**1.2 Transparent solar panels**

A transparent solar panel is a basically challenging idea because sunlight (photons) must be absorbed by solar cells and converted into electrical energy (electrons). Sunlight can pass through the medium in transparent solar glass and it defeats the

conversion purpose. But in transparent solar panels, the absorption happens in a different way. The cell selectively harnesses a portion of the sunlight that is invisible to naked eye and allows the visible light to pass through the device. The researchers have developed the transparent luminescent solar concentrator (TLSC) to achieve the transparent behavior of the cell rather than trying to develop the challenging transparent PV glass cell [6, 7]. Transparent solar panels use transparent luminescent solar concentrators as glass, which is transparent in nature. It uses organic molecules to absorb invisible spectrum of sunlight. So, these organic molecules absorb the specific IR and UV light. The electrons of molecules are excited by the energy of radiation and jump to a higher-level orbit. When they settle at ground state, the energy is released in the form of IR radiation (luminescent or glowing) of the different non-visible spectrum. This emitted IR light is guided through the edge of the plastic, where the strips of traditional solar cells convert it into electrical energy. This technology is feasible on cars windows, buildings, mobile phones, and other devices with a transparent surface. The limiting factor of this technology for commercialization is its low efficiency of about 1%. However, efficiency can be achieved by tuning the properties of the material in device. The first fully transparent solar concentrator was built by researchers at Michigan State University in 2014. This transparent solar panel could turn any glass sheet or window into a photovoltaic cell. The full transparency was achieved for the solar glass by 2020. Transparent solar panel technologies are set to transform the solar energy utilization landscape globally. We may able to generate electricity from windows of building, vehicles, phones, etc. These transparent solar panels can be deployed easily in various devices and systems such as laptops, e-readers, skyscrapers, windows, etc. The glass windows in buildings can be replaced by solar power windows [8, 9].

The organic salts absorb IR and UV light and emit IR in case processes occur outside the visible spectrum. Thus it appears transparent. TLSC is composed of organic salts that absorb specific UV and IR light, which then luminesce (glow) as another invisible wavelength. This wavelength is then guided to the edge of the window-thin PV solar cell strips that convert it into electricity. The mass production of such transparent solar panels can give an efficiency of about 10%. This can help to generate power through every window of home or office buildings and may bring a transformative result [10].

Transparent solar panels are categorized into (i) partially transparent solar panels: Heliatek Gmb, a German manufacturer has developed this technology that can absorb 60% of the sunlight it receives. They have achieved an energy conversion efficiency of about 7.2%. However, the generation of solar power can be increased by adjusting the transmitted and absorbed sunlight, for example, south-facing glass buildings can reduce the transmitted light. (ii) fully transparent solar panels: The researchers at MSU, USA have achieved fully transparent PV glass panels that resemble regular glass. They have achieved energy conversion efficiency of about 10%. Bigger or more windows can generate more solar power [11–13].

Currently, researchers at Michigan State University and MIT as well as manufacturers such as Brite solar, Physee, and Ubiquitous energy are pioneers in transparent solar panel technology as shown in **Figure 1**. Ubiquitous energy has achieved energy conversion efficiency of 9.8% and working toward developing net-zero energy buildings [14, 15]. Physee has introduced power windows that work as building blocks for smartskin. Smartskin can work for sensing, power generation, and regulating the inside climate using an artificial intelligent system. Richard Lunt has proposed making a solar cell that would absorb all the energy from the sun except the part that allows us to see. He has developed highly transparent solar cells that represent the

#### **Figure 1.**

*Vladimir Bulović of electrical engineering and computer science showing their transparent solar cells (upper), and Richard Lunt demonstrates the transparency of the novel solar cell at MIT (lower).*

wave of the future. He has claimed that these solar panels have similar power generation potential as rooftop. This can also be applicable to buildings, cars, mobile, and other such devices. MIT researchers are developing transparent solar cells that could turn every product such as windows and electronic devices into power generators. These cells can absorb only IR and UV light. They have developed a room-temperature fabrication method and can deposit coatings of solar cells on various materials to run electronic displays using ambient light [16–18].

Solar Panel blinds are accessories to transparent solar glass or panels in case of windows for generating electricity. This blocks direct sunlight from entering inside. SolarGaps introduce solar blinds and claim that they can generate about 100 W of power on every 10 sq. ft. of window area. It can be installed from outside or inside and can control its angle according to the sun's position [19, 20].
