ZnO-free organic solar cells with a self-assembled monolayer as an ETL for improved stability

Abstract

Despite constant improvements in the performance, the stability of organic photovoltaic (OPV) solar cells remains hindered. 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) exhibits photocatalytic activity that degrades organic materials, resulting in an altered interface, ultimately affecting the performance upon operation. As a means to address this issue, the use of self-assembled monolayers (SAM) in replacements of ZnO is investigated. Here, we design and synthetize a molecule, 2PAP-SAM, ensuring a favourable molecular dipole, ensuring a suitable shift of the work function from 5.16 eV for pristine cleaned ITO to 4.34 eV 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 an active layer were then fabricated with 2PAP-SAM as an ETL, and compared to a reference device with ZnO. Devices with the SAM exhibited power conversion efficiencies (PCEs) reaching up to 10%, with a V_OC of around 0.78 V, close to the reference devices made with ZnO, which exhibited a PCE of 11% with a V_OC of 0.81 V. In comparison, devices without any ETL show only poor performances. Furthermore, stability studies under continuous light illumination were performed and showed that devices with the SAM as an ETL exhibit improved stability compared to devices with ZnO.