MATS6008 Chap.3 Photovoltaics, Lighting and Nanoelectronics
Photovoltaics, Lighting and Nanoelectronics
This chapter follows photons and charge carriers through photovoltaic cells, OLEDs and nanoscale switches. It compares three photovoltaic generations, calculates conversion efficiency with area-normalised incident power, and shows why recombination is a loss in solar cells but the desired light-producing event in OLEDs.
Molecular switches, single-electron transistors and oxide interfaces extend the carrier logic into nanoelectronics.
What this chapter covers
- 01
Photon absorption, carrier separation and collection
- 02
Photovoltaic generations and architecture trade-offs
- 03
Efficiency from irradiance, area and maximum power
- 04
OLED injection and radiative recombination
- 05
Quantum-dot emission and surface control
- 06
Molecular switches and single-electron transport
Worked example · free
Photovoltaic conversion efficiency
- +1Convert 120 cm2 to 0.0120 m2.
- +1Incident power is 1000 W m-2 multiplied by 0.0120 m2, giving 12.0 W.
- +1Divide 1.80 W by 12.0 W to obtain 0.150.
- +1Report 15.0% and distinguish device efficiency from absorption alone.
Key terms
- Carrier separation
- Movement of photo-generated electrons and holes toward different contacts before recombination.
- Recombination
- An electron and hole annihilation event that can waste solar-cell charge or deliberately produce OLED light.
- Maximum power point
- The operating condition at which a photovoltaic device delivers its greatest electrical power.
- Molecular switch
- A molecule whose conductance changes through contact alignment, conformation or another controlled state change.
- Single-electron transistor
- A device using tunnel junctions and a small island where discrete charging controls current.
Photovoltaics, Lighting and Nanoelectronics FAQ
What are the main photovoltaic loss paths?
Incident light can be reflected or transmitted, below-threshold photons may not be absorbed, excess photon energy can become heat, carriers can recombine before collection, and resistance can reduce useful voltage and current. The best explanation locates the proposed improvement on that conversion chain.
Why is recombination good in an OLED but bad in a solar cell?
A solar cell needs electrons and holes to reach opposite contacts and deliver electrical work, so premature recombination removes useful carriers. An OLED intentionally brings injected carriers together in an emissive layer so radiative recombination releases photons. Device purpose determines whether the process is loss or function.
How should photovoltaic efficiency be calculated?
Multiply irradiance by illuminated area to obtain incident power, then divide maximum electrical output by that input. Convert square centimetres to square metres before multiplication and report a dimensionless percentage. Do not confuse absorption, quantum efficiency and complete device power efficiency.
How should photovoltaic generations be compared?
Compare absorber and junction architecture, manufacturing route, material use, stability and efficiency constraints rather than treating generations as a quality ranking. A thin-film route may reduce material consumption, while an emerging architecture may tune absorption or processing. The relevant choice depends on lifetime, substrate, toxicity, cost and the losses the design actually addresses.
What makes a single-electron transistor distinctive?
A very small island has charging energy large enough for individual electron addition to matter. Source, drain and gate potentials control whether an allowed state lies in the transport window, producing discrete tunnelling behaviour. Explain electrostatic control and the tunnel barriers together; merely calling the device small does not establish single-electron operation or a useful switch.
Which measurements separate photovoltaic loss mechanisms?
Absorption or reflection data test optical capture, time-resolved signals probe carrier lifetime, and current-voltage behaviour reveals collection and resistance. Use matched illumination, temperature and active area. A design can absorb more light yet collect less current, so no single efficiency value identifies whether optics, recombination, contacts or transport changed.
Exam move
Redraw the photovoltaic conversion chain and annotate reflection, thermalisation, recombination and resistance. Rework efficiency calculations with different cell areas until unit conversion is automatic. Compare solar cells and OLEDs through opposite carrier objectives. For nanoelectronics, name the switching event, the contact path and the physical origin of discrete behaviour.
Start every efficiency problem with incident power and keep area units visible until watts cancel to a dimensionless ratio. Distinguish absorption, quantum efficiency and complete device power efficiency. For conceptual questions, draw electron and hole paths and mark where generation, separation, transport, recombination and collection occur.
Use the diagram twice: premature recombination removes solar-cell carriers, while controlled radiative recombination is the function of an OLED. Compare photovoltaic generations through absorber, junction architecture, manufacturing route, stability, material use and the particular loss being addressed rather than treating chronology as quality.
A stronger absorption spectrum does not guarantee more collected current, so pair electrical operation with optical or time-resolved evidence. For a molecular switch, name the stimulus, state change and conductance pathway. For a single-electron transistor, show the small island, two tunnel barriers and gate control, then explain how charging energy produces discrete transport.
Merely calling a device nanoscale is insufficient. Practise identifying contact resistance, disorder, leakage, thermal motion or recombination as limits and propose a controlled change in illumination, bias, layer structure or temperature that would test the diagnosis. Finish by linking every material improvement to a location in the energy or carrier pathway and to a measurable device response.