Broadband Infrared Photodetection Using a Narrow Bandgap Conjugated Polymer

Photodetection spanning the short-, mid-, and long-wave infrared (SWIR-LWIR) underpins modern science and technology. Devices using state-of-the-art narrow bandgap semiconductors require complex manufacturing, high costs, and cooling requirements that remain prohibitive for many applications. We report high-performance infrared photodetection from a donor-acceptor conjugated polymer with broadband SWIR-LWIR operation. Electronic correlations within the π-conjugated backbone promote a high-spin ground state, narrow bandgap, long-wavelength absorption, and intrinsic electrical conductivity. These previously unobserved attributes enabled the fabrication of a thin-film photoconductive detector from solution, which demonstrates specific detectivities greater than 2.10 × 109 Jones. These room temperature detectivities closely approach those of cooled epitaxial devices. This work provides a fundamentally new platform for broadly applicable, low-cost, ambient temperature infrared optoelectronics

Nanoplasmonic Array Enhancement of Two-Photon Absorption in a Dye Film

We investigate the enhancement of the effective two-photon absorption cross section of a film of an organic dye by a plasmonic triangular prism array through finite element calculations and experimental measurements. Hexagonal arrays of plasmonic triangular prism arrays were prepared using nanosphere lithography so that their localized surface plasmon resonance (LSPR) is at 800 nm. A dye, AF455, with significant two-photon fluorescence when excited at 800 nm was spin-coated onto the plasmonic array. Several film thicknesses of AF455 were prepared, ranging from 40 to 184 nm. The dependence of the effective two-photon absorption cross section σ⁽²⁾ of AF455 on the thickness of the dye layer was measured using a newly applied technique. Because two-photon fluorescence is only sensitive to light absorbed by the chromophore, absorption from the nanostructure, thermal effects, and other parasitic optical mechanisms that could indicate anomalously high σ⁽²⁾ enhancement values are eliminated from the measured enhancement. The results quantitatively agreed with the σ⁽²⁾ enhancement values predicted by finite element method (FEM) calculations. The simulations show that the σ⁽²⁾ enhancement observed was due to the plasmonic triangular prism arrays’ LSPR and that the dependence of σ⁽²⁾ of AF455 on the gold nanostructure is influenced by an optical reflection pattern generated by the plasmonic array in addition to the near-field enhancement. The combination of theory and experiments validates the application of the technique to σ⁽²⁾ enhancement measurements

Triplet Sensitization in an Anionic Poly(phenyleneethynylene) Conjugated Polyelectrolyte by Cationic Iridium Complexes

We describe a systematic study of triplet sensitization in a poly­(phenyleneethynylene) conjugated polyelectrolyte (CPE) in methanol solution by using a series of three cationic iridium complexes with varying triplet energy. The cationic iridium complexes bind to the anionic CPE by ion-pairing, leading to singlet state quenching of the polymer, and allowing for efficient back-transfer of triplet excitation energy to the polymer. Efficient (amplified quenching) of the polymer’s fluorescence is observed for each iridium complex, with Stern–Volmer quenching constants in excess of 10⁵ M^–1. Triplet sensitization is confirmed for two of the iridium complexes by monitoring the relative yield of the CPE triplet state by transient absorption spectroscopy. One of the iridium complexes does not sensitize the CPE triplet, and consideration of the energies of the three complexes allows us to bracket the triplet energy of the CPE within the range 1.95–2.26 eV

Solution-Processable Infrared Photodetectors: Materials, Device Physics, and Applications

This review is written to introduce infrared photon detectors based on solution-processable semiconductors. A new generation of solution-processable photon detectors have been reported in the past few decades based on colloidal quantum dots, two-dimensional materials, organics semiconductors, and perovskites. These materials offer sensitivity within the infrared spectral regions and the advantages of ease of fabrication at low temperature, tunable materials properties, mechanical flexibility, scalability to large areas, and compatibility with monolithic integration, rendering them as promising alternatives for infrared sensing when compared to vacuum-processed counterparts that require rigorous lattice matching during integration. This work focuses on infrared detection using disordered semiconductors so as to articulate how the inherent device physics and behaviors are different from conventional crystalline semiconductors. The performance of each material family is summarized in tables, and device designs unique to solution-processed materials, including narrowband photodetectors and pixel-less up-conversion imagers, are highlighted in application prototypes distinct from conventional infrared cameras. We share our perspectives in examining open challenges for the development of solution-processable infrared detectors and comment on recent research directions in our community to leverage the advantages of solution-processable materials and advance their implementation in next-generation infrared sensing and imaging applications

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

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 thin-film materials such as amorphous silicon and vanadium oxide

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

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 thin-film materials such as amorphous silicon and vanadium oxide

Triplet Exciton Diffusion in Platinum Polyyne Films

A time-resolved photoluminescence quenching approach is developed for determining the triplet exciton diffusion coefficient and diffusion length (<i>D</i> and <i>L</i>_D, respectively) of phosphorescent conjugated polymers. This method is applied to a solid-state organometallic conjugated polymer with the structure [−Pt­(PBu₃)₂–CC–C₆H₄–CC−]<i>_n</i> (where Bu = <i>n</i>-butyl and −C₆H₄– is 1,4-phenylene). The approach relies on analysis of the lifetime quenching of the polymer’s phosphorescence by a series of three different quenchers that are dispersed into the polymer phase at varying concentration. The lifetime quenching data are analyzed by using a modified Stern–Volmer quenching expression to determine the diffusion-controlled quenching rate constant, <i>k</i>_q, which is then related to the exciton diffusivity, <i>D</i>, and diffusion length, <i>L</i>_D. The diffusion coefficients that are determined using the three quenchers are consistent, <i>D</i> ≈ 4 × 10^–6 cm² s^–1, and combined with the triplet exciton lifetime of the pristine polymer (τ = 1.05 μs) give an exciton diffusion length <i>L</i>_D ≈ 22 nm. The results are compared to literature studies of singlet exciton diffusion in conjugated polymers and triplet exciton diffusion in molecular materials