Charge-Generating Mid-Gap Trap States Limiting Organic Electronic Devices

Near-infrared photodetectors based on narrow-gap organic semiconductor blends are attractive candidates for light-sensitive applications in industry and consumer electronics. However, these photodetectors currently suffer from large dark current densities, strongly limiting their sensitivity. [1] Organic photodiodes typically show dark saturation current levels that are several orders of magnitude higher than expected from radiative band-to-band transitions only, suggesting the presence of a large non-radiative recombination channel – the origin of which is still debated. In this work [2] we conduct ultra-sensitive external quantum efficiency and temperature dependent dark current measurements on organic photodiodes based on narrow-gap organic semiconductors. The thermal activation energy of the dark current at small reverse bias voltages is found to equal half of the effective bandgap energy revealing that the dominant recombination channel is trap-mediated via mid-gap states which we have recently shown to be relevant to organic solar cells. [3,4] By taking Shockley-Read-Hall statistics into account, we derive an analytical expression which accurately describes the dark current in the reverse bias, allowing for the upper limit of the specific detectivity in organic photodiodes to be calculated. Finally, upon comparing the light-to-dark current ratio of a large number of reported organic photodiodes from the literature, we find that the dark current is universally limited by trap-assisted recombination via mid-gap states in narrow gap systems. Our findings shed new light on the origin of noise in organic light-harvesting applications fundamentally limiting the low light performance and sensitivity in photodiodes and detectors.<br/><br/>[1] Gielen, Sam, et al. "Intrinsic Detectivity Limits of Organic Near-Infrared Photodetectors." Advanced Materials 32.47 (2020): 2003818.<br/>[2] Kaiser, Christina, et al. "Mid-gap Trap State Mediated Dark Current in Organic Photodiodes." DOI: 10.21203/rs.3.rs-710876/v1 (2021).<br/>[3] Zarrabi, Nasim, et al. "Charge-generating mid-gap trap states define the thermodynamic limit of organic photovoltaic devices." Nature communications 11.1 (2020): 1-10.<br/>[4] Zeiske, Stefan, et al. "Direct observation of trap-assisted recombination in organic photovoltaic devices." Nature Communications 12.1 (2021): 1-7.