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Downscaling the Sample Thickness to Sub-Micrometers by Employing Organic Photovoltaic Materials as a Charge-Generation Layer in the Time-of-Flight Measurement.

Liu SW, Lee CC, Su WC, Yuan CH, Lin CF, Chen KT, Shu YS, Li YZ, Su TH, Huang BY, Chang WC, Liu YH - Sci Rep (2015)

Bottom Line: When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly.We also propose a new approach to design the TOF sample using an optical simulation.These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

View Article: PubMed Central - PubMed

Affiliation: Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan.

ABSTRACT
Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, thus rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Here, we use subphthalocyanine (SubPc):C70 as a charge-generation layer (CGL) in the TOF measurement and a commonly hole-transporting layer, N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB), as a standard material under test. When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly. Only 1-μm NPB is required for determining the electron mobility by using the proposed CGL. Both the thicknesses are the thinnest value reported to data. In addition, the flexibility of fabrication process of small molecules can deposit the proposed CGL underneath and atop the material under test. Therefore, this technique is applicable to small-molecule and polymeric materials. We also propose a new approach to design the TOF sample using an optical simulation. These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

No MeSH data available.


Configuration of the proposed TOF measurement. (a) Comparisonbetween emission spectra of 355 and 532 nm from Nd:YAG laser andextinction coefficients of organic materials used in the current study. Theextinction coefficient of NPB was magnified twofold. Optical properties ofOPV device with 6% PCE is provided. (b) Energy-level diagram andmolecular structures of organic materials. (c) Device structures ofTOF samples without (w/o) and with (w/) a CGL (SubPc:C70) forlaser illumination at 355 and 532 nm, respectively. (d)Schematic diagram of the TOF measurement with employment of the CGL. Thesolid and open circles denote electron and hole, respectively. All thestructures are not scaled with a real device.
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f2: Configuration of the proposed TOF measurement. (a) Comparisonbetween emission spectra of 355 and 532 nm from Nd:YAG laser andextinction coefficients of organic materials used in the current study. Theextinction coefficient of NPB was magnified twofold. Optical properties ofOPV device with 6% PCE is provided. (b) Energy-level diagram andmolecular structures of organic materials. (c) Device structures ofTOF samples without (w/o) and with (w/) a CGL (SubPc:C70) forlaser illumination at 355 and 532 nm, respectively. (d)Schematic diagram of the TOF measurement with employment of the CGL. Thesolid and open circles denote electron and hole, respectively. All thestructures are not scaled with a real device.

