Roll-processing of organic solar cells with optimal morphology

Marcial Fernández Castro

Research output: Book/ReportPh.D. thesis

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Abstract

Solar energy is by far the most abundant renewable energy source on Earth. In recent years, an urgent need for a green energy transition has appeared due to the rapid increase of the global population and energy demand. Fossil fuels are not only limited but also the main cause of global warming, which may lead to a climate crisis if not correctly addressed in time. It is our responsibility to future generations to find and work on solutions for this green transition to happen.
Organic Solar Cells (OSCs) can convert sunlight into electricity using only organic materials, usually mixed in a so-called bulk heterojunction (BHJ). OSCs presents several advantages, such as low cost, lightweight, flexibility, semitransparency, easy processing, and compatibility with roll-to-roll (r2r) manufacturing. r2r is a continuous fabrication technique, similar to the conventional industrial printing process for newspapers, that leads to far shorter Energy Payback Times (EPBTs) for solar cells. Due to this, r2r fabrication is a promising candidate for fabrication techniques in the context of a necessity for a fast global green energy transition.
Recently, the efficiency of OSCs has skyrocketed above 18%, mainly due to the development of novel non-fullerene acceptors (NFAs). The energy levels of these NFAs can be fine-tuned through molecular design, allowing modifications to their light-harvesting properties (bandgap) to match their energy levels with their donor counterparts. However, these champion efficiencies have been obtained in small-area lab-scale devices fabricated through the spin-coating technique in a controlled environment. Large-area OSCs still suffer from a considerable efficiency loss when upscaling, especially when the fabrication is done using r2r compatible techniques, such as slot-die coating, in open-air conditions.
The goal of this Ph.D. project is to transfer the reported lab-scale champion solar cell performances to scalable fabrication techniques, closing the so-called “scalability lag”. Therefore, during this journey I was responsible for finding new and innovative solutions to enhance the photovoltaic performance and to control the material properties on the nanoscale, using only up-scalable methods. This required the manipulation of drying kinetics, using different solvent mixtures, the control of drying temperature, coating parameters, and other processing steps, such as post-annealing treatment or encapsulation.
During this project, not only the BHJ of the devices was carefully optimized, but also different interfacial layers and electrodes were used and studied. Moreover, not only photovoltaic performance was considered, but also the stability of the devices was studied as another important step towards the industrialization of the technology.
It is worth mentioning that organic materials and solar energy can also be used without electricity being involved. This is the case of the Molecular Solar Thermal Energy Storage(MOST), where the energy from the Sun is stored in organic molecules as chemical energy, to then be released as thermal energy through a thermal back conversion. MOST was the focus of my External Stay research at the Chalmers University, in Gothenburg.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherDTU Energy
Number of pages256
Publication statusPublished - 2022

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