Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have recently been demonstrated for the visible regime.[1] However, near-infrared photodetection at a high detectivity has been proven to be more challenging and, to date, the true potential of organic semiconductors in this spectral range (800‒2500 nm) remains largely unexplored. We have recently shown that the main factor limiting the specific detectivity ( ) is non-radiative recombination, which is also known to be the main contributor to open-circuit voltage losses in organic photovoltaics.[2] Based on this finding we concluded that OPDs have the potential to be a useful technology up to 2 μm, given that high external quantum efficiencies can be maintained at these low photon energies. To further elaborate on the fundamentals defining these limitations, a next-generation of ‘defect-free’ organic semiconductors is synthesized. This allows us to investigate the influence of material imperfections (end-capping, homocoupling defects, and other impurities) on the material properties and device performance. In this contribution, we give an overview of the synthetic approaches applied and the latest organic semiconductors for high performance NIR OPDs, approaching their intrinsic limits.
[1] N. Li et al., Mater. Sci. Adn Eng., 146, 100643 (2021)
[2] S. Gielen et al., Adv. Mater., 32, 2003818 (2020)
Probing the photovoltaic external quantum efficiency (EQE) at photon energies well below the semiconductor bandgap is an important tool for achieving a better understanding of the contribution of trap and tail states involved in charge generation processes in photovoltaic devices, notably solar cells. In this work, we present an electrical and optical noise-reduced EQE apparatus achieving 100 dB dynamic range. We carefully identify and study several device- and EQE apparatus-related factors limiting the EQE measurement sensitivity. Minimizing these factors allows us to detect photocurrents smaller than a fA, corresponding to EQE signals as small as -100 dB. We use these ultra-sensitive EQE measurements to probe weak sub-bandgap absorption features in organic, inorganic and perovskite semiconductors. In this regard, we directly observe photocurrent-contributing sub-gab trap states in organic solar cells significantly lower in energy than the corresponding charge-transfer state.
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