# Photovoltaic Solar Cells: A Review
## 2 Solar Cells
### p-n Junction Solar Cell  
#### 2.4.1 Formation of the Depletion Region  
A solar cell in a basic term is a semiconductor diode that has been carefully designed to generate power from the sunlight. A diode is a single crystal semiconductor material such as silicon, having one side doped with pentavalent impurities forming n-type and another side doped with trivalent impurities as p-type. The doping process creates additional mobile carriers called majority carriers in each respective region. When n-region and p-region semiconductors are brought into contact, electrons of the n-section will diffuse into the p-section leaving a region of positively charged donor atoms in the p-n interface near the n-zone. Likewise, a net of negatively charged acceptor atoms is left behind in the p-n junction near the p-zone as holes diffuse from p-region to n-region. The consequent jumps of the valence electrons can be noted as a motion of the holes. The diffusion of electrons and holes will create a current called diffusion current \({}^{\text{T}}_{\text{diff}}\)\({}^{\text{}}\) and a depleted area of charge carriers, referred to as the depletion region or space charge region. All electrons and  
Figure 2: The basic components of a PV system.  
holes are swept out of the depletion region by a generated electric field, which prevents any additional flow of the charge carriers [25], as shown Figure 3 below.  
The net flow of electrons and holes in a semiconductor under equilibrium conditions will generate currents, namely diffusion current '\(I_{diff}\)' as a result of the concentration difference and drift current '\(I_{drift}\)' as a result of the generated field at the junction [26]. The diffusion current of electrons '\(I_{n\_diff}\)' is expressed by:  
\[I_{n\_diff}=e\ D_{n}.\ dt/dx \tag{1}\]  
Likewise, the typical diffusion current density of holes is given by:  
\[I_{p\_diff}=-e\ D_{p}.\ dp/dx \tag{2}\]  
where \(dn/dx\) and \(dp/dx\) are the ratios of the change in particles' density to the change in the diffusion distance of electrons and holes, respectively. Due to a built-in electric field, '\(I_{drift}\)' is generated, which is opposite to the '\(I_{diff}\)' [27].  
The drift current due to drift of electrons '\(I_{n\_drift}\)' is given by:  
\[I_{n\_drift}=en\upmu_{n}E=env_{dn} \tag{3}\]  
where '\(\upmu\)' is the electron's mobility, '\(E\)' is the electric field in the depletion region, and '\(\upsilon_{dn}\)' is the drift velocity of electrons.  
Similarly, '\(I_{p\_drift}\)' due to drift of holes is expressed as:  
\[I_{p\_drift}=ep\upmu_{p}E=ep\upsilon_{dp} \tag{4}\]  
'\(\upmu\)' is the holes mobility and '\(\upsilon_{dp}\)' is the average drift velocity of holes.  
The net '\(I_{drift}\)' due to drift of holes and electrons is as follows:  
\[I_{drift}=e\big{(}\upmu_{n}n+\upmu_{p}\ p\big{)}E \tag{5}\]  
The above equation can be written as:  
\[I_{drift}=e\big{(}\upmu_{n}n+\upmu_{p}\ p\big{)}E=\sigma\ E \tag{6}\]  
\[\sigma=e(\upmu_{n}n+\upmu_{p}\ p) \tag{7}\]  
Figure 3: The basic semiconductor form at the instant connection of the p-n sections and generating the electric field in the space charge region.  
'\(\sigma\)' given by (\(\Omega\)/cm) is the conductivity of the electronic semiconductor. For nondegenerate semiconductors, there are no net currents in the depletion region as '\(I_{drift}\)' and '\(I_{diff}\)' are balanced at the state of equilibrium. This leads to the [PERSON] relation:  
\[\frac{D}{\mu}=\frac{kT}{e} \tag{8}\]  
The particle mobility 'u' gives a sign of how well an electron or a hole moves in a semiconductor due to an electric field while the particle diffusion coefficient '\(D\)' indicates how well the particles move in the semiconductor owing to the concentration gradient [28].