Polarization‐Sensitive Photodetectors Based on Directionally Oriented Organic Bulk‐Heterojunctions

Abstract

Polarized spectroscopic photodetection enables numerous applications in diverse areas such as sensing, industrial quality control, and visible light communications. Although organic photodetectors (OPDs) can offer a cost‐effective alternative to silicon‐based technology—particularly when flexibility and large‐area arrays are desired—polarized OPDs are only beginning to receive due research interest. Instead of resorting to external polarization optics, this report presents polarized OPDs based on directionally oriented blends of poly(3‐hexylthiophene) (P3HT) and benchmark polymer or nonfullerene acceptors fabricated using a versatile solution‐based method. Furthermore, a novel postprocessing scheme based on backfilling and plasma etching is advanced to ameliorate high dark‐currents that are otherwise inherent to fibrillar active layers. The resulting polarized P3HT:N2200 OPDs exhibit a broad enhancement across all principal figures of merit compared to reference isotropic devices, including peak responsivities of 70 mA W−1 and up to a threefold increase in 3 dB bandwidth to 0.75 MHz under parallel‐polarized illumination. Polarization ratios of up to 3.5 are obtained across a spectral range that is determined by the specific donor–acceptor combinations. Finally, as a proof‐of‐concept demonstration, polarized OPDs are used for photoelasticity analysis of rubber films under tensile deformation, highlighting their potential for existing and emerging applications in advanced optical sensing.

Gas-assisted blade-coating of organic semiconductors: molecular assembly, device fabrication and complex thin-film structuring

“…Directional orientation is enabled by co-deposition with the crystallisable solvent 1,3,5-trichlorobenzene (TCB) which, under particular deposition conditions described in detail in ref. 27, provides a nucleation surface for subsequent epitaxial solidification of organic semiconductors. Hence, key to achieving a high degree of molecular orientation for a given deposition method [24][25][26][27] is ensuring maximally directional growth of fibrillar TCB crystals in a particular-typically, the coating-direction.…”

“…polarised OPDs with polarisation axes of individual devices oriented in multiple different directions for imaging-based polarimetry analysis. 27 Elsewhere, 2D-oriented organic semiconductor films offer a versatile and highly tuneable test-bed for studying anisotropic thermal transport using the emerging high-resolution optical methods. 51…”

“…27, provides a nucleation surface for subsequent epitaxial solidification of organic semiconductors. Hence, key to achieving a high degree of molecular orientation for a given deposition method [24][25][26][27] is ensuring maximally directional growth of fibrillar TCB crystals in a particular-typically, the coating-direction. The use of accelerated, gas-assisted drying represents an optimal means of obtaining a uniform drying front, which provides a moving gradient in solution supersaturation and, therewith, drives directional crystal growth.…”

“…22 Elsewhere, our recent work reported the use of an electric fan placed adjacent to the doctor blade to obtain, via the use of crystallisable co-solvents, [24][25][26] high-performance polarised organic photodetectors (OPDs) based on directionally oriented bulk heterojunction active layers. 27 However, despite its essential role in obtaining maximally anisotropic organic semiconductor blend films, 27 this 'fan-assisted drying' method generates a non-uniform airflow and remains poorly controllable.…”

“…Using common (macro-)molecular semiconductors, we study the broad scope of the influence of in situ drying on the resulting thin-film microstructure as well as the characteristics of devices based on blade-coated active layers. Specifically, using organic photodetectors (OPDs) as an exemplary class of optoelectronic devices, 8,27–29 we demonstrate that, for a given solution formulation, employing gas-assisted drying improves the overall device performance up to the benchmarks set by spin-coating-based fabrication. Finally, while conventional blade-coating and its ‘N 2 -knife’ extension, as well as spin-coating, are limited to the fabrication of spatially uniform material layers, we show that varying the spatial distribution of drying gas enables one-step deposition of 1D- and 2D-patterned organic semiconductor films for rapid prototyping and fabrication of complex device arrays.…”