Impact of the Resonant Cavity Order on the Performance of the PTB7-Th Polymer and Nonfullerene Acceptor Blend Near-Infrared Organic Photodiodes

Abstract

The development of low-bandgap polymers and nonfullerene acceptors (NFAs) has enabled the fabrication of solution-processable organic photodiodes (OPDs) with improved sensitivity in the near-infrared (NIR) region. However, geminate and bimolecular recombinations in low-bandgap NFAs often limit performance capabilities and restrict the freedom of molecular designs. Exploring alternate strategies, such as charge transfer absorption (CT absorption), is thus necessary to extend the absorption of OPDs fabricated with low-bandgap NFAs further into the deeper NIR region. The overall aim of this study is to combine the CT absorption strategy with low-bandgap NFAs and, at the same time, to apply optical cavity resonance to extend the spectral coverage and maximize the responsivity of NIR OPDs at a certain target wavelength. We systematically explore the CT absorption phenomena and corresponding properties in mixtures of PTB7-Th and IEICO-4F by means of quantum mechanical (QM) and molecular dynamics (MD) simulations. Upon a careful ellipsometry study, the NIR materials used in this study were shown to exhibit high refractive indices, with which we strategically designed and fabricated a device forming high-order resonance while ensuring the active layer thickness not to be too large to maintain proper electrical operation. Our results show that the device with higher cavity resonance order can achieve a better figure of merit. Consequently, the responsivity of the device forming third-order resonance exhibits a ca. 13.7-fold enhancement, from 0.6 mA/W to 8.2 mA/W, at a wavelength of 1,040 nm, compared to a device without resonance. This approach also reduces the dark current and noise power spectral density while maintaining spectral detectivity comparable to those of noncavity devices. Extending this strategy to PTB7-Th:COTIC-4F systems further demonstrated the potential for detecting longer wavelengths, enabling applications in the deeper NIR region.