Insight into Perovskite Solar Cells Based on SnO2 Compact Electron

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Insight into Perovskite Solar Cells Based on SnO Compact Electron-Selective Layer 2

Qingshun Dong, Yantao Shi, Kai Wang, Yu Li, Shufeng Wang, Hong Zhang, Yujin Xing, Yi Du, Xiaogong Bai, and Tingli Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00541 • Publication Date (Web): 15 Apr 2015 Downloaded from http://pubs.acs.org on April 21, 2015

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The Journal of Physical Chemistry

pubs.acs.org/JPCC

Insight into Perovskite Solar Cells Based on SnO2 Compact Electron-Selective Layer Qingshun Dong,† Yantao Shi,*,† Kai Wang,† Yu Li,|| Shufeng Wang,|| Hong Zhang,† Yujin Xing,† Yi Du,† Xiaogong Bai,† and Tingli Ma*,†,‡,§ †

State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, P. R.

China ‡

School Petroleum and Chemical Engineering, Dalian University of Technology, Panjin Campus, Panjin 124221, P. R.

China §

Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Fukuoka,

808-0196, Japan ||

Department of Physics, Peking University, 209 Chengfu Road, Beijing 100871, P. R. China

Abstract Considering the remarkable progress in photovoltaic performance, scholars have focused on perovskite solar cells (PSCs) over the recent two years. TiO2 thin film is a semiconductor with a wide band gap and usually used as an electron-selective layer (ESL) in PSCs. Although SnO2 exhibits higher conductivity than TiO2, its use as a compact ESL in PSCs has not been reported. In this study, nanocrystalline SnO2 thin film was prepared through sol-gel method and then characterized. The prepared SnO2 thin film was composed of small-sized tetragonal rutile nanocrystals. We applied the SnO2 compact ESL into PSCs and compared with that based on TiO2 thin layer. SnO2-ESL-based PSCs (S-PSCs) showed higher short-circuit current density and lower open-circuit voltage, fill factor, and conversion efficiency than the conventional TiO2-ESL-based PSCs (T-PSCs). Furthermore, the photovoltaic performance of S-PSCs was highly dependent on measurement means and this relationship was investigated and discussed in detail.

■ INTRODUCTION

typical 3rd generation solar cells1 that present the

With the advent of energy crisis, the proportion of

highest recorded PCE of more than 13 % to date.2

renewable energy in the energy mix has been increasing.

However, the commercialization of DSSCs is hindered

Solar energy has gained considerable attention among

by their poor durability caused by leakage and

many research institutions because of its clean,

evaporation of liquid electrolytes.3 These limitations

convenient, and inexhaustible characteristics. Solar

could be resolved by developing solid-state DSSCs or

energy can be efficiently converted into electricity by

similar PV devices.4-5

using photovoltaic (PV) devices, such as solar cells that

In 2012, the advent of solid organic-inorganic hybrid

endured three generations. The first two generations of

perovskite solar cells (PSCs) has motivated scholars to

solar cells are based on crystalline silicon and other

redesign solar cells for improved structure and

semiconductors and exhibit high power conversion

efficiency 6-11. Over the recent 2 years, the efficiency of

efficiencies (PCEs). Despite their advantage, the

PSCs has sharply increased close to 20 %.12-13 PSCs are

development and practical applications of these solar

usually composed of an FTO conductive substrate,

cells are limited by their tedious processing, high costs,

compact electron-selective layer (ESL), mesoporous

and environmental unfriendliness. The 3rd generation

scaffold layer (optional), organic–inorganic hybrid

solar cells exhibit many attractive characteristics, such

perovskite layer, hole-transporting material (HTM), and

as low cost, environmental friendliness, and simple

metal

fabrication. Dye-sensitized solar cells (DSSCs) are

photogenerated electrons from a photoabsorber 14 and is

electrode.

ACS Paragon Plus Environment

The

compact

ESL

extracts


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indispensable in PSCs with various configurations. A

compact SnO2 film. The concentration of SnO2 organic

previous study reported that TiO2 or ZnO nanocrystals

sol was regulated to 0.7 M, 0.5 M, 0.25 M and 0.125 M.

are effective ESL materials in PSCs.

