Two words. Very similar etymologies, yet very different meanings: photoelectric and photovoltaic. With the web the way it is (amorphous and shifting), and sources of information getting mingled between similar vocabularies, there needs to be a point of clarification regarding Albert Einstein’s role in the specific realm of photovoltaics. I will say it again: Albert Einstein did not discover photovoltaics, in theory or in practice. He was, however, instrumental in allowing other scientists to perceive the process by his implementation of the concept of the photon.
In 1887, Heinrich Hertz observed that metal electrodes (with a certain electrostatic bias) separated by a small space of air would “spark” across the gap more easily with the addition of ultraviolet electromagnetic waves (which we now also call photons) incident upon the electrode surfaces. In fact, the photons had sufficiently high energy to cause electrons bound within the metal to be ejected from the confines of the metal into space, then jumping to the next nearest metal at the opposite electrode. This observation was expanded by a number of well-recognized experimentalists including J. J. Thomson (1899) and Nikola Tesla (1901). Tesla even submitted and received a patent to charge a capacitor using a metal plate (US685957). This was an example of an optoelectronic effect being used to convert photons to usable electrons, but it is not the photovoltaic effect.
Now where does Einstein come in? Oh yes, in 1905 his paper On a Heuristic Viewpoint Concerning the Production and Transformation of Light was published. This paper (re-)established the concept of electromagnetic radiation as quanta of photons, giving great weight and validity to the idea that a mass-less photon (of a known frequency) could exchange energy with an electron to promote the charged species to an excited state. Later, in 1915, Robert A. Milliken demonstrated experimentally how this exchange was possible. Einstein’s explanation was the attribution for his 1921 Nobel Prize in Physics.
What about the photovoltaic effect? Up until now, we have explained the photoelectric effect wherein the energy from a photon is applied to eject a bound electron into space, then becoming a free electron until it was recaptured by a conductive surface. What if we wanted something more subtle? What if we wanted to absorb a photon of sufficient energy (lower than the ejection energy) to promote a low energy photon into an open shelf of energy (an orbital or band) that is still bound to the attractive force from the atomic nuclei? Such a process, absorbing light and maintaining the excited electron within the material of excitation is precursor to the photovoltaic effect. The final step for the complete photovoltaic effect is to separate excited charge carriers to electrically conductive electrodes (ohmic contacts). The photovoltaic effect was actually observed earlier than the sibling effect of ejecting electrons. In 1839, a 19-year old Alexandre-Edmund Becquerel observed a photocurrent (no sparks here, just electronic power) from light-sensitive electrodes immersed in an acidic solution. The electrodes were platinum metal, coated with AgCl or AgBr–silver salts much like our older photographic film materials. Then in 1877, Adams and Day published their work on selenium metal immersed in an electrolyte bath, and exposed to light. This was followed by C. E. Fritts in 1883, who compacted a selenium photovoltaic cell into a flat plate using gold leaf and brass electrodes.
Perhaps some of the real confusion cropped up fifty years later, when L. O. Grondahl observed the photovoltaic effect in copper/copperoxide materials, but titled his 1933 research: The copper-cupprous-oxide rectifier and photoelectric cell. Yep, there is that problem with similar etymologies. Even in their 1954 paper from Bell Labs research, authors Chapin, Pearson, and Fuller used the more generic title of photocell, and would term their device a photobattery in everyday speak (A new p-n junction photocell for converting solar radiation into electrical power). In fact, the term photobattery is a perfectly accurate way to describe the devices using the photovoltaic effect. I would even venture that a photocapacitor is a more likely descriptor for Tesla’s 1901 device in comparison.
In summary:
Photoelectric Effect:
- Absorb light (photons)
- Excite electrons sufficiently to eject them from their bound state (about nuclei) to a free, kinetic state in space.
This describes an energy transformation: from potential energy to kinetic energy.
Photovoltaic Effect:
- Absorb light (photons)
- Excite charge carriers sufficiently to promote them from a lower energy bound state to a higher energy bound state.
- Separate charge carriers to ohmic contacts.
This describes a different energy transformation: from low potential energy to high potential energy.
**Note: For an excellent review of photovoltaic devices and early observations (as well as the best PV text on-line), please visit Chapter 6 of Honsberg and Bowden’s PVCDROM. I use it in my classes on Design of Solar Energy Conversion Systems and Advanced Photovoltaics.
