Thermally Induced Crystallization of High Quality CH3NH3PbI3 Film with Large Grains for Highly Efficient Perovskite Solar Cells

Article abstract of DOI:10.1002/chem.201605693

Recently, fully covered and smooth perovskite films could be fabricated by optimized coating methods; however, it is still hard to prepare perovskite films with large grain sizes and high crystallinity. Given the fact that thermal energy can promote crystallization, we combine high-temperature crystallization with the application of a solvent featuring a high boiling point, in order to produce high quality perovskite films with micrometer-sized grains. We further investigated the temperature dependence of the thermally induced synthetic strategy, whereby the grains become larger as the temperature is elevated. After solar cell device fabrication, the efficiency of the best cell can attain a high value of 15.53 % with reduced hysteresis behavior.

Full text of DOI:10.1002/chem.201605693

A Journal of
Accepted Article
Title: Thermal Induced Crystallization of High Quality CH3NH3PbI3
Film with Large Grains for Highly Efficient Perovskite Solar Cells
Authors: Yu Hou, Shuang Yang, Xiao Chen, Chunzhong Li, Huijun
Zhao, and Huagui Yang
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To be cited as: Chem. Eur. J. 10.1002/chem.201605693
Link to VoR: http://dx.doi.org/10.1002/chem.201605693
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Thermal InducedCrystallization of High Quality CH3NH3PbI3 Film
with Large Grains for Highly Efficient Perovskite Solar Cells
Yu Hou+,[a,b] Shuang Yang+,[a] Xiao Chen,[a] Chunzhong Li,[a] Huijun Zhao,*[b] and Hua Gui Yang*[a]
Abstract: Recently, fully covered and smooth perovskite films can
be fabricated by optimized coating methods, however, it is still hard
to prepare perovskite films with large grain size and high crystallinity.
Given the fact that thermal energy can promote crystallization, we
combine the application of solvent with high boiling point and high
temperature crystallization, which produce high quality perovskite
films with micrometer sized grains. We further investigated the
temperature dependence of the thermal induced synthetic strategy
that the grains become larger as the temperature elevated. After
solar cell device fabrication, the efficiency of the champion cell can
attain a high value of 15.53% with a reduced hysteresis behavior.
Organic-inorganic hybrid perovskite materials, which owe low
band-gap, high absorption coefficient and high charge mobility,
have triggered a revolution in solar cells.[1-5] Since the 9.7%
solid-state perovskite solar cells (PSCs), employing MAPbI3 (MA
Figure 1. Schematic illustration of the thermal induced growth of perovskite
films.
= CH3NH3) as light absorber and spiro-MeOTAD as hole
transport material, were reported in 2012. In only a few years,
[6]
the power conversion efficiency (PEC) of PSCs has attained a
certified value of 22.1%, reaching the standard of other
commercial thin film photovoltaic. In a typical device, the light
harvesting and carrier generation efficiency in perovskite layer
can almost reach the maximum value by adopting suitable film
thickness, and thus, a critical issue in improving higher efficiency
is to reduce the energy loss in charge transport.[7-9] MAPbI3 layer
in PSCs was normally prepared by solution one-step deposition,
sequential two-step deposition, vapor-phase deposition and so
on.[10-13] Among various deposition methods, sequential two-step
deposition has shown promise for fabricating high-efficiency
PSCs.[14] Unfortunately, the perovskite crystals in films prepared
by two-step method are small with grains at the sub-micron
scale. This kind of perovskite film with small crystals
demonstrates too much grain boundaries, vacancies, dislocation
or other crystal defects. All these defects would cause obvious
charge recombination and would also lead to the electrical
resistance in the whole film.[15,16]
In this paper, we demonstrate a hot sequential deposition with
α-terpineol as the solvent, in which process, thermal energy is
introduced in the reaction between PbI2 and CH3NH3I. It is clear
that thermal energy can promote the formation of large
perovskite grains with a mean size over 1500 nm. More
intriguingly, the films can also fully cover the substrate with
strong light absorbance of visible light. Solar cell devices based
on such film exhibits power conversion efficiency over 15%.
Such method would also provide new avenue for optimization of
perovskite films, and may also serve in other optoelectronic
applications.
The method was on the basis of the sequential deposition
method composed of initial preparation of PbI2 films and
sequential spin-coating of CH3NH3I solution under high
temperature. As depicted in Figure 1, the PbI2 film is pre-heated
to a high temperature, typically 200 oC and then quickly
transferred to the spin-coater to react with CH3NH3I solution. We
selected α-terpineol (TP) as the solvent which can has a high
boiling point of 220.85 oC. All perovskite films were deposited on
TiO2 compact layers.