15

As an oxide

We found that 0.5 M was the optimum concentration. A

semiconductor with a wide bad gap and similar

thin layer of compact anatase TiO2 was formed through

properties to TiO2 or ZnO, SnO2 is used as a

spin coating TiO2 organic sol on the clean substrates at

photoanode in DSSCs.16 SnO2 also exhibits carrier

3000 rpm for 30 s, followed by a sintering process in a

transport capability. Despite their potential, the use of

furnace at 450 oC for 2 h. TiO2 organic sol for TiO2

SnO2 as an ESL in PSCs has not been reported.

compact layer was prepared according to the reported

Therefore, the performance and PV characteristics of

procedure.18 Then, mesoporous TiO2 layer were

S-PSCs must be evaluated.

prepared by spin coating a commercial TiO2 paste

In this study, we prepared nanocrystalline SnO2

(Dyesol-18NR-T, Dyesol) diluted in ethanol (2:7,

through sol-gel technique, used SnO2 thin film as a

weight ratio) at 5000 rpm for 30s. After drying at 125

compact ESL in PSCs, and evaluated PV properties.

o

S-PSCs demonstrated higher short-circuit current

baked at this temperature for 30 min and cooled to

density (JSC) than conventional T-PSCs. Serious charge

room temperature. The mesoporous TiO2 films were

recombination resulted in lower open-circuit voltage

then infiltrated with PbI2 by spin coating at 5000 rpm

(VOC) and fill factor (FF) in S-PSCs than those in

for 5 s and dried at 75 oC for 30 min. After cooling to

T-PSCs, finally leading to lower PCEs. By comparison

room temperature, the films were dipped in a solution

with T-PSCs, the type of measurement method

of CH3NH3I in 2-propanol (10mg ml-1) for 60 s, rinsed

significantly affects the performance evaluation of

with 2-propanol and dried at 75 oC for 30 min The

S-PSCs.

peroviskite films were dried on a hot plate at 70 oC for

C, the TiO2 films were gradually heated to 500 oC,

30 min. The HTMs were then deposited by spin coating

■ EXPERIMENTAL DETAILS

at 3000 rpm for 30 s. HTM was prepared by dissolving

Materials and Reagents. Unless stated otherwise, all

72.3

materials were purchased from Sigma-Aldrich or Alfa

4-tert-butylpyridine, 17.5 ml of a stock solution of 520

Aesar and used as received. The TiO2 paste was

mg ml-1 lithium bis(trifluoromethylsulphonyl)imide in

purchased from Dyesol (18NR-T). Fluorine-doped tin

acetonitrile in 1 ml chlorobenzene. Finally, 50 nm Ag

oxide (FTO, 15 Ω/sq) was purchased from Pilkington,

was thermally evaporated on top of the device to form

in US. Spiro-MeOTAD was purchased from Lumtec.

the back contact. The active area of this electrode was

CH3NH3I was synthesized according to a reported

fixed at 0.06 cm2. All devices were fabricated in glove

procedure.17

box. the

and

28.8

ml

Characterization. The sample morphologies were

Fluorine-doped tin oxide (F: SnO2) coated glass (15

observed using scanning electron microscopy (SEM,

fabrication.

