Polymer chemistry
Conducting and photoactive polymers
Understand how conjugation, morphology and doping govern charge transport and optical response in conducting polymers, then connect these mechanisms to organic electronic devices and current research.
IntuitionA polymer chain that can carry charge
In an ordinary saturated polymer, electrons are localized in σ bonds. A conjugated backbone alternates single and double bonds, allowing π electrons to spread along the chain. Chemical doping adds or removes electronic charge and creates mobile carriers; chain packing determines whether they can travel between chains.
SchoolConjugation, light and doping
Definition: Conjugated polymer
A polymer with a backbone containing overlapping p orbitals and alternating bonding patterns. π-electron delocalization produces electronic states whose energies can be addressed by visible or near-infrared light; disorder and torsion shorten effective conjugation.
Example: Reading an absorption edge
A conjugated polymer absorbs strongly near 620 nm. Estimate the photon energy in eV and state what this does—and does not—tell you about the band gap.
Solution
E ≈ 1240/620 = 2.00 eV. This is the photon energy at that spectral feature. It is not automatically the transport gap: exciton binding, vibronic structure, disorder and the chosen onset-fitting method affect the optical edge.
| Process | Physical picture |
|---|---|
| Photoexcitation | Absorbed photon creates a bound electron–hole exciton; separation is needed for photocurrent. |
| Chemical doping | Oxidation or reduction creates polarons/bipolarons with counterions for charge neutrality. |
| Transport | Intrachain delocalization and interchain hopping compete; morphology and traps matter. |
UndergraduateElectronic structure and conductivity
The Drude-like relation σ = nqμ is a useful bookkeeping model, not a complete microscopic theory for disordered organic solids. Doping changes carrier density n and often mobility μ through structural rearrangement, counterion fields and trap filling. Optical absorption follows A = ε₂ℓ in a homogeneous dilute solution; scattering and thin-film interference complicate film spectra.
Example: Estimating conductivity
For a doped film, take n = 1.0 × 10²⁶ m⁻³, μ = 2.0 × 10⁻⁶ m² V⁻¹ s⁻¹ and q = 1.60 × 10⁻¹⁹ C. Estimate σ using σ = nqμ.
Solution
σ = (1.0 × 10²⁶)(1.60 × 10⁻¹⁹)(2.0 × 10⁻⁶) = 32 S m⁻¹. This estimate assumes one effective carrier charge and a meaningful macroscopic mobility; real films may be anisotropic and spatially heterogeneous.
AdvancedFrom excitons to device function
In many organic semiconductors, photoexcitation first creates a Frenkel-like exciton with appreciable binding energy. At a donor–acceptor interface, energetic offsets and interfacial electric fields can promote charge transfer, but excessive offsets waste voltage. In OLEDs, radiative recombination competes with non-radiative decay; in organic photovoltaics, domain scale must balance exciton diffusion to interfaces against continuous pathways for both carrier types.
ResearchResearch frontier
References
- Synthesis of electrically conducting organic polymers: halogen derivatives of polyacetylene, (CH)x · H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang, A. J. Heeger, 1977
- Polarons, bipolarons, and solitons in conducting polymers · J.-L. Bredas, G. B. Street, 1985
- Organic bioelectronics bridging the signaling gap between biology and technology · D. T. Simon, E. O. Gabrielsson, K. Tybrandt, M. Berggren, 2016