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Advanced aRchitectures for ultra-thin high-efficiency CIGS solar cells with high Manufacturability

Periodic Reporting for period 2 - ARCIGS-M (Advanced aRchitectures for ultra-thin high-efficiency CIGS solar cells with high Manufacturability)

Période du rapport: 2018-06-01 au 2020-11-30

1.1 Objectives and summary of achievements
The aims of ARCIGS-M have been to further develop the CIGS thin film solar cell technology towards higher efficiency by using novel approaches in passivation, patterning and optical design developed by the partners. Key competences regarding equipment for up-scaling of the patterning and deposition techniques have been identified among the partners that were invited to join the ARCIGS-M consortium. The final target of ARCIGS-M has been to demonstrate a new CIGS photovoltaic (PV) architecture, with increased efficiency, improved reliability and stability, at reduced cost and high potential for new applications and markets, with a focus on building-integrated PV (BIPV).

A strong focus has been on scalability, even at the nano-scale. Here the partnership between academia and our industrial partners has resulted in a patterning process, resulting in 100-200 nm holes in dielectric layers with micrometer pitch that is feasible also on square meter areas.

Sustainability has been another strong objective and our life-cycle analysis shows an advantage in carbon dioxide emissions with the ARCIGS-M technology as compared to other thin film technologies.

ARCIGS-M supports European industry (partners and non-partners) related to photovoltaics, directly by potentially enabling savings in active materials and equipment for production of photovoltaics and indirectly by informing power companies and building companies about the possibilities of using the ARCIGS-M results. The final result may lead to increase of the power generation by photovoltaics and reducing green-house gas emissions.

ARCIGS-M consists of 13 partners from University (Uppsala University, University of Ljubljana, Université Catholique de Louvain), research institutes (imec, INL, CNRS and TNO), SME´s (Obducat AB, Midsummer AB), R&D companies (Solibro Research AB, AC&CS) and large companies (EDF and Arcelor Mittal). Out of 55 research-active persons, 18 (32%) are female (among which, the project coordinator and 2 WP leaders) and 37 (68%) are male.
At the end of the project, we have advanced the status of ultrathin CIGS-based solar cells with a highest efficiency of 16.3 % using an ultrathin (Ag,Cu)(In,Ga)Se2 (ACIGS) absorber layer of down to 0.65 µm in thickness. This solar cell was deposited on a back contact with ITO as the contact layer. To our knowledge, this value is the highest value reported for sub-micrometer CIGS absorbers.

Furthermore, a fully reflecting back contact (RBC) has been developed and patented and demonstrated within the project. About 90 % of the current density for a solar cell with 500 nm thick CIGS was obtained as compared to a solar cell with full CIGS thickness.

By electrical and optical modeling, we have been able to characterize and even predict the results and advances in the ARCIGS-M architecture. The parameters have been published and are accessible for the general public. Modeling has been carried out using both commercially available software, such as Comsol, ATLAS 2D, and Sunshine but also open-source, such as SCAPS-1D.

Passivation of the back contact has led to several approaches for passivation that have been developed and studied in depth. The passivation layer materials include alumina or hafnia deposited by atomic layer deposition (ALD) and silica deposited by plasma-enhanced chemical vapor deposition (CVD) resulting in solar cell record efficiencies of 14.8 %. The passivation layers were patterned to provide holes for contacting by scalable methods, such as optical lithography and nano-imprint lithography. Additionally, the use of gallium grading to provide an electron selective contact has been successful as an alternative or complement to other passivation strategies.

A module prototype architecture design was developed, and preliminary first tests carried out, integrating the special requirements of the RBC. We also worked in parallel with module development using conventional scribing on the innovative steel substrates. The results on modules on steel were to some extent promising with close to 15 % efficiency for full thickness CIGS layers, but we identified some challenges regarding module scribing and the surface .

A full life cycle analysis has been carried out and normalized to one Wp, it shows that the ARCIGS-M glass and steel proceses behave better than the reference case for all environmental indicators and thereby a better ecoprofile. The economical assessment shows a limited competitiveness, manly due to the extra cost for the passivation layer steps.

A homepage was launched at an early stage of the project, including filmed interviews of the partners. Daily twitter feeds report about news from the photovoltaics world and from the project. Four scientific papers have been published with results from ARCIGS-M. All papers from ARCIGS-M are gathered at the digital library DiVA at Uppsala University.
In the list below the key results are listed according to partner involvement:

• A low cost steel substrate with isolation coating and demonstrated module performance suitable for thin film solar cell modules and demonstrated for CIGS with a module efficiency of 15 % (AM, AC&CS, SOL)
• Three innovative passivation layer materials suitable for thin film solar cells and demonstrated for CIGS with efficiencies of up to 15 % for 500 nm ACIGS thickness (INL, UU, TNO)
• Nano-imprint lithography demonstrated as a scalable patterning technology, also on steel substrates and for scattering patterns (OBD, INL, TNO, CNRS)
• A patented reflective multilayer structure, able to withstand temperatures above 500 °C during vacuum processing and giving expected current density improvements for ultrathin CIGS according to modelling, resulting in >90 % of the current as compared to fully thick absorberas and with at best 15 % efficiency. (CNRS, UL, UU)
• State-of-the-art processes for fabrication of sub-micron CIGS using sputtering (MS) and for fabrication of submicron CIGS and ACIGS (UU, TNO)
• CIGS and ACIGS processes adapted to minimize parasitic reactions between ACIGS and ITO (ITO is an essential part of the RBC). 16.3 % efficiency achieved for 0.65 µm thick absorber on an ITO-based substrate. This is to the best of our knowledge a world record for a submicron CIGS-based solar cell.
• Optoelectronic modelling based on real experimental data with high predictive power for the architectures in ARCIGS-M, but also general for thin film solar cells with low thickness. (UCL, UL, CNRS)
• Advanced electrical and optical characterization methods combined with modelling to get in-depth understanding of device performance and for failure analysis (UCL, IMEC, UU, UL)
• Modelling and simulations for internal textures analyzed (for further work) External light management foil was used to validate this modelling and led to a demonstrated 6 % current density gain for UT-CIGS (UL, CNRS)
• A life cycle analysis for the ARCIGS-M architecture, but using an approach and input relevant for all thin film technologies. The LCA showed lower environmental impact as compared to conventional CIGS module technology (ALL)
• A consolidated network of experts in thin film technology (ALL)
• High visibility for ARCIGS-M at major international conferences (ALL)
• High impact scientific results in UT-CIGS (ALL)