Remarkable conductivity enhancement in P-doped polythiophenes via rational engineering of polymer-dopant interactions

Molecular doping is an effective approach to tune the charge density and optimize electrical performance of conjugated polymers. However, the introduction of dopants, on the other hand, may disturb the polymer microstructure and disrupt the charge transport path, often leading to a decrease of charge carrier mobility and deterioration of electrical conductivity of the doped films. Here we show that dopant-induced disorder can be overcome by rational engineering of polymer-dopant interactions, resulting in remarkable enhancement of electrical conductivity. Benchmark poly(3-hexylthiophene) (P3HT) and its analogous random polymers of 3-hexylthiophene and thiophene P[(3HT)1-x-stat-(T)x] were synthesized and doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). Remarkably, random P[(3HT)1-x-stat-(T)x] was doped to a far superior electrical conductivity, that in the case of x ≥ 0.24, the conductivity of P[(3HT)1-x-stat-(T)x] is over 100 times higher than that of the doped P3HT, despite both P3HT and P[(3HT)1-x-stat-(T)x] exhibit comparable charge carrier mobility in their pristine state and in spite of their practically identical redox properties. This result can be traced back to the formation of π-stacked polymer-dopant-polymer co-crystals exhibiting extremely short packing distances of 3.13–3.15 \uc5. The mechanism behind these performances is based on a new role played by the dopant molecules that we name “bridging-gluing”. The results are coherently verified by the combination of optical absorption spectroscopy, X-ray diffraction, density functional theory calculations, and molecular dynamics simulations

Hexanary blends: a strategy towards thermally stable organic photovoltaics

Non-fullerene based organic solar cells display a high initial power conversion efficiency but continue to suffer from poor thermal stability, especially in case of devices with thick active layers. Mixing of five structurally similar acceptors with similar electron affinities, and blending with a donor polymer is explored, yielding devices with a power conversion efficiency of up to 17.6%. The hexanary device performance is unaffected by thermal annealing of the bulk-heterojunction active layer for at least 23 days at 130 \ub0C in the dark and an inert atmosphere. Moreover, hexanary blends offer a high degree of thermal stability for an active layer thickness of up to 390 nm, which is advantageous for high-throughput processing of organic solar cells. Here, a generic strategy based on multi-component acceptor mixtures is presented that permits to considerably improve the thermal stability of non-fullerene based devices and thus paves the way for large-area organic solar cells

Synthesis and characterization of spark plasma sintered FeAl and in situ FeAl–Al₂O₃ composite

In the present work, nanocrystalline FeAl and FeAl–Al2O3 composite were synthesized by high energy ball milling and subsequent compaction by spark plasma sintering. Microstructural changes during all stages of processing are studied using X-ray analysis. After 20 h of milling, the disordered FeAl and some amount of Fe rich solid solution was obtained in both of these compositions. Subsequent heat treatment results in formation of ordered FeAl. However, disordering of FeAl was observed in both compositions after spark plasma sintering. Nanocrystallinity is retained in both the compositions even after sintering at high temperature of 1,000°C. Very high hardness of ∼575 HV1 and ∼600 HV1 was exhibited by FeAl and FeAl–Al2O3 composite

Increasing the Ionization Energy Offset to Increase the Quantum Efficiency in Non‐Fullerene Acceptor‐Based Organic Solar Cells: How Far Can We Go?

Abstract Molecular engineering of organic semiconductors provides a virtually unlimited number of possible structures, yet only a handful of combinations lead to state‐of‐the‐art efficiencies in photovoltaic applications. Thus, design rules that guide material development are needed. One such design principle is that in a bulk heterojunction consisting of an electron donor and lower bandgap acceptor an offset (ΔIE) of at least 0.45 eV is required between both materials ionization energies to overcome energy level bending at the donor–acceptor interface, in turn maximizing the charge separation yield and the cell's internal quantum efficiency. The present work studies energy losses associated with ΔIE and, based on 24 blends, finds that losses are minimal up to a ΔIE of 0.6 eV. Electroluminescence spectroscopy shows that low energy losses are achieved when the charge transfer state energy (ECT) is similar to the acceptor's optical bandgap (EgA). Further ΔIE increase lowers ECT with respect to EgA, thus decreasing VOC. Within that 0.45–0.6 eV ΔIE sweet range, the fill factor FF, hence the power conversion efficiency, increases only marginally as the FF is often already close to maximal for ΔIE = 0.45 eV. The results are extended to 76 binary and ternary blends

