DOI: 10.1002/ ((please add manuscript number))
Article type: Full Paper
Impact of monovalent cation halide additives on the structural and
optoelectronic properties of CH
3
NH
3
PbI
3
perovskite
Mojtaba Abdi-Jalebi, M. Ibrahim Dar,* Aditya Sadhanala, Satyaprasad P. Senanayak, Marius
Franckevičius, Neha Arora, Yuanyuan Hu, Mohammad Khaja Nazeeruddin, Shaik M. Zakeeruddin,
Michael Grätzel,* Richard H. Friend*
M Abdi-Jalebi, Dr. Aditya Sadhanala, Dr. S. P. Senanayak, Dr. Y. Hu, Prof. R. H. Friend
Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue,
Cambridge CB3 0HE, UK
E-mail: rhf10@cam.ac.uk; ibrahim.dar@epfl.ch; michael.graetzel@epfl.ch
Dr. M. I. Dar, Dr. N. Arora, Dr. S. M. Zakeeruddin, Prof. M. Grätzel
Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École
Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
Dr. M. Franckevičius,
Center for Physical Sciences and Technology, Savanorių Ave. 231, LT-02300 Vilnius, Lithuania
Prof. M. K. Nazeeruddin
Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and
Engineering, École Polytechnique Fédérale de Lausanne, CH-1015-Lausanne, Switzerland
2
Abstract
We report the influence of monovalent cation halide additives on the optical, excitonic and electrical
properties of CH
3
NH
3
PbI
3
perovskite. Monovalent cation halide with similar ionic radii to Pb
2+
, including
Cu
+
, Na
+
and Ag
+
, were added to explore possibility of doping. We observed significant reduction of sub-
bandgap optical absorption and lower energetic disorder along with a shift in the Fermi level of the
perovskite in the presence of these cations. The bulk hole mobility of the additive based perovskites as
estimated using the space charge limited current method exhibited an increase of up to an order of
magnitude compared to the pristine perovskites with a significant decrease in the activation energy.
Consequentially, enhancement in the photovoltaic parameters of additive-based solar cells was achieved.
We observed an increase in open circuit voltage for AgI (~1.02 vs 0.95 V for the pristine) and
photocurrent density for NaI and CuBr based solar cells (≈23 vs 21 mA.cm
-2
for the pristine). This
enhanced photovoltaic performance could be attributed to the formation of uniform and continuous
perovskite film, better conversion and loading of perovskite as well as the enhancement in the bulk charge
transport along with a minimization of disorder, pointing towards possible surface passivation.
Keywords: Monovalent cation halide, additives, CH
3
NH
3
PbI
3
Perovskite, doping, surface passivation
3
1. Introduction
Nowadays, organic–inorganic metal halide perovskites have been receiving tremendous attention
owing to their facile synthesis
[1]
, low temperature deposition
[2]
, capability to make flexible
devices
[3]
, and extraordinary optical and electronic properties
[4,5]
. Organic-inorganic metal halide
perovskites have a cubic framework structure with general formula ABX
3
(where A is an organic
cation, B a divalent metal ion and X a halide ion Cl, Br or I or any mixture thereof). Perovskite-
sensitized solar cell employing a liquid electrolyte was first documented by Miyasaka and co-
workers, and reported an efficiency of 3.8% for CH
3
NH
3
PbI
3
based solar cell
[1]
. In recent years,
perovskite solar cells (PSC) have shown a paradigm shift in photovoltaic technology, mainly by
adopting device configuration ranging from mesoscopic semiconducting TiO
2
[6,7]
or insulating
Al
2
O
3
scaffolds
[8]
, to the planar heterojunction (PHJ) architecture
[3,9]
. Recently, a certified power
conversion efficiency of over 20%
[10]
has been reported through optimizing device design,
material interfaces, processing techniques and chemical composition of perovskite materials
[11–13]
.
These recent developments further demonstrate the promising potential of PSC to compete with
silicon solar cells in the photovoltaic markets
[14]
.
