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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.


Comparison of CGLs deposited underneath and atop the NPB. Hole andelectron mobility obtained from the various device structures withdepositing CGLs underneath and atop NPB layer. The open circles and dashedlines indicate the hole mobility of the NPB. The solid circles and linesindicate the electron mobility of the NPB. The inset depicts CGL/NPB andNPB/CGL structures. In the inset, the open and solid symbol denote the holeand electron, respectively.
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f6: Comparison of CGLs deposited underneath and atop the NPB. Hole andelectron mobility obtained from the various device structures withdepositing CGLs underneath and atop NPB layer. The open circles and dashedlines indicate the hole mobility of the NPB. The solid circles and linesindicate the electron mobility of the NPB. The inset depicts CGL/NPB andNPB/CGL structures. In the inset, the open and solid symbol denote the holeand electron, respectively.

Mentions: The discussed devices were placed the proposed CGL underneath the material undertest. This approach may limit to the samples prepared by thermal evaporation.For example, polymers are mostly spun-coated onto substrates and thus maydissolve CGLs that are already deposited on the substrates. Therefore, weproposed that another advantage of using the proposed CGL is the flexibility ofthe fabrication process (i.e., the thermal-evaporated CGL). For measuring thecarrier mobility of polymers, the discussed structure can be altered toITO/polymer/CGL/Al to resolve the dissolving problem. To demonstrate this idea,a structure of ITO/NPB (0.3 μm)/CGL(10 nm)/Al was fabricated for comparison with ITO/CGL(10 nm)/NPB (0.3 μm)/Al. Because the CGL ison the top of the NPB, the measurement setup is different from the case in whicha CGL is underneath the NPB (SupplementaryFigure S6). Figure 6 compares the resultsobtained from both the CGL/NPB and NPB/CGL structures. The hole and electronmobility were mostly identical for both device structures, thus indicating thatin the thermal-evaporation system the CGL can place underneath and atopmaterials of interest, both of which showed an accurate measure of the carriermobility. Therefore, for measuring the carrier mobility of polymers, theproposed CGL can be deposited on polymers that were already spun-coated onto thesubstrates without concerning the dissolving problem. To demonstrate thepossibility of confining the charge-generation width by using the proposed CGLfor measuring the carrier mobility of polymers that were used in OPV devices, wechoose three commonly used polymers,poly[(R)-3-(4-(4-ethyl-2-oxazolin-2-yl)phenyl)thiophene] (PEOPT)85, poly(3-hexylthiophene-2,5-diyl) (P3HT)86, and[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)87 as a standardreference. The structure of TOF sample was ITO/polymers/CGL/Ag, in which the Agmust be transparent (the thickness is typically 10-20 nm) for thelight passing through the Ag to excite CGLs88. The calculatedpower-dissipation profiles were shown in the Figure S7 in the Supplementary Information. The measurementconfiguration for measuring the polymers are provided (Supplementary Information Figure S7). Tosimulate a real case, the thicknesses of the polymers were fixed at100 nm and that of the CGL was fixed at 5 nm. BecausePEOPT and P3HT absorb the wavelength of 532 nm, exciting the CGLwith a 355-nm excitation is preferable. The transient signals may be primarilycontributed by the carriers in the CGL because the power dissipation in the CGLwas much higher than that in the polymers. However, for the PCBM, which has ahigh absorption in a wavelength of 355 nm, a 532-nm excitation willbe more appropriate to excite the CGL and the transient signals were primarilycontributed by the carriers in the CGL. Although the charge generation may occurin the polymers, most of the carriers were generated in the CGL and contributesto the transient signals, thus leading to a more accurate carrier mobility. Inthe state of the art, a series of novel polymers was synthesized toward thenear-infrared (NIR) absorption for extending the spectral coverage over thesolar spectrum18899091929394. The polymers were theso called low-bandgap polymers which having absorption within an NIR wavelengthrange. In addition, the proposed CGL has high extinction coefficient than thatof these polymers, especially when these polymers were in a blend with the PCBM.Although the proposed CGL cannot completely confine the charge-generation widthin the polymers, the CGL can contribute most of the carriers to dominate thetransient signals without overestimating the carrier mobility of the polymersbecause of a broad width of charge generation.


