Professor Hanington's Speaking of Science: Record 19.3% efficiency with plastic solar cell

As our energy sources move away from fossil fuels towards more environmentally friendly ones, the quest for low-cost solar cells that can power a home or recharge an electric vehicle (or place electricity in some kind of grid where you can later financially re-claim it) is going on with increasing fervor.

Just this week a large leap in power efficiency has been announced for organic type solar cells. As published in Nature Communications, a team led by Professor Li Gang at Hong Kong Polytechnic University has announced a method of fabricating a polymer solar cell that has a power efficiency of almost 20%. This bites on the heels of the best real-world silicon solar cell to date, the one developed by Kaneka Corporation, which has a recorded 26.7% conversion efficiency. That one, composed of special single crystal array, is very costly to fabricate as opposed to the polymer type that can be easily fabricated by squeegeeing material on a suitable substrate. That was in 2016 and still holds the record.

To understand how the Hong Kong team was able to pull off such a feat, we need to back up a little and quickly go over the concepts involved in turning sunlight into energy. Although the first demonstration of the photovoltaic effect, by Edmond Becquerel in 1839, used an electrochemical cell, probably the kind we are most familiar with is a silicon cell, the standard used for re-chargeable garden lights and calculators. These are all silicon devices because the technology of this element is well known, being over seventy years old.

The silicon solar cell is made of two layers inside, one called a P-type and the other an N-type. The P-type silicon is produced by adding atoms — such as boron or gallium — that have one less electron in their outer energy level than does silicon. Because boron has one less electron than is required to form the bonds with the surrounding silicon atoms, an electron vacancy or "hole" is created.

The n-type silicon is made by including atoms that have one more electron in their outer level than does silicon, such as phosphorus. Phosphorus has five electrons in its outer energy level, not four. Although silicon has four electrons is easily bonds with boron or phosphorus in the crystal structure. But because phosphorus has that extra electron, the crystal there is slightly negatively charged because that electron is not involved in bonding and instead, it is free to move inside the silicon structure. It becomes a charge carrier. Likewise in the boron doped region. The "hole" formed there — actually a missing bond — can act as a charge carrier too because the missing bond can move and act as a positive entity. When an N region is placed next to a P region, some of these extra electrons and holes find each other again and a depletion region forms which is void of these mobile charges.

But here is the interesting part about this no-man's-land depletion zone: Because the boron and phosphorus atoms stay put, the N-type side of the depletion zone now contains positively charged ions (from the phosphorous atoms) and the P side now contains negatively charged ions (from the boron atoms), and this creates an internal electric field that prevents further mingling of the electrons and holes. But when sunlight strikes the PN junction, electrons in the silicon are ejected, forming "holes." When this happens in the electric field of the depletion region, the field will move electrons to the n-type layer and holes to the p-type layer.

If you connect the outer regions of n-type and p-type layers with a metallic wire, electrons will travel, creating a flow of electricity. A typical silicon crystal cell creates about 0.6 volts in bright sunlight and can supply 1 Ampere of current for a cell the area of a typical cellphone. To get higher voltage you just place many cells in series. The average cost per watt for monocrystalline solar cell is about $1.

When you look down at the grey solar cell top you can see right through to the PN junction where all of the above action takes place.

The top layer is usually the N-type region. Putting an N region on top has an advantage because it is more resistant to light-induced degradation due to the presence of phosphorus instead of boron. This factor leads to more charges moving within the cell and a more efficient, powerful output.

Next week we will examine organic solar cells and how they work.

Gary Hanington is Professor Emeritus of physical science at Great Basin College and Chief Scientist at AHV. He can be reached at [email protected].

Mexican kick ball is a fun relay race that is a traditional game in many Mexican villages and played with a ball whose outer surface area number equals the volume number. If this were in the units of inches, what is the ball's diameter?

Answer: SQRT (36)

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