Ammonium-iodide-salt additives induced photovoltaic performance enhancement in one-step solution process for perovskite solar cells

Article abstract of DOI:10.1016/j.jallcom.2016.05.154

Unordered aggregation of perovskite particles on TiO₂ mesoporous film surface is a common problem in one-step solution process for fabricating perovskite solar cells (PSCs). This phenomenon is harmful for homogeneous dispersion of perovskite in mesoporous TiO₂ film and responsible for the low photovoltaic performance of corresponding perovskite solar cells at the same time. Delicate control of perovskite nucleation and growth is an effective route to solve this problem. In this work, we proposed a facile strategy to improve perovskite (CH₃NH₃PbI₃) growth by adding C₆H₅CH₂NH₃I (BAI) or NH₄I ionic compounds in perovskite precursor solution. We investigated perovskite crystal structure and morphology, optical and electrochemical properties of perovskite films or devices using different additives by XRD, SEM, UV-Vis, IPCE, and EIS. We found that these additives could decrease the grain size of perovskite crystal and diminish perovskite particle aggregation on TiO₂ film surface. This effect is benefit for electron transfer on perovskite/TiO₂ interface. Finally, perovskite solar cells using BAI or NH₄I additives obtain the best solar-to-electricity conversion efficiency of 9.05% and 9.49%, respectively, which are much higher than that of the pristine one, 6.83%.

Full text of DOI:10.1016/j.jallcom.2016.05.154

Accepted Manuscript
Ammonium-iodide-salt additives induced photovoltaic performance enhancement in
one-step solution process for perovskite solar cells
Y. Yang, J. Song, Y.L. Zhao, L. Zhu, X.Q. Gu, Y.Q. Gu, M. Che, Y.H. Qiang
PII:
S0925-8388(16)31487-6
10.1016/j.jallcom.2016.05.154
JALCOM 37674
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 21 January 2016
Revised Date: 11 May 2016
Accepted Date: 15 May 2016
Please cite this article as: Y. Yang, J. Song, Y.L. Zhao, L. Zhu, X.Q. Gu, Y.Q. Gu, M. Che, Y.H.
Qiang, Ammonium-iodide-salt additives induced photovoltaic performance enhancement in one-step
solution process for perovskite solar cells, Journal of Alloys and Compounds (2016), doi: 10.1016/
j.jallcom.2016.05.154.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Ammonium-iodide-salt Additives Induced Photovoltaic Performance Enhancement in
One-step Solution Process for Perovskite Solar Cells
Y. Yang, J. Song*, Y. L. Zhao, L. Zhu, X. Q. Gu, Y. Q. Gu, M. Che, Y. H. Qiang*
School of Materials Science and Engineering, China University of Mining and Technology,
Xuzhou 221116, China
Abstract
Unordered aggregation of perovskite particles on TiO2 mesoporous film surface is a
common problem in one-step solution process for fabricating perovskite solar cells
(PSCs). This phenomenon is harmful for homogeneous dispersion of perovskite in
mesoporous TiO2 film and responsible for the low photovoltaic performance of
corresponding perovskite solar cells at the same time. Delicate control of perovskite
nucleation and growth is an effective route to solve this problem. In this work, we
proposed a facile strategy to improve perovskite (CH3NH3PbI3) growth by adding
C6H5CH2NH3I (BAI) or NH4I ionic compounds in perovskite precursor solution. We
investigated perovskite crystal structure and morphology, optical and electrochemical
properties of perovskite films or devices using different additives by XRD, SEM,
UV-Vis, IPCE, and EIS. We found that these additives could decrease the grain size of
perovskite crystal and diminish perovskite particle aggregation on TiO2 film surface.
This effect is benefit for electron transfer on perovskite/TiO2 interface. Finally,
perovskite solar cells using BAI or NH4I additives obtain the best solar-to-electricity
conversion efficiency of 9.05% and 9.49%, respectively, which are much higher than
that of the pristine one, 6.83%.
Keywords: Perovskite solar cells; Additives; One-step process; Crystal growth
* Corresponding authors
E-mail addresses: jsoong@cumt.edu.cn, yhqiang@cumt.edu.cn

