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International audience; An efficient metal-free formulation of a hole transport material (HTM) based on an ionic liquid polymer is developed for n-i-p perovskite solar cells (PSCs), to address reproducibility issues related to the use of complex dopant mixtures based on lithium salts and cobalt coordination complexes. The conductivity of the HTM is thus significantly improved by 4 orders of magnitude, up to 1.9 x 10-3 S.cm-1, using poly(1-butyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide) (PVBI-TFSI) as dopant. Introduced in the FTO/c-TiO2/mp-TiO2/K0.05(MA0.15FA0.85)0.95PbI2.55Br0.45/HTM/Au PSC configuration, PVBI-TFSI-HTM formulation shows power conver-sion efficiency as high as 20.3 %, versus 18.4 % for the standard lithium salt-HTM formulation, with considerably re-duced hysteresis and excellent reproducibility. Mechanistic investigations suggest that PVBI-TFSI acts as a source of protons promoting the HTM oxidation.

We investigated the electrical and optoelectronic characteristics of ambipolar WSe2 field-effect transistors (FETs) via facile p-doping process during the thermal annealing in ambient. Through this annealing, the oxygen molecules were successfully doped into the WSe2 surface, which ensured higher p-type conductivity and the shift of the transfer curve to the positive gate voltage direction. Besides, considerably improved photoswitching response characteristics of ambipolar WSe2 FETs were achieved by the annealing in ambient. To explore the origin of the changes in electrical and optoelectronic properties, the analyses via X-ray photoelectron, Raman, and photoluminescence spectroscopies were performed. From these analyses, it turned out that WO3 layers formed by the annealing in ambient introduced p-doping to ambipolar WSe2 FETs, and disorders originated from the WO3/WSe2 interfaces acted as non-radiative recombination sites, leading to significantly improved photoswitching response time characteristics.

In the research field on materials for energy solutions, the exploitation of organic compounds and polymers is of great interest, especially in view of a more eco-sustainable development. In this context, the 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecule actively participates in the optimization of manyoptoelectronic devicesbased on such elements, thanks to its electrical properties. In this paper, we report a study on its optical properties, in particular demonstrating fluorescence emission from solutions containing F4TCNQ. This phenomenon is equally relevant, together with the effects on electricalconductivity, in the design of systems for energy technology. The results we present here offer new insights in the phenomenological picture of this molecule and can sharpen its applicability.

International audience; Simultaneously optimizing performances, processability and fabrication cost of organic electronic materials is the continual source of compromise hindering the development of disruptive applications. In this work, we identified a strategy to achieve record conductivity values of one of the most benchmarked semiconducting polymers by doping with an entirely solution-processed, water-free and cost-effective technique. High electrical conductivity for poly(3-hexylthiophene) up to 21 S/cm has been achieved, using a commercially available electron acceptor as both a Lewis acid and an oxidizing agent. While we managed water-free solution-processing a three-time higher conductivity for P3HT with a very affordable/available chemical, near-field microscopy reveals the existence of concentration-dependent higher-conductivity micro-domains for which furthermore process optimization might access to even higher performances. In the perpetual quest of reaching higher performances for organic electronics, this work shall greatly unlock applications maturation requiring higher-scale processability and lower fabrication costs concomitant of higher performances and new functionalities, in the current context where understanding the doping mechanism of such class of materials remains of the greatest interest.

The electronic properties of single-layer, CVD-grown graphene were modulated by deep ultraviolet (DUV) light irradiation in different radiation environments. The graphene field-effect transistors (GFETs), exposed to DUV in air and pure O2, exhibited p-type doping behavior, whereas those exposed in vacuum and pure N2 gas showed n-type doping. The degree of doping increased with DUV exposure time. However, n-type doping by DUV in vacuum reached saturation after 60 min of DUV irradiation. The p-type doping by DUV in air was observed to be quite stable over a long period in a laboratory environment and at higher temperatures, with little change in charge carrier mobility. The p-doping in pure O2 showed ~15% de-doping over 4 months. The n-type doping in pure N2 exhibited a high doping effect but was highly unstable over time in a laboratory environment, with very marked de-doping towards a pristine condition. A lateral pn-junction of graphene was successfully implemented by controlling the radiation environment of the DUV. First, graphene was doped to n-type by DUV in vacuum. Then the n-type graphene was converted to p-type by exposure again to DUV in air. The n-type region of the pn-junction was protected from DUV by a thick double-coated PMMA layer. The photocurrent response as a function of Vg was investigated to study possible applications in optoelectronics.

