Incident photon-to-current efficiency (IPCE)Suren Gevorgyan
IPCE or Quantum Efficiency (QE) is a measure of how efficiently the device converts the incident light into electrical energy at a given wavelength. There are two types of QE: External Quantum Efficiency (EQE) and Internal Quantum Efficiency (IQE).
- EQE is the ratio between the number of collected carriers and the number of all the incident photons on the device active area at a given wavelength.
- IQE is the ratio between the number of collected carriers and the number of all the absorbed photons by only the active absorber at a given wavelength.
The principle of EQE measurement is based on illuminating the sample by a monochromatic light and recording the device electrical current (number of generated carriers). By varying the frequency of the light the entire curve of the current as a function of wavelength can then be established. Figure 1 shows an example of an EQE curve for a typical P3HT:PCBM based device. The area under the curve will then represent the total number of carriers created by the device under full spectrum white light illumination. In other words, the integration of the curve will give the electrical current density.
In order to determine IQE the light absorbed by the active layer stack has to be identified. This is typically done by recording and excluding the light reflected from the device and using only the absorbed portion of light for the calculation of quantum efficiency. Since the absorbed light is typically less than the total incident light (as there will always be some loss of light due to reflections) then EQE will typically be less than IQE. The difference between IQE and EQE is important for distinguishing loss mechanisms between optical absorption properties of the entire device and photoconversion properties of the absorbing materials. IPCE is also useful for studying degradation properties of devices. The general reduction in the IPCE during time points to deterioration of photoconversion properties of the active material, while the change of the shape of the IPCE curve may point to morphological alterations in the absorbing layer.