Synthesis, characterization, and crystal structures of molybdenum complexes of unsymmetrical electron-poor dithiolene ligands

Improving the Resistance of Molecularly Doped Polymer Semiconductor Layers to Solvent

The ability to form multi-heterolayer (opto)electronic devices by solution processing of (molecularly doped) semiconducting polymer layers is of great interest since it can facilitate the fabrication of large-area and low-cost devices. However, the solution processing of multilayer devices poses a particular challenge with regard to dissolution of the first layer during the deposition of a second layer. Several approaches have been introduced to circumvent this problem for neat polymers, but suitable approaches for molecularly doped polymer semiconductors are much less well-developed. Here, we provide insights into two different mechanisms that can enhance the solvent resistance of solution-processed doped polymer layers while also retaining the dopants, one being the doping-induced pre-aggregation in solution and the other including the use of a photo-reactive agent that results in covalent cross-linking of the semiconductor and, perhaps in some cases, the dopant. For molecularly p-doped poly(3-hexylthiophene-2,5-diyl) and poly[2,5-bis(3-tetradecyl-thiophene-2-yl)thieno(3,2-b)thiophene] layers, we find that the formation of polymer chain aggregates prior to the deposition from solution plays a major role in enhancing solvent resistance. However, this pre-aggregation limits inclusion of the cross-linking agent benzene-1,3,5-triyl tris(4-azido-2,3,5,6-tetrafluorobenzoate). We show that if pre-aggregation in solution is suppressed, high resistance of thin doped polymer layers to solvent can be achieved using the tris(azide). Moreover, the electrical conductivity can be largely retained by increasing the tris(azide) content in a doped polymer layer.

Chemical approaches for electronic doping in photovoltaic materials beyond crystalline silicon

Electronic doping is applied to tailor the electrical and optoelectronic properties of semiconductors, which have been widely adopted in information and clean energy technologies, like integrated circuit fabrication and PVs. Though this concept has prevailed in conventional PVs, it has achieved limited success in the new-generation PV materials, particularly in halide perovskites, owing to their soft lattice nature and self-compensation by intrinsic defects. In this review, we summarize the evolution of the theoretical understanding and strategies of electronic doping from Si-based photovoltaics to thin-film technologies, e.g., GaAs, CdTe and Cu(In,Ga)Se₂, and also cover the emerging PVs including halide perovskites and organic solar cells. We focus on the chemical approaches to electronic doping, emphasizing various chemical interactions/bonding throughout materials synthesis/modification to device fabrication/operation. Furthermore, we propose new classifications and models of electronic doping based on the physical and chemical properties of dopants, in the context of solid-state chemistry, which inspires further development of optoelectronics based on perovskites and other hybrid materials. Finally, we outline the effects of electronic doping in semiconducting materials and highlight the challenges that need to be overcome for reliable and controllable doping.

Visualisation of individual dopants in a conjugated polymer: sub-nanometre 3D spatial distribution and correlation with electrical properties

While molecular doping is ubiquitous in all branches of organic electronics, little is known about the spatial distribution of dopants, especially at molecular length scales. Moreover, a homogeneous distribution is often assumed when simulating transport properties of these materials, even though the distribution is expected to be inhomogeneous. In this study, electron tomography is used to determine the position of individual molybdenum dithiolene complexes and their three-dimensional distribution in a semiconducting polymer at the sub-nanometre scale. A heterogeneous distribution is observed, the characteristics of which depend on the dopant concentration. At 5 mol% of the molybdenum dithiolene complex, the majority of the dopant species are present as isolated molecules or small clusters up to five molecules. At 20 mol% dopant concentration and higher, the dopant species form larger nanoclusters with elongated shapes. Even in case of these larger clusters, each individual dopant species is still in contact with the surrounding polymer. The electrical conductivity first strongly increases with dopant concentration and then slightly decreases for the most highly doped samples, even though no large aggregates can be observed. The decreased conductivity is instead attributed to the increased energetic disorder and lower probability of electron transfer that originates from the increased size and size variation in dopant clusters. This study highlights the importance of detailed information concerning the dopant spatial distribution at the sub-nanometre scale in three dimensions within the organic semiconductor host. The information acquired using electron tomography may facilitate more accurate simulations of charge transport in doped organic semiconductors.

Double Doping of a Low-Ionization-Energy Polythiophene with a Molybdenum Dithiolene Complex

Doping of organic semiconductors is crucial for tuning the charge-carrier density of conjugated polymers. The exchange of more than one electron between a monomeric dopant and an organic semiconductor allows the polaron density to be increased relative to the number of counterions that are introduced into the host matrix. Here, a molybdenum dithiolene complex with a high electron affinity of 5.5 eV is shown to accept two electrons from a polythiophene that has a low ionization energy of 4.7 eV. Double p-doping is consistent with the ability of the monoanion salt of the molybdenum dithiolene complex to dope the polymer. The transfer of two electrons to the neutral dopant was also confirmed by electron paramagnetic resonance spectroscopy since the monoanion, but not the dianion, of the molybdenum dithiolene complex features an unpaired electron. Double doping allowed an ionization efficiency of 200% to be reached, which facilitates the design of strongly doped semiconductors while lessening any counterion-induced disruption of the nanostructure.

