Physicochemical Studies on Nafion® Modified ZnO Interlayers for Enhanced Electron Transport in the Inverted Polymer Solar Cells

Abstract

Electron selective intermediate layers are crucial for both efficiency and stability of the inverted polymer solar cells (IPSCs). In this paper, an established low-temperature (<130°C) recipe to fabricate nano zinc oxide (ZnO) layers (L-ZnO) exhibited typical values of power conversion efficiency (PCE∼ 3.3%) with the standard P3HT:PC61BM blend. However, PCEs showed further improvement up to ∼24% (∼ 4.08%) when these layers were annealed at 300°C (H-ZnO). This annealing step can be bypassed by adopting a slightly different chemistry using Nafion as a modifier (NM-ZnO). I-PSCs fabricated with NM-ZnO showed nearly 40% increase in the PCE (∼4.62%) with PEDOT:PSS hole transport layers (HTL) that matches the so far highest recorded value for such a system. For the low-bandgap polymer blend PBDTT-FTTE:PC71BM, a peak efficiency of ∼6.77% was obtained compared to ∼5.34% with L-ZnO (∼28% increase). Our experiments revealed that these enhancements were due to a combination of benefits obtained from Nafion modification such as favourable positioning of work function (WF), high band gap (Eg) and better transport characteristics through the layer interfaces.