Understanding and Engineering of Sub-gap States in Photodetection

Emerging applications for light sensing, including wearable electronics, internet of things and autonomous driving, are pushing conventional semiconductors technologies to their limits when it comes to ease of fabrication, power consumption and device design. Organic semiconductors are considered next-generation absorber materials for photodetection in the visible and near infrared part of the electromagnetic spectrum, which hold some promise of addressing the aforementioned problems of conventional materials. So far, only a handful of companies are putting organic semiconductors to the test for commercial photodetectors, however, research on organic photodetectors is thriving – in particular on photodetectors with a diode architecture called photodiodes. The goal is to make flexible, light-weight devices with improved performance metrics and high stability to realize viable alternatives to conventional photodiodes. The performance limits of organic photodiodes are often associated with the presence of electronic states with energies below the bandgap edge – the so-called sub-gap states. A powerful tool to study the properties of sub-gap states is to measure the external quantum efficiency (EQE), however, the subsequent analysis is complicated by the presence of static disorder and optical interference. In the first part of this work, it is shown how the true absorption coefficient can be extracted from a series of interference affected sub-gap EQE spectra of organic photodiodes with different thicknesses. In consequence, the effect of chemical structure modification on the absorption coefficient in the spectral range of charge transfer absorption is demonstrated. By adjusting the molecular energy levels through target chemical substitutions, a redshift and an increase of the oscillator strength are achieved. The increased spectral coverage in the near infrared is then exploited in micro-cavity photodiodes. The second part of this work deals with the sub-gap absorption coefficient of donor and acceptor materials and how it is affected by the molecular energy level offset. For materials with low energetic offset, it is shown that the sub-gap absorption coefficient follows the Urbach rule in the spectral range of excitonic absorption, dictating the broadening of the sub-gap absorption coefficient at energies right below the bandgap. Lastly, the origin of the high dark current in organic photodiodes is identified as non-radiative recombination via mid-gap trap states. An upper limit to the specific detectivity is calculated that is expected viable in organic photodiodes. The findings of this thesis contribute to the understanding of the sub-gap states by studying their absorption features and distinguishing them from the ubiquitous optical interference effects. The spectroscopic observation of mid-gap trap states is linked to the dark current generation dictating the upper performance limits of organic photodiodes.