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)

Comparison of CGLs deposited underneath and atop the NPB. Hole andelectron mobility obtained from the various device structures withdepositing CGLs underneath and atop NPB layer. The open circles and dashedlines indicate the hole mobility of the NPB. The solid circles and linesindicate the electron mobility of the NPB. The inset depicts CGL/NPB andNPB/CGL structures. In the inset, the open and solid symbol denote the holeand electron, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f6: Comparison of CGLs deposited underneath and atop the NPB. Hole andelectron mobility obtained from the various device structures withdepositing CGLs underneath and atop NPB layer. The open circles and dashedlines indicate the hole mobility of the NPB. The solid circles and linesindicate the electron mobility of the NPB. The inset depicts CGL/NPB andNPB/CGL structures. In the inset, the open and solid symbol denote the holeand electron, respectively.
Mentions: The discussed devices were placed the proposed CGL underneath the material undertest. This approach may limit to the samples prepared by thermal evaporation.For example, polymers are mostly spun-coated onto substrates and thus maydissolve CGLs that are already deposited on the substrates. Therefore, weproposed that another advantage of using the proposed CGL is the flexibility ofthe fabrication process (i.e., the thermal-evaporated CGL). For measuring thecarrier mobility of polymers, the discussed structure can be altered toITO/polymer/CGL/Al to resolve the dissolving problem. To demonstrate this idea,a structure of ITO/NPB (0.3 μm)/CGL(10 nm)/Al was fabricated for comparison with ITO/CGL(10 nm)/NPB (0.3 μm)/Al. Because the CGL ison the top of the NPB, the measurement setup is different from the case in whicha CGL is underneath the NPB (SupplementaryFigure S6). Figure 6 compares the resultsobtained from both the CGL/NPB and NPB/CGL structures. The hole and electronmobility were mostly identical for both device structures, thus indicating thatin the thermal-evaporation system the CGL can place underneath and atopmaterials of interest, both of which showed an accurate measure of the carriermobility. Therefore, for measuring the carrier mobility of polymers, theproposed CGL can be deposited on polymers that were already spun-coated onto thesubstrates without concerning the dissolving problem. To demonstrate thepossibility of confining the charge-generation width by using the proposed CGLfor measuring the carrier mobility of polymers that were used in OPV devices, wechoose three commonly used polymers,poly[(R)-3-(4-(4-ethyl-2-oxazolin-2-yl)phenyl)thiophene] (PEOPT)85, poly(3-hexylthiophene-2,5-diyl) (P3HT)86, and[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)87 as a standardreference. The structure of TOF sample was ITO/polymers/CGL/Ag, in which the Agmust be transparent (the thickness is typically 10-20 nm) for thelight passing through the Ag to excite CGLs88. The calculatedpower-dissipation profiles were shown in the Figure S7 in the Supplementary Information. The measurementconfiguration for measuring the polymers are provided (Supplementary Information Figure S7). Tosimulate a real case, the thicknesses of the polymers were fixed at100 nm and that of the CGL was fixed at 5 nm. BecausePEOPT and P3HT absorb the wavelength of 532 nm, exciting the CGLwith a 355-nm excitation is preferable. The transient signals may be primarilycontributed by the carriers in the CGL because the power dissipation in the CGLwas much higher than that in the polymers. However, for the PCBM, which has ahigh absorption in a wavelength of 355 nm, a 532-nm excitation willbe more appropriate to excite the CGL and the transient signals were primarilycontributed by the carriers in the CGL. Although the charge generation may occurin the polymers, most of the carriers were generated in the CGL and contributesto the transient signals, thus leading to a more accurate carrier mobility. Inthe state of the art, a series of novel polymers was synthesized toward thenear-infrared (NIR) absorption for extending the spectral coverage over thesolar spectrum18899091929394. The polymers were theso called low-bandgap polymers which having absorption within an NIR wavelengthrange. In addition, the proposed CGL has high extinction coefficient than thatof these polymers, especially when these polymers were in a blend with the PCBM.Although the proposed CGL cannot completely confine the charge-generation widthin the polymers, the CGL can contribute most of the carriers to dominate thetransient signals without overestimating the carrier mobility of the polymersbecause of a broad width of charge generation.

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.