In general, there are four methods, which includes one step spin deposition
[15]
, vacuum vapor
deposition
[9]
, two-step deposition technique
[6,16]
, and patterning thin film
[17]
to prepare the hybrid
organic-inorganic perovskite film. So far, solution processed PSC show the highest efficiency and
stability
[12,18]
. However, achieving good quality solution processed CH
3
NH
3
PbI
3
films on top of
mesoporous TiO
2
(ms-TiO
2
) with high uniformity and smoothness is a challenge. In many PSC, a
non-continuous perovskite film is usually obtained, where pinholes can introduce shunting
pathways limiting the device performance. Recent studies show that by upon addition of excess
organic component (methyl ammonium iodide) much larger crystalline domains can be created
[8]
,
and smoother films can be formed than those processed from a stoichiometric mixture of
CH
3
NH
3
I and PbI
2
[19]
. In addition, it is found that changing the anions from halide to acetate in
4
the lead source of perovskite solution has an effective influence on the perovskite crystal growth
and therefore improves the film quality
[20]
. On the other hand, hybrid CH
3
NH
3
PbI
3
doped with
Sn
2+
, Sr
2+
, Cd
2+
and Ca
2+
in the position of the Pb
2+
ion, are also known to affect both the
crystalline phase and the band gap energy
[21]
. A complete understanding of these issues is hence
critically important for advancing our understanding of perovskite semiconductors and solar cell
performance.
Despite the rapid rise in the PSC performance, the fundamental properties of organic–inorganic
trihalide perovskites pertaining to the formation of perovskite are not yet well understood.
Various investigations have focused on tuning the band gap of absorber material by changing the
ratio of cations
[22]
, anions
[5]
or the divalent metal
[23,24]
. But, the effect of precursor composition
on the perovskite crystal growth, film formation, coverage and thus on the device performance, is
yet to be investigated in detail
[20,25,26]
.
In this work, we explore the effect of adding small amount of monovalent cation halide based
salts including NaI, CuBr, CuI and AgI into the perovskite precursor solution on morphology,
charge transport, excitonic and optical properties of CH
3
NH
3
PbI
3
perovskite.
To the best of our
knowledge, the incorporation of CH
3
NH
3
PbI
3
with
aforementioned salts has not been reported so
far. Morphological characterization based on field emission scanning electron microscopy (FE-
SEM) determined that shape and coverage of the CH
3
NH
3
PbI
3
structures prepared in the presence
of additives is considerably different from an additive-free reference. In addition, samples were
characterized using X-ray diffraction (XRD) to study crystalline phases present in the samples
and the extent of lead halide conversion into perovskite in presence of additives. Comprehensive
studies on optical properties of additive based perovskite were carried out using photothermal
deflection spectroscopy (PDS), UV-visible absorption and photoluminescence (PL). In addition,
kelvin probe force microscopy (KPFM) and space charge limited current (SCLC) technique were
used to characterise influence of aforementioned additives on electrical properties of CH
3
NH
3
PbI
3
5
perovskite. Finally, fabrication of solar cells based on the incorporated monovalent cation in
perovskite structure reveals an improvement in power conversion efficiency (PCE) reaching
15.6% which can be ascribed to the improvement in the conversion reaction, optical, excitonic
and electrical properties of CH
3
NH
3
PbI
3
.
2. Results and discussion
In this study, CH
3
NH
3
PbI
3
perovskite samples were synthesized using sequential two-steps deposition
process
[6]
in which 0.02 mol.L
-1
of different additives including NaI, CuBr, CuI and AgI were added into
the 1.2 mol.L
-1
PbI
2
solution in N, N-dimethylformamide (DMF). In this procedure, the CH
3
NH
3
PbI
3
is
formed onto ~250 nm thick TiO
2
photoanode films by spin coating DMF solution of PbI
2
followed by dip
coating in a solution of isopropyl alcohol containing methylamonium iodide (MAI) under optimized
conditions.
2.1. Morphological characterization
To understand the impact of these additives on the surface morphology of the PbI
2
and
CH
3
NH
3
PbI
3
deposited on mesoporous TiO
2
photoanode, field emission scanning electron
microscopy (FESEM) was employed. The top view SEM image confirms that the mesoporous
TiO
2
is covered by an overlayer of PbI
2
(Figure 1, left-side). It is noteworthy that a significant
change in the morphology of PbI
2
overlayer is observed while adding NaI to its solution (Figure
1d) and a rough and highly porous overlayer of PbI
2
containing branched large crystals was
formed. This morphological difference was also evident from the macroscopic image (Figure S1a)
as the respective PbI
2
film including NaI is relatively more scattering compared to pristine lead
iodide film (Figure S1b).
In addition, the presence of CuBr does not make any significant difference in the morphology of
PbI
2
(Figure 1b) whereas, in case of CuI and AgI based samples, we, obtain a uniform and pinhole
free overlayer (Figure 1c, e). The right side of Figure 1 displays top-view SEM images of
CH
3
NH
3
PbI
3
films obtained after the conversion of pristine and additive based PbI
2
films.
