Chapter 8. ORGANIC SOLAR CELLS - from and for SET students

1.1. REFRACTION AND DISPERSION CHAPTER 1. LIGHT MANAGEMENT
Figure 1.4: The Law of Snell
When light moves from a medium of a given refractive index n1 into a second medium with refractive index n2,
both reflection and refraction of the light may occur.
Since it has an electromagnetic and a magnetic vector that changes in time, we need to take a closer look.
We can have two cases:
� P-polarization: Electric field parallel to plane of Incident (p=parallel)
� S-polarization: Electric field perpendicular to plane of incident (s=senkrecht)
These two cases can also be seen in figure 1.5.
Figure 1.5: P-Polarization and S-Polarization
For the P- and S-Polarization different equations for transmission and reflectance hold.
For the reflectance is given by:
and the transmission by:
R = ( n1 − n2
)
n1 + n2
2
T = ( 4n1n2
2
) (1.5)
n1 + n2
At one particular angle for a given n1 and n2, the value of Rp goes to zero and a p-polarised incident ray
is purely refracted. This angle is known as Brewster’s angle, and is around 56 for a glass medium in air or
vacuum. Note that this statement is only true when the refractive indices of both materials are real numbers,
as is the case for materials like air and glass. For materials that absorb light, like metals and semiconductors,
n is complex, and Rp does not generally go to zero. When moving from a denser medium into a less dense one
(i.e., n1 ¿ n2), above an incidence angle known as the critical angle, all light is reflected and Rs = Rp = 1. This
phenomenon is known as total internal reflection. The critical angle is approximately 41 for glass in air. This
can also be seen in figure 1.6
The Brewster angle can be calculated from:
θBrewster = arctan( n2
The critical angle can be determined from Snells law using:
5
n1
(1.4)
(1.6)