2P025
Photocurrent Enhancement in Bulk Heterojunction
Organic Photovoltaic Devices
Daniel Ping-Kuen Tsang, Mei-Yee Chan*, Vivian Wing-Wah Yam

Department of Chemistry, The University of Hong Kong,
Pokfulam Road, Hong Kong
Email: chanmym@hku.hkI.

INTRODUCTION
Organic photovoltaic (OPV) devices that can convert sunlight into electricity in organic
semiconductors have been subject of active research over the past two decades. Significant
advances on the development of smart materials and innovative device architecture have boosted
the power conversion efficiency up to 10.6 % [1]. In particular, bulk heterojunction consisting of a
spatially distributed donor/acceptor interface can greatly increase the exciton dissociation
efficiency approaching an unity, yielding a higher short-circuit current (Isc). The major obstacle forachieving high power conversion efficiency is in balancing the absorption needed for photocurrent
generation with the charge transport properties of the blended layer. Especially the charge carrier
mobilites in the blended layer are orders of magnitude lower than those of homogeneous layers,
and it requires a delicate control on the composition and thickness of the blended layer. Zhang et al.
recently modified this bulk heterojunction concept by employing non-absorbing donor material,
1,1-bis(4-bis(4-methly-phenyl)-amino-phenyl)-cyclohexane (TAPC), doped into fullerene (C70)
acceptor, in conjunction with molybdenum oxide as anodic buffer layer [2]. Surprisingly, with avery low concentration of TAPC of only 5%, a three-fold increase in Isc with an extraordinarily
high open-circuit voltage (Voc) of 0.91 V had been obtained. These yielded a high power conversion
efficiency of 5.23%, which is the highest ever reported for a cell with fullerene as the sole absorber.
On the other hand, the exact mechanisms and the underlying physics for the performance
improvement are still unclear. In particular, such high Voc cannot be explained by the common
energy gap law. Here, the effects of different non-absorbing organic materials as donor on theperformance of OPV devices with the modified bulk heterojunction have been studied and thecorrelation between the energetic properties of donor materials and the photovoltaic responses isdiscussed.
II.

EXPERIMENTAL
OPV devices were fabricated with the configuration of indium-tin oxide/WO3/C60/
bathophenoline (BPhen)/aluminum, in which 5% non-absorbing organic materials were doped into
C60 as donor. Two well-known hole-transporting materials, namely, 4,4’,4”-tris(N-carbazolyl)
triphenylamine (TCTA) and N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine(TPD), and two electron-transporting materials, namely, 3-(4-biphenyl)-4-phenyl-5-(4-tertbutylphenyl)-1,2-4-triazole (TAZ) and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB), were used
as donor materials. A control C60-only device had also been prepared for comparison. The
photocurrent of OPV devices was measured under illumination with an intensity of 1 sun from an
Oriel 300 W solar simulator equipped with AM1.5 G filter and the light intensity was calibrated bya Newport reference cell equipped with a KG5 window to minimize the spectral mismatch.

III. RESULTS AND DISCUSSION
Under light illumination, the C60-only device showed a poor performance with Isc = 1.22mA/cm2, Voc = 0.87 V, fill factor = 0.36, and power conversion efficiency of 0.38%. The incorporationof hole-transporting donor material, e.g. TCTA or TPD, can improve the photovoltaic responses.
The Isc was dramatically increased to 3.21 and 3.68 mA/cm2, for the TCTA- and TPD-doped
devices, respectively. More importantly, the Voc of the device increased from 0.87 V to 0.95 V for
the TCTA-doped device. These correspond to high power conversion efficiencies of 1.20% for bothdevices, much higher than that of the C60-only device. In sharp contrast, the doping ofelectron-transporting donor material, e.g. TAZ or TmPyPB, would degrade the device performance.
Particularly, the Isc of the TAZ- and TmPyPB-doped devices dropped to 0.82 and 0.77 mA/cm2,respectively; while the Voc decreased to 0.54 and 0.48 V, respectively. It is quite surprising that theuse of donor material with a very low dopant concentration (i.e. 5%) can lead to a significant

change on the photovoltaic responses. To investigate the role of donor materials, anincident-photon-to-current efficiency (IPCE) measurement was performed. Figure 1 depicts theIPCE spectra of OPV devices doped with different non-absorbing donor materials. Apparently, theincorporation of 5% donor materials can significantly enhance the IPCE and thus the exciton
dissociation efficiency, in which the IPCE dramatically increased from 16.0% for the C60-only
device to 36.7% and 41.2% for the TCTA- and TPD-doped devices at the C60 absorption at 440 nm.
This enhanced exciton dissociation efficiency definitely increases the Isc of OPV devices. It is also
found that the Voc of OPV devices are strongly dependent on the highest occupied molecularorbital (HOMO) level of the donor materials. To investigate the controlling factors governing theVoc, the Voc of the devices were re-plotted as a function of HOMO level of donor materials. As
depicted in Figure 2, the Voc of OPV devices generally increased with increasing the HOMO levelof donor material and reached a maximum at 0.95 V when the HOMO level increased up to 5.7 eV(i.e. TCTA was used as donor material). The Voc increase is due to the increase on the energy
difference between the HOMO level of donor and the lowest unoccupied molecular orbital level ofC60. On the other hand, a further increase in the HOMO level would lead to the reduction in theVoc of the devices. The close proximity of the HOMO levels of donor and C60 may restrict theexciton dissociation in C60 and accounts for the Voc reduction [3].
601.2
Without donor
TCTA
TPD
TAZ
TmPyPB5030
20
100
4000.8

Voc (V)

IPCE (%)

40

TCTA

1.00.6

TPD

TAZ

0.4

TmPyPB

0.2500600

700800 (nm)

Figure 1. IPCE spectra for OPV devices dopedwith different donor materials.

0.0
5.0

5.4

5.8

6.2

6.6

7.0

HOMO (eV)Figure 2. Voc of OPV devices as a function ofHOMO levels of donor materials.

IV. CONCLUSION
In summary, the influences of different non-absorbing organic materials as donor on theperformance of OPV devices with modified bulk heterojunction had been studied. The photovoltaic
responses of OPV devices were found to be sensitive to the choice of the donor materials, even if avery low donor concentration of only 5% was employed. Particularly, the Isc showed a three-foldincrease when a hole-transporting material was doped into C60 to form bulk heterojunction, whichmight be due to an enhanced exciton dissociation efficiency. A strong dependence of HOMO level ofdonor material on the Voc of OPV devices was also found.
ACKNOWLEDGEMENTS
This work was fully supported by a grant from the Research Grants Council of the Hong Kong
Special Administrative Region, China (Project No. HKU 700510).
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[2] M. Zhang, H. Wang, H. Tian, Y. Geng, C.W. Tang, Adv. Mater. 23, 4960 (2011).
[3] S.L. Lai, M.F. Lo, M.Y. Chan, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 95, 153303 (2009).