Interfacial modification for improving inverted
organic solar cells by poly(N-vinylpyrrolidone)
Xuan Yu,†
ab
Xiaoming Yu,†
b
Jianjun Zhang,
*
b
Dekun Zhang,
b
Hongkun Cai
b
and Ying Zhao
b
The effect of the thickness of the poly(N-vinylpyrrolidone) interface
modifier on the photovoltaic performance of inverted organic solar
cells was investigated. Superior interface properties provided efficient
charge transport and decreased the charge recombination due to PVP
interlayer, which reduced the energy barrier for electron extraction by
lowering the hydroxide radical amount. We obtained an enhanced
efficiency of 4.55% (for the P3HT:PCBM device) and 6.18% (for the
PTB7:PC
71
BM device).
Organic photovoltaics (OPVs) based on a conjugated polymer–
fullerene bulk heterojunction (BHJ) are attractive because their
solution processing provides the advantage of low fabrication-
cost and potential in large-scale commercial use.
1–5
Remark-
able progress in the photovoltaic application eld has been
achieved during the last decade; device power conversion
efficiency (PCE) has increased from less than 1–8% (for
conventional structure devices) and over 9% (for inverted
structure devices).
6–12
Inverted organic solar cells (IOSCs) in
which an electron transport layer (ETL) is introduced between
the ITO and BHJ layer, with high PCE and better stability in the
atmosphere, are gaining more attention and are under intense
investigation.
10,11,13–17
Due to their efficient electron transport
properties, zinc oxide (ZnO)
15–19
and doped-ZnO (e.g. Ga-doped
ZnO, Al-doped ZnO(AZO))
20–23
have been explored as an ETL in
IOSCs and are compatible with large-area fabrication methods.
24
However, the surface of ZnO (or AZO) ETLs may cause
incomplete ligand removal and lead to bad control over the
electronic surface properties during the low processing
temperatures,
25
which leads to inferior interface contact quality
between the inorganic ETL and organic BHJ layer; this imposes
a restriction on further improving the performance of IOSCs as
a result of poor electron extraction
26
and bimolecular recom-
bination.
15
To resolve this issue and achieve higher module
efficiencies, interface modication becomes a critical factor in
improving the performance of IOSCs.
27,28
An interfacial layer has
been introduced on the ETL to improve the contact quality and
electron extraction. It is suggested that the interfacial layer can
serve multiple functions, including: ① tuning the energy level
alignment, ② improving charge selectivity, ③ controlling
surface properties, ④ introducing optical spacer and plasmonic
effects and ⑤ improving the interfacial stability.
29
However,
interfacial materials that simultaneously possess these
requirements are rare.
Several classes of interfacial materials have been designed
for improving the interface between the inorganic ETL and
organic BHJ layers in IOSCs such as C
60
-substituted benzoic
acid,
30
phosphoric acid-anchored C
60
self-assembled mono-
layers (SAMs),
25
conjugated polyelectrolyte,
15
and cross-linked
fullerenes (C-PCBSD).
31
More recently, C. Gu et al. demon-
strated that the effi ciency can be further enhanced by inte-
grating optimization of both the anode and cathode interlayer.
32
In our previous study, poly(N-vinylpyrrolidone) (PVP) was
demonstrated to be a promising candidate interface material
for IOSCs.
33
It can provide an excellent interface property and is
fully compatible with roll-to-roll production techniques,
because the approach does not need chemical synthesis and
can be easily conducted at room temperature open to air.
Moreover, unlike some SAMs that may cause damage
34
via
corrosion, the compatibility of PVP interlayer with the upper
and lower layers in the device does not need to be carefully
considered. However, the inuence of PVP thickness on device
performance is not clear. Alex et al. suggested that the thickness
of the inserting layer should be well controlled to tune the
interfacial properties.
29
Furthermore, non-conformal coverage
of PVP
35
has a great inuence not only on the properties of
PVP/AZO layers and the surface morphology of the active layer,
but also on the interface characteristics, which should be
changed with PVP thickness. Moreover, the working mecha-
nism between the PVP interlayer and AZO ETL is unclear.
a
Innovation Application Institute, Zhejiang Ocean University, Zhoushan, Zhejiang,
China, 316022
b
College of Electronic Information and Optical Engineering, Nankai University,
Tianjin, China, 300071. E-mail: jjzhang@nankai.edu.cn; Fax: +86-22-23508032;
Tel: +86-13820739160
† Xuan Yu and Xiaoming Yu contributed equally to this work.
Cite this: RSC Adv.,2015,5, 58966
Received 20th May 2015
Accepted 25th June 2015
DOI: 10.1039/c5ra09427e
www.rsc.org/advances
58966 | RSC Adv.,2015,5, 58966–58972 This journal is © The Royal Society of Chemistry 2015
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