2 research outputs found
Short-Term Environmental Effects and Their Influence on Spatial Homogeneity of Organic Solar Cell Functionality
In
this study, we focus on the induced
degradation and spatial inhomogeneity of organic photovoltaic devices
under different environmental conditions, uncoupled from the influence
of any auxiliary hole-transport (HT) layer. During testing of the
corresponding devices comprising the standard photoactive layer of
poly(3-hexylthiophene) as donor, blended with phenyl-C<sub>61</sub>-butyric acid methyl ester as acceptor, a comparison was made between
the nonencapsulated devices upon exposure to argon in the dark, dry
air in the dark, dry air with illumination, and humid air in the dark.
The impact on the active layer’s photophysics is discussed,
along with the device physics in terms of integral solar cell performance
and spatially resolved photocurrent distribution with point-to-point
analysis of the diode characteristics to determine the origin of the
observed integrated organic photovoltaic device behavior. The results
show that even without the widely used hygroscopic HT layer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate),
humidity is still a major factor in the short-term environmental degradation
of organic solar cells with this architecture, and not only oxygen
or light, as is often reported. Different from previous reports where
water-induced device degradation was spatially homogeneous and formation
of Al<sub>2</sub>O<sub>3</sub> islands was only seen for oxygen permeation
through pinholes in aluminum, we observed insulating islands merely
after humidity exposure in the present study. Further, we demonstrated
with laser beam induced current mapping and point-to-point diode analysis
that the water-induced performance losses are a result of the exposed
device area comprising regions with entirely unaltered high output
and intact diode behavior and those with severe degradation showing
detrimentally lowered output and voltage-independent charge blocking,
which is essentially insulating behavior. It is suggested that this
is caused by transport of water through pinholes to the organic/metal
interface, where they form insulating oxide or hydroxide islands,
while the organic active layer stays unharmed
Influence of Environmentally Affected Hole-Transport Layers on Spatial Homogeneity and Charge-Transport Dynamics of Organic Solar Cells
After
organic photovoltaic (OPV) cells achieved efficiency of more than
10%, the control of stability and degradation mechanisms of solar
cells became a prominent task. The increase of device efficiency due
to incorporation of a hole-transport layer (HTL) in bulk-heterojunction
solar cells has been extensively reported. However, the most widely
used HTL material, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS), is frequently suspected to be the dominating source for
device instability under environmental conditions. Thereby, effects
like photooxidation and electrode corrosion are often reported to
shorten device lifetime. However, often in environmental device studies,
the source of degradation, whether being from the HTL, the active
layer, or the metal cathode is rather difficult to distinguish because
the external diffusion of oxygen and water affects all components.
In this study, different HTLs, namely, those prepared from traditional
PEDOT:PSS and also two types of molybdenum trioxide (MoO<sub>3</sub>) are exposed to different environments, such as oxygen, light, or
humidity, prior to device finalization under inert conditions. This
allows investigating any effects within the HTL and from reactions
at its interface to the indium tin oxide electrode or the active layer.
The surface and bulk chemistry of the exposed HTL has been monitored
and discussed in context to the observed device physics, dynamic charge
transport, and spatial performance homogeneity of the corresponding
OPV device. The results show that merely humidity exposure of the
HTL leads to decreased device performance for PEDOT:PSS, but also
for one type of the tested MoO<sub>3</sub>. The losses are related
to the amount of absorbed water in the HTL, inducing loss of active
area in terms of interfacial contact. The device with PEDOT:PSS HTL
after humid air exposure showed seriously decreased photocurrent by
microdelamination of swelling/shrinkage of the hygroscopic layer.
The colloidal MoO<sub>3</sub> with water-based precursor solution
presents slight decay of solar cell performance, also here caused
by swelling/shrinking reaction, but by a combination of in-plane particle
contact and resistance scaling with particle expansion. However, the
device with quasi-continuous and alcohol-based MoO<sub>3</sub> showed
unharmed stable electrical performance