2 Molecular design of non-fullerene acceptors (NFAs) is of vital importance for highefficiency organic solar cells. The branched alkyl chain modification is often regarded as a counter-intuitive approach as which may introduce undesirable steric hindrance that reduces charge transport in NFAs. Here we show the design and synthesis of a highly efficient NFA family by substituting the beta position of thiophene unit on Y6-based A-DAD-A backbone with branched alkyl chains. It was found that such modification of different alkyl chain length could completely change the molecular packing behavior of NFAs, leading to improved structure order and charge transport in thin films. Unprecedented efficiency of 18.32% (certified value of 17.9%) with a fill factor of 81.5% is achieved for single-junction organic solar cells. This work reveals the importance of branched alkyl chain topology in tuning the molecular packing and blend morphology that leads to improved organic photovoltaic performance.

In organic photovoltaics, morphological control of donor and acceptor domains on the nanoscale is key for efficient exciton diffusion and dissociation, carrier transport, and suppression of recombination losses. To realise this, here, we demonstrated a double-fibril network based on ternary donor:acceptor morphology with multi-length scales constructed by combining ancillary conjugated polymer crystallizers and non-fullerene acceptor filament assembly. Using this approach, we achieved an average power conversion efficiency of 19.3% (certified 19.2%). The success lies in the good match between the photoelectric parameters and the morphological characteristic lengths, which utilizes the excitons and free charges efficiently. This strategy leads to enhanced exciton diffusion length (hence exciton dissociation yield) and reduced recombination rate, hence minimizing photon-to-electron losses in the ternary devices as compared to their binary counterparts. The double-fibril network morphology strategy minimizes losses and maximizes the power output, offering the possibility towards 20% power conversion efficiencies in single-junction organic photovoltaics. MainOrganic semiconductors offer the advantage of high optical absorption and tunable energy levels, enabling thin-film solar cells with high light-to-electron conversion efficiencies over a wide range of wavelengths [1][2][3][4] . Desipte recent progresses, the performance of organic solar cells (OSCs) is still limited by non-ideal exciton and charge transport, which depend not only on the electronic structure of organic semiconductors but also on the nanostructure that is formed by material crystallization and phase separation in a bulk heterojunction (BHJ) setting [5][6][7][8] . A suitable sized phase-separated morphology that balances crystalline region and mixing domain on the nanoscale is therefore needed to further push the power conversion efficiency (PCE) of OSCs, however it is a

Single layered organic solar cells (OSCs) using non-fullerene acceptors have This article is protected by copyright. All rights reserved. 35A power conversion efficiency of 16.88% (certified as 16.4%) is achieved based on PM6:Y6 by morphology optimization, which is the top efficient for organic solar cells. Through the study of single structure and film morphology, a well ordered 2D crystal was found, which helps to enhance ultrafast hole and electron transfer, thus improving performance.

resulting in inefficient charge separation and collection. [7][8][9] However, recent progresses show quite steep climbing of power conversion efficiency (PCE) for all-PSCs, reaching a value over 10%, [10][11][12] under systematic optimization, leaving a large gap to be filled in understanding the mechanism of morphology evolution. Polymer blends that reach nanoscaled phase separation in organic photovoltaic (OPV) application have distinctive advantages. They are more suitable for printing fabrication because of their good filmforming ability and mechanical flexibility, prominent device stability, and readily tunable ink viscosity. [13][14][15][16][17] Such benefits make all-PSC unique in scalable OPV fabrication, which also leads to boosted interest in understanding and manipulating the morphology. Initiative efforts on all-PSC printing have been surveyed, showing PCEs way below device made by spin-coating. [16,18] The mystery lies in morphology control, of which BHJ thin film is typical nonequilibrium nature that film-drying kinetics dictates the final nanostructure composed of crystalline networks and mixed and phase-separated domains. [19][20][21] It has been shown that solar cells using polymer:non-fullerene acceptor blends processed by slot die printing have exceed 11% in PCE, [22][23][24][25][26][27][28] from which we think that fine-tuning of morphology in all-PSCs could reach a similar level once a All-polymer solar cells (all-PSCs) exhibit excellent stability and readily tunable ink viscosity, and are therefore especially suitable for printing preparation of large-scale devices. At present, the efficiency of state-of-the-art all-PSCs fabricated by the spin-coating method has exceeded 11%, laying the foundation for the preparation and practical utilization of printed devices.A high power conversion efficiency (PCE) of 11.76% is achieved based on PTzBI-Si:N2200 all-PSCs processing with 2-methyltetrahydrofuran (MTHF, an environmentally friendly solvent) and preparation of active layers by slot die printing, which is the top efficient for all-PSCs. Conversely, the PCE of devices processed by high-boiling point chlorobenzene is less than 2%. Through the study of film formation kinetics, volatile solvents can freeze the morphology in a short time, and a more rigid conformation with strong intermolecular interaction combined with the solubility limit of PTzBI-Si and N2200 in MTHF results in the formation of a fibril network in the bulk heterojunction. The multilength scaled morphology ensures fast transfer of carriers and facilitates exciton separation, which boosts carrier mobility and current density, thus improving the device performance. These results are of great significance for large-scale printing fabrication of high-efficiency all-PSCs in the future. Polymer Solar Cells

Morphology control in laboratory and industry setting remains as a major challenge for organic solar cells (OSCs) due to the difference in film-drying kinetics between spin coating and the printing process. A two-step sequential deposition method is developed to control the active layer morphology. A conjugated polymer that self-assembles into a well-defined fibril structure is used as the first layer, and then a non-fullerene acceptor is introduced into the fibril mesh as the second layer to form an optimal morphology. A benefit of the combined fibril network morphology and non-fullerene acceptor properties was that a high efficiency of 16.5% (certified as 16.1%) was achieved. The preformed fibril network layer and the sequentially deposited non-fullerene acceptor form a robust morphology that is insensitive to the polymer batches, solving a notorious issue in OSCs. Such progress demonstrates that the utilization of polymer fibril networks in a sequential deposition process is a promising approach towards the fabrication of high-efficiency OSCs.