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Electronic doping of organic semiconductors is essential for their usage in highly efficient optoelectronic devices. Although molecular and metal complex-based dopants have already enabled significant progress of devices based on organic semiconductors, there remains a need for clean, efficient and low-cost dopants if a widespread transition towards larger-area organic electronic devices is to occur. Here we report dimethyl sulfoxide adducts as p-dopants that fulfil these conditions for a range of organic semiconductors. These adduct-based dopants are compatible with both solution and vapour-phase processing. We explore the doping mechanism and use the knowledge we gain to 'decouple' the dopants from the choice of counterion. We demonstrate that asymmetric p-doping is possible using solution processing routes, and demonstrate its use in metal halide perovskite solar cells, organic thin-film transistors and organic light-emitting diodes, which showcases the versatility of this doping approach.

The double ionization of molecular dopants enables the doping efficiency free charges per dopant molecule to rise above 100 . However, the current models of doped organic semiconductors based on Fermi Dirac statistics fail to explain the double ionization of dopants and also the analogous situation of bipolaron formation on a host polymer. Here, we address this shortcoming by considering the renormalization of the state energies upon electron transfer between host and p dopant. We vary the model parameters amp; 9472;the reorganization energy and evolutions of ionization energies and electron affinities upon charging amp; 9472;and plot the fractions of doubly ionized, singly ionized, and neutral species. The model shows good agreement with experimental measurements of doubly ionized p dopants and bipolarons on a p doped polymer. With these insights, we suggest that the state energy renormalization upon charging is the key parameter to be minimized for double ionization of dopants or maximized to avoid formation of bipolarons on the host

The dielectric constant ( εr) is a fundamental material parameter that governs charge transfer processes in organic semiconductors, yet its value is often assumed rather than measured. Here, we use impedance spectroscopy to determine εr in regioregular poly(3-hexylthiophen-2,5-diyl) (P3HT) thin films p-doped with the molecular dopants hexafluoro-tetracyanonaphthoquinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). We fit the impedance spectra using a single RC circuit model to determine the frequency-dependent capacitance and extract εr. The value of the dielectric constant increases by around two-thirds from 2.9 ± 0.1 (undoped polymer) to 4.9 ± 0.6 on the addition of one F4TCNQ molecule per 500 P3HT monomer units. In contrast, the addition of the weak dopant 7,7,8,8-tetracyanoquinodimethane (TCNQ), which does not undergo ground state charge transfer with P3HT, has no effect on the dielectric constant. Our results support the hypothesis that molecular doping has a considerable impact on the materials dielectric constant via polarizable host-dopant complexes.

This article is part of the Organic Bioelectronics special issue., Electronic doping in organic materials has remained an elusive concept for several decades. It drew considerable attention in the early days in the quest for organic materials with high electrical conductivity, paving the way for the pioneering work on pristine organic semiconductors (OSCs) and their eventual use in a plethora of applications. Despite this early trend, however, recent strides in the field of organic electronics have been made hand in hand with the development and use of dopants to the point that are now ubiquitous. Here, we give an overview of all important advances in the area of doping of organic semiconductors and their applications. We first review the relevant literature with particular focus on the physical processes involved, discussing established mechanisms but also newly proposed theories. We then continue with a comprehensive summary of the most widely studied dopants to date, placing particular emphasis on the chemical strategies toward the synthesis of molecules with improved functionality. The processing routes toward doped organic films and the important doping–processing–nanostructure relationships, are also discussed. We conclude the review by highlighting how doping can enhance the operating characteristics of various organic devices., This work was from King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award nos. OSR-2018-CARF/CCF-3079 and OSR-2019- CRG8-4095.3. F.A. acknowledges the support from The Wilkinson Charitable Foundation. C.M. acknowledges financial support from the Swedish Research Council (grant no. 2018- 03824) and the Knut and Alice Wallenberg Foundation through a Wallenberg Academy Fellowship Prolongation grant.

