Deliverable D2.5: Report on modulated surface texture concepts​

Partner: TUDelft

Executive summary:

The study focuses on the process development and evaluation of two innovative modulated surface texturing (MST) strategies, developed to enhance light trapping and reduce reflection losses—key factors in improving solar cell efficiency. This report presents a detailed optical analysis of reflectance-induced current losses in silicon solar cell structures incorporating advanced micro- and nanostructured surfaces.  

Reflectance losses were categorized into front surface reflection (dominant at wavelengths below 800 nm) and secondary escape losses (relevant above 800 nm). Measurements were conducted on MST black silicon (b-Si), MST white silicon (w-Si), and standard random pyramids textured (RPT) reference samples, both with and without dual-layer antireflection coatings (DARC). 

Among the tested structures, MST w-Si demonstrated the best optical performance, with the lowest overall reflectance losses. It achieved an implied photocurrent of 44.45 mA/cm², outperforming all other samples. MST b-Si also showed improvement over uncoated RPT, especially in front reflection losses (~2.5 mA/cm²), although its advantage was reduced when DARC was applied to the RPT, which then slightly surpassed MST b-Si. 

Notably, the implied photocurrent of MST w-Si is within 0.2 mA/cm² of the theoretical Green limit for 260 μm thick crystalline silicon. Since the calculation excludes parasitic absorption at wavelengths above 1100 nm, this small offset validates the effectiveness of the surface texturing approach. Overall, MST w-Si proves capable of achieving 99% of the Green limit, confirming its value as a high-efficiency light management strategy. 

The report addresses the following key performance indicators: 

• Development of two full-area modulated surface texturing (MST) concepts: 

• Black silicon (b-Si) via reactive ion etching (RIE) 

• White silicon (w-Si) via chemical etching, PECVD, and selective etching 

• Demonstration of MST w-Si achieving implied photocurrent within 1% of the Green limit for 260 μm crystalline silicon, fulfilling the project’s optical performance target. 

 

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Deliverable D2.3: Report on optical characteristics of differently prepared photonic crystal structures for light trapping

Partner: LUH MBE

Executive summary:

This deliverable gives an overview about the optical properties of different light trapping (LT) schemes, based on periodic photonic-crystal structures which are theoretically and experimentally investigated in the BURST project. This report serves as a basis for the updated report expected at M30.

Informed by the modelling studies of WP2, two types of periodic micro-structures have been developed and tested, for integration in the silicon front surface: (i) regular inverted pyramids and (ii) metal oxide based photonic coatings. The aim is to attain maximum light confinement (reaching the Lambertian limits) in c-Si absorbers with reduced thickness, at the level of the best-performing random texturing processes, while preventing electrical degradation from the recombination effects caused by increased surface roughness.

To improve the areal yield and quality of the regular inverted pyramids, acting as photonic crystals, different pathways for achieving such are presented. Vacuum contact lithography and direct laser writing seem to be viable preparation methods for obtaining regular structures on larger areas (>2 cm x 2 cm) compared to conventional photolithography. Thus, photonic crystals with a total reflectance close to a random pyramid textured surface are achieved. Additional process improvements, namely pre-etching with reactive ion etching and a process called oxide sharpening, both not fully optimised so far, are applied to further reduce the overall reflectance down to that of a random pyramid textured surface (~10 % @ 600 nm w/o ARC). Another approach for the preparation of LT structures that is pursued in the BURST project is by photonic coatings, consisting of periodic honeycomb arrays of semispheroidal nanovoids on a dielectric film, e.g. TiO2 (and/or AZO). Such structures offer the benefits of an easy and high-quality passivation (due to the planar silicon surface) combined with excellent light in-coupling and scattering. A simulation-based approach is used to optimise the LT effects of these structures with special focus on very thin silicon wafers. Combined optical and electrical simulations are used to model the resulting implied photocurrent and adjust the structure geometry to increase the photocurrent, approaching the Lambertian limit.

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Deliverable D3.1: Report on optical and passivation potential of front-side layers developed for front-surface of the IBC device​

Partner: CEA INES

Executive summary:

In this period (M12), both low-temperature (CEA, TUD) and high-temperature (ISFH) passivation approaches are developed. CEA developed very promising a-SiOx:H, nc-SiOx:H and nc-Si:H layers in terms of high i-V_oc (> 750 mV) and low J_o (< 1 fA/cm²), whereas TUD developed aSi:H/SiN_x/SiO_x multi-layer with outstanding effective carrier minority lifetimes (> 24 ms). However, both low-temperature approaches are lacking the desired optical transparency (in other words higher J_sc loss than targeted) due to absorbant a-Si:H layers deposited for chemical passivation. On the other hand, high-temperature passivation approach developed by ISFH showed no absorption loss in the 300-1200 nm range thanks to transparent ALD-grown AlO_x passivation layers, but with lower passivation properties. It is concluded that utilizing AlO_x layers on thinner wafers has the potential to reach the targeted KPIs.

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Deliverable D4.2: Description of the fabrication processes for a high efficiency solar cell including BURST building blocks​

Partner: ISFH

Executive summary:

This document is written by ISFH, as leader of deliverable 4.2 (Description of the fabrication processes for a high efficiency IBC (Interdigitated Back Contacted) solar cell including BURST building blocks). Its purpose is to describe the fabrication process for the BURST baseline solar cell that has been developed in the first half of the project, including processes for integrating BURST’s building block technologies into these solar cells, taking into account all specifications and requirements that were listed in deliverable 4.1.

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