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:

1. Absorb light (photons)
2. 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:

1. Absorb light (photons)
2. Excite charge carriers sufficiently to promote them from a lower energy bound state to a higher energy bound state.
3. 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.

From the Google SketchUp Team:

“We’ve done some investigation over the weekend, but we don’t actually have any more information to provide. We don’t have a particular reference document, paper, or source for our algorithms, so there isn’t a location to which we could point you for reference. Furthermore, the solar calculations were implemented early on in SketchUp’s development, and the people who initially worked on them are no longer with the company.”

Thank you for considering this unusual failing in an otherwise highly useful software piece.

Dr. Jeffrey R. S. Brownson
Dept. of Energy & Mineral Engineering
The Pennsylvania State University
nanomech@psu.edu

Video interview provided by Eon’s Video Blog on bliptv.

Books on solar energy by Perlin:

A Golden Thread: 2500 Years of Solar Architecture and Technology (1980) Ken Butti and John Perlin (foreward by Amory Lovins). Cheshire Books, Palo Alto.

From Space to Earth: The Story of Solar Electricity (1999, 2002) John Perlin. (Aatec Press) Harvard University Press.

Therefore, I would recommend two web-based books for the curious, right now! The first is an educational project that began as an international collaboration between the University of Delaware and the University of New South Wales, funded by an IGERT grant. The site is called Photovoltaics: Devices, Systems and Applications CD-ROM, and the authors are Christiana Honsberg and Stuart Bowden. This includes interactive diagrams, movie clips of the silicon manufacture process, and a good review of solar energy. You will need to download Shockwave from Adobe. Up until recently, the Shockwave addition did not work for Macintosh systems, so I was more hesitant at recommending the site. But now: go for it! You will be busy for weeks. Note that the site is dedicated to silicon devices, and will not provide a comprehensive description of thin film PV devices and the principles of operation. That being said, the site is a gem.

The second book is not as web savvy, but does contain fantastic fundamental information on solar energy conversion. The resource is Power from the Sun by by William B. Stine and Michael Geyer, at California State Polytechnic University in the USA and IEA SolarPACES in Spain. This text is more like the classic paper text by John Duffie and William Beckman: Solar Engineering of Thermal Processes,¹ in which multiple solar energy conversion technologies are described.

There you go, solar energy enthusiasts! Go to school and get informed on solar energy. But if you are tied up with other things (like life), in the mean time do some winter reading and find out how much potential solar energy has as a sustainable technology!

1. Duffie, J. A.; Beckman, W. A. Solar Engineering of Thermal Processes. (3rd Ed.) 2006 John Wiley & Sons Inc, Hoboken, NJ, USA.

When we say low levels of irradiance, we are estimating a scale of light concentration that is typical of the diffuse and direct component of unconcentrated “global” or “total” solar radiation, or the light from a standard incandescent lamp or fluorescent lamp. This could be anywhere <1000 mW/cm², or 10x the sun’s concentration (remember, this is just a crude scale, not a hard and fast rule–don’t take this back to your classes). The standard for testing solar cells inside the earth’s atmosphere is called Air Mass 1.5 Global (AM 1.5G), because the light from the sun passes through 1.5 lengths of a generic Earth’s atmosphere to generate a convenient irradiance of ~ 100 mW/cm². Low levels of light such as this provide a sufficient number of photons (packets of light) to excite the electrons into an unoccupied level of energy (the conduction band). However, the population distribution of the majority carriers does not change significantly. That’s okay: the key player in a photovoltaic absorber is the minority carrier (n-type semiconductor: a hole; p-type semiconductor: an electron), and the population of minority carriers does change significantly with light absorption. Minority carrier transport gets the job done, in fact, because they are the limiting rate in the absorber reactor. You can find out more about charge carriers and carrier transport in the Photovoltaics CDROM from Honsberg and Bowden, Chapter 3 (although it doesn’t work completely for Macs, sadly)

What is high irradiance? You’ve heard the warnings about strong lasers pointing into others’ eyes? A laser is a coherent, collimated light source (the photons’ waves are in phase and heading the same direction), such that the photons can be very concentrated. If sufficient numbers of photons are absorbed by a semiconductor, the population of photoexcited charge carriers can be much greater than the majority carriers, and there a population inversion occurs, leading to stimulated emission (Light Amplification by Stimulated Emission of Radiation).

The photons from light bulbs and suns are neither coherent nor collimated, although they can be concentrated significantly to potentially cause a population inversion and stimulated emission (yes, there is the possibility for a solar laser). However, before that stage there are other phenomena that occur, making it a bit more complicated.

