Interface Energy Level Alignments And Their Impacts On Charge Separation In Organic Photovoltaics

Organic electronics have been attracted intensive and increasing attention in the past two decades due to their potential advantages of low cost, versatile chemical design and synthesis, and ease of fabrication. Organic photovoltaics(OPVs), as a renewable and clean energy, can effectively alleviate the current energy needs and show great prospects in applications. It is widely recognized that the organic/organic(organic/inorganic) interface plays an important role in determining the performance and lifetime of organic electronic devices. Despite impressive breakthroughs in organic electronics technology have been achieved, a thorough understanding of the underlying physics within organic devices is still lacking because of the formation of interface dipole, band bending and gap states in the organic interface. It is not applicable to accurately describe and predict the interfacial electronic structures and energy level alignment with conventional models in inorganic semiconductors. Therefore, it is necessary to investigate the organic interface to get a deep understanding of the underlying physical mechanisms.In this thesis, the organic/organic(organic/inorganic) interface energy level alignments and their influence on charge separation have been investigated by Ultraviolet and X-ray photoemission spectroscopies(UPS and XPS). Based on perovskite and organic solar cells, we discussed the how the interface energy level offsets affect the charge separation, furthermore, the manipulation of molecular energy levels and molecular structure could be realized by substrate effect. In the last part, we studied the charge transport mechanisms in organic layers by electrical doing and annealing treatment, more details are listed below:(1) Firstly, the energy level offsets at perovskite/organic hybrid interfaces was investigated by UPS and XPS. The measured alignment schemes at perovskite/HTM hybrid interfaces reveal four entirely different energy level alignment with respect to the variation of HTMs, including spiro-OMe TAD, NPB, F16 Cu Pc, HATCN, and Mo O3, and their impacts on charge separation are also elucidated. The experimental findings provide the guideline of not only understanding the interfacial charge separation mechanisms but also optimizing the HTMs in perovskite-based solar cells.(2) In the second part, we mainly discussed the influence of substrate effect on F16 Cu Pc/C60 and Ti OPc/C60 interface. The reults shows the interface energy levels of F16 Cu Pc change from Fermi level pinning to vacuum level alignment with the increase of substrate work function, at the same time, F16 Cu Pc/C60 interface energy level alignment is also affected. However, the Ti OPc materials behave the opposite changes because of the formation of interface dipoles and band bendings at Ti OPc interface. Therefore, Ti OPc/C60 interface energy level alignment keeps fixed with the change of substrates.(3) In the last part, the charge transport mechanism of organic layers(Bphen) was investigated by electrical doping with cesium compounds(Cs N3, Cs2CO3, Cs F). The results showed different interaction strength exists at the dopants/host interface, which leads to different shifts in energy levels of Bphen layer. For the cesium compounds doped Bphen layers, the conductivity was effectively enhanced compared to pristine bphen layer.