Graphene for Transparent Electrodes and Organic Electronic Devices - Organic Solar Cells: Materials and Device Physics - page 84

conductive graphene films by a two-step reduction method that consisted of the
controlled chemical reduction of GO in an aqueous suspension and the thermal
annealing of the resultant films [ 29 ]. OPV devices with P3HT and PCBM as active
layer showed PCE of 1.01 %.

4.2.2 Transparent Electrode Based on CVD graphene

CVD is an important and successful method to obtain high-quality graphene films
[ 30 - 32 ]. Recently, films with sheet resistance of 280 X/sq (80 % transparency)
and 770 X/sq (90 % transparency) have been reported for CVD graphene

synthesized on Ni films [ 20 ].
Wang et al. synthesized a large-area graphene film on Ni-coated SiO 2 /Si wafer
using a CVD process [ 33 ]. For 6-30 nm thick graphene films, the average sheet
resistance varies from 1350 to 210 X/sq with an optical transparency from 91 to
72 % in the visible light wavelength range. As shown in Fig. 4.2 , a BHJ structure
solar cell using the graphene anode was fabricated. The J sc , V oc , FF, and PCE are
2.39 mA/cm 2 , 0.32 V, 0.27, and 0.21 %, respectively. The poor performance was
caused by the hydrophobic property of graphene, which could not form the uniform
coating of PEDOT:PSS. After the UV treatment of graphene film for 10 min to
improve the surface wettability, the device PCE was increased to 0.74 %. In order
to avoid the disruption of the aromatic structures caused by covalent bonding with
oxygen groups after the UV treatment, the graphene anode was modified by self-
assembled pyrene buanoic acid succidymidyl ester (PBASE). A well-improved
performance (V oc = 0.55 V, J sc = 6.05 mA/cm 2 ,FF= 0.51, and PCE = 1.71 %)
was obtained. In contrast, the device made with ITO anode showed V oc , J sc , FF, and
PCE of 0.56 V, 9.03 mA/cm 2 , 0.61, and 3.10 %, respectively.
Similar works employing CVD graphene as transparent electrode for OPV
application have been reported recently. For example, Arco et al. reported a
transparent graphene film by CVD with sheet resistance 230 X/sq and 72 %
transparency at the wavelength of 550 nm [ 34 ]. OPV devices using CVD graphene
and ITO electrodes were fabricated side-by-side on flexible PET substrates and
were confirmed to offer comparable performance, with PCE 1.18 and 1.27 %,
respectively. Loh et al. reported a layer-by-layer (LBL) transfer method of CVD
graphene sheets. The LBL, acid-doped, four layer graphene film exhibited a sheet
resistance of 80 X/sq and a transmittance of 90 % at 550 nm, which is comparable
to the ITO [ 35 ]. OPVs with the structure of graphene/PEDOT:PSS/P3HT:PCBM/
LiF/Al exhibited the best performance with a PCE of 2.5 %, which is comparable
with the PCE of 3 % for ITO-based devices. Lee and his coworkers reported the
preparation of multilayer graphene (MLG) film grown by CVD [ 36 ]. OPV devices
using graphene with sheet resistances of 606 X/sq and transmittances of 87 % as
electrodes showed the best performance with PCE up to 2.58 ± 0.45 %.