High Polymer Molecular Weight Yields Solar Cells with Simultaneously Improved Performance and Thermal Stability.

Simple synthetic routes, high active layer thickness tolerance as well as stable organic solar cells are relentlessly pursued as key enabling traits for the upscaling of organic photovoltaics. Here, the potential to address these issues by tuning donor polymer molecular weight is investigated. Specifically, the focus is on PTQ10, a polymer with low synthetic complexity, with number average molecular weights of 2.4, 6.2, 16.8, 52.9, and 54.4 kDa, in combination with three different non-fullerene acceptors, namely Y6, Y12, and IDIC. Molecular weight, indeed, unlocks a threefold increase in power conversion efficiency for these blends. Importantly, efficiencies above 10% for blade coated devices with thicknesses between 200 and 350 nm for blends incorporating high molecular weight donor are shown. Spectroscopic, GIWAXS and charge carrier mobility data suggest that the strong photocurrent improvement with molecular weight is related to both, improved electronic transport and polymer contribution to exciton generation. Moreover, it is demonstrated that solar cells based on high molecular weight PTQ10 are more thermally stable due to a higher glass transition temperature, thus also improving device stability.

Matching electron transport layers with a non-halogenated and low synthetic complexity polymer:fullerene blend for efficient outdoor and indoor organic photovoltaics.

The desired attributes of organic photovoltaics (OPV) as a low cost and sustainable energy harvesting technology demand the use of non-halogenated solvent processing for the photoactive layer (PAL) materials, preferably of low synthetic complexity (SC) and without compromising the power conversion efficiency (PCE). Despite their record PCEs, most donor-acceptor conjugated copolymers in combination with non-fullerene acceptors are still far from upscaling due to their high cost and SC. Here we present a non-halogenated and low SC ink formulation for the PAL of organic solar cells, comprising PTQ10 and PC61BM as donor and acceptor materials, respectively, showing a record PCE of 7.5% in blade coated devices under 1 sun, and 19.9% under indoor LED conditions. We further study the compatibility of the PAL with 5 different electron transport layers (ETLs) in inverted architecture. We identify that commercial ZnO-based formulations together with a methanol-based polyethyleneimine-Zn (PEI-Zn) chelated ETL ink are the most suitable interlayers for outdoor conditions, providing fill factors as high as 74% and excellent thickness tolerance (up to 150 nm for the ETL, and >200 nm for the PAL). In indoor environments, SnO2 shows superior performance as it does not require UV photoactivation. Semi-transparent devices manufactured entirely in air via lamination show indoor PCEs exceeding 10% while retaining more than 80% of the initial performance after 400 and 350 hours of thermal and light stress, respectively. As a result, PTQ10:PC61BM combined with either PEI-Zn or SnO2 is currently positioned as a promising system for industrialisation of low cost, multipurpose OPV modules.

High-Throughput Screening of Blade-Coated Polymer:Polymer Solar Cells: Solvent Determines Achievable Performance.

Optimization of a new system for organic solar cells is a multiparametric analysis problem that requires substantial efforts in terms of time and resources. The strong microstructure-dependent performance of polymer:polymer cells makes them particularly difficult to optimize, or to translate previous knowledge from spin coating into more scalable techniques. In this work, the photovoltaic performance of blade-coated devices was studied based on the promising polymer:polymer system PBDB-T and PF5-Y5 as donor and acceptor, respectively. Using the recently developed high-throughput methodology, the system was optimized for multiple variables, including solvent system, active layer composition, ratio, and thickness, among others, by fabricating more than 500 devices with less than 24 mg of each component. As a result, the power conversion efficiency of the blade-coated devices varied from 0.08 to 6.43 % in the best device. The performed statistical analysis of the large experimental data obtained showed that solvent selection had the major impact on the final device performance due to its influence on the active layer microstructure. As a conclusion, the use of the plot of the device efficiency in the Hansen space was proposed as a powerful tool to guide solvent selection in organic photovoltaics.

Sequential Doping of Ladder-Type Conjugated Polymers for Thermally Stable n-Type Organic Conductors.

