Organic Bulk Heterojunction Infrared Photodiodes for Imaging Out to 1300 nm

This work studies organic bulk heterojunction photodiodes with a wide spectral range capable of imaging out to 1.3 μm in the shortwave infrared. Adjustment of the donor-to-acceptor (polymer:fullerene) ratio shows how blend composition affects the density of states (DOS) which connects materials composition and optoelectronic properties and provides insight into features relevant to understanding dispersive transport and recombination in the narrow bandgap devices. Capacitance spectroscopy and transient photocurrent measurements indicate the main recombination mechanisms arise from deep traps and poor extraction from accumulated space charges. The amount of space charge is reduced with a decreasing acceptor concentration; however, this reduction is offset by an increasing trap DOS. A device with 1:3 donor-to-acceptor ratio shows the lowest density of deep traps and the highest external quantum efficiency among the different blend compositions. The organic photodiodes are used to demonstrate a single-pixel imaging system that leverages compressive sensing algorithms to enable image reconstruction

Novel Organic Shortwave Infrared Photosensors

A low-cost and scalable short-wavelength infrared (SWIR: λ = 1–3 μm) photosensor will be widely deployable and transformative for a wide range of spectroscopic systems and optoelectronics that form the foundation for scientific, industrial, medical, and defense applications. Conventional SWIR photosensors are cost-prohibitive due to complex fabrication involving epitaxial growth of inorganic crystals. To make SWIR photodetectors affordable for ubiquitous sensing, this thesis aims to realize low-cost organic SWIR bulk heterojunction (BHJ) photodiodes by using a novel class of modular narrow bandgap conjugated polymers, and to demonstrate the direct solution deposition of organic thin films can replace complex manufacturing processes and produce the comparable performance.While polymeric semiconducting materials have been widely used and extensively studied in organic photovoltaics, different device behaviors accompany the progressive shrinkage of polymer bandgap to extend spectral absorption out to SWIR region. A better understanding of the fundamental properties of SWIR organic photodiode (OPD) is therefore needed to predict and advance performance. This thesis focuses on understanding different aspects of OPD performance that centers around specific detectivity, which ultimately defines the signal-to-noise ratio.Firstly, in Chapter 3, the challenges in the dark noise increase associated with the low-bandgap organic system are discussed, followed by an introduction to different noise suppression methods. An emphasis is made on the importance of direct noise measurement to stay out of the pitfall of overestimating detectivity. Two approaches including interface engineering and electrode work function tuning are demonstrated to suppress the noise current in SWIR OPDs. Secondly, in Chapter 4, an improved model to decouple dissociation and collection efficiency is proposed to pinpoint the limiting factor of SWIR OPDs, and exemplary device engineering methods are shown to improve the dissociation bottleneck. Then, in Chapter 5, two recombination mechanisms limiting the device efficiency are shed light upon by connecting the optoelectronic properties to the materials composition. Lastly, in Chapter 6, three applications are demonstrated to show the practicality of solution-processed SWIR OPDs and the potential in realizing the low-cost and large-scale SWIR sensing

Novel Organic Shortwave Infrared Photosensors

A low-cost and scalable short-wavelength infrared (SWIR: λ = 1–3 μm) photosensor will be widely deployable and transformative for a wide range of spectroscopic systems and optoelectronics that form the foundation for scientific, industrial, medical, and defense applications. Conventional SWIR photosensors are cost-prohibitive due to complex fabrication involving epitaxial growth of inorganic crystals. To make SWIR photodetectors affordable for ubiquitous sensing, this thesis aims to realize low-cost organic SWIR bulk heterojunction (BHJ) photodiodes by using a novel class of modular narrow bandgap conjugated polymers, and to demonstrate the direct solution deposition of organic thin films can replace complex manufacturing processes and produce the comparable performance.While polymeric semiconducting materials have been widely used and extensively studied in organic photovoltaics, different device behaviors accompany the progressive shrinkage of polymer bandgap to extend spectral absorption out to SWIR region. A better understanding of the fundamental properties of SWIR organic photodiode (OPD) is therefore needed to predict and advance performance. This thesis focuses on understanding different aspects of OPD performance that centers around specific detectivity, which ultimately defines the signal-to-noise ratio.Firstly, in Chapter 3, the challenges in the dark noise increase associated with the low-bandgap organic system are discussed, followed by an introduction to different noise suppression methods. An emphasis is made on the importance of direct noise measurement to stay out of the pitfall of overestimating detectivity. Two approaches including interface engineering and electrode work function tuning are demonstrated to suppress the noise current in SWIR OPDs. Secondly, in Chapter 4, an improved model to decouple dissociation and collection efficiency is proposed to pinpoint the limiting factor of SWIR OPDs, and exemplary device engineering methods are shown to improve the dissociation bottleneck. Then, in Chapter 5, two recombination mechanisms limiting the device efficiency are shed light upon by connecting the optoelectronic properties to the materials composition. Lastly, in Chapter 6, three applications are demonstrated to show the practicality of solution-processed SWIR OPDs and the potential in realizing the low-cost and large-scale SWIR sensing

