(January 2004)

Discovering Photoelectrics isn't Easy

By Donald Georgi

This started out to be a straightforward review of a story titled NO P-N  INTENDED which appeared on page 88 of the February 2003 issue of Technology Review. The story focused on the efforts of a Bell laboratories researcher, Russell  Ohl, who was trying to find a more effective way to build a radio receiver without vacuum tubes because of their difficulty in operating at higher frequencies. Better amplification deemed to be necessary for the coming importance of technologies such as radar.

Ohl's approach was to focus on the advantages of crystal receivers, which had preceded vacuum tube receivers because of their availability and simplicity in the beginning days of radio.  As his work progressed, Ohl decided that the best approach would be to use semiconductor materials, which had greater purity than crystals.  

In 1940, Ohl had a sample of a silicon semiconductor with a crack in the middle,.  When the crystal was exposed to light, the current flowing through the semiconductor jumped significantly.  Ohl had the advantage of having many colleagues who understood the physics of semiconductors, and did identify that in this special photosensitive semiconductor had what we now known as n-type silicon on one side of the crack and p-type silicon on the other side.  The group developed better understanding to arrive at the conclusion that the energy of an arriving photon of light gave the excess electrons in the n-type material an energy boost across the junction to the p-type material and thereby produce a current to an external circuit. This fundamental idea is the basis of the photovoltaic devices, which are used today.

To bring a bit of historical color to this story, the origins of the photovoltaic phenomena were researched,.  The first scientific observation was documented in 1838 by nineteen-year-old Edmund Becquerel. By observation he noted that electrolytic cells consisting of certain metals in an electrolyte solution would produce small amounts of electric current when exposed to light. This discovery did not have the same energizing effect on the scientific community, which Volta's battery had produced, possibly because the amount of current produced was very small.

It seems that photovoltaics were destined to be discovered more by accident that investigation, because not much of  significance happens until 1873 when Willoughby Smith found bars of selenium had a reduction of resistance when exposed to light.  Smith was really working on methods to better understand conductors submerged for long  lines in telegraph applications. He presented his findings to the Society of Telegraph Engineers, but pursuit of photovoltaics did not continue at this time.

In 1877, Charles Fritts recorded the deliberate construction of a junction solar cell.  He used a substrate of selenium, probably chosen from the reports of Smith's experimental work. It was coated with nearly transparent layer of gold, allowing light to penetrate to the intersection between the two metals.  The efficiency of the cell was less than one percent, but the fundamental method had been established through experimentation.

In this period of time, rigorous understanding of physical phenomenon was beginning to take place. Maxwell had presented his theory of electromagnetics in 1865, concluding that light was a form of electromagnetic waves. Experimental confirmation of Maxwell's theory received substantial reinforcement in the experiments of Heinrich Hertz in 1886. Hertz detected propagated radiation from a spark fifty feet away and also noticed that the spark gap was more vigorous if it was exposed to ultraviolet light.  Hertz offered the phenomenon unexplained, but in the following year, Wilhelm Hallwachs offered an explanation and an experiment to show that electrical charge conductivity was dependent on the ultraviolet light striking a zinc plate. Later, in 1899, J. J.  Thompson was to determine the ultraviolet light impinging on the metal surface caused electrons to be emitted.

The topic was beginning to heat up. Three years later in a 1902, Lenard experimentally show to conduction of electrons through a vacuum were stimulated by blue lights impinging on the emitter plates.  He noted that increasing the light intensity increased to the number of electrons emitted it, but did not affect the energies of the emitted electrons.

History takes on a pattern similar to that of the Yankees during the era of Ruth and Gherig. Thompson had established the explanation with atomic theory, and now it was Einstein's turn to step up to the plate in 1905 to bring all the runners in. Albert decided that the incoming radiation should be thought of a quanta or bundles of energy rather than waves with energy equal to the product of Plank's constant, times the frequency minus the work function of the material.  (E=hf-W). In this approach, the observer can see that an electron which absorbs the quatum, has a resultant higher energy which allows it to flow in an electrical circuit. From Einstein's quantitative prediction, higher frequency, not greater intensity, will produce greater energies but light below the minimum frequency will not cause photo emission. This work was so significant that Einstein received the Nobel Prize for the work in 1921.

Robert Millikan, did not believe Einstein's theory and tried to experimentally disprove it in favor of the wave theory until 1916. He was not to be able to do this, but rather his experiments confirmed Einstein's approach and Millikan later won the Nobel Prize himself for the series of experiments.

With all the photoelectric experimental information and the underlying science understood, the crystal detectors of beginning radio were a stimulus to Ohl's search for better amplification which did progress to the invention of the transistor, again at Bell Labs. On the way it was one of those side benefits, the discovery of semiconductor photovoltaics provided a stepping stone for cleaner terrestrial and space power generation. We may someday be able to include the restoring sight to the list of contributions of photovoltaics. Beyond that, even the sky is no longer the limit.

BD