Sample preparation for scanning Kelvin probe microscopy studies on cross sections of organic solar cells

Scherer, Michael; Saive, Rebecca; Daume, Dominik; Kröger, Michael; Kowalsky, Wolfgang

doi:10.1063/1.4824323

Cited by 18 publications

(16 citation statements)

References 18 publications

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“…8 × 8 μm 2 holes with a depth of about 2 μm were milled into the OLEDs with the FIB at an acceleration voltage of 30 kV and a gallium ion beam current of 1 nA at an angle of 54 degrees versus the surface normal. In other studies, we have shown that measured potential profiles do not differ significantly between samples prepared by ion beam cutting at this angle and samples which were cleaved perpendicular to the surface . The exposed cross section was further polished with an ion current of 50 pA.…”

Section: Methodsmentioning

confidence: 87%

Direct observation of the potential distribution within organic light emitting diodes under operation

Weigel

Kowalsky

Saive

2015

Phys. Status Solidi RRL

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Kelvin probe measurements and orientation polarization Previous Kelvin probe investigations of Alq 3 films have shown a constant surface potential increase with film thickness (up to 5 V per 100 nm) that is linked to spontaneous orientation polarization of the polar Alq 3 molecules but vanishes for various external stresses such as visible light or elevated temperature. [1,2] This permanent polarization density is around -1 mC/m 2 . [3][4][5][6] SKPM measurements reproduced the surface potential shift on bare Alq 3 films as well as in the areas that had been behind a shadow mask during metal evaporation. It corresponds to the potential of the film surface against the substrate in the case of depletion at the transition voltage. After deposition of a metal contact onto the Alq 3 layer no such shift was found neither on the metal surface (before FIB cutting) nor in the Alq 3 beneath (after FIB cutting with floating top contact). This means that deposition of a metal layer on top of the Alq 3 film triggers compensation of the orientation polarization by charge accumulation. SKPM raw dataThe SEM image of the cross section of an OLED was captured by a high resolution and therefore high dose electron beam after exposure of the cross section but before the polishing to avoid electron beam damage to the final, polished cross section. After a final polishing step the probe tip was positioned at the edge of the milled hole using the SEM with high scan rate to avoid carbon deposition. Subsequently, topography and SKPM images were taken. The SEM allowed for accurate identification of the different layers and alignment and calibration of the SPM. The electron beam was blanked for further data acquisition. Figure S1 shows the CPD profiles across the cross section of an OLED with an active layer of 224 nm Alq 3 and a) 224 nm NPB and b) 112 nm NPB at external bias voltage between a) -10 V and 5 V and b) -10 V and +10 V. The Al contact was kept on ground potential and the bias voltage was applied to the ITO contact. The 0 V profiles correspond to the work function difference between the cantilever tip and the sample. As electrostatic force is long-range, not only the very end of the probe tip interacts with the sample. The measured signal is also influenced by the sample interaction with the whole cantilever. [7][8][9] Although contacted to ground potential the measured CPD at the Al contact slightly changes its value upon different biases on the ITO. The corresponding measurement artifact on the ITO contact is even more pronounced because the FIB milled hole is significantly smaller than the cantilever so that most of the cantilever is positioned above and interacting with the Al contact. Therefore, the measured potential at the ITO contact is skewed towards the potential of the Al contact and smaller than the applied bias voltage. To facilitate the comparison of predicted and measured values shown in Fig. 2, we accounted for this measurement artifact by setting the ITO and Al on the nominal applied potential and extending the ...

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Section: Methodsmentioning

confidence: 87%

Direct observation of the potential distribution within organic light emitting diodes under operation

Weigel

Kowalsky

Saive

2015

Phys. Status Solidi RRL

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“…In a direct microscopic measurement, it was possible to visualize the described charge barrier in P3HT:PCBM solar cells [3]. In this measurement, a scanning electron microscopy/focused ion beam system was combined with a scanning probe system [111] to perform in-situ cross section preparation and measurement [112]. Fig.…”

Section: S-shaped I-v Curves In Organic Solar Cellsmentioning

confidence: 99%

S-Shaped Current–Voltage Characteristics in Solar Cells: A Review

Saive

2019

IEEE J. Photovoltaics

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S-shaped current-voltage (I-V) characteristics are a frequently occurring hurdle in the development of new solar cell material combinations and device architectures. Their presence points to the existence of a charge transport bottleneck that needs to be removed in order to unlock high fill factors and power conversion efficiencies. In this review, examples of studies in which s-shaped I-V curves have appeared are presented, and the cause and mitigation are discussed. Different solar cell material systems are often treated by separate communities, thereby, also the physics of s-shaped I-V curves have been treated separately. This review covers the main solar cell technologies-silicon, thin film, organic, hybrid-with the aim to provide an overarching picture of the common mechanisms and universal guidelines for mitigation of s-shaped I-V characteristics in emerging solar cell technologies. Except for a few studies on organic solar cells, s-shaped I-V curves are reported to result from charge transport barriers at one of the (selective) contact layers that can be overcome by interface engineering and doping.Index Terms-Current-voltage characteristics, s-shape, roll over.

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“…Таким образом, комбинируя микротомирование с АСМ, можно получать детальную информацию о структуре органических образцов в объеме, измеряя различные ти-пы взаимодействия зонда АСМ с образцом, содержащим сечение фотоактивного слоя. В недавней работе [12] ком-бинация ультрамикротомирования и АСМ была исполь-зована для измерения распределения потенциала по се-чению ПСЭ. В данной работе мы используем подобный подход, но с применением других, более высокоразреша-ющих методов АСМ и для более актуальных материа-лов.…”

Section: Introductionunclassified

Исследование Структуры Объемного Гетероперехода В Полимерных Солнечных Элементах С Помощью Комбинации Ультрамикротомирования И Атомно-Силовой Микроскопии

Алексеев¹,

Ал-Афееф²,

Хедли³

et al. 2018

Физика И Техника Полупроводников

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A method for visualization via atomic-force microscopy of the internal structure of photoactive layers of polymer solar cells using an ultramicrotome for photoactive layer cutting is proposed and applied. The method creates an opportunity to take advantage of atomic-force microscopy in structural investigations of the bulk of soft samples. Such advantages of atomic-force microscopy include a high contrast and the ability to measure various surface properties at nanometer resolution. Using the proposed method, samples of the photoactive layer of polymer solar cells based on a mixture of PTB7 polythiophene and PC_71BM fullerene derivatives are studied. The disclosed details of the bulk structure of this mixture allow us to draw additional conclusions about the effect of morphology on the efficiency of organic solar cells.

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Sample preparation for scanning Kelvin probe microscopy studies on cross sections of organic solar cells

Cited by 18 publications

References 18 publications

Direct observation of the potential distribution within organic light emitting diodes under operation

Direct observation of the potential distribution within organic light emitting diodes under operation

S-Shaped Current–Voltage Characteristics in Solar Cells: A Review

Исследование Структуры Объемного Гетероперехода В Полимерных Солнечных Элементах С Помощью Комбинации Ультрамикротомирования И Атомно-Силовой Микроскопии

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