Self-assembled diblock conjugated polyelectrolytes as electron transport layers for organic photovoltaics

Abstract

Interfacial morphology is not only paramount for charge extraction and transport but also dramatically affects the morphology of the upper active layer, thereby influencing the ultimate power conversion efficiency. However, detailed investigation of the instinctive self-assembly of conjugated polyelectrolytes (CPEs) as the electron transport layers (ETLs) in polymer solar cells (PSCs) has rarely been investigated. Meanwhile, the correlations between the structural assembly of CPEs ETLs on the crystalline ordering, morphology of the upper active layer and the final photovoltaic performance are mystical stories. Herein, two water/alcohol-soluble diblock CPEs with different backbone PF_EO-b-PCNBr and PF_EO-b-PTNBr are synthesized via Kumada catalyst transfer coupling reactions as ETLs for inverted bulk-heterojunction PSCs. Both PF_EO-b-PCNBr and PF_EO-b-PTNBr offer an ohmic contact between the ITO electrode and the active layer by substantially reducing the work function of the ITO via modulating the interfacial dipoles. More intriguingly, the spontaneous self-assembly of the diblock polymers can act as a template to induce the upper active layer to form ordered wide nanowire and nanofiber morphology. The more ordered morphology is beneficial for charge extraction and transportation. Consequently, the devices based on poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) with ZnO/PF_EO-b-PCNBr and ZnO/PF_EO-b-PTNBr as ETLs deliver notable power conversion efficiencies (PCEs) of 3.6% and 3.8%, respectively, which is distinctly enhanced compared to 3.0% for the device with pure ZnO as an ETL. These findings indicate that the self-assembled diblock CPEs ETLs provide a novel strategy for optimization of the morphology of the upper active layer and performance of the PSCs.