Studies on solution-processed tungsten oxide nanostructures for efficient hole transport in the inverted polymer solar cells

Abstract

Electron/hole transporting interlayer films are vital to photovoltaic devices for enhancing both efficiency and stability. The acidic nature of the standard anode buffer layer (or hole transport layer; HTL) PEDOT:PSS limits the stability of inverted polymer solar cells (IPSCs). High-quality inorganic substitutes for PEDOT:PSS usually needs vacuum deposition and therefore are not preferred choice for the large-scale and low-cost production of IPSCs. Here, we fabricated highly efficient and stable IPSCs with solution-processed tungsten oxide (WO3) anode buffer layer as an alternative. The effect of acidity over device stability was compared with WO3 nanostructures in different solvents. We fabricated IPSCs without encapsulation using both standard P3HT:PC61BM and low-bandgap polymer PTB7:PC71BM bulk heterojunction (BHJ) with acidic (WO3 nanoparticle; WNP) and neutral (WO3 nanosheets; WNS) anodic buffer layers. Cells fabricated with WNS layer exhibit ~11% (η = 5.1%) and ~15% (η = 7.8%) enhancements in the PCE for P3HT:PC61BM and PTB7:PC71BM bulk heterojunction (BHJ) respectively, compared to the standard cells made with PEDOT:PSS. Also, this is the first report where a WNS and P3HT:PC61BM blend is used as the anode buffer layer that yields the highest efficiency of ~5.1% with excellent air stability of ~240 h without encapsulation.