Advanced Characterization of Interfaces of the Chalcogenide-Based Absorbers for the Thin Film Photovoltaic Technologies

Robert Fonoll i Rubio

Guc, Maxim Izquierdo Roca, Víctor

Materials Characterization Laboratory (IREC) Materials and Systems for Solar Energy (IREC)

The doctoral thesis "Advanced Characterization of Interfaces of the Chalcogenide-Based Absorbers for the Thin Film Photovoltaic Technologies", by Robert Fonoll i Rubio (defended in March 2023), was carried out at the Institute for Energy Research of Catalonia (IREC) within the research group "Materials and Systems for Solar Energy (SEMS)", located in Sant Adrià de Besòs, within the framework of the research line of "advanced characterization of materials and electronic devices". Photovoltaic energy is one of the pillars for the energy transition. Within photovoltaic technologies, thin-film technologies represent an interesting evolution that allows the integration of solar energy in a multitude of scenarios (buildings, vehicles, agriculture, etc.) and products (electronic devices, IoT, wearables, etc.) thanks to their adaptability. However, these technologies are formed by stacking layers of various materials that involve a complex structure and manufacturing. In this context, the development of methodologies for obtaining physicochemical information on the materials and structures that form thin-film photovoltaic technologies is fundamental for their development and industrialization. However, the complexity and degree of precision required for the characterization of the interfaces of these solar cells mean that this field has not been explored in depth, so the development of advanced methodologies for the analysis of interfaces in thin-film photovoltaic technologies is of great relevance. With this motivation, the objective of the thesis has been twofold: 1) To develop non-destructive spectroscopic tools for the rapid characterization of the interfaces of photovoltaic materials and adaptable to the industry as process control techniques. 2) The application of these tools to identify physicochemical mechanisms of efficiency loss that occur at the interfaces of thin-film solar cells. The thesis has focused on chalcogenide photovoltaic technologies such as Cu(In,Ga)Se2 (CIGS) and Cu2ZnSn(S,Se)4 (CZTSSe) within the framework of different national and international projects, such as In4CIS and MasterPv (Solar-era.net), Solar-Win (H2020), or Platform-ZERO (HE), however, due to the great flexibility of the methodologies developed during the thesis, its adaptation to other materials and technologies has been explored, such as the case of materials derived from the graphene family (LESGO, H2020) for energy storage applications, or the characterization of advanced plastic materials such as PVB (SUNRISE, H2020) for their reuse and efficient recycling. As a result of the thesis, the scientific work carried out has allowed the publication of 6 articles in high impact journals with a total of 87 citations, as well as the manufacture of 3 semi-industrial prototype systems for online industrial inspection based on methodologies and systems developed in the thesis which have been installed and tested under industrial conditions, allowing their potential industrial transfer to be validated.

