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The low dielectric constant of organic semiconductors limits the efficiency of organic solar cells (OSCs). In an attempt to improve the dielectric constant of conjugated polymers, we report the synthesis of three semiconducting copolymers by combining the thiophene-substituted diketopyrrolopyrrole (TDPP) monomer with three different monomeric units with varying electron donating/accepting strengths: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP), and naphthalene diimide (P(gNDI-TDPP)). Among the series, BBT-3TEG-TDPP and P(gNDI-TDPP) exhibited the highest dielectric constants (∼5) at 1 MHz frequency, signifying the contribution of dipolar polarization from TEG side-chains. Furthermore, transient absorption spectroscopic studies performed on these polymers indicated low exciton diffusion length as observed in common organic semiconducting polymers. Our findings suggest that utilizing the polar side-chains enhances the dielectric constant in a frequency regime of megahertz. However, it is not sufficient to reduce the Coulombic interaction between hole and electron in excitonic solar cells.

Two fused ladder type non-fullerene acceptors, DTCCIC and DTCCIC-4F, based on an electron-donating alkylated dithienocyclopentacarbazole core flanked by electron-withdrawing non-fluorinated or fluorinated 1,1-dicyanomethylene-3-indanone (IC or IC-4F), are prepared and utilized in organic solar cells (OSCs). The two new molecules reveal planar structures and strong aggregation behavior, and fluorination is shown to red shift the optical band gap and down shift energy levels. OSCs based on DTCCIC-4F exhibit a power conversion efficiency of 12.6 %, much higher than that of DTCCIC based devices (6.2 %). Microstructural studies reveal that while both acceptors are highly crystalline, bulk heterojunction blends based on the non-fluorinated DTCCIC result in overly coarse domains, while blends based on the fluorinated DTCCIC-4F exhibit a more optimal nanoscale morphology. These results highlight the importance of end group fluorination in controlling molecular aggregation and miscibility.

It remains a challenge to precisely tailor the morphology of polymer monolayers to control charge transport. Herein, the effect of the dissolution temperature (T dis) is investigated as a powerful strategy for morphology control. Low T dis values cause extended polymer aggregation in solution and induce larger nanofibrils in a monolayer network with more pronounced π–π stacking. The field‐effect mobility of the corresponding monolayer transistors is significantly enhanced by a factor of four compared to devices obtained from high T dis with a value approaching 1 cm2 V−1 s−1. Besides that, the solution kinetics reveal a higher growth rate of aggregates at low T dis, and filtration experiments further confirm that the dependence of the fibril width in monolayers on T dis is consistent with the aggregate size in solution. The generalizability of the T dis effect on polymer aggregation is demonstrated using three other conjugated polymer systems. These results open new avenues for the precise control of polymer aggregation for high‐mobility monolayer transistors., Cool down! The dissolution temperature is an efficient parameter to control the aggregation of conjugated polymers in solution and to influence their microstructure in monolayers. A lower dissolution temperature induces stronger π–π stacking interactions and larger fibrillar domains, resulting in high‐performance monolayer transistors with a mobility approaching 1 cm2 V−1 s−1.

The synthesis of a naphthalene diimide bithiophene copolymer P(EO-NDIT2) with branched, base-stable, and purely ether-based side chains is presented. Stille polycondensation leads to high molecular ...

A major breakthrough in organic solar cells (OSCs) in the last thirty years was the development of the bulk heterojunction (BHJ) solution processing strategy, which effectively provided a nanoscale phase-separated morphology, aiding in the separation of Coulombically bound excitons and facilitating charge transport and extraction. Compared with the application of the layer-by-layer (LbL) approach proposed in the same period, the BHJ spin-coating technology shows overwhelming advantages for evaluating the performance of photovoltaic materials and achieving more-efficient photoelectric conversion. Thus, in this study, we have further compared the BHJ and LbL processing strategies via the doctor-blade coating technology because it is a roll-to-roll compatible high-throughput thin film fabrication route. We systematically evaluated multiple target parameters, including morphological characteristics, optical simulation, physical kinetics, device efficiency, and blend stability issues. It is worth emphasizing that our findings disprove the old stereotypes such as the BHJ processing method is superior to the LbL technology for the preparation of high-performance OSCs and the LbL approach requires an orthogonal solvent and donor/acceptor materials with special solubility. Our studies demonstrate that the LbL blade-coating approach is a promising strategy to effectively reduce the efficiency-stability gap of OSCs and even a superior alternative to the BHJ method in commercial applications.

Doped polymer semiconductors are actively studied for opto- and micro-electronic applications including thermoelectric generators, where a high electrical conductivity is a key factor. In general, n-type doping is more challenging to achieve than p-type doping. Here we study n-type doping of a commonly used electron transporting naphthalene-diimide bithiophene copolymer with a series of air-stable and solution-processable benzimidazole dopants. To understand the role of dopant structure on miscibility and the resulting conductivity, benzimidazoles with different linear and branched alkyl substituents were synthesized, and their doping efficacy compared through combined morphological, electrical and thermoelectric characterization. We observe a clear dependence of the nature of the alkyl substituent on dopant intercalation into the semicrystalline morphology. By increasing the length or the steric hindrance of the alkyl substituents, the miscibility between dopant and copolymer is enhanced leading to optimized electrical conductivity.

