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

Non-Covalent Interaction Enhancement on Active/Interfacial Layers via Two-Dimensional Vermiculite Doping for Efficient Organic Solar Cells

Interface modification plays an important role in improving the power conversion efficiency (PCE) of organic solar cells (OSCs). However, the low non-covalent interaction between the cathode interface layer (CIL) and nonfullerene acceptor (NFA) directly affects the charge collection of OSCs. Here, the non-covalent interaction between the CIL and NFA is enhanced by introducing the 2D vermiculite (VML) in the poly(9,9-bis(3'-(N,N-dimethyl)-Nethylammonium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) dibromide (PFN-Br) interface layer to form an efficient electron transport channel. As a result, the electron extraction efficiency from the active layer to the CIL is increased, and the PCE of OSCs based on PBDB-T:ITIC is boosted from 10.87% to 12.89%. In addition, the strategy of CIL doping VML is proven to be universal in different CIL materials, for which the PCE is boosted from 10.21% to 11.57% for OSCs based on PDINN and from 9.82% to 11.27% for OSCs based on PNDIT-F3N. The results provide a viable option for designing efficient CIL for high-performance non-fullerene OSCs, which may promote the commercialization of OSCs.

N-Type Small Molecule Electron Transport Layers with Excellent Surface Energy and Moisture Resistance Siloxane for Non-Fullerene Organic Solar Cells

Electron transport layers (ETLs) generally contain polar groups for enhancing performance and reducing the work function. Nevertheless, the polar group with high surface energy may cause inferior interfacial compatibility, which challenges the ETLs to balance stability and performance. Here, two conjugated small molecules of ETLs with low surface energy siloxane, namely PDI-Si and PDIN-Si, are synthesized. The siloxane with low surface energy not only enhances the interfacial compatibility between ETLs and active layers but also improves the moisture-proof stability of the device. Impressively, the amine-functionalized PDIN-Si can simultaneously exhibit conspicuous n-type self-doping properties and outstanding moisture-proof stability. The optimization of interfacial contact and morphology enables the PM6:Y6-based OSC with PDIN-Si to achieve a power conversion efficiency (PCE) of 15.87%, which is slightly superior to that of classical ETL PDINO devices (15.27%), and when the PDIN-Si film thickness reaches 28 nm, the PCE remains at 13.19% (≈83%), which indicates that PDIN-Si has satisfactory thickness insensitivity to facilitate roll-to-roll processing. Excitingly, after 120 h of storage in an environment with humidity above 45%, the unencapsulated device with PDIN-Si as ETL remains at 75% of the initial PCE value, while the device with PDINO as ETL is only 50%.

CPE-Na-Based Hole Transport Layers for Improving the Stability in Nonfullerene Organic Solar Cells: A Comprehensive Study

Organic photovoltaic (OPV) cells have experienced significant development in the last decades after the introduction of nonfullerene acceptor molecules with top power conversion efficiencies reported over 19% and considerable versatility, for example, with application in transparent/semitransparent and flexible photovoltaics. Yet, the optimization of the operational stability continues to be a challenge. This study presents a comprehensive investigation of the use of a conjugated polyelectrolyte polymer (CPE-Na) as a hole layer (HTL) to improve the performance and longevity of OPV cells. Two different fabrication approaches were adopted: integrating CPE-Na with PEDOT:PSS to create a composite HTL and using CPE-Na as a stand-alone bilayer deposited beneath PEDOT:PSS on the ITO substrate. These configurations were compared against a reference device employing PEDOT:PSS alone, as the HTL increased efficiency and fill factor. The instruments with CPE-Na also demonstrated increased stability in the dark and under simulated operational conditions. Device-based PEDOT:PSS as an HTL reached T80 after 2500 h while involving CPE-Na in the device kept at T90 in the same period, evidenced by a reduced degradation rate. Furthermore, the impedance spectroscopy and photoinduced transient methods suggest optimized charge transfer and reduced charge carrier recombination. These findings collectively highlight the potential of CPE-Na as a HTL optimizer material for nonfluorine OPV cells.

