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Design of Structural and Chemical Disorder in Metal Oxide Thin Films for Enhanced Light Absorption (EnLight)

Project Duration: 2015 - 2018 (36 months)

Funding Agency: DFG

Funding Scheme: SPP 1839 „Tailored Disorder“

The natural starting point for understanding and tailoring the disorder is to compare ordered (homogeneous) and disordered (heterogeneous) systems. Homogeneous materials typically display similar structural and compositional properties locally as well as globally in a physical sample, whereas large fluctuations in topological and substitutional attributes are inherent to disordered materials. In inorganic materials such as metal oxides, the presence of random disorder is manifested in the formation of surface steps, defects, vacancies, dislocations and grain boundaries, which has shown to create unique electrical and optical properties not found in the corresponding bulk ordered phases1-4. Although chemical or structural imperfections in conjunction with the loss of translational symmetry are generally defined as disorder, a tailored approach to design disorder that is not simply driven by thermodynamics (e.g., surface energy, entropy of mixing) is a challenging avenue for innovative materials engineering. Disorder whether based on perturbed periodic patterns (correlated) or lacking any long range symmetry (random) has been reported to improve the spectral response of semiconductor films used in photovoltaics and solar absorbers, which is attributed to the presence of optical modes formed by two-dimensional multiple light scattering5. In-plane diffraction of light incident on disordered semiconductor grains/surfaces results in the formation of quasi-guided modes that improves the absorption efficiency of disordered films due to more efficient light spreading, when compared to single-crystalline thin films.

In view of the above, this interdisciplinary project – En-Light – will develop innovative materials modification processes to improve the light-management properties of metal oxide semiconductor films by laser-assisted periodic surface structuring (correlated disorder) of SnO2, TiO2, CeO2 films grown by atomic layer deposition (ALD) and chemical vapour deposition (CVD) techniques. In addition, plasma-assisted surface reduction of the above-mentioned metal oxide surfaces will be undertaken to induce valence-exchange and chemical gradient in the films that have been reported to drastically alter the optical and electrical properties. Whereas thin film deposition and characterization of optical properties will performed at the University of Cologne (UNICO), the work devoted to numerical and experimental analyses of pulsed laser patterning that allows fabrication of microstructure having spatial resolution of few micrometres to several hundreds of nanometres will be performed at the Ruhr University, Bochum; (RUB).The focused nature of laser beams and random nature of plasma plume will allow evaluating tailored disorder against random disorder in terms of optical and electrical properties.