International audience; This work reports on the optical feedback dynamics of InAs/GaAs QD lasers epitaxially grown on silicon operating in both the short and long delay regimes. Both undoped and p-doped QD lasers are considered. Whatever the external cavity length, no chaotic oscillations are observed on both samples as a result of the small α-factor observed in the silicon QD lasers. Despite that, experiments conducted in the short-cavity region raise period-one oscillation for the undoped QD laser. In addition, the transition from the short to long delay regimes can be finely covered by varying the external cavity length from 5 cm to 50 cm, and the boundaries associated to the appearance of the periodic oscillation are identified. In the short-cavity region, boundaries show some residual undulations resulting from interferences between internal and external cavity modes; whereas in the long-delay regime, the feedback ratio delimiting the boundaries keeps decreasing, until it progressively becomes rather independent of the external cavity length. Overall, our results showed that the p-doped device clearly exhibits a much higher tolerance to the different external feedback conditions than the undoped one, seeing that its periodic oscillation boundaries are barely impossible to retrieve at the maximum feedback strength of-7 dB. These results show for the first time the p-modulation doping effect on the enhancement of feedback insensitivity in both short-and long-delay configurations, which is of paramount importance for the development of ultra-stable silicon transmitters for photonic technologies.

Derivatives of the hexacyano-[3]-radialene anion radical (CN6-CP•−) emerge as a promising new family of p-dopants having a doping strength comparable to that of archetypical dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ). Here, mixed solution (MxS) and sequential processing (SqP) doping methods are compared by using a model semiconductor poly(3-hexylthiophene) (P3HT) and the dopant CN6-CP•−NBu4 + (NBu4 + = tetrabutylammonium). MxS films show a moderate yet thickness-independent conductivity of ≈0.1 S cm−1. For the SqP case, the highest conductivity value of ≈6 S cm−1 is achieved for the thinnest (1.5–3 nm) films whereas conductivity drops two orders of magnitudes for 100 times thicker films. These results are explained in terms of an interfacial doping mechanism realized in the SqP films, where only layers close to the P3HT/dopant interface are doped efficiently, whereas internal P3HT layers remain essentially undoped. This structure is in agreement with transmission electron microscopy, atomic force microscopy, and Kelvin probe force microscopy results. The temperature-dependent conductivity measurements reveal a lower activation energy for charge carriers in SqP samples than in MxS films (79 meV vs 110 meV), which could be a reason for their superior conductivity. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim EXC 1056 Russian Foundation for Basic Research, RFBR: 18‐03‐00715 Deutsche Forschungsgemeinschaft, DFG: KI‐1094/9‐1 Deutsche Forschungsgemeinschaft, DFG Deutsche Forschungsgemeinschaft, DFG: KR 4364/4‐1 The authors gratefully acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) (Grant KI‐1094/9‐1) and the Cluster of Excellence (EXC 1056) “Center for Advancing Electronics Dresden (cfaed).” T.B. gratefully acknowledges financial support by the Russian Foundation for Basic Research (Project 18‐03‐00715). Y.K., B.D., and M.B. acknowledge the financial support from the German Research Foundation (DFG) through KR 4364/4‐1. M.A.‐H. thanks the University of Jordan and Leibniz‐Institut für Polymerforschung, Dresden (IPF) for financial support.

The development of electrically pumped semiconductor diode lasers emitting at the ultraviolet (UV)-B and -C spectral bands has been an active area of research over the past several years, motivated by a wide range of emerging applications. III-Nitride materials and their alloys, in particular AlGaN, are the material of choice for the development of this ultrashort-wavelength laser technology. Despite significant progress in AlGaN-based light-emitting diodes (LEDs), the technological advancement and innovation in diode lasers at these spectral bands is lagging due to several technical challenges. Here, the authors review the progress of AlGaN electrically-pumped lasers with respect to very recent achievements made by the scientific community. The devices based on both thin films and nanowires demonstrated to date will be discussed in this review. The state-of-the-art growth technologies, such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD); and various foreign substrates/templates used for the laser demonstrations will be highlighted. We will also outline technical challenges associated with the laser development, which must be overcome in order to achieve a critical technological breakthrough and fully realize the potential of these lasers.