Mentions: Figure 2a compares the emission spectra at a wavelength of355 and 532 nm from Nd:YAG laser together with the absorptionproperties of organic materials used in the current study. As shown in thefigure, the NPB has absorption band in a range of 330 to 370 nm,while it is transparent to a wavelength of 532 nm. This explainsthat the typical wavelength of excitation source is 337 nm (fromN2 laser) to induce the photogenerated carriers for measuring thecarrier transient signal of the NPB without using a CGL6372. Asa matter of fact, most of organic semiconductors absorb the ultraviolet (UV)light range because of their molecular nature, and therefore previous studiesmeasured the carrier mobility by using 337 or 355 nm to be theexcitation wavelength.607374757677787980 Some oforganic materials, however, especially materials with a wide band gap thathaving either a low absorption coefficient or an absorption wavelength shorterthan the deep UV light, face difficulties in selecting the excitation source andrender the inaccuracy in determining the carrier mobility.6073757677 Therefore, a wavelength that is transparent tothe wide-bandgap materials while having a high photo-response to the CGL shouldbe a more suitable excitation source. Because the SubPc exhibits wide absorptionband from 500 to 600 nm with a shoulder around 532 nm,it is expected that the mixture of the SubPc and C70 would be apromising CGL upon the incident wavelength of 532 nm, as thephoto-response and charge-generation ability of the OPV device composed of theSubPc and C70 was extraordinarily prominent according to the highabsorption, EQE, and internal quantum efficiency (IQE) (see Supplementary Figure S2 for detail indetermining absorption and IQE). Besides concerning the above properties, theenergy level alignment between a CGL and the material under test is crucial forthe injection of the photogenerated carriers into the tested material. Figure 2b shows an energy level diagram of TOF sample usingthe NPB as the tested material and with a SubPc:C70 mixed layer asthe CGL. The molecular structures of organic materials are provided. Thephotogenerated holes were easily injected into the NPB because of the absence ofenergetic barrier between the highest occupied molecular orbital (HOMO) levelsat the SubPc:C70/NPB interface. However, the lowest unoccupiedmolecular orbital (LUMO) levels between the CGL and the NPB were not matched,thus resulting in large injection barrier that may impede the electron injectionacross the SubPc:C70/NPB interface. This issue may be resolvedthrough inserting an injection layer and/or applying a relatively high bias topromote the electron injection from the CGL into the NPB, the latter one hasbeen proven to be feasible and thereby the electron mobility was determined asshown in the previous study72. Two kinds of experiments wereperformed in the current study; one is the TOF sample without the CGL underillumination of 355 nm, another incorporates with the CGL underillumination of 532 nm, as shown in Fig. 2c.The laser pulse impinged through the ITO electrode for all the samples. Figure 2d illustrates the setup and working mechanism of theTOF measurement with a CGL using a thickness relatively less than the testedmaterial. The laser-pumped and dissociated carriers were transported through thematerial with a thickness of L under an applied voltage (V) from apower supply, which creates an electric field (E) to drive holes orelectrons, depending on the polarity of the bias applied to ITO. Al electrodecollected the carriers, which contribute to the current (I) that beingrecorded by an oscilloscope with a resistor for observing the photovoltagetransient signal. By reading the transit time (tT) from thetransient signal, the carrier mobility can be determined using the followingequation:


Downscaling the Sample Thickness to Sub-Micrometers by Employing Organic Photovoltaic Materials as a Charge-Generation Layer in the Time-of-Flight Measurement.

Liu SW, Lee CC, Su WC, Yuan CH, Lin CF, Chen KT, Shu YS, Li YZ, Su TH, Huang BY, Chang WC, Liu YH - Sci Rep (2015)

Configuration of the proposed TOF measurement. (a) Comparisonbetween emission spectra of 355 and 532 nm from Nd:YAG laser andextinction coefficients of organic materials used in the current study. Theextinction coefficient of NPB was magnified twofold. Optical properties ofOPV device with 6% PCE is provided. (b) Energy-level diagram andmolecular structures of organic materials. (c) Device structures ofTOF samples without (w/o) and with (w/) a CGL (SubPc:C70) forlaser illumination at 355 and 532 nm, respectively. (d)Schematic diagram of the TOF measurement with employment of the CGL. Thesolid and open circles denote electron and hole, respectively. All thestructures are not scaled with a real device.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4441200&req=5

