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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.


Hole transient signals. Hole transient signals for a 2-μmNPB device measured (a) without a CGL at a 355-nm excitation and(b) with a 100-nm CGL at a 532-nm excitation when variouselectric fields were applied. Hole transient signals at various electricfields for a 0.3-μm NPB devices measured (c) without aCGL at a 355-nm excitation and (d) with a 10-nm CGL at a 532-nmexcitation. All insets show corresponding log-log plots. The terms w/o andw/ represent the without and with, respectively.
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f3: Hole transient signals. Hole transient signals for a 2-μmNPB device measured (a) without a CGL at a 355-nm excitation and(b) with a 100-nm CGL at a 532-nm excitation when variouselectric fields were applied. Hole transient signals at various electricfields for a 0.3-μm NPB devices measured (c) without aCGL at a 355-nm excitation and (d) with a 10-nm CGL at a 532-nmexcitation. All insets show corresponding log-log plots. The terms w/o andw/ represent the without and with, respectively.

Mentions: Comparisons of the hole transient signals for the 2- and 0.3-μmNPB without and with the CGL. Under the photoexcitation, excitons weredissociated within the SubPc:C70 CGL and generates holes andelectrons that are transported by SubPc and C70 molecules,respectively. To measure the hole transient signal, a positive bias was appliedto the ITO electrode, photogenerated holes were swept through the NPB layer andreaches the Al electrode. Figure 3a shows the holetransient signal for the structure of ITO/NPB (2 μm)/Al(100 nm) at various electric fields under 355-nm illumination,together with an inset showing a corresponding log-log plot. The NPB is anon-dispersive material, which showed a clear plateau and dramastic drop at theturning point in the transient signal5758. In our case,however, the emergence of cusps were observed, especially in high electricfields. The presence of cusps has been observed in previous studies, which haveattributed this phenomenon to the intrinsic feature between the randomlydisordered transport sites which act as trap states for the carriers, thusleading to the monotonically increased current instead of forming a plateau3749627981. Here, the NPB thickness of2 μm only may also lead to a different results fromprevisous studies that used the NPB thickness of7-10 μm. Although the reason for the emergence of thecusps is not well understood, the transient signal plotted in a log-log scaleenabled determining the transit time from the intersections of the asymptotes tothe increasing signals and tail sections, occasionally coincides with the cuspsin the linear plot. By contrast, the sample configuration of ITO/CGL(100 nm)/NPB (2 μm)/Al (100 nm)under 532-nm excitation showed very different transient signals, as shown inFig. 3b. The cusps were still observed, whereas thetail sections drop more rapidly at different applied electric fields, asobserved in the log-log plot. This observation is ascribed to the well-confinedposition of the charge generation by using the CGL with a thickness relativelyless than that of the NPB layer. In addition, the NPB is transparent to awavelength of 532 nm, and therefore only the thin CGL can generatethe photoexcited carriers and reaches the Al electrode simultaneously because ofthe non-dispersive characteristic of the NPB. Therefore, a clear turning pointbetween the plateau and the tail section well defineed the transit time eitherin a linear or log-log plot, when the degree of dispersion was reduced. In orderto determine the dispersion property, we used the general expression describingthe dispersivity as follows558283:


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)

Hole transient signals. Hole transient signals for a 2-μmNPB device measured (a) without a CGL at a 355-nm excitation and(b) with a 100-nm CGL at a 532-nm excitation when variouselectric fields were applied. Hole transient signals at various electricfields for a 0.3-μm NPB devices measured (c) without aCGL at a 355-nm excitation and (d) with a 10-nm CGL at a 532-nmexcitation. All insets show corresponding log-log plots. The terms w/o andw/ represent the without and with, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Hole transient signals. Hole transient signals for a 2-μmNPB device measured (a) without a CGL at a 355-nm excitation and(b) with a 100-nm CGL at a 532-nm excitation when variouselectric fields were applied. Hole transient signals at various electricfields for a 0.3-μm NPB devices measured (c) without aCGL at a 355-nm excitation and (d) with a 10-nm CGL at a 532-nmexcitation. All insets show corresponding log-log plots. The terms w/o andw/ represent the without and with, respectively.
Mentions: Comparisons of the hole transient signals for the 2- and 0.3-μmNPB without and with the CGL. Under the photoexcitation, excitons weredissociated within the SubPc:C70 CGL and generates holes andelectrons that are transported by SubPc and C70 molecules,respectively. To measure the hole transient signal, a positive bias was appliedto the ITO electrode, photogenerated holes were swept through the NPB layer andreaches the Al electrode. Figure 3a shows the holetransient signal for the structure of ITO/NPB (2 μm)/Al(100 nm) at various electric fields under 355-nm illumination,together with an inset showing a corresponding log-log plot. The NPB is anon-dispersive material, which showed a clear plateau and dramastic drop at theturning point in the transient signal5758. In our case,however, the emergence of cusps were observed, especially in high electricfields. The presence of cusps has been observed in previous studies, which haveattributed this phenomenon to the intrinsic feature between the randomlydisordered transport sites which act as trap states for the carriers, thusleading to the monotonically increased current instead of forming a plateau3749627981. Here, the NPB thickness of2 μm only may also lead to a different results fromprevisous studies that used the NPB thickness of7-10 μm. Although the reason for the emergence of thecusps is not well understood, the transient signal plotted in a log-log scaleenabled determining the transit time from the intersections of the asymptotes tothe increasing signals and tail sections, occasionally coincides with the cuspsin the linear plot. By contrast, the sample configuration of ITO/CGL(100 nm)/NPB (2 μm)/Al (100 nm)under 532-nm excitation showed very different transient signals, as shown inFig. 3b. The cusps were still observed, whereas thetail sections drop more rapidly at different applied electric fields, asobserved in the log-log plot. This observation is ascribed to the well-confinedposition of the charge generation by using the CGL with a thickness relativelyless than that of the NPB layer. In addition, the NPB is transparent to awavelength of 532 nm, and therefore only the thin CGL can generatethe photoexcited carriers and reaches the Al electrode simultaneously because ofthe non-dispersive characteristic of the NPB. Therefore, a clear turning pointbetween the plateau and the tail section well defineed the transit time eitherin a linear or log-log plot, when the degree of dispersion was reduced. In orderto determine the dispersion property, we used the general expression describingthe dispersivity as follows558283:

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.