orders of magnitude higher than in the red. Therefore the penetration depth of blue light
into the absorber layer is rather small. In crystalline silicon, the blue light is already fully
absorbed within a few nanometres. The red light requires an absorption path length of 60
μm to be fully absorbed. The infrared light is hardly absorbed, and after an optical path
length of 100 μm only about 10% of the light intensity is absorbed.
As the absorption of photons generates excited charge carriers, the wavelength
dependence of the absorption coefficient determines the local generation profile of the
charge carriers. At the front side where the light enters the absorbing film, the generation
of charge carriers is significantly higher than at the back. It follows that the EQE values
measured in the blue correspond to charge carriers generated close to the front of the solar
cell, whereas the EQE in the red part represents charge carriers generated throughout the
entire absorber layer.
Further, it is important to reduce the optical loss mechanisms such as shading losses,
reflection, and parasitic absorption that we already discussed in Section 10.3.1.
For reducing the reflection, anti-reflective coatings (ARC) can be used. Light that is
impinging upon a surface between two media with different refractive indices will always
be partly reflected and partly transmitted. In order to reduce losses, it is important to
minimise these reflective losses.
The first method is based on a clever utilization of the Fresnel equations that we
introduced in Eqs. (4.12) and (4.13). For understanding how this can work, we will first
take a look at interfaces with silicon, the most commonly used material for solar cells. Let
us consider light of 500 nm wavelength falling onto an air–silicon interface
perpendicularly. At 500 nm, the refractive index of air is n 0 = 1 and that of silicon is n s =
4.3. With the Fresnel equations we hence find that the optical losses due to reflection are
significant at 38.8%.
The reflection can be significantly reduced by introducing an interlayer with a
refractive index n 1 with a value in between that of n 0 and n s . If no multiple reflection or
interference is taken into account, it can easily be shown that the reflectivity becomes
minimal if
This is also seen in Figure 10.11, where n 1 takes all the values in between n 0 and n s . In this
example, including a single interlayer can reduce the reflection at the interface from
38.8% down to 22.9%. If more than one interlayer is used, the reflection can be reduced
even further. This technique is called refractive index grading.
into the absorber layer is rather small. In crystalline silicon, the blue light is already fully
absorbed within a few nanometres. The red light requires an absorption path length of 60
μm to be fully absorbed. The infrared light is hardly absorbed, and after an optical path
length of 100 μm only about 10% of the light intensity is absorbed.
As the absorption of photons generates excited charge carriers, the wavelength
dependence of the absorption coefficient determines the local generation profile of the
charge carriers. At the front side where the light enters the absorbing film, the generation
of charge carriers is significantly higher than at the back. It follows that the EQE values
measured in the blue correspond to charge carriers generated close to the front of the solar
cell, whereas the EQE in the red part represents charge carriers generated throughout the
entire absorber layer.
Further, it is important to reduce the optical loss mechanisms such as shading losses,
reflection, and parasitic absorption that we already discussed in Section 10.3.1.
For reducing the reflection, anti-reflective coatings (ARC) can be used. Light that is
impinging upon a surface between two media with different refractive indices will always
be partly reflected and partly transmitted. In order to reduce losses, it is important to
minimise these reflective losses.
The first method is based on a clever utilization of the Fresnel equations that we
introduced in Eqs. (4.12) and (4.13). For understanding how this can work, we will first
take a look at interfaces with silicon, the most commonly used material for solar cells. Let
us consider light of 500 nm wavelength falling onto an air–silicon interface
perpendicularly. At 500 nm, the refractive index of air is n 0 = 1 and that of silicon is n s =
4.3. With the Fresnel equations we hence find that the optical losses due to reflection are
significant at 38.8%.
The reflection can be significantly reduced by introducing an interlayer with a
refractive index n 1 with a value in between that of n 0 and n s . If no multiple reflection or
interference is taken into account, it can easily be shown that the reflectivity becomes
minimal if
This is also seen in Figure 10.11, where n 1 takes all the values in between n 0 and n s . In this
example, including a single interlayer can reduce the reflection at the interface from
38.8% down to 22.9%. If more than one interlayer is used, the reflection can be reduced
even further. This technique is called refractive index grading.