Periodic Reporting for period 1 - SITA (Stable Inorganic TAndem solar cell with superior device efficiency and increased durability)
Période du rapport: 2022-09-01 au 2024-02-29
SITA aims to explore an innovative concept for tandem solar cells with a 2-terminal (2T) approach based on two technologies with strong competence base in Europe: Silicon Heterojunction (SHJ) and high bandgap Cu(In,Ga)(Se,S)2 (CIGS).
The novel tandem concept – that requires no additional cables or electronics - is enabled by recent developments in wide-gap CIGS devices and the SITA project goal is to reach 18% solar-to-electric power conversion efficiency for the CIGS-based top cell. SITA will demonstrate the durability of the new modules under realistic outdoor conditions delivering the next generation of stable inorganic tandem solar modules with superior device efficiency with the goal of reaching up to 30 %.
A key challenge, where the development has to exceed state-of-the-art, is to demonstrate a high bandgap CIGS-based top cell (>1.5 eV) with high transparency in the CIGS sub-bandgap region, combined with the 18 % efficiency. Another challenge is to match the top and the bottom cells with minimized opto-electronic losses.
SITA’s tandem modules, combining high bandgap CIGS-based top cells with SHJ bottom cells have potential to enable module efficiencies of 30 %, with only a small increase in the bill of materials (BOM). This, in turn, leads to a considerable reduction in area-related system costs of up to 25% per installed power and a corresponding decrease in the levelized cost of electricity (LCOE).
Tandem-junction efficiencies have recently approached or even surpassed the single-junction Shockley-Queisser limit for prototype devices. SITA will address the remaining limitations in terms of stability, scaling, manufacturing costs, and environmental impact.
The novel tandem concept – that requires no additional cables or electronics - is enabled by recent developments in wide-gap CIGS devices and the SITA project goal is to reach 18% solar-to-electric power conversion efficiency for the CIGS-based top cell. SITA will demonstrate the durability of the new modules under realistic outdoor conditions delivering the next generation of stable inorganic tandem solar modules with superior device efficiency with the goal of reaching up to 30 %.
A key challenge, where the development has to exceed state-of-the-art, is to demonstrate a high bandgap CIGS-based top cell (>1.5 eV) with high transparency in the CIGS sub-bandgap region, combined with the 18 % efficiency. Another challenge is to match the top and the bottom cells with minimized opto-electronic losses.
SITA’s tandem modules, combining high bandgap CIGS-based top cells with SHJ bottom cells have potential to enable module efficiencies of 30 %, with only a small increase in the bill of materials (BOM). This, in turn, leads to a considerable reduction in area-related system costs of up to 25% per installed power and a corresponding decrease in the levelized cost of electricity (LCOE).
Tandem-junction efficiencies have recently approached or even surpassed the single-junction Shockley-Queisser limit for prototype devices. SITA will address the remaining limitations in terms of stability, scaling, manufacturing costs, and environmental impact.
The work within SITA is divided into six work packages. The work during the 18 first months of SITA has aimed to reach the goals with a combination of experiments, characterization and theory and with close collaboration between the 14 academic and industrial partners.
During SITA period 1, the consortium has met in four on-site consortium meetings (with hybrid possibility for persons not able or wanting to travel), approximately every six months. The consortium members have met in on-line WP meetings approximately every two months and the WP leaders have met with about the same frequency to summarize progress and plan for actions as part of WP1. A HAL repository has been set up for journal articles and for data-sets. Several abstracts from the consortium have been accepted for major conferences and a dedicated session for solar cells on transparent contacts is planned for the E-MRS fall meeting with contributions and co-arranged by SITA.
In WP2, which deals with novel high bandgap top cells, the research aims to provide top cells with bandgaps higher than 1.5 eV and with a transparency of 80% below the absorption edge. We aim for an efficiency of 18%, which is required to reach the goal of a 30% efficient tandem device with a silicon bottom cell. The progress towards the goal was initially slower than expected, but recently has taken on a strongly increasing trend regarding efficiency as well as transparency. At the time of deliverable D2.1 in month 18, we did not reach the 17% goal, but could demonstrate an 11% efficient cell with the required transparency and bandgap. This has recently been increasing to 13% and we expect the efficiency to catch up with the targets. We are now on a steep learning curve based on insights in interface and processing methods, achieved by successful collaboration mainly between WP2, WP3 and WP4.
