High performence visble-near infrared photovoltaic detector based on narrow bandgap polymer

Abstract

Polymer-based visible-near infrared photodetectors have attracted considerable attention in the recent years due to their unique advantages of low cost of fabrication, compatibility with lightweight/flexible electronics, and wide material sources. Current researches mainly focus on high performence visble-near infrared photovoltaic detector based on narrow bandgap polymer. Device structure of the photodetector is ITO/PEDOT:PSS/photosensitive layer/Ca/Al. The weak light (0.4 mW/cm2, 800 nm) and reverse bias (-2 V) induce insignificant differences in photocurrent among the devices. Current values of 1.69×10-4 A/cm2, 7.96×10-5 A/cm2 and 6.98×10-5 A/cm2 are obtained with photosensitive layer thickness values of 100, 200 and 300 nm, respectively. However, the dark current density-voltage characteristics of the detectors with various thickness values of the photosensitive layer show that reverse bias (-2 V) induces significant differences in current among the devices. Current values of 1.35×10-6 A/cm2, 1.13×10-7 A/cm2 and 2.98×10-8 A/cm2 are obtained with photosensitive layer thickness values of 100 nm, 200 nm and 300 nm, respectively. Meanwhile, all detectors possess high rectification ratios over 105(±2 V), indicating good diode rectification characteristics. Photosensitivity measurements show that detection spectral regions of the detectors are extended from 380 nm to 960 nm. The values of detectivity (D*) of detectors with various thickness values of photosensitive layers are investigated, and the obtained values of D* of tested detectors are found to be very stable in a range from 400 nm to 860 nm, and the average D* value for the 300 nm thick device in this spectral range is as high as 6.89×1012 Jones. The latter compares well with values obtained with silicon detectors. In a range from 800 nm to 900 nm, the estimated detectivities of the 300 nm and 200 nm thick detectors are slightly higher than those obtained with InGaAs devices. Through analyzing energy band diagrams of the polymer photodetectors under reverse voltage bias it could be argued that the relatively weak electric field in the thicker device is the origin of the lower noise current density. The capacitance characteristics of polymer based detectors at high frequency (100 kHz) are examined through capacitance-voltage curves, and the resulting data show that capacitances of all devices at reverse and even small positive voltage are constant. This indicates that the device photosensitive layers are fully depleted and fast signal detections are theoretically possible. The time responses of detectors under near-infrared stimulation are also examined. The output signal appears to rise and fall periodically according to the input signal, suggesting a good repeatability. The rise and fall times for the devices are recorded to be ~5 μs and ~50 μs, indicating that the polymer photodetectors have quick response capabilities.