Bi3+-doped CH3NH3PbI3: Red-shifting absorption edge and longer charge carrier lifetime

Article abstract of DOI:10.1016/j.jallcom.2016.11.125

Ion doping is an effective approach to improve the properties of hybrid perovskite CH₃NH₃PbI₃, such as chemical stability and solar absorption. Here Bi³⁺-doped CH₃NH₃PbI₃was synthesized via cooling crystallization process in aqueous solution. Along with the Bi³⁺doping, the bandgap of CH₃NH₃PbI₃could be significantly narrowed, by the maximum value of 140 meV at the optimal doping level of 1.6 M %. Transient photovoltage measurement (TPV) revealed the Bi³⁺-doped CH₃NH₃PbI₃could achieve a charge carrier lifetime of 280 μs, which was almost twice longer than that of pristine CH₃NH₃PbI₃(157 μs). The prolonged carrier lifetime of Bi³⁺-doped CH₃NH₃PbI₃was supposed to be attributed to its lower packing factor (PF), which was caused by the CH₃NH₃⁺vacancy (V_MA) after doping This work provides a new approach to tune the band gap and charge carrier lifetime of the hybrid perovskite CH₃NH₃PbI₃, which should be promising to further improve the performance of perovskite-based photovoltaic devices.

Full text of DOI:10.1016/j.jallcom.2016.11.125

Accepted Manuscript
3+
Bi -doped CH NH PbI : Red-shifting absorption edge and longer charge carrier
3
3
3
lifetime
Ruiqi Wang, Xian Zhang, Jianqiao He, Cheng Ma, Li Xu, Peng Sheng, Fuqiang
Huang
PII:
S0925-8388(16)33593-9
10.1016/j.jallcom.2016.11.125
JALCOM 39621
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 27 June 2016
Revised Date: 7 November 2016
Accepted Date: 9 November 2016
3+
Please cite this article as: R. Wang, X. Zhang, J. He, C. Ma, L. Xu, P. Sheng, F. Huang, Bi -doped
CH NH PbI : Red-shifting absorption edge and longer charge carrier lifetime, Journal of Alloys and
3
3
3
Compounds (2016), doi: 10.1016/j.jallcom.2016.11.125.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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ACCEPTED MANUSCRIPT
3
+
Bi -doped CH NH PbI : red-shifting absorption edge
3
3
3
and longer charge carrier lifetime
†
†
‡
†
§
§
*, †, ‡
Ruiqi Wang, Xian Zhang, Jianqiao He, Cheng Ma, Li Xu , Peng Sheng, and Fuqiang Huang
†
Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Rare Earth Materials
Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University,
Beijing 100871, China.
‡
CAS Key Laboratory of Materials for Energy Conversion and State Key Laboratory of High
Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy
of Sciences, Shanghai 200050, P. R. China.
§
State Key laboratory of Advanced Transmission Technology, State Grid Smart Grid Research Institute.

ACCEPTED MANUSCRIPT
ABSTRACT. Ion doping is an effective approach to improve the properties of hybrid perovskite
3+
CH NH PbI , such as chemical stability and solar absorption. Here Bi -doped CH NH PbI was
3
3
3
3
3
3
3
+
synthesized via cooling crystallization process in aqueous solution. Along with the Bi doping, the
bandgap of CH NH PbI could be significantly narrowed, by the maximum value of 140 meV at the
3
3
3
3+
optimal doping level of 1.6 molar %. Transient photovoltage measurement (TPV) revealed the Bi -
doped CH NH PbI could achieve a charge carrier lifetime of 280 µs, which was almost twice longer
than that of pristine CH NH PbI (157 µs). The prolonged carrier lifetime of Bi -doped CH NH PbI
3
3
3
3
+
3
3
3
3
3
3
+
was supposed to be attributed to its lower packing factor (PF), which was caused by the CH NH
3
3
vacancy (VMA) after doping This work provides a new approach to tune the band gap and charge carrier
lifetime of the hybrid perovskite CH NH PbI , which should be promising to further improve the
3
3
3
performance of perovskite-based photovoltaic devices.
Keywords. Hybrid perovskite, Doping, Band gap energy, Charge carrier lifetime.

