The alcohol and water-based processing of a perylene diimide (PDI) organic semiconductor into large area and solvent resistant films is reported. The compound, PDIN-H, is an N-annulated PDI dye with...
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.
Electroluminescence
(EL) degradation mechanisms in solution-coated
(SOL) host:guest (H:G) systems commonly used in phosphorescent organic
light-emitting diodes (OLEDs) are investigated and compared to their
vacuum-deposited (VAC) counterparts. Changes in the EL, photoluminescence
(PL), and time-resolved PL (TRPL) characteristics of devices comprising
SOL or VAC H:G light-emitting layers (EMLs) made of the same materials
and in the same device architectures during prolonged electrical driving
are compared and analyzed. Hole-only devices are also utilized to
study the effects of charges and excitons, separately and combined.
Moreover, devices with double EMLs comprising SOL and VAC components
are tested to glean additional insights into the role of host excitons
in device degradation. The results indicate that the faster degradation
of SOL EML devices relative to their VAC EML counterparts under electrical
stress is due–at least in part–to the less efficient
host-to-guest (H → G) energy transfer in these systems, which
accelerates molecular aggregation in the EML. Interactions between
excitons and polarons in the EMLs induce this aggregation phenomenon
which occurs more strongly in the case of SOL EMLs compared to their
VAC counterparts because of the higher host exciton concentration
in the former as a result of the less efficient H → G energy
transfer. The findings shed light on one of the root causes of the
limited stability of SOL OLEDs.
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