Organic electronics based on organic semiconductors offer tremendous advantages compared to traditional inorganic counterparts such as low temperature processing, light weight, low manufacturing cost, high throughput and mechanical flexibility. Many key electronic processes in organic electronic devices, e.g. charge injection/extraction, charge recombination and exciton dissociation, occur at interfaces, significantly controlling performance and function. Understanding/modeling the interface energetics at organic-electrode/organic-organic heterojunctions is one of the crucial issues for organic electronic technologies to provide a route for improving device efficiency, which is the aim of the research presented in this thesis.
Integer charge transfer (ICT) states pre-existed in the dark and created as a consequence of Fermi level equilibrium at donor-acceptor interface have a profound effect on open circuit voltage in organic bulk heterojunction photovoltaics. ICT state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor-acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.
Doping and insertion of interlayer are two established methods for enhancing charge injection/extraction properties at organic-electrode interface. By studying the energy level alignment behavior at low to intermediate doping levels for molecule-doped conjugated polymer/electrode interfaces, we deduce that two combined processes govern the interface energetics: (i) equilibration of the Fermi level due to oxidation (or reduction) of polymer sites at the interface as per the ICT model and (ii) a double dipole step induced by image charge from the dopant-polymer charge transfer complex that causes a shift of the work function. Such behavior is expected to hold in general for low to intermediate level doped organic semiconductor systems. The unified model is further extended to be suitable for conjugated electrolyte/electrode interfaces, revealing the design rules for achieving the smallest charge injection/extraction barrier for both thin tunneling and thick charge transporting conjugated electrolyte interlayers.
To probe into the energy level spatial extension at interfaces, we employ the original approach of building and characterizing multilayers composed of a well-defined number of polymer monolayers with the Langmuir-Shäfer method to control polymer film uniformity and thicknesses, avoiding the problems associated with spin-coating ultrathin films. The disordered/amorphous films feature smaller, and in fact negligible, energy level bending compared to the more well-ordered films, in contradiction with existing models. It is found that that energy level bending depends on the ICT state distribution rather than the density of states of the neutral polymer chains in relation to the Fermi energy, thus taking into account the Coulomb energy associated with charging the polymer chain and transferring a charge across the interface. Based on this work, a general model for energy level bending in absence of significant doping of conjugated polymer films is proposed.
Organic semiconductors are sensitive to ambient atmosphere that can influence the energetics. The degradation effects of common PCBM film induced by oxygen and water are found to be completely different. Upon exposure to oxygen, the work function is down-shifted by ~ 0.15 eV compared to the ICT curve of the pristine PCBM film, originating from the weak interaction between the fullerene part of PCBM and oxygen, and this can be reversed by thermal treatment in vacuum. The down-shift in energetics will cause a loss in open circuit voltage at electrode interface, but aids free charge generation at donor-acceptor interface. Upon exposure to water, there is irreversible extensive broadening and bleaching of the valence electronic structure features as well as a substantial decrease of work function and ionization potential, severely degrading the transport properties.
Overall, the research results in this thesis thus give a deeper understanding of interface phenomena in organic electronics, especially regard to organic solar cells, aimed to further improve the device operation efficiency and lifetime.
Linköping: Linköping University Electronic Press, 2015. , 56 p.
Turbiez, Mathieu, Dr.
Fahlman, Mats, ProfessorLiu, Xianjie, Dr.Braun, Slawomir, Dr.