Miscibility and Acid Strength Govern Contact Doping of Organic Photovoltaics with Strong Polyelectrolytes

Le, Thinh P.; Shang, Zhengrong; Wang, Lizhu; Li, Nanwen; Kesava, Sameer Vajjala; O’Connor, Joseph W.; Chang, Ying; Bae, Chulsung; Zhu, Chenhui; Hexemer, Alexander; Gomez, Esther W.; Salleo, Alberto; Hickner, Michael A.; Gomez, Enrique D.

doi:10.1021/acs.macromol.5b00724

Cited by 13 publications

(13 citation statements)

References 62 publications

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“…Interfacial (IF) modification is currently recognized as one of the most efficient ways to improve the performance of OPV devices. Various materials have been demonstrated as efficient IF‐modified layer for OPV applications, including self‐organized molecules, conjugated materials,[2b] ionic compounds (e.g., electrolytes), metal oxides,[1g,5] and nonconjugated organic materials . Typically, the mechanism through which the PCE is improved relies on (i) passivation of interfaces to suppress carrier recombination, (ii) enhanced carrier extraction, (iii) enhancing interfacial dipoles and modifying the work function (built‐in dipole) of the electrode to ensure better energy levels alignment, (iv) smoothing the surface of the substrate, and (v) optimizing the blend film morphology …”

Section: Introductionmentioning

confidence: 99%

Carbonized Bamboo‐Derived Carbon Nanodots as Efficient Cathode Interfacial Layers in High‐Performance Organic Photovoltaics

Juang

Kao

Wang

et al. 2018

Adv Materials Inter

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have been demonstrated as efficient IFmodified layer for OPV applications, [2] including self-organized molecules, [3] conjugated materials, [2b] ionic compounds (e.g., electrolytes), [4] metal oxides, [1g,5] and nonconjugated organic materials. [6] Typically, the mechanism through which the PCE is improved relies on (i) passivation of interfaces to suppress carrier recombination, [7] (ii) enhanced carrier extraction, [8] (iii) enhancing interfacial dipoles and modifying the work function (built-in dipole) of the electrode to ensure better energy levels alignment, [9] (iv) smoothing the surface of the substrate, [10] and (v) optimizing the blend film morphology. [11] Carbon nanodots (CNDs) are interesting carbon nanomaterials for optoelectronic applications because of their strong luminescence emissions, good water solubility, tunable optical and electronic properties, and photochemical stability. [12] CNDs typically comprise sp 2 -hybridized carbon atoms and present oxygen-containing functional groups (e.g., epoxy, ether, carbonyl, hydroxyl, carboxylic acid) on their surfaces. These oxygen-rich functionalities impart the CNDs with high water dispersibility and make them ready for modification with organic, inorganic, or biological materials. [13] CNDs exhibit interesting photoluminescence (PL) properties because of their sizes, edge shapes, and surface functional groups, and have demonstrated potential applicability in sensing, catalysis, biolabeling, photovoltaic devices, optoelectronic devices, and energy storage. [13d,e,14] CNDs have been employed as active layers, electrolytes, dopants, hole transporting layers, and interfacialmodified layers (IFLs) for dye-sensitized organic and perovskite solar cells. [12] Chen and co-workers first demonstrated the use of amino-based CNDs as efficient zinc oxide (ZnO) or Al-doped ZnO IFLs for OPVs and observed improvements in PCE of up to 10.24%. [13c] Yang et al. used CNDs as a cathode [indium tin oxide (ITO)]-modifying layer that enhanced the interfacial dipole, thereby decreasing the work function (WF) of the electrode and increasing the performance of derived OPV devices from 4.14% to 8.13% when using a poly [4,8bis(5-(2-ethylhexyl)thien-2-yl)benzo [1,2-b:4,5-b′-dithiopheneco-3-fluorothieno[3,4-b] thiophene-2-carboxylate](PTB7-Th)/ phenyl-C 71 -butyric acid methyl ester (PC 71 BM) blend film as the active layer. [15] These previous studies highlight the potential emerging applications of CNDs as IFLs. CNDs can be prepared

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Section: Introductionmentioning

confidence: 99%

Carbonized Bamboo‐Derived Carbon Nanodots as Efficient Cathode Interfacial Layers in High‐Performance Organic Photovoltaics

Juang

Kao

Wang

et al. 2018

Adv Materials Inter

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“…It is however not clear whether at such low doping levels one should still expect a mobility enhancement. Furthermore, as interfacial doping is adopted for transport layers and tandem cells, a study of the effect of bulk doping will be of interest in order to predict the effect of unintentional dopant diffusion into the active layer …”

