A new-generation of thin-film photovoltaics have emerged in the past two year with the introduction of solution-processable organometallic halide perovskites (e.g. CH3NH3PbI3) as light absorbers. Owing to the extraordinary crystallization kinetics of such organic-inorganic hybrid perovskites from precursor solutions at elevated temperatures of about 70 to 150 oC, the reproducible deposition of high quality perovskite thin films has been a challenge. To address this issue, we describe here a universal low-temperature one-step solution processing method which enables the deposition of organometallic halide perovskite thin films with superior uniformity over square-centimeter area and smoothness at the nanometer scale. Using this method, CH3NH3PbI3-based photovoltaics can be reproducibly fabricated with high efficiencies up to 15%. It is also demonstrated that ultrathin perovskite films with thickness below 100 nm can form in complete coverage on substrates and their based full devices deliver surprisingly high efficiency, which are promising for building-integrated photovoltaic applications with low lead levels. Furthermore, partial or full substitution of iodine with bromine in the precursor solution allows band-gap tuning of the perovskite thin films, resulting in tunable vivid colors in the solar cells for potential application as decorative or tandem photovoltaics. The simple and mild deposition conditions used here also expand the selection of materials used in electron/hole selective layers and substrates. This makes the resulting solar cells more versatile in terms of multifunctionality and the types of fundamental studies that can be carried out. This new solution-processing method for the deposition of organometallic halide perovskite thin films is potentially low-cost, considering the low temperature, ambient pressure and one-step nature involved. Also, the rapid perovskite conversion from precursor solutions makes this process amenable to roll-to-roll continuous fabrication and automation for high throughput manufacturing.