Investigating Device Physics in Bulk-Heterojunction Organic Solar Cells through Materials Engineering of Interfaces

Thesis (Ph.D.)--University of Washington, 2013We have designed and implemented several organic photovoltaic materials with the goal of engineering interfaces within bulk-heterojunction organic solar cells. In one project, we synthesized a C₆₀ bis-adduct surfactant for use as a buffer layer between the photoactive layer and the thermally evaporated metal top contact of conventional structure, bulk-heterojunction organic solar cells. By systematically varying the work function of the contact metal, with and without the surfactant buffer layer, we gained insight into the physics governing the photoactive layer/metal interface and vastly improved the device performance. By applying Mott-Schottky analysis to the capacitance-voltage data obtained for these devices we were able to conclude that the surfactant modifies the metal work function to an appreciable extent, and allows for efficient charge extraction and significantly enhanced open-circuit voltage regardless of the chosen contact metal. This enhancement allowed us to use more air-stable metals that would ordinarily be prohibited due to suboptimal energy level alignment at the electron-collecting electrode. In a second line of investigation, we used impedance spectroscopy to probe the charge carrier recombination dynamics and their effects on device performance in organic solar cells composed of poly(indacenodithiophene-co-phananthrene-quinoxaline), as well as its fluorinated derivatives, and various fullerenes. We find that the morphology of the blended photoactive layer has a strong influence on the electronic density-of-states distribution, which in turn directly affects the recombination rate as well as the achievable open-circuit voltage. We show that attempting to increase the open-circuit voltage through structurally tuning the energy levels of polymer and fullerene inadvertently introduces different bulk phase separation that leads to a reduction in photocurrent. We observe that the recombination lifetime decreases more dramatically with increasing excess photogenerated charge carrier density for blends with more finely separated phases and propose that the resulting increase in recombination surface area leads directly to reduced overall device performance, despite a marked increase in open-circuit voltage