沉积方法和Craft.io对钙钛矿CH3NH3PbI3薄膜中带隙，带间电子跃迁和光吸收的影响
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Effects of deposition methods and processing techniques on band gap,
interband electronic transitions, and optical absorption in perovskite
CH
3
NH
3
PbI
3
films
Wenwu Li (李文武),
1,a)
Tingting Sha (沙婷婷),
1
Ya n Wang (王琰),
2
Wenlei Yu (余温雷),
3
Kai Jiang (姜凯),
1
Hang Zhou (周航),
2,a)
Chuan Liu (刘川),
4
Zhigao Hu (胡志高),
1,a)
and Junhao Chu (褚君浩)
1
1
Key Laboratory of Polar Materials and Devices (Ministry of Education), Technical Center for Multifunctional
Magneto-Optical Spectroscopy (Shanghai), East China Normal University, Shanghai 200241, China
2
School of Electronic and Computer Engineering, Peking University Shenzhen Graduate School,
Peking University, Shenzhen 518055, China
3
Department of Biomedical Engineering, Wenzhou Medical University, Zhejiang 325035, China
4
State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of
Display Material and Technology, School of Electronics and Information Technology, Sun Yat-Sen University,
Guangzhou 510275, China
(Received 1 April 2017; accepted 23 June 2017; published online 6 July 2017)
Despite intensive studies on the improvements of conversion efficiencies in solar cells, many
questions regarding the effects of deposition techniques on optical properties and electronic band
structures of CH
3
NH
3
PbI
3
(MAPbI
3
) remain unresolved. Here, perovskite MAPbI
3
films were
prepared using different deposition methods and processing techniques. The effects of deposition
and processing parameters on dielectric functions and optical absorption were investigated by
fitting the reflectance spectra in the photon energy range of 0.5–5.16 eV. It is found that the
bandgap (E
g
) of the films deposited by two-step spinning (1.591 eV) is larger than that prepared by
evaporations (1.514 eV), due to different Pb-I orbital hybridization and spin-orbit coupling.
Moreover, the E
g
value of the films increase s from 1.543 eV to 1.591 eV after toluene solution
dripping. Five interband electronic transitions (E
p1
; E
p2
; E
p3
; E
p4
, and E
p5
) are observed, and the
origins of E
p2
; E
p3
, and E
p4
are assigned to the direct transitions between the highest valence band
and the lowest lying conduction band at the R, M, and X symmetry points. Further, the transition
energies of the films deposited by evaporation are less than those prepared by two-step spinning.
The present results shed light on preparing more reliable and reproducible high performance
MAPbI
3
-based solar cells. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4991864]
Perovskite solar cells (PSCs) have attracted considerable
interest because of their potential for low-cost, large-scale,
and high efficiency solar energy production. After only 5 years
of development, lead halide perovskite solar cells have
reached certified power conversion efficiencies over 22.1%,
which is an unprecedented achievement compared to the
counterparts using conventional materials.
1–3
More impor-
tantly, there is no sign indicating that the energy conversion
efficiency of PSCs has reached a bottleneck. As light-
harvesting materials, organic-inorganic hybrid perovskites
such as CH
3
NH
3
PbX
3
(MAPbX
3
)(X¼ Cl, Br, and I) have
been widely studied, with promising properties such as a suit-
able optical bandgap (E
g
)of1.57eV,
4
large absorption coeffi-
cient (10
4
cm
1
),
5
high carrier mobility, and long charge
carrier diffusion length (1 lm).
6,7
These excellent optical and
electrical properties make CH
3
NH
3
PbI
3
(MAPbI
3
) an efficient
absorber material for applications in perovskite solar cells.
Developing a deeper understanding of the optical prop-
erties is important both practically and theoretically for solar
cells. A suitable bandgap and a high absorption coefficient
for absorber materials play an important role in determining
the energy conversion efficiency of photovoltaic devices.
Bandgap tuning is required to extend the absorption to longer
wavelengths without sacrificing the absorption coefficient.
Many efforts have been made to optimize the bandgap and
absorption of the MAPbI
3
layer.
5,8
Modification of the bond
distance and/or angle of X-Pb-X in MAPbI
3
is one of the
strategies to affect bandgap energy. A theoretical study
suggests that the bandgap can also be adjusted by the p
orbital of I and p orbital of Pb.
9
Low-cost and facile deposition techniques are desired to
fabricate stable and high quality MAPbI
3
films. Compared to
one-step methods, two-step spin-coating processing
10,11
and
thermal evaporation methods,
12
which are very important
techniques for absorption layer fabrications, exhibit better
photovoltaic performance owing to better morphology and
interfaces.
13
It should be noted that the phase transition,
microstructure, defect states, and optical properties of
MAPbI
3
films are strongly affected by temperatures and
deposition methods. Jiang et al. reported the temperature
dependent optical properties of MAPbI
3
films from 77 K
to room temperature.
14
Furthermore, the bandgap, optical
constants, and optical absorption can also be affected by
processing techniques such as toluene dripping treatment.
However, a fundamental experimental understanding for the
a)
Authors to whom correspondence should be addressed: wwli@ee.ecnu.
edu.cn; zhouh81@pkusz.edu.cn; and zghu@ee.ecnu.edu.cn
0003-6951/2017/111(1)/011906/5/$30.00 Published by AIP Publishing.111, 011906-1
APPLIED PHYSICS LETTERS 111, 011906 (2017)
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