**1. Introduction**

The increasing demand on clean and inexpensive energy has led to the emergence of solar cells in the early 1950s, where the main source of the world's power is fossil fuels. The creation of photovoltaics (PV) has opened a new era on exploiting solar radiation for the production of electricity. However, the development of the PV industry is not sufficient to cover the market demand on solar panels due to their low efficiency. Therefore, cheaper and higher-efficiency technologies are required for the solar power market. These requirements have induced the researchers to find an alternative solution by replacing the current solar cells with optical antennas integrated to diodes forming a rectifying antenna (rectenna) using the wave nature of light [1, 2]. Most of the recent researches are focused on developing solar rectennas to convert the visible region of solar spectrum efficiently to electric power and exploiting the unused portion of solar radiation (i.e., infrared region) [3]. The proposed solar rectennas are expected to exhibit higher efficiency (theoretically 100% for monochromatic

illumination) than current solar cells [4]. Rather than the low efficiency, solar rectennas overcome the other drawbacks of PVs which include the dependence on the bandgap energy and the narrowband operation (visible region only). However, several challenges contribute to make the actual conversion efficiency much lower than expected such as the poor coupling between the optical antenna and the diode [5].

Each photon in semiconductor solar cells produces electron hole pair to generate electrical power. However, the device absorbs only those photons that have energy higher than the band gap energy. This limits the conversion efficiency to 44% or even less in real devices. On the other hand, classical rectifiers receive the electromagnetic energy and convert it into DC power with a conversion efficiency reaching 100%. Solar rectennas are designed to operate in a similar way with the expectation to obtain very high efficiencies at a wide range of the electromagnetic spectrum. The field of solar rectennas appears to be promising and attractive due to the fact that high efficiency is theoretically obtainable and the material used is inexpensive and available.

Why solar rectennas?


In contrast, there are several drawbacks and challenges associated with solar rectennas such as [6]:


It is obvious that the technology of solar rectennas is still young in the early stage of research and faces numerous challenges and limitations. Thus, in this chapter, the theoretical understanding is presented highlighting the development of each part of a solar rectenna.
