also suggest that 5,6-dialkyloxynaphthodithiophene is a useful
electron-rich building unit of a donor material for efficient cells.
The Institute of Molecular Functional Materials and this
work are supported by a grant from the University Grants
Committee, Areas of Excellence Scheme (AoE/P-03/08).
Notes and references
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Fig. 3 The external quantum efficiency of (a) PNB-3 and (b) PNB-4
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(Fig. 3). It is well known that a solvent additive can have
tremendous impact on the morphology of the blend film and
hence affecting the device performance.15 Fig. 2 shows the J–V
characteristics of PNB-3 and PNB-4 based devices fabricated
with a solvent additive, 1,8-diiodooctane (DIO) (3% by
volume). Indeed, the PNB-4 based device shows a dramatic
enhancement in the photovoltaic performance in which the
PCE reaches 5.3% with Voc = 0.64 V, short-circuit current (Jsc) =
13.5 mA cmꢀ2 and FF = 0.62. Such a device exhibits a broad and
strong photoresponse with EQE over 50% in the spectral range
from 350 nm to 640 nm and a maximum of EQE (EQEmax
)
reaching 67% at 457 nm. The wavelength integration of the
product of the EQE curve and the standard AM 1.5G solar
spectrum affords a calibrated Jsc of 12.66 mA cmꢀ2, which is 6%
smaller than that measured from J–V measurement due to a
spectral mismatch. In contrast, there is no apparent improvement
in device performance for the PNB-3 based PV cell fabricated
from chlorobenzene with the DIO additive.
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To get insight into the origin of such dramatic enhancement
in device performance for the PNB-4 based device fabricated
from solvent additive processing, the morphologies of the
blend films were investigated by atomic force microscopy
(AFM). As shown in Fig. S3(a) and (b) in ESIw, discrete and
large domains (B500 nm in diameter) are formed in the blend
film prepared from chlorobenzene without using DIO, which
is detrimental to the efficient exciton dissociation and charge
transport. The large domain hinders the extraction of
the carrier and results in low photocurrent. As shown in
Fig. S3(c) and (d) (ESIw), the addition of DIO dramatically
modified the phase segregation of the blend film. It is mainly
attributed to the improved miscibility of PNB-4 and PC71BM.
The formation of nanoscale fine networks between donor and
acceptor materials facilitates both the exciton dissociation and
charge transport in the blend film. This is consistent with the
large improvement of photocurrent from 4.3 mA cmꢀ2 to
13.5 mA cmꢀ2 in OPV devices. Such phenomenon is also
observed in recently reported efficient OPV devices.3d,4c
In summary, we have successfully developed new alternating
low bandgap copolymers, PNB, based on 5,6-bis(20-ethyl-
hexyloxy)naphthodithiophene and 2,1,3-benzothiadiazole or
4,7-bis(30-decylthiophen-2-yl)-2,1,3-benzothiadiazole with the
optical bandgap of B1.64 eV for organic solar cells. The bulk
heterojunction solar cell fabricated from the blend of PNB-4
and PC71BM using diiodooctane as a solvent additive afforded
a power conversion efficiency of 5.3% with an external
quantum efficiency over 60% in a broad spectral range. As a
result, PNB-4 shows promising potential for further development
and device optimization for practical applications. Our results
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9471–9473 9473