Operational Stability and Charge Transport in Fullerene-Based Organic Solar Cells

Abstract

Organic photovoltaic cells are approaching commercially-viable levels of performance for a variety of applications--particularly those which make use of the unique transparency, flexibility, and ultra-thin form factor that organic solar cells can achieve. With state-of-the-art solar to electric power conversion efficiencies now exceeding 15%, operational stability of organic photovoltaics is perhaps their most significant remaining challenge, as the presence of intrinsic photochemical and morphological degradation modes have thus far limited device lifetimes to several years or months. Thermally evaporable fullerenes (C60 and C70), with their remarkable optical and semiconducting properties, have enabled many of the most efficient and reliable organic photovoltaic cells over the past 15 years and remain central to state-of-the-art devices today.

After introducing the fundamentals of organic photovoltaic cell operation and characterization, this thesis focuses on the discovery and exploration of extremely long-range electron diffusion currents in fullerene-based organic heterostructures. It is shown that an energy barrier can be used to confine photogenerated electrons to a thin channel of C60 or C70, where they can persist for several seconds. During this time, the electrons can diffuse laterally over centimeters, which allows for unprecedented study of charge diffusion processes in an organic semiconductor. Organic photovoltaic cells are demonstrated that make use of these channels to achieve high transparency by employing a C60 layer to collect and transport electrons to a sparse metal grid in place of a conventional continuous metal electrode.

The remainder of this dissertation explores the reliability of fullerene-based organic solar cells by monitoring their performance during long-term aging, and studying the stability of the individual layers which comprise the cells. The performance of organic solar cells with planar C60 layers degrades rapidly under illumination, which is found to result from photo-oligomerization of adjacent C60 monomers. An analytical model based on reduced exciton diffusion length in the oligomerized C60 layer is developed to describe the device degradation, which fits the observed loss. Blending C60 with a second material and replacing C60 with C70 are both found to effectively stabilize photovoltaic performance.

The stability of blended tetraphenyldibenzoperiflanthene (DBP):C70-based organic photovoltaics is found to follow the morphological stability of the device's non-photoactive cathode buffer layer. Stable cathode buffer layers based on 2,2',2''-(1,3,5-benzenetriyl tris-[1-phenyl-1H-benzimidazole] (TPBi):C70 are developed, which produce the most robust organic photovoltaics reported to-date. Even under constant simulated illumination at temperatures up to 130C, no performance degradation is observed over more than 2500 hours. Under exposure to high intensity illumination (up to 37 suns equivalent), the devices degrade slowly, with an extrapolated outdoor lifetime of 54+/-14 years.