Significant Photostability Enhancement of Inverted Organic Solar Cells by Inserting an N-Annulated Perylene Diimide (PDIN-H) between the ZnO Electron Extraction Layer and the Organic Active Layer
Mozhgan Sadeghianlemraski, Cayley R. Harding, Gregory C. Welch, Hany Aziz
Abstract
We investigate the effect of adding an N-annulated perylene diimide dye with a pyrrolic NH functional group (PDIN-H) onto an electron extraction layer (EEL) in a bilayer configuration (ZnO/PDIN-H) on the photostability of inverted organic solar cells (OSCs). To do so, we insert a thin layer of PDIN-H in between the ZnO layer and the bulk heterojunction (BHJ) active layer. Results show that under prolonged ultraviolet (UV) irradiation, the cells with the ZnO/PDIN-H EEL exhibit substantially higher photostability compared to the reference cells with only ZnO, leading to respective T80 values of ≳780 h versus only ∼124 h, where T80 is the time before the power conversion efficiency (PCE) decreases to 80% of its initial value. The higher PCE photostability arises primarily from the more stable open-circuit voltage (Voc) and fill factor (FF) under UV stress. Changes in the dark reverse current characteristics of the cells show that the higher Voc stability acquired upon adding PDIN-H on top of ZnO is mainly due to the ability of the ITO/ZnO/PDIN-H contact to maintain the blockage of hole injection even after UV stress. Analysis of the voltage dependence of dark and light ideality factors verifies that inserting PDIN-H prevents to a significant extent the UV-induced surface recombination that is observed at the ZnO/active-layer interface, thus enhancing the cells’ photostability. Results from hole-only devices and ultraviolet photoelectron spectroscopy reveal that the passivation of ZnO surface defects by PDIN-H is the primary origin of suppressing the UV-induced surface recombination and thereby increasing the photostability. The findings provide not only critical insights into the substantial role of the electron collection contact in the photodegradation of OSCs but also strategies to control them that can be utilized well beyond the specific material system being studied here.