The application of the results obtained by Dr. Fonoll i Rubio during the development of his doctoral thesis and their transference to the industry are demonstrated by the real products produced in the several national and international projects that involved his work. All the following projects employ the characterization methodologies developed by Dr. Fonoll i Rubio to design and implement prototype systems for industrial process monitoring applications, or they employ the fundamental research of CIGS and kesterite PV technologies performed during the thesis to develop new commercial applications for these technologies. In4CIS — New in-line optical methodologies for advanced assessment of CIGS industrial processes (Solar-era.net, PCI 2019-111837-2) In4CIS project, which used a technology with TRL 5, aimed to establish and demonstrate at pre-industrial level optical advanced methodologies for the in-line assessment of advanced CIGS processes. These were applied to the monitoring of innovative postdeposition treatments that are developed for the production of very high efficiency CIGS devices, in order to ensure a successful transfer of these CIGS process concepts from cell (lab) level to a pre-industrial module level. This transfer requires for a detailed assessment of their uniformity when up-scaled to module device dimensions, which implies the need for high sensitivity tools and methodologies for the advanced assessment at in-line monitoring level of the uniformity of the surface region of the CIGS absorbers after deposition of the buffer layers in the device structure. Availability of suitable in-line monitoring tools is critical to ensure high throughput/high yield industrial processes as required for competitive CIGS PV production lines. Dr. Fonoll i Rubio, in the framework of both In4CIS project and the doctoral thesis, developed an optical methodology based in the use of 638 nm (red) excitation Raman scattering and PL techniques for the advanced non-destructive quantitative assessment of the quality of the CIGS surface absorber region at early process steps in the CIGS production line. This methodology was applied in an in-line process monitoring tool that was developed by IREC and the Spanish company Lenz and that was installed at the CIGS pre-industrial pilot line of the German institute ZSW. MasterPV — Innovative manufacturing solutions for cost-efficient semitransparent BIPV (Solar-era.net, PCI 2018-092945) MasterPV, with a technology at TRL 5, proposed the development of low cost innovative processes for cost efficient semi-transparent CIGS building-integrated photovoltaics (BIPV) solutions. The project involves the replacement of the Mo back contact in the traditional CIGS device architecture by transparent conductive oxide electrodes, which will allow achieving a significant improvement in the aesthetic quality of the semi-transparent devices, with the elimination of the back mirror effect that is determined by the remaining Mo regions in the modules. Improvement of the aesthetic quality of CIGS devices is strongly relevant to ensure a higher level of acceptance of these solutions in the BIPV market. Solar-Win — Next generation transparent solar windows based on customized integrated photovoltaics (H2020, 870004) Solar-Win project aimed to develop and commercialize a new transparent solar cell that will give windows in homes and other buildings the ability to generate electricity while still allowing people to see outside. The technology will be compatible with window manufacturing technologies and its lifespan will be the same as standard windows. The new smart window will cost 30 % more and will deploy solar energy in a non-intrusive way. This project resulted in the development by IREC and the Spanish company Lenz of a new optical system for the industrial inspection of the roll-to-roll manufacturing processes of flexible photovoltaic devices that was installed at the Austrian company Sunplugged,. This tool is based in the use of 532 nm (green) excitation Raman scattering and PL techniques, a methodology developed applying the knowledge obtained during the doctoral thesis of Dr. Fonoll i Rubio. LESGO — Light to Store chemical Energy in reduced Graphene Oxide for electricity generation (H2020, 952068) LESGO project seeks to store energy in the C-H bond of reduced graphene oxide, whose advantages include safe storage, easy transportation, an energy density more than one hundred times that of H2 gas, and no CO2 emissions during the electricity generation process. This will promote an environmentally friendly and affordable way to supply on-demand energy wherever it is required. This system includes an in-line inspection tool developed by IREC that performs real-time Raman spectroscopy measurements of the liquid solution of reduced graphene oxide that flows through the system during the hydrogenation process, and it applies a methodology developed by Dr. Fonoll i Rubio to evaluate the hydrogenation of the reduced graphene oxide by analyzing variations in the measured Raman spectra of the graphene oxide.

Doctoral Thesis; Advanced Characterization; Interfaces; Chalcogenide-Based Absorbers; Thin Film Photovoltaic Technologies; Robert Fonoll i Rubio; Institute for Energy Research of Catalonia (IREC); Materials and Systems for Solar Energy (SEMS); Sant Adrià de Besòs; Advanced Characterization of Materials; Electronic Devices; Photovoltaic Energy; Energy Transition; Thin-film technologies; Solar Energy Integration; Buildings; Vehicles; Agriculture; Electronic Devices; IoT; Wearables; Complex Structure; Manufacturing; Physicochemical Information; Materials; Structures; Development Methodologies; Industrialization; Precision; Efficiency Loss; Physicochemical Mechanisms; Non-destructive spectroscopic tools; Rapid characterization; Process control techniques; Cu(In,Ga)Se2 (CIGS); Cu2ZnSn(S,Se)4 (CZTSSe); National Projects; International Projects; In4CIS; MasterPv (Solar-era.net); Solar-Win (H2020); Platform-ZERO (HE); Graphene family materials; LESGO (H2020); Energy storage applications; Advanced plastic materials; PVB (SUNRISE, H2020); Reuse; Efficient recycling; High impact journals; Citations; Semi-industrial prototype systems; Online industrial inspection; Industrial conditions; Industrial transfer.