Band alignment engineering towards 10.71%‐efficiency ...static.tongtianta.site/paper_pdf/7904266c-60b3-11e9-9c44-00163e08… · energy. Recently, there is an ever-increasing attention

www.chemsuschem.org

Accepted Article

A Journal of

Title: Band alignment engineering towards 10.71%-efficiency carbon-based, all-inorganic planar CsPbIBr2 perovskite solar cells

Authors: Weidong Zhu, Zeyang Zhang, Wenming Chai, Qianni Zhang,Dazheng Chen, Zhenhua Lin, Jingjing Chang, JinchengZhang, Chunfu Zhang, and Yue Hao

This manuscript has been accepted after peer review and appears as anAccepted Article online prior to editing, proofing, and formal publicationof the final Version of Record (VoR). This work is currently citable byusing the Digital Object Identifier (DOI) given below. The VoR will bepublished online in Early View as soon as possible and may be differentto this Accepted Article as a result of editing. Readers should obtainthe VoR from the journal website shown below when it is publishedto ensure accuracy of information. The authors are responsible for thecontent of this Accepted Article.

To be cited as: ChemSusChem 10.1002/cssc.201900611

Link to VoR: http://dx.doi.org/10.1002/cssc.201900611

http://crossmark.crossref.org/dialog/?doi=10.1002%2Fcssc.201900611&domain=pdf&date_stamp=2019-03-26

1

DOI: 10.1002/ ((please add manuscript number))

Article type: Full Paper

Band alignment engineering towards 10.71%-efficiency carbon-based, all-inorganic

planar CsPbIBr2 perovskite solar cells

Weidong Zhu,* Zeyang Zhang, Wenming Chai, Qianni Zhang, Dazheng Chen, Zhenhua Lin,

Jingjing Chang, Jincheng Zhang, Chunfu Zhang,* and Yue Hao

Dr. W. Zhu, Z. Zhang, W. Chai, Q. Zhang, Dr. D. Chen, Prof. Z. Lin, Prof. J. Chang, Prof. J.

Zhang, Prof. C. Zhang, Prof. Y. Hao

State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology & Shaanxi

Joint Key Laboratory of Graphene, School of Microelectronics, Xidian University, Xi’an,

710071, China.

E-mail: [email protected], [email protected]

Keywords: CsPbIBr2; band alignment; interfacial modification; all-inorganic perovskite solar

cells; efficiency

Perovskite CsPbIBr2 is particularly attracting ever-increasing attention for carbon-based,

all-inorganic solar cells, owing to its well-balanced bandgap and stability features. Yet, the

serious interfacial recombination of charge carriers in the cells has limited their efficiencies

significantly, which is intrinsically associated with the unwanted conduction band

misalignment between TiO2 electron transport layer (ETL) and CsPbIBr2 absorber. Herein,

we demonstrate the successful conduction band alignment engineering of TiO2/CsPbIBr2

heterojunction by modifying TiO2 ETL with CsBr clusters. Such modification triggers the

beneficial increase in conduction band minimum (CBM) from -4.00 to -3.81 eV and decrease

in work function from 4.11 to 3.86 eV for TiO2 ETL. It thus promotes the favorable band

alignment of the heterojunction, facilitating the markedly suppressed recombination as well as

improved extraction and transport of charge carriers. As a result, the carbon-based, all-

inorganic CsPbIBr2 solar cells exhibit over 20% enhancement in average efficiency. And, the

champion one achieves a record-high efficiency of 10.71% among pure CsPbIBr2-based cells,

coupled with superior open-circuit voltage (Voc) of 1.261 V and excellent stability. Hence, our

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem

This article is protected by copyright. All rights reserved.

2

work presents a facile way of band alignment engineering to attain record efficiency for pure

CsPbIBr2-based solar cells.

1. Introduction

Organic-inorganic hybrid halide perovskites have established themselves as efficient

absorber materials for photovoltaic cells that are named as perovskite solar cells (PSCs).[1,2]

In

less than a decade, the cells have already achieved the certified power conversion efficiency

(PCE) of 23.3%,[2,3]

thus setting great promise for effective delivery of inexpensive solar

energy. Recently, there is an ever-increasing attention to the PSCs with inorganic halide

perovskites because of their upgraded stability against heat, moisture, and light attacks.[4-

9,10,11-15] In particular, the carbon-based, all-inorganic PSCs free of hole-transport layers

(HTLs) have attracted the extra interest.[4-9,11,12,16]

The cells are built by substituting hybrid

halide perovskites with the more stable inorganic analogs, as well as replacing expensive

organic HTLs and noble-metal electrodes with cheap, inert carbon.[4-6]

Hence, they are

highlighted by not only excellent stability but also the specialties of cost-effectiveness and

simple fabrication.

