Periodic Reporting for period 1 - ATACAMA (Affordable TAndem Cell Architecture by Multi-thin-epitaxy Approach)
Okres sprawozdawczy: 2024-05-01 do 2026-04-30
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.
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.
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.
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.
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.
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.