bias in Fig. 1 has only qualitative meaning and needs more scientific approach to identify the presence of different wavelength components in the transmitted light when the device is seen in a visible light. Perceived color by human eye must be cross checked with scientific results (with optical tools) before making any conclusions. To understand the color changing phenomena in CBG device the best suitable technique is Ultra-violet visible (UV-Vis) spectroscopy. From UV-Vis spectra one can understand the actual wavelength of light which is more affected by the reduction of viologen molecule.

![Figure 5: (a) Optical image of OFG with and without bias. (b) Schematic cross section view of OFG device with biasing arrangement. (c)In-situ Raman spectra recorded on the various position of the OFG device.](figure5){width="10cm"}

In-situ UV-Vis spectroscopy has been performed to investigate optical property of the device which are clearly visible from naked eyes. Figure 2 represent transmission spectra with respect to change in bias voltage using UV-Vis spectroscopy. Transmittance (%) of the film depends upon the band-gap of the material so the band-gap modulation is carried out by the application of voltage During in-situ measurement, bias was increased gradually upto -3.5V(in steps of 0.5 V) and transmittance 