  Wavelength discriminating systems typically consist of heavy benchtop-based
instruments, comprising diffractive optics, moving parts, and adjacent
detectors. For simple wavelengths measurements, such as lab-on-chip light
source calibration or laser wavelength tracking, which do not require
polychromatic analysis and cannot handle bulky spectroscopy instruments,
light-weight, easy-to-process, and flexible single-pixel devices are attracting
increasing attention. Here, we propose a device for wavelength tracking with
room-temperature phosphorescence at the heart of its functionality that
demonstrates a resolution down to one nanometer and below. It is
solution-processed from a single host-guest system comprising organic
room-temperature phosphors and colloidal quantum dots. The share of excited
triplet states within the photoluminescent layer is dependent on the excitation
wavelength and determines the afterglow intensity of the film, which is tracked
by a simple photodetector. Finally, an all-organic thin-film wavelength sensor
and two applications are demonstrated where our novel measurement concept
successfully replaces a full spectrometer.
