Organic solar cells (OSCs) have made dramatic advancements during the past decades owing to the innovative material design and device structure optimization, with power conversion efficiencies surpassing 19% and 20% for single-junction and tandem devices, respectively. Interface engineering, by modifying interface properties between different layers for OSCs, has become a vital part to promote the device efficiency. It is essential to elucidate the intrinsic working mechanism of interface layers, as well as the related physical and chemical processes that manipulate device performance and long-term stability. In this article, the advances in interface engineering aimed to pursue high-performance OSCs were reviewed. The specific functions and corresponding design principles of interface layers were summarized firstly. Then we discussed the anode interface layer (AIL), cathode interface layer (CIL) in single-junction OSCs, and interconnecting layer (ICL) of tandem devices in separate categories and analyzed the interface engineering-related improvements on device efficiency and stability. Finally, the challenges and prospects associated with application of interface engineering were discussed with the emphasis on large-area, high-performance and low-cost device manufacturing. This article is protected by copyright. All rights reserved.

Interface Engineering for Highly Efficient Organic Solar Cells.

Organic solar cells (OSCs) have experienced rapid progress with the innovation of near‐infrared (NIR)‐absorbing small‐molecular acceptors (SMAs), while the unique electronic properties of the SMAs raise new challenges in relation to cathode engineering for effective electron collection. To address this issue, two fluorinated perylene‐diimides (PDIs), PDINN‐F and PDINN‐2F, are synthesized by a simple fluorination method, for application as cathode interlayer (CIL) materials. The two bay‐fluorinated PDI‐based CILs possess a lower lowest unoccupied molecular orbital (LUMO) energy level of ≈−4.0 eV, which improves the energy level alignment at the NIR‐SMAs (such as BTP‐eC9)/CIL for a favorable electron extraction efficiency. The monofluorinated PDINN‐F shows higher electron mobility and better improved interfacial compatibility. The PDINN‐F‐based OSCs with PM6:BTP‐eC9 as active layer exhibit an enhanced fill factor and larger short‐circuit current density, leading to a high power conversion efficiency (PCE) exceeding 18%. The devices with PDINN‐F CIL retain more than 80% of their initial PCE after operating at the maximum power point under continuous illumination for 750 h. This work prescribes a facile, cost‐effective, and scalable method for the preparation of stable, high‐performance fluorinated CILs, and instilling promise for the NIR‐SMAs‐based OSCs moving forward.

Fluorinated Perylene‐Diimides: Cathode Interlayers Facilitating Carrier Collection for High‐Performance Organic Solar Cells

Ternary strategy has attracted extensive attention for bulk heterojunction organic solar cells (BHJ OSCs) owing to their potentially improved light harvesting, cascaded energy levels, and optimized film morphology of binary BHJs. Herein, three novel star‐shaped unfused ring electron acceptors (SSUFREAs), i.e., H1–3, with and without fluorine‐substituent in phenyl core or peripheral group are designed and synthesized as third components via direct C–H arylation to incorporate into PM6:Y6 BHJ films. The structure–property–performance dependence study reveals that the isotropic charge transfer, complementary star‐shape structure and light absorption, and energy‐level cascades of H1–3 with PM6 and Y6 allow ternary BHJs to have higher power conversion efficiency (PCE) compared to the PM6:Y6 binary BHJ. Among them, the ternary BHJ involving fluorine‐free H1, i.e., H1:PM6:Y6, possesses the highest PCE (16.57%) owing to the high‐lying frontier molecular orbital and the enlarged torsion angle, which enhances open‐circuit voltage, inhibits the excessive crystallization of Y6, and facilitates exciton dissociation as well as collection. The findings indicate that SSUFREAs have great potential to serve as third components to optimize morphology and improve the open‐circuit voltage of BHJ OSCs.

Structurally Complementary Star‐Shaped Unfused Ring Electron Acceptors with Simultaneously Enhanced Device Parameters for Ternary Organic Solar Cells

Although ternary organic solar cells (OSCs) have unique advantages in improving device performance, the morphology assembly in ternary-phase would be more uncertain or complex than that in binary-phase. Here, we propose a new concept of oligomer-assisted photoactive layers for high-performance OSCs. The formed alloy-like phase of the oligomer:host polymer blend enabled the oligomer-assisted OSCs fusing the advantages of both binary and ternary devices, exhibiting substantially enhanced performance and stability compared to the control devices. With the addition of oligomers, outstanding efficiencies of 17.33% for PM6:Y6 device, 18.32% for PM6:BTP-eC9 device, and 17.13% for PM6/Y6 pseudo-bilayer device were achieved, all of which are one of the highest values in their corresponding fields. The improved performance originated from the downshift energy levels, enhanced light absorption, optimized blend morphology, favorable charge dynamics, and reduced non-radiative energy loss.

