Hierarchical nanostructures for light management in thin-film optoelectronic devices

• Author / Creator

  Mahpeykar, Seyed M

• Despite offering advantages such as physical flexibility and cost-effectiveness, thin-film optoelectronic devices are held back by their lackluster efficiency in competition with their traditional counterparts. Since one major factor in low performance of these devices is known to be their deficiency in light absorption or extraction which is the result of their thin-film materials’ properties, seeking for better light management is an intuitive approach to boost their performance. This thesis is focused on leveraging nanotechnology to develop simple and low-cost solutions for addressing light management deficiencies in thin-film photovoltaic devices and light emitting diodes using hierarchical nanostructures by taking advantage of theory, simulation and experimental design, fabrication and characterization. First, the application of nanostructured indium-doped tin oxide (ITO) electrodes as diffraction gratings for light absorption enhancement in thin-film solar cells is studied using finite-difference time-domain (FDTD) simulation. Resonant coupling of the incident diffracted light with supported waveguide modes in light absorbing layer is shown to provide superior light trapping for nanostructured devices as compared to a planar structure. Among various technologically feasible nanostructures, a two-dimensional nano-branch array is demonstrated as the most promising structure and proven to be able to maintain its performance despite structural imperfections common in fabrication. Additionally, a stretchable hexagonal diffraction grating, which has the potential to act as an optical diffuser, is proposed. Leveraging the simplicity of self-assembly, the photon manipulation capability of polystyrene nanospheres, and elastomeric properties of polydimethylsiloxane (PDMS), the proposed device is capable of in-situ tuning of both diffraction efficiency and spectral range and displays highly efficient and broadband light diffusion independent of incident light polarization and angle of incidence which enables integration of cheap and widely used materials with simple cost-effective fabrication for light management in optoelectronic devices. Furthermore, I will leverage a modeling approach for optoelectronic engineering of colloidal quantum-dot (CQD) thin-film solar cells. The presence of a strong efficiency loss mechanism, called the “efficiency black hole”, that can hold back the improvements achieved by any efficiency enhancement strategy is demonstrated. The results suggest that for CQD solar cells to come out of the mentioned black hole, incorporation of an effective light trapping strategy and a high quality CQD film at the same time is an essential necessity. Using the developed optoelectronic model, the requirements for this incorporation approach and the expected efficiencies after its implementation are predicted as a roadmap for solar cell community. Ultimately, a novel volumetric optical diffuser based on cellulose nano-crystals (CNCs) embedded in PDMS is reported. By offering a very simple and low-cost fabrication process as well as compatibility with large-scale production using an earth-abundant material, the proposed optical diffuser is an ideal choice for integration into optoelectronic devices due to the lack of requirement for an index-matching layer. It is demonstrated that CNCs can provide broadband and highly efficient light diffusion at very low concentrations while maintaining a high degree of transparency. Finally, light management capabilities of CNP hybrid optical diffusers are leveraged to show their potential for light absorption enhancement in thin-film solar cells and light extraction improvement in thin-film LEDs.

• Subjects / Keywords

  • Optoelectronics
  • Nanotechnology
  • Semiconductors

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3ZC7S790

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.