## 3 Perovskite Photovoltaic: Basics and Related Work  
Solar power is a vital renewable energy solution for meeting global energy demands. Solar panels, also called photovoltaics (PVs), convert sunlight into electricity and are critical for solar energy systems. These cells typically generate an open circuit voltage (\(V_{\infty}\)) over 0.9 V under artificial sunlight (AM 1.5) or visible light [32]. The main PV types are monocrystalline, polycrystalline, thin-film, and perovskite. Monocrystalline cells offer higher efficiency but are more expensive, while polycrystalline options are more affordable for large-scale use but less efficient. A monocrystalline silicon cell with an n-type structure has achieved a 21.3% conversion efficiency using a passivated emitter and rear-diffused design [33]. Figure 4: MIMO system in RF energy harvesting. Silicon (Si) solar cells dominate the current PV market, with a theoretical efficiency limit of around 29.4% for single-junction cells. Multi-junction structures using absorbers with suitable bandgaps have been developed to enhance efficiency and reduce thermalisation losses [34]. Tandem configurations of III-V materials with Si have achieved efficiencies over 32% [35; 36], but their high cost limits widespread adoption. Consequently, organic-inorganic metal halide lead PSCs have gained attention for their impressive 25.2% efficiency [37]. Despite facing durability and stability challenges, PSCs offer comparable efficiency to Si cells at a lower production cost, enhancing their appeal as alternatives in the PV market [38]. Researchers are drawn to perovskite materials for their unique crystal structure, offering excellent light absorption, performance, low-temperature processability, and versatility as light energy harvesters [39]. PSCs utilise perovskites with an \(ABX_{3}\) crystal structure. In this structure, organic cations (methylammonium or formamidinium) or inorganic cations (\(Cs^{+}\) and \(Rb^{+}\)) serve as A cations. B cations are divalent ions ( \(Pb^{2+}\), \(Sn^{2+}\), or \(Ge^{2+}\)), while X represents halide anions ( \(I^{-}\), \(Cl^{-}\), or \(Br^{-}\)) [40]. Two typical designs are employed in PSC-Si tandem solar cells (Figure 5): mechanically or optically stacked four-terminal (4T) and monolithically integrated two-terminal (2T) architectures. In 2T tandems, high-bandgap perovskite cells are stacked directly on silicon cells, requiring precise current matching due to their electrical series connection. Conversely, 4T tandems involve separate production of the two cells, which are then connected at the module level using optical coupling, which eases current matching but increases production complexity. Both configurations require a semitransparent upper cell with minimal parasitic absorption [34]. Recent advances in 2T and 4T perovskite-Si tandem cells have achieved power conversion efficiencies (PCEs) of 28.27% [41] and 31.34% [42], respectively. However, these configurations face current and spectrum matching challenges, resulting in lower short-circuit current density (\(I_{sc}\)) in bottom Si cells, typically ranging from 14 to 19 mA/cm\({}^{2}\), significantly lower than the original Si cells, which had a \(I_{sc}\) of approximately 40 mA/cm\({}^{2}\). Consequently, half of the output power would be lost [42]. Single-junction PSCs show strong efficiency, but semitransparent versions are less efficient due to difficulties in applying transparent electrodes without damaging underlying layers [43; 44]. Moreover, single-junction PSCs reached 38.18% efficiency under Sun conditions and 13.18% under light emitting diode's (LED) light [45], while concentrated PV cells can exceed 40% under optimal conditions, making them suitable for regions with ample direct sunlight, surpassing the Shockley-Queisser limit of 33% [45]. Figure 5: Schematic diagram of common PSC architectures: 2-terminal (**A**) and 4-terminal (**B**). The study in [39] introduced a cost-effective indoor sensing method using PSCs for powering backscatter sensors.