PhD projects

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

Over the past two decades, organic photovoltaics (OPV) has been dominated by fullerene acceptors. However, their development faces some drawbacks as the materials show limitations in terms of synthetic flexibility, long-term stability (e. g. easy aggregation), large energy offsets for free charge carrier generation and weak absorption in the visible and near infrared (NIR) region.

Non-fullerene acceptors (NFA) mainly overcome these drawbacks. Their optical and photophysical properties can be easily modified due to molecular engineering which allows for the tunability of the absorption and energy levels which open up a wide range of opportunities for applications. Choosing specific donor and acceptor combinations with complementary absorption thus allows for higher current output and tuning of the energy levels will lead to an increase in device voltage. Therefore, NFAs are very interesting candidates for industrial OPV as they promise a high power conversion efficiency (PCE).

According to literature, PCEs of current state of the art NFA based OPV over 18% have been reported. Having said this, such efficiencies have solely been obtained for small lab-scale devices (<< 1 cm²), commonly fabricated under inert atmosphere, using non-benign solvents and a combination of solution-processing and evaporation techniques. All the aforementioned is not compatible with industrial manufacturing.

The aim of this PhD project is the fabrication of long-lasting and large-scale organic photovoltaics based on high-efficiency NFA systems in a roll-to-roll operational environment. The focus will be on the evaluation of new material systems for possible industrial use which includes optimisation of the device architecture, layer morphology, coating uniformity and long-term stability. Optical, electrical and morphological characterisations will be performed to understand charge carrier dynamics of the studied NFA systems.

This project is an industrial PhD project at Armor solar power films GmbH and conducted in collaboration with SDU.

PhD student: Le Lena Maria Nguyen Ngoc

University supervisor: Morten Madsen
Industrial supervisor: Dr. Sebastian Meier

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