Transparent back contacts for CIGS were intensively researched and novel materials were tested in WP3. Structuring of cells to modules on these new transparent back contacts was newly developed resulting in a Si bottom cell with above 20% efficiency and with a shingle design. First widegap CIGS modules on transparent back contacts were demonstrated. The development of silicon bottom modules for the Circuitry 2T concept was completed. Initial experiments on the Bonded 2T concept have been performed. WP3 is within time planning.
By smart combination of standard and advanced analytics of materials and interfaces as well as optical, electrical, and outdoor characterization of corresponding devices and modelling of their performance, WP4 will continue in period 2 to provide the required insights to establish the structure-property-function relationships, that are a crucial pre-requisite for a knowledge-based optimization of materials and interfaces. Ultimately tandem solar cell layer stacks will show the potential of the developed tandem technology when employed in the field. Computer models for the two concepts Bonded 2T and Circuitry 2T have been developed to aid in determining the optimal thickness and bandgap for the CIGS top module. A testing site for stability assessment at ZSW in southern Germany has been established, equipped for high-accuracy testing of small-area devices.
The main work in WP 5 is planned for period 2, but preparatory work has been carried out, where the complete process flows for the SITA tandems have been agreed upon and discussed. Partner VITO, responsible for the LCA work, has been participating in the Consortium meetings and on-line WP meetings in order to pick up topics to be discussed in the LCA. All partners have also been assigned to consider the LCA aspects and sustainability aspects in all processes.
During SITA period 1, the consortium has met in four on-site consortium meetings (with hybrid possibility for persons not able or wanting to travel), approximately every six months. The consortium members have met in on-line WP meetings approximately every two months and the WP leaders have met with about the same frequency to summarize progress and plan for actions as part of WP1. A HAL repository has been set up for journal articles and for data-sets. Several abstracts from the consortium have been accepted for major conferences and a dedicated session for solar cells on transparent contacts is planned for the E-MRS fall meeting with contributions and co-arranged by SITA.
In WP2, which deals with novel high bandgap top cells, the research aims to provide top cells with bandgaps higher than 1.5 eV and with a transparency of 80% below the absorption edge. We aim for an efficiency of 18%, which is required to reach the goal of a 30% efficient tandem device with a silicon bottom cell. The progress towards the goal was initially slower than expected, but recently has taken on a strongly increasing trend regarding efficiency as well as transparency. At the time of deliverable D2.1 in month 18, we did not reach the 17% goal, but could demonstrate an 11% efficient cell with the required transparency and bandgap. This has recently been increasing to 13% and we expect the efficiency to catch up with the targets. We are now on a steep learning curve based on insights in interface and processing methods, achieved by successful collaboration mainly between WP2, WP3 and WP4.
Transparent back contacts for CIGS were intensively researched and novel materials were tested in WP3. Structuring of cells to modules on these new transparent back contacts was newly developed resulting in a Si bottom cell with above 20% efficiency and with a shingle design. First widegap CIGS modules on transparent back contacts were demonstrated. The development of silicon bottom modules for the Circuitry 2T concept was completed. Initial experiments on the Bonded 2T concept have been performed. WP3 is within time planning.
By smart combination of standard and advanced analytics of materials and interfaces as well as optical, electrical, and outdoor characterization of corresponding devices and modelling of their performance, WP4 will continue in period 2 to provide the required insights to establish the structure-property-function relationships, that are a crucial pre-requisite for a knowledge-based optimization of materials and interfaces. Ultimately tandem solar cell layer stacks will show the potential of the developed tandem technology when employed in the field. Computer models for the two concepts Bonded 2T and Circuitry 2T have been developed to aid in determining the optimal thickness and bandgap for the CIGS top module. A testing site for stability assessment at ZSW in southern Germany has been established, equipped for high-accuracy testing of small-area devices.
The main work in WP 5 is planned for period 2, but preparatory work has been carried out, where the complete process flows for the SITA tandems have been agreed upon and discussed. Partner VITO, responsible for the LCA work, has been participating in the Consortium meetings and on-line WP meetings in order to pick up topics to be discussed in the LCA. All partners have also been assigned to consider the LCA aspects and sustainability aspects in all processes.
A key performance indicator for tandem solar cells is long term stability. The final outcome of this project will contribute to the development of a long term stable tandem solar cell technology suitable for applications in e.g. buildings or in other applications with requirements of high efficiency and high stability. The on-going work on fundamental limitations and computer modeling based on real datasets and samples will help developing this innovative technology further. A competitive strength of the SITA project is its close collaboration between the academic and the solar industry partners (silicon and thin film), which will enable a fast take-up of the technologies. Our prototypes will be the first of their kind, paving the way for further improvements and innovations.