Highly Conductive, Extremely Transparent ZnO Electrodes for Efficient ITO-Free
Polymer Solar Cells
Ermioni Polydorou, Anastasia Soultati, Panagiotis Argitis, Dimitris Davazoglou and
Maria Vasilopoulou
1
Institute of Nanoscience and Nanotechnology, National Center for Scientific Research Demokritos,
15310, Aghia Paraskevi, Attiki, Greece
*email
Transparent conducting oxide (TCO) films have been extensively applied to optoelectronic devices,
1
chemical sensors,
2
and solar cells.
3
Doped zinc oxide (ZnO) thin films, in particular, have been
investigated as promising transparent conducting layers for photovoltaic devices as an alternative to
indium tin oxide thin films because the impurity doping significantly improves the electrical and
optical properties of these films.
The group III elements, such as boron (B), aluminum (Al), gallium
(Ga), and indium (In) have been intensively investigated for the doping of ZnO. It was demonstrated
that the group III elements could work as n-type dopants for ZnO to replace Zn sites and generate free
electrons. Because the indium element in ITO films is comparatively rare, the doped ZnO films have
been investigated as an alternative to ITO for their excellent compromise on electrical and optical
properties, low price, abundance of raw materials and simplicity of fabricating process.
Due to their
obvious benefits, ZnO-based TCO films have been introduced into inverted polymer solar cells as
either charge transport interfacial layers
3
or bottom electrode materials.
4
Among the various doping
methods, aluminum and gallium doping are well-known approaches to enhance the conductivity of
ZnO layers for use as bottom electrodes in inverted PSCs.
5
To nowadays, well-performing inverted
PSCs based on Al or Ga doped bottom electrodes with modified crystallinities and optical properties
have been demonstrated. However, up to now the successful application of boron doped ZnO (BZO)
layers as transparent electrodes in PSCs has not been demonstrated.
In this work, we report on the electrical, optical and structural/electronic properties of boron-doped
ZnO (termed hereafter as BZO) thin films before and after hydrogen post annealing for use as bottom
cathode electrodes in inverted PSCs. It was found that hydrogen annealing at temperatures around 200
o
C (and above) resulted in the formation of hydrogen-doped BZO electrodes (termed hereafter as
BZO:H) which exhibited significantly improved conductivity, carrier concentration, Hall mobility and
optical transparency whereas they also presented reduced work function value when compared with
their non-annealed counterparts. As a result, PSC devices using BZO:H films as cathode electrodes
and the well-known poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C71butyric acid methyl ester
(PC
71
BM) system as the photoactive layer exhibited a power conversion efficiency (PCE) of 3.9%
which was 20% improved relative to that exhibited by the reference device with the conventional FTO
cathode (PCE=3.30%) whereas these devices also exhibited exceptional environmental stability. In
addition,
by
using
poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-
benzothiadiazole-4,7-diyl-2,5-thiophenediyl]
(PCDTBT):PC
71
BM
or
poly[[4,8-bis[(2-
ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)
carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7:PC
71
BM) as the active component in the solar cell a
high PCE of 5.90 % or 7.25%, respectively, was achieved, which were higher than those of the
reference devices using either the non-annealed BZO or FTO cathodes. Based on these results, we
believe that our prescription for the control of optical and electronic properties of ZnO-based
electrode materials using a hydrogen post-annealing treatment, constitute a step forward in the
engineering of electrode materials which is a necessary achievement for the selection of the most
suited properties to be exploited not only to PSCs but also in the different kinds of optoelectronic
applications.
References
1.
A. Stadler,
Materials
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Y.-S. Shim, H. G. Moon, D. H. Kim, H. W. Jang, C.-Y. Kang, Y. S. Yoon and S.-J. Yoon,
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S. Trost, K. Zilberberg, A. Behrendt, A. Polywka, P. Görrn, P. Reckers, J. Maibach, T. Mayer and T. Riedl,
Adv. Energy Mater.
,
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H.-K. Park, J.-W. Kang, S.-I. Na, D.-Y. Kim and H.-K. Kim,
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