Tuning the Work Function of Polar Zinc Oxide Surfaces using Modified Phosphonic Acid Self‐Assembled Monolayers

Lange, Ilja; Reiter, Sina; Pätzel, Michael; Zykov, Anton; Nefedov, Alexei; Hildebrandt, Jana; Hecht, Stefan; Kowarik, Stefan; Wöll, Christof; Heimel, Georg; Neher, Dieter

doi:10.1002/adfm.201401493

Cited by 174 publications

(224 citation statements)

References 75 publications

(110 reference statements)

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“…Figure 1 shows the molecular structure of the aromatic PAs used for surface modification (Figure 1a−c), as well as the oligophenylene consisting of a three spiro-bridged ladder-type quarterphenyl (L4P-sp3, see Figure 1d), which was subsequently deposited as overlayer. (Pyrimidin-2-yl) methyl-phosphonic acid (PyPA, see Figure 1 a) was synthesized as described by Lange et al 14 and has a methylene group (alkyl spacer) between the PA anchoring group and the pyrimidine phenyl ring. Phenyl-phosphonic acid (PhPA, see Figure 1b) and p-(trifluoromethyl)phenyl-phosphonic acid (pCF 3 PhPA, see Figure 1c) were obtained from Aculon, Inc. and used as received.…”

Section: Methodsmentioning

confidence: 99%

Energy-Level Engineering at ZnO/Oligophenylene Interfaces with Phosphonate-Based Self-Assembled Monolayers

Timpel

Nardi

Ligorio

et al. 2015

ACS Appl. Mater. Interfaces

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We used aromatic phosphonates with substituted phenyl rings with different molecular dipole moments to form self-assembled monolayers (SAMs) on the Zn-terminated ZnO(0001) surface in order to engineer the energy-level alignment at hybrid inorganic/organic semiconductor interfaces, with an oligophenylene as organic component. The work function of ZnO was tuned over a wide range of more than 1.7 eV by different SAMs. The difference in the morphology and polarity of the SAMmodified ZnO surfaces led to different oligophenylene orientation, which resulted in an orientation-dependent ionization energy that varied by 0.7 eV. The interplay of SAM-induced work function modification and oligophenylene orientation changes allowed tuning of the offsets between the molecular frontier energy levels and the semiconductor band edges over a wide range. Our results demonstrate the versatile use of appropriate SAMs to tune the energy levels of ZnO-based hybrid semiconductor heterojunctions, which is important to optimize its function, e.g., targeting either interfacial energy-or chargetransfer.

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Section: Methodsmentioning

confidence: 99%

Energy-Level Engineering at ZnO/Oligophenylene Interfaces with Phosphonate-Based Self-Assembled Monolayers

Timpel

Nardi

Ligorio

et al. 2015

ACS Appl. Mater. Interfaces

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“…5 Experiments on welldefined single crystalline ZnO (0001) and (000-1) surfaces proved that etching during immersion can be fully suppressed if dried ethanol (<0.01% H 2 O) is used as the solvent. A preparation protocol was developed which results in a very dense and homogenous molecular monolayer of oriented molecules on ZnO substrates.…”

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confidence: 98%

“…Following the protocol described in Ref. 5, SAMs from various BPAs and PyPA (see Figure 2(a)) were prepared on top of these ZnO substrates. We again find that the WF shift is largest for a specific optimum of immersion time, between 20 min and 2 h. Smaller WF shifts for nonoptimized immersion are supposedly due to an incomplete coverage for shorter times or stable multilayer formation at longer times.…”

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confidence: 99%

“…For this purpose, a recently developed protocol for the formation of SAMs on highly defined single-crystalline ZnO was adopted to the less defined surfaces of sol-gel processed ZnO. 5 This modification caused the WF to vary over an exceptionally large range spanning more than 2.3 eV, which enabled us to apply ZnO as hole injecting/extraction interlayer in regular solar cell structures. Efficiency measurements of regular and inverted P3HT:PCBM solar cells showed that these fully solution processed interlayers lead to a competitive or even increased device performance, compared to the reference cells on ITO/ PEDOT:PSS.…”

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confidence: 99%

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Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures

