Squaraine Dye for a Visibly Transparent All-Organic Optical Upconversion Device with Sensitivity at 1000 nm

Abstract

Efficient light detection in the near-infrared (NIR) wavelength region is central to emerging applications such as medical imaging and machine vision. An organic upconverter (OUC) consists of a NIR-sensitive organic photodetector (OPD) and an visible organic light-emitting diode (OLED), connected in series. The device converts NIR light directly to visible light, allowing imaging of a NIR scene in the visible. Here, we present an OUC composed of a NIR-selective squaraine dye-based OPD and a fluorescent OLED. The OPD has a peak sensitivity at 980 nm and an internal photon-to-current conversion efficiency of ∼100%. The OUC conversion efficiency (0.27%) of NIR to visible light is close to the expected maximum. The materials of the OUC multilayer stack absorb very little light in the visible wavelength range. In combination with an optimized semitransparent metal top electrode, this enabled the fabrication of transparent OUCs with an average visible transmittance of 65% and a peak transmittance of 80% at 620 nm. Visibly transparent OUCs are interesting for window-integrated electronic circuits or imaging systems that allow for the simultaneous detection of directly transmitted visible and NIR upconverted light.

Recent advances with optical upconverters made from all-organic and hybrid materials

“…When using a nontransparent top electrode the luminance must be measured from the same side as the incident NIR light. In this case, a fraction of emitted light can be re-absorbed in the photodetector unit thereby changing the (often) known electroluminescence spectrum of a stand-alone light-emitting device [25]. Therefore, it is advisable to measure the electroluminescence spectrum of the upconverter under operation.…”

“…Several all-organic upconverters (sometimes abbreviated as OUCs or OUDs) used a NIR organic photodiode in combination with an OLED based on fluorescent materials [25,[34][35][36][37]. An organic upconverter consisting of a single titanyl phthalocyanine (TiOPc, see Figure 3) absorber layer on indium tin oxide (ITO) was combined with an OLED containing tris-(8-hydroxy quinoline) aluminium (Alq 3 ) as the emissive layer including a hole transporting layer, and light at 650 nm was upconverted to green light at 530 nm [35].…”

“…A photodetector with peak sensitivity at 980 nm and an internal photon-to-current conversion efficiency of close to 100% under reverse bias was demonstrated with a NIR-selective squaraine dye (SQ-880):PC 61 BM blend, where PC 61 BM denotes [6,6]-phenyl-C 61 -butyric acid methyl ester [25]. The photodetector was integrated in an upconverter by using a MoO 3 hole transport layer to facilitate charge injection into the Alq 3 emitter.…”

“…A final processing challenge is that in an upconverter the top electrode should preferably be visibly transparent, in order that the emitted visible light can be detected at the opposite side of the incident NIR light. So far, semitransparent cathodes of upconverters were deposited as thin metal/dielectric layer via thermal evaporation [25][26][27]. However, several concepts for solution-processable transparent electrodes can be borrowed from related fields of research, such as spray-coating of metal nanowires or lamination of carbon nanotubes and graphene [55,56].…”

“…In addition to manufacturing benefits, organic and hybrid materials have the advantage over broadband absorbing inorganic semiconductors that their optical bandgap and absorption bandwidth can be tailored, to a greater or lesser extent. For the photodetector part in an upconverter, it is particularly desirable to have narrowband NIR-selective sensitive materials that do not absorb in the visible [4,[25][26][27][28]. Substantial progress is being made with individual organic and hybrid photodetectors, and lightemitting devices [3,4,[29][30][31] and it is anticipated that inspiration can be obtained from these standalone devices for the design and realization of improved upconverters.…”