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) 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.