Evaluation of Carbon Based Molecular Junctions as Practical Photosensors

Molecular junctions with partially transparent top contacts permit monitoring photocurrents as probes of transport mechanism and potentially could act as photosensors with characteristics determined by the molecular layer inside the device. Previously reported molecular junctions containing nitroazobenzene (NAB) oligomers and oligomers of two different aromatic molecules in bilayers were evaluated for sensitivity, dark signal, responsivity, and limits of detection, in order to determine the device parameters which have the largest effects on photodetection performance. The long-range transport of photogenerated charge carriers permits the use of molecular layers thick enough to absorb a large fraction of the light incident on the layer. Thick layers also reduce the dark current and its associated noise, thus improving the limit of detection to a few nanowatts on a detector area of 0.00125 cm(2). Since the photocurrents have much lower activation energy than dark currents do, lowering the detector temperature significantly improves the limit of detection, although the present experiments were limited by environmental and instrumentation noise rather than detector noise. The highest specific detectivity (D*) for the current molecular devices was 3 x 10(7) cm s(1/2) /W (similar to 10(9), if only shot noise is considered) at 407 nm in a carbon/NAB/carbon junction with a molecular layer thickness of 28 nm. Although this is in the low end of the 10(6)-10(12) range for commonly used photodetectors, improvements in device design based on the current results should increase D* by 3-4 orders of magnitude, while preserving the wavelength selectivity and tunability associated with molecular absorbers. In addition, operation outside the 300-1000 nm range of silicon detectors and very low dark currents may be possible with molecular junctions.

Automated summary

Molecular junctions with partially transparent top contacts can be used as photosensors, and the device performance is determined by the molecular layer inside. By evaluating molecular junctions containing nitroazobenzene (NAB) oligomers and two different aromatic molecules in bilayers, device parameters affecting photodetection performance were determined. The use of thick molecular layers can reduce dark current and noise, improving detection limits, while lower temperature enhances detection limits due to the lower activation energy of photocurrents compared to dark currents.