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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.


Related in: MedlinePlus

Thickness and electric-field dependence of carrier mobility.(a) Hole mobility measured with different configurations in thecurrent study compared with the values obtained by the SCLC model on thebasis of hole-only devices. (b) Hole mobility at various NPBthicknesses measured using the TOF measurement with a CGL. (c)Electron mobility at various NPB thicknesses measured using the TOFmeasurement with a CGL. The inset shows electron dispersion parameters as afunction of applied electric fields for the TOF samples with variousthicknesses of the NPB. The terms w/o and w/ represent the without and with,respectively.
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f5: Thickness and electric-field dependence of carrier mobility.(a) Hole mobility measured with different configurations in thecurrent study compared with the values obtained by the SCLC model on thebasis of hole-only devices. (b) Hole mobility at various NPBthicknesses measured using the TOF measurement with a CGL. (c)Electron mobility at various NPB thicknesses measured using the TOFmeasurement with a CGL. The inset shows electron dispersion parameters as afunction of applied electric fields for the TOF samples with variousthicknesses of the NPB. The terms w/o and w/ represent the without and with,respectively.

Mentions: Figure 5a shows the hole mobility estimated at various filmthicknesses and excitation sources. For comparison, the mobility obtained from ahole-only device, in a structure of ITO/MoO3 (15 nm)/NPB(150 or 300 nm)/Au, detertemed using a SCLC model is provided (Supplementary Figure S4)4142. The 0.3-μm NPB without the CGL cannot determinethe hole mobility because of the lack of a turning point in the transientsignals, as shown in Fig. 3c. The 2-μm NPBwithout using the CGL at 355-nm excitation has a compable hole mobility of 2-4 x10−4 cm2 V−1 s−1to previous studies57586382. When the CGL and 532-nmexcitation were used in the 2-μm NPB device, the hole mobilitybecomes lower in a range of 1-2 x10−4 cm2 V−1 s−1and exhibited high dependence on the electric field. We attribute thisphenomenon to the confinement effect of the charge generation because of the useof the CGL, thus leading to a longer distance the carriers require to travel incomparison to the carriers generated with a broad carrier distribution in thesamples without the CGL. As a matter of fact, the carriers generated in thebroad distribution may overestimate the carrier mobility because the averagedtravelling distance is shorter than the case if the carriers are generated in asheet. The 2-μm NPB sample used the 100-nm CGL, which isone-twentieth of the 2-μm NPB, for generating the carriers. The CGLwas the SubPc:C70 mixture, which may hinder the carrier transport atsuch thick layer because of the incomplete interpenetrating network and/or shortcarrier lifetime. Therefore, a CGL possessed a better carrier transport isexpected to obtain more reliable data. This can be achieved by reducing thethickness of the CGL to increase the thickness ratio of NPB to CGL. In the TOFsample composed of a 0.3-μm NPB with a 532-nm excitation, thecarrier mobility remains almost the same of approximately 1 x10−4 cm2 V−1 s−1with a slight dependence on electric field. Because the CGL was reduced to10 nm which is one-thirtieth of the NPB layer, the photogeneratedcarriers were appropriately-confined in a narrow width and can transport throughthe NPB layer without being trapped in the SubPc:C70 mixture.Although the hole mobility obtained based on the use of the CGL differed betweenthe devices with varous thicknesses, the values were on the order of10−4 cm2 V−1 s−1(Fig. 5b), thus showing a thickness-iudependentproperty. By contrast, the carrier mobility estimated by the SCLC model lead toa substantial thickness-to-thickness variation, as observed in the current studyand in previous studies414265. Therefore, the TOF measurementwith the employment of the CGL and specific excitation source is more reliableand becomes a valuable tool in determining the carrier mobility once the samplethickness can be reduced to a value near that used in a real device. The holemobility was determined because of an appropirate energy-level alignment betweenthe CGL and the NPB. However, the electron mobility was yet to determine becuaaelarge barrier height for electron at the CGL/NPB interface may impede theelectron injection (Fig. 2b) and limits the proposedtechnique to estimating the hole transport only. To address this issue, wedetermined the electron mobility by applying a negative bias to ITO, thusallowing electrons to be swept toward the Al electrode. The samples with the10-nm CGL at 532-nm excitation showed non-dispersive properties in a wide rangeof the NPB thicknesses from 2 to 1 μm (Supplementary Figure S5). The electrontransient signals showed clear turning points differentiated from the plateausand tail sections, defining the transit times without ambiguities (Supplementary Table S1). Figure5c shows the electron mobility estimated from these transit signals,together with an inset presenting the dispersion parameter as defined inEquation 2. The electron mobility measured in the currentstudy was 5-8 x10−4 cm2 V−1 s−1,which agrees well with the value reported by Tse et al.72,thus proving that the NPB is an ambipolar material with a higher electronmobility than the hole moblity. In addition, the dispersion parameters forelectron were lower than 0.1 for all thicknesses and electric fields, indicatingthe highly non-dispersive electron transport of the NPB. We inferred that thehigh barrier for electrons at the CGL/NPB interface could be overcome by highelectric fields that promotes electron injection, thus permitting the measure ofelectron transport in the NPB. Although the thickness for determining electronmobility was limited to 1 μm, this thickness is thethinnest value to date. We have successfully demonstrated and re-evaluated thatthe TOF technique is a valuable and practical tool in determining carriermobility using a combination of the highly efficient OPV materials SubPc andC70. In the near future, carrier-injection layers or structuraldesigns are required to reduce the TOF sample thickness to achieve thereal-device thickness.