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1. Introduction
In the past few years, organic-inorganic lead halide perovskite based solar cells
(PSCs) have become a research hotspot, and its power conversion efficiency has
increased from 3% to as high as 20% [1-4], which is comparable to traditional
photovoltaic devices, such as mc/a-Si, CdTe, etc. Organic-inorganic lead halide
perovskite materials generally have suitable direct band gap (~ 1.5eV) [5], high light
absorption coefficient (> 104 cm-1) [5], fast electron and hole transfer rate, and long
carrier transport length (100~1000 nm) by modulating its chemical composition
[6-10]. All these superiorities make this material an outstanding light harvester for
photovoltaic devices. Perovskite material is infiltrated in mesoporous TiO2 film in
conventional perovskite solar cells. Electron-hole pairs generated under light
illumination in perovskite layer could be separated easily due to its low binding
energy (~50 meV). Whilst, as the suitable energy level of TiO2 and hole transfer
materials (HTM) to CH3NH3PbX3, electrons and holes could inject into TiO2 layer
and HTM layer, respectively, and finally they recombine in external circuit.
Organic-inorganic lead halide perovskite film could be prepared by
solution-processed and vapor-assisted techniques, such as one-step solution process
[11], sequential deposition process [12], dual-source thermal evaporation process [13],
vapor-assisted solution processes [14-16], and solvent post-treatment process [17], etc.
In all these techniques, one-step solution method is the simplest and cheapest one,
which has great potential in large-scale production.
It has been demonstrated that the morphology and crystallinity of perovskite films
can significantly affect the photovoltaic properties of devices [12, 13, 18].
Unfortunately, perovskite film fabricated by one-step solution method usually
presents inhomogeneous morphology [12]. The poor quality of perovskite layer would
lead to low light harvesting, poor charge carrier mobility and lifetime, and even
serious electron leakage. These disadvantages will induce low photovoltaic
performance of the device [2]. Therefore, specific techniques are required for
controlling perovskite crystal growth process. Additive-assisted method has been
demonstrated to be an effective approach to improve perovskite layer morphology

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[19-24]. By adding PbCl2 or CH3NH3Cl in the perovskite precursor solution, the
crystallinity of the resulting perovskite thin films could be enhanced greatly as the
addition of Cl can slow down perovskite crystallization rate during the annealing
process [6, 25, 26]. As a result, the mixed halide perovskites, CH3NH3PbIxCl3-x,
present greatly improved photo-physical properties compared to those derived from
pure iodide-based perovskite, especially in charge carrier diffusion process. HI
additive in perovskite precursor solution could prohibit the decomposition of
CH3NH3PbI3 and obtain pure perovskite film without PbI2 [27]. Solvent additives are
also applied to modulate perovskite crystal process. The use of a mixed solvent of
γ-butyrolactone and dimethylsulphoxide (DMSO) in perovskite precursor solution
could form homogeneous perovskite films via a CH3NH3I–PbI2–DMSO intermediate
phase which is favorable for retarding the rapid reaction between PbI2 and CH3NH3I
[28].
In this work, we detailedly investigated the influence of benzylammonium iodide
(C6H5CH2NH3I) and ammonium iodide (NH4I) additives on CH3NH3PbI3
crystallization behavior in one-step solution process. We found that the additives
consisted of different cations had varied impacts on perovskite crystal formation. By
adding proper amount of benzylammonium iodide or ammonium iodide, the
efficiencies of corresponding perovskite solar cells increased by 33% and 39%,
respectively, compared to the one without any additives.
2. Experimental Section
2.1. Synthesis of CH3NH3I and C6H5CH2NH3I
The methylammonium iodide (CH3NH3I, MAI) was synthesized in accordance with
an early report [23]. Firstly, hydriodic acid (30 mL, 57 wt % aqueous solution, J&K)
was slowly added into the methylamine solution (35 mL, 33 wt % in ethanol, J&K) in
flowing N2 atmosphere under stirring in an ice bath for 2 h. Afterward, the solution
was rotary evaporated at 60 °C for 1 h. The resulting white precipitate was collected
and re-dissolved in ethanol, and then recrystallized by adding diethyl ether in ethanol
solution. The powder was re-dissolved and re-crystallized for three times. Finally, the

