Investigation of p-dopant diffusion in polymer films and bulk heterojunctions: Stable spatially-confined doping for all-solution processed solar cells

Dai, An; Wan, A.; Magee, C. W.; Zhang, Yadong; Barlow, Stephen; Marder, Seth R.; Kahn, Antoine

doi:10.1016/j.orgel.2015.04.023

Cited by 42 publications

(53 citation statements)

References 31 publications

(48 reference statements)

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“…One of the main challenges in utilizing selective molecular doping in organic optoelectronic devices has been the diffusion problem of dopant molecules in the host materials, which has reduced the device stability . The dopant diffusion will especially be significant if one adopts selective bulk‐doping on the contact regions of OFETs due to a large dopant concentration gradient of dopant molecules between the doped regions and active channel (non‐doped) regions.…”

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confidence: 99%

Enhanced Charge Injection Properties of Organic Field‐Effect Transistor by Molecular Implantation Doping

et al. 2019

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memories, and organic field-effect transistors (OFETs), have various advantages including mechanical flexibility, low cost, solution-processed fabrication, and tunable material functionalities by molecular design compared with silicon-based materials. [1][2][3][4][5][6][7][8][9][10][11][12][13] However, the contact resistance problem arising between organic materials and metal electrodes has been one of the dominant obstacles for adopting organic semiconducting devices instead of silicon-based devices. Diverse attempts, for instance, self-assembled monolayer (SAM) treatment on metal electrodes, [14][15][16][17][18][19] inserting a charge injection layer between OSC and metals, [20][21][22][23][24][25][26][27] choice of metals for better injection properties, [28,29] adopting carbon-based conductor like graphene as electrodes, [30] have been introduced to improve carrier injection across typically a non-ohmic contact. Especially, considering large operation voltages required for OFETs, improving contact properties of organic/metal interface is an essential step for practical applications of OSCs.Contact doping is one of the most effective techniques to reduce contact resistance and has been widely employed in silicon-based devices and recently in OSCs to reduce the contact resistance. [31][32][33][34][35][36] In order to avoid undesirable OFF currents, it needs to be performed selectively, i.e., in localized regions at the source-drain contacts only and not in the channel region. The doped regions have been usually confined to the top surface of the OSC film by depositing a small amount of dopants on the top of the organic film by thermal evaporation. As a result, the position of the gate dielectrics was normally restricted to the top side of devices (i.e., FETs in a top-gate structure) in order to enhance the charge injection from metal electrodes to the accumulation layer formed on the top surface of the polymer. [31,32] Recently, the combination of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ) as host and dopant material, respectively, has produced a highly conducting polymer that has been studied as a candidate for a synthetic metal and high power-factor thermoelectric material. [37][38][39][40][41] Interestingly, this combination achieved an efficient bulk-doping of PBTTT by solid-state diffusion which implied that the F 4 -TCNQ Organic semiconductors (OSCs) have been widely studied due to their merits such as mechanical flexibility, solution processability, and large-area fabrication. However, OSC devices still have to overcome contact resistance issues for better performances. Because of the Schottky contact at the metal-OSC interfaces, a non-ideal transfer curve feature often appears in the low-drain voltage region. To improve the contact properties of OSCs, there have been several methods reported, including interface treatment by self-assembled monolayers and introducing charge injection layers. Here, a selectiv...

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confidence: 99%

Enhanced Charge Injection Properties of Organic Field‐Effect Transistor by Molecular Implantation Doping

et al. 2019

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“…Using UPS and Kelvin probe force microscopy, they did not observe discontinuities at the interface between the layers, which was confirmed by I–V measurements which were indistinguishable from films with increased thickness . Dai et al extended this concept to doped films, studying P3HT films doped by Mo(tfd) 3 laminated to undoped P3HT films . Using this technique they formed inverted OPV devices using 4 wt% Mo(tfd) 3 doped P3HT as the hole transport layer (HTL), obtaining similar PCEs to devices with a PEDOT:PSS hole transport layer .…”

Section: Doping Gradientsmentioning

confidence: 92%

“…Forming doped homojunctions in polymer OSCs is more difficult. Evaporation of dopants, such as F4TCNQ, into polymer films does not lead to the formation of a vertical doping gradient because dopant diffusion is fast on the length scale of typical film thicknesses (<100 nm) . However, this technique may prove successful with suitably engineered dopants with bulky 3D structures or side groups.…”

Section: Doping Gradientsmentioning

confidence: 99%

Controlling Molecular Doping in Organic Semiconductors

2017

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The field of organic electronics thrives on the hope of enabling low-cost, solution-processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution-processed p-type doped polymeric semiconductors. Highlighted topics include how solution-processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.

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“…Unintentional p-type doping of the active layer has frequently been encountered in organic solar cells, especially in thicker devices, and has been generally attributed to the presence of oxygen and water inside the active layer, or impurities due to residues from synthesis [36][37][38][39]. However, also diffusion of molecules from the electrode contacts and/or the electrode interlayers has been observed to cause doping of the active layer [40][41][42][43][44]. The effect of doping is to create a depleted space-charge region (SCR) within the active layer, adjacent to one of the contacts (a Schottky junction), at moderate doping levels [45].…”

Section: Introductionmentioning

confidence: 99%

Impact of a Doping-Induced Space-Charge Region on the Collection of Photogenerated Charge Carriers in Thin-Film Solar Cells Based on Low-Mobility Semiconductors

et al. 2019

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Unintentional doping of the active layer is a source for lowered device performance in organic solar cells.The effect of doping is to induce a space-charge region within the active layer, generally resulting in increased recombination losses. In this work, the impact of a doping-induced space-charge region on the current-voltage characteristics of low-mobility solar cell devices has been clarified by means of analytical derivations and numerical device simulations. It is found that, in case of a doped active layer, the collection efficiency of photo-generated charge carriers is independent of the light intensity and exhibits a distinct voltage dependence, resulting in an apparent electric-field dependence of the photocurrent. Furthermore, an analytical expression describing the behavior of the photocurrent is derived. The validity of the analytical model is verified by numerical drift-diffusion simulations and demonstrated experimentally on solutionprocessed organic solar cells. Based on the theoretical results, conditions of how to overcome charge collection losses caused by doping are discussed. Furthermore, the presented analytical framework provides tools to distinguish between different mechanisms leading to voltage dependent photocurrents.

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Investigation of p-dopant diffusion in polymer films and bulk heterojunctions: Stable spatially-confined doping for all-solution processed solar cells

Cited by 42 publications

References 31 publications

Enhanced Charge Injection Properties of Organic Field‐Effect Transistor by Molecular Implantation Doping

Enhanced Charge Injection Properties of Organic Field‐Effect Transistor by Molecular Implantation Doping

Controlling Molecular Doping in Organic Semiconductors

Impact of a Doping-Induced Space-Charge Region on the Collection of Photogenerated Charge Carriers in Thin-Film Solar Cells Based on Low-Mobility Semiconductors

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