The analog VOC response with illumination time is attributed to a gradual accumulation of electrons in the TiO2 film caused by an increase in dye regeneration rate with light exposure duration. This hypothesis is supported by the general trend that the VOC rises more slowly at lower light intensities for all three types of pixels previously discussed (Figures 2a, b, and c). Let’s consider the devices containing no initial oxidizing agents (Figure 2a and 2c). At low intensities where dye excitation is minimal, only a few oxidizing species are being generated via dye regeneration. These Co3+ oxidizing shuttles are too low in concentration to diffuse from the mesoporous TiO2 film into the bulk electrolyte\cite{michael2008}. As a consequence, the Co3+ compounds are localized near the dye molecules, prohibiting fast dye regeneration by the surrounding Co2+ reducing complexes\cite{michael2018}. As illumination continues, more Co3+ manage to diffuse from the dye molecules, facilitating the majority Co2+ complexes to participate in dye regeneration. This process increases the Co3+ concentration with time, leading to gradually enhanced Co3+ diffusion from the mesoporous TiO2 film into the bulk Co2+ electrolyte, and thus increasing the dye regeneration rate. Consequently, the number of electrons injected into the TiO2 rises with illumination time, resulting in the observed analog VOC growth.

1.2.3.     Case studies of tracking moving occluded objects