Conductive Ionenes Promote Interfacial Self-Doping for Efficient Organic Solar Cells

Interfacial modification in organic solar cells

Organic solar cells (OSCs), consisting of several layers of organic semiconductors stacked between electrodes, have flourished in recent years. However, the energy barrier at the organic semiconductor/electrode interface remains a great challenge, limiting further advancements in device performances. In general, polar and even charged electronically active materials are recognized for their ability to modify the contact between the electrodes and organic semiconductors. Although numerous interlayer materials have been developed, there are still open questions about the mechanisms of interfacial modifications and molecular design strategies. This review focuses on the organic semiconductor/electrode interface in devices, starting with the working mechanism of the interlayers and followed by analyzing various interfacial electronic characteristics, such as the energy level arrangement, based on the basic principles of organic semiconductors. Then, we take the representative interlayer materials as examples and examine their specific working modes and functions in promoting device performance. The combination of mechanistic analysis and case studies provided in this review offer new insights into the development of more efficient organic solar cells for various applications.

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%.

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.

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.

Self-Aggregated Light-Trapping Nanodots for Highly Efficient Organic Solar Cells

The typical thickness of the photoactive layer in organic solar cells (OSCs) is around 100 nm, which limits the absorption efficiency of the incident light and the power conversion efficiency (PCE) of OSCs. Therefore, light-trapping schemes to reduce the optical losses in the thin photoactive layers are critically important for efficient OSCs. Herein, light-trapping and electron-collection dual-functional small organic molecules, N,N,N',N'-tetraphenyloxalamide (TPEA) and N,N,N',N'-tetraphenylmalonamide (TPMA), are designed and synthesized by a one-step acylation reaction. Driven by strong intermolecular force, TPEA and TPMA tend to self-aggregate into hemispherical light-trapping nanodots on the photoactive layer, resulting in enhanced light harvesting. Meanwhile, TPEA and TPMA demonstrate high electron mobility and excellent electron-collection ability.  Compared with the device without cathode buffer layer (CBL, PCE = 14.09%), PM6:BTP-eC9 based OSCs with TPEA and TPMA light-trapping CBLs demonstrate greatly enhanced PCE of 16.21% and 17.85%, respectively. Furthermore, a record PCE of 19.02% can be achieved for PM6:BTP-eC9:PC₇₁ BM based ternary OSC with TPMA light-trapping CBL. Moreover, TPMA exhibits a low synthesis cost of only 0.61 $ g^-1 with high yield. These findings could open a window for the rational design of multifunctional CBLs for efficient and stable OSCs.

Optimizing Interfacial Energetics for Conjugated Polyelectrolyte Electron Injection Layers in High Efficiency and Fast Responding Polymer Light Emitting Diodes

Modification of the π-conjugated backbone structure of conjugated polyelectrolytes (CPEs) for use as electron injection layers (EILs) in polymer light emitting diodes (PLEDs) has previously brought conflicted results in the literature in terms of device efficiency and turn-on response time. Herein, we determine the energetics at the CPE and the light emitting polymer (LEP) interface as a key factor for PLED device performance. By varying the conjugated backbone structure of both the LEP and CPE, we control the nature of the CPE/LEP interface in terms of optical energy gap offset, interfacial energy level offset, and location of the electron-hole recombination zone. We use a wide gap CPE with a shallow LUMO (F8im-Br) and one with a smaller gap and deeper LUMO (F8imBT-Br), in combination with three different LEPs. We find that the formation of a type II heterojunction at the CPE/LEP interfaces causes interfacial luminance quenching, which is responsible for poor efficiency in PLED devices. The effect is exacerbated with increased energy level offset from ionic rearrangement and hole accumulation occurring near the CPE/LEP interface. However, a deep CPE LUMO is found to be beneficial for fast current and luminance turn-on times of devices. This work provides important CPE molecular design rules for EIL use, offering progress toward a universal PLED-compatible CPE that can simultaneously deliver high efficiency and fast response times. In particular, engineering the LUMO position to be deep enough for fast device turn-on while avoiding the creation of a large energy level offset at the CPE/LEP interface is shown to be highly desirable.

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.

Electroactive Ionenes: Efficient Interlayer Materials in Organic Photovoltaics

ConspectusOrganic photovoltaics (OPVs) have the advantages of being lightweight, mechanically flexible, and solution-processable over large areas, and for decades, they have been the focus of the academic and industrial communities. Recent progress in the design of high-performance organic semiconductors and device optimization has promoted solar cell efficiencies of up to 19%, showing great promise for commercialization. Optimally designed OPVs are achieved using a bicontinuous interpenetrating network of donor and acceptor materials in between two charge-collecting electrodes. Charge extraction and transport between metal electrodes and organic semiconductors are crucial to device operation. The energy-level mismatch when metal electrodes and organic semiconductors are in contact usually induces additional energy barriers and resultant inefficient charge transport and collection, leading to charge carrier recombination at the interface and inferior device performance. To align energy levels at the interface, interlayer materials and their integration into devices have emerged as a widely used strategy to promote the performance of solar cell devices. Interlayer materials have the ability to modify the work functions (WFs) of metal electrodes, holding the potential to enhance the built-in electrostatic field (V_bi) of the devices and suppress the charge recombination loss, which is beneficial to improving the open circuit voltage (V_OC), short circuit current density (J_SC), and fill factor (FF) of the solar cells.Organic interlayer materials have recently come into focus for fundamental study and practical development because of their diverse molecular design and superior solution processability. Tremendous effort has been devoted to exploring novel organic interlayer materials to achieve all-solution-processed multilayer solar cells. Such interlayer materials usually have orthogonal solubilities relative to the photoactive layer materials, working as multifunctional interfacial layers to manipulate the mechanical and electrical contacts in solar cell devices. Ionenes are a unique class of polyelectrolytes wherein the ionic species reside within the polymer backbone rather than as pendant groups. In ionenes, the charge density is high in comparison to that of other polyelectrolytes, and the periodicity of the charges is easily controlled, providing a tunable density of dipole moments. Ionenes can be readily synthesized from 3° diamines and α,ω-dihaloalkanes to generate polymer chains of ammonium cations connected by flexible hydrocarbon linkages with mobile anions. However, the requisite building blocks of ionenes are not limited to such molecules. Recent advances in combining ionenes with conjugated molecules to generate electroactive ionenes have catalyzed a great amount of interest in such polymers for organic electronic devices.In this Account, we first introduce the molecular design and synthesis of electroactive ionenes. Following this, we will discuss the mechanism and effect of ionenes on the modification of metal electrodes. We then review the strategies for controlling the morphology of ionene interlayers. Finally, we compare the doping effect, conductivity, and charge transport of some representative ionenes and their performance as interlayers in solar cell devices. We present our current understanding based on recent progress and outstanding issues of interlayer materials in OPVs and to propose future directions and opportunities.