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

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 T 80 values of ≳780 h versus only ∼124 h, where T 80 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 (V oc) and fill factor (FF) under UV stress. Changes in the dark reverse current characteristics of the cells show that the higher V oc 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.

The Use of Green‐Solvent Processable Molecules with Large Dipole Moments in the Electron Extraction Layer of Inverted Organic Solar Cells as a Universal Route for Enhancing Stability

“…In a previous work, [25] we showed that UV irradiation does not appreciably alter the morphology or the UV-vis absorption www.advsustainsys.com characteristics of P3HT:PC60BM films coated on ZnO, indicating that morphological changes or photocatalytic effects [40] are not primary root causes in the observed low photostability of P3HT:PC60BM-based OSCs with ZnO EEL. Knowing from the literature that low morphological stability might be a factor in case of non-fullerene-based devices, [41] we conducted atomic force microscopy (AFM) and UV-vis measurements on the PM6:Y6 films.…”

“…The results of the ZnO cells are consistent with our recent findings that increased surface recombination is the primary cause of degradation in P3HT:PC60BM-based cells with ZnO under prolonged irradiation. [25] The observations also show that the higher photostability of the cells with DM-EELs originates primarily from an unchanged recombination mechanism in these cells. Since, apart from the EELs, all devices are structurally identical, differences in recombination mechanisms among them after the UV-irradiation must be associated with the electron collection contacts.…”

“…[21][22][23] Recently, we and others have found that exposure of ZnO EELs to UV irradiation (in the solar spectrum) leads to a loss in their electron-selectivity and that the phenomenon plays a major role in the low photostability of OSCs. [21][22][23][24][25] Some approaches have been proposed to overcome this UV-induced degradation, such as applying UV treatments to the ZnO layer before active layer deposition, [26] adding a bilayer EEL, [25] or doping the ZnO layer. [20,27] These methods, however, increase the manufacturing complexity and cost.…”

Air‐Processed Organic Photovoltaics for Outdoor and Indoor Use Based upon a Tin Oxide‐Perylene Diimide Electron Transporting Bilayer

“…Chemical structures of the BHJ photoactive materials a) PTQ10, Y6, and electron transport layer modifiers b) PDIN‐Boc, and PDIN‐H. c) Energy level diagram of the materials used (ITO, [ 19 ] SnO 2 [ 19 ] , PDIN‐H, [ 21 ] Y6, [ 19 ] PTQ10, [ 23 ] MoO x , [ 19 ] Ag [ 24 ] ) in the OPV device. d) Schematic diagram of the champion OPV device structure identified.…”

“…[ 16–19 ] Our team has recently demonstrated that an N‐annulated PDI with a functional N‐H bond (PDIN‐H) can be solution processed onto ZnO, decreasing the surface hydrophobicity and lowering the work function, leading to improvements in both OPV device PCE and stability. [ 20–22 ]…”