Lecture 09 Solar Energy : Part 6

Summary

The lecture introduces the semiconductor principles behind solar photovoltaic (PV) cells: energy bands, material types, p–n junctions, and the photovoltaic effect. It then explains how a PV cell produces electricity and how cells are combined into larger systems.

1. Energy bands and material classification

• A material’s valence band contains electrons that are generally bound to atoms and do not move freely through the material.
• The conduction band contains electrons that can move through the material and carry electric current.
• The band gap (or energy gap) is the energy range between these bands in which electrons cannot exist. An electron must gain enough energy—such as from heat, light, or electrical energy—to move from the valence band to the conduction band.
• The lecture classifies materials by the size of this gap:
  • Conductors: The valence and conduction bands overlap, allowing electrons to move readily. The lecture notes their low electrical resistance and gives silver, copper, and aluminum as examples.
  • Insulators: They have a wide band gap, described as roughly 5–10 electron volts or more. Electrons normally cannot reach the conduction band. Examples include rubber, glass, and dry wood.
  • Semiconductors: They have a comparatively narrow gap, described as roughly 0.7–1.4/1.5 electron volts, and intermediate conductivity. Silicon, germanium, and gallium arsenide are examples. Semiconductors are the key materials used in PV cells.

2. P-type and N-type semiconductors

Semiconductors can be doped—combined with small amounts of other elements—to change their charge-carrier properties:

• N-type material: Made by adding a pentavalent impurity, such as phosphorus, arsenic, or antimony, to silicon or germanium. The extra electron becomes available for conduction. Electrons are the majority carriers; holes are the minority carriers.
• P-type material: Made by adding a trivalent impurity, such as boron, aluminum, gallium, or indium. The resulting electron deficiency is described as a hole. Holes are the majority carriers; electrons are the minority carriers.

3. The p–n junction and biasing

• Joining P-type and N-type materials forms a p–n junction, a foundational structure in diodes and solar cells.
• Near the interface, charge carriers exchange and a depletion region forms.
• The junction’s behavior depends on how it is connected:
  • Forward bias: P is connected to the positive terminal and N to the negative terminal. The depletion region narrows, resistance decreases, and current flows more readily.
  • Reverse bias: P is connected to the negative terminal and N to the positive terminal. The depletion region widens, resistance increases, and only a small current flows.

4. Photovoltaic effect

• The photovoltaic effect is the generation of voltage or electric current when a material absorbs light.
• The lecture distinguishes it from the photoelectric effect: in the latter, electrons are knocked out of a material; in the photovoltaic effect, light excites electrons within the material, producing electron–hole pairs without those electrons leaving the material.
• The lecture attributes the discovery of the photovoltaic effect to Edmond Becquerel in 1839, when he observed increased voltage in an electrochemical cell exposed to sunlight.

5. How a PV cell generates electricity

A PV cell uses a semiconductor p–n junction to turn light into electrical output:

1. Photons strike the semiconductor.
2. Absorbed energy excites electrons and creates electron–hole pairs.
3. The junction’s electric field separates the charges: electrons move toward the N-type side, while holes move toward the P-type side.
4. This separation creates a potential difference, or voltage.
5. When the cell’s front and back contacts are connected through a load, the completed circuit allows current to flow and deliver electrical power.

A PV cell is the basic unit of a solar installation. The lecture describes a typical cell as having a P-type base, an N-type top layer, a p–n junction between them, and electrical contacts.

6. Scaling cells into a PV system

• Individual cells produce limited power, so they are electrically connected in groups.
• Multiple cells form a PV module; modules are linked to make panels, and panels are arranged into a PV array.
• The array provides the primary electricity-generating source in a solar PV system.

7. Cell specifications mentioned in the lecture

The lecture presents the following approximate progression in wafer sizes and cell output:

Period or cell label Wafer size described Voltage Power Before 2010 125 × 125 mm 0.5–0.54 V 2.5–3 W M0/M2 156 mm Around 0.6 V 4.5–5.2 W M6/G1 (2019–2021) About 10 mm larger than the prior size; some described as rectangular Around 0.64 V 6–7 W M10 (described as current in the lecture) 182 mm square Up to around 0.7 V Up to 9.5 W G12/M12 210 mm Not specified 10–12 W

The lecture’s figures are approximate examples of how individual cell sizes and power ratings have increased over time.

Speakers and sources featured

• Lecturer: The speaker introducing and explaining the material; not named in the subtitles.
• Edmond Becquerel: Mentioned as the discoverer of the photovoltaic effect in 1839. He is referenced historically, but is not a speaker in the video.