Near-infrared absorbing cyanine dyes and organic-inorganic perovskites for electronic applications

Abstract

Light-absorbing materials with excellent semiconducting properties are of great importance for the application in solar cells, transistors or light-emitting devices, which are emerging technologies. This thesis encompasses the synthesis and analysis of new materials for this purpose. Heptamethine cyanine dyes are strong absorbers of light in the near-infrared (NIR) energy regime with synthetically tunable absorption and redox properties, making them excellent candidates as light harvesters in organic solar cells. In particular, the absorption of sunlight invisible to the human eye allows for the fabrication of visibly transparent solar cells. In here, the exchange of counterions from cyanine iodide salts to PF6 – and $\Delta$– TRISPHAT– is presented and the effect of the counterion on the material properties of the dye is discussed. While the counterion had little to no influence on the optical and electrochemical properties in solution, a tremendous effect on the properties in the solid state was found. Thin films of the dyes with varying counterions showed differently shaped absorption bands indicating alterations in the dye aggregation behavior. Tendencies in the solid state packing of the salts with different counterions are further highlighted by X–ray crystal structures. Also the formation of bulk heterojunction blend films with [60]PCBM was strongly affected by the counterion. With the PF6 – counterion large fully phase-separated domains were obtained, whereas the $\Delta$–TRISPHAT– counterion gave a fully intermixed dye–fullerene phase. These findings will stimulate the further development of cyanine bulk heterojunction solar cells. When the heptamethine dyes were applied in semitransparent bilayer organic solar cells together with C60, a power conversion efficiency of 2.2% was achieved while maintaining a high average visible transparency of 66%. Organic–inorganic hybrid perovskites are an uprising class of compounds which have recently attracted tremendous attention due to the rapid increase in power conversion efficiency of perovskite solar cells (PSCs), reaching more than 20% in 2016. This work aimed at the enhancement of photon–to–current generation of PSCs into the NIR energy regime by utilizing heptamethine dyes as co–sensitizers. For this purpose, new heptamethine dyes with electron-donating substituents were synthesized and characterized in order to tune their redox levels to be compatible with the perovskite material. While the cyanine dyes were able to perform reasonably well as hole-transporting materials – supporting high short–circuit currents of up 15 mA cm–2, co– sensitization beyond 800 nm was not successful. Further synthetic adaptations of the dyes and improvements of the perovskite/cyanine interface might be necessary. Further investigations on organic-inorganic hybrid materials addressed the incorporation of large organic cations into low–dimensional inorganic networks. Such systems allow for the combination of desirable properties from both components, such as the structural and functional versatility of organic compounds with the stability and electrical properties of inorganic materials. Two–dimensional lead halide perovskite materials with the general formula (R-NH3)2PbX4 contain layers of PbX4 sheets alternating with layers of organic cations R-NH3 and have interesting properties such as room– temperature photo– and electroluminescence as well as high charge carrier mobilities, making them promising candidates for the application in devices such as light emitting diodes or field–effect transistors (FET). In this thesis, a method to synthesize 2D– perovskite films consisting of highly ordered crystallites is presented. In XRD spectra of the thin films very intense (00l) reflexes were found from l=2 up to l=20, suggesting that the alternating layers of inorganic sheets and organic cations are perfectly arranged in parallel to the substrate plane. This structure is promising especially for the application in FET devices as the inorganic sheets are aligned in the direction of charge transport from source to drain electrode. Finally, an original hybrid material is presented, which incorporates NIR– heptamethine cations in an inorganic network consisting of infinite chains of face–sharing lead iodide octahedra. The structure of this hybrid was solved using X–ray crystal structure analysis and structural aspects are discussed. This “cyanine perovskite” represents the first one–dimensional lead halide perovskite incorporating a functional NIR–absorbing dye as the organic cation, which is predicted to lead to unusual optical and electrical properties through the synergistic interaction between the components.