On the Origin of Dark Current in Organic Photodiodes

Giulio Simone, Matthew J. Dyson, Christ H. L. Weijtens, Stefan C. J. Meskers, Reinder Coehoorn, René A. J. Janssen, Gerwin H. Gelinck
2019 Advanced Optical Materials  
percolating networks. Whereas OPDs operate under forward bias in photovoltaic mode for power conversion, they typically operate under reverse bias for light detection. The all-important light sensitivity is widely parameterized by the specific detectivity, defined as where R is the spectral responsivity in A W −1 , A is the device active area in m 2 , B is the detection band width in Hz, and i noise is the noise current in A. [1, 7] An important contribution to i noise is the dark current
more » ... dark current density (J d ), which can span multiple orders of magnitude depending on the material properties and device architecture. [2, 4] Since minimizing J d while maintaining a high responsivity is a prerequisite for high detectivity OPDs, an enhanced understanding of the mechanism that determines J d can thus direct further detectivity improvement strategies. The intrinsic dark current of OPDs is typically attributed to either charge carrier injection from the metal contacts into the organic semiconductor [4, 5, 8] or to bulk thermal generation within the active layer. [9, 10] Whereas thermal generation typically makes a limited contribution to J d as organic materials have a relatively large bandgap (>1 eV), [2] charge injection may not be negligible under an applied reverse bias voltage. As such, modifying the metal-semiconductor interfaces by introducing electron and hole blocking layers (EBLs and HBLs) to suppress charge injection is a common strategy to reduce J d , [5, 11, 12] along with increasing the active layer thickness. [13] Photomultiplication is another strategy to enhance OPD performance. [14] [15] [16] However, despite the importance of J d , a quantitative relation between its magnitude and BHJ properties, such as the energetic landscape and charge transport characteristics, remains unestablished. This study aims to clarify the relationship between J d and the properties of the organic semiconductor blend, with a view to providing material selection guidelines to improve BHJ OPD sensitivity. Herein, we investigate J d in BHJ OPDs based on five different polymer donors with widely varying optical bandgaps, each combined with a common fullerene acceptor. The active layer thickness (≈280 nm) and the contact layers are kept the same in all OPDs. We show that at −2 V reverse bias, J d depends substantially on the polymer donor, differing by five orders of magnitude. Furthermore, for the OPDs analyzed in this work the current density, measured in the dark at −2 V, correlates exponentially with the open-circuit voltage (V oc ) measured under a simulated solar spectrum. This suggests that J d Minimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias ( J d ) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of J d in state-of-the-art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of J d are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating J d to material parameters, insights into the origin of J d are obtained that enable the future selection of successful OPD materials. The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.201901568. Solution processed organic photodiodes (OPDs) are attracting attention for use as photodetectors since they possess several advantages over their inorganic counterparts. These include a higher absorption coefficient, greater color selectivity, and compatibility with low temperature solution processing that enables cost-effective, large-area image detectors. [1] [2] [3] The bulk heterojunction (BHJ) architecture, comprising a phaseseparated blend of a donor polymer and a fullerene acceptor sandwiched between charge extraction layers, is widely employed [4] [5] [6] to enhance photocarrier generation at the donoracceptor interface and charge extraction via bicontinuous
doi:10.1002/adom.201901568 fatcat:xnnrfyzomfav7pxlcqsj2sez2u