The development of organic photoactive materials, especially the newly emerging non-fullerene electron acceptors (NFAs), has enabled rapid progress in organic photovoltaic (OPV) cells in recent years. Although the power conversion efficiencies (PCEs) of the top-performance OPV cells have surpassed 16%, the devices are usually fabricated via a spin-coating method and are not suitable for large-area production. Here, we demonstrate that the fine-modification of the flexible side chains of NFAs can yield 17% PCE for OPV cells. More crucially, as the optimal NFA has a suitable solubility and thus a desirable morphology, the high efficiencies of spin-coated devices can be maintained when using scalable blade-coating processing technology. Our results suggest that optimization of the chemical structures of the OPV materials can improve device performance. This has great significance in larger-area production technologies that provide important scientific insights for the commercialization of OPV cells.

achieved power conversion efficiencies (PCEs) greater than 15% in singlejunction binary devices. [13,14] However, the performance of the binary system is still largely limited by the material's own properties (narrow absorption, small crystallinity, low charge mobility, strong recombination, etc.). In order to overcome these limitations, the strategy of adding a third component to the binary system, e.g., ternary solar cell approach, has come into being and shown wideranging applicability in improving the solar cell device function. The addition of a structurally similar third component can either extend the absorption range of solar emission spectrum, tune the frontier molecular orbital (FMOs) levels such as through forming homogeneous donor or acceptor phases, [15,16] modulate the active layer's electric property by improving the film morphology, [17][18][19] or tune the acceptor phase optical property, [20] which can promote either the device short-circuit current density (J sc ), or open-circuit voltage (V oc ), or fill factor (FF), and finally, boost power conversion efficiencies with the values reaching over 13% recently. [21][22][23][24][25][26] The ternary approach, by introducing a smaller bandgap nonfullerene acceptor as a near infrared (NIR) absorber to increase the device J sc of fullerene-free binary blended material systems, hereafter named as the "current-increased" Ternary approaches to solar cell design utilizing a small bandgap nonfullerene acceptor as the near infrared absorber to increase the short-circuit current density always decreases the open-circuit voltage. Herein, a highly efficient polymer solar cell with an impressive efficiency of 16.28 ± 0.20% enabled by an effective voltage-increased ternary blended fullerene-free material approach is reported. In this approach, the structural similarity between the host and the higher-LUMO-level guest enables the two acceptors to be synergized, obtaining increased open-circuit voltage and fill factor and a small increase of short-circuit current density. The same beneficial effects are demonstrated by using two host binary systems. The homogeneous fine film morphologies and the π-π stacking patterns of the host blend are well maintained, while larger donor and acceptor phases and increased lamellar crystallinity, increased charge mobilities, and reduced monomolecular recombination can be achieved upon addition of the guest nonfullerene acceptor. The increased charge mobilities and reduced monomolecular recombination not only contribute to the improved fill factor but also enable the best devices to be fabricated with a relatively thicker ternary blended active layer (110 vs 100 nm). This, combined with the absorption from the added guest acceptor, contribute to the increased short-circuit current.

Ternary solar cells with an efficiency of 16.7% were enabled through the use of PCBM as a higher LUMO-level acceptor additive to concurrently increase Voc, Jsc and FF values.

Synergistic optimization of donor-acceptor blend morphologyis a hurdle in the path of realizing efficient non-fullerene small-molecule organic solar cells (NFSM-OSCs) due to the anisotropic conjugated backbones of both donor and acceptor. Therefore, developing a facile molecular design strategy to effectively regulate the crystalline properties of photoactive materials, and thus, enable the optimization of blend morphology is of vital importance. In this study, a new donor molecule B1, comprising phenyl-substituted benzodithiophene (BDT) central unit, exhibits strong interaction with the non-fullerene acceptor BO-4Cl in comparison with its corresponding thiophene-substituted BDT-based material, BTR. As a result, the B1 is affected and induced from an edgeon to a face-on orientation by the acceptor, while the BTR and the acceptor behave individually for the similar molecular orientation in pristine and blend films according to grazing incidence wide angle X-ray scattering results. It means the donor-acceptor blend morphology is synergistically optimized in the B1 system, and the B1:BO-4Cl-based devices achieve an outstanding power conversion efficiency (PCE) of 15.3%, further certified to be 15.1% by the National Institute of Metrology, China. Our results demonstrate a simple and effective strategy to improve the crystalline properties of the donor molecule as well as synergistically optimize the morphology of the all-small-molecule system, leading to the high-performance NFSM-OSCs.