Elucidating the Detectivity Limits in Shortwave Infrared Organic Photodiodes

p\u3eWhile only few organic photodiodes have photoresponse past 1 µm, novel shortwave infrared (SWIR) polymers are emerging, and a better understanding of the limiting factors in narrow bandgap devices is critically needed to predict and advance performance. Based on state‐of‐the‐art SWIR bulk heterojunction photodiodes, this work demonstrates a model that accounts for the increasing electric‐field dependence of photocurrent in narrow bandgap materials. This physical model offers an expedient method to pinpoint the origins of efficiency losses, by decoupling the exciton dissociation efficiency and charge collection efficiency in photocurrent–voltage measurements. These results from transient photoconductivity measurements indicate that the main loss is due to poor exciton dissociation, particularly significant in photodiodes with low‐energy charge‐transfer states. Direct measurements of the noise components are analyzed to caution against using assumptions that could lead to an overestimation of detectivity. The devices show a peak detectivity of 5 × 1010 Jones with a spectral range up to 1.55 µm. The photodiodes are demonstrated to quantify the ethanol–water content in a mixture within 1% accuracy, conveying the potential of organics to enable economical, scalable detectors for SWIR spectroscopy

Elucidating the Detectivity Limits in Shortwave Infrared Organic Photodiodes

p\u3eWhile only few organic photodiodes have photoresponse past 1 µm, novel shortwave infrared (SWIR) polymers are emerging, and a better understanding of the limiting factors in narrow bandgap devices is critically needed to predict and advance performance. Based on state‐of‐the‐art SWIR bulk heterojunction photodiodes, this work demonstrates a model that accounts for the increasing electric‐field dependence of photocurrent in narrow bandgap materials. This physical model offers an expedient method to pinpoint the origins of efficiency losses, by decoupling the exciton dissociation efficiency and charge collection efficiency in photocurrent–voltage measurements. These results from transient photoconductivity measurements indicate that the main loss is due to poor exciton dissociation, particularly significant in photodiodes with low‐energy charge‐transfer states. Direct measurements of the noise components are analyzed to caution against using assumptions that could lead to an overestimation of detectivity. The devices show a peak detectivity of 5 × 1010 Jones with a spectral range up to 1.55 µm. The photodiodes are demonstrated to quantify the ethanol–water content in a mixture within 1% accuracy, conveying the potential of organics to enable economical, scalable detectors for SWIR spectroscopy

Temperature-Dependent Detectivity of Near-Infrared Organic Bulk Heterojunction Photodiodes.

Bulk heterojunction photodiodes are fabricated using a new donor-acceptor polymer with a near-infrared absorption edge at 1.2 μm, achieving a detectivity up to 1012 Jones at a wavelength of 1 μm and an excellent linear dynamic range of 86 dB. The photodiode detectivity is maximized by operating at zero bias to suppress dark current, while a thin 175 nm active layer is used to facilitate charge collection without reverse bias. Analysis of the temperature dependence of the dark current and spectral response demonstrates a 2.8-fold increase in detectivity as the temperature was lowered from 44 to -12 °C, a relatively small change when compared to that of inorganic-based devices. The near-infrared photodiode shows a switching speed reaching up to 120 μs without an external bias. An application using our NIR photodiode to detect arterial pulses of a fingertip is demonstrated

Temperature-Dependent Detectivity of Near-Infrared Organic Bulk Heterojunction Photodiodes

Bulk heterojunction photodiodes are fabricated using a new donor–acceptor polymer with a near-infrared absorption edge at 1.2 μm, achieving a detectivity up to 1012 Jones at a wavelength of 1 μm and an excellent linear dynamic range of 86 dB. The photodiode detectivity is maximized by operating at zero bias to suppress dark current, while a thin 175 nm active layer is used to facilitate charge collection without reverse bias. Analysis of the temperature dependence of the dark current and spectral response demonstrates a 2.8-fold increase in detectivity as the temperature was lowered from 44 to −12 °C, a relatively small change when compared to that of inorganic-based devices. The near-infrared photodiode shows a switching speed reaching up to 120 μs without an external bias. An application using our NIR photodiode to detect arterial pulses of a fingertip is demonstrated

Temperature-Dependent Detectivity of Near-Infrared Organic Bulk Heterojunction Photodiodes

Bulk heterojunction photodiodes are fabricated using a new donor-acceptor polymer with a near-infrared absorption edge at 1.2 μm, achieving a detectivity up to 1012 Jones at a wavelength of 1 μm and an excellent linear dynamic range of 86 dB. The photodiode detectivity is maximized by operating at zero bias to suppress dark current, while a thin 175 nm active layer is used to facilitate charge collection without reverse bias. Analysis of the temperature dependence of the dark current and spectral response demonstrates a 2.8-fold increase in detectivity as the temperature was lowered from 44 to -12 °C, a relatively small change when compared to that of inorganic-based devices. The near-infrared photodiode shows a switching speed reaching up to 120 μs without an external bias. An application using our NIR photodiode to detect arterial pulses of a fingertip is demonstrated