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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.


Performance of the OPV devices. (a) J-Vcharacteristics for the devices with various SubPc:C70thicknesses in a structure of ITO/SubPc(5 nm)/SubPc:C70 (1:5; xnm)/C70 (43-x nm)/BCP (8 nm)/Al(100 nm), x = 10, 15, 20, 25, and30 under AM 1.5G solar illumination at100 mW cm−2.(b) Corresponding EQE spectra of these OPV devices.
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f1: Performance of the OPV devices. (a) J-Vcharacteristics for the devices with various SubPc:C70thicknesses in a structure of ITO/SubPc(5 nm)/SubPc:C70 (1:5; xnm)/C70 (43-x nm)/BCP (8 nm)/Al(100 nm), x = 10, 15, 20, 25, and30 under AM 1.5G solar illumination at100 mW cm−2.(b) Corresponding EQE spectra of these OPV devices.

Mentions: In the TOF measurement, a thickness-tunable property is preferable withoutcompensation on both the absorption and transport for controlling thecharge-generation width because the thickness of a CGL must be considerablythinner than that of the material under test. Therefore, in the current study,OPV devices were fabricated based on a structure of indium-tin-oxide(ITO)/MoO3 (15 nm)/SubPc(5 nm)/SubPc:C70 (1:5; x nm)/C70(43-x nm)/bathocuproine (BCP) (8 nm)/Al, where xis 10, 15, 20, 25, and 30 nm, for studying the effects of theSubPc:C70 thickness on the charge generation and transportproperties. The result may be misinterpreted if a single layer of C70was not used because the first optical-field maximum is unlikely to occur withinthe active layer, as predicted by the optical modeling (Supplementary Fig. S1). Figure1a shows the current density-voltage (J-V)characteristics under AM 1.5G illumination at100 mW cm−2 for thedevices with various structures. The error bar representing the standarddeviation is provided at each data point. The error of fill factor (FF) waslower when the SubPc:C70 thickness increased, probably because of theinhomogeneity of the thin film formation at a thinner thickness. Nevertheless,the thickness variation has a minor influence on FF. The difference inopen-circuit voltage (VOC) is probably due to the logarithmicrelation between VOC and short-circuit current density(JSC) and the carrier recombination caused by anincomplete interpenetrating network between the SubPc and the C70.Such issue, however, is beyond the scope of the current study. The mainparameter of special interest here is JSC, which represents aphoton absorption and charge transport properties The parameterJSC showed a considerable variation when variousSubPc:C70 thickness were used. Table 1summarizes the photovoltaic parameters for all the devices, showing that thebest power conversion efficiency (PCE) of 6.0% that can be achieved by a 25-nmSubPc:C70. To determine the variation in JSC,external quantum efficiency (EQE) spectra of the OPV devices were measured, asshown in Fig. 1b. Because JSC is theintegral of the product of the EQE and AM 1.5G solar spectrum, the averaged EQEin a wavelength range of 500 to 600 nm was directly proportional toJSC. Although the EQE decreased with theSubPc:C70 thickness, these values seem sufficient to allow thecarriers for flowing through the material under test in the TOF measurement.These results indicate that the mixture of the SubPc and C70 is apromising candidate for the use in CGLs, and its thickness allows room formodulating according to the tested material 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)

Performance of the OPV devices. (a) J-Vcharacteristics for the devices with various SubPc:C70thicknesses in a structure of ITO/SubPc(5 nm)/SubPc:C70 (1:5; xnm)/C70 (43-x nm)/BCP (8 nm)/Al(100 nm), x = 10, 15, 20, 25, and30 under AM 1.5G solar illumination at100 mW cm−2.(b) Corresponding EQE spectra of these OPV devices.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Performance of the OPV devices. (a) J-Vcharacteristics for the devices with various SubPc:C70thicknesses in a structure of ITO/SubPc(5 nm)/SubPc:C70 (1:5; xnm)/C70 (43-x nm)/BCP (8 nm)/Al(100 nm), x = 10, 15, 20, 25, and30 under AM 1.5G solar illumination at100 mW cm−2.(b) Corresponding EQE spectra of these OPV devices.
Mentions: In the TOF measurement, a thickness-tunable property is preferable withoutcompensation on both the absorption and transport for controlling thecharge-generation width because the thickness of a CGL must be considerablythinner than that of the material under test. Therefore, in the current study,OPV devices were fabricated based on a structure of indium-tin-oxide(ITO)/MoO3 (15 nm)/SubPc(5 nm)/SubPc:C70 (1:5; x nm)/C70(43-x nm)/bathocuproine (BCP) (8 nm)/Al, where xis 10, 15, 20, 25, and 30 nm, for studying the effects of theSubPc:C70 thickness on the charge generation and transportproperties. The result may be misinterpreted if a single layer of C70was not used because the first optical-field maximum is unlikely to occur withinthe active layer, as predicted by the optical modeling (Supplementary Fig. S1). Figure1a shows the current density-voltage (J-V)characteristics under AM 1.5G illumination at100 mW cm−2 for thedevices with various structures. The error bar representing the standarddeviation is provided at each data point. The error of fill factor (FF) waslower when the SubPc:C70 thickness increased, probably because of theinhomogeneity of the thin film formation at a thinner thickness. Nevertheless,the thickness variation has a minor influence on FF. The difference inopen-circuit voltage (VOC) is probably due to the logarithmicrelation between VOC and short-circuit current density(JSC) and the carrier recombination caused by anincomplete interpenetrating network between the SubPc and the C70.Such issue, however, is beyond the scope of the current study. The mainparameter of special interest here is JSC, which represents aphoton absorption and charge transport properties The parameterJSC showed a considerable variation when variousSubPc:C70 thickness were used. Table 1summarizes the photovoltaic parameters for all the devices, showing that thebest power conversion efficiency (PCE) of 6.0% that can be achieved by a 25-nmSubPc:C70. To determine the variation in JSC,external quantum efficiency (EQE) spectra of the OPV devices were measured, asshown in Fig. 1b. Because JSC is theintegral of the product of the EQE and AM 1.5G solar spectrum, the averaged EQEin a wavelength range of 500 to 600 nm was directly proportional toJSC. Although the EQE decreased with theSubPc:C70 thickness, these values seem sufficient to allow thecarriers for flowing through the material under test in the TOF measurement.These results indicate that the mixture of the SubPc and C70 is apromising candidate for the use in CGLs, and its thickness allows room formodulating according to the tested material 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.