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1
. Introduction
The organic-inorganic hybrid perovskite CH
extraordinary photovoltaic properties. Since the first report about its photovoltaic performance in
009,[1] the record of the photoelectric conversion efficiency (PEC) of the perovskite-based solar cells
PSCs) has been broken continuously in the past few years,[2-9], and the highest certified efficiency
3 3 3
NH PbI has attracted much scientific interests for its
2
(
now comes to 22.1%.[10] Recently a PCE of 19.6 % has been achieved with an aperture area exceeding
2
1
cm , which paves the way to realize highly efficient large-area PSCs for practical deployment. [11] In
addition to the facile deposition techniques, the suitable band gap value (1.52 eV), long carrier diffusion
length (over 175 µm in single crystals and over 1 µm in optimized thin film) are the main factors
determining the excellent photovoltaic performance of CH NH PbI . [4, 9, 11-18] Recent study revealed
3
3
3
that the photon recycling can make the travel distance of charge carriers even longer than the charge-
carrier diffusion length in perovskite layer [19] Moreover, the defect-tolerated talent of the perovskite
framework makes it possible to optimize its performance by isovalent doping at different sites. For
+
+
+
+
instance, monovalent cations (CH(NH
2
)
2
, Cs , Rb ) doping at the CH
3
NH
3
site can not only increase
-
the thermal stability, but broaden the absorption range of hybrid perovskites. [7, 20-26] Cl doping at the
-
I site can significantly improve the crystallization behavior and morphology of the polycrystalline thin
film.[27-35] Now the highest-efficiency perovskites are with mixed MA/FA cations and Br/I halides.[3,
2
+
2+
1
1, 36]. Besides, low-toxic bivalent ion Sn doping at the Pb site can also broaden the absorption
range of hybrid perovskites.[37-45] And the PCE of Sn-based planar perovskite solar cell now achieves
5%.[46] In contrast, the heterovalent doping is rare to be investigated. Ahmed et al. first reported the
1
3
+
3+
3+
2+
heterovalent doping of Bi , In or Au at the Pb site in CH NH PbBr by a rapid crystallization
3
3
3
3
+
method.[47] It was remarkable that Bi -doped CH
3
NH
3
PbBr
3
showed a narrower band gap, a
significant increase of carrier concentration and an electronic conduction transformation from p- to n-

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type semiconductor. Although the rapid crystallization method benefits the incorporation of heterovalent
dopant, it would also cause an explosive growth of defects in the crystal structure. Such high defect
density can impede the charge carrier diffusion. In order to suppress the formation of defects, doped
3
+
perovskite should be grown in near-equilibrium state. Al -doping was also reported to be able to
3
+
improve the quality of the perovskite film, as well as the PCE of related PSC.[48] However, Al
dopants are unlikely to be incorporated within the perovskite crystal lattice. Ion doping can always
modify the electronic structure of the host compound, along with the charge carrier properties. For
3 3 3
example, Cl-doping resulted in longer carrier lifetime of CH NH PbI -based thin film devices, because
it can improve the carrier transport characteristics of the interface between the perovskite layer and the
-
charge transport layers.[49] Theoretical calculations also showed that Cl doping can suppress electron-
2
+
hole recombination in perovskites.[50, 51] However, dopants such as Sn were evidenced to be able to
2
+
reduce the charge carrier lifetime of perovskite, because Sn -doping tended to result in defects which
could act as the non-radiative recombination centers.[46, 52]
Efficient charge separation and transportation can be expected in compounds with more open
structures due to their stronger internal electric field caused by larger lattice distortion. Packing factor
(PF) can be used to describe the structural opening degree, and lower PF always results in better
photoelectric related properties.[54-57] In order to decrease the PF of CH NH PbI , one strategy could
3
3
3
4
-
be the introduction of specific structural defects. Perovskite framework constructed by [PbI ] octahedra
6
is responsible for the charge carrier diffusion. Defects such as Pb vacancy (V ) or I vacancy (V ) can
Pb
I
+
impede the charge carrier diffusion and thus should be avoided. Cations CH NH make no contribution
3
3
+
to the electron transition between valence band and conduction band, thus the generation of CH
vacancy (VMA) can lower the PF of CH NH PbI without disturbing the charge carrier diffusion.
According to electroneutrality, lattice VMA can be generated if Pb sites are partially substituted by
3 3
NH
3
3
3
2
+