Highly Stretchable and Air-Stable PEDOT:PSS/Ionic Liquid Composites for Efficient Organic Thermoelectrics

Thermoelectric (TE) generators that are capable of providing sustainable energy conversion under dynamic mechanical stresses have been explored for realizing autonomous wearable electronics. However, finding extremely deformable, efficient, and air-stable TE materials is still a major challenge. Here, we report highly stretchable and efficient organic TE materials from aqueous composites of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and ionic liquids (ILs). In this composite, ILs simultaneously enhance the Seebeck coefficient and electrical conductivity of PEDOT:PSS (up to 35 μV K -1 and 538 S cm -1 , respectively) by controlling its oxidation level and nanostructure. Moreover, the resulting fibrous structure with IL-assisted soft domains leads to outstanding mechanical deformability and durability, enabling that the PEDOT:PSS/IL films simply coated on elastomeric substrates maintain the TE functionality under tensile strain (ϵ) up to 70% and repetitive stretching cycles with 30% ϵ without severe degradation in TE performance. Furthermore, we also demonstrate the long-term TE stability of PEDOT:PSS/IL composites maintaining >80% of the initial performance after 10 days under ambient conditions. Our finding proves the potential of this novel composite as a stretchable and air-stable organic TE material

Correlating Acceptor Structure and Blend Nanostructure with the Photostability of Nonfullerene Organic Solar Cells

The formation of photoinduced traps resulting in the loss of electron mobility deteriorates the performance of organic solar cells under continuous light soaking. The genesis of these loss mechanisms is elucidated by examining the structural stability of halogenated ITIC derivative films and the phase behavior of the respective binary systems by blending with the donor polymer PBDBT-2F. Under constant illumination, ITIC-4Cl is found to maintain its structural integrity, whereas fluorine on the peripheral moieties of ITIC-4F undergoes chemical substitution to form a mixture of ITIC and ITIC-4F. Thermal analysis of the light-soaked binary films reveals that ITIC-4Cl loses its crystalline phase while the crystallinity of ITIC-4F does not undergo changes. Further, it is shown that the addition of a small amount of ITIC-4F as a third component hinders the loss of ITIC-4Cl crystalline phase in bulk heterojunction blends through the formation of cocrystals. These results suggest that long-range ordering of NFAs does not necessarily improve the photostability of organic solar cells and that the addition of a third component, irrespective of the crystalline nature, can prevent changes in bulk heterojunction blend nanostructure

Electrically Programmed Doping Gradients Optimize the Thermoelectric Power Factor of a Conjugated Polymer

Functionally graded materials (FGMs) are widely explored in the context of inorganic thermoelectrics, but not yet in organic thermoelectrics. Here, the impact of doping gradients on the thermoelectric properties of a chemically doped conjugated polymer is studied. The in-plane drift of counterions in moderate electric fields is used to create lateral doping gradients in films composed of a polythiophene with oligoether side chains, doped with 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ). Raman microscopy reveals that a bias voltage of as little as 5 V across a 50 µm wide channel is sufficient to trigger counterion drift, resulting in doping gradients. The effective electrical conductivity of the graded channel decreases with bias voltage, while an overall increase in Seebeck coefficient is observed, yielding an up to eight-fold enhancement in power factor. Kinetic Monte Carlo simulations of graded films explain the increase in power factor in terms of a roll-off of the Seebeck coefficient at high electrical conductivities in combination with a mobility decay due to increased Coulomb scattering at high dopant concentrations. Therefore, the FGM concept is found to be a way to improve the thermoelectric performance of not yet optimally doped organic semiconductors, which may ease the screening of new materials as well as the fabrication of devices.</p

Suppressing Co-Crystallization of Halogenated Non-Fullerene Acceptors for Thermally Stable Ternary Solar Cells

While photovoltaic blends based on non-fullerene acceptors are touted for their thermal stability, this type of acceptor tends to crystallize, which can result in a gradual decrease in photovoltaic performance and affects the reproducibility of the devices. Two halogenated indacenodithienothiophene-based acceptors that readily co-crystallize upon mixing are studied, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization, which results in a fine-grained ternary blend with nanometer-sized domains that do not coarsen due to a highT(g)approximate to 200 degrees C. As a result, annealing at temperatures of up to 170 degrees C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. The results indicate that the ternary approach enables the use of high-temperature processing protocols, which are needed for upscaling and high-throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 degrees C for at least 205 h, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability