Interfaces in Organic Solar Cells

Sammanfattning: Organic semiconductors (OSs), the promising candidates for the next generation electronic devices, have shown their advantages of light weight, flexibility, semi-transparency, tunable optical gaps, and energy levels, in the development history of over 70 years. OSs have been widely used in versatile applications such as organic light-emitting diodes, organic field-effect transistors, organic photodetectors, and organic solar cells (OSCs) which are mainly studied in this thesis. Nowadays, OSCs have been developed rapidly and reached a new era of high efficiencies with new records close to 20%, since the development of non-fullerene-based donor (D) -acceptor (A) systems. Despite the impressive progress in the field of OSCs, fundamental understandings on the interface energetics are lagging far behind the development of materials and device engineering, which dampens the further material design, device optimization, and scalable production.Interface energetics take charge of many key electronic processes in organic electronic devices, such as charge injection or extraction at the electrode/ OS interface, charge generation or recombination at the D/A interface, and ambient optoelectronic stability resulted from the OS/air interface, all of which significantly affect the device performance. In this thesis, we systematically study the energy level alignment (ELA) at interfaces of inorganic/organic, organic/organic, and organic/air in OSCs, correlate the investigated energetic landscape with the device performance, and provide new understandings on the optoelectronic process in organic electronic devices.Firstly, we determine the pinning energies of a wide variety of newly developed donors and non-fullerene acceptors (NFAs) through ultraviolet photoelectron spectroscopy, to provide the critical characteristics of the material for ELA prediction at either electrode/OS or D/A interfaces. The ELA for inorganic/organic interfaces follows the predicted behavior based on integer charge transfer model, but for organic-organic heterojunctions where both the donor and the NFA feature strong intramolecular charge transfer, the pinning energies often underestimate the experimentally obtained interface vacuum level (VL) shift. To explore the origin of the extra VL shift, we map the ELA at a range of D/NFA interfaces by fabricating and characterizing D-A bilayer heterojunctions monolayer-by-monolayer with the Langmuir-Schäfer technique. We find that the abrupt and significant VL shifts at the D-A interfaces are attributed to interface dipoles induced by D-A electrostatic potential differences. The VL shifts result in reduced interfacial energetic offsets and increased charge transfer (CT) state energies which reconcile the conflicting observations of large energy level offsets inferred from neat films and large CT energies of D-NFA systems. Furthermore, we investigate the influence of H2O and O2 molecules from ambient air on the work functions (WFs) of OS films. We find that OS films generally show higher WFs measured in ambient air, but lower WFs measured in high vacuum, compared to the WFs measured in ultrahigh vacuum. Two mechanisms are proposed to explain this phenomenon: (1) Competition between p-doping induced by O2 or H2O/O2 complexes, and n-doping induced by H2O clusters; (2) Polar H2O molecules preferentially modifying the ionization energy of one of the frontier molecular orbitals over the other. Finally, we fabricate the charge-transport-layer (CTL) free OSCs based on a newly developed NFA molecule with minimum performance degradation. Based on the determined D-A composition and ELA at the Anode/OS and the Cathode/OS interface, we propose several interface design rules for the efficient CTL-free devices, shedding new light on the simplified device structure for achieving more efficient optoelectronic applications.