Over the last few years the efforts devoted to the research on low-temperature processing of metal oxide thin films have increased notably. This has enabled the direct integration of metal oxide layers (amorphous semiconductors) on low-melting-point polymeric substrates for flexible electronic systems, which adds to the economic and environmental benefits of the use of these processes with reduced energy consumption. More challenging is the preparation of crystalline complex oxide films at temperatures compatible with their direct integration in flexible devices. However, the usually high crystallization temperatures (>600 °C) impede the development of devices that take full advantage of the large variety of oxide functionalities available. This tutorial review analyzes a number of strategies based on wet chemical methods for inducing the crystallization of metal oxide thin films at low temperatures. The key mechanisms are explained in relation to the specific step of the fabrication process reached in an earlier stage: the formation of a defect-free, highly densified amorphous metal-oxygen network or the actual crystallization of the metal oxide. The role of photochemistry, where light can be used as a complementary energy source to induce crystallization, is particularly highlighted. This requires the synthesis of novel photosensitive solutions (modified metal alkoxides, charge-transfer metal complexes or structurally designed molecular compounds) and a precise control over the reactions promoted by UV irradiation (photochemical cleavage, ozonolysis, condensation or photocatalysis). Relevant examples derived from the integration of crystalline metal oxide thin films on flexible substrates (≤350 °C) illustrate the most recent achievements in this field.

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Functional ferroelectric oxides for flexible electronics are achieved from activated solutions enabling low-temperature processing and large-area deposition directly on polymeric substrates. This processing technology reaches the lower limit temperature of crystallization at 300 °C, using a strategy that combines seeded diphasic precursors and photochemical solution deposition. Properties of these materials are comparable to those of high-temperature-processed counterparts and organic ferroelectrics.

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Sol-gel derived polycrystalline La-doped PbTiO3 films are investigated by scanning force microscopy (SFM) in a piezoelectric contact mode. The SFM signal proportional to the effective piezoelectric coefficient, deff, is measured inside individual domains as a function of the mechanical force exerted by the SFM tip on the film’s surface. It is found that the piezoelectric signal can be fully suppressed under sufficiently high force (∼20–22 μN). The suppression is qualitatively different for domains of opposite polarities where deff may change its sign depending on the orientation of polarization vector. The piezoelectric hysteresis loops acquired under increasing force gradually shrink with an asymmetric decrease of the piezoelectric coefficients. This leads to a pronounced vertical shift of the loops (deff offset). The obtained results are discussed in terms of the competing electrostatic and piezoelectric contributions to the measured signal and stress-induced polarization instabilities in the vicinity of the SFM tip.

Lead titanate (PT) sols were prepared using propanediol, butanediol, or pentanediol solutions of lead acetate trihydrate and titanium diisopropoxide bis(acetylacetonate). Precursor sols for PbZr0.53Ti0.47O3 (PZT) films were prepared from propanediol solutions, with zirconium tetrapropoxide being used as the zirconium source. Films were formed by spin‐coating the sols onto silicon and platinized silicon substrates; the resulting gel layers were converted to ceramic films by adopting a two‐stage heating schedule with final firing temperatures of 600–700°C. Information on film crystallization, microstructure development, and electrical properties is presented for both compositions. The limiting thickness of surface‐smooth crack‐free single‐layer films was ∼1 μm. The PT films exhibited a “linear” polarization‐electric field (P‐E) response, while the PZT films gave rise to characteristic ferroelectric P‐E hysteresis loops. A 0.5 μm thick single‐layer PZT film exhibited remanent polarization (Pr) values of ∼34 μC·Cm−2, with a coercive field (Ec) of ∼45 kV·Cm−2; the relative permittivity (ɛr) and the dissipation factor (D) were ∼1250 and 0.07. For a 1 μm single‐layer PZT film, the respective values were Pr∼19 μC∼Cm−2, Ec∼40 kV∼Cm−2, ɛr∼750, and D= 0.03.