Correlating Electronic Structure and Device Physics with Mixing Region Morphology in High-Efficiency Organic Solar Cells.

The donor/acceptor interaction in non-fullerene organic photovoltaics leads to the mixing domain that dictates the morphology and electronic structure of the blended thin film. Initiative effort is paid to understand how these domain properties affect the device performances on high-efficiency PM6:Y6 blends. Different fullerenes acceptors are used to manipulate the feature of mixing domain. It is seen that a tight packing in the mixing region is critical, which could effectively enhance the hole transfer and lead to the enlarged and narrow electron density of state (DOS). As a result, short-circuit current (JSC ) and fill factor (FF) are improved. The distribution of DOS and energy levels strongly influences open-circuit voltage (VOC ). The raised filling state of electron Fermi level is seen to be key in determining device VOC . Energy disorder is found to be a key factor to energy loss, which is highly correlated with the intermolecular distance in the mixing region. A 17.53% efficiency is obtained for optimized ternary devices, which is the highest value for similar systems. The current results indicate that a delicate optimization of the mixing domain property is an effective route to improve the VOC , JSC , and FF simultaneously, which provides new guidelines for morphology control toward high-performance organic solar cells.

Capture the high-efficiency non-fullerene ternary organic solar cells formula by machine-learning-assisted energy-level alignment optimization.

Appropriate energy-level alignment in non-fullerene ternary organic solar cells (OSCs) can enhance the power conversion efficiencies (PCEs), due to the simultaneous improvement in charge generation/transportation and reduction in voltage loss. Seven machine-learning (ML) algorithms were used to build the regression and classification models based on energy-level parameters to predict PCE and capture high-performance material combinations, and random forest showed the best predictive capability. Furthermore, two sets of verification experiments were designed to compare the experimental and predicted results. The outcome elucidated that a deep lowest unoccupied molecular orbital (LUMO) of the non-fullerene acceptors can slightly reduce the open-circuit voltage (V OC) but significantly improve short-circuit current density (J SC), and, to a certain extent, the V OC could be optimized by the slightly up-shifted LUMO of the third component in non-fullerene ternary OSCs. Consequently, random forest can provide an effective global optimization scheme and capture multi-component combinations for high-efficiency ternary OSCs.

Preaggregation Matching of Donors and Acceptors in Solution Accounting for Thermally Stable Non-Fullerene Solar Cells.

The mechanism of how the solvent type influences photovoltaic performance and thermal stability of non-fullerene organic solar cells remains unexplored. In this article, the well-known PTB7-Th was selected as a donor, while F8IC was used as an acceptor. The PTB7-Th:F8IC processed from chloroform (CF) exhibited a superiorly higher power conversion efficiency (PCE) of 10.5%, in contrast to the specimen processed from chlorobenzene (CB) of 6.8%. In addition, upon thermal annealing at 160 °C for 120 min, the device processed from CF was more stable than that processed from CB. The incorporation of perylene diimide derivative TBDPDI-C11, serving as the third additive, could also obviously improve the PCE value and thermal stability of PTB7-Th:F8IC processed from CB. According to ultraviolet spectroscopy, atomic force microscopy, transmission electron microscopy, and grazing incidence wide-angle X-ray scattering analyses, the enhanced photovoltaic performance and thermal stability are mainly attributed to formation of PTB7-Th nanofibers and appropriate aggregation of F8IC. The interaction free energy calculated using water and diiodomethane contact angles reveals that PTB7-Th well disperses in CB and tends to aggregate in CF, while F8IC aggregates strongly in CB. The preaggregation matching of the donor and acceptor in solution is essential for the optimization of morphology, efficiency, and thermal stability. The findings in this article could provide useful guidelines to fabricate efficient and thermally stable organic solar cells simply by analyzing the surface energy of components processed from different solvents.

Plasmonic Metal Nanoparticles with Core-Bishell Structure for High-Performance Organic and Perovskite Solar Cells.

To maximize light coupling into the active layer, plasmonic nanostructures have been incorporated into both active layers of organic solar cells (OSCs) and perovskite solar cells (PSCs) with the aim of increasing light absorption, but reports have shown controversial results in electrical characteristics. In this work, we introduce a core-bishell concept to build plasmonic nanoparticles (NPs) with metal-inorganic semiconductor-organic semiconductor nanostructure. Specifically, Ag NPs were decorated with a titania/benzoic-acid-fullerene bishell (Ag@TiO2@Pa), which enables the NPs to be compatible with fullerene acceptors or a perovskite absorber. Moreover, coating the Ag@TiO2 NP with a fullerene shell can activate efficient plasmon-exciton coupling and eliminate the charge accumulation, thus facilitating exciton dissociation and reducing the monomolecular recombination. The improved light absorption and enhanced carrier extraction of devices with Ag@TiO2@Pa nanoparticles are responsible for the improved short-circuit current and fill factor, respectively. On the basis of the synergistic effects (optical and electrical), a series of plasmonic OSCs exhibited enhancement of 12.3-20.7% with a maximum power conversion efficiency of 13.0%, while the performance of plasmonic PSCs also showed an enhancement by 10.2% from 18.4% to 20.2%. This core-bishell design concept of plasmonic nanostructures demonstrates a general approach to improving the photovoltaic performance with both optical and electrical contributions.

Room-Temperature and Solution-Processable Cu-Doped Nickel Oxide Nanoparticles for Efficient Hole-Transport Layers of Flexible Large-Area Perovskite Solar Cells.

Flexible perovskite solar cells (PSCs) using plastic substrates have become one of the most attractive points in the field of thin-film solar cells. Low-temperature and solution-processable nanoparticles (NPs) enable the fabrication of semiconductor thin films in a simple and low-cost approach to function as charge-selective layers in flexible PSCs. Here, we synthesized phase-pure p-type Cu-doped NiOx NPs with good electrical properties, which can be processed to smooth, pinhole-free, and efficient hole transport layers (HTLs) with large-area uniformity over a wide range of film thickness using a room-temperature solution-processing technique. Such a high-quality inorganic HTL allows for the fabrication of flexible PSCs with an active area >1 cm2, which have a power conversion efficiency over 15.01% without hysteresis. Moreover, the Cu/NiOx NP-based flexible devices also demonstrate excellent air stability and mechanical stability compared to their counterpart fabricated on the pristine NiOx films. This work will contribute to the evolution of upscaling flexible PSCs with a simple fabrication process and high device performances.