Interfacial energetics and charge carrier dynamics in organic and hybrid photo electron sensors

Harvesting solar light by conversion of photons into electric charges is one of the most promising strategy to tackle energetic crisis and global warming providing a clean source of power. At the same time organic semiconductors can also act as wavelength-selective radiation sensors when operated in reverse bias. While traditional solar cells and photodetectors are still relying on silicon and to the well understood mechanisms of charge transport in the ordered molecular structure of inorganic crystals, in the last decades substantial progresses have been achieved for a profitable manufacture of organic semiconductors. These materials offer a wide range of advantages such as the possibility of solution-based and cheap processing, transparency and light weight together with flexibility and biocompatibility for integrable and wearable devices.
Energy production in organic solar cells (OSCs) and organic photodetectors (OPDs) is based on the light-excitation of molecules and hopping of the formed localised excitons along the pi-conjugated structure. Moreover, the most common active layer for OSCs nowadays is comprised of an interpenetrating blend of a donor (D) and an acceptor (A) material, the so-called Bulk Heterojunction, where the continuous D:A interface provides the energy offset for exciton separation and charge extraction. All these processes are still not thoroughly understood and they present numerous scientific issues due to the loss mechanisms such as geminate and bimolecular recombination.
Because of these phenomena specific to organic materials, OSCs lag behind silicon cells in terms of Power Conversion Efficiency (PCE) and OPD performances are still difficult to control as for detectivity and dark current. However, these devices can offer a reliable and economically profitable technology if they can guarantee an extended period of operativity. Therefore, one of the key challenges in the field is to identify and control the efficiency loss mechanisms in order to improve their stability in various operational conditions.
This project aims to tackle this stability issue by identifying the loss mechanisms. We will focus on a specific class of molecules applied in photovoltaics and photodetectors, which will include new Non-Fullerene Acceptors (NFAs) and novel small molecule donor materials.
Since these families of materials offer a rich toolbox of molecular design strategies to tailor their properties, an important aim of this study is to investigate optoelectronic properties and thin film morphology of different molecules by techniques like Raman spectroscopy for vibrational mode analysis, Photoemission Spectroscopy for investigation of energetics and Atomic Force Microscopy to visualise surface topography.
Alongside the characterisation of neat materials and respective blends, device fabrication represents an essential part of the work, allowing to assess how properties of different species have outcomes on device efficiency.
The core objective of the project is the understanding of light-induced degradation mechanisms in thin films and operational devices in order to identify ways to improve the OSC lifetime and OPD robustness. By doing so, we aim at identifying the best molecular engineering routes for the fabrication of highly stable devices - e.g. highlighting which structural properties of molecules are critical and beneficial to avoid loss mechanisms of radiative and non-radiative recombination.
With regards to OSCs, the realisation of devices with novel NFAs and suitable donors providing both good PCE and long operational stability under illumination and thermal conditions comparable to the real working environment would be a satisfactory result to demonstrate how organic-based electronics can be a reliable technology for forthcoming commercialisation on the market.