A new design of carbon source using a S-shaped glassy carbon filament for p-doping and carbon deposition is presented. Due to the higher resistivity of the glassy carbon, lower current is needed to produce the same flux as in a source based on pyrolytic graphite filament. The source design is very compact and the use of water cooling is not mandatory. The results and simulations of the new design show a higher flux than the previously reported glassy carbon based source while avoiding hot spots that can cause an early filament degradation. Hole mobility vs doping level relation data on p-doped GaAs layers grown by molecular beam epitaxy using this new carbon source are shown. With this new C source GaAs (001) surface morphology flatness is preserved after depositing C, showing a reduced substrate heating radiation essential for operate in III-V solid source molecular beam epitaxy (MBE) systems., The authors wish to acknowledge the financial support by the Spanish MINECO (MAT2015-67021-R), by MCIU-AEI-FEDER-UE (ENE2017-91092-EXP, RTI2018-096937-B-C22) and by Comunidad de Madrid (P2018/EMT-4308). We acknowledge the service from the MiNa Laboratory at IMN, funding from CM (project S2018/NMT-4291 TEC2SPACE), and from the X-SEM Laboratory, funding from MINECO (project CSIC13-4E-1794), both with support from EU (ERDF, ESF).

In this study, a near‐infrared photodetector featuring a high photoresponsivity and a short photoresponse time is demonstrated, which is fabricated on rhenium diselenide (ReSe2) with a relatively narrow bandgap (0.9–1.0 eV) compared to conventional transition‐metal dichalcogenides (TMDs). The excellent photo and temporal responses, which generally show a trade‐off relation, are achieved simultaneously by applying a p‐doping technique based on hydrochloric acid (HCl) to a selected ReSe2 region. Because the p‐doping of ReSe2 originates from the charge transfer from un‐ionized Cl molecules in the HCl to the ReSe2 surface, by adjusting the concentration of the HCl solution from 0.1 to 10 m, the doping concentration of the ReSe2 is controlled between 3.64 × 1010 and 3.61 × 1011 cm−2. Especially, the application of the selective HCl doping technique to the ReSe2 photodetector increases the photoresponsivity from 79.99 to 1.93 × 103 A W−1, and it also enhances the rise and decay times from 10.5 to 1.4 ms and from 291 to 3.1 ms, respectively, compared with the undoped ReSe2 device. The proposed selective p‐doping technique and its fundamental analysis will provide a scientific foundation for implementing high‐performance TMD‐based electronic and optoelectronic devices., A rhenium diselenide (ReSe2) photodetector featuring long wavelength detection, short response time, and high photoresponsivity is demonstrated by applying a selective HCl p‐doping technique to the ReSe2 channel region. The effects of the HCl p‐doping on the ReSe2 material and device are thoroughly investigated via Raman spectroscopy, Kelvin probe force microscopy, and electrical measurements in dark and under light illumination conditions.

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 host–dopant 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.

Poly(3-hexylthiophene) (P3HT) films and P3HT / fullerene photovoltaic cells have been p-doped with very low levels (< 1 wt. %) of molybdenum tris[1-(trifluoromethylcarbonyl)- 2-(trifluoromethyl)-ethane-1,2-dithiolene]. The dopants are inhomogenously distributed within doped P3HT films, both laterally and as a function of depth, and appear to aggregate in some instances. Doping also results in subtle changes in the local and long range order of the P3HT film. These effects likely contribute to the complexity of the observed evolutions in conductivity, mobility and work function with doping levels. They also negatively affect the open-circuit voltage and fill factor of solar cells in unexpected ways, indicating that dopant aggregation and non-uniform distribution can harm device performance.