Oligomer-assisted Photoactive Layers Enable >18% Efficiency of Organic Solar Cells.

Active layer material plays a critical role in promoting the performance of an organic solar cell (OSC). Small-molecule (SM) materials have the merits of well-defined chemical structures, few batch-to-batch variations, facile synthesis and purification procedures, and easily tuned properties. SM-donor and non-fullerene acceptor (NFA) innovations have recently produced all-small-molecule (ASM) devices with power conversion efficiencies that exceed 17% and approach those of their polymer-based counterparts, thereby demonstrating their great future commercialization potential. In this review, recent progress in both SM donors and NFAs to illustrate structure-property relationships and various morphology-regulation strategies are summarized. Finally, ASM-OSC challenges and outlook are discussed.

Recent Progress in All-Small-Molecule Organic Solar Cells.

The ternary blending strategy of introducing a third component into binary organic solar cells (BOSCs) is a feasible and efficient strategy to optimize the performance of BOSCs. Ternary organic solar cells (TOSCs) have an adjustable absorption band for matching the solar light characteristics, easily adjustable photoelectric performance and tunable micro-morphology; therefore, they have become a hotspot in the field of OSC research. Currently, the power conversion efficiency (PCE) of TOSCs has been boosted over 17%. Herein, we give a review on the four possible working mechanisms of TOSCs, roles of the third component in TOSCs, recent progress made in different kinds of active layer materials of TOSCs based on solution-processed non-fullerene acceptors (NFAs), and recent advances in large-area TOSCs and their long-term stability. This review can provide some guidance for the further development of TOSCs. Finally, to help researchers understand the future of TOSCs, the challenges and prospects of TOSCs based on solution-processible NFAs have been provided.

Recent progress in ternary organic solar cells based on solution-processed non-fullerene acceptors

The electron transport layer (ETL) exerts a dramatic influence on the power conversion efficiency (PCE) of the nonfullerene organic solar cells (NOSCs). Currently, the majority of the organic ETLs possess a relatively poor conductivity, which is not conducive to carrier transport and collection. Herein, we design and develop a novel hyperbranched conjugated polyelectrolyte (CPE) based on n-type perylene diimide (PDI) as the center core and quaternary ammonium salt as the side polar groups. The lone pair electrons of the nitrogen atoms can transfer to the electron deficient PDI core and endow the molecule with an efficient n-type self-doping effect. Moreover, the hyperbranched structure makes the molecule functionalized with more side polar groups, favoring forming more dipoles and stronger dipole moments. Therefore, the CPE PTPAPDINO possesses a high conductivity and can notably decrease the work function (WF) of the electrode, contributing to the carrier transport and collection of the device. The NOSC with PTPAPDINO as ETL delivers an excellent PCE of 15.62%, which is even superior to the device using the classical PDINO ETL. Moreover, the PCE can retain 82.6% of the optimal device when the thickness has been increased to 28 nm. These results manifest that it is a feasible strategy to design an n-type self-doping hyperbranched CPE as efficient ETL, and PTPAPDINO is a promising alternative ETL for high performance NOSCs.

N-Type Self-Doped Hyperbranched Conjugated Polyelectrolyte as Electron Transport Layer for Efficient Nonfullerene Organic Solar Cells.

Surface doping of non-fullerene photoactive layer by soluble polyoxometalate for printable organic solar cells.

Adjusting the Active Layer Morphology via an Amorphous Acceptor Solid Additive for Efficient and Stable Nonfullerene Organic Solar Cells

The invention discloses a preparation method of a hyperbranched conjugated polymer electrolyte electron transport layer. For the first time, 2,7-dibromo-9,9'-di(1-sodium sulfonate butyl)fluorene and tris-[4-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-phenyl]-amine are subjected to a simple Suzuki coupling reaction to prepare a conjugated polymer electrolyte of fluorene containing a hyperbranched sodium sulfonate polar side chain and triphenylamine. Due to containing a plurality of polar sodium sulfonate side chains, the polymer can form dipoles at an interface, the interface potential barrier is reduced, the interface contact is improved, and the polymer can realize environment-friendly water/alcohol soluble processing. In addition, due to the hyperbranched property of the hyperbranchedpolymer (containing a plurality of polar side chains), a strong interfacial interaction between the hyperbranched polymer and an underlying substrate can occur to self-assemble into ordered arrays, then an upper active layer is induced to form ordered arrays by using the hyperbranched polymer as a template, the problem of poor morphology of the active layer is solved, and the carrier mobility canbe improved, so that the short-circuit current and the filling factor of a device is improved, and finally the device efficiency is improved.

Preparation method of hyperbranched conjugated polymer electrolyte electron transport layer

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