High performance solid-state mesoscopic solar cells

Mesoscopic sensitized solar cells are one of the most promising third-generation photovoltaic technologies. Dye-sensitized solar cells (DSCs), imitating the photosynthesis of green plants, were the first photovoltaic devices to utilize a mesoscopic heterojunction for the conversion of solar irradiation into electrical power. Solid-state dye-sensitized solar cells (ssDSCs), that employ an organic hole-transporting material in place of a liquid redox electrolyte, have evolved as viable contenders to conventional liquid DSCs. A typical ssDSC is composed of a mesostructured wide-bandgap metal oxide semiconductor that is sensitized with a light-absorbing chromophore and infiltrated with a molecular organic hole-transporter, usually by solution-processing. In this device, photoexcitation of the sensitizer and subsequent ultrafast electron injection into the conduction band of the metal oxide semiconductor is followed by hole transfer from the oxidized sensitizer to the organic hole-transporter. Charge transport of both the electron and the hole through the two bicontinuous phases, followed by charge migration through the external circuit, completes the photovoltaic operation of the cell. Despite more than 15 years of development, ssDSCs have always been lagging behind their liquid counterparts in terms of both power conversion efficiency and long-term stability. My thesis presents three different approaches that are aimed at contributing to the development of high-performance solid-state mesoscopic solar cells. Firstly, I present a new class of Co(III) complexes as solution-processable p-type dopants for triarylamine-based hole-conductors such as the commonly employed 2,2’,7,7’-tetrakis-N,N-di-para-methoxyphenylamine-9,9’-spirobifluorene (spiro-MeOTAD). The proposed Co(III) complexes were characterized in detail using optical and electrochemical techniques. The application of the new p-dopants in ssDSC rendered it possible to directly relate the conductivity of the doped hole-transporter to the photovoltaic performance of the devices. The work shows that chemical p-doping is a powerful tool to control the charge transport properties of spiro-MeOTAD in ssDSCs, capable of replacing the commonly employed photo-doping, i.e. the light-assisted one-electron oxidation of spiro-MeOTAD by ambient oxygen. Combining this strategy with a state–of–the–art organic D–π–A sensitizer allowed us to achieve power conversion efficiencies of up to 7.2%—a new record for this kind of device architecture. Secondly, I investigated a series of new molecular hole-transporters based on the triarylamine-substituted 9,9’-spirobifluorene core. In total, we synthesized seven new materials and characterized them in detail using electrical and electrochemical techniques. Organic-field effect transistors were fabricated employing these materials in order to evaluate their hole mobilities. From the comparison of different substituents on the hole-conducting triarylamine moieties, a structure–property relationship was derived, highlighting the importance of processability and hole mobility of the hole-transporting material for an efficient application in a ssDSC configuration. We encountered the purity of the new materials as a key parameter for a proper functioning of the device and identified the use of functionalized metal-scavenging polystyrene beads as a valuable purification step. Thirdly, a new technique is introduced that allows to realize functional composites of a mesoporous metal oxide and the organic–inorganic hybrid perovskite CH3NH3PbI3. CH3NH3PbI3 has recently attracted great attention as a light-harvesting pigment in mesoscopic solar cells. We propose a two-step technique to deposit the perovskite absorber: PbI2 is first applied on a substrate by solution-processing and subsequently exposed to a solution of CH3NH3I. It was evidenced that the desired hybrid perovskite forms within seconds after contacting the two precursors. Scanning electron microscopy was employed to elucidate the changes in crystal morphology during the transformation reaction. The kinetics of the reaction was investigated in detail using X-ray diffraction and optical spectroscopy. Employing the two-step technique for the fabrication of mesoscopic perovskite-based solar cells enabled new record power conversion efficiencies of up to 15%. By following three different strategies, this thesis could contribute significantly to the development of high-performance solid-state mesoscopic solar cells. The power conversion efficiencies of 7.2% and 15%, achieved with a molecular dye and an organic–inorganic perovskite pigment, respectively, represent important milestones for the design of third-generation photovoltaic cells that hopefully one day can compete with the conventional silicon-based solar cell technology.