The Study On The Structures And Properties Of Metals/Polymer Interfaces In Polymer-based Solar Cells

Polymer solar cells （PSCs） have attracted considerable attention, because PSCs have the potential to become a renewable energy source. In recent years, the development of PSCs is very promising. However, the power conversion efficiency （PCE） is only around 12% for lab-scale devices and the lifetime is not sufficient compared to inorganic solar cells. In addition, electronic and chemical structures at metal/organic and organic/organic interfaces are the key factors that strongly influence the performance of these （opto-）electronic devices. Therefore, it’s urgently needed to deeply understand the relationship between the interfacial structure and the performance of the devices involved. This will guide us to design rational interfaces to fabricate devices with higher efficiency and stablility. The interfacial structures and band alignments between the low-work-function metal electrodes （Ca and Al） and organic thin films were investigated in-situ/ex-situ through synchrotron radiation photoemission spectroscopy （SRPES）, near-edge x-ray absorption fine structure （NEXAFS）, x-ray photoelectron spectroscopy （XPS） and ultraviolet photoelectron spectroscopy （UPS）.These fundamental studies on the electronic/chemical structure evolution during the interface formation provide useful information for optimal design of novel devices. The main achievements in this dissertation are summarized as follows:（1） SRPES and XPS have been applied to in-situ investigate the chemical reactions and electronic structures during the interface formation of Ca on the regioregular poly[（9,9-dioctylfluorenyl-2,7-diyl）-co-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole] APFO₃ thin films. Upon Ca adsorption onto APFO₃ at 300 K, Ca dopes electrons into APFO₃, inducing the occurrence of the APFO₃ band bending. Moreover, Ca diffuses into the subsurface and reacts with N, S and C atoms in the APFO₃, leading to the formation of CaS and Ca-C complex. Through the investigation of the evolution of core level and valence band spectra together with secondary electron cut-off, an energy level alignment diagram at the Ca/APFO₃ interface is derived. NEXAFS has been applied to investigate the molecular orientation of APFO₃, it is shown that the APFO₃ film has a high degree of orientational ordering with its aromatic ring tilted at an angle of 43’from the substrate, and the 9,9-dioctyl fluorene unit （F8） is almost in the same plane as the benzothiazole unit （BT）.（2） We report on the electronic structures and chemical reactions of the interfaces of metal electrodes and APFO₃, a promising low-band-gap donor material for efficient bulk heterojunction organic solar cells. Electronic properties of interfaces formed between APFO₃ and prototypical electrodes （Al and Ca） are evaluated. The interfacial band structures indicate that the strong dipole moment produced by the Ca and Al interlayers can facilitate the extraction of electrons and drive holes away from the metal /polymer interface, resulting in the suppression of interfacial recombination losses. These results help explain the higher efficiencies of devices made with Ca interlayers as compared to that with the Al interlayer. The chemical reactions occurring in the metal /polymer system may affect the charge extraction at the corresponding interfaces. For further research, an insertion layer between the metal cathode and the organic functional layer should be introduced to prevent the reaction between these two materials and to tune the metal/polymer interfacial electronic structures.