Polymer:Fullerene Bimolecular Crystals for Near‐Infrared Spectroscopic Photodetectors

Abstract

Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable and bio-integrated electronics is hampered by the use of bulky dispersive optics. Here, we employ solution-processed organic donor-acceptor blends in a resonant optical cavity device architecture for wavelength-tunable photodetection. While conventional photodetectors respond to above-gap excitation, the cavity device exploits weak subgap absorption of intermolecular charge-transfer states of the intercalating Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT):[6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) bimolecular crystal. This enables a highly wavelength selective, nearinfrared photoresponse with a spectral resolution down to 14 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. A miniaturized spectrophotometer, comprising an array of narrowband photo-detectors is fabricated using blade-coated PBTTT:PCBM thin films with a thickness gradient. As application example, we demonstrate water transmittance spectral measured by this device.

Determining Ultralow Absorption Coefficients of Organic Semiconductors from the Sub‐Bandgap Photovoltaic External Quantum Efficiency

“…From a Gaussian fit to the absorption coefficient of PBTTT‐PC 71 BM, a charge transfer energy ( E CT ) of 1.16 eV and a reorganization energy λ of the donor–acceptor complex of 163 meV were determined as shown in Figure S3 of the Supporting Information. E CT of PBTTT‐PC 61 BM was previously reported to be 1.15 eV in a thin solar cell, where the active layer absorptance A in the device was approximated by A ≈ αt . As both E CT values are in excellent agreement, we performed the Gaussian fit on EQE exp spectra for different active layer thicknesses under the assumption of A ≈ αt .…”

“…The active layer absorptance spectrum A (λ) depends on the absorption coefficient α(λ), but also the wave‐optics of the multilayer stack device that is the solar cell or the photodetector. In the case of an optically thin layer with αt ≪ 1 (layer thickness t ) and in the limit of negligible cavity effects (such as a semiconductor film deposited on a transparent substrate), it is often assumed that A ≈ αt (or A ≈ 2 αt ) for hv < E g . Therefore the spectral line shape of the EQE should follow α via A .…”

Color‐Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication

“…As such, these devices have relied on device engineering approaches to limit their spectral responsivity range. Recently, solution‐processed wavelength selective OPDs have been realized by means of: i) optical filtering,5,22 ii) charge collection narrowing,23–25 and iii) cavity enhanced absorption 26–28. All of these techniques have demonstrated wavelength selective responsivity and enabled successful application in color reconstruction or IR‐spectroscopy.…”

High‐Throughput Multiparametric Screening of Solution Processed Bulk Heterojunction Solar Cells

“…In fact, the high control of the layer gradient slope has made possible the fabrication of organic near-IR microcavity based spectrophotometers where the spectral response is strongly determined by the active layer thickness. [35] For the PCDTBT:PC70BM thickness gradient sample an optical picture of the reflection already shows relatively strong interference patterns from white light as it can be seen in Figure 6a. When the optical image is compared to the corrected photocurrent measured along the thickness wedge, some correlation can be perceived.…”

“…Interestingly, this method can be used for other applications beyond OPV optimization, including the study of the geometrical confinement on phase transition temperatures, [32] determination of exciton diffusion lengths, [33] fabrication of position sensitive photodetectors, [34] or as miniature spectrophotometers based on microcavity resonators with a wedged active layer. [35] We extend here this method beyond thickness gradients to also include the preparation of films with D:A composition gradients. A simple method to achieve this is by employing the aforementioned doctor blade technique and casting simultaneously two or more drops of the pristine materials.…”