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product was dried in vacuum at 60 °C overnight. The benzylammonium iodide
(C6H5CH2NH3I, BAI) was synthesized by the similar process except replacing
methylamine with benzylamine (20mL, 99%, J&K).
2.2. Preparation of perovskite precursor solution
To prepare the perovskite precursor solutions with or without additives, firstly,
MAI (0.75 mmol) and PbI2 (0.75 mmol, Sigma-Aldrich) powders were mixed in
anhydrous dimethylformamide (DMF, 1.5 mL, J&K), and then C6H5CH2NH3I or
ammonium iodide (NH4I, J&K) were added with molar rations of 0.05, 0.1, 0.2, and
0.3 to MAI. The final solutions were stirred at 70 °C until clear solutions were
obtained, and they were filtered before using in perovskite thin-film deposition.
2.3. Cell fabrication
FTO-coated glass (15 ꢀ per square, Nippon Sheet Glass) with 20*20 mm size was
etched by (Zn+HCl) solution to form two separated electrodes. The etched substrates
were cleaned by ultrasonication in an alkaline aqueous washing solution, and then
rinsed with deionized water, acetone, isopropanol, and ethanol, successively，as our
early report [29]. After drying, a dense blocking layer of TiO2 was deposited onto the
substrate by spin-coating a TiO2 colloid solution at 4500rpm for 30s and then
calcinated at 450oC for 1h. After cooling to room temperature, a mesoporous TiO2
film was fabricated by spin-coating at 5000rpm for 30s using commercial TiO2 paste
(Dyesol 18NRT) which was diluted in ethanol (1:3.5, weight ratio). The film was
prebaked at 100 oC for 30min, and then sintered at 500 oC for 1h in a muffle furnace.
o
The prepared TiO2 film was preheated at 70 C before spin-coating perovskite
precursor solutions. The perovskite film was deposited by spin-coating at 3000rpm for
30s using the prepared precursor solutions which were maintained at 70 oC during the
whole procedure. After that, the perovskite film was heated at 100 oC for 30min. After
cooling to room temperature, the hole transport material (HTM) layer was deposited
by spin coating HTM solution at 3000 rpm for 30 s. The HTM solution was prepared
by dissolving 72.3 mg (2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobi-
fluorene) (spiro-MeOTAD), 28.8ꢁL 4-tert-butylpyridine (TBP), 17.5 ꢁL of
bis(trifluoromethylsulphonyl)imide (LiTFSI) in acetonitrile (520 mg/mL) in 1 mL

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chlorobenzene. After standing at dried air over night, 80nm of silver was thermally
evaporated on top of the device to form the back contact.
2.4. Characterization
The structure and morphology of the as-prepared films were measured by X-ray
diffraction (XRD, D8 ADVANCE, Bruker), scanning electron microscopy (SEM,
1530VP, LEO). The photo-absorption properties of different perovskite films are
characterized by UV-Vis spectrophotometer (CARY 300 Conc). Photocurrent–voltage
(J–V) characteristic curves of PSCs were recorded by an electrochemical workstation
(Keithley, 2420 Source Meter). The cell was illuminated by a solar simulator (Oriel
Sol 3A, Newport) under 100 mW·cm-2 irradiation, calibrated by a standard silicon
solar cell (Oriel Instrument). A non-reflective metal aperture of 0.1 cm2 was used to
define the active area of the device and avoid light scattering through the sides. The
scan range was 1.0V to 0V. Electrochemical impedance spectroscopy (EIS) was
recorded using IM6ex electrochemical workstation. EIS spectra were measured in
dark, and the frequency range was from 100 kHz to 100 mHz with an AC modulation
signal of 10 mV and bias DC voltage of 0.60 V. The IPCE was measured using a
power source (Newport 300W Xenon lamp, 66902) with a monochromator (Newport
Cornerstone 260) and a power meter (Newport 2936-C).
3. Results and Discussion
3.1 Microstructure and optical property of perovskite films
XRD patterns of different perovskite films are shown in Fig. 1. For the pristine
perovskite film, peaks at 13.9°, 19.8°, 28.2°, 31.7°, 40.4°, and 42.8° are in good
agreement with the tetragonal phase of the CH3NH3PbI3 perovskite [28, 30],
corresponding lattice plane are marked in Fig. 1a. The sharp diffraction peaks indicate
that perovskite film has good crystallization. As the addition of BAI, the most
remarkable change is the variation of peak strength at 13.9°. For perovskite film using
BAI/MAI of 0.05, corresponding diffraction peaks are similar to the pristine one.
However, as BAI/MAI ratio increases, the peak at 13.9° turns weak and wide. The
relatively low diffraction intensity indicates small sized perovskite grains or poor