Figure 2a show the surface characteristics of PbI2 films. As
expected, such films demonstrated an almost full coverage of
the substrate. The PbI2 crystals exhibit sheet-like shape with the
mean grain size of about 200 nm. To reveal the growth
mechanism, we quenched the film in pure TP solution at room
temperature with the reaction time of 20 s. The grain size
increased rapidly to about 1-4 μm whichshould be as the result
of high temperature treatment. In this process, the PbI2 was
partially dissolved and thus gave round shaped film with a
relative low surface coverage. After the complete reaction at 40
s, smaller grains around the large ones can be seen as the
further precipitation when the temperature gradually reduced.
[a]
Dr. Y. Hou,Dr. S. Yang, X. Chen,Prof.C. Li, Prof.H. G. Yang
Key Laboratoryfor Ultrafine Materials ofMinistryof Education
School ofMaterials Science and Engineering
EastChina Universityof Science and Technology
130 Meilong Road,Shanghai 200237,China
E-mail:hgyang@ecust.edu.cn
[b]
[+]
Dr. Y. Hou,Prof. H. Zhao
Centre for Clean Environmentand Energy
Gold CoastCampus,Griffith University
Queensland 4222,Australia
E-mail:h.zhao@griffith.edu.au
These authors contributed equally.
Supporting information for this article is given via a link at the end of
the document.
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Such phenomenon finally leads to the fully covered film with
microscale grains.[17]
Figure 3. (a) SEM images and (b) crystal size distribution of the as-prepared
perovskite films prepared under differenttemperature.
could not convert to tetragonal perovskite phase completely, and
turned to brown colour which may form a compact perovskite
layer on the surface. Such compact layer would hinder the
further solute diffusion and reaction in the bulk film. At 100 oC,
the grain size increased to 300 ± 81 nm which is similar to
o
pristine PbI2 films. When the reaction temperature is at 130 C,
the grains improved obviously to 699 ± 195 nm. Meanwhile,
small amount of lead was dissolved as indicated by the voids
between each perovskite particles. Then at 160 oC, the grains
reached the microscale with the mean grain size of 945 ± 219
nm, and the maximum size of 1522 ± 292 nm can be obtained at
200 oC. Regarding the film thickness of about 500 nm, the
aspect ratio of such grain is 3. We finally raised the reaction
temperature to 300 oC, and the obtained perovskite
filmsexhibited a low coverage and similar grain size of 1551 ±
295 nm. This may be because the violent dissolution of lead
under high temperature. Based on above results, the heat
treatment can promote the growth of perovskite film significantly
and arrive the maximum at 200 oC. Further higher temperature
would trigger the dissolution of lead and the anisotropic growth
perpendicular to the films.
We also compare the TP film with the conventional method
(expressed as Control) which uses the 10 mg/mL CH3NH3I
isopropanol solution under room temperature. The TP sample
was using 10 mg/mL CH3NH3I TP solution under 200 oC. As
shown in Figure 4, both films have around 100% coverage, but
with a considerable different grain size. The Control sample
delivers a mean grain size of 353 ± 96 nm, which is about only
one fifth of that of the as-prepared TP film.
Figure 2. SEMimages of (a) PbI2 film, (b) intermediate film and (c) perovskite
film by thermal induced growth.
The XRD patterns of the CH3NH3PbI3 films by using
conventional and hot deposition methods are shown in Figure
S1a (see Supporting Information). Diffraction peaks at 12.67°
can be assigned to lattice planes of hexagonal 2H PbI2 polytype
(space group: P3m1(164), JCPDS file No. 07-0235), which
implies the existence of trace amount of unreacted PbI2 in the
To investigate the influence of reaction temperature, we
carried out control experiments under different reaction
temperature. The SEM images of resulted films are shown in
Figure 3 and, to estimate the mean grain size, over 100 grains
are counted for each sample. When heated to 70 oC, the film
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Control sample.[18] This suggest that the thermal energy can
accelerate the conversion from PbI2 to perovskite phase even
the TP solution can not dissolve lead efficiently. The UV-vis
absorption spectra of TP film appears to have much stronger
light absorption capacity in the visible light range between 400-
800 nm (Figure S1b). In contrast, the control sample has two
absorption terrace between the wavelength of 400-450 nm and
750-800 nm as a result of remained PbI2 residue.
Figure 5. (a) Typical J-V curves under AM 1.5 simulated illumination and (b)
Power conversion efficiency distribution of the control and thermal induced
devices. (c) Photocurrent density and PCE as a function of time for the typical
device based on thermal induced perovskite film held at a forward bias of 0.77
V. The cell was placed in the dark prior to the start of the measurement. (d)
Hysteresis curves of a typical device based on thermal induced perovskite film.
the PCE by boosting the crystallinity of perovskite films, but also
possess lower hysteresis and good productivity.
In conclusion, we have demonstrated that the high-quality
CH3NH3PbI3 perovskite films can be fabricated by a hot
sequential deposition with α-terpineol as the solvent. By inducing
a high temperature condition, the grains can be increased from
hundreds of nanometers to over 1500 nanometers, and more
importantly, the film still maintains a nearly 100% coverage.
