AbstractsEngineering

Understanding the impact of order-disorder phenomena on the optical properties of photoactive semiconducting materials

by Fabian Panzer




Institution: Universität Bayreuth
Department:
Year: 2016
Posted: 02/05/2017
Record ID: 2127311
Full text PDF: https://epub.uni-bayreuth.de/2988/


Abstract

Until today, organic semiconductors as well as hybrid organic-inorganic perovskites have emerged as promising classes of semiconductors. This is obvious, for instance from the significantly increase in efficiencies within the last years in optoelectronic devices such as solar cells or light emitting diodes based on these two classes. Improved device efficiencies were realised mainly due to increased efforts in the field of device engineering. However it is also clear that for further improvement of devices, a fundamental understanding on the nature and dynamics of excited states within the semiconductor material is essential. Irrespective of whether organic- or hybrid organic-inorganic perovskite semiconductors are considered, a major factor controlling device performance is the morphology of the semiconductor, since the morphology of the material is correlated in a sensitive way with its electric properties. So in the case of organic semiconductors the questions arise how the relative alignment of individual constituents or chromophores of conjugated polymers impact on the electronic structure, how major changes in structure, such as order-disorder transitions, occur and how they can be influenced. In contrast to organic semiconductors, i.e. conjugated polymers, oligomers or small molecules, hybrid organic-inorganic perovskites have a crystalline nature. Therefor the correlation between morphology and electronic structure transforms into the aim to understand how structural changes of the perovskite unit cell impact on corresponding electronic structure. On the one hand it is known that the structure of the unit cell of hybrid perovskites can undergo minor and also major changes dependent on temperature. On the other hand, a deep understanding on the origin of these temperature dependent changes has not yet established. Therefore relevant questions in that context are concerned with: what contributes to the spectral shift of the optical spectra of perovskites? How does the nature of the excited states evolve as a function of temperature and is it possible to gain control on the morphological state of the perovskite and even manipulate the latter? This thesis contributes to the tasks and questions raised above and thus is concerned with the issue how changes in morphology modify the electronic structure and electronic interactions among chromophores and constituents of organic- and hybrid organic-inorganic semiconductors. Chapters 4.2 – 4.6 focus on organic semiconductors. Chapter 4.2 first reveals the nature of temperature induced order-disorder transitions in the conjugated polymer P3HT to be a first order rather than a second order transition. This study is conducted by temperature dependent emission and absorption measurements of different batches of P3HT with different molecular weight and polydispersity. Both last-mentioned material properties are also identified to impact on the order-disorder transition, where the molecular weight is correlated with the critical temperature for the transition and the… Advisors/Committee Members: Köhler, Anna (advisor).