Optical detection is an often used technique for recognition of potentially dangerous materials. Hydrogenated amorphous
silicon (a-Si:H) technology provides an inexpensive alternative material compared to crystalline silicon for being used in
photonic devices operating in the visible spectrum. Further materials' key benefits are the high light absorption, the
voltage-tunable spectral sensitivity and the high space efficiency. Present research efforts concentrate on the
determination of the color information in a-Si:H photodiodes. This work presents an approach to improve color
recognition of a-Si:H photodiodes by modifying the layer sequence.
The maximum of the spectral response (SR) of a single i-layer a-Si:H photodiode can be shifted by varying its bias
voltage. In this case, the shift is not more than some nanometers. Precise color recognition requires different SR maxima
(e.g. RGB-model). One possibility to accomplish a separation of the SR is to engineer the bandgap; another idea, which
is presented here, is based on a layer sequence modification. Normally, the SR at higher reverse bias voltages, with the
maximum at longer wavelengths, encloses that at lower voltages. Splitting the SR leads to an improvement of color
recognition and is achieved by depositing an additional interior anode. The SR maximum shift amounts to 100nm, from
570nm by contacting the interior anode, to 670nm at the top anode. Furthermore, the curves are clearly split. The
presented approach should lead to a tunable multi-spectral photodiode for high quality color recognition. Such a diode
can be used in photonic devices, e.g. for safety and security applications.