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3
+
trivalent or even more highly charged ions. Among the different heterovalent ions, Bi has the same
2
+
3+
electron configuration and similar ion radii with Pb .Thus Bi doping could be promising to lower the
PF of CH NH PbI and to improve its charge carrier properties.
3
3
3
3
+
Here we report the synthesis of Bi -doped hybrid perovskite CH NH PbI by slowly cooling
3
3
3
crystallization method. All the doped products well maintained the perovskite structure of CH NH PbI ,
3
3
3
and the highest doping content of 1.6 molar % can be achieved in our experiment. The band gap values
3
+
3+
decreased significantly along with the increasing of Bi content, and the products with the highest Bi
content showed a band gap of 1.38 eV which is 140 meV narrower than that of the pristine CH NH PbI .
3
3
3
3+
Transient photovoltage (TPV) results showed that the incorporation of Bi ions could nearly twice
3
+
increase the charge carrier lifetime of CH NH PbI , which implied that Bi doping could be an effective
3
3
3
strategy to promote the performance of PSCs.
2
2
. Experimental
.1 Reagent
Unless stated otherwise, all the starting materials were obtained from Sinpharm Chemical Reagent
Beijing Co. Ltd without further purifications. (i) Meghylamine (CH NH ) (40 wt.% in ethanol); (ii)
3
2
hydroiodic acid (HI) (57 wt.% in water); (iii) PbI
99.99%); (v) H PO (99.9%). CH NH I was synthesized in the following procedure. HI was dropped
into the equimolar aqueous CH NH in a flask placed in an iced bath for 2 h under stirring. Then the
2 2
(99%); (iv) Bismuth powder (99.99%); (iv) I
(
3
3
3
3
3
2
mixture was evaporated at 60 °C in a rotary evaporator to remove the solvent. The product was washed
with anhydrous diethyl ether for three times, then dried in a vacuum oven at 60 °C for 12 h. Bismuth
iodide (BiI ) was synthesized by heating element Bi powder and I in the radio 2:3 under 300 °C in an
3
2
evacuated quartz tubes for 10 h.

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3
+
2
3 3 3
.2 Synthesis of Bi -doped CH NH PbI .
The PbI powder and H PO were dissolved in aqueous HI to form a bright yellow solution at 100 °C,
2
3
3
then a certain amount of BiI
Stoichiometric amount of CH NH
C afforded highly crystalline product. The product was filtered from the mother liquor, then washed
with diethyl ether for three times. The final product was dried in a vacuum oven at 80 °C overnight.
3
was added and the color of the solution turned to red immediately.
3
3
I was added into the red solution. Slow cooling of the solution to 40
°
2
.3 Materials characterization
The phase purities of the samples were determined by powder X-ray diffraction (XRD). performed on a
Bruker D2 phaser diffractometer equipped with a monochromatized source of Cu Kα radiation (λ =
0
.15406 nm) at 4 kW (40 kW, 100 mA). The crystalline samples were ground into fine powder for the
measurement. The optical diffuse-reflectance spectra of different samples were measured on a UV-4100
spectrophotometer operating form 1500 nm to 400 nm at room temperature. BaSO was used as a non-
4
absorbing reflectance reference. Inductively coupled plasma optical emission spectrometry (ICP-OES)
3+
was used to quantify the Bi doped into the products.
2
.4 Transient Photovoltaic (TPV) measurement
TPV measurements were carried out in devices described by previous reports.[58, 59] The crystal
products were ground into fine powder then pressed into thin pellets. Each device like a parallel-plate
capacitor was composed of sample pellet placed on a ITO substance, a piece of mica and a platinum
wire electrode. The devices were illuminated by laser pulses (532 nm, 7 ns width from an Opolette
3
3
55(HE) LD laser). The energy of each laser pulse is 1.5
µ
J and the calculated density of photon is
11
-2
×10 /mm each pulse. A large resistance of 1 MΩ was connected in parallel with the devices to

ACCEPTED MANUSCRIPT
simulate the open circuit condition. The transient photovoltage was recorded by a 300 MHz Tektronix
digital phosphor oscilloscope. The transient photovoltage curves were fitted with single exponential
decay functions.
3
. Results and discussion
Ion doping has been proved to be an effective way to improve the performance of hybrid perovskite.
+
2+
-
Compared with isovalent dopants such as Cs for A site, Sn for B site and Cl for X site, heterovalent
dopants are more difficult to be introduced into the host crystal lattice and thus are rarely reported.
Cooling crystallization process has been developed into a mature method for either the growth of single
crystals or the synthesis of doped perovskites.[37, 40, 60] In this work we used this method to synthesize
3+
Bi -doped CH NH PbI . SEM images of the pure and doped CH NH PbI crystals are presented in
3
3
3
3
3
3
3
+
Fig.1. Obvious variations of the crystal morphology after doping imply the incorporation of Bi into the
perovskite structure.