Till now, various inorganic halide perovskites including CsPbI3,[9,13]

CsPbI2Br,[7]

CsPbIBr2,[6,14,15]

and CsPbBr3[4,5,8,11,16]

have been explored for carbon-based, all-inorganic

PSCs. Among them, CsPbBr3 offers a good phase stability but has an intrinsically wide

bandgap of ~2.3 eV that confines its absorption edge to ~540 nm.[4,8,11]

This character

questions the possibility of high cell efficiency. By contrast, CsPbI3 and CsPbI2Br show the

narrower bandgap (~1.73 eV and ~1.92 eV), but they easily transform into the undesired

yellow phase (δ phase) spontaneously in ambient room-temperature condition.[7,9,12,17]

Although a variety of strategies, including refined grains,[18]

doping,[19]

or surface termination

modifications,[20,21]

were proven useful for stabilizing the perovskite α phase, their stability is

still not satisfactory.[22,23]

Moreover, the extra high-density grain boundaries or long-chain

surfactants will lead to even inferior PCEs.[22]

As the compromise one, CsPbIBr2 shares a

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


3

narrower bandgap (~2.05 eV) than CsPbBr3 together with a better stability than both CsPbI3

and CsPbI2Br; thus it becomes a rather good choice in view of its well-balanced bandgap and

stability characters.[6,14,15,24-33]

At present, an impressive efficiency of 9.16% has been

achieved for carbon-based, all-inorganic PSC with pure CsPbIBr2 in our previous work.[24]

And, doping CsPbIBr2 with Mn2+

or Sn2+

has been shown to further boost the cell efficiency

to over 11%.[6,14]

In particular, some works have also suggested that the efficient cells can be

also fabricated at low a temperature such as 100 oC.

[15,21] In spite of these rapid progresses, the

PCEs achieved to date for carbon-based, all-inorganic CsPbIBr2 solar cells are still low,

leaving much room for further improvements.

In terms of carbon-based, all-inorganic PSCs, it is essential to obtain satisfactory

efficiency that reduces the recombination of charge carriers at electron transport layer

(ETL)/absorber interface.[23,34-36]

In general, interface recombination of PSC depends not only

on interfacial defects, but also more importantly on interfacial band structure,[37,38]

which is

both theoretically supported and experimentally demonstrated.[39,40]

For typical carbon-based,

all-inorganic CsPbIBr2 cells with the configuration of FTO/TiO2/CsPbIBr2/Carbon, there

exists large energy difference between the conduction band minimums (CBMs) of CsPbIBr2

absorber and TiO2 ETL,[14,24,27]

which causes an undesirable band alignment. Although such

large energy difference can boost the short-circuit current density (Jsc) of the cell, the electrons

in TiO2 ETL can easily back-transfer to the interface of TiO2/CsPbIBr2 heterojunction.[38]

They finally recombine with holes through interfacial deep-level defects, thereby causing

large Voc deficit. Similar phenomenon has also been demonstrated in other types of cells.[38,39]

To this end, the optimization of interface band alignment of TiO2/CsPbIBr2 heterojunction,

aiming to facilitating efficient interfacial extraction and injection of charge carriers, as well as

suppressing their recombination, becomes crucial to further improve the PCEs of carbon-

based all-inorganic CsPbIBr2 cells.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


4

Herein, we report the band alignment engineering of TiO2/CsPbIBr2 heterojunction in

carbon-based, all-inorganic planar CsPbIBr2 solar cells by modifying the TiO2 ETL with CsBr

clusters, targeting to improve their photovoltaic performance. Further studies show that CsBr

modifiers profitably trigger an increase in CBM from -4.00 to -3.81 eV and a decrease in

work function from 4.11 to 3.86 eV for TiO2 ETL, thus facilitating the favorable band

alignment the heterojunction. Consequently, the corresponding cells yield over 20%

enhancement in average efficiency. And, the best-performing one delivers a record-high PCE

of 10.71% among the pure CsPbIBr2-based cells reported so far, coupled with exceptional Voc

of 1.261 V and excellent stability. Furthermore, the detailed dynamic behaviors of charge

carriers in the cells are carefully studied to insight the attributions behind their superior

performance.

The preparation procedures for TiO2 ETL with CsBr modifiers are described in detail in

the Experimental Section. In brief, compact TiO2 layer that serves as ETL was firstly

deposited on the pre-cleaned FTO substrate by spin-coating TiO2 sol, followed by thermal

annealing at 480 oC for 60 min in air. Then, 100 μL 2.5 mg mL

-1 CsBr solution with isometric

deionized water and isopropanol as solvents was spin-coated on TiO2 ETL, and thermal

annealed at 90 oC for 30 min in air. Thus, the TiO2 ETL with CsBr modifiers was obtained for

further use. And, the TiO2 ETL without any modifiers was also prepared as control sample.

Hereinafter, we mark the TiO2 ETLs as well as the cells with and without CsBr modifiers to

“TiO2/CsBr ” and “TiO2”, respectively.

Figure S1 gives the atomic force microscope (AFM) topographical images of TiO2 and

TiO2/CsBr ETLs. The appearance of bright dots in TiO2/CsBr ETL indicates that CsBr salts

precipitate and aggregate to form clusters on TiO2 ETL rather than a continuous film after the

evaporation of solvents, similar to that of Cs2CO3 or Cs4SnO4.[41,42]

This can be further

corroborated by the AFM phase images in Figure 1a-b, wherein the randomly-scattered,

spheroidal clusters can be easilyfound from TiO2/CsBr ETL. Surficial compositions of the

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


5

concerned samples are studied by X-ray photoelectron spectroscopy (XPS). The results were

calibrated with C 1s peak energy of 284.5 eV. As presented in Figure S2a, clear characteristic

peaks of C, Ti, and O are detected and no others such as that from FTO substrate, can be

identified from TiO2 ETL, which is indicative of its full coverage to FTO substrate. Such

feature is highly imperative to avoid shutting paths in ultimate cell. And, it also excludes the

possible influence of surface coverage variation of TiO2 ETL after CsBr modification on

ultimate cell performance in our following discussion. For TiO2/CsBr ETL, similar

composition traits are observed from it, as displayed in Figure S2b. Besides, the characteristic

peaks corresponding to Cs element can be distinguished. The XPS core spectra in Figure 1c-d

confirm that there are a certain amount of Cs and Br elements on surface of TiO2/CsBr ETL

in contrast to TiO2 ETL. Together, these observations provide clear evidences to the

successful load of spheroidal CsBr clusters on TiO2/CsBr ETL.

Previous works have revealed that some interfacial modifiers could act as nucleation

sites and alter the crystal growth of upper perovskite photovoltaic films, finally enabling the

variation of their microstructures.[35,41,42]

In order to exclude whether CsBr modifier has the

similar effect, the morphology, crystallinity, and absorption property of resulting CsPbIBr2

films on TiO2 and TiO2/CsBr ETLs are studied. The films are deposited via one-step spin-

coating method with the modification of intermolecular exchange recipe proposed in our

previous work.[24]

We can see from Figure 2a-b that both CsPbIBr2 films are consisted of

closely-packed crystalline grains. And, no any pores can be found in them over the whole

scan areas. Moreover, there seems no obvious difference in grain sizes between them.

Likewise, XRD patterns and UV-vis spectra in Figure 2c-d suggest the identical crystallinity

and absorbance property of them.[24,25,27]

Thus, the potential influence of CsBr modifiers on

the microstructure of upper CsPbIBr2 film can be ignored rationally.

Thus far, the above studies reveal that CsBr modifiers are successfully loaded on

TiO2/CsBr ETL; moreover, they have a negligible influence on the upper CsPbIBr2 film.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


6

Hence, we focus our investigations on the energy band structure of TiO2 and TiO2/CsBr ETLs

below. On the one hand, Kelvin probe force microscopy (KPFM) was used to test their work

functions with the surface potential images shown in Figure 3a-b. It is clear that the surface

potential of TiO2/CsBr ETL is much higher than that of TiO2 ETL, indicating the decrease in

work function of TiO2 ETL after CsBr modification.[41,43]

Such phenomenon can be further

supported by the ultraviolet photoelectron spectroscopy (UPS) measurement results, wherein

the work function can be derived by subtracting the UPS Ecutoff from 21.22 eV.[14,22,27]

As

given in Figure 3c, the Ecutoff values are linearly fitted to 17.11 and 17.36 eV for TiO2 and

TiO2/CsBr ETLs, respectively. It again conveys a significant drop of work function from 4.11

to 3.86 eV of TiO2 ETL triggered by CsBr modification. We attribute such result to the

passivation surface states of TiO2 ETL by CsBr species or charge transfer between

them.[35,41,42]

Similar phenomenon was also demonstrated with Cs2CO3 and Cs4SnO4 that are

employed as surface modifiers for TiO2 ETL in both organic light emitting diodes and

photovoltaics. [35,41,42]

On the other hand, the valence band maximum (VBM) values are calculated to be 7.20

and 7.01 eV for TiO2 and TiO2/CsBr ETL according to VBM=21.22-(Ecutoff-Eonset),

respectively. Meanwhile, Figure S3 reveals that the samples possess similar absorption onsets,

indicating that optical bandgap of TiO2/CsBr ETL does not change in spite of obvious

decrease in work function. At such, the energy band structures of ultimate carbon-based, all-

inorganic planar CsPbIBr2 PSC with TiO2 and TiO2/CsBr ETLs can be established. The cell

configuration is illustrated in the left panel of Figure 3d, and the corresponding energy band

diagrams are given in the right panel. It can be seen that the CBM rises from -4.00 eV for

TiO2 ETL to -3.81 eV for TiO2/CsBr ETL. Thus, the CBM difference at the interface between

TiO2 ETL and CsPbIBr2 absorber is estimated to 0.51 eV. In contrast, the value for TiO2/CsBr

ETL significantly decreases to 0.32 eV.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


7

Overall, the above results manifest that TiO2/CsBr ETL has both smaller work function

and narrower interfacial CBM difference with respect to TiO2 ETL, hence it gives rise to the

better interfacial band alignment in ultimate cell. In detail, the smaller work function of

TiO2/CsBr ETL indicates a higher built-in electric field in the resulting cell. In turns, it

provides an enhanced driving force for separation of photo-generated electron-hole pairs as

well as an extended depletion region for substantially suppressing the recombination of

charge carriers, which are extremely desirable for obtaining high Voc and FF for the

cell.[22,34,35,41-43]

On the other hand, the properly narrower CBM difference is greatly beneficial

to reduce interface recombination of charge carriers, without any loss in photo-current

collection.[34,36-38]

Both these two factors can facilitate efficient interfacial extraction and

transport of charge carriers together with decreased their energy loss in carbon-based, all-

inorganic planar CsPbIBr2 cells. So, it can be expected that the cells fabricated with

TiO2/CsBr ETL will deliver a better efficiency.

Further, the photovoltaic properties of carbon-based, all-inorganic planar CsPbIBr2 PSCs

with TiO2 and TiO2/CsBr ETLs are carefully studied. First of all, the CsBr modification

condition is optimized according to ultimate cell efficiency. We vary the spin-coating speed

of CsBr solution from 5000, 4000, 3000, to 2000 rpm during the preparation of TiO2/CsBr

ETLs. And, photocurrent density-voltage (J-V) curves in Figure S4 reveal that 3000 rpm

enables the best cell efficiency. Therefore, this condition is adopted to prepare TiO2/CsBr

ETLs. In this manner, 25 cells with the TiO2 ETLs and optimized TiO2/CsBr ETLs are

fabricated, respectively. Figure 4a presents the statistic PECs of corresponding cells, which

were tested under standard AM 1.5G illumination scanning from 1.5 to -0.1 V with 100 mV s-

