Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 142

indium diffuses through all layers in the device and ends up on the outer surface of
the counter electrode (Al).
It is clear that the interface between ITO and active layer is playing an
important role in device performance and lifetime. Therefore, one effective
solution is to incorporate poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS) as anode buffer layer between ITO and photoactive layer, which
results in obvious improvement of lifetime and luminous efficiency [ 25 ]. Choice of
polyaniline doped with conducting PSS could partially overcome the issue asso-
ciated with the ITO/organic interface instability. Consequently, PEDOT:PSS has
been widely used in organic electronic devices with the functions of (1) a barrier to
prevent oxygen reaction, (2) a planarizing layer to inhibit electrical short points,
(3) an interfacial layer to shift the work function of ITO, and (4) an adhesive layer
to facilitate the adhesion between ITO and active layer [ 27 ]. The incorporation of
PEDOT:PSS layer could also prevent the diffusion of indium containing etch
products into organic active layer by trapping them.
However, the interface instability caused by PEDOT:PSS exists as well. de
Jong et al. studied the interface stability between ITO and PEDOT:PSS by using
Rutherford backscattering (RBS) [ 28 ]. It is found that the indium concentration in
PEDOT:PSS increases from 0.02 to 0.2 at.% upon annealing in nitrogen at 100 C
for 2500 h. A more serious and faster degradation of ITO/PEDOT:PSS interface is
observed when exposed to air, and the indium concentration reaches a saturated
value of 1.2 at.% after a few days. Therefore, it is explained that the strong acidic
nature of PEDOT:PSS makes the ITO etched. Furthermore, Girtan et al. compared
two sets of devices with ITO/PEDOT:PSS or only PEDOT:PSS as transparent
anode using MDMO-PPV:C 60 BM or P3HT:PC 60 BM blend system as the active
layer [ 29 ]. It was found that V oc of cells without ITO stays unchanged after aging
(40 days in open atmosphere at room temperature) and this is not valid for cells
with ITO. Meanwhile, for cells without ITO, the J sc decreases by one or two orders
of magnitude after aging, by contrast, for cells with ITO, the J sc decreases by four
to seven orders of magnitude. The results show that the work function of ITO
changes with surface modification during aging. Kawano et al. studied the deg-
radation of nonencapsulated OPVs (ITO/PEDOT:PSS/MDMO-PPV:PCBM/Al)
under different ambient conditions such as white light irradiation, in dark, expo-
sure to air, dry oxygen, and humid nitrogen atmospheres [ 30 ]. By comparing the
devices with and without PEDOT:PSS, the main reason for degradation under air
exposure is ascribed to water adsorption by the hygroscopic PEDOT:PSS layer,
independent of light, therefore, the resistance at the PEDOT:PSS/blend layer
interface is increased with degrading, proved by the change of charge mobility and
hole injection after air exposure. Spatially inhomogeneous degraded PEDOT:PSS
layer is correlated with insulating domains causing current loss.
Thus, it is summarized that the interface degradation caused by PEDOT:PSS
should be as follows: (1) the ITO etching due to acidic nature of PEDOT:PSS; (2)
the resistance increase due to the water adsorption by hygroscopic PEDOT:PSS;
(3) the change of ITO work function due to aging.