to solar cells, numerical simulation has proven to be a viable tool over the years to study and comprehend the features and properties of solar cell devices, like the electrical, optical, and mechanical properties of complicated device structures of solar cell. The advantages of simulation are calculating the electrical behavior before the manufacturing process, calculating and visualizing internal electronic values that cannot be measured. It also helps to reduce processing costs and time spent on manufacturing solar cell devices by providing useful information on how the parameters of production can be varied to improvise the behavior of the device.
In this chapter, different simulation softwares are discussed. It presents the physical basis for modeling of semiconductor devices. Basic equations required for semiconductor modeling are also given here. Then after, it discusses in detail various recombination phenomenon taking place in semiconductors. After this, some softwares used for simulation of different types of solar cells are detailed one by one.
1.2 Preliminary Steps
(I) Physical Basis for Semiconductor Device Modeling
A. Semiconductor at Thermal Equilibrium
Every operation of a semiconductor device relies on the carriers carrying charge within the semiconductor and creating electrical currents. It is crucial to understand the exact number of these carriers so as to interpret the functioning of the device. First, we assume the thermal equilibrium of the semiconductor. This means that there are no external forces acting on the semiconductor, such as voltage, electrical fields, magnetic fields, or temperature gradients. In this case, all of the semiconductor’s properties are independent of time [1].
Carrier Concentration
First, we consider the intrinsic condition applied to the semiconductor without any impurities. The current in a semiconductor is contributed by two types of charge carrier, the electron and the hole. To evaluate the carrier concentration the function of the density of the allowed electron energy states and the occupation function of the allowed energy states must be established. The function of energy density states, N(E), defines the number of allowed electronic states per unit volume and energy. The function describing occupation is the well-established distribution function given by Fermi-Dirac, f(E), which defines the ratio of filled states having an electron to maximum states allowed with the given energy E.
The density of states at an energy E in the conduction band, which is given as NC(E) is close to EC, and in the valance band, NV(E) is close to EV and are given by [2]:
with me and mh being electron and hole effective mass respectively. EC denotes the conduction band bottom, whereas EV represents the valance band top.
The Fermi-Dirac distribution function is expressed as follows:
(1.2)
with kB representing the Boltzmann constant, the absolute temperature is represented by T and EF the Fermi energy level.
For energy much greater than kBT (i.e. E – EF ≫ kBT), the Fermi function can be approximated by the Boltzmann function:
The density of free electrons (n) (conduction-band levels which have been occupied) is found after multiplying the total number of states NC(E) with occupation function f(E) and then integrating over the conduction band:
By replacing equations 1.1(a) and 1.3 into 1.4 the resultant expression for n is found after resolving the equation:
The parameter NC is termed the conduction band effective density of states which is:
(1.6)
We can derive an expression for the concentration of holes in a similar manner. The probability that a state of energy is captured by a hole is the same as its not being held by an electron, i.e., 1-f(E). Thus,
Applying the Boltzmann approximation, we can get:
Substituting Equations 1.1(b) and 1.8 into Eq. 1.7 yields
(1.10)
NV shows the valance band effective density of states. The difference in the values of NC and NV is only because of the difference in me and mh. The variation in NC and NV from one semiconductor to another is also due to the dissimilarity in the effective masses, also.
Intrinsic carrier concentration
The number of electrons present in the conduction band and the number of holes present in the valance band are equal in an intrinsic (undoped) semiconductor which is in thermal equilibrium; n = p = ni, where ni is the concentration of carriers of intrinsic semiconductors.
When multiplying the corresponding sides of Equations 1.5 and 1.9 one obtains:
where Eg is the forbidden gap: