ZnO-free Organic Solar Cells with a Self-Assembled Monolayer as ETL for Improved Stability

Abstract

Albeit constant improvement of performances, stability remains hindered for organic photovoltaics (OPV) solar cells. One of the limiting factors is the degradation happening at the interface between the metal oxide electron transport layer (ETL) and the organic active layer. In particular, well known zinc oxide (ZnO) presents photocatalytic activity that degrades organic materials, resulting in altered interface, ultimately affecting the performances upon operation. As a mean to address this issue, the use of self-assembled monolayers (SAM) in replacement of ZnO is investigated. Here we design and synthetize a molecule, 2PAP-SAM, insuring a favourable molecular dipole ensuing a suitable shift of work function from 5.16 eV for pristine cleaned ITO to 4.34eV after SAM functionalisation. This shift contributes to a better energy level alignment between the cathode and the LUMO level of the acceptor molecule. Solar cells with PTQ10:Y6 as active layer were then fabricated with 2PAP-SAM as ETL and compared to a reference device with ZnO. Devices with SAM reached up to 10% power conversion efficiency (PCE) with VOC around 0.78V, close to the reference devices made with ZnO which presented 11% PCE with a VOC of 0.81V. In comparison, devices without any ETL show only poor performances. Furthermore, stabilities studies under continuous light illumination were performed and showed that devices with SAM as ETL present an improved stability than devices with ZnO.