# What is the Difference Between Internal and External Quantum Efficiency

The key difference between internal and external quantum efficiency is that internal quantum efficiency is calculated using the photons that are shinning from inside of the solar cell, whereas external quantum efficiency is calculated using the photons that are shinning from outside of the solar cell.

Quantum efficiency or QE is a concept in quantum mechanics. This term has two major applications; it can be applied to the incident photon in converted electron ratio concept (IPCE) of photosensitive devices and to the TMR effect of a magnetic tunnel junction. When considering the quantum efficiency in solar cells, there are two types as internal and external quantum efficiency.

### CONTENTS

1. Overview and Key Difference
2. What is Quantum Efficiency in Solar Cells
3. What is Internal Quantum Efficiency
4. What is External Quantum Efficiency
5. Internal vs External Quantum Efficiency in Tabular Form
6. Summary – Internal vs External Quantum Efficiency

## What is Quantum Efficiency in Solar Cells?

The Quantum efficiency of a solar cell defines the amount of current the solar cell can produce upon irradiation by photons of a particular wavelength. We can evaluate the amount of current that is produced by the solar cell when it is exposed to sunlight if the quantum efficiency of the cell is integrated over the whole solar electromagnetic spectrum. If we can get the ratio between the energy production value of the solar cell and the highest possible energy production value of the solar cell, we can get the overall energy conversion efficiency value. Moreover, we can get quantum efficiencies that are higher than 100% in multiple exciton generation. This is because the energy of an incident photon is twice higher than the bandgap energy, which leads to the creation of two or more electron-hole pairs per incident photon.

## What is Internal Quantum Efficiency?

Internal quantum efficiency is the ratio between the number of charge carriers that are collected by the solar cell to the number of photons having a given energy that can shine on the solar cell from the inside (these photons are absorbed by the cell). The determination of internal quantum efficiency can be done as follows:

## What is External Quantum Efficiency?

External quantum efficiency is the ratio between the number of charge carriers that are collected by the solar cell to the number of photons having a given energy value that is shinning from outside (named as incident photons). The determination of external quantum efficiency can be done as follows:

As the above description indicates, the external quantum efficiency is always calculated using the photons that are shinning from outside the solar cell. Moreover, external quantum efficiency depends on the absorption of light.

## What is the Difference Between Internal and External Quantum Efficiency?

When the quantum efficiency in solar cells is considered, there are two types as internal and external quantum efficiency. The key difference between internal and external quantum efficiency is that internal quantum efficiency is calculated using the photons that are shinning from inside of the solar cell, whereas the external quantum efficiency is calculated using the photons that are shinning from outside of the solar cell. Moreover, internal quantum efficiency always has a higher value than external quantum efficiency. In addition, internal quantum efficiency does not depend on the absorption of light, whereas external quantum efficiency depends on the absorption of light.

The following infographic lists the differences between internal and external quantum efficiency in tabular form for side by side comparison.

## Summary – Internal vs External Quantum Efficiency

When considering the quantum efficiency in solar cells, there are two types as internal and external quantum efficiency. The key difference between internal and external quantum efficiency is that internal quantum efficiency is calculated using the photons that are shinning from the inside of the solar cell, whereas external quantum efficiency is calculated using the photons that are shinning from outside of the solar cell.