Rob Snyder Science and Engineering Saturday Seminar April 11, 2015 How do photovoltaic cells produce electricity?

Each renewable energy technology has a unique way of transforming an energy resource into forms of energy that heat interior spaces or power machinery and appliances.

The NGSS Standard HS-P53-3 suggests that a study of renewable energy systems are to be included in the High School STEM curriculum. Design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy.* [Clarification Statement: Emphasis is on both qualitative and quantitative evaluations of devices. Examples of devices could include Rube Goldberg devices, wind turbines, solar cells, solar ovens, and generators. Examples of constraints could include use of renewable energy forms and efficiency.] [Assessment Boundary: Assessment for quantitative evaluations is limited to total output for a given input. Assessment is limited to devices constructed with materials provided to students.]

The challenge is to determine when and to what extent students can develop a genuine understanding of how each renewable energy technology transforms energy. For Example: The similar NGSS Middle School Performance Expectation is that “Students apply scientific principles to design, construct, and test a device that minimizes or maximizes thermal energy transfer.”

Students have many experiences with sunlight being transformed into thermal energy. They would have little difficulty developing an understanding of how solar thermal systems transform energy.

A study of solar thermal systems also lends itself well to a science class where student learn concepts that include: Nuclear fusion reactions in the sun transform matter into energy in the form of photons. Photons of visible light have specific wavelengths, frequencies and energies. Materials, such as glass, are transparent because they absorb and reemit visible light photons with no changes in wavelengths, frequencies or energy. Visible light photons can be absorbed by an opaque materials in the interior of a structure and reemitted as thermal energy (commonly referred to as heat). The amount of thermal energy gained in the interior of a structure can be managed by controlling the amount of heat loss by radiation, conduction and/or convection.

Students can easily select materials to design, construct and evaluate of the performance of a solar thermal device. A cardboard box, transparent food wrap, and tape can be used. Thermometers can be used to analyze the performance of a student designed solar thermal system. The design of the system can be altered meet a variety of goals. Simple components can convert the design to an active solar thermal system.

A study of solar thermal systems also provides an opportunity for students to conduct in-depth studies of a renewable energy technology A research report published by Schwartz, Sadler and Tai concluded that “Students who reported covering at least 1 major topic in depth, for a month or longer, in high school were found to earn higher grades in college science than did students who reported no coverage In depth.

Photovoltaic cell kits can provide opportunities for students to design and conduct investigations that include: Determining the best arrangement of cells to operate appliances Analyzing the affect of tilt and direction on the performance of photovoltaic cells Learning how daily and seasonal changes in the position of the sun in the sky affects electricity production Designing support structures for arrays Etc.

10 Students can also study the geometry of a tilted solar array and locate suitable sites for a solar array. Massachusetts Mathematic Framework Standards include: Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. (Geometry; Grade 8: Page 68) Explain and use the relationship between the sine and cosine of complementary angles.. (High School Geometry: Page 96)

However, it would be very difficult for students to select materials, design, assemble and modify a photovoltaic cell.

A photovoltaic cell is one example of the many “Black Box” devices we use. The term black box is used by scientists and engineers to describe a device for which inputs to and outputs of the device are understood but processes that take place in the device are not understood. Many students can describe the energy input to and energy output of a photovoltaic cell but probably can not describe how the energy is transformed in a photovoltaic cell.

What do you do if a student asks: How do photovoltaic cells produce electricity? What learning experiences would students need to have in order to understand how photovoltaic cells transform energy ?

Understanding how a photovoltaic cell transforms energy requires a breadth of knowledge that includes: Nuclear fusion reactions in the sun transform matter into energy in the form of electromagnetic radiation. Visible light consists of bundles of electric and magnetic fields (photons). Photons of different colors of visible light have specific wavelengths, frequencies and energies. The geometry of a crystal lattice structure is determined by the electron configuration of individual atoms. Silicon is a semiconducting element used to produce many photovoltaic cells. Silicon atoms have 4 outer shell electrons and form a tetrahedral crystal lattice structure. 4 covalent bonds are associated with each silicon atom. Chemical elements with 5 outer shell electrons are added to a layer of silicon creating an N-Type layer. This results in free electrons atoms moving about in the N-Type layer. Chemical elements with 3 outer shell electrons are added to a different layer of silicon creating a P-Type layer. This results in vacancies (called “holes”) moving about in the P-Type layer. Electrons migrate from the N-Type layer to the P-Type layer when a photovoltaic cell is manufactured before the cell is exposed to sunlight until a maximum separation of charge is reached. A separation of electric charge produces an electric field with a potential difference (voltage) across the P-N junction. An external electric circuit that connects the N-Type and P-Type layers provide a lower resistance pathway for electrical charges (electrons) to move from the N-Type layer to the P-Type layer. Electrons in the photovoltaic cell absorb energy as photons of visible shine on the surface of the photovoltaic cell. A continuous electric current flows through the external circuit when sunlight is available. Appliances in the external circuit can transform electrical energy into other forms of energy.

Lets look at small groups of the concepts associated with the energy transformation that takes place in a photovoltaic cell. The concepts were selected provide a basic understanding of how photovoltaic cells produce electricity. The concepts would probably be studied in a several different middle school and high school science classes.