The fabrication of n-type diamond is essential for the realization of electronic components for extreme environments. We report on the growth of a 66 μm thick homoepitaxial phosphorus-doped diamond on a (110)-oriented diamond substrate, grown at a very high deposition rate of 33 μm/h. A pristine diamond lattice is observed by high resolution transmission electron microscopy, which indicates the growth of high quality diamond. About 2.9 × 10^16 cm−3 phosphorus atoms are electrically active as substitutional donors, which is 60% of all incorporated dopant atoms. These results indicate that P-doped (110)-oriented diamond films deposited at high growth rates are promising candidates for future use in high-power electronic applications. This work was financially supported by the EU through the FP7 Collaborative Project “DIAMANT,” the “H2020 Research and Innovation Action Project” “GreenDiamond” (No. 640947), and the Research Foundation-Flanders (FWO) (Nos. G.0C02.15N and VS.024.16N). J.V. acknowledges funding from the “Geconcentreerde Onderzoekacties” (GOA) project “Solarpaint” of the University of Antwerp. The TEM instrument was partly funded by the Hercules fund from the Flemish Government. We particularly thank Dr. J. E. Butler (Naval Research Laboratory, USA) for the sample preparation by laser slicing for TEM analysis, Dr. J. Pernot (Universite Grenoble Alpes/CNRS-Institut Neel, France) for helpful discussions, Ms. C. Vilar (Universite de Versailles St. Quentin en Yvelines, France) for technical help on SEMCL experiments, and Dr. S. S. Nicley (Hasselt University, Belgium) for improving the language of the text. P.P. and S.T. are Postdoctoral Fellows of the Research FoundationFlanders (FWO).

Transition metal oxides (TMOs) on organic semiconductors (OSs) structure has been widely used in inverted organic optoelectronic devices, including inverted organic light-emitting diodes (OLEDs) and inverted organic solar cells (OSCs), which can improve the stability of such devices as a result of improved protection of air sensitive cathode. However, most of these reports are focused on the anode modification effect of TMO and the nature of TMO-on-OS is not fully understood. Here we show that the OS on TMO forms a two-layer structure, where the interface mixing is minimized, while for TMO-on-OS, due to the obvious diffusion of TMO into the OS, a doping-layer structure is formed. This is evidenced by a series of optical and electrical studies. By studying the TMO diffusion depth in different OS, we found that this process is governed by the thermal property of the OS. The TMO tends to diffuse deeper into the OS with a lower evaporation temperature. It is shown that the TMO can diffuse more than 20 nm into the OS, depending on the thermal property of the OS. We also show that the TMO-on-OS structure can replace the commonly used OS with TMO doping structure, which is a big step toward in simplifying the fabrication process of the organic optoelectronic devices. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore)

Polyfluorene PF•γCD rotaxane copolymer, composed of randomly distributed 9,9-dioctylfluorene, methyltriphenylamine (electron-donating) and 9-dicyanomethylenefluorene complexed with γ-cyclodextrin (γCD) (electron-accepting) structural units, has been synthesized by Suzuki cross-coupling reaction. The chemical structures were proved by FTIR and 1H NMR spectroscopy. The surface morphology, thermal, optical, electrochemical behavior, and adhesion characteristics of the obtained rotaxane copolymer have been investigated and compared with those of the nonrotaxane counterpart (PF). Relatively high fluorescence efficiency, almost identical normalized absorbance maximum in solution and solid-state of PF•γCD rotaxane copolymer, and a more uniform and smoother surface with lower adhesion forces provides the role of γCD encapsulation on the lower aggregation propensity. PF•γCD and PF copolymers exhibit n- and p-doping processes and blue-light emission in the film state. The optical and electrochemical band gaps (ΔEg), as well as the highest occupied molecular orbital/lowest unoccupied molecular orbital positions in an energetic diagram indicate that both copolymers are promising blue-emitting electroluminescent materials. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Here, GaN/AlxGa1-xN heterostructures with a graded AlN composition, completely lacking external p-doping, are designed and grown using metal-organic-chemical-vapour deposition (MOCVD) system to realize three-dimensional hole gas (3DHG). The existence of the 3DHG is confirmed by capacitance-voltage measurements. Based on this design, a p-doping-free InGaN/GaN light-emitting diode (LED) driven by the 3DHG is proposed and grown using MOCVD. The electroluminescence, which is attributed to the radiative recombination of injected electrons and holes in InGaN/GaN quantum wells, is observed from the fabricated p-doping-free devices. These results suggest that the 3DHG can be an alternative hole source for InGaN/GaN LEDs besides common Mg dopants. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

The addition of strain and p-doping in high-speed GaAs-based MQW lasers are assessed experimentally in terms of their potential for both increasing the differential gain, /spl part/g//spl part/N, and reducing the modulus of the differential refractive index, |/spl part/n//spl part/N|, in order to decrease the linewidth enhancement factor, /spl alpha/. The increased differential gain with strain alone is found to be offset by a corresponding increase of |/spl part/n//spl part/N|. The further addition of p-doping, on the other hand, simultaneously increases /spl part/g//spl part/N and decreases |/spl part/n//spl part/N|, yielding a substantial reduction in /spl alpha/. >