---
title: "Blue LED: Recombination and Efficiency Interactive Lab | Sandbox Physics"
description: "Why can a blue LED become brighter while its efficiency falls? More current can produce more photons in total while each injected carrier has a smaller chance to emit one. In this model, high carrier density strengthens the cubic loss channel faster than radiative recombination, so external efficiency falls even as light output rises."
canonical_url: "https://sandboxphysics.com/labs/blue-led-efficiency/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/blue-led-efficiency/"
last_modified: "2026-10-24"
---

C-X1c · Where injected carriers go

# Blue LED: Recombination and Efficiency

**Why can a blue LED become brighter while its efficiency falls?** More current can produce more photons in total while each injected carrier has a smaller chance to emit one. In this model, high carrier density strengthens the cubic loss channel faster than radiative recombination, so external efficiency falls even as light output rises.

Inject electrons and holes into a blue-emitting well. Watch radiative recombination compete with defect and high-density losses, then record the external efficiency curve.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

A uniform steady-state ABC model assumes equal electron and hole densities in a three-nanometre active well. Synthetic coefficients define defect, radiative and cubic recombination. The well emits at a fixed wavelength of four hundred and fifty nanometres. Extraction is a prescribed fraction of generated photons. The cubic loss illustrates one possible high-density channel; this model does not establish the cause of droop in an actual LED. Injection leakage, heating, polarization fields, localization, wavelength shifts, optical interference and spatial transport are omitted. The cross-section and particle motion are schematic.

Synthetic model acquisition

## Efficiency across injection levels

The stored sweep retains the settings used to generate it.

> Synthetic sweep data. Axis definitions and model assumptions are retained in the text.

$J/(\mathrm A\,\mathrm{cm}^{-2}),\quad\eta_{\rm EQE}/\%$

The current-density axis is logarithmic. The dot marks the operating point stored with this sweep.

### Where carrier pairs recombine

Teal: radiative. Amber: defect. Pink: cubic loss. These fractions sum to unity before optical extraction.

Nobel · Condensed matter

## What did the original experiments establish?

Follow the apparatus, observations and interpretation in the historical evidence. Each article explains where this teaching model fits.

[Read the historical evidence](https://sandboxphysics.com/stories/condensed-matter/blue-led/) [All six reading paths](https://sandboxphysics.com/stories/condensed-matter/)

Physics tutorial

## Blue LED: Recombination and Efficiency

**Background** In a blue LED, electrons and holes meet in an active well. Some pairs emit photons, while other recombination channels dissipate energy without useful light.

**Why it matters** A rising light output does not by itself show better efficiency. Relate the photon output to the same injected current.

Start with the essentials

- **Focus question**: Why can a blue LED become brighter while its efficiency falls?
- **One-sentence intuition**: Brightness counts emitted photons; efficiency counts emitted photons per injected carrier.

### Core mathematical model

#### Steady carrier balance

$$
\frac{J}{qd}=An+Bn^2+Cn^3
$$

The three channels together consume every injected carrier pair in the uniform steady-state model.

#### External quantum efficiency

$$
\eta_{\rm EQE}=\eta_{\rm ext}\frac{Bn^2}{An+Bn^2+Cn^3}
$$

Only radiative recombination makes photons, and only the extracted fraction escapes.

### Common difficulties

#### A common inference to test

**Typical misconception** The brightest operating point must also have the best quantum efficiency.

**Better mental model** A larger injection can compensate for a smaller radiative fraction. Compare photons per injected carrier as well as total optical power.

### Run the experiment

1. 01

   #### Find the bright well

   Choose Bright well and change Injection current density.

   **What to observe:** Electrons and holes arrive at the well and blue photons escape.

2. 02

   #### Increase the defect loss

   Choose Defect losses and compare the recombination fractions.

   **What to observe:** More injected pairs disappear through the defect channel, reducing useful light.

3. 03

   #### Follow the droop

   Choose Efficiency droop, record a sweep, then compare photon output with efficiency.

   **What to observe:** At high injection the cubic channel grows faster than radiative recombination. Light output can rise while efficiency falls.

Physical conclusion

Brightness counts emitted photons; efficiency counts emitted photons per injected carrier.

Fields & matter · Starter path

## From a field to an electron microscope

Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface.

[Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)