Engineers and architects increasingly turn to organic photovoltaics to satisfy building functionality and aesthetics
By: J. Patrick Thompson, VP of business and technology development at New Energy Technologies
Architectural design is changing. Energy costs and social trends are making sustainable buildings more attractive and valuable for designers, developers and owners. Sustainable buildings can be more efficient for net-energy usage through conservation technologies such as insulation and low-emissivity windows. These conservation technologies can be used in combination with energy-generating technologies, such as solar PV.
Sustainable building practices, taken to their ultimate potential, can create net-zero buildings that use electricity and other resources extremely efficiently. Net-zero buildings generate the electricity they need from their structures. This trend reflects the need to combine aesthetics with functionality to optimize overall building operations.
Current systems sometimes intrude on the aesthetics of a building and will require complementary technologies. Building engineers and architects are looking to solar engineers to increase their options.
Emergence of Organic Photovoltaics
Engineers and architects are excited about using organic photovoltaics (OPV) for net-zero buildings because they allow for design flexibility and can provide additional functionality for fixtures and materials. Rather than a standard p-n junction used in inorganic semiconductor modules, OPV uses a blend of an organic conjugated polymer and a fullerene to form what is commonly referred to as an active layer. These compounds, composed primarily of carbon and hydrogen, act as an electron donor and an electron acceptor to provide electron mobility. With this system, a photon interacts with the active layer to produce an excited electron-hole pair called an exciton, which consists of the negative electron and positive hole and delivers the electrons to an electron transport layer (ETL). At the same time, it delivers the holes to a hole-transport layer (HTL).
There is more than just exciton physics that differentiate OPV from conventional PV. OPV has attributes that can be tweaked to adhere to specific performance criteria. Predominantly, these attributes are light absorption, open-circuit voltage, good performance under indoor-lighting conditions, low-capital expenditure and potentially low-energy production costs using printable techniques and methods. By fine-tuning light absorption, it is possible to absorb wavelengths of specific light, which in turn affects color and visual light transmission (VLT). By balancing light absorption and VLT, with the ability to specify the open circuit voltage (Voc), the electrical performance of the device can be optimized. Interconnecting cells in series and parallel strings allow voltage and current to build power (wattage) within an OPV solar module.
OPV does not need direct solar (i.e., natural sunlight) irradiation to produce electrical power. OPV devices can produce electrical energy when exposed to artificial and reflected light. An example of this can be observed in prototypes such as New Energy Technologies’ SolarWindow, which operates by using natural and artificial light sources to produce electricity. SolarWindow technology, which is still under development, uses OPV to maintain high levels of VLT to produce a see-through coating that can be applied to windows. Since the technology can be tweaked for relatively high VLT and an architecturally neutral color, it can be used in building windows to generate electricity — unlike conventional, opaque, solar PV modules. Since the window is see-through, it can collect light from both sides of the window.
Another attribute of OPV is that the coating can be applied on flexible substrates. This makes possible many potential applications and uses in building systems, and it’s expected to contribute as an option for net-zero buildings and provide other electricity-generating options in the non-architectural space.
There are also manufacturing advantages to OPV applications. With OPV it is possible to apply these materials in a solution process. While there are many coating methods available for OPV, New Energy’s SolarWindow coating can be applied at ambient conditions that do not require high temperature, pressure or vacuum processes. This has the potential for low cost, high-speed manufacturing. Conventional silicon and thin-film PV, on the other hand, typically requires high electrical energy and high temperature, pressure or vacuum deposition techniques.
OPV Contributions To Sustainability
OPV is expected to be a good fit for building-integrated photovoltaics (BIPV) and net-zero building construction and renovations. Since glass is a primary component of most building envelopes, architects and builders are already familiar with its uses and function. With OPV, it will be possible to add functionality to building components without compromising aesthetics. This increases the value of sustainable architecture and net-zero buildings, as well as the value of the building itself. It is expected that OPV can be an important addition to the growing list of available technologies used in sustainable-building design. Conventional solar PV arrays in tall commercial structures must compete for space with other roof structures such as access points, and electrical and mechanical systems. Power generation is a function of power density and available real estate (area or space). If we consider redefining “real estate” to include the vertical surfaces of tall buildings, urban environments may have an entirely new dimension to its potential electrical generation.
As market demand evolves, new technologies must continue to evolve as well to address these and other net-zero building requirements. OPV, and other products being developed for eventual commercialization, show that innovative companies are prepared to meet the challenging sustainable energy and market demands with new solutions. Whatever the future may bring, we can expect that there will be technologies developed to address current and future sustainability goals as we move toward an energy autonomous society. SPW