4.2. CELL THEORETICAL CONSIDERATIONS
Under steady state analysis, (ω= 0), a small voltage change,”dV” is expected to produce a small current change,”dI “, which implies that, Z (ω=0) = dV/dI = Rdc the same applies to capacitor, where, C= dQ/dV. This gives the dc resistance and capacitance at the test voltage bias. When a system is under influence of voltage or current ripples, this relationship no longer holds. In fact it becomes more pronounced for a non-linear device such as the solar cell. Thus, ac analysis is required. For the ac analysis, we perturb the solar cell with ac voltage “V(jω)”(0 ≤ ω< ∞), and state the impedance as, Z(jω)=V(jω)/I(jω) .Likewise, C*(jω)=V*(jω)/I*(jω) . Since the ac analysis suffice to achieve results from steady state over the dynamic state, ac analysis becomes a preferred choice. The ac analysis can be implemented either in the time domain (transient analysis) or in the frequency domain (impedance studies). When properly implemented, both techniques produce the same validity in result. In solar cell applications, the transient study usually presents a decay profile that was analyzed. When the profile involves more than a single time constant (relaxation time), it becomes difficult to distinguish the different processes under the transient studies [27-28]. Impedance spectroscopy has been applied by simulation of impedance data, to know what microscopic phenomena are translated by dielectric elements such as CPE constant phase element, R resistance thus C the geometric capacity of the cell. For example, the CPE is used in this work to provide a more suitable description of the relationship between the actual capacitance and ideality of the solar cell diode junctions.