1 scanning rate. The average PCE for the cells fabricated with TiO2 ETLs is estimated to

7.88±0.45%. Remarkably, the value increase significantly to 9.57±0.50% for the ones with

TiO2/CsBr ETLs, yielding an over 20% enhancement.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


8

Figure 4b gives the J-V characteristics measured under reverse scan for the best-

performing cells with TiO2 and TiO2/CsBr ETLs, respectively. The cell with the former

exhibits the Jsc of 11.37 mA cm-2

, Voc of 1.212 V, and FF of 0.63, yielding a PCE of 8.68%

(reverse scan). In comparison, the one with TiO2/CsBr ETL achieves a much enhanced PCE

of 10.71% with the Jsc of 11.80 mA cm-2

as well as the particularly high Voc of 1.261 V and

FF of 0.72. To the best of our knowledge, such PCE is the highest value achieved for pure

CsPbIBr2-based PSCs reported so far, no matter the ones with or without HTLs, as listed in

Table 1.[6,14,15,21,24-33]

Under forward scan, the cells with TiO2 and TiO2/CsBr ETLs yield the

PCEs of 6.44% and 8.11%, respectively. These results indicate that J-V hysteresis inevitably

happpen in them, which seems to be a common phenomenon for CsPbIBr2 solar cells.[6,29]

Then, the photocurrent densities of the cells at their respective maximum power points were

tested as a function of time, as given in Figure 4c. The stabilized photocurrent densities of

9.32 and 9.82 mA cm-2

are obtained for the cells with TiO2 and TiO2/CsBr ETLs, respectively,

which correspond to the PCEs of 8.20% and 9.82%. It should be mentioned that the

photocurrent outputs of both cells are extremely stable during the relatively long-term

measurements in ambient atmosphere, suggesting an excellent operation stability of them.

Finally, the external quantum efficiency (EQE) characters of the cells are tested, as provided

in Figure 4d. Both of them exhibit a similar cut-off wavelength at ~608 nm, in well

accordance with their absorption spectra. By contrast, the cell with TiO2/CsBr ETL produces

the higher EQEs than the one with TiO2 ETL in the whole response region, further

consolidating its better photo-electric conversion ability. As shown in Figure S5, the

integrated Jsc values from the EQE spectra are 8.84 and 9.51 mA cm-2

for the cells with TiO2

and TiO2/CsBr ETLs, respectively. Both of them are lower than those of measured from J-V

curves, which is mainly ascribed to the lack of light soaking during the EQE

measurements.[6,21,25]

Light soaking that stabilizes the current density of the cell typically

takes a few seconds, which can be detected by steady-state photoluminescence (PL)

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


9

technology. As demonstrated in Figure S6, after the soaking of a 325 nm laser the I-rich

regions with smaller bandgap are formed in CsPbIBr2 film. The reduced bandgap could cause

the increase of current density during J-V measurement, while light soaking is absent in EQE

measurement. It is noting that although both the integrated Jsc are slightly lower, their

difference is in basic agreement with the J-V results. Overall, the above investigations

indicate that TiO2/CsBr ETLs can greatly boost the efficiencies of carbon-based, all-inorganic

CsPbIBr2 PSCs, in particular of Voc and FF.

In order to understand the improved photovoltaic performance, typical J-V features of

the cells with TiO2 and TiO2/CsBr ETLs are analyzed in depth. Figure 5a presents the

corresponding dark J-V curves. It is clear that the cell with TiO2/CsBr ETL has a much lower

leak current in contrast to the one with TiO2 ETL. It thus suggests that the recombination of

charge carriers is suppressed significantly in it, which is beneficial to improve its Voc and FF,

in concert well with the light J-V measurement results.[8,16,24,44] Then, the photocurrent

density (Jph)-effective voltage (Veff) features are studied. Jph is defined as Jph=Jlight-Jdark. Herein,

Jdark and Jlight are the current densities measured under dark and AM 1.5 G illumination,

respectively. And, Veff is expressed as Veff=V0 -Va, wherein V0 is the voltage when Jph is equal

to 0 mA cm-2

, and Va is bias voltage.[24]

Figure 5b shows that each Jph increases linearly and

becomes saturated finally with increased Veff, wherein the saturated Jph comes from all the

photo-generated carriers that are collected by electrodes. By contrast, the slightly higher Jsat

can be identified in the cell with TiO2/CsBr ETL. In consideration of the identical absorption

of CsPbIBr2 films in both cells, the higher Jsat means more effective extraction and transport

of charge carriers, which thus contributes to the higher Jsc of the cell with TiO2/CsBr ETL.

Overall, we can speculate that the impressive performance of the cells with TiO2/CsBr ETLs

mainly comes from the suppressed recombination as well as improved extraction and

transport of charge carriers in them.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


10

Aming to further insight the dynamics of charge carriers in the concerned cells, transient

photocurrent (TPC)/photovoltage (TPV) technology are used. The former can measure

extraction and transport of charge carriers, and the latter provides information on their

recombination process in solar cell.[20,24,36,45]

On the one hand, Figure 5c reveals the much

faster photocurrent decay of the cell with TiO2/CsBr ETL than the one with TiO2 ETL, which

is indicative of the more effective extraction and transport of charge carriers in the former.

Such desirable feature mainly originates from the properly narrower CBM difference at the

interface between TiO2/CsBr ETL and CsPbIBr2 as is evident by the UPS results, which is

beneficial to form a more favorable energy band alignment and thereby allows for energetic

extraction and transfer of charge carriers.

