Periodic Reporting for period 5 - ENERGYMAPS (Revealing the electronic energy landscape of multi-layered (opto)electronic devices)
Reporting period: 2022-10-01 to 2024-03-31
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 developed 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. We demonstrated that the new spectroscopic method not only allows us to probe the energetic landscape of emerging photovoltaic (and other optoelectronic) devices, but also is a powerful tool to understand the physical principles of new device architectures and photovotlaic concepts. Importantly, we have shown that the method goes beyohnd the visualisation of the energetic landscape of devices, also offering detailed compositional information on a nanometre scale. Througout the project we applied the new method to the study of a wide range of material systems and devices types, demonstrating its efficacy also in the context of the study of device stability. This is made possible by the ability to apply the method at any point in the device's lifetime, thus enabling the tracking of the evolution of the energetic landscape upon the application of different stimuli.Taken together, these results led to significant advances in the efficiency and stability of the devices, accelerating their transition towards industrial applications.
In this project, we developed 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. We demonstrated that the new spectroscopic method not only allows us to probe the energetic landscape of emerging photovoltaic (and other optoelectronic) devices, but also is a powerful tool to understand the physical principles of new device architectures and photovotlaic concepts. Importantly, we have shown that the method goes beyohnd the visualisation of the energetic landscape of devices, also offering detailed compositional information on a nanometre scale. Througout the project we applied the new method to the study of a wide range of material systems and devices types, demonstrating its efficacy also in the context of the study of device stability. This is made possible by the ability to apply the method at any point in the device's lifetime, thus enabling the tracking of the evolution of the energetic landscape upon the application of different stimuli.Taken together, these results led to significant advances in the efficiency and stability of the devices, accelerating their transition towards 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. In the second half of the project, we focused on the application of the new method to the study of perovskite based and quantum dot based solar cells. The new method made it possible to explore the impact of ion migration on the electronic structure and energy level alignment in perovskite photovoltaic devices, revealing that ions can become trapped at the devices' interfaces due to pronounced perovskite lattice reorganization. The trapped ions lead to a significant band bending, resulting in an increased built-in potential and consequently device performance. Another highlight was the demonstration of phase heterojunction solar cells - a new concept in photovotlaics - in which two different crystallographic phases of the same perovskite composition are interfaced. UPS depth profiling experiments revealed that the formation of a phase heterojunction leads to an advantegous energy level alignment, increasing the maximum possible open-circuit voltage and fill fator of the device. These and other results of the project were disseminated in academic literature, leading to the emergence of new collaborations with other researchers who were interested in the application of our method to their types of materials and devices.
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. Throughout the project, we applied the method to a broad range of material systems and device configurations, thus demonstrating its broader applicability and efficacy. This made it possible to elucidate new phenomena in emerging photovoltaic devices and develop a fundamentally new concept for photovoltaics.