Layered Composite thin Films for Cost Effective Transparent Organic Solar Cell Electrodes
In this dissertation two main themes are explored. In the first part some phenomenologies of light transport across the ZnO/Al/ZnO (ZAZ) thin film multilayer structures are investigated. The multilayered ZAZ thin film composite structures are explored for potential applications as transparent electrodes (TEs) alternatives to the costly transparent indium-doped-tin-oxide (ITO) anodes that are used currently in organic solar cells (OSCs), organic light emitting devices (OLEDs) and variety of other layered optoelectronic structures. The transmission characteristics and the optimum interlayer thicknesses of ZAZ thin film structures were numerically predicted in this part of the project. Computational modeling and simulations were used successfully to study the transmittances (Ts) of the multilayered ZAZ thin film composite stacks with intermediate aluminum (Al) layer thicknesses between ~ 1 – 100 nm. Multilayered ZAZ thin film composite structures with mid-Al layer thicknesses between ~ 1 – 10 nm are shown to have average Ts between ~ 75 – 90%, which decreased further to ~ 63 and 41% for the mid-layer Al thicknesses of 20 and 40 nm, respectively. A further decrease of the T values down to ~ 35 and 15% was observed for the mid-layer Al thicknesses of 50 and 100 nm, respectively. The actual multilayered ZAZ thin film composite structures were then successfully synthesized with the predicted optimal interlayer thicknesses and tested accordingly. These were produced via radio frequency (RF) magnetron sputtering (MS). Both computational modeling and experimental studies examined the effects of Al nanolayers on the TE properties of ZAZ film composites. The experimental study clarified the role of the Al mid-layer thickness in a multilayered ZAZ thin film composite with a ZnO(25 nm)/Al/ZnO(25 nm) structure and an optimum mid-layer Al thickness between ~ 1 – 10 nm. Within this range, the numerical simulations are comparable with experimental optical T measurements in multilayered ZAZ thin film composite structures produced with similar ZnO layer thicknesses and the predicted optimum intermediate Al layer thicknesses between ~ 1 – 10 nm. The electrical properties of multilayered ZAZ thin film composite structures were also investigated for structures produced with optimum intermediate Al layer thicknesses. Multilayered ZAZ thin film composite structures, with resistivity values as low as ~ 3.62 × 10−4 Ωcm at average Ts between ~ 85 – 90% (in the visible region of the solar spectrum), were produced. The results show further that the best multilayered ZAZ thin film composite structures that were produced have the highest Haacke Figure of Merit (HFoM) of 4.72 × 10−3 Ω−1 and electrical sheet resistances as low as ~ 7.25 Ω/sq. These transparent conductive properties of multilayered ZAZ thin film composite structures are shown to be comparable to the performance characteristics of ITO-coated anodes that are used currently in organic solar cells, light emitting devices and other electronics and optoelectronic components in passive and active technological systems. The highest HFoM above was obtained for a multilayered ZAZ thin film composite structure with an intermediate Al layer thickness of ~ 8 nm. Furthermore, the combined apparent optical bandgap energy of the multilayered ZAZ thin film composite structures changed from ~ 3.26 to 3.85 eV, an increase of ~ 0.60 eV for intermediate Al layer thicknesses between ~ 1 – 10 nm. This optical bandgap energy widening led to shifts in the optical absorption edges to shorter wavelengths in the solar spectrum. Such shifts are shown to be in agreement with the Moss-Burstein effect. Generally, the structural, optical and electrical properties of the obtained multilayered ZAZ thin film composite structures revealed realistic physics of TEs. These were also comparably in good agreement with the transparent conductive properties of the standard ITO thin film coated-substrates. The second theme of the project explores the effects of contact on charge-carrier transport across the interface between the photoactive organic layer and the TE layer of organic photovoltaic (OPV) solar cell systems. The photo-current-density versus voltage (J-V) characteristics of an OSC as function of contact height for different contact lengths were studied by numerical modeling. The obtained results are used to assess the prospects of charge transport and/or collection across the photoactive/TE interfaces of OPV solar cell systems and other electronics/optoelectronic devices and components. The results show that optimum contact length above ~ 80% is needed for the organic solar cell to have the performance characteristics that resemble closely to those of organic solar cell systems with perfect planar interfacial/interlayer contacts.