Interface Engineering for Highly Efficient Organic Solar Cells

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 are reviewed. The specific functions and corresponding design principles of interface layers are summarized first. Then, the anode interface layer, cathode interface layer in single-junction OSCs, and interconnecting layer of tandem devices are discussed in separate categories, and the interface engineering-related improvements on device efficiency and stability are analyzed. Finally, the challenges and prospects associated with application of interface engineering are discussed with the emphasis on large-area, high-performance, and low-cost device manufacturing.

Impact of solvents on doctor blade coatings and bathocuproine cathode interlayer for large-area organic solar cell modules

Efforts to commercialize organic solar cells (OSCs) by developing roll-to-roll compatible modules have encountered challenges in optimizing printing processes to attain laboratory-level performance in fully printable OSC architectures. In this study, we present efficient OSC modules fabricated solely through printing methods. We systematically evaluated the impact of processing solvents on the morphology of crucial layers, such as the hole transport, photoactive, and electron transport layers, applied using the doctor blade coating method, with a particular focus on processability. Notably, deposition of charge transport layer using printing techniques is still a challenging task, mainly due to the hydrophobic characteristic of the organic photoactive layer. To overcome this issue, we investigated the solvent effect of a well-studied cathode interlayer, bathocuproine (BCP). We were able to form a uniform thin BCP film (∼10 nm) on a non-fullerene based organic photoactive layer using the doctor bladed coating method. Our results showed that the use of volatile alcohols in the BCP processing required a delicate balance between wettability and vaporization, which contrasted with the results for spin-coated films. These findings provide important insights into improving the efficiency of printing techniques for depositing charge transport layers. The fully printed OSC modules, featuring uniform and continuous BCP layer formation, achieved an impressive power conversion efficiency of 10.8% with a total area of 10.0 cm2 and a geometrical fill factor of 86.5%.

Bay-Functionalized Perylene Diimide Derivative Cathode Interfacial Layer for High-Performance Organic Solar Cells

The field of organic solar cells (OSCs) has acquired rapid progress with the development of nonfullerene acceptors. Interfacial engineering is also significant for the enhancement of the power conversion efficiency (PCE) in OSCs. Among the cathode interfacial materials (CIMs), perylene diimide (PDI) small molecules are promising owing to the excellent electron affinity and electron mobility. Although the well-known PDINN molecule has excellent properties, it has a high planarity formed by an extensive rigid π-conjugated backbone. Because the PDI molecular backbone has a strong tendency to aggregate, it causes the problem of excessive molecular aggregation and stacking, which directly leads to excessive crystallinity. Proper accumulation is beneficial for charge transport, but oversized crystals formed by overaggregation will hinder charge transport, ultimately affecting the film morphology and charge transport efficiency. Modifying the bay position of PDINN is an effective strategy to reduce the planarity, modulate the molecular aggregation, optimize the morphology, and enhance the charge-collecting efficiency. Therefore, PDINN-S was synthesized from PDINN by substituting the hydrogen with thiophene. The optimal PCE in the PM6:Y6 active layer was 16.18% and remained at 80% of the initial value after 720 h in a glovebox. This provides some guidance for exploring CIMs and preparing large-scale OSCs in the future.

Small-Molecule Electron Transport Layer with Siloxane-Functionalized Side Chains for Nonfullerene Organic Solar Cells

