Abstract
Organic photodetectors offer distinct advantages over their inorganic analogues, most notably through optical transparency and flexibility, yet their figures-of-merit still lag behind those of inorganic devices, and optimization strategies generally encounter a trade-off between device responsivity and bandwidth. Here we propose a novel photodetector architecture in which an organic photoactive semiconductor layer (S) is sandwiched between two thick insulating layers (I) that separate the semiconductor from the metallic contacts (M). In this architecture a differential photocurrent response is generated purely from the polarization of the active layer under illumination. Especially for an asymmetric MISIM design, where one insulating layer is a high-k ionic liquid IIL and the other a low-k polymer dielectric Ip, the responsivity/bandwidth trade-off is broken, since the role of the IIL in efficient charge separation is maintained, while the total device capacitance is reduced according to Ip. Thus the benefits of single insulating layer differential photodetectors (MISM) using either IIL or Ip are combined in a single device. Further improvements in device performance are also demonstrated by decreasing the series resistance of the photoactive layer through semiconductor:metal blending and by operation under strong background light.
Highlights
The importance of insulating materials in the advancement of organic photodiodes is often overshadowed by the plethora of literature devoted to the design and processing of the light harvesting organic semiconductor component(s) that serve as the locus of light to energy conversion in such devices[1,2]
This trade-off has been demonstrated by Hu et al for MISM devices based on solid-state dielectrics[20], and while the ionic liquid-based devices have shown improved responsivities compared to solid-state devices, their bandwidths are significantly reduced[19]
In this article we demonstrate a strategy to break the responsivity/bandwidth trade-off in differential photodetectors, exploiting the photocurrent enhancement observed in the ionic liquids (ILs) based MISM devices while maintaining the bandwidth of the low-k polymer devices
Summary
Further information on all the fabrication and testing methods can be found in the Supplementary Information. For the device transmission measurement, a perforated plate of ø = 1.0 mm was used to ensure the incident light passed through the patterned ITO electrode. A customised laser (spot size = 2 × 1 mm, λmax = 848 ± 10 nm, OPG1000PL-850, Scientex) was used to generate pulsed light signals of approximately square-wave shape with a duty cycle of about 50% over a frequency range of 10 kHz to 1 GHz, controlled by a function generator (Tektronix AWG2041) (see Supplemental Fig. SX1). In the case of the responsivity-bandwidth product (RBP), the responsivity was calculated using the pk-pk value of the photocurrent response for a square-wave device signal recorded at an appropriate modulation frequency, while the bandwidth was estimated from the rise time of this square-wave signal, as described in the manuscript above. The responsivity spectra were determined by dividing the photocurrent spectra by the wavelength-dependent light power (see Supplementary Fig. SX3)
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