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.
