Supporting Information for “Organic Photovoltaic Cells Based on an Acceptor of
Soluble Graphene”
Qian Liu, Zunfeng Liu, Xiaoyan Zhang, Nan Zhang, Liying Yang, Shougen Yin*, and
Yongsheng Chen*
Experimental details:
Preparation of graphene oxide: Graphene oxide was prepared using a modified
Hummers method from flake graphite (average particle diameter of 4m, 99.95%
purity, Qingdao Tianhe Graphite co., Ltd.).1 Briefly, 5 g of graphite and 3.75 g of
NaNO3 (A. R.) were placed in a flask. Then, 375 mL of H2SO4 (A. R.) was added
with stirring in an ice water bath. 22.5 g of KMnO4 (A. R.) was slowly added over
about 1 h. Cooling was completed in 2 h, and the mixture was allowed to stand for
five days at room temperature with vigorous stirring. During the five days, the
mixture turns green, then dark brown, and then brick brown. The liquid obtained was
added to 700 mL of 5 wt% H2SO4 aqueous solution over about 1 h with stirring and
the temperature was kept at 98oC, the resultant mixture was further stirred for 2 h.
Then the temperature was cooled down to 60oC and 15 mL of H2O2 (30 wt% aqueous
solution) was added to the above liquid and the mixture was stirred for 2 h at room
temperature.
In order to remove ions of oxidant origin, especially manganese ions, the
resultant liquid was purified by repeating the following procedure cycle 15 times:
centrifugation, removal of the supernatant liquid, addition of 2 L of a mixed aqueous
solution of 3 wt% H2SO4/0.5 wt% H2O2 and disperse using vigorous stirring and bath
ultrasonication for 30 min at a power of 140 W. Then the procedure was cycled for 3
times using 3 wt% HCl aqueous solution and for 1 time using H2O. The resultant
solution was transferred to acetone to remove the remained acid. Then through a
drying process the graphene oxide was obtained.
Isocyanate functionalization of graphene oxide: The functionalization process of
the graphene oxide was as follows: 200 mg of dried graphite oxide was suspended in
20 mL of anhydrous DMF, treated with 20 g of phenyl isocyanate (A.R) for 1 week.
After a work up procedure to remove the impurities, the isocyanate treated graphene
oxide is obtained. The reaction mixture was added dropwise into 25 mL of DCB and
centrifuged at 1000 rpm for 10 min. The upper solution was dropped into 50 mL of
CHCl3 and centrifuged at 11000 rpm to collect the deposit at the bottom. Repeat the
above procedure for 2 times and get the purified functionalized graphene with a yield
of 70%.
Fabrication of the photovoltaic device: Poly (3-hexylthiophene) was purchased
from Aldrich Chemical Co. The materials were synthesized by the Rieke method. The
HT (HT= head-to-tail) regioregularity reported by the company was > 98.5%. The
number average molecular weight of P3HT is reported as Mn=60 000-70 000 with a
PDI ~ 1.8. The devices were prepared from P3HT/graphene solution in 1,
2-dichlorobenzene (P3HT content of 15mg.mL-1, graphene content 0 to 15 wt%). Poly
(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Baytron P,
Bayer Germany) was then spin (4000 rpm, ~ 40 nm of thickness) coated on top ITO
coated glass. The active layer was prepared by spin-coating (2000 rpm, 9 s) on the
above modified ITO glass using a solution of P3HT/graphene with a P3HT
concentration of 15 mg mL-1 and different graphene content (2 to 15 wt%) in DCB.
The active layer thickness was measured at ~ 100 nm using Profilometer (Ambios,
XP-2TM). The annealing was carried out at 160oC for 10 min in glove box. Then ~ 1
nm of lithium fluoride (LiF) and a 70 nm thick Al electrode was deposited onto the
blend film by thermal evaporation at ca. 5 × 10–6 mbar. Eight polymer solar cell
devices were fabricated in one cut ITO glass, and the effective area of each cell is ~ 8
mm2. All current–voltage (J-V) characteristics of the photovoltaic devices were
measured in air using a Keithley SMU 2400 unit. A Xeon lamp was used as the
excitation source with a power of 100 mW cm–2 white light illumination (AM1.5G
conditions) from the ITO side. Light source illumination intensity was measured using
a calibrated broadband optical power meter. All the fabrication and measurement is in
air at room temperature. The calculation of the power conversion efficiency, η, has
been performed using the following equation:
 =Voc Jsc FF/Pin
where Voc, Jsc, FF, and Pin are the open circuit voltage, the short circuit current, the fill
factor and the incident light power, respectively. The fill factor (FF) is determined
according to FF = (Vm Jm)/(Voc Jsc), where Vm and Jm are the voltage and the current in
the maximum power point of the J–V curve in the fourth quadrant. All the fabrication
and measurement is in air at room temperature.
AFM and TGA-DSC: For Atomic force microscopy measurement, the graphene
oxide sample was prepared by spin coating (2000 rpm, 30 s) graphene oxide solution
(0.2 mg mL-1) in H2O on a mica surface. AFM studies were performed using a Digital
Instruments Dimension 3100 in the tapping mode. Thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) analysis were performed using a
TGA/DSC STA904 at a heating rate of 5 °C min–1 under Ar flow.
References
1
M. Hirata, T. Gotou, S. Horiuchi, M. Fujiwara, and M. Ohba, Carbon 42, 2929
(2004).