## 4 System Architecture and Design Procedures
### Perovskite Photovoltaic Energy Harvesting System  
Significant research has focused on enhancing the performance and efficiency of PV energy harvesting cells, particularly PSCs, which have made remarkable strides over the past decade, achieving efficiencies between 5% and 33% [51; 52]. The PV characteristics, represented by I-V and P-V curves, are essential for optimising system performance and assessing the efficiency and reliability of solar energy conversion. While PSCs and Si cells show minor differences in hysteresis and temperature sensitivity, both aim to convert sun light into electricity, possess a maximum power point, and focus on efficiency optimisation. These characteristic curves can be generated using single or double-diode modelling, with the latter being more complex to develop and implement [53]. This section presents the mathematical modelling of PV systems, both cell and module, utilising the one-diode model. The equivalent circuit model for the cell, as shown in Figure 7, includes components such as a photocurrent (\(I_{ph}\)), diode (\(D1\)), series resistor (\(R_{s}\)), and shunt resistor (\(R_{sh}\)). \(R_{s}\) represents the resistance in the current circuit, primarily caused by losses from metal grids, semiconductor materials, and interconnections. At the same time, \(R_{sh}\) is associated with current leakage due to cell thickness and surface effects. The impact of \(R_{s}\) is more pronounced at the cell level due to the multiplication of resistances within a module, whereas \(R_{sh}\) becomes significant when multiple PV modules are involved [54]. Figure 7 illustrates the cell's \(V_{out}\) and \(I_{ph}\), which are directly proportional to incident light on the cell and expressed in (10). \[I_{ph}=[I_{sc}+K_{i}(T_{o}-T_{s})]\times\frac{G}{1000}, \tag{10}\]  
where irradiance (G) is 1000 W/m\({}^{2}\) in standard test conditions (STC), \(I_{sc}\) is the short-circuit current at STC, \(K_{i}\) is temperature coefficient of \(I_{sc}\), \(T_{o}\) is cell temperature, and \(T_{s}\) is standard temperature (298 K). The reverse saturation current for a single PSCs, \(I_{s,r}\), is as follows:  
\[I_{s,r}=\frac{I_{sc}}{e^{\frac{(r+V_{out})}{(I_{sc}\times A\times I_{p})}}-1}. \tag{11}\]  
Referring to Figure 7, the expressions for the current through the diode (D), (\(I_{d}\)), the current through \(R_{sh}\), \(I_{p}\), the output current (\(I_{ppv}\)), and the output power (\(P_{ppv}\)) of a PV cell at a load (\(R_{L}\)) are as follows:  
\[I_{d}=I_{s}[e^{(\frac{V_{out}+I_{ppv}\times R_{s}}{A\times V_{T}})}-1], \tag{12}\]  
\[I_{p}=\frac{V_{out}+I_{ppv}\times R_{s}}{R_{sh}}, \tag{13}\]  
\[I_{ppv}=I_{ph}-I_{d}-I_{p}, \tag{14}\]  
\[P_{ppv}=\frac{I_{ppv}^{2}}{R_{L}}. \tag{15}\]  
In (10) and (11), \(V_{T}\) is the thermal voltage equivalent (V), \(V_{T}=\frac{\mathrm{I}\times\mathrm{T}_{o}}{q}\), \(q\) is electron charge (\(1.6\times 10^{-19}\) C), \(V_{oc}\) is the open circuit voltage, \(k\) represents the Boltzmann constant (\(1.3806\times 10^{-23}\) J/K), and \(A\) is the ideality factor for PSCs. Figure 7: Single–diode PV cell equivalent circuit. A single PSC produces low output power, but its voltage or current can be increased to meet IoT device requirements by combining multiple PSCs. This can be carried out by connecting cells in a series to form a perovskite module or by creating a perovskite array by connecting modules in parallel. The performance of these configurations can be assessed by calculating the voltage from a single cell multiplied by the number of series-connected cells for module voltage and the current multiplied by the number of parallel-connected cells or modules for the array current. The configuration of the general PSCs, shown in Figure 8, varies based on the number of series diodes \((Y_{s})\) and parallel diodes \((X_{p})\). A PSC is represented as (\(X_{p}=Y_{s}=1\)), a module as (\(X_{p}=1,Y_{s}\
eq 1\)), and an array as (\(X_{p}=Y_{s}\
eq 1\)).