fabricate

spiro-MeOTAD

devices,

Device

To

mg

Ω/sq, Pilkington, US) was patterned by etching with Zn

FEI QUANTA 450). J–V curves of PSCs were

powder and 2 M HCl diluted in deionized water. The

measured using a Keithley 2601 source meter scanning

etched substrate was then cleaned with 2 % hellmanex

the devices at 80mV/s under simulated AM 1.5

diluted in deionized water, rinsed with deionized water,

illumination (100 mW cm-2, PEC-L15, Japan). IPCE

acetone, isopropanol and deionized water and dried

values were recorded using the monochromatic light

with clean dry air. A SnO2 compact ESL was formed

from a system made of a xenon lamp, a monochromator,

through spin coating SnO2 organic sol on the clean

and appropriate filters. Steady-state and time-resolved

substrates at 5000 rpm for 30 s, followed by a sintering

photoluminiscence (PL) measurements were acquired

process in a furnace at 450 oC for 2 h. SnO2 organic sol

using an optically triggered streak camera system

was prepared by dissolved SnCl2 •2H2O in absolute

(C5410,

ethyl alcohol, the solution was stirred on a magnetic

photoexcited using a 517 nm laser with a repetition rate

stirring apparatus for 3 h at the temperature of 80

of 76 MHz (Mira900, Coherent).19 X-ray diffraction

o

C ,then aged for 3 h at 30 oC, 24 h at room temperature,

(XRD) measurements were carried out using an

the SnO2 organic sol was then used for the coating of a

automatic X-ray diffractometer (D/Max 2400, Rigaku)

Hamamatsu).


All

the

samples

were

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with Cu K α radiation ( γ =0.154 nm).

incorporation

of

a

mesoporous

TiO2

electron-

transporting layer provides stable reproducibility with

■ RESULTS AND DISCUSSION

regard to PV performance. In this work, mesoporous and

TiO2 layers were used to fabricate T-PSCs and S-PSCs.

cross-sectional morphologies of the two types of

The cross-sectional SEM image of the PSC device is

compact ESL were characterized through SEM. Figure

shown in Figure 3, in which FTO, SnO2 compact ESL,

1a shows that the compact SnO2-ESL can completely

mesoporous TiO2, perovskite, HTM (Spiro-OMeTAD),

cover FTO, thereby avoiding the direct contact of the

and Ag electrode are clearly marked. The mesoporous

Morphology

and

structure.

The

surface

upper layers with FTO and minimizing the charge shunt pathway. The

Figure 2. X-ray diffraction patterns of the SnO2 compact layers. (b) Linear sweep voltammetry curves of TiO2 ESL /HTM/TiO2 ESL and SnO2 ESL/HTM/SnO2 ESL.

Figure 1. Top-view and cross-sectional SEM images of SnO2 compact ESL (a, b) and TiO2 compact ESL (c, d), respectively.

thickness

of

the

optimized

SnO2-ESL

was

approximately 100 nm as demonstrated in Figure 1b. By

contrast,

Figures

1c

and

1d

show

that

conventionally prepared TiO2-ESL (ca. 50 nm) can still cover the FTO substrate very well. From the images we

Figure 3. Cross-sectional SEM image of S-PSCs.

can found large SnO2 nanoparticles unable to fully cover FTO like TiO2. Therefore, we need a thick layer

TiO2 layer was sufficiently filled and capped with

than TiO2 to completely cover FTO. The crystallinity of

perovskite

the compact SnO2-ESL sintered at 450 °C was

Spiro-OMeTAD layer and Ag electrode were uniformly

investigated through X-ray diffraction as shown in

prepared.

CH3NH3PbI3,

on

top

of

which

the

Figure 2a. The SnO2 sample was identified as the

PV performance tests on S-PSCs. As demonstrated

tetragonal rutile phase in accordance with JCPDS

in Figure 3, two sorts of PSCs were fabricated using

41-1445 data, and the diffraction peaks at 2θ = 26.6°,

TiO2-ESL and SnO2-ESL, respectively. Current–voltage

33.9°, 37.9°, 51.8° correspond to the (110), (101), (200),

(J-V) properties were measured under AM 1.5G with 1

and (211) crystal planes, respectively. In figure 2b,

sun illumination (100 mW cm−2). The average PV

SnO2 shows a higher current density, which indicated

parameters of about 20 PSC devices for each group are

that SnO2 ESL possessed higher electrical conductivity

shown in Figure 4a and summarized in Table 1. Despite

than TiO2, favorable to the transport of electrons in

the high JSC, S-PSCs exhibited lower VOC and FF than

ESL. This result was consistent with the fact that SnO2

T-PSCs. For example, the two typical J-V curves in

demonstrated higher electron mobility than TiO2.

