Impact of a Low Concentration of Dopants on the Distribution of Gap States in a Molecular Semiconductor

Lin, Xin; Purdum, Geoffrey E.; Zhang, Yadong; Barlow, Stephen; Marder, Seth R.; Loo, Yueh‐Lin; Kahn, Antoine

doi:10.1021/acs.chemmater.6b00165

Cited by 31 publications

(47 citation statements)

References 28 publications

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“…E a decreases from 0.29 to 0.24 eV with as little as 1.0 mol% F6-TCNNQ, and further to 0.18 eV with 10.0 mol%. In analogy to previous work on n-doping of C 60 [33] and p-doping of CuPc, [25] this evolution is attributed to the filling and deactivation of trap and tail states by holes generated from the F6-TCNNQ molecules. As more dopants are introduced, E F moves toward the Spiro-TAD HOMO into a region of higher density of states where the activation energy for the hopping process is lower.…”

Section: Introductionsupporting

confidence: 62%

“…Lin et al reported increased disorder in CuPc films doped with Mo(tfd) 3 . [25] Doping-induced change in morphology was also reported for solution processed polymer thin films, such as poly(3-hexylthiophene) (P3HT). Duong et al [26] directly visualized the dopant distribution in P3HT:F4-TCNQ blend films using conductive atomic force microscopy (C-AFM), and showed that both dopant distribution and film morphology depend on the strength of the doping process.…”

Section: Introductionmentioning

confidence: 85%

“…Doping also correlates with the broadening of the Spiro-TAD HOMO region of the UPS spectrum and the progressive loss of the resolution of the doublet (Figure 4a). As discussed by Lin et al [25] in the case of p-doped CuPc, such broadening of the valence density of states results from increased structural disorder as well as from the electrostatic disorder due to the random position of the ionized dopants. [27] In contrast, the introduction of F6-TCNNQ barely changes the morphology of the TCTA films.…”

Section: Introductionmentioning

confidence: 93%

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Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

Zhang

Kahn

2017

Adv Funct Materials

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2,2′-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ) is investigated as a molecular p-type dopant in two hole-transport materials, 2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene (Spiro-TAD) and tris(4-carbazoyl-9-ylphenyl)amine (TCTA). The electron affinity of F6-TCNNQ is determined to be 5.60 eV, one of the strongest organic molecular oxidizing agents used to date in organic electronics. p-Doping is found to be effective in Spiro-TAD (ionization energy = 5.46 eV) but not in TCTA (ionization energy = 5.85 eV). Optical absorption measurements demonstrate that charge transfer is the predominant doping mechanism in Spiro-TAD:F6-TCNNQ. The hostdopant interaction also leads to a significant alteration of the host film morphology. Finally, transport measurements done on Spiro-TAD:F6-TCNNQ as a function of dopant concentration and temperature, and using a highly doped contact layer to ensure negligible hole injection barrier, lead to an accurate measurement of the film conductivity and hole-hopping activation energy.

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Section: Introductionsupporting

confidence: 62%

Section: Introductionmentioning

confidence: 85%

Section: Introductionmentioning

confidence: 93%

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Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

Zhang

Kahn

2017

Adv Funct Materials

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“…[ 40 ] External quantum effi ciency measurements were performed using a Newport TLS-300X tunable light source system. Ultraviolet photoelectron spectroscopy and inverse photoelectron spectroscopy were used to characterize the electronic structure of each fi lm, using a setup described elsewhere.…”

