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In this work, a drop-on-demand piezoelectric ink-jet system has been employed to fabricate disposable photodiodes as
the detectors on the integrated lab-on-chip lasers. J-aggregated films of a cyanine dye, NK-1952 doped into a conductive
polymer, poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) on the indium tin oxide (ITO)
substrate have made by the ink-jet method, respectively. With the thin Al layer on the top of cyanine dye films as the
cathode, while ITO substrate as the anode, the photodiodes have been demonstrated to be wavelength sensitive under the
excitation of a tunable pico-second laser, which corresponds to the characteristic red-shifted, sharp and narrow J-aggregate
absorption peak of each cyanine dye employed. The influence of ink-jet fabrication parameters, presence of
metal ions and pH value of dye solutions on the J-aggregate formation and also the wavelength sensitivity of the
photodiodes have been systematically investigated and the mechanisms involved have been discussed. It is found that by
optimizing the ink-jet fabrication parameters such as UV exposure dose amount, and substrate temperature, or by
introducing ions such as K+, Na+, or H+, the wavelength resolution of the ink-jet printed photodiodes can be improved
significantly, and wavelength resolution of less than 0.1nm may be expected.
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