On the other hand, Figure 5d indicates the greatly slower photovoltage decay of the cell

with the TiO2/CsBr ETL in comparison with the one with TiO2 ETL. It means that the

recombination of charge carriers in the former is weakened significantly. This can be also

evident by the electrochemical impedance spectra (EIS) measurement. Figure 5e gives the

Nyquist plots of cells with TiO2 and TiO2/CsBr ETLs at a forward bias of 1 V under dark

condition. The clear arc is involved to the recombination of charge carriers at the

ETL/CsPbIBr2 interface, reflecting the recombination resistance Rrec.[8,36,43,44]

The cell with

TiO2/CsBr ETL yields larger Rrec than the one with TiO2 ETL, indicative of a lower

recombination rate. The suppressed charge carrier recombination could be connected to the

smaller work function of TiO2/CsBr ETL that endows a higher built-in electric field in the

cell.[35,41,43]

It thus creates a higher driving force for separation of photo-generated electron-

hole pairs along with a wider depletion region for significantly suppressing the recombination

of charge carriers, finally promoting higher Voc and FF. This can be supported by the

capacitance-voltage (C-V) measurements with the 1/C2-V plots being given in Figure 5f. For

the cell with TiO2 ETL, the built-in voltage (Vbi) is estimated to 1.27 V from the intercept of

the extrapolated 1/C2-V straight line with the V-axis, while the value increases remarkably to

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


11

1.56 V for the one with TiO2/CsBr ETL. This larger Vbi well consolidates the higher built-in

electric field in the cell with TiO2/CsBr ETL.[8,22,29]

Overall, all the above investigations

reveal that the excellent photovoltaic property of the cells with TiO2/CsBr ETLs can be

attributed to the suppressed recombination as well as improved extraction and transport of

charge carriers. Such desirable features inevitably rely on the favorable band alignment of

TiO2/CsPbIBr2 heterojunction realized by CsBr modification, which specifies to the narrowed

CBM difference at the interface between TiO2/CsBr ETL and CsPbIBr2, as well as the

decreased work function of TiO2/CsBr ETL.

2. Conclusion

In summary, in setting out to promote the band alignment of TiO2/CsPbIBr2

heterojunction for efficient carbon-based, all-inorganic PSCs, we carry out an innovative

study on the effects of modifying the interface between TiO2 ETL and CsPbIBr2 absorber

with CsBr species. It is found out that CsBr modifiers can profitably trigger an increase of

CBM of TiO2 ETL from -4.00 to -3.81 eV and a decrease of its work function from 4.11 to

3.86 eV. They thus promote the favorable band alignment of TiO2/CsPbIBr2 heterojunction

with suppressed recombination as well as improved extraction and transport of charge carriers.

Hence, the carbon-based, all-inorganic PSCs with them deliver an over 20% enhancement in

average efficiency. Notably, the champion one shows a record-high PCE of 10.71% among

the pure CsPbIBr2-based PSCs reported so far, coupled with outstanding Voc of 1.261 V and

excellent stability. Our work highlights the potential to achieve more efficient carbon-based,

all-inorganic CsPbIBr2 PSCs through careful optimization of interfaces by CsBr modification.

3. Experimental Section

Materials. All the materials and reagents such as PbBr2 (ultra-dry, 99.999%), CsI (ultra-

dry, 99.998%), PbCl2 (ultra-dry, 99.999%), and CsBr (ultra-dry, 99.9%), dimethylsulfoxide

(DMSO, anhydrous, 99.8+%), isopropanol (anhydrous, 99.5+%) were purchased from Alfa

Aesar and used without further purification. Patterned FTO glass substrates (Pilkington, TEC-

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


12

8, 8 Ω/sq) are received from Yingkou OPV Tech New Energy Co., Ltd., China. Conductive

carbon paste is purchased from Shanghai MaterWin New Materials Co., Ltd., China.

Preparation of TiO2/CsBr ETL. Firstly, FTO glass substrate with size of 2×2.5 cm2 was

ultrasonically cleaned with detergent, deionized water, acetone, and ethanol successively.

Then, compact TiO2 layer with the thickness of ~60 nm that acts as ETL was deposited on the

FTO substrate by spin-coating the normal TiO2 sol, which was thermal annealed at 480 oC for

60 min in air. Thus, the TiO2 ETL is obtained. For CsBr modification, 100 μL 2.5 mg mL-1

CsBr in isometric deionized water and isopropanol was directly spin-coated on TiO2 ETL at

3000 rpm for 60 s, and then thermal annealed at 90 oC for 30 min in air. Thus, the TiO2/CsBr

ETL was obtained for further use.

Deposition of CsPbIBr2 film. 70 μL CsPbIBr2 precursor that contains 330.0 mg PbBr2,

27.8 mg PbCl2, and 260.0 mg CsI in 1 mL DMSO was coated on TiO2 or TiO2/CsBr ETL at

1500 rpm for 20 s and 5000 rpm for 60 s. At the last 20 s, 20 μL CsI in methanol with the

concentration of 30 mg mL-1

was dropped onto the substrate for fulfilling the intermolecular

exchange. Finally, the crystalized CsPbIBr2 film was formed by annealed the sample at 280

oC for 10 min. It should be noted that during deopsition of CsPbIBr2 film, the DMSO solvent

in the precursor could not induce severe dissolution of CsBr modifiers, considering that the

precursor is almost saturated when 1 M PbBr2 and 1 M CsI are dissolved. This can also be

supported by Figure S7.

