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An electrochromic device containing heptyl viologen, poly(3,4-ethylenedioxythiophene) (PEDOT), TiO2 and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was fabricated. Both heptyl viologen and PEDOT are electrochromic materials with different spectra at its colored states. The transmission spectrum was measured in different voltages in a wide wavelength range from near infrared to near ultraviolet. By combining PEDOT and heptyl viologen, a high on/off contrast was obtained with almost zero transmittance in a dark state. Also, the TiO2 layer increased optical transmittance change, as it provides a large surface area where the heptyl viologen was reduced to be colored. On the other hand, the TiO2 layer did not decrease the on/off switching time. [ABSTRACT FROM AUTHOR]