Transport Effects on Capacitance-Frequency Analysis for Defect Characterization in Organic Photovoltaic Devices

Xu, Liang; Wang, Jian; Hsu, Julia W. P.

doi:10.1103/physrevapplied.6.064020

Cited by 40 publications

(49 citation statements)

References 64 publications

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“…On the one hand, this enables the characterization of defects by a straightforward technique, such as IS. On the other hand, such a technique probes many different effects in the device, especially in an organic one, which can show identical capacitive spectra to those generated by traps, inspiring discussion in the community whether such a measurement can in fact reveal the trap characteristics 45 – 47 . Special caution should be taken when using this method in low-mobility materials and in devices where energetic transport barriers are present.…”

Section: Resultsmentioning

confidence: 99%

Reverse dark current in organic photodetectors and the major role of traps as source of noise

et al. 2021

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Organic photodetectors have promising applications in low-cost imaging, health monitoring and near-infrared sensing. Recent research on organic photodetectors based on donor–acceptor systems has resulted in narrow-band, flexible and biocompatible devices, of which the best reach external photovoltaic quantum efficiencies approaching 100%. However, the high noise spectral density of these devices limits their specific detectivity to around 1013 Jones in the visible and several orders of magnitude lower in the near-infrared, severely reducing performance. Here, we show that the shot noise, proportional to the dark current, dominates the noise spectral density, demanding a comprehensive understanding of the dark current. We demonstrate that, in addition to the intrinsic saturation current generated via charge-transfer states, dark current contains a major contribution from trap-assisted generated charges and decreases systematically with decreasing concentration of traps. By modeling the dark current of several donor–acceptor systems, we reveal the interplay between traps and charge-transfer states as source of dark current and show that traps dominate the generation processes, thus being the main limiting factor of organic photodetectors detectivity.

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Section: Resultsmentioning

confidence: 99%

Reverse dark current in organic photodetectors and the major role of traps as source of noise

et al. 2021

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“…At further increased forward-bias voltages, the hole current motion across the device proceeds towards the drift-dominant, space-charge-limited regime, while the capacitance rolls off towards three-quarters (0.75) of the geometric capacitance C geo [38,131,134]. Moreover, the frequency-dependent capacitance response of semiconductor diodes in the presence of localized defect states [22,40,135,136] has been well documented in the literature. In conjunction with probing the intrinsic C − V caused by the gradual transition from diffusion-to drift-governed transport, because of the frequency-tunable small-signal ac modulation, the C − V measurements inherently capture an extra capacitance contribution from charges thermally released from trap states.…”

Section: Capacitance-voltage Characteristicsmentioning

confidence: 87%

“…In order to correlate the distribution of sub-bandgap density of states (DOS) with its impact on charge transport, a variety of time-resolved, frequency-resolved, and temperaturedependent (opto)electrical probes has been used to extract information about the sub-bandgap DOS and its relation to charge transport. For instance, transient current measurements including time-of-flight (TOF) photocurrent may be interpreted in terms of the rate of charge-carrier release from trap states [2][3][4]26,27], thermally stimulated current (TSC) [28][29][30][31] or luminescence (TSL) [32,33] measurements analyzed in terms of thermal activation out of trap states, and frequency-domain methods [34][35][36][37][38][39][40] interpreted in terms of electrical perturbation of charge induced by a frequency-variable small-signal ac voltage superimposed on a dc bias, such that carrier motion as well as trapping and detrapping processes can be observed. However, these varying experimental methods generally expose devices to different measurement conditions (e.g., applied bias, device thickness, choice of contacts, use of background illumination or laser pulse), and data interpretation involves different theoretical approximations.…”

Section: Introductionmentioning

confidence: 99%

Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β -phase in Poly(9,9-dioctylfluorene)

