"Photovoltaic cells based on zinc oxide nanorod arrays and poly(3-hexylthiophene)."
Mankind's addiction to energy, combined with the scarcity of fossil resources, has brought us to a point where the quest for renewable (and clean) energy is no longer an act of idealism, but a bare necessity. Beside a number of alternatives, photovoltaic technology provides a compelling solution for many energy applications.
The well-established and decently efficient silicon-based solar cells rule the market, however their wafer-based nature renders their fabrication energy-inefficient (thus costly). This calls for substitutes, where organic solar cells are being deemed the most promising, supported by the possibility to print them. The active layer of such cells delivers the most power if their donor and acceptor molecules exhibit an optimal degree of phase separation. At typical operating temperatures, the interdiffusion of these materials creates a deviation from this optimum, making this type of solar cells prone to morphological degradation.
A possible solution comprises the replacement of the organic acceptor material by an inorganic one. The inorganic material can be tuned to form nanorod array morphologies, offering a template for the deposition of the organic donor material. The nanorod array thereby provides sufficient interfacial area with the donor, while simultaneously acting as a rigid backbone. This dissertation focuses on such "hybrid" solar cells, based on nanorod arrays of zinc oxide in combination with the polymer poly(3-hexylthiophene) (or: P3HT). Efforts are made to evaluate and improve the degree of infiltration of the polymer into the nanorod array (which is a prerequisite for successful device operation) as well as to optimize morphological parameters, ultimately resulting in a significantly improved energy conversion efficiency. Furthermore, impedance spectroscopy is introduced as a characterization tool, here specifically used to investigate the electronic impact of interface morphology of P3HT near ZnO. The technique is also used to probe the recombination kinetics of the ZnO nanorod array/P3HT solar cell, and to explore the relation between morphology, recombination and device performance. This study allows to draw interesting parallels between hybrid and organic solar cells, assisted by experiments that rely on the controllable morphology of the former. To further understand the operation of ZnO/P3HT solar cells, their energy level alignment is investigated by ultraviolet photoelectron spectroscopy (UPS), using improved methodology compared to former studies. In conclusion, insights are acquired regarding the interplay of (bulk and interface) morphology, recombination dynamics, electronic structure and device performance of ZnO nanorod array/P3HT solar cells, which leads to improved efficiencies and many recommendations for further studies.