Concentrating cells allow an increased flux of photons to the smaller receiver/absorber using a larger aperture to collect the solar light. The geometric concentration ratio is the ratio of the area of an aperture to that of the absorber (C=A_apt/A_abs).^1,2 For a perfect concentrator (as a point on the surface of Earth), the radiation from the Sun on the aperture-receiver assembly is only a fraction of the total radiation emitted by the Sun, given a half-angle subtended by the Sun of 0.27°. Assuming a blackbody, the absorber would have a maximum theoretical concentration ratio of 45,000 (for a circular concentrator) or 212 (for a linear,trough concentrator).¹ The higher the concentration,the higher the photon flux (including increasing temperature),and the more precise the optics of the collector must be to deliver. This is an extreme energy flux for any semiconductor. Under high illumination levels, one will observe a decrease in minority carrier lifetimes and related diffusion path lengths. However, 45.6% of the suns power is contained in the infrared band (the part that makes things “hot”). Thermally, an imaging concentrator (C>> 10; analogous to camera lenses) can produce temperatures from 500 to 1500 °C at the absorber.² This increased temperature can be used to drive thermal work (steam generation) or thermophotoelectrochemical reactions for concentrating solar power (CSP, not to be confused with CPV), but is not necessarily good for photovoltaic performance. High temperatures tend to decrease the efficiency of a photovoltaic device. In particular, this is why members of the microelectronics industry are getting into the concentrating photovoltaics field (CPV)–they know how to cool superhot microelectronics, and will do the same with CPV devices.

It is so interesting to see how this is all a great spread of possibilities that one can derive from our nearest fusion reactor!

Text sources:
1. Rabl, A. Active Solar Collectors and Their Applications. 1985 Oxford University Press, New York

2. Duffie, J. A.; Beckman, W. A. Solar Engineering of Thermal Processes. (3rd Ed.) 2006 John Wiley & Sons Inc, Hoboken, NJ, USA.

3. Andreev, V. M.; Grilikhes, V. A.; Rumyantsev, V. D. Photovoltaic Conversion of Concentrated Sunlight. 1997, John Wiley & Sons Ltd, Chichester, England.

Analogy:
I want to use the familiar example of technologies for indoor air quality and thermal comfort: HVAC systems (Heating, Ventilation, and Air Conditioning). Think about how many air conditioning units are now an integral part of buildings in the country. Consider the labor force that is required for AC/heating installation, duct installation, monitoring and control systems (e.g. thermostats), and maintenance or repairs (hint: it is a huge industry). Now think about how little you think about these systems (because they just work). There is similar (perhaps even greater) potential for green collar jobs–earning a paycheck and helping society and the environment!

The Very Near Future:
Green collar jobs for solar technologies are here! Training is in full gear in states like California, New Jersey, and Florida, and is ramping up in Wisconsin and Pennsylvania. At Penn State, we are already working on a training course for PV installation, as well as an upper level college course in solar energy technology design.

Additional reading: NYT article on PV installers as the new wave of green collar jobs.

Here are some ideas beyond PV and concentrating PV (CPV):

1. Passive/Active Solar Water Heating Systems (in your showers, dishwashers, heating your floors)
2. Commercial/Distributed Space Heating Systems (using Solar Walls, Phase Change materials, Pebble-bed hot air storage).
3. Solar Cooling (Yes! you can cool with the sun and heat pumps, dessicants, refrigeration cycles).
4. Solar Industrial Process Heat and Solar Ponds (Do you own a mine or a refinery? Look into ways that you could dramatically reduce your energy bills!)
5. Solar Thermal Power Systems (Also called Concentrating Solar Power–CSP–this is the technology with the best odds at being the next wave of electric power from the sun).
6. Don’t forget solar chemistry (not just growing plants) to make hydrogen and other fuels!

Solar is very close to breaking out. Why not invest in solar tech?

A quantum dot is a nanoparticle in which the excited states (high energy electrons and holes) are “confined” by the very small dimensions of the particle. This leads to increased energy in the excited states (no where to go but up in energy), and has resulted in many new technologies. One proposed technology would use quantum dots as light absorbers for a photovoltaic effect, where one could collect mulitiple electrons (increased photocurrent) or very high energy electrons (increased photovoltage). The up side is that quantum dots sound sooo cool, why not make them into PV devices? The down side is that the rates of charge carrier extraction (collecting the electrons to do work) are still way too high to get much efficiency out of them. A lot of research needs to occur before you start seeing purely quantum dot PV. The disruption appears to be far away.

On the other side, if you concentrate the sun’s power, you can use it effectively for multiple applications, and often you don’t need radical new technologies. Rather, a combination of straight forward technologies in a new way may lead to something disruptive. You can concentrate the sun’s visible light (48% of the suns power, or 656 W/m2) for photovoltaics, OR you can concentrate the sun’s infrared light (45.6% of the total power, or 623 W/m2) and use the thermal heat to do work! Either way, by concentrating you take a diffuse source and, well, concentrate it. Certainly, you would need to cool a PV collector, but what about a thermal collector powering a turbine to generate electricity? In 1878, a solar power collector was exhibited at the World’s Fair in Paris, France. Between 1907 and 1913, an American engineer (F. Shuman) developed solar powered hydraulic pumps with a concentration ratio of about 4.5:1.¹

And the kicker, CSP is getting closer and closer to being the first economically viable solar technology–opening the doors to the following technologies? Is this disruption, by opening the possibilities of solar power beyond the single junction photovoltaic device?

1. D. Y. Goswami, F. Kreith, and J. F. Kreider Principles of Solar Engineering 2nd Ed. (2000) Taylor & Francis, Philadelphia, PA.