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.