Electronic and Chemical Properties of Interfaces in Organic Photovoltaic Devices

Author: Anirudh Sharma

Sharma, Anirudh, 2014 Electronic and Chemical Properties of Interfaces in Organic Photovoltaic Devices, Flinders University, School of Chemical and Physical Sciences

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Organic Photovoltaics is a promising technology, which can potentially be a cheap source of clean and renewable energy in the near future. Despite tremendous research and development efforts in this field, organic solar cells still take a back stage in the mainstream photovoltaic market. Though the efficiencies have gradually increased to up to 12 %, device stability still remains a challenge limiting large-scale commercialization of this technology. This dissertation is devoted primarily to the study of stability and performance-limiting electronic properties of device interfaces in both conventional and inverted OPVs. Given the importance of electrode workfunction in interfacial charge transport in devices, special focus was on better understanding the workfunction measurements on heterogeneous surfaces and precise measurement of lateral variations in workfunction on a nanoscale. In particular, the interfacial instability of ITO-PEDOT:PSS interface in conventional OPVs was investigated and it was shown for the first time that the migration of indium and tin into the PEDOT:PSS was strongly driven by the presence of moisture and is not merely a diffusive process, as prior beliefs. It was systematically demonstrated that indium and tin contaminants can adversely affect the device performance by increasing the interfacial dipole at the ITO-PEDOT:PSS interface. For inverted OPVs, a strong correlation between the processing conditions of ZnO and the device performance has been established. Changes in the electronic or structural properties of ZnO were demonstrated to be the driving force behind the strong dependence of device performance on the processing conditions of ZnO. ZnO prepared via a range of techniques was studied and for all cases a minimum of 25 nm layer thickness was found to be essential to achieve optimum device performance. For sol-gel prepared ZnO, the workfunction was found to be independent of the layer thickness, whereas for ZnO layer casted from a colloidal solution, post annealing temperature was found to be critical and a minimum temperature of 200 °C was found to be essential in order to achieve desirable workfunction and electron affinity. As in case of pulsed laser deposited ZnO, stoichiometric ratio of Zn and O was also found to be dependent on the layer thickness and thicker layer (up to 100 nm) were found to get oxygen deficient with increasing thickness. A fully evolved band structure of ZnO was found to be absent for layers of thickness 12 nm or less, which explains the poor performance of such devices. This work also establishes a clear understanding of workfunction measurements of heterogeneous surfaces with UPS. Surfaces with heterogeneity on a nanoscale were artificially created with a combination of energetically different materials. It was demonstrated that materials having relatively low workfunction have an enhanced secondary electron emission, which can be misleading in deriving absolute workfunction values from UPS measurements. This behaviour was found to be valid even for polycrystalline materials with nanoscale variations in workfunction such as ZnO. While nano-domains of different workfunctions across a nano-roughned ZnO surface were clearly demonstrated using KPFM, UPS results were found to be more representative of the domains corresponding to low workfunction regions.

Keywords: Organic Solar Cells,Interfacial Energetics,XPS,UPS,KPFM,Workfunction,ZnO,ITOPEDOT: PSS
Subject: Chemistry thesis

Thesis type: Doctor of Philosophy
Completed: 2014
School: School of Chemical and Physical Sciences
Supervisor: Prof. David Lewis