and start all over again. A self-cleaning technology that has been developed to help rovers do this can also be applied to the panels here on the deserts of the Earth. It uses the principle of repelling dust particles using electrostatic forces. As you can see, even if deserts provide us with maximum sunlight, dust can stifle all our efforts of putting it into use. To add to the problem is lack of water. Water is scarce in such arid regions, hence water-cleaning is not a feasible option. NOMADD is an automated and mechanical technology that claims to solve these woes.
Next to dust is the problem of reflection of light off the surfaces of the solar panels. We require the light to be absorbed and not reflected. Anti-reflection coatings are hence applied.
Other factors influencing solar panel efficiency are rather inconspicuous. These are related to the intrinsic properties of materials such as band gap. Band gap energy is the minimum amount of energy required by the incoming photons to kick loose the electrons in the material (kicking the electrons by the photons is scientifically called as photoexcitation). Band gap is the gap between the valence band of the material and its conduction band. Conduction band is where the electrons flow freely. Please know that when the electrons start flowing, not all of these electrons constitute the electricity generated. This is because some of these electrons recombine with nearby holes. Hence, not all electrons are ‘collected’. The band gap energy is different for different materials. Any less, no electrons move. Any more, light energy is converted into heat energy instead of electrical energy. To overcome these energy-efficiency limits, scientists have devised several ways.
How were scientists able to increase photovoltaic efficiency?
Structure of the solar cell: Take the example of the world record beater solar cell. It employed a procedure called wafer bonding in which two desired semiconductor crystals can be connected together without loss of crystal quality. Also, multi-junction cells (more than one sandwich) are reported to have higher efficiency as compared to single junction cells. Multi-junction cells overcome the Shockley–Queisser limit over the single-junction cells.
New materials: Ferroelectric ceramic material – perovskite crystals made with a combination of potassium niobate and barium nickel niobate, was developed by the team at the University of Pennsylvania and Drexel University. The additional materials in the crystals were inserted to alter the energy band gap. The magic of these being ferroelectric is in the fact that they are able to channel the haphazard movement of kicked electrons in a much better way. In other words, consistent polarity is maintained in a much better way without much loss.
Better cooling: IBM’s developed a liquid metal cooling system for concentrated photovoltaic cells so that they can endure highly focused solar energy without overheating.
Using up all the sunlight: Sunlight is made of a range of wavelengths of lights – the electromagnetic spectrum. Silicon absorbs light of wavelengths close to the visible range. Photovoltaic response curve – solar cell output plotted against wavelength, will show the wavelengths absorbed. In order to harness the entire spectrum, scientists have to engineer new materials. New nanomaterials can now do just that.
Upping the existing materials: Semiconductor materials do not absorb light all that well when they are made thinner. To improve this without compensating on the thickness of the material, scientists have identified a concept called ‘nanocavity’. With this, we can have thinner yet better solar cells.
Biomimicry: Bio-inspired materials’ best example will be one that has taken after the process of photosynthesis. This gave rise to dye-sensitized solar cells. On the other hand, bio-inspired mechanisms such as moving solar cells are inspired from sunflowers that track the sun.
This ends the third part of the series of Basics of Solar Power.
This blog post was first published at GreenHatters on March 18, 2014. Version edited for minor corrections. It’s a part of a series on solar power fundamentals.
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