Doping of organic semiconductors is a powerful tool to optimize the performance of various organic (opto)electronic and bioelectronic devices. Despite recent advances, the low thermal stability of the electronic properties of doped polymers still represents a significant obstacle to implementing these materials into practical applications. Hence, the development of conducting doped polymers with excellent long-term stability at elevated temperatures is highly desirable. Here, we report on the sequential doping of the ladder-type polymer poly(benzimidazobenzophenanthroline) (BBL) with a benzimidazole-based dopant (i.e., N-DMBI). By combining electrical, UV-vis/infrared, X-ray diffraction, and electron paramagnetic resonance measurements, we quantitatively characterized the conductivity, Seebeck coefficient, spin density, and microstructure of the sequentially doped polymer films as a function of the thermal annealing temperature. Importantly, we observed that the electrical conductivity of N-DMBI-doped BBL remains unchanged even after 20 h of heating at 190 °C. This finding is remarkable and of particular interest for organic thermoelectrics.

Impact of Singly Occupied Molecular Orbital Energy on the n-Doping Efficiency of Benzimidazole Derivatives.

We investigated the impact of singly occupied molecular orbital (SOMO) energy on the n-doping efficiency of benzimidazole derivatives. By designing and synthesizing a series of new air-stable benzimidazole-based dopants with different SOMO energy levels, we demonstrated that an increase of the dopant SOMO energy by only ∼0.3 eV enhances the electrical conductivity of a benchmark electron-transporting naphthalenediimide-bithiophene polymer by more than 1 order of magnitude. By combining electrical, X-ray diffraction, and electron paramagnetic resonance measurements with density functional theory calculations and analytical transport simulations, we quantitatively characterized the conductivity, Seebeck coefficient, spin density, and crystallinity of the doped polymer as a function of the dopant SOMO energy. Our findings strongly indicate that charge and energy transport are dominated by the (relative) position of the SOMO level, whereas morphological differences appear to play a lesser role. These results set molecular-design guidelines for next-generation n-type dopants.

Reduction of Charge Traps and Stability Enhancement in Solution-Processed Organic Field-Effect Transistors Based on a Blended n-Type Semiconductor.

Solution-processed n-type organic field-effect transistors (OFETs) are essential elements for developing large-area, low-cost, and all organic logic/complementary circuits. Nonetheless, the development of air-stable n-type organic semiconductors (OSCs) lags behind their p-type counterparts. The trapping of electrons at the semiconductor-dielectric interface leads to a lower performance and operational stability. Herein, we report printed small-molecule n-type OFETs based on a blend with a binder polymer, which enhances the device stability due to the improvement of the semiconductor-dielectric interface quality and a self-encapsulation. Both combined effects prevent the fast deterioration of the OSC. Additionally, a complementary metal-oxide semiconductor-like inverter is fabricated depositing p-type and n-type OSCs simultaneously.

Role of Polymorphism and Thin-Film Morphology in Organic Semiconductors Processed by Solution Shearing.

Organic semiconductors (OSCs) are promising materials for cost-effective production of electronic devices because they can be processed from solution employing high-throughput techniques. However, small-molecule OSCs are prone to structural modifications because of the presence of weak van der Waals intermolecular interactions. Hence, controlling the crystallization in these materials is pivotal to achieve high device reproducibility. In this perspective article, we focus on controlling polymorphism and morphology in small-molecule organic semiconducting thin films deposited by solution-shearing techniques compatible with roll-to-roll systems. Special attention is paid to the influence that the different experimental deposition parameters can have on thin films. Further, the main characterization techniques for thin-film structures are reviewed, highlighting the in situ characterization tools that can provide crucial insights into the crystallization mechanisms.

Electrolyte-Gated Organic Field-Effect Transistor Based on a Solution Sheared Organic Semiconductor Blend.

This communication presents a novel electrolyte gated field-effect transistor based on a blend of dibenzo-tetrathiafulvalene and polystyrene deposited through bar-assisted meniscus shearing. This technique allows the fabrication of high performing electronic devices suitable for (bio)sensing applications and might capture industrial interest due to its scalability. The reported devices can operate in aqueous solution with comparable complexity to real samples.

Structural and electronic characterisation of π-extended tetrathiafulvalene derivatives as active components in field-effect transistors.

The electronic and structural properties of two tetrathiafulvalene derivatives bearing aromatic benzene rings are reported. Thin film transistors of these materials show p-type characteristics with comparable mobility values. It is found that the rigidification of the molecule is beneficial for reducing the reorganisation energy but also has an unfavorable impact on the electronic structure dimensionality.