Transparent Solar PV Panels

Abstract

Global warming is increasing emissions of greenhouse gases. It damages the environment of Earth. Solar energy is the cleanest source of renewable energy. It is an abundant source of clean energy. It has tremendous scope to generate electricity. Solar cells are devices that convert solar energy into electrical energy. Transparent solar panels are made up of transparent solar cells or transparent luminescent solar concentrators. A transparency of about 80% has been achieved with power conversion efficiency of about 12–15% in transparent solar cells. These cells can be used in buildings, vehicles, and other desired applications to generate solar power. We discuss solar energy basics and its conversion technologies. Transparent solar panels may bring a revolution in low-power display devices and mobile applications.

1. Introduction

1.1 Conventional solar panels

Solar energy is the cleanest source of renewable energy on earth. Solar cell works on the principle of photoelectric effect that converts solar energy into electrical energy. Solar cells are mainly categorized into four generations. The conventional mono and polycrystalline wafer-based solar cells are first-generation [1, 2]. Thin films solar cells such as CIGS, CdTe, and CZTS are second-generation of solar cells. Multijunction solar cells are third-generation solar cells. Quantum dot and hybrid solar cells are fourth generation of solar cells. Theoretically, each generation of solar cells has energy conversion efficiency. Experimentally, multijunction solar cells have achieved the highest energy conversion efficiency, about 47.5%. Silicon solar cells are mostly commercialized technology due to abundant and cheap materials. However, its fabrication involves expensive and toxic processes. Currently, solar panels are used as off-grid and rooftop on-grid applications. Therefore, novel solar panel technology is required for multiple applications such as windows, displays, surface, etc. [3, 4, 5].

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, 12, 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 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, 17, 18].

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).

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].

2. Device configuration of transparent photovoltaic device

The schematic representation of a transparent photovoltaic device is shown in Figure 2. This shows the key component of transparent solar cell, which transmits visible light and captures NIR and UV light. The thick layer is glass, plastic, or other transparent material. The coating of PV materials on top of the device. These PV materials are photoactive layers. The active layers include semiconducting materials that get excited upon falling sunlight and interact, creating an electric field that causes electric current to flow in a device through a circuit. This current can be taken out of the device to an external circuit via connecting sandwiched electrodes. Both electrodes must be transparent and they are anti-reflective coatings to reduce light reflection. Therefore, a combination of optical design, molecular engineering, and optimization of the device are used to design transparent PV devices [21].

Figure 2.

Schematic of transparent photovoltaic device.

3. Spectral response of conventional and transparent PV cells

The spectral response of conventional and transparent solar cells is shown in Figure 3. The absorptive response (black curve) is superimposed on the solar spectrum (gray curve). In the conventional cell, the wavelengths at which absorption is relatively high include the visible spectrum (400–700 nm). The transparent cell absorbs well in the near-infrared and the ultraviolet spectrum but the absorption drops off and approaches zero in visible range [22].

Figure 3.

Spectral response for conventional (upper) and transparent PV devices (bottom).

The current transparent solar cells transmit more than 70% of the visible light, which is within the range of tinted glass used in the windows of buildings with a power-conversion efficiency of about 2%. Lunt and Bulović claimed that it can able to reach over 12% efficiency on basis of theory. Lunt already has demonstrated transparent cells integrated into series can power the liquid crystal display on a small clock, relying entirely on ambient light [23].

4. Transparent flexible solar cells

A flexible transparent solar cell was first developed at MIT as shown in Figure 4. Researchers have developed a novel technique using graphene to prepare solar cells on surfaces of glass, plastic, paper, and tape. This device contains carbon-based organic materials with graphene electrodes and transparent material. This device involves a layer of graphene on the solar cell. The flexible transparent solar cells would be lightweight and cheap technology as well as low-cost materials [23]. Organic materials absorb the UV and IR components of the solar spectrum but transmit visible light. The most affordable option is to use indium tin oxide (ITO) coated flexible substrate. But ITO is brittle and may break during deposition on flexible substrate. A one-atom thick layer of graphene can be developed as alternative to ITO in transparent solar cells. This alternative material is flexible, highly conductive, and transparent. The most important thing is that it is made up of inexpensive carbon material [24].

Figure 4.

Transparent solar cell on flexible substrate developed at MIT.

There are some problems during the deposition of graphene electrodes on solar cells. The first problem is depositing 1 atom layer thick of graphene. A bottom layer of graphene is deposited directly on the substrate and then the top layer. The fabrication of graphene on the top layer in solar cells as a hole transport layer is very tricky. The other main problem is preparing graphene electrodes for different applications. One electrode should let electrons flow out easily in solar cells. So, here both graphene electrodes have different work functions. Therefore, changing the work function is not an easy task. A layer of ethylene-vinyl acetate (EVA) can be incorporated into the graphene layer to make the device more flexible. This would differ both graphene electrodes to work for different purpose [25].

In practical, a transparent solar cell was developed using graphene, ITO, and aluminum materials. The performance of this solar cell was lower than solar cells with one aluminum electrode. Aluminum electrode on the bottom reflects some incoming light back into solar cell and increases the absorbance of sunlight more than a transparent cell. The PCE for graphene/graphene devices was comparable to existing commercial solar panels, about 4.1% with transparency of about 61%.

The organic solar cell has the advantage to deposit on any type of surface, such as rigid or flexible, transparent, etc. The graphene/graphene devices have been demonstrated on various flexible substrates such as plastic, opaque paper, translucent kapton tape, etc. The performance was nearby in all these devices [26].

5. Manufacturing of transparent solar panels

Polysolar is developing transparent solar panels for buildings, canopies, and greenhouses applications. These panels can be used on walls with non-directional ambient sunlight. They have installed transparent solar panels in some buildings and bus shelters. This has allowed manufacturing of PV powering interactive displays, lighting, and signage. They have achieved power conversion efficiency of up to 12–15% in gray-tinted panels. Polysolar is trying to cover maximum area for generation of solar power and increase the transparent PV footprint in various sectors [27].

Thin-film PV cells (in orange color) are deposited as a naturally translucent layer onto the glass (blue) before another glass sheet is laminated on top as shown in Figure 5. They allow the panels to work at a much higher efficiency at varying angles than regular solar panels and can generate solar power at low levels of sunlight [28, 29, 30].

Figure 5.

Thin-film photovoltaic cells (in orange) are deposited as a naturally translucent layer on to the glass (blue).

Ubiquitous energy is also developing transparent PVs using semiconductor with higher efficiencies and transparencies. They are also trying to assemble panels on electronic devices such as mobile phone display to self charge the devices.

6. Conclusion

Transparent solar panels are very challenging to find their real time applications. They have huge scope to generate solar power and making electronic devices for self charging applications. Currently, they have limitations on large applications due to low efficiency and cost. These solar cells can block the most of IR. Researchers have developed graphene based organic solar cells with improved efficiency with significant transparency. Increasing the amount of active area would push up the PCE, but transparency would drop. The highest transparency of about 80% has been measured in transparent solar cells with maximum power conversion efficiency of about 12–15%. The various materials have been utilized to improve the transparency and performance of solar cells. Transparent solar panel would bring a remarkable change in electronic and optical applications.

Acknowledgments

The authors are thankful to Shiyani Research Institute, Rajkot, and Shiyani Enterprise (OPC) Private Limited, Rajkot for providing technical support to publish this chapter.