Periodic Reporting for period 4 - ENERGYMAPS (Revealing the electronic energy landscape of multi-layered (opto)electronic devices)
Période du rapport: 2021-04-01 au 2022-09-30
The field of emerging photovoltaics (PV) is experiencing unprecedented progress with remarkable advances in power conversion efficiencies recently reported organic, quantum dot and perovskite based devices. Despite these major breakthroughs, many aspects of device physics of emerging PVs remain unknown. One of the most common aspects of the device, routinely used for device physics interpretation is the energy level diagram (energetic landscape) of the solar cell, with such diagrams being ubiquitous in literature, appearing in almost every publication. Despite the importance of energy level diagrams in determining the elementary processes taking place in the device (e.g. charge generation, transport and extraction), accurately determining these diagrams is extremely challenging, especially for solution-processed systems. Most commonly, these diagrams are constructed by combining energy values for the individual components as obtained by different methods, resulting in a large scatter of reported values even for the same material systems. In addition, this approach neglects to account for interfacial effects such as formation of dipoles or band bending. Consequently, the current approach hinders further advancement in the field of emerging photovoltaics in particular in material design, interfacial engineering and development of novel device architectures.
In this project, we develop a new method that can directly measure the vertical energetic landscape of solution-processed photovoltaic systems. Our methodology is based on UPS depth profiling, made possible by the use of a gas-cluster ion beam that allows essentially damage-free sputtering of semiconducting materials. Our goal is to probe the energetic landscape of emerging photovoltaic (and other optoelectronic) devices and exploit them in order to understand energy losses in photovoltaic devices. Moreover, it is our aim to track the evolution of the energetic landscape throughout the device lifetime in order to gain insights into the mechanisms of its degradation. These results will not only lead to further advances in the efficiency of the devices, but will also allow us to develop suitable mitigation strategies to supress their degradation, leading to better performing, more stable PVs that can be used in industrial applications.
In this project, we develop a new method that can directly measure the vertical energetic landscape of solution-processed photovoltaic systems. Our methodology is based on UPS depth profiling, made possible by the use of a gas-cluster ion beam that allows essentially damage-free sputtering of semiconducting materials. Our goal is to probe the energetic landscape of emerging photovoltaic (and other optoelectronic) devices and exploit them in order to understand energy losses in photovoltaic devices. Moreover, it is our aim to track the evolution of the energetic landscape throughout the device lifetime in order to gain insights into the mechanisms of its degradation. These results will not only lead to further advances in the efficiency of the devices, but will also allow us to develop suitable mitigation strategies to supress their degradation, leading to better performing, more stable PVs that can be used in industrial applications.
In the first half of the project we focused on the development of the UPS depth profiling methodology for organic materials. We investigated a range of organic materials including conjugated polymers and small molecules and identified the optimal etching conditions that lead to negligible damage. We applied these conditions to characterise a range of bilayer and bulk heterojunction systems and measure the evolution of their energy levels as a function of depth (energetic landscape). These energetic landscapes were used in order to quantify the photovoltaic gap of these systems in comparison to the measured open-circuit voltage of the devices. The results revealed that UPS depth profiling leads to an accurate quantification of the photovoltaic gaps, far superior to the conventional approach of characterising each material on its own. We also applied UPS depth profiling to degraded organic photovoltaic devices and tracked the changes in the energetic landscape during degradation. Finally, we extended the methodology to be used also in ternary organic photovoltaic systems, in which we tracked the evolution of the photovoltaic gap as a function of the active layer composition. Moreover, we discovered that UPS depth profiling is a powerful tool capable of tracking compositional profiles in multi-layers and mixed systems relying on differences in the electronic structure of materials. This approach has two key benefits to standard methodologies (such as x-ray photoemission spectroscopy depth profiling): a superior depth resolution of 1-2 nm due to the surface sensitivity of UPS and the ability to distinguish materials of similar atomic composition. Beyond these studies on organic systems, we began exploring quantum dot and perovskite systems with promising preliminary results.
The development of UPS depth profiling goes beyond state of the art since it allows for the first time to directly measure the evolution of energy levels in mixed material systems and across multi-layers providing accurate information about the electronic structure and composition with a 1-2 nm vertical resolution. Until the end of the project we expect to extend our methodology also to quantum dot and perovskite based photovoltaic devices and investigate cluster etching as a tool for nanopatterning of soft materials.