One set of concepts focuses on sunlight. (The energy source for most renewable energy technologies) Nuclear fusion reactions in the sun transform matter into energy in the form of electromagnetic radiation. Visible light consists of photons that have both wave and particle characteristics. Photons of different colors of visible light have specific wavelengths, frequencies and energies. In what science classes might students use spectrometers to study the properties of visible light?

When do students learn about the electron configurations of atoms? water.me.vccs.edu

These two concepts focus on the “doping” of an N-Type layer of PV cell. Silicon is a semiconducting element used to produce many photovoltaic cells. Silicon atoms have 4 outer shell electrons and form a tetrahedral crystal lattice structure with 4 covalent bonds associated with each silicon atom. In this “doping” process, atoms of an element with 5 outer shell electrons are added to a layer of silicon creating an N-Type layer. The result is that one of the outer shell electrons of each dopant atoms move about in the tetrahedral lattice structure.

A closer look at the N-Type doping process Red spheres in the diagram below left illustrate how silicon atoms form 4 covalent bonds with neighboring atoms in the crystal lattice. Yellow spheres are dopant atoms that have 5 valence electrons The diagram below right illustrates that one of a dopant atom’s outer shell electrons does not participate in the covalent bonding of the tetrahedral crystalline structure. Is there a way for students to model this process?

The result of doping a layer of atoms silicon with atoms of antimony that have 5 outer shell electrons is that many free electrons are moving about in the N- Type Layer. Does doping silicon with atoms that have 5 outer shell electrons produce a N-Type layer that is: Negatively charged? Positively charged? Electrically neutral?

How would students use The Periodic Table of the Elements to suggest what elements other than Antimony could be used to “dope” an N-Type Layer?

This next concept focuses on the doping of a P-Type layer of a common photovoltaic cell. In this doping process, an element with 3 outer shell electrons is added to a different layer of silicon creating a P-Type layer. This results in vacancies in the tetrahedral lattice structure that are referred to as “holes” which move about in the lattice structure. ecee.colorado.edu

A Closer Look at the P-Type Doping Process Red spheres in the diagram below left illustrate how silicon atoms form 4 covalent bonds with neighboring atoms in the crystal lattice. Yellow spheres are now dopant atoms that have 3 valence electrons. The diagram below right illustrates how a vacancy (called a hole) forms. How might students model the P-Type doping process?

The result of doping a layer of atoms silicon with atoms of elements that have 3 outer shell electrons is that many vacant spaces (holes) are moving about in the P Layer. Holes are moving about in the P-Type Layer Does doping silicon with atoms that have 3 outer shell electrons produce a P-Type layer that is: Negatively charged? Positively charged? Electrically neutral?

How can students use The Periodic Table of the Elements to explore what elements other than Boron could be used to “dope” a P-Type Layer?

Make a Prediction: What might happen when an N-Type layer comes into contact with a P-Type layer to form a “P-N Junction”? Free electrons are moving about in the N-Type Layer Holes are moving about in the P-Type Layer Note: The two doping processes usually occur simultaneously in a single sheet of silicon (or other semiconductor).

These two concepts focus on how a “built-in” voltage develops when a photovoltaic cell is manufactured. Electrons tend to migrate from the N-Type layer to the P- Type layer when a photovoltaic cell is manufactured and before the cell is exposed to sunlight. A separation of electric charge produces an electric field with an electric potential difference across the P-N junction (0.5 volts in a common silicon based photovoltaic cell). What will be the electric charge of each layer of a photovoltaic cell as a result of the migration of electrons?

When do students typically learn that electrons can gain or lose energy? en.wikipedia.org

Sunlight can really excite electrons! If electrons absorb enough energy they can overcome the attractive forces that exist between positively charged nuclei and the negatively charged electrons. The result is the production of a large number of mobile “conduction” electrons. Note: This (as can be expected) is a simplified explanation of a much more complex process that occurs in a photovoltaic call.

These two concepts focus on what happens when sunlight shines on a photovoltaic cell. Electrons in the photovoltaic cell absorb the energy of photons of visible light that shine on the N-Type layer. An external electric circuit that connects the N-Type and P-Type layers provides a pathway for electric charge (electrons) to move from the N-Type layer to the P-Type layer. The red arrows indicate the flow of electrical charge (electrons).

What if a very curious student asks: How do photons excite electrons? The short explanation: Photons are electric and magnetic fields that constantly change strength and direction as they move through space. Energy transfer occurs as magnetic and electric fields of photons interact with negatively charged electrons. Source:

These two concepts focus on what continues to happen as long as sunlight shines on a photovoltaic cell. Appliances in the external circuit can transform electrical energy into other forms of energy. There will be a continuous electric current in the complete circuit that includes the photovoltaic cell as long as sunlight is available

What happens when there is no sunlight? In the absence of sunlight, the flow of electrons in the external circuit stops and the electric force field with a “built in” voltage is re-established.

You have also been given a document with examples of science concepts associated with common renewable energy technologies. This document is a work in progress and comments would be appreciated – my is included in the document.

The document can also be used to compare and contrast renewable energy systems. Determine which technologies use sunlight as the energy source. Determine which technologies use a nuclear reaction as the energy source. Determine which technologies rely on a device that has the characteristics of a “black box”. Rate the renewable energy technologies from the least complex to the most complex. Decide which renewable energy devices they could construct. Etc