ACCEPTED MANUSCRIPT
3 3 3
Fig.1. SEM images of (a) pure and (b)~(f) doped CH NH PbI , (b) ~ (f) for Bi % solution= 1%, 2%, 4%,
6
%, 10% respectively.
In order to check the phase purities of the products, their powder XRD patterns (Fig.2a) were
3
+
collected. All the Bi -doped samples show same XRD pattern with that of pristine CH NH PbI ,
3
3
3
indicating the perovskite structure is well maintained. The maintaining of the perovskite symmetry after
doping is the precondition for further improving the electronic properties of CH NH PbI .[16] Lattice
parameters calculated by powder XRD show no significant difference between the pristine and doped
3
3
3
samples with
a
around 8.86 Å and
c
around 12.63 Å. This is mainly resulted from the similar radii of
3
+
2+
3+
Bi and Pb and small amount of the dopant in the crystal. The accurate Bi contents of the doped
samples are studied by ICP-OES. Fig.2b shows the positive correlation between the Bi molar % in the
samples and those in the solution. Detailed Bi % values in solution and crystals are shown in Table S1 in
3
+
Supporting Information, and the Bi content of the optimal doped sample is 1.6 % with a nominal
composition 10% in solution. These results indicate that while the cooling crystallization is nearly an
3
+
equilibrium process, Bi ions can be incorporated into the crystal lattice of hybrid perovskite due to the
2
+
same electron configuration and similar ion radii with Pb ions. The perovskite framework can tolerate
2
+
3+
this doping and keep the host structure unchanged. Along with the decreasing temperature, Pb and Bi
ions show the same tendency to precipitate from the solution which acts as a strong force to incorporate
3
+
the Bi ions into the crystal lattice.

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3
+
Fig.2. (a) Powder XRD patterns of Bi -doped CH NH PbI , (b) Bi molar % in crystals as a function
3
3
3
of those in solution.
3
+
The optical absorption properties of Bi -doped CH
3
NH
3
PbI
3
samples were investigated by solid-state
NH PbI , a significant broadness of
absorption range can be observed in Bi -doped samples, as shown in Fig.3. The absorption edges show
diffuse-reflectance spectroscopy. Compared to the pristine CH
3
3
3
3+
3+
red shift along with the increasing amounts of Bi . The optimal doping (Bi % =1.6 %) has a band gap of
1
.38 eV, which is 140 meV narrower than that of pure CH NH PbI (1.52 eV) (Table S1 in Supporting
3
3
3
Information). This bandgap narrowing can be attributed to the generation of impurity bands within the
3
+
2+
3+
band gap. In Bi -doped CH NH PbI , parts of Pb could be substituted by Bi , considering their
3
3
3
physical similarities. That substitution results in Bi defects in the lattice. Yan et al. predicted that the
Pb
defects of BiPb tend to be shallow donors which could generate energy levels below the conduction band
3
+
3 3 3
minimum.[61] These energy levels in Bi -doped CH NH PbI could account for its enhanced optical
response property.

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3
+
Fig.3. Electronic absorption spectra of Bi -doped CH NH PbI , inset: the variation of band gap
3
3
3
energies as a function of Bi molar % in crystals.
By regulating the growth condition, either n- or p-type CH
3
NH
3
PbI
3
can be obtained, depending on
3
7
3+
the different dominated defects. It is worth noting that different growth conditions of Bi -doped
3+
CH NH PbI can also result in different absorption properties of the final products. The reported Bi -
3
3
3
doped CH NH PbI synthesized via a rapid crystallization process showed a 300 meV bandgap
3
3
3
3
+
3+
narrowing with Bi content of 0.3%,[47] while the doped sample we synthesized with 1.6% Bi
content only showed a 140 meV narrowing. We attribute this deviation to different concentrations and
varieties of defects in crystals grown under different conditions. Generally, faster crystallization leads to
higher concentration of defects in the crystals. Besides the BiPb defects, there are some other defects
2
+
such as Pb vacancy (V ) existing in the crystal synthesized by fast crystallization method. V can
Pb
Pb
result in mid-gap states which are responsible for the large extent of bandgap narrowing.[16, 53]