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crystallization. In the case of perovskite films using NH4I additive, the diffraction
peaks turn weak a little as the increase amount of NH4I additive, but they are
relatively sharper even when NH4I/MAI is up to 0.3 than BAI based perovskite films.
This result indicates that NH4I presents weaker influence on CH3NH3PbI3 perovskite
growth compared to BAI. The varied impact of BAI and NH4I on perovskite growth
+
may be induced by the larger ion size of BA+ than NH4 .
The scanning electron microscopy (SEM) is employed to observe surface
morphology of different perovskite films. As shown in Fig. 2, for the pristine
perovskite film without any additives, perovskite nanoparticles aggregate into
irregular rods and scatter on TiO2 film. The morphologies of perovskite films are
similar to the pristine one when additive of BAI or NH4I are few (BAI/MAI=0.05-0.1
and NH4I/MAI=0.05-0.2), except that the density of perovskite rods on TiO2 film
reduce slightly. When BAI/MAI ratio increases to 0.2, perovskite rods are cracked
into different sizes of pieces with relatively poor crystallization. As BAI/MAI ratio
increases further to 0.3, small perovskite particles gather densely. In regard to NH4I
additive, large perovskite rods disappear and particle aggregations arise when
NH4I/MAI ratio reaches to 0.3. Briefly, BAI and NH4I additives could diminish the
size of perovskite particles on TiO2 surface while weaken crystallinity of perovskite
films. This morphology transformation concurs with XRD patterns above. In fact,
pristine perovskite film with well-crystalline is formed due to the strong ionic
+
interactions under evaporation and convective process during spinning. BA+ and NH4
+
in precursor solution would intercalate between CH3NH3 and [PbI6]4- and prevent the
fast deposition of CH3NH3PbI3. The slow deposition process could suppress
agglomeration of perovskite grains, but reduce crystallization quality of perovskite in
the subsequent heat procedure. From XRD and SEM results, we find that BAI present
stronger ability in surpressing crystallization process of perovskite than NH4I additive.
+
It might derive from the larger size of BA+ than NH4 .
Fig. 3 shows the UV−vis absorption spectra of the perovskite films processed with
or without additives. Apparently, the perovskite films based on BAI additive have
higher absorbance than the pristine one. Usually, high coverage and continuous

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perovskite crystalline film could absorb more sunlight. However, we could see from
SEM images that coverage of perovskite crystalline films did not increase as the
addition of BAI. On the contrary, the surface coverage of perovskite films decrease as
the addition of BAI arised from 0.05 to 0.3. These results demonstrate that the
increased light absorbance should not be induced by improvement of perovskite film
coverage. In our opinion, the enhanced light absorbance may be attributed to the
better penetration of smaller perovskite grains in TiO2 porous film. In the case of
perovskite films based on NH4I additive, light absorbance is close to each other under
different amount of additive. It could be attributed to the unconspicuous change of
perovskite crystal according to XRD results. However, the light absorbance is
enhanced greatly in the long wavelength ranging from 550 to 750 nm. In fact, light
absorbance enhancement of perovskite film is the prerequisite for a high short-circuit
current in perovskite solar cells.
3.2 Photovoltaic performance of perovskite solar cells
The J-V curves of different devices are shown in Fig. 4, and the corresponding
photovoltaic performance parameters are summarized in Table 1. The pristine
perovskite solar cell exhibits a conversion efficiency of 6.83% with Jsc of 11.51
mA/cm2, Voc of 0.85 V, and FF of 0.70. As BAI is introduced in perovskite precursor
solution, corresponding devices show significant improvement in Jsc. The BAI
additive based devices achieve the best photovoltaic performance when BAI/MAI is
0.2, presenting a conversion efficiency of 9.05% with Jsc of 14.61 mA/cm2, Voc of
0.86 V, and FF of 0.72. Perovskite morphology optimization and light absorption
enhancement owing to BAI addition are the main reasons for photovoltaic property
improvement of the device. However, when BAI/MAI reaches to 0.3, conversion
efficiency of the device drop down dramatically to 7.84%, although the perovskite
film has high performance in light absorbance. This could be induced by the poor
crystallization of perovskite and residual impurity of BAI in perovskite film. For
NH4I additive based devices, the photovoltaic properties are enhanced when
increasing NH4I/MAI from 0 to 0.1, but declined as NH4I/MAI is raised further to 0.3.

