PhD projects

Over two decades, an enormous amount of effort has been made to improve the efficiency and lifetime of organic solar cell single junction and tandem devices. With the emergence of non-fullerene acceptors and high mobility polymer donors, the efficiency of devices has improved significantly, and their unique property of selective tuning of bandgap helped to achieve highly efficient tandem solar cells. The state of art, small area - spin coated, organic solar cell single junction efficiency reached 19% and two terminal tandem junction efficiency reached 20%. The main bottleneck of commercialisation of this technology is of upscaling the small devices from laboratory compatible processes to industry compactable roll-to-roll processes and achieving high efficiency tandem solar cells and high lifetime.

This Ph.D. project focuses on roll-to-roll printed tandem solar cells mainly using energy converting organic semiconducting materials. The focus is on using industry compatible printing techniques like slot die, flexography, gravure, and sputtering, to coat the different organic solar cell layers. Different tandem configurations, including two-terminal and four-terminal, will be explored to print highly efficient tandem solar cells. In all cases environmental-friendly solvents will be explored and printing processes developed. A comprehensive study of morphological and corresponding photo-electrical properties will be conducted and analyzed.

Sheet-to-sheet slot die coating technique will be used to print various layers of organic photovoltaic (OPV) devices. Coating processes will be developed for highly efficient polymer donors (PM6, PM7, PBDB-T etc.) and non-fullerene acceptors (Y6, IT-4F, N3 etc.). Same slot die coating method will be used to develop coating processes for different hole transport layers (HTLs- PEDOT:PSS, MoOx) and electron transports (ETLs- ZnO, PDININ, PDINO) as well. For printing metal electrodes (Ag, Cu) flexography printing technique will be used. Two different configurations of photoactive layers namely bulk heterojunction and completely new pseudo-bilayer will be explored to fabricate devices, and their performance will be compared. During process development, various processing parameters such as temperature (ink, die and substrate), ink concentration, and coating speed will be optimized to achieve proper thicknesses and morphology. In all cases, green solvents like water, O-xylene, toluene, alcohols will be prioritised for process development.

PhD student: Eswaran Jarayaman

Supervisor: Morten Madsen

Organic photovoltaics (OPVs) are an emerging light-conversion technology for clean energy generation. Non-fullerene acceptors (NFAs)-based OPVs have achieved over 19% efficiency but they suffer from short shelf lives and limited operational lifetimes. The decline in device performance with time can be partly attributed to morphological degradation and photodegradation.

NFAs can be categorized as small molecules, polymers, and larger structures. While degradation mechanisms and stabilization methods have been extensively studied in polymer donor: small-molecule NFA systems, less emphasis has been placed on other organic photoactive materials, such as small molecular donors and polymer acceptors.

The project aims to investigate the photochemical and morphological degradation mechanisms of state-of-the-art OPV materials, including small molecule donors and polymeric acceptors. The feasibility of using antioxidants in the active layer will also be investigated by analysing the interactions their interactions with both the active layer and the OPV interlayers. Overall, the project seeks to extend the operational lifetime of OPV modules and develop a structural-dependent degradation understanding of degradation mechanisms. This research is part of the OPVStability Marie Skłodowska-Curie Actions Doctoral Network (MSCA-DN).

PhD student: Hsuan-yu Wang

Supervisor: Vida Engmann

Energy production by renewable energy sources that can generate sufficiently large amounts of energy to supply the world’s needs is urgently needed. Solar energy, as one of the most promising renewable sources, can be harnessed by photovoltaics (PVs) that directly convert sunlight into electricity. Recently, organic photovoltaics (OPVs) are emerging due to their low-cost fabrication, mechanical flexibility, low environmental impact, short energy payback times and high demonstrated Power Conversion Efficiency (PCE) of 19%.

OPVs based on low-gap non-fullerene acceptors (NFAs) absorb primarily in the near infrared (NIR) region and can be designed to let visible light pass through the cell. Hence, these solar cells can potentially be used in a semitransparent device, which can be integrated into windows of buildings or greenhouses to generate substantially large amounts of energy with net zero energy consumption. However, stability issues remain, which still hinder the large-scale commercialization of OPV materials.

The PCE of a solar cell can be improved by forming a tandem structure that stacks two or more semiconductors with complementary band gaps. In the case of a semitransparent solar cell, tandem devices have to be visually transparent. One way of constructing such devices is to have a bottom cell based NIR NFAs and a top cell that absorbs primarily in the near ultraviolet (NUV) range, so that photons in the visible range are not absorbed. Metal oxides are typically stable, non-toxic and can be manufactured with low-cost techniques, making them appealing candidates as a top cell in a tandem transparent solar cell. Nevertheless, most oxides are n-type semiconductors and exhibit a band gap of >3 eV that is too wide to absorb a significant portion of the solar spectrum. Band structure engineered low gap (2<Eg ≲2.5 eV), stable and non-toxic oxide semiconductors using earth-abundant elements could potentially be used as a top cell in an OPV-based tandem device to harvest UV photons, and thus construct a unique low-cost, stable and highly efficient semitransparent solar cells with PCEs >11% and average visible transmission (AVT) of >53% (beyond state-of-art).

