On the Role of Mid-Gap Trap States in Organic Photodetectors

Photodetectors derived from organic, semi-crystalline and amorphous semiconductors receive significant interest in the optoelectronic research community, industry, and consumer electronics. Their UV-to-NIR tuneable and narrow spectral responses, detectivities comparable with silicon and other crystalline semiconductors, and low embodied energy manufacturing with the potential of flexible application form factors offer great potential in next generation light sensing and imaging applications, such as wearable and building integrated electronics, or the internet of things. In any photodetector, the dark current is a critical parameter limiting the sensitivity at low light intensity and thus, defining the maximum achievable signal-to-noise. Organic photodetectors, in particular, are characterized by intrinsically high dark currents orders of magnitude higher than expected for thermally excited band-to-band transitions, the origins of which are still subject of current debate. Here, we find the dark current in organic semiconductor photodetectors to be universally mediated and thus limited by mid-gap trap states. We derive a diode equation based on Shockley-Read-Hall statistics and determine a revised upper limit for the specific detectivity of organic photodetectors. Finally, we present a method to overcome the limitations of those mid-gap trap states. Our results deliver new insight on the origin of performance- and sensitivity-limiting noise in organic light-harvesting applications, such as photodiodes and detectors.