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The best performance is obtained when NH4I/MAI is 0.1 with Jsc of 15.31 mA/cm2,
Voc of 0.86 V, FF of 0.72, and conversion efficiency of 9.49%. Compared to BAI
additive based PSCs, these devices present relatively higher Jsc, deriving from their
larger absorption in long wavelength region and comparable crystallinity to the
pristine one. Finally, perovskite solar cell using optimal amount of NH4I additive has
relatively higher photovoltaic performance than that based on BAI additive.
The incident photon-to-current conversion efficiency (IPCE) spectra are shown in
Fig. 5. The onset of photocurrent at 800 nm is consistent with the band gap of
CH3NH3PbI3 (~1.5 eV). The pristine device presents conversion efficiency from 45%
to 65% in the range of 400-700 nm. As BAI additive is introduced in precursor
solution, the quantum efficiency increase evidently. When BAI/MAI reaches to 0.2,
corresponding devices show the highest quantum efficiency, which is in accordance
with its highest Jsc value. For NH4I addtive based devices, similar results are
proceeded. However, quantum efficiency reduces more slowly from 450 nm to 700
nm for NH4I addtive based devices than BAI additive based devices and the pristine
one. This result should be attributed to the relatively stable absorbance of perovskite
films using NH4I additive from 550 nm to 750 nm, as shown in UV-Vis spectra.
Besides, we could see that quantum yield of perovskite solar cells do not always
coincide with light absorption strength of corresponding perovskite films from
UV-Vis and IPCE results. For example, perovskite film with BAI/MAI=0.3 exhibits
the highest absorbance in all the perovskite films, but the corresponding solar cell
presents a relative lower performance than the film using BAI/MAI=0.2. In fact,
quantum yield of solar cells concerns not only light harvesting of active layer but also
charge transfer process both in perovskite film and on TiO2/perovskite or
perovskite/HTM interfaces. As demonstrated in Fig. 1, perovskite film with
BAI/MAI=0.3 presents poor crystallinity, which is bad for the charge transfer in
perovskite film.
3.3 Electrochemical property of perovskite solar cells
To investigate the electrochemical properties of interfaces with or without different

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additive treatments, electrochemical impedance spectrometry (EIS) of the pristine cell,
BAI and NH4I based cells with the highest photovoltaic performance are conducted
under dark conditions with a bias voltage of 0.6 V in this work. As shown in Fig. 6,
Nyquist plots are usually composed of two irregular semicircles, including the small
one at high frequency and the large one at low frequency [31-33]. The equivalent
circuit is inserted in Fig. 6. The value of the starting point at the real part of the
Nyquist plot corresponds to the series resistance Rs. Rs is mainly related to the
resistance of FTO substrate [34, 35]. The first semicircle is attributed to the resistance
(R1) of hole transporting on the HTM/perovskite interface. The second semicircle in
the low frequency range is associated with the recombination resistance (R2) at the
TiO2/perovskite interface, a large R2 value indicates a hard recombination. The
devices with or without additive treatments exhibit a similar resistance Rs and R1.
The main difference of perovskite devices lies in R2. It could be observed that both
BAI additive-based and NH4I additive-based perovskite devices present a much
higher resistance (R2) than the pristine one. The increased resistance will retard
charge recombination at TiO2/perovskite interface. Because of the low recombination
rate at TiO2/perovskite interface, it is reasonable that additive-based PSCs show a
higher Jsc and VOC than that of the pristine one [36].
In fact, the decreased recombination rate at TiO2/perovskite interface is closely
related to morphology modification of perovskite film in the device. BAI additive
could diminish perovskite grains and further enhance the infiltration process of
perovskite in TiO2 pores. This modification would improve the interaction between
perovskite and TiO2 particles and promote the charge transfer in TiO2/perovskite
interface. For NH4I based perovskite film, perovskite grains present relatively smaller
changes in size compared to BAI. This result may be the main reason that NH4I based
perovskite solar cells have a smaller recombination resistance (R2) on TiO2/perovskite
interface than BAI based one. The light absorption enhancement of NH4I based
perovskite films in long wavelength is the main reason for their high photovoltaic
performance.

