Zn-doping for reduced hysteresis and improved performance of
methylammonium lead iodide perovskite hybrid solar cells
Wangen Zhao
a
, Dong Yang
a
, Zhou Yang
a
, Shengzhong (Frank) Liu
a
,
b
,
*
a
Key Laboratory for Applied Surface and Colloid Chemistry, National Ministry of Education, Shaanxi Engineering Lab for Advanced Energy Technology,
Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi'an 710062, China
b
Dalian National Laboratory for Clean Energy, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian, 116023, China
article info
Article history:
Received 17 February 2017
Received in revised form
2 May 2017
Accepted 25 June 2017
Keywords:
Perovskite
Doping
Divalent metal
Non-radiative recombination
Carrier lifetime
abstract
The high efficiency and low-toxicity are the two main objectives for all perovskite solar cells (PSCs)
towards large scale commercialization. Herein, ecology-benign Zn was introduced to partially replace the
noxious Pb in the typical Pb-based perovskite absorber layer owing to its low-toxicity, high-richness in
the crust, suitable valence states, and good stability against oxidation and reduction. It is found that the
addition of Zn with a certain amounts significantly improves the crystalline quality, enlarges the grain
sizes in the perovskite thin films, reduces the non-radiative recombination loss and lengthens the carrier
lifetime, leading to the improved photovoltaic performance and eliminated hysteresis. It is demonstrated
that Zn doping is an effective strategy to replace Pb to achieve superior solar cell performance with
reduced Pb usage.
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
The photovoltaics has caught considerable attention for its
sustainable clean energy supply owing to its being able to convert
freely available solar energy directly into electricity. Among the
various types of photovoltaic materials, organic-inorganic halide
perovskites (OHP) polycrystalline material features ease of fabri-
cation, strong sunlight absorption, low defect density, small exciton
binding energy (~20e50 meV), long charge carrier lifetime
(>100 ns) and long carrier diffusion length (>1
m
m), in spite of the
material being solution process [1e6], making it a superior
competitor for the low-cost solar cell production. And furthermore,
the variability of the perovskite ABX
3
crystal structure facilitates it
going through a rapid advance in efficiency and stability in just few
years. Especially, the efficiency from 3.8% [7] reported by the first
time has soared to nowadays certified 22.1% [8]. The stability has
also been significantly improved by introducing some hydrophobic
protective layer, element substitute or dimension-reduction in so-
lar cell architecture [9e14]. These intense investigations in
perovskite materials prompt it the most dazzling superstar among
photovoltaic semiconductor materials.
In the typical perovskite CH
3
NH
3
PbI
3
(MAPbI
3
), the organic
cation CH
3
NH
3
þ
is mainly responsible for the structural stability of
the perovskite film. Large-size organic ion CH
3
NH
3
þ
can effectively
stabilize this perovskite structures while it makes little contribu-
tion to the electronic structure of CH
3
NH
3
PbI
3
around the band
edge due to the rather weak vander Waals interactions between
MA cation and three dimensional (3D) Pb-I framework. And it was
found that partial Cs substitution of MA can obviously enhance the
resistance against environmental humidity due to smaller size
resulting to entropic stabilisation [15,16]. The electronic properties
are largely determined by metal, especially the Pb 6s
2
lone pair, and
halide hybridized orbitals [17e19]. The Pb-site have a vital effect on
the band gap of halide perovskite since its conduction band is
mainly derived from the unoccupied Pb p orbitals, while the upper
valence band is constituted mainly by halogen p orbitals mixed
with Pb s orbitals. Therefore, X is usually employed to adjust the
band gap of the perovskite material in a broad range due to Pb often
being fixed [6,20,21]. Not only the nature of material itself, but also
the chemical composition can tune the optical and electrical
properties of lead halide perovskites [22]. Hence, partial substitu-
tion of lead or doping with a homo- or heterovalent cations can
substantially change the photoelectronic properties of the
* Corresponding author. Dalian National Laboratory for Clean Energy, iChEM
(Collaborative Innovation Center of Chemistry for Energy Materials), Dalian Insti-
tute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China.
E-mail addresses: szliu@dicp.ac.cn, Liusz@snnu.edu.cn (S. Liu).
Contents lists available at ScienceDirect
Materials Today Energy
journal homepage: www.journals.elsevier.com/materials-today-energy/
http://dx.doi.org/10.1016/j.mtener.2017.06.009
2468-6069/© 2017 Elsevier Ltd. All rights reserved.
Materials Today Energy 5 (2017) 205e213