This PhD project aims to develop novel low-cost and stable oxide/OPV semitransparent tandem PV with beyond state-of-the-art performance. New oxide alloys with tunable band gaps will be synthesized using magnetron sputtering. The electronic, optical and transport properties of the alloys will be characterized with a particular focus on tuning the band gap and band edges by varying the alloy composition. Then n- and p-type doping of the alloys will be explored to optimize them for PV devices and semi-transparent tandem solar cells. Finally, an oxide/OPV transparent tandem solar cell will be fabricated and evaluated.

PhD student: Kun Wang

Fossil fuels are the cause of smog, oil spills, ocean acidification and climate change. Their extraction breaks and poisons the earth. When we buy them, we fund dictators and autocrats the world over. Fossil fuels are also running out.

There therefore is a great need for new ways of producing large amounts clean renewable energy. A long running favorite for this is solar. However current solar cells, based on silicon, is expensive, have a long energy payback time and are commonly seen as an eyesore.

A promissing alternative is semi-organic solar cells based on organic semiconductors and metal oxides. They require far less energy to produce and are as a result cheaper with a shorter energy payback time. They can also be made transparent such that they can just replace windows in building design, no more eyesores!

Transparent solar cells are not just for aesthetics though. They would also be great materials for building greenhouses since plants only use a thin spectrum of red light meaning we could use land for both farms and clean energy production.

Semi-organic solar cells can be transparent by using the large bandgap of the metal oxide to absorb the UV-light and the small bandgap of the organic semiconductor to absorb the infrared light converting both into electricity while letting the visible light pass through. However, producing them will require some combination of doping, control over interface morphology, controlled stoichiometry, insertion of thin interlayers and possibly other means. This project seeks to unite all these in order to produce high quality semi-organic solar cells for scalable production.

PhD student: Martin Asbjørn Krehbiel

Supervisor: Morten Madsen

In the age of climate change and the scarcity of fossil raw materials, the renewable energies must increasingly be used. Although classic inorganic solar cells have proven themselves and production costs are steadily being reduced, research interest is increasingly directed towards alternative materials for converting light energy into electrical energy.

Organic semiconductors offer several advantages over inorganic variants: low production costs, high absorption coefficients, easy processability and flexibility; which make them one of the most promising candidates for photovoltaics and photocatalysis. The key challenge that remains to be overcome to make organic photovoltaic (OPV) devices competitive is their limited photostability and hence relatively short lifetimes.

This PhD project is part of the Carlsbergfondet project Artplast - Artificial Chloroplasts: Nature-inspired electronic molecular nanoparticle platform with the focus on developing highly efficient material systems for hydrogen evolution and solar electricity generation by mimicking the energy converting and self-healing mechanism of a leaf.

The goal of the project is to develop an artificial chloroplast using conjugated donor:acceptor:antioxidant nanoparticles. By testing and controlling the fundamental photophysical and photochemical properties, the morphology vs. electrical properties relations in these materials will be identified and it will be used to tailor highly efficient material systems for hydrogen evolution and solar electricity generation, thus contributing to the transition of the current energy system to zero emission society based on renewable resources.

PhD student: Rovshen Atajanov

Supervisor: Vida Engmann

Solar intermittency is a great drawback for solar energy conversion systems. By harvesting solar irradiation in the form of hydrogen bonds, the energy can be stored and transported. The active layer in organic solar cells (OSC) is the backbone of the devices; it is responsible for light absorption, charge separation and migration.

The use of nanoparticles (NP) as active layers made of conjugated polymer donor (D), non-acceptor-fullerene (NFA) and antioxidant (Aox) bulk heterojunction have demonstrated great photo conversion efficiencies and enhanced stability in OSC devices. These NP developed for organic solar cells require the same characteristics as hydrogen production photocatalysts: efficient light-harvesting, charge separation, charge transfer and long-term stability. Yet, this new class of NP materials has not been much explored despite of their extremely high HER activity under visible light.

In this project, we strive to find D:A:Aox NP with suitable thermodynamic and kinetic properties to carry out water splitting. The study and characterisation of these NP will provide the knowledge in this regard to fill the gaps in the development of organic NP for hydrogen production.

PhD student: Miguel Angel Leon Luna

Supervisor: Vida Engmann