A team of researchers from the University of Surrey and Imperial College London collaborated with Amsterdam’s research institute AMOLF to develop a method to help achieve a 25% increase in energy levels absorbed by wafer-thin solar photovoltaic (PV) panels.
The researchers claim that their solar panels, just one micrometer thick (1μm), convert light into electricity more efficiently than others as thin and pave the way to make it easier to generate more clean, green energy.
Hyperuniform disordered patterns
In the paper published in the American Chemical Society’s (ACS) journal Photonics, the team demonstrates the power of hyperuniform disordered (HUD) patterns for lightweight, flexible, and efficient PVs by focusing on the absorption properties in ultrathin (∼1 μm) silicon.
The HUD pattern consists of a two-dimensional network of silicon walls resembling the underlying honeycomb structure in black butterfly wings. The current 3D nanophotonic wafer designs can only prevent light reflection via impedance matching of the solar cell, but it fails to extend the light paths in silicon cells required for photon absorption.
Dr. Marian Florescu from the University of Surrey’s Advanced Technology Institute (ATI) said, “One of the challenges of working with silicon is that nearly a third of light bounces straight off it without being absorbed and the energy harnessed. A textured layer across the silicon helps tackle this, and our disordered yet hyperuniform honeycomb design is particularly successful.”
Hyperuniform disordered media are isotropic (having the same properties in all directions) and possess constraint randomness — such that the density fluctuations on large scales behave more like those of ordered solids. HUDs are highly flexible mediums to control light transport, emission, and absorption in unique ways.
In the study, the research team achieved light absorption in a 1 micrometer (∼1 μm) thick silicon slab, over twofold in the wavelength range from 400 to 1000 nm when textured with optimized HUD patterns compared to slabs that are without patterns.
The level of absorption obtained is the highest demonstrated until now in a silicon slab as thin as one micrometer. To achieve this, researchers pursued the approach of k-space engineering (an array of numbers representing spatial frequencies in the MR image) of HUD patterns with a tailored scattering spectrum and diffractive coupling of solar irradiation into guided modes of the silicon slab.
How does diffraction help light absorption?
The team focused on the trade-off between light trapping and increased carrier recombination given by the nanostructures. On investigation, they discovered efficiencies above 20% could be obtained for several optimized HUD designs and state-of-the-art silicon PV technologies.
The team used the diffraction approach in the absorber to enhance light trapping into the ultrathin photovoltaic. Guided modes of thin silicon slab tend to become leaky (quasi-guided) in the presence of textures, which can in and out couple to the electromagnetic modes supported by the surrounding medium. Total absorption is achieved by summing the coupling contributions of each mode.
To maximize the sunlight absorption in the slab, the team efficiently coupled the loose modes for wavelengths ranging from 350 nm to 1100 nm. Due to several modes in a one μm silicon slab, a pattern structure that diffracts incident light to the range from ~15μm to ~20μm raise to minus one ensures all sunlight has a mode to couple to.
The team decorated the two-dimensional HUD point pattern with 200 nm tall silicon walls following a Delaunay tessellation protocol (fundamental computational geometric structure) that forms a continuous silicon network.
However, the light absorption with the two-phase design was no longer expected to be optimal as the 3D texture strongly disrupted the wavelengths of the silicon slab. Researchers then considered the power spectral density (PSD), which is the Fourier transform of the 2D design, to represent scattering strength better. The tessellation protocol causes the resulting 3D network to become nearly hyperuniform.
The team successfully demonstrated light trapping in the thinnest silicon slab by decorating the point pattern with two materials in a wall network fashion.
In the laboratory, absorption rates of 26.3 mA/cm2 were achieved, a 25% increase from the previous record of 19.72 mA/cm2 in 2017. They secured an efficiency of 21% but anticipate that further improvements will push the figure higher, resulting in significantly better efficiencies than many commercially available photovoltaics.
Besides improving solar power generation, the findings could also benefit other industries where light management and surface engineering are crucial, such as photo-electrochemistry, solid-state light emission, and photodetectors.
Previously, an international team of researchers from ICFO, Imperial College London, and University College London had claimed to have developed a new disorder-engineering technique to develop high-efficiency ultra-thin solar cells.