Effects of solvent additives on
trap-assisted recombination in
P3HT:ICBA based polymer solar cells
Shi Yan, Longfeng Lv, Yu Ning, Liang Qin, Chunhai Li, Xiande Liu, Yufeng Hu, Zhidong Lou,
Feng Teng
**
, and Yanbing Hou
*
Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044,
P. R. China
Received 24 March 2015, revised 29 April 2015, accepted 20 May 2015
Published online 22 June 2015
Keywords ideality factor, polymer solar cells, recombination, solvent additives, transient photocurrent
*
Corresponding auttors: e-mail ybhou@bjtu.edu.cn, Phone: þ86 10 5168 4860, Fax: þ86 10 5168 3933
**
e-mail fteng@bjtu.edu.cn, Phone: þ86 10 5168 4860, Fax: þ86 10 5168 3933
In this work, the effects of solvent additives on the trapping and
recombination of carriers in polymer solar cells based on the
blend of poly(3-hexylthiophene) (P3HT) and indene-C
60
-
bisadduct (ICBA) are investigated by measuring the ideality
factor and transient photocurrent. The addition of a few percent
of 4-bromoanisole (BrAni) and 1-chloronaphthalene (CN) can
give rise to the preferable morphology with well-ordered
interpenetrating networks, which is beneficial for charge
transport. Efficiency enhancement of polymer solar cells with
solvent additives originates from the decreased defects and
suppressed trap-assisted recombination at the interface
between the donor and acceptor materials.
ß 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Polymer solar cells (PSCs) ha ve
attracted much attention as a promising future photovoltaic
technology owing to their low cost, light weight, a nd
mechanical flexibility [1]. Poly(3-hexylthi ophene) (P3HT)
and [6,6]-phenyl-C
60
butyric ac id methylester (PCBM) ar e
the most commonly used donor and acceptor materials,
respectively. However, because of the limitation o f the
lowest unoccu pied molecular orbital (LUMO) energy level
of PCB M, it is difficult to further improve the photovoltaic
performance of P3H T:PCBM-bas ed PSCs [ 2]. Recently, a
new fullerene derivative indene-C
60
-bisadduct (ICBA)
with the LUMO level of 3.74 eV, which is u p-shifted by
0.17 eV than PCBM, has demonstrated excellent photo-
volt aic performance when used as an acceptor in P3HT-
base d PSCs [3]. The efficiencies of PSCs have been
significantly increased by perfor ming thermal treatments,
using different solvents, vapor anne aling , and applying
solvent additives [4, 5]. The in corporation of solvent
additives into the active layer solution is the most attractive for
the convenience of fabrication process. The additives can
assist in the crystallization of polymer chains and the suitable
formationofthe fullerenedomains to developinterpenetrating
networks [6]. Blend films with fewer defects can suppress the
trap-assisted recombination and promote charge transport. As
an important indicator in solar cells, the ideality factor can
reflect the charge trap-assisted recombination process [7].
Moreover, the transient photocurrent can provide the direct
information on the dynamics of the generation, transport, and
recombination of photocarriers [8].
Herein, we demonstrate that the performance enhance-
ment in P3HT:ICBA-based PSCs with solvent additives is
attributed to the improved morphology of active layers with
fewer defects. Investigating the trap-assisted recombination
in the devices by combination with the ideality factor and
transient photocurrent techniques indicates that the pref-
erable morphology can help to boost the charge transport.
2 Materials and methods The device configuration
of the PSCs based on P3HT:ICBA is shown in the inset of
Fig. 1(a). ZnO precursor was spin coated onto the
precleaned ITO substrates with a typical thickness of
40 nm [9]. Dichlorobenzene solutions of P3HT and ICBA
(1:1 w/w, polymer concentration of 17 mg/ml) without
additive and with 3% volume ratio of 4-bromoanisole
Phys. Status Solidi A 212, No. 10, 2169–2173 (2015) / DOI 10.1002/pssa.201532250
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ß 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim