Highly efficient dye-sensitized solar cells (DSSCs) with excellent long-term stability were fabricated based on tin(IV) oxide (SnO2) nanocrystals with tunable morphologies and band energy levels. The nanocrystals were prepared by a facile, fast, and energy-saving microwave-assisted solvothermal reaction. Through variation of the precursor base used during nanocrystal synthesis control over morphology was achieved—precursor metal cations are known to have a strong influence on the growth process of SnO2 nanostructures. A simple and economic way to prepare semiconducting pastes for photoanodes was devised. The photovoltaic performance of dye-sensitized solar cells based on SnO2 photoanodes was investigated. A very high power conversion efficiency of up to 3.2%, based on very high Voc and comparable Jsc and FF [under 1 Sun condition (AM 1.5, 100 mW cm−2, with shading masks)] was achieved, reporting the highest efficiency value for the cells based on unmodified SnO2 nanocrystals so far. In order to elucidate the efficient cell behavior, electrochemical properties such as the charge transport in the photoanodes as well as SnO2/electrolyte interfacial properties were investigated. Uncharacteristically for DSSCs, all devices tested in the present study show an unusual long-term stability under ambient conditions over several weeks.