f2: Configuration of the proposed TOF measurement. (a) Comparisonbetween emission spectra of 355 and 532 nm from Nd:YAG laser andextinction coefficients of organic materials used in the current study. Theextinction coefficient of NPB was magnified twofold. Optical properties ofOPV device with 6% PCE is provided. (b) Energy-level diagram andmolecular structures of organic materials. (c) Device structures ofTOF samples without (w/o) and with (w/) a CGL (SubPc:C70) forlaser illumination at 355 and 532 nm, respectively. (d)Schematic diagram of the TOF measurement with employment of the CGL. Thesolid and open circles denote electron and hole, respectively. All thestructures are not scaled with a real device.
Mentions: Figure 2a compares the emission spectra at a wavelength of355 and 532 nm from Nd:YAG laser together with the absorptionproperties of organic materials used in the current study. As shown in thefigure, the NPB has absorption band in a range of 330 to 370 nm,while it is transparent to a wavelength of 532 nm. This explainsthat the typical wavelength of excitation source is 337 nm (fromN2 laser) to induce the photogenerated carriers for measuring thecarrier transient signal of the NPB without using a CGL6372. Asa matter of fact, most of organic semiconductors absorb the ultraviolet (UV)light range because of their molecular nature, and therefore previous studiesmeasured the carrier mobility by using 337 or 355 nm to be theexcitation wavelength.607374757677787980 Some oforganic materials, however, especially materials with a wide band gap thathaving either a low absorption coefficient or an absorption wavelength shorterthan the deep UV light, face difficulties in selecting the excitation source andrender the inaccuracy in determining the carrier mobility.6073757677 Therefore, a wavelength that is transparent tothe wide-bandgap materials while having a high photo-response to the CGL shouldbe a more suitable excitation source. Because the SubPc exhibits wide absorptionband from 500 to 600 nm with a shoulder around 532 nm,it is expected that the mixture of the SubPc and C70 would be apromising CGL upon the incident wavelength of 532 nm, as thephoto-response and charge-generation ability of the OPV device composed of theSubPc and C70 was extraordinarily prominent according to the highabsorption, EQE, and internal quantum efficiency (IQE) (see Supplementary Figure S2 for detail indetermining absorption and IQE). Besides concerning the above properties, theenergy level alignment between a CGL and the material under test is crucial forthe injection of the photogenerated carriers into the tested material. Figure 2b shows an energy level diagram of TOF sample usingthe NPB as the tested material and with a SubPc:C70 mixed layer asthe CGL. The molecular structures of organic materials are provided. Thephotogenerated holes were easily injected into the NPB because of the absence ofenergetic barrier between the highest occupied molecular orbital (HOMO) levelsat the SubPc:C70/NPB interface. However, the lowest unoccupiedmolecular orbital (LUMO) levels between the CGL and the NPB were not matched,thus resulting in large injection barrier that may impede the electron injectionacross the SubPc:C70/NPB interface. This issue may be resolvedthrough inserting an injection layer and/or applying a relatively high bias topromote the electron injection from the CGL into the NPB, the latter one hasbeen proven to be feasible and thereby the electron mobility was determined asshown in the previous study72. Two kinds of experiments wereperformed in the current study; one is the TOF sample without the CGL underillumination of 355 nm, another incorporates with the CGL underillumination of 532 nm, as shown in Fig. 2c.The laser pulse impinged through the ITO electrode for all the samples. Figure 2d illustrates the setup and working mechanism of theTOF measurement with a CGL using a thickness relatively less than the testedmaterial. The laser-pumped and dissociated carriers were transported through thematerial with a thickness of L under an applied voltage (V) from apower supply, which creates an electric field (E) to drive holes orelectrons, depending on the polarity of the bias applied to ITO. Al electrodecollected the carriers, which contribute to the current (I) that beingrecorded by an oscilloscope with a resistor for observing the photovoltagetransient signal. By reading the transit time (tT) from thetransient signal, the carrier mobility can be determined using the followingequation:

Bottom Line: When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly.We also propose a new approach to design the TOF sample using an optical simulation.These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

View Article: PubMed Central - PubMed

Affiliation: Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan.

ABSTRACT
Time-of-flight (TOF) measurements typically require a sample thickness of several micrometers for determining the carrier mobility, thus rendering the applicability inefficient and unreliable because the sample thicknesses are orders of magnitude higher than those in real optoelectronic devices. Here, we use subphthalocyanine (SubPc):C70 as a charge-generation layer (CGL) in the TOF measurement and a commonly hole-transporting layer, N,N'-diphenyl-N,N'-bis(1,1'-biphenyl)-4,4'-diamine (NPB), as a standard material under test. When the NPB thickness is reduced from 2 to 0.3 μm and with a thin 10-nm CGL, the hole transient signal still shows non-dispersive properties under various applied fields, and thus the hole mobility is determined accordingly. Only 1-μm NPB is required for determining the electron mobility by using the proposed CGL. Both the thicknesses are the thinnest value reported to data. In addition, the flexibility of fabrication process of small molecules can deposit the proposed CGL underneath and atop the material under test. Therefore, this technique is applicable to small-molecule and polymeric materials. We also propose a new approach to design the TOF sample using an optical simulation. These results strongly demonstrate that the proposed technique is valuable tool in determining the carrier mobility and may spur additional research in this field.

No MeSH data available.