The energy level tuning effect, observed when mixing halogenated versions of similar molecules, has recently been identified as a new way to modify the bulk ionisation energy and electron affinity of an organic semiconducting thin film, and increase the open-circuit voltage in organic solar cells. In this thesis, the energy level tuning effect is extended to organic field-effect transistors (OFETs). A method for the fabricating ambipolar devices with balanced hole and electron transport is established. This is achieved by using blends of two host molecules, zinc phthalocyanine (ZnPc) and its eight-times fluorinated derivative (F8ZnPc), with energy levels that shift based on mixing ratio. The semiconducting behaviour of the bottom-gate bottom-contact OFETs can be tuned continuously from unipolar p-type, through ambipolar, and finally to unipolar n-type. For the ambipolar devices, the optimum balance between the hole and electron mobilities is found for the blend of 1:1.5 weight ratio of ZnPc:F8ZnPc with hole and electron mobilities of (8.3 ± 0.2)×10 −7 cm2V−1s−1 and (5.5 ± 0.1)×10−7 cm2V−1s−1 , respectively. An application of the ambipolar devices in a complementary-like voltage inverter circuit, with performance comparable to an inverter based on separate ZnPc and F8ZnPc OFETs, is demonstrated. Next, a p-dopant (F6-TCNNQ) is introduced into the blend of ZnPc: F8ZnPc to investigate the impact of the energy level tuning effect on the molecular doping process. The doping efficiency is investigated using photothermal deflection spectroscopy and electron paramagnetic resonance spectroscopy, and is found to depend upon host mixing ratio. The experimentally observed trend is explained with a statistical model that includes both shifts of the host’s ionisation energies, and, importantly, the electron affinity of the dopant. The energy level shift in the model is in close agreement with the 0.86 eV shift in ionisation energy observed experimentally. As further validation, the model reproduces the measured trend at low temperature. The energy level tuning effect therefore has a crucial impact on the molecular doping process. The practice of comparing host and dopant energy levels must consider the long-range electrostatic shifts to consistently explain the doping mechanism in organic semiconductors.

The quenching of excitons in organic solar cells can play a significant role in limiting their power conversion efficiency (PCE). In this article, we investigate the effect of a thin layer of hexapropyltruxene inserted at the interface between the electron donor boron subphthalocyanine chloride (SubPc) and its underlying hole contact in planar heterojunction solar cells. We find that a 3.8 nm hexapropyltruxene interlayer between the molybdenum oxide (MoOx) hole contact and SubPc is sufficient to improve PCE in SubPc/C60 fullerene solar cells from 2.6% to 3.0%, a ∼20% performance improvement. While the absorption stays roughly the same, the comparison of external and internal quantum efficiencies reveals a significant increase in SubPc's contribution to the current for light with wavelengths between 520 and 600 nm. Microstructure and surface morphology assessed with in situ Grazing-Incidence Wide-Angle X-Ray Scattering (GIWAXS) and Atomic Force Microscopy (AFM), are evaluated alongside in situ spectroscopic ellipsometry, and photoluminescence measurements. The microstructural investigations demonstrate changes to the surface and bulk of SubPc grown atop a hexapropyltruxene interlayer indicating that the latter acts as a template layer in a similar way as MoOx. However, the improvement in PCE is found to be mainly via reduced exciton quenching at the MoOx contact with the insertion of the hexapropyltruxene layer.

Bacterial cellulose (BC) has attracted a great deal of interest due to its green synthesis and biocompatibility. The nanoscale dimension of BC nanofibers generates an enormous surface area that enhances interactions with water and soluble components within aqueous solution. Recent work has demonstrated that BC is a versatile platform for the formation of metal/metal oxide nanocomposites. Copper oxide (CuO) is a useful material to compare nanomaterial deposition on BC with other cellulosic materials because of copper&rsquo, s colorimetric reaction as it forms copper hydroxide (Cu(OH)2) and transitions to CuO. In this research, we found that never-dried BC readily deposits CuO into its matrix in a way that does not occur on cotton, dried BC, or regenerated cellulose fibers. We conclude that hydroxyl group availability does not adequately explain our results and that intrafibrillar pores in never-dried BC nanofibers play a critical role in CuO deposition.