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.

Bulk heterojunction (BHJ) polymer solar cells (PSCs) based on composites of conjugated polymers (electron donor) and fullerene derivatives (electron acceptor) have attracted attention due to their potential as renewable energy sources. [1 -7 ] The major challenges for BHJ solar cells are the achievement of competitive power conversion effi ciencies (PCEs) and the demonstration of long-term air stability. [8][9][10][11][12][13][14][15][16] BHJ solar cells are typically fabricated with a transparent conductive anode (e.g. indium tin oxide, ITO), a low-work-function metal cathode (e.g., Al, Ca), and an active layer (a mixture of conjugated polymer and fullerene derivative) sandwiched between the anode and cathode. The BHJ layer and cathode dramatically affect the stability. In particular, the cathode is susceptible to degradation by oxygen and water vapor. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is often used as an anode buffer layer. Long-term stability is a problem because PEDOT:PSS is hygroscopic and acidic. [17][18][19][20][21] In order to circumvent these problems, inverted polymer solar cells have been developed; air-stable high-work-function metals (e.g., Au, Ag) are used as the anode to collect holes and ITO is used as the cathode to collect electrons. In the inverted architecture, n-type metal oxides such as titanium oxide (TiO x ), zinc oxide (ZnO), and cesium carbonate (Cs 2 CO 3 ) are deposited onto the ITO electrode to break the symmetry. [ 22 − 24 ] The elimination of the PEDOT:PSS layer improves the device stability. Moreover, in the inverted cell, the anode is a highwork-function metal such as Ag, which can be formed using coating or printing technology to simplify and lower the cost of manufacturing. [ 25 ] Among the n-type metal oxides used in inverted cells, ZnO is a promising candidate due to its relatively high electron mobility, environmental stability, and high transparency. A variety of fabrication methods have been employed to grow thin fi lms of ZnO. Sol-gel method has been extensively investigated as a solution-based thin-fi lm deposition process. [ 26 ] Sol-gelderived ZnO fi lm is widely used in inverted solar cells. However, a high annealing temperature, usually over 200 ° C and incompatible with fl exible substrates, is used to promote crystallization and removal of residual organic compounds. [27][28][29] Although solution-processed ZnO nanoparticles have been shown to be easily processed into thin fi lms via spin coating or roll-to-roll printing at room temperature, [ 23 , 30 , 31 ] ZnO nanoparticles are not very stable in solution and a ligand is usually used to stabilize them. [ 32 ] We report here that uniform sol-gel-derived ZnO fi lms can be obtained at relatively low annealing temperatures ( ≤ 200 ° C) and they can function as the effi cient electron transporting layer in inverted solar cells.Despite a dramatic improvement of stability, inverted solar cells suffer from relatively lower PCEs compared to conventional solar cells, mainly due to the ...

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

Organic photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial laboratories because of their potential to enable mass production of flexible and cost-effective devices. Most of the attention has been focused on solution-processed polymer bulk-heterojunction (BHJ) solar cells. A combination of polymer design, morphology control, structural insight and device engineering has led to power conversion efficiencies (PCEs) reaching the 6-8% range for conjugated polymer/fullerene blends. Solution-processed small-molecule BHJ (SM BHJ) solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts. Here, we report efficient solution-processed SM BHJ solar cells based on a new molecular donor, DTS(PTTh(2))(2). A record PCE of 6.7% under AM 1.5 G irradiation (100 mW cm(-2)) is achieved for small-molecule BHJ devices from DTS(PTTh(2))(2):PC(70)BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25% v/v of 1,8-diiodooctane, DIO) during the film-forming process, which leads to reduced domain sizes in the BHJ layer. These results provide important progress for solution-processed organic photovoltaics and demonstrate that solar cells fabricated from small donor molecules can compete with their polymeric counterparts.