Imagine a world where your smartphone is charged by a thin film of solar cell embedded on the smartphone casing. Such personal energy provision devices and technologies could dramatically revolutionize mobile digital technology for communication, given the availability of an unlimited power supply from the sun. But, the route for getting there remains arduous with many problems hampering the realization of low cost printable flexible solar cells with performance comparable to conventional solar photovoltaic, and with little health risks from additives necessary for manufacturing flexible and printable solar energy capture thin films (see Nature commentary, Link).
Traditional solar cells use large area silicon for capturing sunlight and converting it into electricity. With increasing economies of scale and improved manufacturing processes for solar photovoltaic panels, the cost of providing solar energy has declined significantly, leading to the creation of many solar farms around the world. While pure single crystal silicon is the preferred material for manufacturing silicon solar cells, improvement in design has enabled the use of inferior polycrystalline silicon for solar photovoltaic applications. In general, the efficiency of converting sunlight into electricity has improved for polycrystalline solar cells, but it remains lower than that of single crystal silicon (~22%, which is the theoretical conversion efficiency).
Silicon is a substrate not amenable to printing or creating flexible solar cells; thus, scientists and engineers have harnessed the utility of electrically conductive organic polymers as well as light sensitive dye molecules for creating light capturing organic polymers capable of converting sunlight into electricity, and this class of materials has been heavily investigated for their potential use in organic solar cells.
Specifically, the concept revolves around integrating a light sensitive and capture module with an electrically conductive organic polymer. For example, the light capturing component could be a quantum dot or a light sensitive dye molecule. Together, the light capturing module would harness specific wavelength region of the solar insolation, and pass it on to the electrically conductive polymer for electricity generation. Typically, such electrically conductive polymeric materials exist as thin films, which suggests their potential use as printable flexible solar cells.
However, many obstacles stand in the way of designing cost efficient processes for printing a hierarchical structure comprising the light sensitive component on top of the flexible thin film of electrically conductive polymer. First, a self-assembly process is used to bond the light sensitive molecular layer with the underlying electrically conductive polymer. If the above could be achieved, the next question queries the availability of printing technologies able to sequentially deposit different layers onto the substrate. Finally, questions of environmental health and safety also arises given the use of metal additives in thin film photovoltaic devices.
In solving the above problems and creating a safe as well as highly efficient and flexible solar energy harnessing device, many barriers to the reproducible manufacture of thin film inorganic materials would have been overcome. This helps open the doors to other flexible electrically conductive substrates such as incorporating biomolecules on flexible inorganic components for biosensing. But, the widespread adoption of such flexible electronics rest on the twin requirements of economics as well as health and safety. High cost and potential safety issues would significantly dent the prospects of utilizing flexible solar cells for generating small amounts of power in mobile applications.
Currently, the best in class dye sensitized solar cells have achieved 12% efficiency in solar energy conversion, which is higher than that of quantum dot solar cells and organic polymer solar cells. Hence, research into printing such dye sensitized solar cells as thin film substrate on a variety of materials such as clothing would help popularize solar energy harnessing technologies to the mass population. However, the question remains in technology development, what is the killer application? Or more specifically, what would prompt a person to purchase and wear a piece of fabric capable of generating electricity for a smartphone or a wearable computer? Are there safety concerns?
Hence, potential use of flexible solar cells would most likely come from coating thin films of flexible solar cells on surfaces of cars or other mobile devices which has regular exposure to fluorescent light or sunlight. Future roll to roll printing of flexible solar cells is the goal of many researchers interested in bringing solar cell technology to curvilinear surfaces and flexible materials. However, current limitations in printing technology hamper the high precision deposition of different substrate layers needed to incorporate solar energy harnessing layers onto electrically conductive polymers. Specific problems are the tendency of defect generation during the printing process and the resultant loss of conversion efficiency. Additionally, there is also a safety issue in the possible leak of metal additives and skin exposure. Therefore, lack of suitable printing technology is the major bugbear towards the realization of printable flexible solar cells, while redesigning the concept of solar cells that utilizes more environmentally friendly metals and additives would remove a potential safety risk at source.
Category: materials, renewable energy,
Tags: dye sensitized solar cells, roll to roll printing, flexible solar cells, electrically conductive polymers, hierarchical materials,