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Organic semiconductor design for increased performance and stability of perovskite solar cells
Schloemer, Tracy H.
Schloemer, Tracy H.
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2019
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Abstract
Perovskite solar cells (PSCs) have received considerable attention due to the absorbing layer’s excellent semiconducting properties and simple deposition methods that allow for impressive device-level efficiencies at potentially much lower cost than state-of-the-art technologies. Most PSCs use organic hole- and electron-transport materials for highly efficient devices. While many organic charge-transport materials used allow for high device-level efficiencies, they often are the bottleneck with regard to thermal stability in the context of the materials in the entire device stack. This dissertation primarily describes the design, synthesis, purification, and characterization of new charge-transport materials and dopants to develop molecular structure and performance relationships for application in highly durable PSCs. Our current hole-transport layer (HTL) research focuses on developing both hole-transport materials and dopants. HTL design is one method with a high return on investment for PSC stability and lifetime. We found radical triarylamine salts as dopants must contain two electron-donating groups para to the triarylamine to prevent dimerization that renders the dopant ineffective. A stable dopant combined with a triarylamine-based, high glass-transition temperature HTM allows for improved device-level stability at elevated temperatures (50 C). To extend this system to further improve device-level properties, HTMs with thiomethyl substituents for perovskite active layer (PAL) passivation and cross-linkable HTMs were synthesized and characterized with the aim to improve device-level efficiency and operational stability at elevated temperatures respectively. Upon device-level optimization, these routes were not ultimately achieved due to unexpected fabrication considerations. With regard to electron-transport layers (ETLs), naphthalene diimide-based (NDI) materials have recently emerged as a potential competitor for C60, the state-of-the-art organic ETL due to their impressive morphological stability and intrinsic electron mobilities. Due to poor solubility in solvents orthogonal to the perovskite active layer, NDIs are not compatible with solution-processing. A library of NDI-based ETLs was synthesized and characterized for improved solubility and frontier orbital energetic alignment with a variety of PALs. Optoelectronic properties were characterized which show successful frontier molecular orbital energetic tuning. Structural motifs for moderately improved solubility in desired solvents were also identified. The PAL contains a number of species that can participate in a variety of chemical reactions (e.g., Brønsted acidic organic species like methylamine, redox active halides, etc.). The organic charge-transport layers must survive these conditions during device operation. Little work has been done to develop design criteria for charge-transport materials based upon compatibility with the PAL. Towards this end, perovskite precursor ink impurities were identified and characterized. Acid-base reactions between aliphatic amine-lead coordination complexes and free aliphatic amines produce an aliphatic ammonium and lead-amide species. These findings highlight the importance of perovskite precursor ink chemistry and potential impurities in the thin film after deposition.
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