We demonstrated efficient electron injection and transport in organic light-emitting diodes using an electron-transport layer (ETL) composed of a Cs and phenyldipyrenylphosphine oxide (POPy2) co-deposited layer. In particular, an ETL composed of a Cs:POPy2 layer with an atom:molar ratio of 1:2 demonstrated an extremely low driving voltage, resulting in a high current density of 100mA∕cm2 at an applied voltage of only 3.9 V. The results of Kelvin probe and absorption measurements indicated that the formation of a CsAl alloy layer at the Cs:POPy2/Al cathode interface and the charge-transfer complex between the Cs and POPy2 contributed to enhancing the efficiency of electron injection and transport, respectively.

Electrochromism of various transition-metal phthalocyanine thin films is studied in a sodium nitrate solution. Copper and nickel phthalocyanine show reversible electrochromism, while, zinc, cobalt, and iron phthalocyanine show the irreversible type. The transition-metal phthalocyanine film, which gives irreversible electrochromism, exhibits a new absorption peak in the NIR region under oxidation. This result indicates that some chemical changes of molecular structure occur with oxidation, and consequently the cyclic voltammograms and the absorbance are diminished with repeated scans, even under the well-designed experimental conditions. As for composite phthalocyanine, the high dispersibility of the phthalocyanine molecules in the codeposited thin film can improve the reversibility in electrochromism. In this case, chemical changes of the molecular structure of metal phthalocyanine with irreversible electrochromism can be prevented to some extents by the codeposition with another phthalocyanine with reversible electrochromism. Sequentially deposited double-layered thin films, in contrast, give completely different results. The electrochromism depends on the order of the oxidation potentials of simple phthalocyanine thin film, and the designed hole transfer compatible with the concept of “sequential potential field” can be accomplished in the double-layered thin film.

The electrochromism of copper phthalocyanine thin films was examined in detail under various conditions. The application of a positive voltage to copper phthalocyanine thin film exhibited two oxidation peaks in the range from 0 to 1.4 V. Although the color of the film changed from sky blue to pale gray upon scanning the voltage in this region, irreversible electrochromism was observed, leading to a deterioration of the film. The two oxidation peaks were concluded to represent two types of appearances of one reaction with different overpotentials caused by the structure of the crystal grains of the film. Control of the scan range to admit only the first oxidation peak provided a reversible electrochromism between sky blue and bluish purple. Electrolyte anions are also of great importance to accomplish reversible electrochromism. Thus, the anions should be stable for the application of a positive voltage, and their Stokes radii should be neither too large nor too small in order to achieve reversible electrochromism. Rather small anions provide almost Nernstian cyclic voltammograms and the quick color change.

The reversible electrochromism in the thin films of single-transition metal phthalocyanines is reported. Copper and nickel phthalocyanine films can be oxidized and rereduced reversibly, when the film is enough thin, the scan range is well controlled, and the appropriate anion is used as the electrolyte. Especially, the cyclic voltammogram of nickel phthalocyanine films in a sodium nitrate solution is almost Nernstian.

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