Fabrication of PSC. Carbon back-electrode with the active area of 0.09 cm2 was coated

on CsPbIBr2 film by screen-printing the conductive carbon paste at room temperature,

followed by heating treatment at 120 oC for 15 min. Thus, the carbon-based, all-inorganic

planar CsPbIBr2 solar cell was obtained for further characterizations.

A detailed description of characterizations of the sample together with the performance

measurement of the cells can be found in our previous works.[15,24]

Supporting Information

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


13

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

Weidong Zhu and Zeyang Zhang contributed equally to the work. All the authors gratefully

acknowledge the financial support from the National Natural Science Foundation of China

(61334002, 61804113, and 61874083), the Fundamental Research Funds for the Central

Universities (JB181107 and JBX171103), and the National Natural Science Foundation of

Shaanxi Province (2018ZDCXL-GY-08-02-02 and 2017JM6049).

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff))

Published online: ((will be filled in by the editorial staff))

References

[1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050; J.-P.

Correa-Baena, A. Abate, M. Saliba, W. Tress, T. J. Jacobsson, M. Grätzel, A. Hagfeldt,

Energy Environ. Sci. 2017, 10, 710; M. Saliba, J.-P. Correa-Baena, M. Grätzel, A.

Hagfeldt, A. Abate, Angew. Chem., Int. Ed. 2018, 57, 2554; J.-P. Correa-Baena, M.

Saliba, T. Buonassisi, M. Grätzel, A. Abate, W. Tress, A. Hagfeldt, Science 2017, 358,

739; J. Huang, Y. Yuan, Y. Shao, Y. Yan, Nat. Rev. Mater. 2017, 2, 17042.

[2] Y. Rong, Y. Hu, A. Mei, H. Tan, M. I. Saidaminov, S. I. Seok, M. D. McGehee, E. H.

Sargent, H. Han, Science 2018, 361, eaat8235.

[3] H. Zhang, M. Nazeeruddin, W. Choy, Adv. Mater. 2019, 31, 1805702

[4] J. Liang, C. Wang, Y. Wang, Z. Xu, Z. Lu, Y. Ma, H. Zhu, Y. Hu, C. Xiao, X. Yi, J. Am.

Chem. Soc. 2016, 138, 15829.

[5] X. Chang, W. Li, L. Zhu, H. Liu, H. Geng, S. Xiang, J. Liu, H. Chen, ACS Appl. Mater.

Interfaces 2016, 8, 33649.

[6] J. Liang, P. Zhao, C. Wang, Y. Wang, Y. Hu, G. Zhu, L. Ma, J. Liu, Z. Jin, J. Am. Chem.

Soc. 2017, 139, 14009.

[7] C. Dong, X. Han, Y. Zhao, J. Li, L. Chang, W. Zhao, Solar RRL 2018, 2, 1800139.

[8] X. Liu, X. Tan, Z. Liu, H. Ye, B. Sun, T. Shi, Z. Tang, G. Liao, Nano Energy 2018, 50,

201.

[9] S. Xiang, W. Li, Y. Wei, J. Liu, H. Liu, L. Zhu, H. Chen, Nanoscale 2018, 10, 9996; S.

Xiang, Z. Fu, W. Li, Y. Wei, J. Liu, H. Liu, L. Zhu, R. Zhang, H. Chen, ACS Energy Lett.

2018, 3, 1824.

[10] L. Zhang, B. Li, J. Yuan, M. Wang, T. Shen, F. Huang, W. Wen, G. Cao, J. Tian, J. Phys.

Chem. Lett. 2018, 9, 3646; J. Yuan, L. Zhang, C. Bi, M. Wang, J. Tian, Solar RRL 2018,

2, 1800188.

[11] J. Duan, Y. Zhao, B. He, Q. Tang, Angew. Chem., Int. Ed. 2018, 130, 3849.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


14

[12] J. Liang, J. Liu, Z. Jin, Solar RRL 2017, 1, 1700086.

[13] P. Wang, X. Zhang, Y. Zhou, Q. Jiang, Q. Ye, Z. Chu, X. Li, X. Yang, Z. Yin, J. You, Nat.

Comm. 2018, 9, 2225.

[14] J. Liang, Z. Liu, L. Qiu, Z. Hawash, L. Meng, Z. Wu, Y. Jiang, L. K. Ono, Y. Qi, Adv.

Energy Mater. 2018, 8, 1800504.

[15] W. Zhu, Q. Zhang, C. Zhang, Z. Zhang, D. Chen, Z. Lin, J. Chang, J. Zhang, Y. Hao,

ACS Appl. Energy Mater. 2018, 1, 4991.

[16] J. Duan, Y. Zhao, X. Yang, Y. Wang, B. He, Q. Tang, Adv. Energy Mater. 2018, 8,

1802346.

[17] C. Liu, W. Li, C. Zhang, Y. Ma, J. Fan, Y. Mai, J. Am. Chem. Soc. 2018, 140, 3825.

[18] A. Swarnkar, A. R. Marshall, E. M. Sanehira, B. D. Chernomordik, D. T. Moore, J. A.

Christians, T. Chakrabarti, J. M. Luther, Science 2016, 354, 92.

[19] Y. Hu, F. Bai, X. Liu, Q. Ji, X. Miao, T. Qiu, S. Zhang, ACS Energy Lett. 2017, 2, 2219.

[20] Y. Wang, T. Zhang, M. Kan, Y. Zhao, J. Am. Chem. Soc. 2018, 140, 12345.

[21] Y. Jiang, J. Yuan, Y. Ni, T. Jiu, M. Yuan, J. Chen, Joule, 2018, 2, 1356.

