The Organic Stack That Actually Emits
Five organic layers, a few hundred nanometres total — the sequence is not decorative; it is the device.
The Stack, Layer by Layer
An OLED emitter is not a bulb, not a phosphor, and not an LED in the semiconductor sense. It is a thin-film sandwich, grown by vacuum thermal evaporation onto a glass or plastic substrate, in which two charge populations — holes from the anode, electrons from the cathode — are steered toward each other until they meet, bind, and release a photon. The steering is the engineering. Get the layer sequence wrong and recombination happens at the wrong place, against the wrong interface, and the device either emits poorly or dies quickly.
The conventional five-layer sequence runs: hole-injection layer (HIL), hole-transport layer (HTL), emissive layer (EML), electron-transport layer (ETL), and then the cathode. The anode — typically indium tin oxide on the glass side — sits beneath all of them and is not organic.
Hole injection layer. The anode's work function and the organic material's ionisation potential rarely match well. An HIL — often a transition-metal oxide such as molybdenum trioxide, or a doped organic like PEDOT:PSS in solution-processed devices — steps down the energy barrier, reducing the voltage needed to inject holes into the stack. Without it, you are trying to push charge over a cliff rather than down a ramp.
Hole transport layer. Once holes are in the stack, they need to move efficiently toward the EML without recombining early. HTL materials are selected for high hole mobility, a HOMO level that aligns with the EML, and chemical stability. The layer also functions as an electron blocker: its LUMO sits high enough that electrons arriving from the cathode side cannot penetrate it. Triarylamine compounds and carbazole derivatives are common here, chosen because their molecular geometry resists the crystallisation that would otherwise open conduction pathways and accelerate degradation.
Emissive layer. This is where the physics earns its pay. The EML hosts a small-molecule emitter doped into a wider-gap host material, typically at a few percent by mass. Holes and electrons meet in the host, form an exciton — a bound electron-hole pair — and the exciton's energy then transfers by Förster or Dexter coupling to the dopant. The dopant emits. Using a host-guest architecture spatially separates the emitting species from the charge-transport environment, slows concentration quenching, and allows the dopant's emission colour to be tuned independently of the transport chemistry. Phosphorescent and thermally activated delayed fluorescence (TADF) emitters exploit triplet excitons to push internal quantum efficiency toward 100 %; a naive fluorescent emitter wastes 75 % of its excitons to heat because only singlet states radiate.
Electron transport layer. The ETL is the HTL's mirror image: high electron mobility, HOMO deep enough to block holes, LUMO aligned with the EML and cathode. Phenanthroline derivatives and oxadiazoles appear frequently. The ETL sometimes incorporates a thin lithium fluoride or caesium carbonate interlayer at its interface with the cathode — these narrow work-function materials improve electron injection in the same way the HIL improves hole injection at the other end.
Cathode. Low work function is the primary requirement; magnesium-silver alloys and calcium-aluminium bilayers are standard. The cathode is deposited last, through a shadow mask in monochrome sub-pixels or as a blanket layer in white-OLED-plus-colour-filter architectures.
Why the Order Is Non-Negotiable
The sequence exists to control where recombination occurs. Ideally, excitons form well within the EML, far from both electrode interfaces. Interface proximity accelerates non-radiative quenching: electrode metal surfaces couple to excitons via Förster energy transfer and dissipate the energy as heat rather than light. Keeping the EML shielded by transport layers on both sides preserves quantum yield and — critically — delays the point at which that interface-driven quenching degrades the organic material.
Longevity is dominated by chemistry at the EML interfaces. The emissive molecules themselves are comparatively stable; it is the charge-accumulation zones at the EML/HTL and EML/ETL junctions where molecular reactions occur under operational field stress, producing traps and scattering centres that raise drive voltage and shift colour over time. This is the mechanism behind the differential decay rates between red, green and blue sub-pixels — a fact that feeds directly into OLED burn-in as the device ages unevenly across its aperture.
The entire functional stack — five layers, two electrodes — occupies somewhere between 100 and 300 nanometres. That the physics of light, charge, and molecular energy transfer is compressed into a film thinner than a bacterium is either humbling or clarifying, depending on your disposition.