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)

Thickness and electric-field dependence of carrier mobility.(a) Hole mobility measured with different configurations in thecurrent study compared with the values obtained by the SCLC model on thebasis of hole-only devices. (b) Hole mobility at various NPBthicknesses measured using the TOF measurement with a CGL. (c)Electron mobility at various NPB thicknesses measured using the TOFmeasurement with a CGL. The inset shows electron dispersion parameters as afunction of applied electric fields for the TOF samples with variousthicknesses of the NPB. The terms w/o and w/ represent the without and with,respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: Thickness and electric-field dependence of carrier mobility.(a) Hole mobility measured with different configurations in thecurrent study compared with the values obtained by the SCLC model on thebasis of hole-only devices. (b) Hole mobility at various NPBthicknesses measured using the TOF measurement with a CGL. (c)Electron mobility at various NPB thicknesses measured using the TOFmeasurement with a CGL. The inset shows electron dispersion parameters as afunction of applied electric fields for the TOF samples with variousthicknesses of the NPB. The terms w/o and w/ represent the without and with,respectively.
Mentions: Figure 5a shows the hole mobility estimated at various filmthicknesses and excitation sources. For comparison, the mobility obtained from ahole-only device, in a structure of ITO/MoO3 (15 nm)/NPB(150 or 300 nm)/Au, detertemed using a SCLC model is provided (Supplementary Figure S4)4142. The 0.3-μm NPB without the CGL cannot determinethe hole mobility because of the lack of a turning point in the transientsignals, as shown in Fig. 3c. The 2-μm NPBwithout using the CGL at 355-nm excitation has a compable hole mobility of 2-4 x10−4 cm2 V−1 s−1to previous studies57586382. When the CGL and 532-nmexcitation were used in the 2-μm NPB device, the hole mobilitybecomes lower in a range of 1-2 x10−4 cm2 V−1 s−1and exhibited high dependence on the electric field. We attribute thisphenomenon to the confinement effect of the charge generation because of the useof the CGL, thus leading to a longer distance the carriers require to travel incomparison to the carriers generated with a broad carrier distribution in thesamples without the CGL. As a matter of fact, the carriers generated in thebroad distribution may overestimate the carrier mobility because the averagedtravelling distance is shorter than the case if the carriers are generated in asheet. The 2-μm NPB sample used the 100-nm CGL, which isone-twentieth of the 2-μm NPB, for generating the carriers. The CGLwas the SubPc:C70 mixture, which may hinder the carrier transport atsuch thick layer because of the incomplete interpenetrating network and/or shortcarrier lifetime. Therefore, a CGL possessed a better carrier transport isexpected to obtain more reliable data. This can be achieved by reducing thethickness of the CGL to increase the thickness ratio of NPB to CGL. In the TOFsample composed of a 0.3-μm NPB with a 532-nm excitation, thecarrier mobility remains almost the same of approximately 1 x10−4 cm2 V−1 s−1with a slight dependence on electric field. Because the CGL was reduced to10 nm which is one-thirtieth of the NPB layer, the photogeneratedcarriers were appropriately-confined in a narrow width and can transport throughthe NPB layer without being trapped in the SubPc:C70 mixture.Although the hole mobility obtained based on the use of the CGL differed betweenthe devices with varous thicknesses, the values were on the order of10−4 cm2 V−1 s−1(Fig. 5b), thus showing a thickness-iudependentproperty. By contrast, the carrier mobility estimated by the SCLC model lead toa substantial thickness-to-thickness variation, as observed in the current studyand in previous studies414265. Therefore, the TOF measurementwith the employment of the CGL and specific excitation source is more reliableand becomes a valuable tool in determining the carrier mobility once the samplethickness can be reduced to a value near that used in a real device. The holemobility was determined because of an appropirate energy-level alignment betweenthe CGL and the NPB. However, the electron mobility was yet to determine becuaaelarge barrier height for electron at the CGL/NPB interface may impede theelectron injection (Fig. 2b) and limits the proposedtechnique to estimating the hole transport only. To address this issue, wedetermined the electron mobility by applying a negative bias to ITO, thusallowing electrons to be swept toward the Al electrode. The samples with the10-nm CGL at 532-nm excitation showed non-dispersive properties in a wide rangeof the NPB thicknesses from 2 to 1 μm (Supplementary Figure S5). The electrontransient signals showed clear turning points differentiated from the plateausand tail sections, defining the transit times without ambiguities (Supplementary Table S1). Figure5c shows the electron mobility estimated from these transit signals,together with an inset presenting the dispersion parameter as defined inEquation 2. The electron mobility measured in the currentstudy was 5-8 x10−4 cm2 V−1 s−1,which agrees well with the value reported by Tse et al.72,thus proving that the NPB is an ambipolar material with a higher electronmobility than the hole moblity. In addition, the dispersion parameters forelectron were lower than 0.1 for all thicknesses and electric fields, indicatingthe highly non-dispersive electron transport of the NPB. We inferred that thehigh barrier for electrons at the CGL/NPB interface could be overcome by highelectric fields that promotes electron injection, thus permitting the measure ofelectron transport in the NPB. Although the thickness for determining electronmobility was limited to 1 μm, this thickness is thethinnest value to date. We have successfully demonstrated and re-evaluated thatthe TOF technique is a valuable and practical tool in determining carriermobility using a combination of the highly efficient OPV materials SubPc andC70. In the near future, carrier-injection layers or structuraldesigns are required to reduce the TOF sample thickness to achieve thereal-device thickness.

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.


Related in: MedlinePlus