Affordable TAndem Cell Architecture by Multi-thin-epitaxy Approach

Project Information

ATACAMA

Grant agreement ID: 101106002

DOI

10.3030/101106002

Project terminated on 4 April 2025

EC signature date 21 April 2023

Start date 1 May 2024

End date 30 April 2026

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 211 754,88

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - ATACAMA (Affordable TAndem Cell Architecture by Multi-thin-epitaxy Approach)

Reporting period: 2024-05-01 to 2026-04-30

Summary of the context and overall objectives of the project

Climate change is one of the greatest challenges of our time. It is essential to drastically reduce greenhouse gas emissions. While coal (800-1200 gCO2eq per kWh), oil (800-900 gCO2eq per kWh), and natural gas (600-750 gCO2eq per kWh) release considerable amounts of greenhouse gases, photovoltaics (PV) can lower this value to below 20 gCO2eq per kWh. PV systems will need to be widely distributed to be able to replace the enormous (and growing) amounts of conventional electrical energy. Availability of surface areas allocated to PV, preferably close to the end use of the electricity, will become an issue. Reliability and a high energy yield per area will also become important factors for the success of PV products. Europe presents higher complexity than other regions because of its limited space and high-density population.

Current Silicon technology is basically limited by its single junction (one semiconductor bandgap) to efficiencies below 30%. Multi-junctions/Tandems using more than one semiconductor diode (aiming at different energies in the spectrum) are the demonstrated path to overpass this limit and achieve efficiencies close to 40%, as done for three junctions (3J) on III-V and with 35% achievable by using a Si bottom junction (3J GaInP/GaAs/Si). Two Hybrid Tandems technologies have emerged as promising to upgrade the current Si technology: perovskites/Si and III-V/Si. III-V/Si presents the highest efficiency and stability but two orders of magnitude higher cost.

The ATACAMA project will develop an affordable tandem cell architecture by dramatically lowering the cost of the top junction for a III-V/Si tandem (AlGaAs/Si) using: (1) ultra-thin absorbers, (2) substrate multi- epitaxial use, and (3) maintaining a cost-effective high-efficiency (>30%). Nevertheless, the tandem must ensure circularity (recyclable) to split the Si from the III-V. The goal of the project is to glue with transparent polymers, with a melting point of 200 °C, ensuring the possibility of easily recycling in the future the AlGaAs cell from the Si cell.

We estimate ATACAMA will reduce the cost by a ten-fold in the near-term (multi-thin-epitaxy of 10 cells is comparable to moderate-high growth rate); opening markets for III- V/Si on high-density applications, and opening a novel path for lowering the cost to reach competitive €/W with Si.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The action has ended early, only 8 months. We have performed the development of the work package one and two as described in the Gantt Chart.

The WP1: Simulation was centered on the development of the photonic crystal towards ultrathin III-V/Si tandems. The progress was oriented towards thinning the Si cell as the III-V cells were already thinned down during the initial tests with GaAs. Si cells of only 12 µm in thickness were simulated using rigorous coupled wave analysis, as a single junction and as a tandem made of III-V/Si. We reached current densities of 37 mA/cm2 for the single junction, and demonstrated current-matching with a top III-V AlGaAs solar cell. Finally, a tandem cell with 500 nm AlGaAs and 12 µm Si with double nanostructured was simulated. We observed the more favorable device design is the 2 µm AlGaAs and 12 µm Si.

The WP1: Fabrication was done through polymer blend lithography and dry plasma etching. We explored different approaches using polystyrene (PS), Poly(methyl methacrylate) (PMMA), and with resists (AZ/NLOF-2070). We used Propylene glycol methyl ether acetate (PGMEA) as a common solvent. The optimization was done by adding plasma oxygen, changing the dilution ratio, and spin-coating speed. The patterns realized during the action were bigger than expected (~2 µm), and the process performed was under optimization towards reaching the average size proposed by the simulation (0.85 µm).

The WP2 was centered on developing the epitaxial lift-off through photo-chemical etching. The setup was developed prior to the action through the internship of a student. The initial steps were performed with satisfactory results when combining 850 nm light and HNO3. The solution ratio was optimized to minimized dark etching. The process to perform the lift-off was under optimization, with surface breakage and small pieces of max 1x1 mm2 by the end of December 2024.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The simulation of the Si structures is very promising, achieving through simple design rules current densities of an ultrathin Si cell (12 µm) comparable to a standard thick cell (150 µm). The fabrication process through polymer blend was done in the past for III-V and Si through dry etching; nevertheless, future optimization for ultrathin Si cells (with the challenges on size and device compatibility) is fundamental to achieve this type of structure.

The photochemical lift-off is in its infancy, with very small areas so far, but the technology is feasible, and we are currently filling a patent for it, as it can be later used for commercialization.