Fire Retardant and Conducting Polymers, OLEDs and Dye-Sensitised Solar Cells

Fire retardant polymers

Polymers are hydrocarbons and therefore burn. Where they are used in electrical enclosures, cable insulation and building materials, that matters, and additives are used to interrupt combustion.

Halogenated — containing bromine or chlorine. Examples: PVC, decabromodiphenyl ether, tetrabromobisphenol A, chlorinated paraffins.

They act in the gas phase. On heating they release halogen radicals that scavenge the highly reactive H• and OH• radicals sustaining the flame:

Very effective at low loading — but they produce dense corrosive smoke including hydrogen halides, which in a real fire kills by inhalation and destroys electronic equipment far from the flames. Some brominated compounds are persistent and bioaccumulative, and several are now restricted.

Non-halogenated — metal hydroxides such as aluminium trihydroxide and magnesium hydroxide, phosphorus compounds, and intumescent systems.

Aluminium trihydroxide works by endothermic decomposition:

It absorbs heat, releases water vapour that dilutes the flammable gases, and leaves an oxide layer. Intumescent systems swell into a thick insulating char that starves the polymer beneath of heat and oxygen.

The trade-off is honest and worth stating: non-halogenated additives are cleaner and safer in a fire but need much higher loading — often 50–60% by weight — which degrades the mechanical properties of the polymer. The industry is shifting towards them anyway, because smoke toxicity turned out to matter more than flame spread.

Conducting polymers

Polymers are normally insulators. A conducting polymer conducts because its backbone is conjugated — alternating single and double bonds — so the p orbitals overlap continuously along the chain and the π electrons delocalise, exactly as in graphene but in one dimension.

Conjugation alone is not enough. An undoped conjugated polymer is at best a semiconductor. Conductivity requires doping, which here means oxidation or reduction of the chain rather than the substitutional doping of silicon:

• p-doping — oxidation, removing electrons, using I₂ or FeCl₃.
• n-doping — reduction, adding electrons, using alkali metals.

Doping creates charge carriers on the chain — polarons and bipolarons — which move along it and hop between chains. Conductivity can rise by ten orders of magnitude, from insulator to near-metallic.

Classification:

• Intrinsically conducting — conduct through their own conjugated backbone: polyacetylene, polyaniline, polypyrrole, polythiophene, PEDOT.
• Extrinsically conducting — ordinary polymers filled with conductive particles such as carbon black or metal powder. The polymer contributes nothing electrically; it is a binder.

Polyaniline (PANI)

Synthesis: oxidative polymerisation of aniline in acid, using ammonium persulphate as oxidant. Chemical or electrochemical.

Properties: uniquely, its conductivity is controlled by acid-base chemistry as well as by oxidation. Treating with acid protonates the chain and makes it conducting; treating with base deprotonates it and makes it insulating — reversibly. It is cheap, air-stable and easy to make.

Applications: anticorrosion coatings, sensors — particularly pH and ammonia, exploiting that acid-base switching — antistatic coatings, supercapacitor electrodes, and EMI shielding.

Polypyrrole (PPy)

Synthesis: oxidative polymerisation of pyrrole, chemically with FeCl₃ or electrochemically as a film on an electrode.

Properties: good conductivity, good environmental stability, and biocompatible, which distinguishes it.

Applications: biosensors, neural electrodes and tissue engineering scaffolds, drug release triggered electrically, supercapacitors, and gas sensors.

The 2000 Nobel Prize in Chemistry went to Heeger, MacDiarmid and Shirakawa for the discovery of conducting polymers.

The OLED

Organic Light Emitting Diode.

Construction, from the bottom: glass or flexible substrate → transparent anode (ITO) → hole transport layer → emissive organic layer → electron transport layer → metal cathode of low work function such as calcium or aluminium.

Working:

1. Under forward bias the cathode injects electrons and the anode injects holes.
2. Both drift through their transport layers into the emissive layer.
3. They meet and form an exciton, a bound electron-hole pair.
4. The exciton recombines and releases the energy as a photon.

Colour is set by the band gap of the organic emitter — the same $\lambda \approx 1240/E_g$ relation as an inorganic LED — so it is tuned by choosing the molecule.

Advantages over LCD, and the reason they matter: an OLED emits light, whereas an LCD blocks a backlight. So a black OLED pixel is simply switched off, giving true black and effectively infinite contrast, and saving power on dark content. OLEDs are also thin, need no backlight, are flexible, and have wide viewing angles and fast response.

Limitations: shorter lifetime, particularly for blue emitters, which degrade fastest and cause colour shift with age; sensitivity to moisture and oxygen, so encapsulation is critical; and higher cost.

Applications: phone and television displays, wearables, flexible and rollable screens, lighting panels.

The dye-sensitised solar cell

The Grätzel cell — a photovoltaic that separates light absorption from charge transport, unlike a silicon cell where the semiconductor does both.

Construction: transparent conducting glass anode → a porous layer of TiO₂ nanoparticles → a monolayer of dye adsorbed on them → iodide/triiodide electrolyte → platinum-coated counter electrode.

Working:

1. The dye absorbs a photon and an electron is promoted to its excited state.
2. The excited electron is injected into the conduction band of the TiO₂ within femtoseconds.
3. It travels through the porous TiO₂ network to the anode and out through the external circuit.
4. The oxidised dye is regenerated by iodide from the electrolyte.
5. The resulting triiodide is reduced back at the platinum counter electrode, completing the cycle.

Why TiO₂ is nanoporous. A flat electrode could carry only a monolayer of dye over its geometric area, absorbing almost nothing. The nanoparticle network multiplies the internal surface area by a factor of around a thousand, so a thousand times more dye sits within a few micrometres of the surface. The nanostructure is what makes the cell work at all — this is the surface-area argument from the first topic of this module, doing real work.

Advantages: cheap materials and low-temperature processing; works in diffuse and indoor light, where silicon performs poorly; semi-transparent and available in colours, so it can be built into windows; and tolerant of high temperature, unlike silicon whose efficiency falls as it heats.

Limitations: efficiency around 11–13% against silicon's 20%+; the liquid electrolyte can leak or freeze and is the main durability problem; and long-term stability remains inferior.

Materials for emerging technologies

Named in the syllabus, and worth a paragraph each.

Quantum computing — superconducting circuits of niobium and aluminium with Josephson junctions, operating at millikelvin temperatures; trapped ions of ytterbium or calcium; silicon and diamond with nitrogen-vacancy centres for spin qubits; topological materials. The common requirement is long coherence, which means isolating the qubit from everything, including the material defects around it.

Supercapacitors — activated carbon, graphene and carbon nanotubes for electrical double-layer capacitors, where charge is stored electrostatically at a very large surface; metal oxides such as RuO₂ and MnO₂ and conducting polymers for pseudocapacitors, which add fast surface redox. They deliver far higher power than a battery and far lower energy, so they complement rather than replace it.

Spintronics — devices using electron spin rather than charge: ferromagnetic layers and giant magnetoresistance in hard disk read heads, magnetic tunnel junctions in MRAM, and half-metals and dilute magnetic semiconductors. The promise is non-volatile memory that needs no power to retain data.