[22] J. Zhang, D. Bai, Z. Jin, H. Bian, K. Wang, J. Sun, Q. Wang, S. Liu, Adv. Energy Mater.

2018, 8, 1703246.

[23] J. K. Nam, D. H. Chun, R. J. K. Rhee, J. H. Lee, J. H. Park, Adv. Sci. 2018, 5, 1800509.

[24] W. Zhu, Q. Zhang, D. Chen, Z. Zhang, Z. Lin, J. Chang, J. Zhang, C. Zhang, Y. Hao, Adv.

Energy Mater. 2018, 8, 1802080.

[25] C. F. J. Lau, X. Deng, Q. Ma, J. Zheng, J. S. Yun, M. A. Green, S. Huang, A. W. Ho-

Baillie, ACS Energy Lett. 2016, 1, 573.

[26] Q. Ma, S. Huang, X. Wen, M. A. Green, A. W. Ho‐Baillie, Adv. Energy Mater. 2016, 6,

1502202.

[27] C. Liu, W. Li, J. Chen, J. Fan, Y. Mai, R. E. Schropp, Nano Energy 2017, 41, 75.

[28] W. Li, M. U. Rothmann, A. Liu, Z. Wang, Y. Zhang, A. R. Pascoe, J. Lu, L. Jiang, Y.

Chen, F. Huang, Adv. Energy Mater. 2017, 7, 1700946.

[29] N. Li, Z. Zhu, J. Li, A. K. Y. Jen, L. Wang, Adv. Energy Mater. 2018, 8, 1800525.

[30] M. Aamir, T. Adhikari, M. Sher, N. Revaprasadu, W. Khalid, J. Akhtar, J.-M. Nunzi, New

J. Chem. 2018, 42, 14104.

[31] J. Lin, M. Lai, L. Dou, C. S. Kley, H. Chen, F. Peng, J. Sun, D. Lu, S. A. Hawks, C. Xie,

Nat. Mater. 2018, 17, 261.

[32] J. Lu, S.-C. Chen, Q. Zheng, ACS Appl. Energy Mater., 2018, 1, 5872..

[33] Z. Guo, S. h. H. Teo, Z. Xu, C. Zhang, Y. Kamata, S. Hayase, T. Ma, J. Mater. Chem. A,

2019, 7, 1227-1232.

[34] H. Li, W. Shi, W. Huang, E.-P. Yao, J. Han, Z. Chen, S. Liu, Y. Shen, M. Wang, Y. Yang,

Nano Lett. 2017, 17, 2328.

[35] W. Li, W. Zhang, S. Van Reenen, R. J. Sutton, J. Fan, A. A. Haghighirad, M. B. Johnston,

L. Wang, H. J. Snaith, Energy Environ. Sci. 2016, 9, 490.

[36] L. Yan, Q. Xue, M. Liu, Z. Zhu, J. Tian, Z. Li, Z. Chen, Z. Chen, H. Yan, H. L. Yip, Adv.

Mater. 2018, 30, 1802509.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


15

[37] T. Minemoto, M. Murata, Sol. Energy Mater. Sol. Cells 2015, 133, 8.

[38] C. Ding, Y. Zhang, F. Liu, Y. Kitabatake, S. Hayase, T. Toyoda, K. Yoshino, T. Minemoto,

K. Katayama, Q. Shen, Nano Energy 2018, 53, 17.

[39] T. Minemoto, Y. Hashimoto, W. Shams-Kolahi, T. Satoh, T. Negami, H. Takakura, Y.

Hamakawa, Sol. Energy Mater. Sol. Cells 2003, 75, 121.

[40] T. Minemoto, T. Matsui, H. Takakura, Y. Hamakawa, T. Negami, Y. Hashimoto, T.

Uenoyama, M. Kitagawa, Sol. Energy Mater. Sol. Cells 2001, 67, 83.

[41] Q. Hu, J. Wu, C. Jiang, T. Liu, X. Que, R. Zhu, Q. Gong, ACS Nano 2014, 8, 10161.

[42] B. Zhao, G. Niu, Q. Dong, J. Liu, N. Li, J. Li, L. Wang, J. Mater. Chem. A 2018, 6, 23797.

[43] D. Yang, X. Zhou, R. Yang, Z. Yang, W. Yu, X. Wang, C. Li, S. F. Liu, R. P. Chang,

Energy Environ. Sci. 2016, 9, 3071.

[44] X. Liu, Z. Liu, B. Sun, X. Tan, H. Ye, Y. Tu, T. Shi, Z. Tang, G. Liao, Electrochim. Acta

2018, 283, 1115.

[45] J. Shi, Y. Li, Y. Li, D. Li, Y. Luo, H. Wu, Q. Meng, Joule 2018, 2, 879; H. Zhang, J. Xiao,

J. Shi, H. Su, Y. Luo, D. Li, H. Wu, Y. B. Cheng, Q. Meng, Adv. Funct. Mater. 2018, 28,

1802985.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


16

Figure 1. AFM phase images of (a) TiO2 and (b) TiO2/CsBr ETLs. Core-level (c) Cs 3d and

(d) Br 3d XPS spectra of TiO2 and TiO2/CsBr ETLs, respectively.