Section: Methodsmentioning

confidence: 99%

Morphological Tuning of the Energetics in Singlet Fission Organic Solar Cells

Lin

Fusella

Kozlov

et al. 2016

Adv Funct Materials

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6489wileyonlinelibrary.com conversion effi ciency (PCE) of a single junction solar cell is 41.9%-a signifi cant improvement over the so-called Shockley-Queisser limit of 32% for conventional devices. [ 6,7 ] The theoretical maximum PCE can be boosted even further if a long wavelength absorber is incorporated to take advantage of solar photons with energies between the singlet and triplet levels of the singlet fi ssion material. [ 1,3,8 ] Because of these exciting prospects, singlet fi ssion solar cells have attracted signifi cant research interest in the last several years.Ultimately, the functionality of this class of solar cells hinges on two requirements: (1) the ability to undergo rapid and effi cient singlet fi ssion, and (2) the ability to dissociate and extract the triplets that are generated by singlet fi ssion. Regarding the fi rst criterion, signifi cant efforts have been made to identify and engineer materials that readily undergo singlet fi ssion. The polyacenes (e.g., tetracene, pentacene) stand out in terms of usage, and have been incorporated in singlet-fi ssion-based solar cells [ 1,2,5 ] and photodetectors with great success. [ 3,9 ] Rubrene, a tetracene derivative, is capable of fast singlet fi ssion [ 10,11 ] but has not experienced the same levels of usage as other acenes, perhaps on the basis of its morphological variability and inferior performance in solar cells. As for the second requirement, little effort has been made to gain control over triplet extraction, yet it remains fundamental to the functionality of singlet fi ssion solar cells. In particular, only charge transfer processes from the triplet, and not the singlet, of the singlet fi ssion material are able to contribute to the effi ciency gains outlined above.We demonstrate that the tunable morphology of rubrene can be used to substantially change the behavior of devices in favor of more effi cient photocurrent generation. Specifically, the charge transfer (CT) state of a rubrene/C 60 solar cell can be shifted by over 300 meV simply by crystallizing the asdeposited amorphous fi lms. This shift in CT state energy is able to transform the amorphous device from one that blocks charge transfer of rubrene triplets into one that readily dissociates rubrene triplets. We conduct a comprehensive study of the impact of morphology on exciton dynamics in rubrene/C 60 systems and analyze its implications on device performance. Ultimately, the ability to morphologically fi ne-tune the interfacial energetics and the singlet fi ssion kinetics in such devices opens Effective singlet fi ssion solar cells require both fast and effi cient singlet fi ssion as well as favorable energetics for harvesting the resulting triplet excitons. Notable progress has been made to engineer materials with rapid and effi cient singlet fi ssion, but the ability to control the energetics of these solar cells remains a challenge. Here, it is demonstrated that the interfacial charge transfer state energy of a rubrene/C 60 solar cell can be modifi ed dramatically by the mo...

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“…This undesired consequence is often observed in the distinct case of dopants, which are chosen for their ability to directly tune the electronic properties, rather than for their similarity in chemical structure to the host. Since doping in organic‐semiconductor systems is far less efficient than in the inorganic systems from which the technique was borrowed, dopant molar concentrations in excess of 1% are often required to effect the desired electronic changes in the host material . At such high concentrations, the anticipated electronic benefits of doping are often thwarted by insufficient consideration of the structural compatibility of the dopant and host material.…”

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confidence: 99%

Understanding the Crystal Packing and Organic Thin‐Film Transistor Performance in Isomeric Guest–Host Systems

et al. 2017

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In order to understand how additives influence the structure and electrical properties of active layers in thin-film devices, a compositionally identical but structurally different guest-host system based on the syn and anti isomers of triethylsilylethynyl anthradithiophene (TES ADT) is systematically explored. The mobility of organic thin-film transistors (OTFTs) comprising anti TES ADT drops with the addition of only 0.01% of the syn isomer and is pinned at the mobility of OTFTs having pure syn isomer after the addition of only 10% of the isomer. As the syn isomer fraction increases, intermolecular repulsion increases, resulting in a decrease in the unit-cell density and concomitant disordering of the charge-transport pathway. This molecular disorder leads to an increase in charge trapping, causing the mobility of OTFTs to drop with increasing syn-isomer concentration. Since charge transport is sensitive to even minute fractions of molecular disorder, this work emphasizes the importance of prioritizing structural compatibility when choosing material pairs for guest-host systems.

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Impact of a Low Concentration of Dopants on the Distribution of Gap States in a Molecular Semiconductor

Cited by 31 publications

References 28 publications

Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

Morphological Tuning of the Energetics in Singlet Fission Organic Solar Cells

Understanding the Crystal Packing and Organic Thin‐Film Transistor Performance in Isomeric Guest–Host Systems

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