Active layer materials with silicone side chains have been broadly reported to have excellent long-term stability in harsh environments. However, the application of conjugated materials with silicone side chains in electron transport layers (ETLs) has rarely been reported. In this research, we synthesized for the first time a siloxane-modified perylene-diimide derivative (PDI-OSi) consisting of a side-chain substituent of siloxane and a conjugated group of perylene-diimide (PDI). The inserted siloxane functional groups not only can strengthen the light transmittance of PDI-OSi but also can remarkably expand its solubility and improve the film-forming ability and air stability of the material. Second, introducing siloxane-containing side chains can dramatically lower the work function and interfacial barrier of the electrode, thereby achieving a favorable ohmic contact. In addition, the moderate surface energy of siloxane functional groups makes PDI-OSi hydrophobic, which is conducive to forming excellent miscibility with hydrophobic active layers to promote charge transfer. When PDI-OSi is used as an ETL in organic solar cells (OSCs), operative exciton dissociation and more favorable surface morphology enable OSCs to realize a power conversion efficiency (PCE) of 13.99%. These results indicate that side-chain engineering with siloxane pendants is a facile strategy for constructing efficient OSCs.

Electron-donating amine-interlayer induced n-type doping of polymer:nonfullerene blends for efficient narrowband near-infrared photo-detection

Inherently narrowband near-infrared organic photodetectors are highly desired for many applications, including biological imaging and surveillance. However, they suffer from a low photon-to-charge conversion efficiencies and utilize spectral narrowing techniques which strongly rely on the used material or on a nano-photonic device architecture. Here, we demonstrate a general and facile approach towards wavelength-selective near-infrared phtotodetection through intentionally n-doping 500-600 nm-thick nonfullerene blends. We show that an electron-donating amine-interlayer can induce n-doping, resulting in a localized electric field near the anode and selective collection of photo-generated carriers in this region. As only weakly absorbed photons reach this region, the devices have a narrowband response at wavelengths close to the absorption onset of the blends with a high spectral rejection ratio. These spectrally selective photodetectors exhibit zero-bias external quantum efficiencies of ~20-30% at wavelengths of 900-1100 nm, with a full-width-at-half-maximum of ≤50 nm, as well as detectivities of >10¹² Jones.

Fluorinated Perylene-Diimides: Cathode Interlayers Facilitating Carrier Collection for High-Performance 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.

Hole/Electron Transporting Materials for Nonfullerene Organic Solar Cells

Nonfullerene acceptor based organic solar cells (NF-OSCs) have witnessed rapid progress over the past few years owing to the intensive research efforts on novel electron donor and nonfullerene acceptor (NFA) materials, interfacial engineering, and device processing techniques. Interfacial layers including electron transporting layers (ETL) and hole transporting layers (HTLs) are crucially important in the OSCs for facilitating electron and hole extraction from the photoactive blend to the respective electrodes. In this review, the lates progress in both ETLs and HTLs for the currently prevailing NF-OSCs are discussed, in which the ETLs are summarized from the categories of metal oxides, metal chelates, non-conjugated electrolytes and conjugated electrolytes, and the HTLs are summarized from the categories of inorganic and organic materials. In addition, some bifunctional interlayer materials served as both ETLs and HTLs are also introduced. Finally, the prospects of ETL/HTL materials for NF-OSCs are provided.

Polymerized naphthalimide derivatives as remarkable electron-transport layers for inverted organic solar cells

Two polymerized naphthalimide derivatives, named as N-TBHOB and N-DBH, have been prepared by quaternization, which exhibit excellent performance as electron-transport layers (ETLs) in inverted organic solar cells (i-OSCs). The results indicate N-TBHOB with a reticulated structure owns a superior performance on electron extraction, electron transport, thickness tolerance and less carrier recombination compared with N-DBH with linear structure. The i-OSCs based on N-TBHOB with PTB7-Th:PC₇₁ BM as the active layer achieve PCEs of 10.72% and 10.03% under the thickness of 11 and 48 nm respectively, which indicates N-TBHOB possesses better thickness tolerance than most of organic ETLs in i-OSCs. N-TBHOB also shows more competent performance than N-DBH and ZnO in non-fullerene i-OSCs for comprehensively improved J_sc , V_oc and FF values. Its i-OSC with PM6:Y6 blend presents a high PCE of 16.78%. The study provides an efficient strategy to prepare ETLs by combining conjugated and nonconjugated backbone with a reticulated structure for high-performance i-OSCs. This article is protected by copyright. All rights reserved.