Graphene for Transparent Electrodes and Organic Electronic Devices - Organic Solar Cells: Materials and Device Physics - page 83

temperatures (Fig. 4.1 c). The reduced graphene films could have a sheet resis-
tance of 17.9 KX/sq (transmittances of 69 % at 550 nm) and a conductivity of
22.3 S/cm. The OPV device under illumination of simulated solar light (AM
1.5G) shows a short-circuit photocurrent density (J sc ) of 1.18 mA/cm 2 with an
open-circuit voltage (V oc ) of 0.46 V, a filling factor (FF) of 0.25, and PCE of
0.13 %. The low PCE is due to the high sheet resistance of graphene films and the
hydrophobic graphene film surface, which makes it rather hard to get a uniform
PEDOT:PSS layer.
Wu et al. used rGO as transparent conductive anodes for an organic bilayer
small molecule OPV cells [ 26 ]. The transparent electrodes based on graphene were
obtained by vacuum annealing of graphene oxide or by a combination of a
hydrazine treatment and argon annealing at 400 C. The thickness of graphene
films used to fabricate OPV cells is between 4 and 7 nm, and the corresponding
values of the transmittance and sheet resistance are 95-85 %, and 100-500 kX/sq,
respectively. Devices with structure of anode/CuPc/C 60 /BCP/Ag were fabricated.
The J sc , V oc , FF, and PCE are 2.1 mA/cm 2 , 0.48 V, 0.34, and 0.4 %, respectively,
for the cell on graphene, and 2.8 mA/cm 2 , 0.47 V, 0.54, and 0.84 %, respectively,
for the cell on ITO for comparison. The poor solar cells performance is mainly
caused by the high sheet resistance of the graphene thin films, which need to be
reduced without compromising transmittance.
Wu et al. also demonstrate OLEDs with solution-processed graphene film as
transparent conductive anodes (Fig. 4.1 d, e) [ 6 ]. The graphene electrodes were
deposited on quartz substrates by spin coating of an aqueous dispersion of func-
tionalized graphene, followed by a vacuum anneal step to afford the graphene films
with resistance and transmittance of 800 X/sq and 82 % (550 nm). OLED device
with structure of anode/PEDOT:PSS/N,N 0 -di-1-naphthyl-N,N 0 -diphenyl-1,1 0 -
biphenyl-4,4 0 -diamine(NPD)/tris(8-hydroxyquinoline)aluminum(Alq 3 )/LiF/Al was
fabricated. The OLEDs on graphene exhibited a current drive and light emission
intensity comparable to those of ITO-based devices when the current density was
\10 mA/cm 2 . Meanwhile, the external quantum efficiency (EQE) and the lumi-
nous power efficiency (LPE) of graphene-based OLEDs nearly matched that of the
ITO-based device. The turn-on voltage of the OLED with graphene-based trans-
parent electrode was 4.5 V, slightly higher than the 3.8 V of the ITO-based device.
Similar, works have been reported and comparable PCE results were achieved
using rGO as transparent electrode in OPVs. For example, Eda et al. reported the
preparation of transparent and conductive graphene film by vacuum filtration of
graphene oxide to form a film, followed by a combination of hydrazine vapor and
low-temperature annealing (200 C) in nitrogen or vacuum [ 27 ]. Using above rGO
film as the transparent electrode, OPV device with P3HT and PCBM as active
layer gave the PCE of approximately 0.1 %, which is limited by the large resis-
tance with the order of 10 5 X/sq for the rGO electrodes. Yin et al. fabricated
flexible OPV devices by using a transferred rGO film as the transparent electrode.
The highest PCE obtained is 0.78 %, employing the flexible rGO/polyethylene
terephthalate (PET)-based transparent electrodes with a transparency of 55 % and
resistance of 1.6 kX/sq [ 28 ]. Geng et al. reported the preparation of transparent