Acceptor-Substituted Squaraine Dyes for Organic Optical Upconversion Devices

Near infrared optical sensing and imaging are essential to an increasing number of next-generation photodetector applications in communications, process control or medical imaging. Recently, 1,3-squaraine dyes have been established as near-infrared absorbing or emitting dyes for these type of applications. However, most squaraine dyes show peak absorptions well below 1000 nm, a threshold wavelength below which organic photodetectors can hardly compete with the current inorganic silicon-based photodetector technology. Here, we show an approach to shift the peak absorption wavelength of squaraine dyes above 1000 nm through substitution on the central squaric acid core. The acceptor strength of this central moiety is enhanced by incorporation of dicyano- and rhodanine electron accepting groups. In addition, the central squaraine core is flanked by two benz[<i>cd</i>]indoles to generate a pronounced donor-acceptor-donor π-system. The feasibility of these core-substituted squaraine dyes for NIR detection above 1000 nm is demonstrated by integration in all-organic upconverter devices. These devices consist of an organic NIR sensitive photodetector and an organic light-emitting diode, connected in series. Organic upconverters convert NIR photons to visible photons and allow for pixel-free direct NIR imaging. Our NIR squaraine-based upconverters show a response well below the band gap edge of silicon and provide a low-cost opportunity for novel consumer and low-end imaging applications far into the shortwave infrared range.<br/><br/>[1] K. Strassel, W. Hu, S. Osbild, D. Padula, D. Rentsch, S. Yakunin, Y. Shynkarenko, M. Kovalenko, F. Nueesch, R. Hany, M. Bauer, Sci. Technol. Adv. Mater., 2021, 22, 194-204.<br/>[2] K. Strassel, A. Kaiser, S. Jenatsch, A. C. Véron, S. B. Anantharaman, E. Hack, M. Diethelm, F. Nueesch, R. Aderne, C. Legnani, S. Yakunin, M. Cremona, R. Hany, ACS Appl. Mater. Interfaces 2018, 10, 11063−11069