The chemistry of dithietes, 1,2,5,6-tetrathiocins and higher oligomers

The synthesis and reactivity patterns of the strained dithiete ring are compared with their dimeric tetrathiocin counterparts and higher oligomers, highlighting: (i) their cycloaddition chemistry with organic dienophiles as a route to sulfur-containing heterocycles; (ii) their oxidative addition chemistry to low valent transition metal complexes to generate transition metal dithiolate complexes and; (iii) the base-catalysed isomerizations between different dithiete oligomers.

Understanding how Lewis acids dope organic semiconductors: a "complex" story

We report on computational studies of the potential of three borane Lewis acids (LAs) (B(C6F5)3 (BCF), BF3, and BBr3) to form stable adducts and/or to generate positive polarons with three different semiconducting π-conjugated polymers (PFPT, PCPDTPT and PCPDTBT). Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations based on range-separated hybrid (RSH) functionals provide insight into changes in the electronic structure and optical properties upon adduct formation between LAs and the two polymers containing pyridine moieties, PFPT and PCPDTPT, unravelling the complex interplay between partial hybridization, charge transfer and changes in the polymer backbone conformation. We then assess the potential of BCF to induce p-doping in PCPDTBT, which does not contain pyridine groups, by computing the energetics of various reaction mechanisms proposed in the literature. We find that reaction of BCF(OH2) to form protonated PCPDTBT and [BCF(OH)]-, followed by electron transfer from a pristine to a protonated PCPDTBT chain is highly endergonic, and thus unlikely at low doping concentration. The theoretical and experimental data can, however, be reconciled if one considers the formation of [BCF(OH)BCF]- or [BCF(OH)(OH2)BCF]- counterions rather than [BCF(OH)]- and invokes subsequent reactions resulting in the elimination of H2.

A polymeric bis(di-p-anisylamino)fluorene hole-transport material for stable n-i-p perovskite solar cells

Half-devices made with a norbornene homopolymer with hole-transporting 2,7-bis(di-p-anisylamino)fluorene side chains exhibit improved light and heat stability in comparison to those incorporating spiro-OMeTAD.

High Thermoelectric Power Factor of Poly(3-hexylthiophene) through In-Plane Alignment and Doping with a Molybdenum Dithiolene Complex

We report a record thermoelectric power factor of up to 160 μW m-1 K-2 for the conjugated polymer poly(3-hexylthiophene) (P3HT). This result is achieved through the combination of high-temperature rubbing of thin films together with the use of a large molybdenum dithiolene p-dopant with a high electron affinity. Comparison of the UV-vis-NIR spectra of the chemically doped samples to electrochemically oxidized material reveals an oxidation level of 10%, i.e., one polaron for every 10 repeat units. The high power factor arises due to an increase in the charge-carrier mobility and hence electrical conductivity along the rubbing direction. We conclude that P3HT, with its facile synthesis and outstanding processability, should not be ruled out as a potential thermoelectric material.

Enhanced Thermoelectric Power Factor of Tensile Drawn Poly(3-hexylthiophene)

The thermoelectric power factor of a broad range of organic semiconductors scales with their electrical conductivity according to a widely obeyed power law, and therefore, strategies that permit this empirical trend to be surpassed are highly sought after. Here, tensile drawing of the conjugated polymer poly(3-hexylthiophene) (P3HT) is employed to create free-standing films with a high degree of uniaxial alignment. Along the direction of orientation, sequential doping with a molybdenum tris(dithiolene) complex leads to a 5-fold enhancement of the power factor beyond the predicted value, reaching up to 16 μW m^-1 K^-2 for a conductivity of about 13 S cm^-1. Neither stretching nor doping affect the glass transition temperature of P3HT, giving rise to robust free-standing materials that are of interest for the design of flexible thermoelectric devices.

Hybrid Doping of Few-Layer Graphene via a Combination of Intercalation and Surface Doping

Surface molecular doping of graphene has been shown to modify its work function and increase its conductivity. However, the associated shifts in work function and increases in carrier concentration are highly coupled and limited by the surface coverage of dopant molecules on graphene. Here we show that few-layer graphene (FLG) can be doped using a hybrid approach, effectively combining surface doping by larger (metal-)organic molecules and intercalation of smaller molecules, such as Br₂ and FeCl₃, into the bulk. Intercalation tunes the carrier concentration more effectively, whereas surface doping of intercalated FLG can be used to tune its work function without reducing the carrier mobility. This multimodal doping approach yields a very high carrier density and tunable increase in the work function for FLG, demonstrating a new versatile platform for fabricating graphene-based contacts for electronic, optoelectronic, and photovoltaic applications.