Advanced Level

The advanced level deepens the connection between photon absorption and the flow of electrical current through external circuits. Students observe how light energy is converted into separated charges within the P-N junction and how these charge carriers propagate through the semiconductor and metallic contacts to deliver power.

To understand the sequence of events, the concept of band bending at the P-N junction is introduced: when N-type and P-type regions contact, electrons diffuse from N to P and holes diffuse from P to N until an equilibrium is reached. This creates a depletion region depleted of mobile carriers, with fixed positive charges on the N-side and fixed negative charges on the P-side. The resulting electric field (~10⁵ V/cm) is strong enough to separate photo-generated electron-hole pairs before they recombine.

The propagation of charge carriers and the generation of electrical current is illustrated through an Augmented Reality simulation developed with Delightex Studio – Spaces (Marker):
https://edu.delightex.com/Studio/Spaces

Using mobile devices, students explore a three-dimensional model of a solar cell:

  1. Observe photons striking the surface and penetrating the semiconductor
  2. Visualize electron-hole pair generation when photon energy exceeds the bandgap
  3. Follow electrons swept toward the N-layer and holes toward the P-layer by the junction field
  4. See electrons flowing through the external circuit (wire, load, return path)
  5. Track the complete current loop from generation to load and back

During the activity, guiding questions are proposed:
– At what point in the cell does light become separated electrical charges?
At the P-N junction, where the built-in electric field separates photo-generated electrons and holes before they recombine.

– Why does current flow through the external circuit when the cell is illuminated?
Because the junction continuously generates a voltage difference (photovoltage) between the N-side and P-side, driving electrons through the circuit to minimize the potential difference.

– How does the solar cell produce power without consuming fuel or moving parts?
By directly converting photon energy into charge separation through quantum absorption, then extracting those separated charges as current through metallic contacts-a solid-state energy conversion with no mechanical or chemical transformations.

Answers emerge through group discussion and comparison between quantum-scale carrier generation and macroscopic electrical measurements.

The lesson ends with a reflection on the evolution of photovoltaic technology. Students compare first-generation crystalline silicon cells with emerging technologies:

Technology

Efficiency

Advantages

Challenges

Crystalline Silicon

15–22%

Mature, stable, scalable

Resource-intensive, rigid

Thin-Film (CdTe, CIGS)

10–15%

Flexible, lightweight

Lower efficiency, toxic materials

Perovskite

20–26% (lab)

High efficiency, low-cost processing

Stability, degradation, lead content

Multi-junction (GaAs)

30–47%

Ultra-high efficiency

Expensive, used in space/concentrators

Let’s reflect together. Do modern solar technologies truly approach fundamental physical limits, or is there room for breakthroughs in materials science and device architecture? Can we engineer cells that convert all solar wavelengths efficiently, or will spectral mismatch always constrain single-junction systems? How do economic and environmental factors (manufacturing energy, rare materials, recycling) influence which technologies scale globally?