Mitigating the Undesirable Chemical Reaction between Organic Molecules for Highly Efficient Flexible Organic Photovoltaics

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.

High-Performance Inverted Organic Solar Cells via the Incorporation of Thickness-Insensitive and Low-Temperature-Annealed Nonconjugated Polymers as Electron Transport Materials

Developing new electron transport layers has been an effective way to fabricate high-performance bulk-heterojunction organic solar cells (OSCs). Resolving the longstanding problems associated with commonly used zinc oxide (ZnO), such as electron traps and light-induced device deterioration, however, is still a great challenge. In this study, glycerol diglycidyl ether (GDE) and 1,4-butanesultone (BS) are blended with polyethyleneimine (PEI) to produce cross-linkable PEI-based materials, PEI-GDE and PEI-GDE-BS, which can function as alternative electron transport layers to replace conventional ZnO cathode-modifying layers in inverted OSCs. PEI-GDE and PEI-GDE-BS are amendable to low-temperature annealing processes to produce cross-linked films. The inverted device structure of ITO/ETL/PM6:BTP-BO-4F:PC₇₁BM/MoO₃/Ag was used to evaluate the effects of incorporating PEI-GDE and PEI-GDE-BS as electron transport materials. Compared with ZnO-based devices, the PEI-GDE- and PEI-GDE-BS-based devices exhibit significant improvements in photovoltaic performances due to smoother surface roughness, higher charge collection and exciton dissociation efficiencies, higher electron mobilities, and stronger π-π interactions. In particular, a PEI-GDE-BS-based device shows an outstanding power conversion efficiency (PCE) of 17.55% with a V_OC of 0.83 V, a J_SC of 27.88 mA/cm², and an FF of 75.96%, which offers great possibilities in the applications of flexible solar cells.

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.

On the interface reactions and stability of nonfullerene organic solar cells

Long-term stability is critical for organic solar cells (OSCs) for practical applications. Several factors affect the stability of OSCs, including materials stability, morphology stability of bulk-heterojunctions and interface stability. In this perspective, we focus on interface stability due to interfacial reactions between the emerging acceptor-donor-acceptor (A-D-A) type nonfullerene active layers and interfacial layers. The description covers the initial phenomena of interfacial instability, mechanism of interfacial reactions, and strategies adopted to suppress interfacial reactions between the nonfullerene active layers and interfacial layers. Methods to test and analyze the chemical instability of nonfullerene acceptors are also included. The C[double bond, length as m-dash]C vinyl linker between the donor moiety and acceptor moiety is chemically or photochemically reactive and is a weak point for interface stability. The interface stability of OSCs could be enhanced by reducing the reactivity of the C[double bond, length as m-dash]C vinyl linker or removing it directly, modifying the surface of interfacial layers, and developing other novel interfacial materials.

Hybrid Cathode Interlayer Enables 17.4% Efficiency Binary Organic Solar Cells

With the emergence of fused ring electron acceptors, the power conversion efficiency of organic solar cells reached 19%. In comparison with the electron donor and acceptor materials progress, the development of cathode interlayers lags. As a result, charge extraction barriers, interfacial trap states, and significant transport resistance may be induced due to the unfavorable cathode interlayer, limiting the device performances. Herein, a hybrid cathode interlayer composed of PNDIT-F3N and PDIN is adopted to investigate the interaction between the photoexcited acceptor and cathode interlayer. The state of art acceptor Y6 is chosen and blended with PM6 as the active layer. The device with hybrid interlayer, PNDIT-F3N:PDIN (0.6:0.4, in wt%), attains a power conversion efficiency of 17.4%, outperforming devices with other cathode interlayer such as NDI-M, PDINO, and Phen-DPO. It is resulted from enhanced exciton dissociation, reduced trap-assisted recombination, and smaller transfer resistance. Therefore, the hybrid interlayer strategy is demonstrated as an efficient approach to improve device performance, shedding light on the selection and engineering of cathode interlayers for pairing the increasing number of fused ring electron acceptors.