Section: Introductionmentioning

confidence: 99%

Trade‐Off between Trap Filling, Trap Creation, and Charge Recombination Results in Performance Increase at Ultralow Doping Levels in Bulk Heterojunction Solar Cells

Shang

Heumueller

Prasanna

et al. 2016

Advanced Energy Materials

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Doping of organic bulk heterojunction solar cells has the potential to improve their power conversion efficiency (PCE). Deconvoluting the effect of doping on charge transport, recombination, and energetic disorder remains challenging. It is demonstrated that molecular doping has two competing effects: on one hand, dopant ions create additional traps while on the other hand free dopant-induced charges fill deep states possibly leading to V OC and mobility increases. It is shown that molar dopant concentrations as low as a few parts per million can improve the PCE of organic bulk heterojunctions. Higher concentrations degrade the performance of the cells. In doped cells where PCE is observed to increase, such improvement cannot be attributed to better charge transport. Instead, the V OC increase in unannealed P3HT:PCBM cells upon doping is indeed due to trap filling, while for annealed P3HT:PCBM cells the change in V OC is related to morphology changes and dopant segregation.In PCDTBT:PC70BM cells, the enhanced PCE upon doping is explained by changes in the thickness of the active layer. This study highlights the complexity of bulk doping in organic solar cells due to the generally low doping efficiency and the constraint on doping concentrations to avoid carrier recombination and adverse morphology changes.

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“…In this case, the location of the dopant remains stable under considerable thermal stress, which implies that reduced mixing can hinder undesired doping [16]. This relationship between ionic dopant miscibility and contact doping across a polar/non-polar interface was recently quantified for a variety of self-doped polymers [17].…”

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The effect of thermal annealing on dopant site choice in conjugated polymers

Rochester

Jacobs

et al. 2016

Organic Electronics

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a b s t r a c tSolution-processed organic electronic devices often consist of layers of polar and non-polar polymers. In addition, either of these layers could be doped with small molecular dopants. It is extremely important for device stability to understand the diffusion behavior of these molecular dopants under the thermal stress and whether the dopants have preference for the polar or the non-polar polymer layers. In this work, a widely used molecular dopant 2, 3,5,7,8, was chosen to investigate dopant site preference upon thermal annealing between the polar thiophene poly(thiophene-3-[2-(2-methoxy-ethoxy)ethoxy]-2,5-diyl) (SeP3MEET) and non-polar thiophene poly(3-hexylthiophene) (P3HT). F4TCNQ is able to p-type dope both P3HT and SeP3MEET. Further doping studies of SeP3MEET using near edge X-ray absorption fine structure spectroscopy, conductivity measurements and atomic force microscopy show that the F4TCNQ additive competes for doping sites with the covalently attached dopants on the SeP3MEET. Calorimetry measurements reveal that the F4TCNQ interacts strongly with the side-chains of the SeP3MEET, increasing the melting temperature of the side-chains by 30 C with 5 wt% dopant loading. Next, the thermal stability of doping in the polar/ non-polar (SeP3MEET/P3HT) bilayer architectures was investigated. Steady-state absorbance and fluorescence results show that F4TCNQ binds much more strongly in SeP3MEET than P3HT and very little F4TCNQ is found in the P3HT layer after annealing. In combination with reflectometry measurements, we show that F4TCNQ remains in the SP3MEET layer with annealing to 210 C even though the sublimation temperature for neat F4TCNQ is about 80 C. In contrast, F4TCNQ slowly diffuses out of P3HT at room temperature. We attribute this difference in binding the F4TCNQ anion to the ability of the ethyl-oxy side-chains of the SeP3MEET to orient around the charged dopant molecule and thereby to stabilize its position. This study suggests that polar side-chains could be engineered to increase the thermal stability of molecular dopant position.

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Miscibility and Acid Strength Govern Contact Doping of Organic Photovoltaics with Strong Polyelectrolytes

Cited by 13 publications

References 62 publications

Carbonized Bamboo‐Derived Carbon Nanodots as Efficient Cathode Interfacial Layers in High‐Performance Organic Photovoltaics

Carbonized Bamboo‐Derived Carbon Nanodots as Efficient Cathode Interfacial Layers in High‐Performance Organic Photovoltaics

Trade‐Off between Trap Filling, Trap Creation, and Charge Recombination Results in Performance Increase at Ultralow Doping Levels in Bulk Heterojunction Solar Cells

The effect of thermal annealing on dopant site choice in conjugated polymers

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