Figure 2. (a-b) SEM images, (c) XRD spectra, and (d) UV-vis adsorption spectra of CsPbIBr2

films deposited on TiO2 and TiO2/CsBr ETLs, respectively.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


17

Figure 3. KPFM images of (a) TiO2 and (b) TiO2/CsBr ETLs. (c) The corresponding UPS

spectra. The linear interpolations reveal the photoemission cut-off energy boundary (Ecutoff)

along with VBM onset (Eonset), respectively. (d) Schematic architecture of the cell, together

with the corresponding energy band diagrams with TiO2 and TiO2/CsBr ETLs.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


18

Figure 4. (a) Statistical PCEs of 25 independent cells with TiO2 and TiO2/CsBr ETLs,

respectively. (b) Light J-V characteristics measured under forward scan (FS) and reverse scan

(RS), (c) Steady-state current densities at maximum power outputs, and (d) EQE spectra for

the champion cells with TiO2 and TiO2/CsBr ETLs, respectively.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


19

Figure 5. (a) Dark J-V curves, (b) Jph-Veff curves, (c) TPC curves, and (d) TPV curves for the

typical cells with TiO2 and TiO2/CsBr ETLs, respectively. (e) The corresponding Nyquist

plots measured at a forward bias of 1 V and dark condition. (f) 1/C2-V plots of the same cells.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


20

Table 1. Summary of photovoltaic performance parameters for pure CsPbIBr2-based PSCs

both with and without HTLs reported so far.

Cell configuration Voc (V) Jsc(mA cm-2

) FF PCE (%) Refs.

FTO/TiO2(CsBr)/CsPbIBr2/

Carbon 1.261 11.80 0.72 10.71 This

work

FTO/c-TiO2/CsPbIBr2/Carbon 1.245 10.66 0.69 9.16 [24]

ITO/SnO2/CsPbIBr2/

spiro-OMeTAD/Ag 1.15 10.61 0.70 8.54 [21]

FTO/c-TiO2/m-

TiO2/CsPbIBr2/Carbon 1.08 12.32 0.62 8.25 [6]

FTO/c-TiO2/CsPbIBr2/

spiro-OMeTAD/Au 1.227 9.69 0.674 8.02 [28]

ITO/SnO2/C60/CsPbIBr2/

spiro-OMeTAD/Au 1.18 8.32 0.748 7.34 [29]

FTO/TiO2/CsPbIBr2/

spiro-OMeTAD/Ag 1.28 8.80 0.649 7.31 [32]

ITO/Passivated SnO2/

CsPbIBr2/Carbon 1.23 8.50 0.67 7.00 [33]

FTO/c-TiO2/CsPbIBr2/Carbon 1.142 9.11 0.63 6.55 [15]

FTO/c-TiO2/m-TiO2/

CsPbIBr2/spiro-OMeTAD/Au 1.121 7.9 0.70 6.30 [25]

FTO/c-TiO2/m-

TiO2/CsPbIBr2/Carbon 0.96 12.15 0.53 6.14 [14]

FTO/NiOx/CsPbIBr2/ZnO/Al 1.01 8.65 0.636 5.57 [31]

FTO/NiOx/CsPbIBr2/MoOx/Au 0.85 10.56 0.62 5.52 [27]

ITO/ZnO/CsPbIBr2/

spiro-OMeTAD/Ag 1.04 8.78 0.525 4.8 [30]

FTO/c-TiO2/CsPbIBr2/Au 0.959 8.7 0.56 4.70 [26]

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


21

The innovative modification of TiO2 electron transport layer (ETL) with CsBr can

enable the carbon-based, all-inorganic planar CsPbIBr2 perovskite solar cell with the record-

high efficiency of 10.71% among the pure CsPbIBr2-based cells reported so far. This mainly

benefits from the favorable band alignment of TiO2/CsPbIBr2 heterojunction as a result of the

elevated conduction band minimum and decreased work function of TiO2 ETL, which thus

effectively suppresse the recombination as well as improve the extraction and transport of

charge carriers.

Keywords: CsPbIBr2; band alignment; interfacial modification; all-inorganic perovskite solar

cells; efficiency

Weidong Zhu, * Zeyang Zhang, Wenming Chai, Qianni Zhang, Dazheng Chen, Zhenhua Lin,


Title: Band alignment engineering towards 10.71%-efficiency carbon-based, all-inorganic


ToC figure

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


22

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2016.

Supporting Information

Band alignment engineering towards 10.71%-efficiency carbon-based, all-inorganic


Weidong Zhu,* Zeyang Zhang, Wenming Chai, Qianni Zhang, Dazheng Chen, Zhenhua Lin,


Figure S1. AFM images of images of (a) TiO2 and (b) TiO2/CsBr ETLs, respectively.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


23

Figure S2. XPS survey spectra of TiO2 and TiO2/CsBr ETLs, respectively.

Figure S3. UV-vis spectra of TiO2 and TiO2/CsBr ETLs, respectively.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


24

Figure S4. Light J-V curves for the best-performing cells with TiO2/CsBr ETLs prepared

with varied spin-coating speeds of CsBr solution. The cells were fabricated from one batch.

Figure S5. Integrated Jsc of the champion cells with TiO2 and TiO2/CsBr ETLs, respectively.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


25

Figure S6. PL spectra of CsPbIBr2 film excited with an excitation of 325 nm laser, which

were collected under different laser soaking time.

Figure S7. Optical image of CsPbIBr2 precursor after adding of 5 mg/mL CsBr. It is clear

that CsBr is insoluble in the precursor.

10.1002/cssc.201900611

Acc

epte

d M

anus

crip

t

ChemSusChem


Documents

Band alignment engineering towards 10.71%‐efficiency ...static.tongtianta.site/paper_pdf/7904266c-60b3-11e9-9c44-00163e08… · energy. Recently, there is an ever-increasing attention