Boosting the Efficiency of Non-fullerene Organic Solar Cells via a Simple Cathode Modification Method

Brominated Quaternary Ammonium Salt-Assisted Hybrid Electron Transport Layer for High-Performance Conventional Organic Solar Cells

Interlayer engineering is crucial for achieving efficient and stable organic solar cells (OSCs). Herein, by introducing a commercialized brominated quaternary ammonium salt, hexamethonium bromide (HB), into a perylene diimide (PDI)-structured electron transport layer (ETL), a PDINN:HB hybrid ETL with enhanced charge collection ability and environmental/operational stability is realized. Molecular dynamics simulations and Kelvin probe force microscopy indicate that strong polar bromine and amine groups can form extra interfacial dipoles in the hybrid interlayer, while X-ray photoelectron spectroscopy and electron paramagnetic resonance suggest the hybrid ETL can interact with the Ag cathode, thereby regulating the energy level arrangement at the interface. As for the results, the PDINN:HB hybrid ETL enables improved power conversion efficiency (PCE) from 17.8 to 18.4% and 18.8 to 19.4% in PM6:C5-16 bulk heterojunction- and PM6/L8-BO pseudobulk heterojunction-based OSCs, respectively. The versatility of this method is further verified by introducing a range of brominated quaternary ammonium salts into PDINN, in which a superior PCE and stability are all obtained compared to the reference device.

Double-Dipole Induced by Incorporating Nitrogen-Bromine Hybrid Cathode Interlayers Leads to Suppressed Current Leakage and Enhanced Charge Extraction in Non-Fullerene Organic Solar Cells

The cathode interlayer plays a vital role in organic solar cells, which can modify the work function of electrodes, lower the electron extraction barriers, smooth the surface of the active layer, and remove solvent residuals. However, the development of organic cathode interlayer lags behind the rapidly improved organic solar cells because their intrinsic high surface tension can lead to poor contact with the active layers. Herein, a double-dipole strategy is proposed to enhance the properties of organic cathode interlayers, which is induced by incorporating nitrogen- and bromine-containing interlayer materials. To verify this approach, the state-of-the-art active layer composed of PM6:Y6 and two prototypical cathode interlayer materials, PDIN and PFN-Br is selected. Using the cathode interlayer PDIN: PFN-Br (0.9:0.1, in wt.%) in the devices can reduce the electrode work function, suppress the dark current leakage, and improve charge extractions, leading to enhanced short circuit current density and fill factor. The bromine ions tend to break from PFN-Br and form a new chemical bond with the silver electrode, which can adsorb extra dipoles directed from the interlayer to silver. These findings on the double-dipole strategy provide insights into the hybrid cathode interlayers for efficient non-fullerene organic solar cells.

Simple Solvent Treatment Enabled Improved PEDOT:PSS Performance toward Highly Efficient Binary Organic Solar Cells

PSS is the most popular hole-transporting material (HTM) for conventional structural organic solar cell (OSC) devices, whose performance is of great importance for realizing high power conversion efficiency (PCE). However, its performance in OSC devices has been continuously challenged by various replacing materials and different doping strategies, for better conductivity, work function, and surface property. Here, we report a simple dopant-free method to tune the phase separation of the PEDOT:PSS layer, which results in better charge transport and extraction in devices. Specifically, high PCEs for binary polymer-small-molecule (>18%) and polymer-polymer (>17%) systems are simultaneously achieved. This work engineeringly provides encouraging improvement for OSC device performance with easy modification and scientifically offers insights into tuning the property of the PEDOT:PSS layer.

Multicomponent Solar Cells with High Fill Factors and Efficiencies Based on Non-Fullerene Acceptor Isomers

Multicomponent organic solar cells (OSCs), such as the ternary and quaternary OSCs, not only inherit the simplicity of binary OSCs but further promote light harvesting and power conversion efficiency (PCE). Here, we propose a new type of multicomponent solar cells with non-fullerene acceptor isomers. Specifically, we fabricate OSCs with the polymer donor J71 and a mixture of isomers, ITCF, as the acceptors. In comparison, the ternary OSC devices with J71 and two structurally similar (not isomeric) NFAs (IT-DM and IT-4F) are made as control. The morphology experiments reveal that the isomers-containing blend film demonstrates increased crystallinity, more ideal domain size, and a more favorable packing orientation compared with the IT-DM/IT-4F ternary blend. The favorable orientation is correlated with the balanced charge transport, increased exciton dissociation and decreased bimolecular recombination in the ITCF-isomer-based blend film, which contributes to the high fill factor (FF), and thus the high PCE. Additionally, to evaluate the generality of this method, we examine other acceptor isomers including IT-M, IXIC-2Cl and SY1, which show same trend as the ITCF isomers. These results demonstrate that using isomeric blends as the acceptor can be a promising approach to promote the performance of multicomponent non-fullerene OSCs.

Sequential Processing Enables 17% All-Polymer Solar Cells via Non-Halogen Organic Solvent

All-polymer solar cells (All-PSCs), whose electron donor and acceptors are both polymeric materials, have attracted great research attention in the past few years. However, most all-PSC devices with top-of-the-line efficiencies are processed from chloroform. In this work, we apply the sequential processing (SqP) method to fabricate All-PSCs from an aromatic hydrocarbon solvent, toluene, and obtain efficiencies up to 17.0%. By conducting a series of characterizations on our films and devices, we demonstrate that the preparation of SqP devices using toluene can effectively reduce carrier recombination, enhance carrier mobility and promote the fill factor of the device.

Achieving the low interfacial tension by balancing crystallization and film-forming ability of the cathode interlayer for organic solar cells

A series of molecules with imide units bridged by the core of thiophene-based groups, namely N-dimethylaminopropyl-4-thiophene-1,8- naphthalimide (NT), bis(N-dimethylaminopropyl)-4-thiophene-1,8-naphthalimide (NTN), and bis(N-dimethylaminopropyl)-4-bithiophene-1,8-naphthalimide (N2TN), have been reported as cathode interfacial materials (CIMs) to realize low interfacial tension with the blend in organic solar cells (OSCs). We evaluated the Ohmic contact between the active layer and these cathode interlayers basedon various characterizations, which is of great significance for further understanding these imide-based interlayers. It turned out that the homogeneous and continuous NTN interlayer as a CIM balanced the factors of crystallization and film-forming property, and broke through the limitation of poor conductivity and high aggregation in our previous work. Moreover, compared with NT and N2TN, the NTN interlayer achieve a combination of good solubility in methanol, efficient electron mobility, and aligned work function. These advantages of NTN are conducive to the realization of high-efficient interfacial electron collection and transfer, thus improving the short-circuit current density (J_SC) and filling factor (FF) of devices. Therefore, the binary OSCs (PM6:Y6) based on NTN engineered aluminium-cathode with excellent stability demonstrate a maximum power conversion efficiency (PCE) of 16.56 %, which is higher than NT (PCE = 1.34 %) and N2TN (PCE = 13.90 %). The enhanced performance is ascribed to the improvement of JSC and FF, which is originated from the outstanding conductivity and high-quality interface of NTN. Surprisingly, the PM6:Y6-based semitransparent device with NTN obtain a PCE of 13.43 % with an average visible transmittance of 17.79 %, which is better than traditional PDINO. This study highlights a potential strategy for enhancing the performance of OSCs by the interface engineering via decreasing the interfacial intension.

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.