Interface Stability of Polymer and Small-Molecule Organic Photovoltaics - Organic Solar Cells: Materials and Device Physics - page 145

saturation of surface density and possible increase in chemical degradation. In
addition, the effect of temperature on device degradation was studied by setting up
a temperature-controlled platform via a fan for the measurement of the devices. It
is confirmed that a higher temperature accelerates device degradation, which is
correlated with glass transition temperature (T g ) of the polymer. It means if a fast
degradation of devices at an operation temperature close to T g could be observed,
the motion of polymer chains is increased and the electrode diffuses.
BCP is also an effective electron-transporting material as an exciton-blocking
layer and electron-transporting layer in organic optoelectronic devices. Vogel et al.
[ 41 ] incorporated BCP buffer layer at the C 60 /Al interface in small-molecule OPVs
(ITO/PEDOT:PSS/Pc/C 60 /BCP/Al) to reduce exciton quenching at the C 60 /Al
interface and to enhance electron transport from C 60 to Al due to established
Ohmic contact between C 60 and Al, resulting in a significant improvement of
efficiency. However, BCP does not have effect on blend geometry based OPVs
(ITO/PEDOT:PSS/Pc:C 60 /BCP/Al). Moreover, BCP prevents the diffusion of Al
during deposition on top of C 60 film, creating a highly structured interface. BCP
also blocks the exciton recombination caused by chemical reactions which forms
recombination centers at the interface.
An alternative to current organic electron-transporting materials is an insulator,
such as Al 2 O 3 or LiF, as cathode buffer layer. Zhang et al. found Al 2 O 3 in a role of
improving electron injection at the organic/cathode interface in Alq 3 based OLEDs
[ 42 ]. The fact that holes are injected and accumulated at the organic/buffer
interface could be beneficial to the enhancement of electron injection due to the
tunneling barrier reduction by changing the Fermi level of the cathode and carrier
transporting models. Hung et al. [ 43 ] demonstrated that an ultrathin LiF layer at
the organic/Al interface enhances electron injection and electroluminescence
efficiency in an OLED of ITO/CuPc/NPB/Alq 3 /LiF/Al, which is primarily attrib-
uted to band bending of the organic active layer by more than 1 eV when Alq 3
contacts with the dielectric LiF, resulting in the reduction of electron injection
barrier height at Alq 3 /Al interfaces.
Brabec et al. summarized a few mechanisms of LiF used in organic electronic
devices [ 44 ]: (1) lower the effective work function of Al; (2) dissociated LiF reacts
with the organic layer via chemical doping; (3) form a dipole layer to make a
vacuum level offset between organic layer and Al; (4) protect the Al atoms from
diffusing to organic layer during thermal deposition. Subsequently, Brabec et al.
[ 44 ] inserted an ultrathin LiF layer between BHJ active layer and metal cathode
(Al and Au) to largely enhance the FF and to stabilize high V oc . They proposed a
combined mechanism of the formation of a dipole moment across the junction, due
to either orientation of the LiF or chemical reactions leading to charge transfer
across the interface. An alignment of LiF is ascribed to the fact that the Li + adheres
preferably to the organic surface and the F - points toward the Al surface.
Furthermore, van Gennip et al. [ 45 ] studied and confirmed the mechanism of LiF
at the interface of metal and active layer by using secondary ion mass spectrometry
(SIMS) and X-ray photoelectron spectroscopy (XPS) measurements. SIMS spectra
measured at the organic/LiF interface to determine the chemical state of LiF layer