Non-fullerene electron acceptors for solar cell applications
Lead Research Organisation:
University of Oxford
Department Name: Oxford Chemistry
Abstract
Non-fullerene electron acceptors for use in organic solar cell applications have rapidly progressed in recent years to the point where they are now outperforming fullerenes in both device efficiency and stability. These molecules offer potential advantages over fullerenes, such as their ease of synthetic modification and facile purification, their potential for low cost, improved spectral absorption coverage and strength, and inherent morphological stability. This year, the McCulloch group has developed a series of rhodanine terminated small molecule acceptors with exceptional performance in combination with a range of electron-donating, hole transporting polymers, offering an attractive pathway for industrial development of solution processed bulk heterojunction organic photovoltaic devices. We demonstrated that an active layer with two electron acceptors can have a synergistic effect on device efficiency and even stability. Much fundamental work however remains to explore the origins of this increased performance. For example, the relationship between the energy levels in a three component active layer and the optimum energetic landscape is not fully understood. In addition, understanding the role that phase separation, molecular diffusion and partitioning, chi parameters and surface energy, plays in determining the complex microstructure is critical to develop new materials and processes. This studentship will explore new molecular design, and develop an understanding of the interrelationships between molecular structure, thin film microstructure and device performance.
We will design and synthesise a range of of calamitic shaped small molecules to be employed as electron acceptors in bulk heterojunction blends. The design strategy will focus on optimizing each tunable aspect of the molecular structure with respect to intrinsic properties such as molecular orbital energy levels, solubility and crystallinity, in addition to thin film heterojunction blend properties and absorption complementarity. The conjugated unit design strategy is based on discrete separation of electron rich and poor aromatic sections, drawing on a symmetric indacenodithiophene design template used in donor polymer synthesis. This has been an established and successful strategy to aid efficient materials optimisation and is a core area of expertise in the McCulloch group. A conjugated push-pull structure is achieved by combining this electron rich unit fluorene with an electron poor peripheral unit comprising benzothiadiazole and a terminal rhodanine. This molecular orbital hybridization reduces the bandgap, which helps to extend the absorption, to lower energies. Additionally, it provides control over the separation of the HOMO and LUMO electron density in the molecule thus influencing the molecular extinction coefficient, and the location for charge transfer.
The initial target of this studentship is to initially explore a range of molecular design modifications (illustrated in red in Figure 1) of the IDTBR acceptor unit, and the subsequent effects that these changes have on the performance of bulk heterojunction solar cell devices. Six examples are illustrated, which can independently tune specific performance features and provide insight into structure property relationships.
The function of these acceptors will be evaluated in blends with donor polymers, including steady state and transient spectroscopic studies of electron and hole transfer in order to evaluate the impact of changes in materials design. The specific targets will be to minimise geminate recombination losses whilst maintaining a high acceptor LUMO level and therefore cell voltage.
We will design and synthesise a range of of calamitic shaped small molecules to be employed as electron acceptors in bulk heterojunction blends. The design strategy will focus on optimizing each tunable aspect of the molecular structure with respect to intrinsic properties such as molecular orbital energy levels, solubility and crystallinity, in addition to thin film heterojunction blend properties and absorption complementarity. The conjugated unit design strategy is based on discrete separation of electron rich and poor aromatic sections, drawing on a symmetric indacenodithiophene design template used in donor polymer synthesis. This has been an established and successful strategy to aid efficient materials optimisation and is a core area of expertise in the McCulloch group. A conjugated push-pull structure is achieved by combining this electron rich unit fluorene with an electron poor peripheral unit comprising benzothiadiazole and a terminal rhodanine. This molecular orbital hybridization reduces the bandgap, which helps to extend the absorption, to lower energies. Additionally, it provides control over the separation of the HOMO and LUMO electron density in the molecule thus influencing the molecular extinction coefficient, and the location for charge transfer.
The initial target of this studentship is to initially explore a range of molecular design modifications (illustrated in red in Figure 1) of the IDTBR acceptor unit, and the subsequent effects that these changes have on the performance of bulk heterojunction solar cell devices. Six examples are illustrated, which can independently tune specific performance features and provide insight into structure property relationships.
The function of these acceptors will be evaluated in blends with donor polymers, including steady state and transient spectroscopic studies of electron and hole transfer in order to evaluate the impact of changes in materials design. The specific targets will be to minimise geminate recombination losses whilst maintaining a high acceptor LUMO level and therefore cell voltage.