Lange

Reiter

Kniepert

et al. 2015

Applied Physics Letters

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An approach is presented to modify the WF of solution-processed sol-gel derived ZnO over an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using P3HT:PCBM as the active layer. These devices compete with or even exceed the performance of the reference cell on ITO/PEDOT:PSS. Our finding challenges the current view that bottom electrodes in inverted solar cells need to be electron-blocking for good device performance.Transparent metal oxides (TMOs) are integral parts of today's optoelectronic devices, either as electron conducting electrodes or intrinsically-doped semiconductors. Among these, zinc oxide (ZnO) is becoming increasingly important, as it is composed of earth-abundant elements. Also, a wide range of vapor-and solution based methods is available for ZnO deposition, ranging from chemical or physical vapor deposition (CVD, PVD) for the preparation of epitaxially grown single crystalline ZnO films to sol-gel procedures for low cost solution-based deposition. ZnO is already intensively used as a low-cost, transparent electrode in inorganic devices, [1][2] but more recently also as a charge selective interlayer material in efficient organic solar cells (OSCs) or organic light-emitting diodes (OLEDs). [3][4] However, the moderate work function (WF) of untreated ZnO of about 4.3 eV causes injection barriers to almost all conventional organic semiconductors [5][6][7][8][9] and the WF of such ZnO layers is poorly reproducible due to the physisorption of contaminations. [10] Finally, its poor chemical stability in an acidic environment [11][12][13] makes the application particularly in organic devices challenging. Therefore, for some of the recent organic record solar cells, the WF of the ZnO forming the bottom electrode was

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“…Some effective approaches proposed so far include employing self-assembled monolayers, small molecules or high-molar-mass polymers such as poly(ethylene oxide) and poly(ethylene glycol), and nonconjugated polymers at the interfaces between the photoactive layers and ZnO. [22][23][24][25][26][27][28] Development of doped ZnO-based materials with a variety of different dopants, such as aluminum, indium, cesium, nitrogen, and hydrogen, has also been shown to improve the efficiency and stability of the fabricated cells. [29][30][31][32][33][34] On the other hand, atomic layer deposition (ALD) of dielectric oxides has been established as a viable and effective approach for the surface passivation of several metal oxides (including ZnO) in order to improve their optoelectronic properties; [35,36] it has also been implemented in silicon solar cell technology.…”

Section: Introductionmentioning

confidence: 99%

Improved Stability of Polymer Solar Cells in Ambient Air via Atomic Layer Deposition of Ultrathin Dielectric Layers

Polydorou

Botzakaki

Sakellis

et al. 2017

Adv Materials Inter

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Polymer solar cells have attracted tremendous interest in the highly competitive solar energy sector, due to the practical advantages they exhibit, such as being lightweight, flexible, and low cost, in stark contrast to traditional photovoltaic technologies. However, their successful commercialization is still hindered by issues related to device instability. Here, atomic layer deposition (ALD) is employed to deposit conformal ultrathin dielectrics, such as alumina (Al2O3) and zirconia (ZrO2), on top of ZnO electron extraction layers to address problems that arise from the defect‐rich nature of these layers. The deposition of dielectrics on ZnO significantly improves its interfacial electronic properties, manifested primarily with the decrease in the work function of ZnO and the concomitant reduction of the electron extraction barrier as well as the reduced recombination losses. Significant efficiency enhancement is obtained with the incorporation of six ALD cycles of Al2O3 into inverted devices, using photoactive layers, that consist of poly(3‐hexylthiophene):indene‐C60‐bisadduct or poly({4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl}{3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl] thieno[3,4‐b] thiophenediyl}):[6,6]‐phenyl‐C70‐butyric acid methyl ester. More importantly, upon performing lifetime studies (over a period of 350 h), a strong improvement in polymer solar cell stability is observed when using the ALD‐modified ZnO films.

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Tuning the Work Function of Polar Zinc Oxide Surfaces using Modified Phosphonic Acid Self‐Assembled Monolayers

Cited by 174 publications

References 75 publications

Energy-Level Engineering at ZnO/Oligophenylene Interfaces with Phosphonate-Based Self-Assembled Monolayers

Energy-Level Engineering at ZnO/Oligophenylene Interfaces with Phosphonate-Based Self-Assembled Monolayers

Zinc oxide modified with benzylphosphonic acids as transparent electrodes in regular and inverted organic solar cell structures

Improved Stability of Polymer Solar Cells in Ambient Air via Atomic Layer Deposition of Ultrathin Dielectric Layers

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