Conducting polymers and organic electronics

A conducting polymer needs conjugation: a backbone of alternating single and double bonds, so that each carbon has a p orbital and these overlap all along the chain. Electrons in the resulting π system are delocalised, much as they are in benzene, except that the delocalisation runs for many repeat units.

Polyethene has only σ bonds and holds its electrons tightly; it is an excellent insulator. Poly(ethyne), better known as polyacetylene, (\mathrm{CH{=}CH})_n, has a conjugated backbone, yet on its own it is only a poor semiconductor. The breakthrough came in 1977 when it was exposed to iodine vapour. Iodine oxidises the chain, removing some π electrons. The positive charges left behind can move along the backbone, and conductivity rose by many orders of magnitude, towards that of metals. This process is called doping, by analogy with silicon.

The longer the conjugated system, the smaller the gap between the highest occupied and lowest unoccupied orbitals. That is why short polyenes absorb in the UV, β-carotene (eleven conjugated C=C bonds) absorbs blue light and looks orange, and long conjugated polymers absorb across the visible.

Useful examples include polythiophenes and polypyrrole, which are more stable in air than polyacetylene, and PEDOT:PSS, a blue conducting polymer dispersion used as a transparent electrode layer. In an OLED, electrons and holes are injected into a thin organic layer from opposite electrodes; when they meet, they release energy as light whose colour depends on the molecule's energy gap.

Bands from orbitals

Combine N p orbitals along a chain and Hückel theory gives N π molecular orbitals spread over an energy range of about 4\beta. For a polyene with N carbon atoms the HOMO–LUMO gap is

which tends to zero as N \to \infty (\beta is negative). Simple Hückel theory therefore predicts that infinite polyacetylene is a metal with a half-filled band. It is not: trans-polyacetylene has a gap of roughly 1.5 eV. The chain lowers its energy by localising the double bonds, alternating short and long C–C bonds rather than having all bonds equal. This Peierls distortion opens a gap at the Fermi level, exactly as a band-structure treatment of a one-dimensional half-filled band predicts. Bond-length alternation, not just chain length, sets the band gap; donor–acceptor copolymers exploit this by alternating electron-rich and electron-poor units to push gaps below 1.5 eV.

Doping is redox

Oxidation (p-doping, with I_2, FeCl_3 or electrochemically) removes electrons from the valence band; reduction (n-doping, with alkali metals) adds them to the conduction band. Unlike silicon, the charge does not sit in a rigid band. The chain relaxes around it, creating a polaron, a radical cation with a local quinoid distortion and new mid-gap states visible as new low-energy absorptions. Two polarons can combine to a spinless bipolaron. In degenerate trans-polyacetylene the charge carriers are solitons, domain walls between the two equivalent bond-alternation patterns. A counter-ion (I_3^-, for example) enters the film to balance each charge, so doping is a solid-state electrochemical reaction.

Devices

In an OLED, holes injected from indium tin oxide and electrons from a low-work-function cathode recombine to form excitons. Spin statistics give one singlet for every three triplets, so purely fluorescent emitters waste about 75% of excitons. Iridium(III) complexes such as fac-Ir(ppy)_3 use strong spin–orbit coupling to emit from the triplet (phosphorescence), and thermally activated delayed fluorescence emitters harvest triplets by reverse intersystem crossing across a small singlet–triplet gap.

In an organic photovoltaic, a donor polymer and an acceptor are blended as a nanoscale bulk heterojunction, because excitons in organic solids diffuse only about 10 nm before decaying and must reach an interface to split into free charges. In an organic field-effect transistor, a gate voltage accumulates charge in the first few molecular layers of the semiconductor next to the dielectric, switching the channel on.

Organic electronics is now a mature industry in one area, OLED displays, and an active research field in several others.

For emitters, the outstanding problem is a blue that is efficient, deep in colour and long-lived together. Blue phosphors degrade quickly because the high-energy triplets drive bond-breaking, and many displays still use fluorescent blue. Multiple-resonance TADF emitters, rigid boron and nitrogen doped polycyclic frameworks, give very narrow emission bands, and "hyperfluorescence" pairs a TADF sensitiser with a fluorescent terminal emitter to combine triplet harvesting with colour purity. Excited-state lifetimes, triplet–polaron annihilation and the chemistry of degradation products are studied by transient spectroscopy and mass spectrometry of aged devices.

Organic photovoltaics were reshaped by non-fullerene acceptors. The fused-ring acceptor Y6 (2019) and its descendants raised single-junction efficiencies from about 12% to around 20%, with smaller voltage losses than fullerene blends. Open questions concern why charge separation is so efficient with little energetic offset, and how to stop the blend morphology coarsening over years of operation.

Organic electrochemical transistors, often based on PEDOT:PSS or glycolated polythiophenes, take up ions from an electrolyte throughout their bulk and give large transconductance at low voltage. They are used to record neural signals and as biosensors. Mixed ionic–electronic conduction, and the stability of n-type materials in water and air, are active topics.

Modern work leans on grazing-incidence X-ray scattering to see packing and orientation in thin films, on machine-learning screens of candidate molecules, and on doping strategies such as molecular dopants introduced by sequential processing so the polymer's order is not disrupted.