Development And Interface/Surface Characterization Of Titanium Dioxide And Zinc Oxide Electron-Collection Interlayer Materials For Organic Solar Cells
My research on metal oxide electron-harvesting interlayers for organic solar cells was focused as three interrelated projects in this dissertation: i) development of a chemical vapor deposition (CVD) system for titanium dioxide (TiO₂) film; ii) an electrochemical methodology to evaluate ZnO thin film charge (hole) blocking ability; iii) the effects of plasma modifications on sol-gel ZnO and sol-gel ZnO/organic (active layer) interfaces. In i), we showed that nanoscale (12-36 nm) CVD TiO₂ film deposited at 210 °C from our system obtains properties of conformal growth with ITO substrate, superior hole blocking ability, stoichiometric metal to oxide ratio, and close energetic alignment with electron acceptors, e.g., fullerenes. The introduction of CVD TiO₂ film as an electron transport layer (ETL) into organic solar cell significantly improves its J-V characteristics compared to bare ITO electrode. The optimum TiO₂ thickness in the OPV device applications was found to be 24 nm with a high fill factor (0.58) and power conversion efficiency (3.7%) obtained. In ii), simple electrochemical methods, i.e., cyclic voltammetry, impedance spectroscopy have been used to evaluate sol-gel derived ZnO (sg-ZnO) and sputtered ZnO (sp-ZnO) porosity and pinhole density. We showed that sg-ZnO with high surface area porous structure allows the probe molecules and poly-thiophene (P3HT) thin layer to direct contact ITO substrate, whereas sp-ZnO with dense structural property efficiently eliminates the probe molecule diffusion and the penetration of P3HT layer to ITO substrate. This electrochemical property difference also directly reflects on the device shunt resistance (Rp), where we observed larger leakage current for the devices using sg-ZnO than that of devices using sp-ZnO. We envision these simple electrochemical characterizations can be applied into other similar metal oxide interlayers as well as on flexible TCO substrates, in which pinholes and physical imperfections, e.g., cracking may occur after multiple bending processes. In iii), we demonstrated low power (10.5 watts) radio frequency (RF) O₂ and Ar plasma treatments have significant impacts on sg-ZnO near-surface chemical compositions, which in turn influence the onset potential of sg-ZnO electron injection from the underlying ITO substrate and its energetic alignment with electron acceptors, e.g., C₆₀. Using UPS, we found the presence of localized mid-gap states near the Fermi-level (Ef) of sg-ZnO, which induces the most favorable band bending and the largest vacuum level shift due to significant electron transfer from sg-ZnO to C₆₀. As a result, the resultant solar cells show the best device performance. Upon the plasma treatments, the passivation effects eliminate the mid-gap state. Therefore, we observed less degree of band bending at ZnO/C₆₀ interface and poorer device performance for the plasma treated sg-ZnO. The study demonstrates the importance of oxide/organics interface in operations of organic solar cells and provides a modification method to tune surface properties of oxide materials which can apparently be applied in other organic electronic devices, e.g., field effect transistors (FETs), organic light emitting diodes (OLEDs), etc.
organic solar cells, titanium dioxide, zinc oxide, interlayer materials, Chemistry
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