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Materials Science News: 2-D Phosphorus-The Future for Solar Cells?

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Materials Science News: 2-D Phosphorus-The Future for Solar Cells?

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Like most industries, the semiconductor industry is not impervious to economic high and lows.  After having a few rough years the industry is recovering, and along with this recovery, a wealth of development and development dollars have been spent.  This week materials science researchers announced that 2-dimensional phosphorus could be part of the future for the semiconductor industry.  One theory is that 2-dimensional phosphorus could eventually replace the more commonly used silicon; how will this affect the future of semiconductors?

Silicon in Semiconductors

Silicon atoms (specifically in crystalline form) are able to create perfect covalent bonds with each other.  This means that once the bond is made, the atom does not gain or lose electrons easily.  When four silicon atoms bond with each other they form what is called a lattice.  Pure silicon crystals are naturally an insulator, and do not allow much electricity to flow through it.  It is possible to change the behavior of silicon by doping it.  Doping is when you add a small amount of impurity into the silicon, which destabilize the covalent bonds.  There are two different types of doping that are done to silicon:

  • N-type: (When phosphorus or arsenic are added) creates a good negative conductor
  • P-type: (when boron or gallium are added) creates a good positive conductor

Adding either an N-type or P-type dopant turns silicon from a good insulator into a good (not great) conductor, and therefore creates a semiconductor.  While both the N-type and P-type doping is not novel, when they are together they create a diode (simplest semiconductor device).  A diode allows a current to flow in one direction but not the other.

New Research

While phosphorus is not in the same group as silicon or carbon (see periodic table)

[1], materials scientists at Rice University have found it to be a promising candidate for “Nano-electronic applications” that require stability [2].  Now to be clear this is not the common element phosphorus.  Rather it is a “two-dimensional phosphorus, [made] through exfoliation from black phosphorus” [2].  Black phosphorus is believed to be the most stable form of phosphorus.  It is created when phosphorus is put at “higher temperatures about 590 °C and higher pressures” or when phosphorus is combined with a “catalyst at ordinary pressures and a temperature of about 200°C” [3].

Researchers at Rice University compared 2-dimensional phosphorus with other 2-dimensional metal dichalcogenides like molybdenum disulfide because of their inherent conductive properties (metals are natural conductors).  Issues have arisen, however, where these other compounds bond-the point where the elements meet (point defect).  A disturbance is created in the flow of the current.  In doped silicon, this doesn’t occur because the negative and positive silicon work together to fill in these gaps therefore eliminating a disruption in flow.  When there are “multiple point defects or grain boundaries-where the sheets of a 2-D material merge at angles” the device is no longer useful [2].

Advantages of Phosphorus

2-dimensional phosphorus does not exhibit the same issues at the point defects that other materials tested experienced.  According to calculations done by theoretical physicist Boris Yakobson and his colleagues at Rice University, the point where 2-dimensional phosphorus point defects or grain boundaries exist, the materials semiconducting properties remain stable.  This transpires at the point defects because “atoms jut out of the matrix, this complexity gives rise to more variations among defects” [2].  Also, 2-D phosphorus bonds with itself, this therefore eliminates the recombining of electrons that occurs between hetero-elemental bonds.  2-dimensional phosphorus is very similar to 3-dimensional silicon because they both don’t have issues with band-gap changes at ground boundaries.  The key difference however between the two is that 3-dimensional silicon can change its properties from positive to negative at the point defects, and this does not occur in phosphorus.  Another benefit of 2-dimensional phosphorus is that phosphorus exists in abundance on Earth, and the black phosphorus is relatively easy to make.  No production worthy semiconductor equipment available yet for this material.

Future of Phosphorus Semiconductors

The researchers at Rice University believe that 2-dimensional phosphorus semiconductors could potentially be used to harvest sunlight in solar cells because their band-gap matches well with the solar spectrum.  Due to the way this new phosphorus responds at the point defects, the materials performance would not deteriorate as it has with other materials tested [2].  This is great news for the solar industry that is constantly looking for new ways to improve their products and make them more durable and efficient.

2-dimensional phosphorus has already been tested in “high-performing electronics, and has already shown it can be a better transistor than 2-D metal dichalcogenides” [2].

So far the future looks bright for the use of 2-dimensional phosphorus in semiconductors instead of silicon.  Semiconductors and their success effects our lives every day without people even realizing it.  Semiconductors are in all of our electronic devices, from our smartphones to the computers in our cars.  Their effectiveness is what keeps us connected in today’s technology dependent society.  If phosphorus is the answer to fewer interruptions in our devices, then it will be welcomed with open arms because let’s be honest nothing is more upsetting than when your smartphone malfunctions.

[1] http://www.mpoweruk.com/images/periodic_table.gif

[2] http://www.rdmag.com/news/2014/09/phosphorus-promising-semiconductor

[3] http://www.britannica.com/EBchecked/topic/68159/black-phosphorus

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By | 2016-12-15T22:25:17+00:00 September 13th, 2014|Engineering Consulting, Mechanical Engineering|0 Comments

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