a-SiC:H/a-Si:H tandem structure analysis for RGB color recognition in LSP devices

A device structure based on a a-SiC:H/a-Si:H pin/pin tandem structure is proposed for a proper color separation process that takes advantage of the different light spectrum filtering properties of the a-Si:H and a-SiC:H absorbers. We have analyzed a tandem pin–pin device with the following structure deposited using the plasma enhanced chemical vapor deposition (PECVD) technique: ITO/p-type a-SiC:H (20 nm)/a-SiC:H (200 nm)/n-type a-SiC:H (20 nm)/p-type a-SiC:H (20 nm)/a-Si:H (1500 nm)/n-type a-Si:H (50 nm)/ITO. Carbon concentration in the top a-SiC:H cell produces an optical gap of about 2.0 eV. A numerical simulation of the internal electrical configuration of the device under different wavelength radiation supports our analysis of the device operation. The bias dependence of the device output is explained by taking into account, from the point of view of the color sensor applications, considerations about an asymmetric reaction of the internal electric fields to the externally imposed forward bias and a self-biasing effect of the sub-cells under certain unbalanced light generation of carriers. The simulation shows that this structure permits a good recognition of blue color under reverse bias and red color under forward bias conditions. The acquisition of a satisfactory RGB image mapping by controlling the applied bias is possible but remains problematic due to the poor separation of the green component at any value of the applied bias. A simple algorithm is proposed to deduce the green component of the light by combining the previously acquired information about the total intensity of the incident radiation and about the blue and red components.