Document Type

Dissertation - Open Access

Award Date


Degree Name

Doctor of Philosophy (PhD)


Electrical Engineering and Computer Science

First Advisor

Qiquan Qiao


counter electrodes, Dye-Sensitized solar cell, oxygen vacancies, Urea


Photovoltaic cells have shown great promise as an alternative to fossil fuel-based energy sources. Dye-sensitized solar cells (DSSCs) have shown potential as low-cost replacement to silicon solar cells owing to their reduced material costs and simple fabrication techniques. Platinum (Pt) was used as a catalyst in the counter electrode for DSSCs. Metal oxides have been used as an alternative material to Pt. The introduction of oxygen vacancies inside metal oxides helps to facilitate electron transport to the electrolyte to enhance the reduction process of triiodide ions. Annealing n-type metal oxides under a reducing agent gas such as hydrogen (H2) at temperature ≥400 oC helps to introduce more oxygen vacancies. In this dissertation, a novel method was developed to convert the electrocatalytically inactive commercial n-type WO3, SnO2, and ZnO into highly active WO3-x, SnO2-x and ZnO1-x as counter electrodes (CEs) for DSSCs. These new metal oxides replaced Pt by controlling the number of introduced oxygen vacancies. All the metal oxides including WO3, SnO2, and ZnO were pre-treated with urea at different wt% before annealing under N2 environment at 470 oC for 2 hr. At high temperatures (e.g., 300-400 oC), urea easily decomposes to ammonia which then decomposes to H2 and N2 gases. Higher wt% of urea leads to more reducing H2 gas and hence helps to create more oxygen vacancies. The urea treatment significantly improved the catalytic activity of all metal oxides, and solar cell power conversion efficiency (PCE) of DSSCs was increased by urea pre-treatment. All other characterizations including SEM, EDS, and Mott-Schottky performed for urea pre-treatment of WO3, SnO2 and ZnO support the hypothesis that urea treatment helps create oxygen vacancies (shallow defects states) in metal oxides. These oxygen vacancies facilitate the redox process in the iodide/triiodide electrolyte. The density of these oxygen vacancies can be engineered by controlling the urea wt% during the treatment. Compared to previous work by only annealing metal oxides in N2 or H2 without any pre-treatment, this new method using urea pre-treatment is more efficient to maximize the performance of DSSC devices.

Library of Congress Subject Headings

Dye-sensitized solar cells.

Transition metal oxides.



Photovoltaic cells.


Includes bibliographical references (pages 110-121)



Number of Pages



South Dakota State University


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