## 1 Introduction  
Today, sustainability is a major global issue, worsened by the increasing use of fossil fuels. The necessity for sustainable and efficient energy sources has become increasingly important with the rapidly evolving landscape of IoT applications. As a result, smart energy and carbon-free energy generation methods, such as energy harvesting from environmental sources like wind, solar, and electromagnetic waves, are emerging as key enablers for IoT devices [1] and the expansion of smart cities. These smart cities rely heavily on smart energy, smart sensing, and resilient infrastructure to support their interconnected systems. As shown in Figure 1, common energy harvesting technologies include solar, thermal, vibrational, and RF harvesters. Each is designed for specific environments, contributing to improved sustainability in IoT ecosystems with unique capabilities [2]. IoT devices have transformed numerous sectors, such as healthcare, agriculture, manufacturing, and smart cities [3]. With the growing number of IoT devices, RF waves are rising in the environment. Hence, an efficient method is required to extract energy from the available spectrum and power IoT devices [4; 5]. The emergence of 5G/6G satellite-cellular networks with terrestrial and non-terrestrial networks (NTNs) promises seamless, ubiquitous communication capabilities [6; 7; 8; 9; 10]. The convergence of 5G/6G/Wi-Fi 6/6E/7 will enhance massive IoT device deployments at an unprecedented rate [11; 12; 13; 14; 15]. Future IoT technologies in smart cities and heterogeneous TN-NTN ecosystems will require energy efficiency, spectrum efficiency, and reconfigurable signal coverage across various network ranges, from the wireless personal area network (WPAN) (10-100 m) to the wireless vast area network (WWAN) (up to 100 km) [16; 17; 15]. Wi-Fi 6/6E, with its additional 6-GHz band, boosts data rates to 9607 Mb/s, surpassing Wi-Fi 5's 3476 Mb/s by using 1024-QAM OFDMA and uplink/downlink MU-MIMO (UL/DL MU-MIMO), enabling massive IoT growth. As 5G/6G networks continue to evolve with high speed, low latency, and increased bandwidth, the demand for wireless power transfer (WPT) and energy harvesting solutions will grow [18], offering a practical means to sustain the massive deployment of IoT devices in smart cities. Integrating energy harvesting technologies with modern wireless communication networks offers a sustainable power solution for IoT devices, especially in smart cities and remote areas [6]. Since the 1990s, RF energy harvesting has become more feasible due to the widespread use of TV, radio, cellular, satellite, and Wi-Fi signals, which create ambient RF energy [19; 20]. This energy is captured and converted into usable electrical power by a rectifier and regulated by a power management unit [21; 22], a typical RF energy harvesting block diagram is given in Figure 2. While RF energy harvesting alone may not always meet the full energy demands of IoT devices, the adaptability and versatility of these solutions mean that they can be combined with other technologies, like perovskite photovoltaic cells, to provide sufficient power for ultra-low-powered IoT applications, such as security surveillance and smart sensing in smart buildings, smart infrastructures, and intelligent agriculture [22; 23]. Figure 1: Common types of ambient energy harvesting. Perovskite solar cells (PSCs) are highly efficient solar cells made from a perovskite-structured compound, typically a hybrid organic-inorganic material containing lead or tin halide. Over the past five years, PSCs have surpassed most thin-film technologies in power conversion efficiency, maintaining a consistently high efficiency even in low-intensity or diffuse light [22]. These attributes make them ideal for hybrid energy harvesting applications, including indoor and outdoor environments [24]. A recent technological breakthrough has facilitated the development of hybrid energy harvesting systems by evaluating the features and trade-offs linked to these harvesters.