awei Hu

Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells

Although the field of polymer solar cell has seen much progress in device performance in the past few years, several limitations are holding back its further development. For instance, current high-efficiency (>9.0%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullerene acceptor. Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains. This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes. The uncovered aggregation and design rules yield three high-efficiency (>10%) donor polymers and will allow further synthetic advances and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility.

High-efficiency non-fullerene organic solar cells enabled by a difluorobenzothiadiazole-based donor polymer combined with a properly matched small molecule acceptor

Here we report high-performance small molecule acceptor (SMA)-based organic solar cells (OSCs) enabled by the combination of a difluorobenzothiadiazole donor polymer named PffBT4T-2DT and a SMA named SF-PDI2. It is found that SF-PDI2 matches particularly well with PffBT4T-2DT and non-fullerene OSCs with an impressive VOC of 0.98 V, and a high power conversion efficiency of 6.3% is achieved. Our study shows that PffBT4T-2DT is a promising donor material for SMA-based OSCs, and the selection of a matching SMA is also important to achieve the best OSC performance.

A Tetraphenylethylene Core‐Based 3D Structure Small Molecular Acceptor Enabling Efficient Non‐Fullerene Organic Solar Cells

A tetraphenylethylene core-based small molecular acceptor with a unique 3D molecular structure is developed. Bulk-heterojunction blend films with a small feature size (≈20 nm) are obtained, which lead to non-fullerene organic solar cells (OSCs) with 5.5% power conversion efficiency. The work provides a new molecular design approach to efficient non-fullerene OSCs based on 3D-structured small-molecule acceptors.

High-Performance Non-Fullerene Polymer Solar Cells Based on a Pair of Donor-Acceptor Materials with Complementary Absorption Properties.

A 7.3% efficiency non-fullerene polymer solar cell is realized by combining a large-bandgap polymer PffT2-FTAZ-2DT with a small-bandgap acceptor IEIC. The complementary absorption of donor polymer and small-molecule acceptor is responsible for the high-performance of the solar-cell device. This work provides important guidance to improve the performance of non-fullerene polymer solar cells.

Efficient non-fullerene polymer solar cells enabled by tetrahedron-shaped core based 3D-structure small-molecular electron acceptors

Here we report a series of tetraphenyl carbon-group (tetraphenylmethane (TPC), tetraphenylsilane (TPSi) and tetraphenylgermane (TPGe)) core based 3D-structure non-fullerene electron acceptors, enabling efficient polymer solar cells with a power conversion efficiency (PCE) of up to ∼4.3%. The results show that TPC and TPSi core-based polymer solar cells (PSCs) perform significantly better than that based on TPGe. Our study provides a new approach for designing small molecular acceptor materials for polymer solar cells.

Reduced Intramolecular Twisting Improves the Performance of 3D Molecular Acceptors in Non‐Fullerene Organic Solar Cells

A small-molecular acceptor, tetraphenylpyrazine-perylenediimide tetramer (TPPz-PDI4 ), which has a reduced extent of intramolecular twisting compared to two other small-molecular acceptors is designed. Benefiting from the lowest extent of intramolecular twisting, TPPz-PDI4 exhibits the highest aggregation tendency and electron mobility, and therefore achieves a highest power conversion efficiency of 7.1%.

Quantitative relations between interaction parameter, miscibility and function in organic solar cells

Although it is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymer donors and small-molecule acceptors, and thus largely control the achievable performance of organic solar cells, quantifying interaction–function relations has remained elusive. Here, we first determine the temperature-dependent effective amorphous–amorphous interaction parameter, χaa(T), by mapping out the phase diagram of a model amorphous polymer:fullerene material system. We then establish a quantitative ‘constant-kink-saturation’ relation between χaa and the fill factor in organic solar cells that is verified in detail in a model system and delineated across numerous high- and low-performing materials systems, including fullerene and non-fullerene acceptors. Our experimental and computational data reveal that a high fill factor is obtained only when χaa is large enough to lead to strong phase separation. Our work outlines a basis for using various miscibility tests and future simulation methods that will significantly reduce or eliminate trial-and-error approaches to material synthesis and device fabrication of functional semiconducting blends and organic blends in general.This work reports a quantitative investigation of the interaction parameter and miscibility of donor and acceptor organic molecules and their relationship with the fill factor and photovoltaic performance of bulk-heterojunction organic solar cells.

