Contribution of Sub-Gap States to Broadband Infrared Response in Organic Bulk Heterojunctions

Abstract

This work studied a series of infrared detectors comprised of organic bulk heterojunctions to explain the origin of their broadband spectral response from the visible to the infrared spanning 1 to 8 μm and the transition from photonic to bolometric operation. Through comparisons of the detector current and the sub-bandgap density of states, the mid-and long-wave infrared response was attributed to charge trap-and-release processes that impact thermal charge generation and the activation energy of charge mobility. We further demonstrate how the sub-bandgap characteristics, mobility activation energy, and effective bandgap are key design parameters for controlling the device temperature coefficient of resistance, which reached up to −7%/K, better than other thinfilm materials such as amorphous silicon and vanadium oxide.

Infrared-Light Visualization by Organic Upconversion Devices

“…3,14,30 It is reported that organic semiconductors with abundant midgap states tend to respond to thermal signals with wavelengths located in the midwave and long-wave infrared. 31 These results are encouraging for the design of organic upconversion imagers for different wavebands. To facilitate human eye vision, green organic LEDs are most frequently employed in upconversion devices since human eyes are naturally sensitive to green photons.…”

“…Device designs such as incorporating etalon structures, photonic structures, and so on, are also effective ways to adjust the response spectrum of organic detectors. − The light absorption of organic semiconductors can cover a very broad spectrum ranging from ultraviolet to SWIR wavelengths. Benefiting from the photovoltaic field, the organic materials sensitive to visible and near-infrared (NIR) light are very mature. ,,, Organic materials with sensitivity to SWIR wavelengths are still under development. ,, It is reported that organic semiconductors with abundant midgap states tend to respond to thermal signals with wavelengths located in the midwave and long-wave infrared . These results are encouraging for the design of organic upconversion imagers for different wavebands.…”

Infrared Light Detection Technology Based on Organics

“…The response for λ < 1.7 μm is photonic, and the response for the 38 μm is bolometric. Parts (k) and (l) are reproduced with permission from ref .…”

“…Recently, Li et al demonstrated a promising thermal response in organic BHJ detectors . The BHJ detectors combined photonic and bolometric mechanisms to achieve broadband detection spanning the visible to longwave infrared (0.4–8 μm), with photonic charge generation for visible to shortwave infrared light (0.4–1.5 μm) and bolometric effects for the midwave infrared to longwave infrared (3–8 μm).…”

“…Some other organic materials are also investigated as the active layer in bolometers, 94 while an in-depth explanation of the origins of the long-wavelength response in organic materials remains nascent. 112 The BHJ detectors combined photonic and bolometric mechanisms to achieve broadband detection spanning the visible to longwave infrared (0.4−8 μm), with photonic charge generation for visible to shortwave infrared light (0.4−1.5 μm) and bolometric effects for the midwave infrared to longwave infrared (3−8 μm). The photonic response is realized by the direct photon absorption, as schematically shown in Figure 2k (left), while the bolometric response is correlated to the distribution of subgap states.…”