Organic Bulk Heterojunction Infrared Photodiodes for Imaging Out to 1300 nm

Advanced Energy Materials

et al. 2019

Self Cite

IntroductionSupercapacitors are electrochemical energy storage devices that store charge through fast, reversible redox reactions, enable load-leveling, regenerative energy harvesting, and high power applications. [1][2][3][4][5][6] The energy density of a supercapacitor (E = CV 2 /2) is linearly dependent on the specific capacitance C and proportional to the square of the operational voltage V. The main strategies to increase energy depend upon the mechanism of charge storage, whether capacitive or Faradaic. Capacitive electrodes store charge on the surface; hence, research is focused on strategies to increase the surface area, typically based on high surface area carbons. [7][8][9] Faradaic materials undergo redox reactions and offer the ability to tune the operating voltage. There has been considerable success in tuning the cathode voltage and delivering high capacitance systems operating at the upper limit of prototypical nonaqueous electrolytes. [1,7] However, the device voltage and energy remain limited by a lack of complementary high power electrodes for the anode. [10,11] In the absence of new high voltage electrolytes, there is no room to further extend the cathode potential, because over-potential may lead to hazardous runaway reactions between the highly oxidized cathodes and flammable nonaqueous electrolytes. In contrast, it is generally safe for nonaqueous electrolytes to operate down to −2 V relative to the Ag/AgCl electrode, and the development of anode materials is critical to increase the energy densities of supercapacitors.Consequently, there is an urgent need for strategies aimed at extending the operational voltage toward negative potentials to improve the energy density and cell voltage of supercapacitors. As an alternative to metal oxides, redox-active macromolecules offer mechanical flexibility, low-cost, and scalability relevant for small and large scale applications. [12][13][14][15][16] In radical polymers, [17][18][19][20] the redox sites are at pendant radical groups distributed along a polymer backbone. Due to the insulating nature of the backbone and low conductivity, devices based on these materials require blending with additional conductive materials for electron transport, ultimately lowering the electrode energy Supercapacitors have emerged as an important energy storage technology offering rapid power delivery, fast charging, and long cycle lifetimes. While extending the operational voltage is improving the overall energy and power densities, progress remains hindered by a lack of stable n-type redox-active materials. Here, a new Faradaic electrode material comprised of a narrow bandgap donor−acceptor conjugated polymer is demonstrated, which exhibits an open-shell ground state, intrinsic electrical conductivity, and enhanced charge delocalization in the reduced state. These attributes afford very stable anodes with a coulombic efficiency of 99.6% and that retain 90% capacitance after 2000 charge-discharge cycles, exceeding other n-dopable organic materials. Redox cycling proce...

Section: Methodsmentioning

confidence: 99%

Wide Potential Window Supercapacitors Using Open‐Shell Donor–Acceptor Conjugated Polymers with Stable N‐Doped States

Wang

Huang

Advanced Energy Materials

et al. 2019

Self Cite

“…In short, a rapid change in the slope ∂C(ω)/∂ln(ω) reflects an increase of the subbandgap DOS. The DOS measured at the energy E ω is expressed as an exponential distribution of bandtail shallow traps 26,27 , and it may include a Gaussian distribution of deep traps 25,28 , if they are present:…”

Section: Energy Level Alignment and Degree Of Disorder In Bhjs With Dmentioning

confidence: 99%

Noise and detectivity limits in organic shortwave infrared photodiodes with low disorder

et al. 2020

npj Flex Electron

Self Cite

To achieve high detectivity in infrared detectors, it is critical to reduce the device noise. However, for non-crystalline semiconductors, an essential framework is missing to understand and predict the effects of disorder on the dark current. This report presents experimental and modeling studies on the noise current in exemplar organic bulk heterojunction photodiodes, with 10 donor-acceptor combinations spanning wavelength between 800 and 1600 nm. A significant reduction of the noise and higher detectivity were found in devices using non-fullerene acceptors (NFAs) in comparison to those using fullerene derivatives. The low noise in NFA blends was attributed to a sharp drop off in the distribution of bandtail states, as revealed by variable-temperature density-of-states measurements. Taking disorder into account, we developed a general physical model to explain the dependence of thermal noise on the effective bandgap and bandtail spread. The model provides theoretical targets for the maximum detectivity that can be obtained at different detection wavelengths in inherently disordered infrared photodiodes.

“…The incident light power is measured with a Ge photodiode (818-IR, Newport Corporation) for 780−1400 nm range and a Si photodiode (DET36A, Thorlabs) for 460−780 nm. Electrochemical impedance spectroscopy was performed with a potentiostat (SP200, Bio-logic) over the frequency range of 100 Hz to 2 MHz with a 100 mV amplitude, 39 with the phototransistor gate electrode floating.…”

Section: ■ Experimental Proceduresmentioning

confidence: 99%

Solution-Processed Phototransistors Combining Organic Absorber and Charge Transporting Oxide for Visible to Infrared Light Detection

Kim

ACS Appl. Mater. Interfaces

et al. 2019

Self Cite

This report demonstrates high-performance infrared phototransistors that use a broad-band absorbing organic bulk heterojunction (BHJ) layer responsive from the visible to the shortwave infrared, from 500 to 1400 nm. The device structure is based on a bilayer transistor channel that decouples charge photogeneration and transport, enabling independent optimization of each process. The organic BHJ layer is improved by incorporating camphor, a highly polarizable additive that increases carrier lifetime. An indium zinc oxide transport layer with high electron mobility is employed for rapid charge transport. As a result, the phototransistors achieve a dynamic range of 127 dB and reach a specific detectivity of 5 × 10 12 Jones under a low power illumination of 20 nW/cm 2 , outperforming commercial germanium photodiodes in the spectral range below 1300 nm. The photodetector metrics are measured with respect to the applied voltage, incident light power, and temporal bandwidth, demonstrating operation at a video-frame rate of 50 Hz. In particular, the frequency and light dependence of the phototransistor characteristics are analyzed to understand the change in photoconductive gain under different working conditions.