In bulk-heterojunction polymer solar cells (PSC), the molecular-level mixing between conjugated polymer donors and small-molecule acceptors plays a crucial role in obtaining a desirable morphology and good device stability. It has been recently shown that the thermodynamic limit of this mixing can be quantified by the liquidus miscibility, the composition of the small-molecule acceptor in amorphous phases in the presence of small-molecule crystals, and then converted to the Flory- Huggins interaction parameter χ. This conversion maps out the amorphous miscibility. Moreover, the quantitative relations between χ and the fill factor of PSC devices were established recently. However, the commonly used measurement of this liquidus miscibility, scanning transmission X-ray microscopy, is not easily and readily accessible. Here, we delineate a method based on common visible light microscopy and ultraviolet-visible absorption spectroscopy to replace the X-ray measurements. To demonstrate the feasibility of this technique and methodology, a variety of conjugated polymers (PffBT4T-C 9 C 13 , PDPP3T PBDT-TS1, PTB7-Th, and FTAZ) and their miscibility with fullerenes or nonfullerene small molecules (PC 71 BM, PC 61 BM, and EH-IDTBR) are characterized. The establishment of this methodology will pave the way to a wider use of the liquidus miscibility and the critical miscibility-function relations to optimize the device performance and obtain good stability in PSCs and other devices.

Diketopyrrolopyrrole (DPP) has been drawing considerable attention for constructing semiconducting materials used in organic optoelectronic applications, mainly for organic field effect transistor (OFET) and organic photovoltaic (OPV) devices. In the present work, we study the effects of varying the number of thiophene units (from four to six) attached to DPP on the physical, chemical, and optoelectronic properties by designing and synthesizing a series of small molecule organic semiconductors. The thermal and optical properties, and electronic energy levels of these molecular semiconductors are studied, and their performance in organic field effect transistor devices (OFETs) compared. These small molecules exhibit promising charge carrier mobility and behave as p-type semiconductors. Hole mobility increases with conjugation length and degree of symmetry of the backbone. By adjusting the number of thiophene units on each side of DPP, the hole mobility is enhanced by almost one order of magnitude, from 1.18 × 10-4 to 1.11 × 10-3 cm2 V-1 s-1. Density functional theory (DFT) calculations indicate that increasing the number of homo-coupled thiophene results in a relatively planar configuration, while the terminal alkoxyl benzene unit causes significant torsional rotation which could hamper electron and hole transport in active layer. MOE (Min. of Education, S’pore)

Alternative electron acceptors are being actively explored in order to advance the development of bulk-heterojunction (BHJ) organic solar cells (OSCs). The indene-C-60 bisadduct (ICBA) has been regarded as a promising candidate, as it provides high open-circuit voltage in BHJ solar cells; however, the photovoltaic performance of such ICBA-based devices is often inferior when compared to cells with the omnipresent PCBM electron acceptor. Here, by pairing the high performance polymer (FTAZ) as the donor with either PCBM or ICBA as the acceptor, we explore the physical mechanism behind the reduced performance of the ICBA-based device. Time delayed collection field (TDCF) experiments reveal reduced, yet field-independent free charge generation in the FTAZ:ICBA system, explaining the overall lower photocurrent in its cells. Through the analysis of the photoluminescence, photogeneration, and electroluminescence, we find that the lower generation efficiency is neither caused by inefficient exciton splitting, nor do we find evidence for significant energy back-transfer from the CT state to singlet excitons. In fact, the increase in open circuit voltage when replacing PCBM by ICBA is entirely caused by the increase in the CT energy, related to the shift in the LUMO energy, while changes in the radiative and nonradiative recombination losses are nearly absent. On the other hand, space charge limited current (SCLC) and bias-assisted charge extraction (BACE) measurements consistently reveal a severely lower electron mobilitiy in the FTAZ:ICBA blend. Studies of the blends with resonant soft X-ray scattering (R-SoXS), grazing incident wide-angle X-ray scattering (GIWAXS), and scanning transmission X-ray microscopy (STXM) reveal very little differences in the mesoscopic morphology but significantly less nanoscale molecular ordering of the fullerene domains in the ICBA based blends, which we propose as the main cause for the lower generation efficiency and smaller electron mobility. Calculations of the JV curves with an analytical model, using measured values, show good agreement with the experimentally determined JV characteristics, proving that these devices suffer from slow carrier extraction, resulting in significant bimolecular recombination losses. Therefore, this study highlights the importance of high charge carrier mobility for newly synthesized acceptor materials, in addition to having suitable energy levels.

One of the most employed morphology optimization methods to design better performing organic photovoltaic devices is ink formulation engineering with additives. In this work, by employing a suboptimal host solvent mixture and 1,8-diiodooctane (DIO) as a very optimal solvent for both components in poly-thieno[3,2b]thiophene-diketopyrrolopyrrole-co-thiophene (DPP-TT-T)-based bulk-heterojunctions (BHJ), an alternative, previously unknown mechanism of additive behavior on BHJ microstructure formation is presented. In situ characterization methods involving grazing incidence X-ray diffraction, white-light reflectometry, laser light scattering and photoluminescence during film drying reveal that the microstructure formation under the influence of DIO is led towards thermodynamic equilibrium during host solvent drying, and the kinetics of morphology formation (i.e. polymer crystallization, fullerene aggregation…) are controlled dominantly by the additive during its evaporation. Ex situ X-ray-based characterization methods, such as scanning transmission X-ray microspectroscopy (STXM) and resonant soft X-ray scattering (R-SoXS), additionally reveal that the microstructure of dried films favors smaller domain sizes with purer domains, smaller fullerene aggregates, bimodal polymer crystallization relative to the substrate and more face-on molecular orientation relative to the donor/acceptor interface, which at the end lead to better performing devices with power conversion efficiencies ranging from 1.25% to 4.68%.