Plastic NIR Photodetector Created with Low Bandgap Polymer

Michael A. Greenwood

Although inorganic photodetectors are finding their way into myriad applications, relatively little has been done with their close cousins, polymer-based near-infrared sensors.

Despite a range of potentially valuable uses, such as chemical and biological sensing, optical communications, and spectroscopic and medical instrumentation, these organic photodetectors remain hampered by slow speed and high power consumption.

The current voltage (I-V) characteristics of a plastic near-IR photodetector were measured in the dark and under 850-nm monochromatic illumination. Reprinted with permission of Advanced Materials.

Researcher Yang Yang and colleagues at the University of California, Los Angeles, at the University of Chicago and at Solarmer Energy Inc. in El Monte, Calif., are seeking to overcome these limitations with the recent fabrication of a low-bandgap polymer for use as the active layer in a near-IR photodetector. The prototype polymer has the potential to result in lower costs, and it could be used for the construction of large, flexible sensors.

The team used an ester group modified polythieno[3,4-b]thiophene (PTT) polymer, lowering the highest occupied molecular orbital energy level of the low bandgap. The long side chain of the ester group also made the polymer soluble and allowed it to be easily processed. The active layer of the photodetector consisted of the electron acceptor (6,6)-phenyl C₆₁-butyric acid methyl ester.

The PTT polymer alone absorbed in the near-IR range with a peak at 750 nm and another slight peak at 850 nm. The polymer had an absorption onset at 970 nm, with an optical bandgap of 1.3 eV.

Photodetector responsiveness was tested with an Optek Technology near-IR laser diode working at 850 nm. It achieved an external quantum efficiency of 19 percent, increasing to more than 38 percent at –5 V.

This result is comparable to inorganic photodetectors, said researcher Yan Yao of the University of California. The device had a response time of 4 MHz.

By optimizing the effect of solvent selection, film coating parameters and phase separation, the investigators believe that the dark current can be further reduced and that still higher external quantum efficiencies can be achieved.

Preliminary data also suggest that the plastic photodetector degrades by nearly 20 percent when stored in air over a six-month period. The device is not fully optimized, however, and the researchers believe that, by modifying the film morphology and processing conditions, the resilience of the photodetector likely can be improved.

Advanced Materials, November 2007, pp. 3979-3983.