Periodic Reporting for period 1 - UL-Flex-Cell (High-performance, ultra-light flexible CIGS Solar Cell)
Reporting period: 2021-02-16 to 2023-02-15
Photovoltaic (PV), a sustainable and renewable source of energy, is the best alternative to suppress the emission of greenhouse gases and limit global warming. The cost of PV systems is dropping continuously, and their energy generation price is not affected by future fuel price increases
Among different thin-film PV technologies, the compound semiconductors of the family I-III-VI2, Cu(In,Ga)Se2 (CIGS), exhibit higher power conversion efficiency. CIGS is a polycrystalline direct bandgap material with tunable bandgap between 1.04eV and 1.68eV (depending on gallium concentration), a high absorption coefficient in the visible spectrum, a thin absorber thickness that is 1/100 of that of silicon (Si) solar cells, and excellent radiation hardness. In comparison to Si solar cells (which dominate the present market), they are lighter, significantly cheaper to produce, and can be made flexible. Therefore, I proposed to develop high-efficiency, flexible single-junction CIGS solar cells using the innovative combination of pulsed hybrid reactive magnetron sputtering (PHRMS) and laser pulses.
PHRMS is a world-unique, single-step deposition process in which the amount of selenium (Se) in the deposition chamber is precisely controlled, while simultaneously sputtering a Cu-In-Ga thin film. Additionally, with controlled Se pulses, the atomic mobility of anion and cation species was promoted to enhance phase transformation.
Among different thin-film PV technologies, the compound semiconductors of the family I-III-VI2, Cu(In,Ga)Se2 (CIGS), exhibit higher power conversion efficiency. CIGS is a polycrystalline direct bandgap material with tunable bandgap between 1.04eV and 1.68eV (depending on gallium concentration), a high absorption coefficient in the visible spectrum, a thin absorber thickness that is 1/100 of that of silicon (Si) solar cells, and excellent radiation hardness. In comparison to Si solar cells (which dominate the present market), they are lighter, significantly cheaper to produce, and can be made flexible. Therefore, I proposed to develop high-efficiency, flexible single-junction CIGS solar cells using the innovative combination of pulsed hybrid reactive magnetron sputtering (PHRMS) and laser pulses.
PHRMS is a world-unique, single-step deposition process in which the amount of selenium (Se) in the deposition chamber is precisely controlled, while simultaneously sputtering a Cu-In-Ga thin film. Additionally, with controlled Se pulses, the atomic mobility of anion and cation species was promoted to enhance phase transformation.
The project has achieved some of its objectives and milestones
• Objective #1. Development of low-temperature growth of CIGS by a pulsed selenization process of simultaneously sputtered metal compound Cu-In-Ga.
Without using a rotating substrate holder during the CIGS deposition at low substrate temperature, a uniform process was developed that minimised CIGS alloys' gradients in 5 cm Χ 5 cm soda-lime glass (SLG). This achievement enables the deposition of CIGS thin film on flexible substrates.
• Objective # 2. Controlled alkali doping of the CIGS thin film.
A new method of incorporating heavy alkali metal (especially cesium) was disclosed to improve the efficiency of CIGS thin film solar cells. As a typical example, the efficiency of CIGS solar cell developed using the PHRMS system was improved by > 33 % using the newly developed solution process.
• Objective # 3. Se-free annealing of Cs-treated CIGS thin film and solar cell performance
The effect of Se-free annealing of Cs treatment on CIGS thin film was investigated. It is disclosed that annealing of CsF-treated CIGS thin films changes the surface morphology and the chemical composition, and reduces the Urbach energy. However, the full benefit of the reduced Urbach energies of the annealed samples is not obtained because of an increased buffer/CIGS interface recombination as a consequence of re-evaporation of the alkali-containing layer from the surface region upon annealing.
• Objective #1. Development of low-temperature growth of CIGS by a pulsed selenization process of simultaneously sputtered metal compound Cu-In-Ga.
Without using a rotating substrate holder during the CIGS deposition at low substrate temperature, a uniform process was developed that minimised CIGS alloys' gradients in 5 cm Χ 5 cm soda-lime glass (SLG). This achievement enables the deposition of CIGS thin film on flexible substrates.
• Objective # 2. Controlled alkali doping of the CIGS thin film.
A new method of incorporating heavy alkali metal (especially cesium) was disclosed to improve the efficiency of CIGS thin film solar cells. As a typical example, the efficiency of CIGS solar cell developed using the PHRMS system was improved by > 33 % using the newly developed solution process.
• Objective # 3. Se-free annealing of Cs-treated CIGS thin film and solar cell performance
The effect of Se-free annealing of Cs treatment on CIGS thin film was investigated. It is disclosed that annealing of CsF-treated CIGS thin films changes the surface morphology and the chemical composition, and reduces the Urbach energy. However, the full benefit of the reduced Urbach energies of the annealed samples is not obtained because of an increased buffer/CIGS interface recombination as a consequence of re-evaporation of the alkali-containing layer from the surface region upon annealing.
A uniform deposition of CIGS thin film using the PHRMS system at low substrate temperature was demonistrated. Low-temperature CIGS growth is of utmost importance for the fabrication of flexible (and building-integrated) PV. Additionally, the developed solution method of heavy alkali metal treatment secures the low-cost post-treatment of CIGS thin film to improve the performance.