How to catch all available photons by thin solar cell?

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A new publication from Opto-Electronic Advances, DOI 10.29026/oea.2022.210086, discusses Lambertian light trapping for large-area silicon solar cells.

In the quest for a sustainable life on Earth, the use of available green energy resources and efficiency of their harnessing is of paramount (existential) importance. The abundant Sun light which reaches the Earth can meet energy needs when efficiently captured. Hence, light trapping is important, and it defines how much of light energy is absorbed by a solar cell and has to be maximized for the high light-to-electrical power conversion. Refractive index of material, n, defines how much light is reflected (portion R) and transmitted (portion T) at the interface onto which light is impinging. The art (and business) of high efficiency solar cells is to maximise the absorbed portion A of Sun light in the overall energy balance A+R+T = 100%, which implies that no light should be escaping a slab of a solar cell engineered to deliver R = 0 and T = 0, hence, aiming at a perfect light trapping with A = 100%. For Sun light entering solar cell from air (from low index into high), reflection is high and is reduced in solar cells by a thin sub-micrometer anti-reflective coating and a random surface roughening (up to tens-of-micrometers) to reduce the portion R of the escaping light. Once light is inside the cell’s medium of high index, it tends to stay inside the high index medium, what is utilised in optical fibers and, e.g., is a cause of mirage phenomenon. The best possible light trapping using multi-reflections inside solar cell defines the ray trapping limit called the Lambertian limit.

In this paper the authors show how to further enhance light trapping beyond the ray optics limit by trapping light with photonic crystal (PhC) periodic patterns. Interference of light which is trapped inside micro-thin Si layer can reach further absorption towards the ideal full absorption limit of A = 100%. Importantly, strong absorption due to interference and light localization enhances light absorption even in micrometers thin solar cells. The principle of PhC light trapping is applicable for other materials and applications where enhancement of light absorption is beneficial, e.g., in wide range of sensors and photo-catalytic applications. Large area direct laser writing is shown to deliver solution for fabrication of light trapping surfaces beyond the currently achievable limit. Laser writing is industrially scalable formacroscopically large area patterning of solar cells and can be carried out in the ambient room conditions.

Research group of Prof. S. Juodkazis established Australia’s first high precision 3D laser machining system based on ultra-short pulse, high average power and high repetition laser at the nanotechnology facility opened in 2011 at Swinburne University of Technology. Direct laser writing is now becoming an increasingly popular 3D computer numerical control (CNC) micro-nano-machining technique when ultra-short sub-1ps pulsed lasers are used. We showed that the average power of ultra-short pulse lasers is doubling every two years from 2000 [Eng. Proc. 2021, 11(1), 44; https://doi.org/10.3390/ASEC2021-11143]. This exponential growth by a Moore’s law like scaling is embraced by applications where fast scanning and tight focusing now can be used together for a large area machining/modification of materials. Both modes of additive and subtractive micro-fabrication become amenable using ultra-short pulsed lasers. It delivers high resolution and nanoscale precision and was used for the fabrication of masks on Si for solar cell patterning. Solar cells which are synonymous with large area application, can benefit from laser texturing by direct laser writing. As compared with standard photolithography or high resolution electron beam lithography techniques, the direct laser writing is more promising due to its simplicity and vacuum free mode of operation. In the next stage of this project, high performance solar cells which currently use only ray trapping will be patterned with PhC structures as introduced in this current study. There is a large room of improvement accessible between the current Si single junction solar cell efficiency approaching to the ray optics (Lambertian) limit of 27% and reach the ultimate limit of ~32% when all Sun’s spectrum absorbed by Si is transferred to the useful electrical power. The proposed PhC texturing can be applied for surfaces of other type of solar cells. The largest breakthrough of such light trapping is expected for thin solar cells of tens of micrometers. Such cells become flexible and small weight which widens application potential for handheld, autonomous, remote, space, etc. applications.

Article reference: Maksimovic J, Hu JW, Ng SH, Katkus T, Seniutinas G et al. Beyond Lambertian light trapping for large-area silicon solar cells: fabrication methods. Opto-Electron Adv 5, 210086 (2022). doi: 10.29026/oea.2022.210086

Keywords: silicon solar cells / laser ablation / light trapping / Lambertian limit.

This project assembled a team to experimentally develop new surface texturing methods for light trapping, which is based on theoretical photonic crystal (PhC) light harvesting approach demonstrated by Prof. Sajeev John (Uni of Toronto). Direct laser writing by ultra-short pulsed laser was applied for surface patterning at Swinburne’s Nanolab (nanotechnology facility) in group of Prof. Saulius Juodkazis (Swinburne Uni of Technol). PhC patterning via projection lithography was made at Nano-Processing Facility in Tsukuba, Japan, in collaboration with Prof. Yoshiaki Nishijima (Yokohama National Uni). The electron beam lithography based approach for PhC trapping surface was developed with team at Melbourne Center for Nanofabrication (MCN). The 3D direct laser writing capability established at Swinburne’s Nanolab is applicable to challenging micro-scale cutting, dicing, drilling fabrication tasks.

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