Study of trap states in polyfluorene based devices by using TSC technique

Renaud, Cédric; Huang, Chen‐Hung; Lee, C.W.; Rendu, P. Le; Nguyen, Thien‐Phap

doi:10.1016/j.tsf.2007.12.063

Cited by 7 publications

(9 citation statements)

References 29 publications

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“…The floor-level DOS deep in the bandgap is estimated as 10 16 -10 17 cm −3 eV −1 . This value is in good agreement with values determined from the TSC technique on a polyfluorene-type material by Renaud et al [29].…”

Section: B Thin-film Dos Mapping Using Energy-resolved Electrochemicsupporting

confidence: 91%

“…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%

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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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Section: B Thin-film Dos Mapping Using Energy-resolved Electrochemicsupporting

confidence: 91%

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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“…The device shows relatively low hysteresis with an I on /I off ratio of 10 1 (Figure 5) due to the intrinsic traps present in the polyfluorene itself. 41 3.5. Tunable Properties of PFONPN.…”

Section: Resultsmentioning

confidence: 99%

Solution Processed Donor–Acceptor Polymer Based Electrical Memory Device with High On/Off Ratio and Tunable Properties

Arunagirinathan

Gopikrishna

Das

et al. 2019

ACS Appl. Electron. Mater.

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A nonvolatile “resistive random access memory” (ReRAM) device is reported with a series of four conjugated polymers (CPs) containing poly[2,7-(9,9′-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide (PFO–NPN) as donor and acceptor, respectively. A single layer, thin film of PFO–NPN copolymer that is sandwiched between indium tin oxide (ITO) and aluminum shows bistable property with a remarkably high I on/I off ratio of 108. The charge transport of the polymer is studied by fitting I–V curves with various conduction models to realize that the trap charge limited current (TCLC) assists in switching and exhibiting bistable property of the memory device. Theoretical calculations were also performed on the polymer to elucidate the presence of traps on the carbonyl oxygen atoms of NPN moiety. In addition, ReRAM properties like I on/I off ratio and write voltage were also tuned by changing the concentration of the acceptor moiety. Four different copolymers of acceptor concentrations (5%, 10%, 35%, and 50%) with respect to donor concentrations were characterized as a memory device. The device with high acceptor concentration (50%) showed the lowest I on/I off ratio (103) and write voltage (0.8 V). It was also observed experimentally that the I on/I off ratio and write voltage decrease sequentially with an increase in the acceptor concentration, thereby providing flexibility in tuning the memory parameters by allowing a molecular level change in the active material. The optical studies were performed to elucidate the mechanism of the tunable memory characteristic of the polymer, and the results reveal that the tunability is achieved due to the variation in the injection barrier and the strength of ICT for the different polymers.

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“…Considering that the TSC techniques are available for the investigation of traps in the OLED [3], the measurement procedure used in this study was as follow: from a thermodynamic equilibrium state at room temperature, a forward bias V H was applied to the device for a charging time of about 5 minutes. The sample was cooled to initial temperature T 0 by keeping this bias and then was short-circuited.…”

Section: Methodsmentioning

confidence: 99%

“…However, the electrically active defects resulting from the amorphous structure or/and impurities affect strongly the charge transport properties of polymer-based devices [3][4][5]. Trapped charge carriers take part no longer in the transport, the trapped charges also influence the electric field distribution and hence the charge transport [6,7].…”

Section: Introductionmentioning

confidence: 99%

Study of traps in P3HT:PCBM based organic solar cells using fractional thermally stimulated current (FTSC) technique

Migan‐Dubois

Alvarez

et al. 2012

Journal of Non-Crystalline Solids

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Study of trap states in polyfluorene based devices by using TSC technique

Cited by 7 publications

References 29 publications

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)

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)

Solution Processed Donor–Acceptor Polymer Based Electrical Memory Device with High On/Off Ratio and Tunable Properties

Study of traps in P3HT:PCBM based organic solar cells using fractional thermally stimulated current (FTSC) technique

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