Shortwave infrared-absorbing squaraine dyes for all-organic optical upconversion devices

Karen Strassel,Wei-Hsu Hu,Sonja Osbild,Daniele Padula,Daniel Rentsch,Sergii Yakunin,Yevhen Shynkarenko,Maksym Kovalenko,Frank Nüesch,Roland Hany,Michael Bauer

doi:10.1080/14686996.2021.1891842

Abstract

ABSTRACT Shortwave infrared (SWIR) optical sensing and imaging are essential to an increasing number of next-generation applications in communications, process control or medical imaging. An all-organic SWIR upconversion device (OUC) consists of an organic SWIR sensitive photodetector (PD) and an organic light-emitting diode (OLED), connected in series. OUCs directly convert SWIR to visible photons, which potentially provides a low-cost alternative to the current inorganic compound-based SWIR imaging technology. For OUC applications, only few organic materials have been reported with peak absorption past 1000 nm and simultaneous small absorption in the visible. Here, we synthesized a series of thermally stable high-extinction coefficient donor-substituted benz[cd]indole-capped SWIR squaraine dyes. First, we coupled the phenyl-, carbazole-, and thienyl-substituted benz[cd]indoles with squaric acid (to obtain the SQ dye family). We then combined these donors with the dicyanomethylene-substituted squaraine acceptor unit, to obtain the dicyanomethylene-functionalized squaraine DCSQ family. In the solid state, the absorbance of all dyes extended considerably beyond 1100 nm. For the carbazole- and thienyl-substituted DCSQ dyes, even the peak absorptions in solution were in the SWIR, at 1008 nm and 1014 nm. We fabricated DCSQ PDs with an external photon-to-current efficiency over 30%. We then combined the PD with a fluorescent OLED and fabricated long-term stable OUCs with peak sensitivity at 1020 nm, extending to beyond 1200 nm. Our OUCs are characterized by a very low dark luminance (<10−2 cd m−2 at below 6 V) in the absence of SWIR light, and a low turn-on voltage of 2 V when SWIR light is present.

Highlights

Shortwave infrared (SWIR) photodetection and imaging offer new application fields in passive night vision, airborne remote sensing or machine vision solutions, including silicon wafer inspection, product quality control and sorting [1,2]
We synthesized a series of squaraines with the aim to obtain dyes with selective light absorption extending considerably into the SWIR wavelength range
We demonstrated efficient SWIR-sensitive PDs and organic SWIR upconverter (OUC)

Summary

Introduction

SWIR photodetection and imaging offer new application fields in passive night vision, airborne remote sensing or machine vision solutions, including silicon wafer inspection, product quality control and sorting [1,2]. For an all-organic SWIR upconverter (OUC) it is advantageous to use an organic PD material with selective absorption in the SWIR region This is because visible light absorption of a broad-band absorber material results in a non-selective response of the device, and visible emitted light from the OLED can be reabsorbed by the PD unit. We fabricated DCSQ-fullerene PDs with a maximum external photon-to-current conversion efficiency (EQE) at 1025 nm By combining this PD with a fluorescent Alq3-based OLED, OUCs were obtained that convert SWIR photons directly to visible green photons, with good performance in terms of a low turn-on voltage, a low dark luminance and a highly linear optical response

Synthesis

Molecular properties

Conclusions

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