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4. Conclusion
We presented a facile procedure to improve the photovoltaic performance of
perovskite solar cell by introducing BAI or NH4I additives in perovskite precursor
solution in one-step solution process. Proper amount of BAI or NH4I additives in
precursor solution was crucial for perovskite crystallization and growth in TiO2
porous film. The devices showed the best performance when the molar ratio of
BAI/MAI and NH4I/MAI were 0.2 and 0.1, respectively. Perovskite films fabricated
under assistance of these additives presented better light harvesting property and low
charge recombination at perovskite/TiO2 interface. Finally, BAI or NH4I additives
based perovskite solar cells obtained solar-to-electricity conversion efficiency of 9.05%
and 9.49%, respectively, which were higher than that of the pristine one, 6.83%.
Acknowledgments
This work was supported by the Fundamental Research Funds for the Central
Universities (2015XKZD01).
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Figure captions
Fig. 1 XRD patterns of perovskite films using different amounts of BAI (a) or NH4I (b) additives.
Fig.2 SEM images of pristine perovskite film or films with BAI and NH4I additives. The values
mean molar ratio of BAI/MAI or NH4I/MAI. The scale bars are 1ꢁm.
Fig. 3 UV-Visible absorption spectra of different perovskite films based on BAI additive (a) and
NH4I additive (b).
Fig. 4 Current density−voltage curves of devices with different amounts of BAI (a) or NH4I (b)
additives in the perovskite films. All the devices were measured under illumination of 100
mW/cm2 and active area of 0.1cm2.
Fig. 5 IPCE spectra of perovskite devices based on BAI additive (a) and NH4I additive (b).
Fig. 6 Nyquist plots of PSCs based on pristine perovskite film, and films with BAI (BAI/MAI=0.2)
or NH4I additive (NH4I/MAI=0.1) in dark at 0.6 V. The inset is corresponding equivalent circuit.

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Table caption
Table 1 Photovoltaic parameters of perovskite solar cells based on different additives in perovskite
films. Jsc, Voc, FF, and η represent short-circuit current density, open-circuit voltage, fill factor,
and conversion efficiency, respectively.

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Samples
Jsc (mA·cm-2)
Voc (V)
0.85
0.83
0.85
0.86
0.86
0.86
0.86
0.83
0.84
FF
0.70
0.70
0.72
0.72
0.70
0.69
0.72
0.71
0.70
η (%)
6.83
7.18
8.11
9.05
7.84
7.91
9.49
8.69
7.54
Pristine One
11.51
12.26
13.41
14.61
13.67
13.24
15.31
14.86
12.83
BAI/MAI=0.05
BAI/MAI=0.1
BAI/MAI=0.2
BAI/MAI=0.3
NH4I/MAI=0.05
NH4I/MAI=0.1
NH4I/MAI=0.2
NH4I/MAI=0.3
Table 1

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(b)
(a)
BAI/MAI=0.3
NH4I/MAI=0.3
NH4I/MAI=0.2
BAI/MAI=0.2
BAI/MAI=0.1
NH4I/MAI=0.1
NH4I/MAI=0.05
BAI/MAI=0.05
TiO2
∆ FTO
∆
∆
∆
Pristine One
10
15
20
25
30
35
40
45
50 10
15
20
25
30
35
40
45
50
2Theta (degree)
2Theta (degree)
Fig. 1

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Fig. 2

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(a)
(b)
1.0
1.0
0.8
0.6
0.4
0.8
0.6
0.4
NH4I/MAI=0.05
NH4I/MAI=0.1
NH4I/MAI=0.2
NH4I/MAI=0.3
Pristine One
BAI/MAI=0.05
BAI/MAI=0.1
BAI/MAI=0.2
BAI/MAI=0.3
200
300
400
500
600
700
800
200
300
400
500
600
700
800
Wavelength / nm
Wavelength / nm
Fig. 3

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16
12
8
(a)
(b)
16
12
8
Pristine One
NH4I/MAI=0.05
NH4I/MAI=0.1
NH4I/MAI=0.2
NH4I/MAI=0.3
BAI/MAI=0.05
BAI/MAI=0.1
BAI/MAI=0.2
BAI/MAI=0.3
4
4
0
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Voltage / V
Voltage / V
Fig. 4

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(a) 80
(b)80
70
70
60
50
40
30
20
10
0
60
50
40
NH4I/MAI=0.05
NH4I/MAI=0.1
NH4I/MAI=0.2
NH4I/MAI=0.3
Pristine One
30
20
10
0
BAI/MAI=0.05
BAI/MAI=0.1
BAI/MAI=0.2
BAI/MAI=0.3
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength / nm
Wavelength / nm
Fig. 5

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2000
1500
1000
500
0
Pristine One
BAI/MAI=0.2
NH4I/MAI=0.1
0
500
1000
1500
2000
2500
3000
3500
4000
Z'/ O
Fig. 6

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Highlights
1. Perovskite films were deposited in one-step solution process using BAI and NH4I
additives
2. Photovoltaic performance increased by 33% and 39% for BAI and NH4I additive
based perovskite solar cells compared to the pristine one
3. BAI and NH4I additive could improve perovskite morphology and perovskite/TiO2
interface