Photodetectors are present in a vast variety of devices used in the fields of communication systems, safety and security, process control and environmental sensing. Organic photodetectors may be well-suited for applications that require large active area, mechanical flexibility, low-cost processing, or wavelength specificity. We fabricated transparent (average visible transmittance of 75%), sensitive (specific detectivity at 850 nm = 3 x 10¹² cm Hz^½ W^-1 at -0.1 V) and fast responding (< 1 microsecond) organic photodetectors and demonstrated such devices on large area (1.5 cm²) and on flexible substrates. Visibly transparent photodetectors can be combined with other optoelectronic devices and allow for new functionalities, such as image sensors or touchless interactive displays.

The picture shows a flexible organic photodiode with an active area of 1.5 cm² and an average visible transmittance of 75%.

 

By combining a near infrared-sensitive photodetector with a visible light-emitting diode, an all-organic upconverter can be fabricated. Such a device absorbs NIR light and emits light in the visible. Thereby, a NIR scene can be converted into a visible picture that can be captured by a conventional camera. In a Joint Brazilian-Swiss Research Project together with the Cremona group from Rio de Janeiro, we synthesize NIR dyes and fabricate organic upconverters that convert NIR photons beyond 1.1 µm into visible light.

 

The figure shows a schematic cross-section view of an organic upconverter.

 

(a) We synthesized a NIR selective squaraine dye with a film absorption maximum at 980 nm. An all-organic upconverter was fabricated by integrating the NIR squaraine photodetector with a fluorescent organic light-emitting diode. (b) the upconverter efficiency (0.27%) of NIR to visible light was close to the expected maximum. (c) The materials of the upconverter 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 upconverters with an average visible transmittance of 65% and a peak transmittance of 80% at 620 nm.

 

References:

H. Zhang et al., Scientific Reports 2015, 5, 9439; K. Strassel et al., ACS Appl. Mater. Interfaces 2018, 10, 11063.

Poster: Squaraine