Photovoltaics Nanophysics Light-Matter Interaction
Overview
Exploring and tuning light matter interaction has led to advances in fundamental science as well as in many applications. Micro and nanostructured materials can be used in order to manipulate optical properties and to confine light to a desired volume. In order to push energy conversion efficiency in solar energy conversion devices to the theoretical limits it is important to gain full control over the optical properties. The Saive research group develops light-management strategies and photonic materials that enable record-high light-energy conversion efficiencies. Furthermore, we combine different material systems with complementary properties to develop hybrid devices that exhibit functionalities beyond existing technology.
Solar energy
Sunlight is by far the largest energy resource on earth – freely available and in great abundance. 5000 times the energy that we globally consume reaches earth’s surface in the form of sunlight. Technologies that harvest and convert sunlight to electricity are on the brink of creating a revolutionary shift away from unsustainable fossil fuel resources. Costs of solar energy have fallen at an incredible rate, and are now on par with incumbent technologies. However, there is always a large inertia to overcome when altering the infrastructure of entire societies. The best lever available to solar energy to overcome this inertia is the power conversion efficiency of solar devices. Break-through technologies are required to accelerate efficiency increases.
Light-management strategies for solar energy conversion
Increasing the efficiency of solar energy conversion devices is a crucial step for energy cost reduction. The development of solar energy conversion devices such as solar cells and water splitting devices undergoes different stages. It usually starts with a new material system and the investigation of its fundamental properties and capabilities. In the second stage a device with the desired functionality is designed and material and interface properties are optimized to improve the performance. In order to push the performance to its theoretical limits it is necessary to optimize the light-management in a further step. Light-management strategies include all measures that are taken in order to achieve maximum light absorption and maximum quasi Fermi level splitting within the active absorber materials. It includes the design of optimized contacts that are as transparent as possible while ensuring good conductivity and also light-trapping strategies that are extremely important in thin film solar cells.
Effectively Transparent Contacts (ETCs)
All PV devices have a basic requirement to collect the generated electrical current, and have conventionally done so through use of necessary electrical contacts. However, these same contacts compromise the intrinsic device performance through parasitic light reflection, scattering, and absorption. The most common example are the silver gridlines present on nearly all solar cells. These silver grids reflect 4-10% of the impinging sunlight, immediately compromising device power conversion efficiency and reducing electricity output. Front contact losses constitute the largest individual efficiency loss mechanism in PV devices. Alternative strategies have been developed to mitigate front contact losses, such as transparent conductive oxides (TCOs), nanowire networks, or interdigitated back contacts (IBC). Nanowires experience resonant interaction with incoming light, acting as antennas, and thereby scatter and absorb part of the incoming light. In addition, their nanoscale cross-section does not allow for efficient charge carrier transport in large scale devices. Despite some success with IBC contacts by the company SunPower, market penetration has been very low and is expected to remain low due to the complex processes involved.
Through extensive research effort we succeeded in creating a front contact technology that eliminates all optical and electrical challenges: effectively transparent contacts (ETCs). ETCs are triangular cross-section micro-scale silver contacts that redirect light toward the active area of a solar cell rather than reflecting it. Their excellent optical and electrical properties mitigate almost all losses related to front contact shading, absorption, and charge transport. Key features of this technology are the precise triangular shape, the smooth side walls enabling mirror-like optical properties, thin line width and a high aspect ratio – features not encompassed by any other technology.
All R&D activities related to ETCs have been moved to our spin-off company ETC Solar (ETC-Solar.com)
Bifacial Solar Cells
Strong industrial interest in rising in the field of bifacial solar cells. Due to the acceptance of photons at front and rear side, the power output can be significantly enhanced without increasing the use of expensive absorber materials. The properties of the surroundings such as the spectral dependent albedo play a crucial role for the power output. We use our strong background in light-management to develop industrially relevant strategies for performance enhancement of bifacial solar cells.
Spectro-Angular Solar Irradiance Measurements
Crucial for developing photonic materials that use the solar irradiance most efficiently, is knowing the solar irradiance. This might seem trivial at first glance but when considering how cloud coverage and ground properties (grass, concrete, …) influence the appearance of objects, it should become clear that determining the exact spectral and angular solar irradiance is rather complex. We have developed a portable setup to determine the spectro-angular irradiance. We are happy to collaborate if you are interested in taking measurements at your location or would like to take a look at our data.
Light-Powered Nanotech Beyond Solar
We are working on some exciting new ideas, stay tuned!
Methods
In all our projects, we combine computational optical and device simulations with experimental prototyping for most efficient device design. State-of-the art nanotechnology such as electron beam lithography, imprint lithography and nano-3D-printing is applied in order to push the performance to its limits.