Such high-quality perovskite thin film led to an improved power
conversion efficiency of 14.46% than the conventional method of
merely 9.81% with less hysteresis. This synthetic strategy will
have great potential to provide new approach towards the goal
of high-performance, low-cost and reproducible devices
compatible with simple solution processes.
Figure 4. (a) SEM images and (b) crystal size distribution of the as-prepared
perovskite films byconventional and thermal induced methods.
The prepared perovskite layers were utilized in solar cells with
a regular planar heterojunction configuration (FTO/compact
TiO2/perovskite/HTM/Ag).[19,20]
The
photocurrent
density-
photovoltage (J-V) curves were measured under irradiation with
air mass (AM)-1.5 global simulated sunlight at an intensity of
100 mW cm–2 (Figure 5a).[21,22] All tests were measured from
forward bias to short circuit with a scan rate of 0.15 V s−1.[23,24]
The device based on TP film yields a higher open-circuit voltage
(Voc = 1015 mV), short-circuit current density (Jsc = 21.9 mA cm–
2), and fill factor (FF = 0.7) than the device based on Control film
(Voc = 1000 mV; Jsc = 18.9 mA cm–2; FF = 0.66). The device
based on TP film demonstrated a power conversion efficiency
(PCE) of 15.53%, whereas the PCE of device based on pristine
Control was 12.54%. The PCE distribution extracted from 20
devices are shown in Figure 5b. The mean PCE of solar cells
based on Control and TP are 9.81 ± 2.31% and 14.46 ± 0.58%,
respectively. To further confirm the reliability of above PCE, we
held the devices at a forward bias of 0.77 V as a function of time
to measure the stabilized power output (Figure 5c). As a result,
the exhibited photocurrent density stabilizes within seconds to
approximately 19.2 mA cm-2, yielding a stabilized PCE of 14.7%,
measured after 600 s.[18,25] The hysteresis behavior of this
device was also investigated. It was shown that the forward scan
delivered slightly lower PCE value (13.78%) than that of the
reverse scan (14.20%) under a scan rate of 0.15 V s−1 (Figure
5d). Therefore, the hot deposition method can not only enhance
ExperimentalSection
Materialssynthesis
CH3NH3I was prepared by a solution reaction. The reaction of 24 mL
methylamine solution (33 wt% in ethanol, Aldrich), 10 mL hydroiodic acid
(HI, 57 wt%, Aldrich) and 100 mL ethanol (99.7%, Shanghai Lingfeng
Chemical reagent) were carried out in 250 mL round bottomed flask
under nitrogen atmosphere at 0 oC for 2 h under stirring. The product was
recovered by evaporation at 50 oC for 2 h. The white power was formed
o
after washed with diethyl ether three times and finally dried at 60 C in
vacuum oven overnight.[26]
Device fabrication
Firstly, FTO glass substrates (Nippon Sheet Glass, Japan, 8 Ω/square)
were patterned by etching process using Zn powder and 2 M HCl diluted
in deionized water. Secondly, the etched FTO substrates were cleaned
by ultrasonication with deionized water for 20 min, acetone for 20 min,
and ethanol for 20 min. Then, mildly acidic solution of titanium
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isopropoxide in ethanol was coated on FTO substrates by the spin-
coating method at 3000 rpm for 30 s, followed by a thermal treatment at
500 °C for 30 min, to make a TiO2 compact layer. The PbI2 layer was
formed on TiO2 compact layer by spin-coating 1 M PbI2 (99.9985%, Alfa
Aesar)/N,N-dimethylformamide (DMF, 99.9%, Alfa Aesar) solution at
Keywords: crystallization • film deposition • micrometer size •
thermal energy • perovskite solar cell
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for ROCS, SEM, SRFDP, Fundamental Research Funds for the
Central Universities (WD1514301), China Postdoctoral Science
Foundation Funded Project (2015M581547, 2016T90342),
“Chen Guang” Project supported by Shanghai Municipal
Education Commission and Shanghai Education Development
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Natural Science Foundation of China (91534202), 111 Project
(B14018), Shanghai Sailing Program (16YF1402100) and ARC
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Entry for the Table of Contents
COMMUNICATION
A hot sequential deposition with α-
terpineol as the solvent is applied to
the deposition of perovskite films and
a mean crystal size over 1500 nm can
be obtained. Experimental results
clearly illustrates that thermal energy
can promote the formation of large
perovskite crystals and enhance the
efficiency of solar cells.
Yu Hou, Shuang Yang, Xiao Chen,
Chunzhong Li, HuijunZhao,* and Hua
Gui Yang*
Page No. – Page No.
Thermal Induced Crystallization of
High Quality CH3NH3PbI3 Film with
Large Grains for HighlyEfficient
Perovskite Solar Cells
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