Ask A Scientist Chemistry Archive

(Created prior to 1993)

Question: Solar cells use Phosphorous to capture light energy. If 
Chlorophyll B was used instead of Phosphorous, would it capture the same 
amount of energy or greater?
------------------------------------------------
Chlorophyll B is, chemically, very similar to hemoglobin (the 
stuff that carries oxygen in your bloodstream and gives it that lovely red 
color).  However, hemoglobin is an iron porphyrin (Fe surrounded by four 
pyrroles, which are organic ring molecules with nitrogen atoms surrounding the 
Fe), whereas chlorophyll (A&B) is a magnesium porphyrin.  Chlorophyll is an 
effective photoreceptor (capture light energy efficiently) because they have 
alternating single and double bonds.  Because of a slight difference in their 
structures, A and B tend to absorb light over different ranges of wavelengths, 
and therefore the two complement each other in biological systems (i.e. in 
plants).  However, it is not enough to just absorb the light energy; there 
must also be efficient means of transferring that energy to chemical potential 
(i.e. to transform it into a form that it can be used to do some work).  In 
biological systems, a complex sequence of reactions within a cellular 
structure, called a chloroplast, uses the light energy to make chemical energy 
by reducing CO2.  In other words, your "bio-solar cell" would have to somehow 
either (1) mimic this process, which sounds pretty tough, or (2) somehow 
transfer energy directly into the motion of electrons (which could cause a 
current, etc.).  Then, you would have to figure out a way to make a cell which 
would prevent the chlorophyll from degrading. Anyway, what I wanted to say
was, it is tough to make a solar cell that is more efficient than a 
chloroplast; it absorbs photons with near-unit efficiency (almost every photon 
that hits a chlorophyll molecule gets absorbed) and transforms light energy to 
chemical energy very efficiently as well.  But a plant can always make more of 
it when it starts to degrade, and a solar cell made by man cannot repair 
itself.  I hope these meandering observations helped!
Robert Topper
=========================================================
I came across an interesting piece of history and it reminded me 
of this question in NEWTON.  Melvin Calvin, who won the Nobel Prize for his 
work in photosynthesis, discovered that plant chlorophyll under the influence 
of sunlight could give up electrons to a semiconductor such as zinc oxide.  
Calvin and his associates made a "green photoelement," which actually produced 
a 0.1 microampere per square cm current.  The addition of hydroquinone to the 
salt solution was necessary to extend the plant's life.  But the chlorophyll 
seemed to act as an electron pump passing electrons from the hydroquinone to 
the semiconductor.  Calvin further calculated that a chlorophyll photoelement 
with an area of 10 square meters could produce a kilowatt of power.  He even 
theorized that such "green photoelements" would be manufactured on an 
industrial scale by the year 2000, which would be many times less expensive 
and efficient than the silicone solar batteries now being researched.  So, 
this technology has been envisioned and though we are far from making Calvin's 
dream reality, it definitely is possible!
=========================================================
Absolutely! It is theoretically possible....but there are 
practical issues.  However, I did not mean to squelch the possibility of 
achieving this someday.  I met Mel Calvin back in 1985, and he is a really 
nice guy who loves to talk to students.  Also, he is an excellent public 
speaker.
Topper
=========================================================