Due to the attraction of optimizing the electronic structure beyond chemical synthesis, molecular doping has recently aroused wide interest in the field of organic solar cells. However, the selection of limited dopants confines its successful application. Inspired by the Lewis base characteristics of the photovoltaic materials, the Lewis acid as novel dopant is introduced in organic solar cells. In both fullerene and nonfullerene based blends, Lewis acid doping leads to increased photovoltaic performance. Detailed experiments reveal that Lewis acid doping has a synergistic effect on modifying the polymer's electronic properties and the acceptor's nanostructure even at low doping concentration, and these are simultaneously responsible for the device improvements. Based on the mechanism studies, it is proposed that the Lewis acid‐doped polymers anions produce induced dipole on the acceptor, this increases the intermolecular interaction and facilitates the morphology optimization. It is believed that the synergistic effect by Lewis acid doping greatly expands the application of doped organic solar cells, in concert with other existing methods to yield higher efficiency values.

Optimizing the photovoltaic processes directly by electric technique attracts the exploration of molecular doping in organic photovoltaics (OPVs). However, the inappropriate and inhomogeneous dopant distribution in the bulk heterojunction (BHJ) film inhibits the performance improvement and mechanism understanding in doped OPVs. A strategy to solve these critical problems is reported here. By employing a planar heterojunction (PHJ) device structure, the role of dopant location and photovoltaic performance is clarified. Dopants tris(pentafluorophenyl)borane (BCF) distributing at the heterojunction rather than in the single component (poly [(2,6-(4,8-bis(5-(2-ethylhexyl)) produces the best performance with quantum efficiency (QE) enhancement. Mechanism studies reveal the facilitated exciton splitting and suppressed bimolecular recombination as the causes for device improvement. The sequential coating procedure is further developed for the doped BHJ cells, and the devices based on this method exhibit a short-circuit current (J sc ) increase of 1.3 mA/cm 2 , which is an impressive value compared with the previous reports.

The uncertain dopant location in the bulk heterojunction (BHJ) film hinders the wide application of molecular doping in polymer solar cells (PSCs) as is in other organic devices. It is known that the interaction between the dopant and component governs the dopant distribution in the BHJ film and thus largely controls the effectiveness of molecular doping. After excluding the strong dopant/component interaction by forming the charge-transfer complex in the solution, we estimate the dopant/component miscibility by calculating the difference of Hansen’s total solubility parameters (△δi‑Hansen) and prove its correctness by contact angle measurements, and two model systems of poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl)­thiophe-2-yl)-benzo­[1,2-b:4,5-b′]­dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′-c:4′,5′-c′]­dithiophene-4,8-dione))] (PBDB-T)/poly­{[N,N′-bis­(2-octyldodecyl)-naphthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (N2200) and poly­[4,8-bis­(5-(2-ethylhexyl)-thiophene-2-yl)­benzo­[1,2-b;4,5-b′]­dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno­[3,4-b]­thiophene-)-2-carboxylate-2-6-diyl]] (PCE10)/N2200 are selected to reveal the miscibility–photovoltaic performance relations. Only the material combination with large △δi‑Hansen between the n-dopant (4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)­phenyl)­dimethylamine (N-DMBI) and the donor polymer achieves enhanced photovoltaic performance. After that, we examine the doped morphology of polymer blends. Since the polymers’ crystallizations are negatively affected by N-DMBI addition, we ensure the significance of n-doping on the enhanced device performance. Besides the dopant/polymer interaction, the solvent/polymer and solvent/dopant interactions are also considered to evaluate the kinetic effect on N-DMBI distribution by drawing the ternary phase diagram. We conclude that the kinetic morphological evolution does not change the miscibility-governed N-DMBI distribution in the BHJ film. Finally, we provide a direct relationship between the N-DMBI position and the device property by fabricating the bilayer devices. The enhancement of photovoltaic performances is observed in both material systems only if the N-DMBI distributes in N2200. Our work outlines a basis for using the dopant/component interaction and ternary phase diagram to predict the dopant distribution before extensive experiments. It significantly reduces the trial-and-error work and increases the reliability of molecularly doped PSCs.

The electron donor/acceptor (D/A) heterojunction is the core for photocharge generation and recombination in organic photovoltaics (OPVs). Developing practical methods for the D/A heterojunction modification remains challenging and is rarely discussed in OPV research. Herein, the roles of molecular doping at the D/A heterojunction in the charge-transfer exciton dissociation and detailed energy loss are investigated, and new insights are gained into the functions of doping on the OPV performance. Heterojunction doping simultaneously enhances all three OPV parameters, especially the short-circuit current (J sc ). It is shown that the J sc improvement is due to the combined effects of strengthened electric field and reduced activation energy, which is regulated via an entropy-related mechanism. The performance enhancement is further demonstrated in homojunction devices showing the great potential of interfacial doping to overcome the intrinsic limitation between high J sc and open-circuit voltage (V oc ) in OPVs.