Polymer:Fullerene Bimolecular Crystals for Near-Infrared Spectroscopic Photodetectors

Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable and biointegrated electronics, is hampered by the use of bulky dispersive optics. Here, solution-processed organic donor-acceptor blends are employed in a resonant optical cavity device architecture for wavelength-tunable photodetection. While conventional photodetectors respond to above-gap excitation, the cavity device exploits weak subgap absorption of intermolecular charge-transfer states of the intercalating poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bimolecular crystal. This enables a highly wavelength selective, near-infrared photoresponse with a spectral resolution down to 14 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. Based on this concept, a miniaturized spectrophotometer, comprising an array of narrowband cavity photodetectors, is fabricated by using a blade-coated PBTTT:PCBM thin film with a thickness gradient. As an application example, a measurement of the transmittance spectrum of water by this device is demonstrated.

Enhancing Sensitivity of a Miniature Spectrometer Using a Real-Time Image Processing Algorithm

A real-time image processing algorithm is developed to enhance the sensitivity of a planar single-mode waveguide miniature spectrometer with integrated waveguide gratings. A novel approach of averaging along the arcs in a curved coordinate system is introduced which allows for collecting more light, thereby enhancing the sensitivity. The algorithm is tested using CdSeS/ZnS quantum dots drop casted on the surface of a single-mode waveguide. Measurements indicate that a monolayer of quantum dots is expected to produce guided mode attenuation approximately 11 times above the noise level.