Design of Donor Polymers with Strong Temperature-Dependent Aggregation Property for Efficient Organic Photovoltaics

Bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted intensive research attention over the past two decades owing to their unique advantages including mechanical flexibility, light weight, large area, and low-cost fabrications. To date, OSC devices have achieved power conversion efficiencies (PCEs) exceeding 12%. Much of the progress was enabled by the development of high-performance donor polymers with favorable morphological, electronic, and optical properties. A key problem in morphology control of OSCs is the trade-off between achieving small domain size and high polymer crystallinity, which is especially important for the realization of efficient thick-film devices with high fill factors. For example, the thickness of OSC blends containing state-of-the-art PTB7 family donor polymers are restricted to ∼100 nm due to their relatively low hole mobility and impure polymer domains. To further improve the device performance and promote commercialization of OSCs, there is a strong demand for the design of new donor polymers that can achieve an optimal blend morphology containing highly crystalline yet reasonably small domains. In this Account, we highlight recent progress on a new family of conjugated polymers with strong temperature-dependent aggregation (TDA) property. These polymers are mostly disaggregated and can be easily dissolved in solution at high temperatures, yet they can strongly aggregate when the solution is cooled to room temperature. This unique aggregation property allows us to control the disorder-order transition of the polymer during solution processing. By preheating the solution to high temperature (∼100 °C), the polymer chains are mostly disaggregated before spin coating; as the temperature of the solution drops during the spin coating process, the polymer can strongly aggregate and form crystalline domains yet that are not excessivelylarge. The overall blend morphology can be optimized by various processing conditions (e.g., temperature, spin-rates, concentration, etc.). This well-controlled and near-optimal BHJ morphology produced over a dozen cases of efficient OSCs with an active layer nearly 300 nm thick that can still achieve high FFs (70-77%) and efficiencies (10-11.7%). By studying the structure-property relationships of the donor polymers, we show that the second position branched alkyl chains and the fluorination on the polymer backbone are two key structural features that enable the strong TDA property. Our comparative studies also show that the TDA polymer family can be used to match with non-fullerene acceptors yielding OSCs with low voltage losses. The key difference between the empirical matching rules for fullerene and non-fullerene OSCs is that TDA polymers with slightly reduced crystallinity appear to match better with small molecular acceptors and yield higher OSC performances.

Influence of fluorination on the properties and performance of isoindigo–quaterthiophene-based polymers

Here a series of isoindigo (ID) and quaterthiophene (T4)-based donor–acceptor copolymers are synthesized and compared. The polymer with fluorination on the donor unit exhibits the strongest extent of temperature-dependent aggregation, which leads to a higher hole mobility of the polymer and PSCs with efficiencies up to 7.0% without using any processing additives. Our results provide important insights into how fluorination affects the aggregation properties and performance of isoindigo-based polymers.

CsPbBr3 perovskite nanocrystals as highly selective and sensitive spectrochemical probes for gaseous HCl detection

A rapid and facile gaseous anion-exchange reaction between CsPbBr3 perovskite nanocrystals and HCl vapor was carried out under the ambient conditions. The resultant CsPb(Br/Cl)3 nanocrystals preserved the morphological features and crystal structure of the original CsPbBr3 nanocrystals and exhibited a significant blue-shift in the ultraviolet-visible light absorption and photoluminescence spectra. Inspired by the visual observations in the fluorimetry upon gaseous anion-exchange reaction, the CsPbBr3 nanocrystals were developed as highly sensitive and selective spectrochemical probes for the detection of toxic and corrosive HCl vapor with a concentration as low as 5 ppm.