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However, the existence of VPb is always detrimental to charge transport because it breaks down the
perovskite framework. In contrast, slowly cooling crystallization can suppress the formation of defects
and produce more perfect crystals. Although the resulted sample has larger band gap, some undesired
defects such as VPb can be avoid. That is critical to maintain the completeness of the perovskite
framework and ensure the charge carrier diffusion.
Charge carrier lifetime
materials, and the formula LD = (kBT/e⋅
parameters. Longer always leads to longer LD thus better photovoltaic performance. In some cases, ion
τ
, mobility
µ
and diffusion length
D
L are three key parameters of photovoltaic
µ⋅τ
) (k : Boltzmann’s constant) shows the relationship of these
B
τ
doping can obviously influence the charge carrier behaviors of hybrid perovskite. For instance, by
-
adding Cl during the film fabricating, the final polycrystalline film would have longer carrier diffusion
length and thus exhibit better photovoltaic performance. In contrast with isovalent doping, hetervalent
doping cannot avoid inducing more defects into the crystal lattice. These defects could have different
3
+
effects on the charge carrier behavior. In order to clarify the impacts of dopant Bi toward the charge
3+
carrier lifetime, transient photovoltages (TPV) of samples with different Bi contents were measured.
All samples were assembled into devices with the same structure. The transient photovoltage curves
were fitted with single exponential decay functions. As shown in Fig.4a, the pristine CH NH PbI
3
3
3
possesses a photo-induced charge carrier lifetime of 157
µs. Along with the increasing amounts of
3
+
3+
dopant Bi , the carrier lifetime increases significantly until the Bi content reaches 0.8%. After that,
the curve comes to a plateau with a carrier lifetime around 280
µs which is almost twice longer than that
of the pristine CH NH PbI , as shown in Fig.4b. The non-normalized TPV values are shown in Fig.S1 in
3
3
3
Supporting Information. Theoretical calculations revealed that BiPb defect levels are just 173 meV lower
than the conduction band minimum which are too shallow to act as Shockley-Read-Hall non-radiative
4
1
recombination centers. Our samples were grown by slowly cooling method which could avoid the

ACCEPTED MANUSCRIPT
formation of V , and ensure the completeness of the carrier-transporting framework. A site vacancy
Pb
VMA can lead to a more open structure and thus decrease the PF of the compound which results in more
3
+
efficient charge separation and transportation. So both Bi doping and the slowly cooling method are
critical to achieving longer charge carrier lifetime.
3+
Fig.4. (a) Transient photovoltaic curves of the devices made with Bi -doped CH NH PbI . (b) the
3
3
3
variation of carrier lifetimes as a function of Bi % in crystals.
4
. Conclusions
3
+
In summary, Bi -doped CH NH PbI was successfully synthesized through a cooling crystallization
3
3
3
3
+
method. It was demonstrated that Bi -doping can not only extend the optical absorption range of the
hybrid perovskite, but also facilitate the process of charge separation and transportation. The prolonged
3
+
3 3 3
carrier lifetime of Bi -doped CH NH PbI could be ascribed to the reduced structural PF caused by
V . Longer carrier lifetime always means longer carrier diffusion length, which is critical for a
MA
superior absorber of solar cells. Our results provide a new approach to tune the bad gap and carrier
properties of perovskites, which is promising for further improving the performance of perovskite-based
photovoltaic devices.

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ASSOCIATED CONTENT
Supporting Information.
3
+
Bi molar % in solution and in crystals and band gap energies of related samples.
AUTHOR INFORMATION
Corresponding Author
*
E-mail: huangfq@pku.edu.cn Phone: +86 62758225
Notes
The authors declare no competing financial interest.
Acknowledgment
Financial support from 863 Program of China (Grant no. 2011AA050505), the NSF of China (Grants
nos. 91122034, 51125006, 51121064, 61376056), the CAS/SAFEA International Partnership Program
for Creative Research Teams, “Strategic Priority Research Program (B)” of the Chinese Academy of
Sciences (Grants XDB04040200), the Science and Technology Projects of State Grid
(SGRIDGKJ[2015]452) and Chinese Academy of Sciences (Grants nos.KJCX2-EW-W11, KGZD-EW-
3
03) are acknowledged.
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Highlights
3
+
Bi -doped CH NH PbI crystals were synthesized via a cooling crystallization
3
3
3
process in aqueous solution.
This doping lead to obvious bandgap narrowing.
3
+
The doping of Bi lead to an almost twice longer charge carrier lifetime.