et al. 2019

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Charge transport in π-conjugated polymers is characterized by a strong degree of disorder in both the energy of conjugated segments and the electronic coupling between adjacent sites. This disorder arises from variations in the structure and conformation of molecular units, as well as the weak intermolecular binding interactions. Although disorder in molecular conformation can be expected to influence the density of states (DOS) distribution-and hence, optoelectronic properties of the material-until now, there has been no direct study of the relationship between a distinct conformational defect and the charge transport properties of a conjugated polymer. Here, we investigate the impact of introducing an extended, planarized chain geometry, known as the "β-phase," on hole transport through otherwise amorphous films of poly(9,9dioctylfluorene) (PFO). We show that while β-phase introduces a striking drop of about a hundredfold in time-of-flight (TOF) hole mobility (μ h ) at room temperature, it reduces the steady-state μ h measured from hole-only devices by a factor of less than about 5. In order to reconcile these observations, we combine high-dynamic-range TOF photocurrent spectroscopy and energy-resolved electrochemical impedance spectroscopy to extract the hole DOS of the conjugated polymer. Both methods show that the effect of the β-phase content is to introduce a sharp sub-bandgap feature into the DOS of glassy PFO lying about 0.3 eV above the highest occupied molecular orbital. The observed energy of the conformational trap is consistent with electronic structure calculations using a tight-binding approach. Using the obtained DOS with a driftdiffusion model capable of resolving charge carriers in both time and energy, we show how the seemingly contradictory transport phenomena obtained via the time-resolved, frequency-resolved, and steady-state methods are reconciled. The results highlight the significance of energetic redistribution of charge carriers in affecting transport behavior. This work demonstrates how charge-carrier mobility in organic semiconductors can be controlled via molecular conformation, and it resolves a long-standing debate over how different (equilibrium versus nonequilibrium) transport techniques reveal electronic properties of disordered solids in a unified manner.

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“…where ∆E is the energy difference of the trap energy level to the conduction/valence band is the conduction/valence band effective density of states, v th the thermal velocity, and σ n,p the electron/hole capture cross section. [34]- [37] When an AC signal is applied in capacitance measurements, the AC frequency determines which trap levels can be probed. Thus, there is maximal frequency at which the defect states are still able to respond to the voltage modulation.…”

Section: A Negative Capacitance In Siqd-ledsmentioning

confidence: 99%

“…The trap states need to fulfill the condition ω AC = τ −1 trap with the angular frequency of the AC signal ω AC = 2πf AC , while f AC is the measurement frequency. [34], [37] From this condition, an energy level known as demarcation [34], [36], [38]- [41] can be determined by rearranging Equation. 1:…”

Section: A Negative Capacitance In Siqd-ledsmentioning

confidence: 99%

Revealing the Negative Capacitance Effect in Silicon Quantum Dot Light-Emitting Diodes via Temperature-Dependent Capacitance-Voltage Characterization

et al. 2022

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In this study, quantum dot light-emitting diodes based on non-toxic silicon quantum dots functionalized with hexyl and dodecyl organic ligands showed a negative capacitance effect. Current density-voltage (J −V ) measurements revealed the charge transport mechanisms in the QLEDs. The capacitancevoltage (C − V ) characteristics were measured with an LCR meter over a wide range of frequencies (200 Hz to 1 MHz) and at temperatures from −40 • C to 60 • C. The classical heterojunction theory can describe the operation of quantum LEDs, but an effect not predicted by Shockley's theory was observed. Negative capacitance values were recorded in both fabricated LEDs, which were not observed in SiQD-LEDs before. Hence, we investigate the negative capacitance origin and its influence on the device performance. We attribute the negative capacitance to trapmediated recombination, where charges at defect sub-bandgap states contribute to the recombination but cannot be replenished fast enough. As a result, a current flows to re-establish the equilibrium, which lags behind the applied voltage, and the NC appears. By comparing the two functionalizations, we also observed a different temperature dependence of the positive capacitance peak and a stronger negative capacitance effect for dodecyl functionalized SiQDs. This effect is attributed to their improved charge carrier confinement abilities.Index Terms-Silicon quantum dot (SiQD), light-emitting diode (LED), negative capacitance (NC). I. INTRODUCTIONQ ANTUM dot light-emitting diodes (QLEDs) are highly researched as promising electroluminescence (EL) devices. QDs itself combine the advantages of inorganic and organic materials by offering size-tunable emission, [1] high color purity, [2] easy processibility, [3] and high photoluminescence quantum efficiency. [4] Additionally, QLEDs are expected to have improved stability compared to OLEDs because of their inorganic crystalline materials. [5] Many conventional QDs consist of toxic or heavy metal materials, such as Cd or Pb, and therefore, alternative materials are researched. A promising alternative is nanosized silicon,

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Transport Effects on Capacitance-Frequency Analysis for Defect Characterization in Organic Photovoltaic Devices

Cited by 40 publications

References 64 publications

Reverse dark current in organic photodetectors and the major role of traps as source of noise

Reverse dark current in organic photodetectors and the major role of traps as source of noise

Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β -phase in Poly(9,9-dioctylfluorene)

Revealing the Negative Capacitance Effect in Silicon Quantum Dot Light-Emitting Diodes via Temperature-Dependent Capacitance-Voltage Characterization

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