Figure 4b show that T-PSCs presented a JSC of 18.59

Among various methods in designing PSC devices,

mA cm−2, VOC of 0.95 V, and FF of 0.71, yielding a



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PCE of 12.5 %. Despite the slightly higher JSC (20 mA

attributed to the cell architectures in which mesoporous

cm−2) of S-PSCs than that of T-PSCs, the former still

TiO2 layers as framework were used for both

exhibited a lower VOC of 0.83 V, FF of 0.42, and PCE of

TiO2-ESL-based and SnO2-ESL-based devices. Thus,

7.02 %. To be noted, the difference in VOC could be

rather than TiO2 or

Figure 4. (a) J−V characteristics, (b) J-V characteristics curves and dark curves, (c) Incident photon-to-electron conversion efficiency (IPCE) spectrum of S-PSCs and T-PSCs, and (d) Photoluminescence decay curves of perovskite films prepared on SnO2 and on TiO2 compact ESLs. Table 1 Photovoltaic parameters of S-PSCs and T-PSCs VOC (V)

JSC(mA/cm2)

S-PSCs

0.84±0.02

T-PSCs

0.95±0.02

electrons from perovskite by conducting time-resolved

FF

PCE (%)

photoluminescence (PL) on a sample with a perovskite/

19.80±0.79

0.41±0.01

6.87±0.39

ESL/FTO structure. Figure 4d shows that the sample

18.92±1.56

0.70±0.02

12.67±1.22

ontaining SnO2-ESL displayed a faster rate of PL decay than the sample containing TiO2-ESL. This finding

SnO2 compact ESL, it were the mesoporous TiO2 layers

indicated

that contacted with CH3NH3PbI3 predominantly and

charge-transfer kinetics. The more efficient carrier

directly, in turn determining VOC. On the basis of

extraction in SnO2-based

smaller shunt resistance (Rsh) and higher dark-current

TiO2-based devices probably resulted from the higher

curves (Figure 4b), the lower VOC and FF of S-PSCs

carrier mobility or lower conduction band level of

were

SnO2-ESL. This advantage can be regarded as a factor

highly

associated

with

severe

charge

recombination that occurred at the interface between

that

SnO2-ESL

exhibited

faster

devices than that

in

that results in high JSC.

SnO2 and CH3NH3PbI3, similar to SnO2-based DSSCs

Figure 5 presents the J-V curves obtained through

in which faster interfacial electron recombination was

forward and reverse scan modes for PSCs fabricated

caused by more positive shift of SnO2 conductive band.

with SnO2- and TiO2-ESLs. The detailed PV parameters

The incident photon-to-current conversion efficiency

are summarized in Table 2. Hysteresis was also

(IPCE) spectra of S-PSCs and T-PSCs are shown in

observed in S-PSCs and T-PSCs as indicated by the

Figure 4c. S-PSCs performed better before 550 nm,

better PV performance evaluated from the reverse scan

whereas T-PSCs prevailed at 550 nm to 750 nm. In

than that from the forward scan. In T-PSCs, the JSC, VOC,

general, S-PSCs produced higher photocurrent than

and FF values obtained from the J-V curve of the

T-PSCs,

J-V

reverse scan were 18.12 mA cm−2, 0.95 V, and 0.71,

characterizations mentioned above. We also evaluated

respectively, yielding a PCE of 12.18 %. By contrast,

and compared the capability of the ESL to extract

the corresponding values from the forward scan were

in

accordance

with

the

JSC

in


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19.18 mA cm−2, 0.91 V, and 0.60, with a lower PCE of

because of the thinness of the cells and the relatively

10.54 %. In S-PSCs, the PCEs from the reverse and

low conductivity of TiO2 and spiro-OMeTAD21. Similar

forward scans were 7.43 % and 5.91 %, respectively.

to other studies, our experiment (Figure 6 and Table 3)

Such hysteresis and discrepancy may result from the

also showed that the influence of mask on the final

defect states in the perovskite absorbers and the

result of T-PSCs was less pronounced. T-PSCs

ferroelectric properties of perovskite or excess ions

measured with a mask provided a PCE of 10.97 %,

(e.g., iodide or methylammonium) in the device.

20

whereas those measured without a mask yielded a PCE of 11.18 %. By contrast, in the absence or presence of a mask, the photocurrent measured from the J-V curves and the subsequent PCE considerably differed in S-PSCs. As shown in Figure 6a, S-PSCs measured with a mask yielded a JSC of 19.22 mA cm−2, and those measured without a mask reached approximately 30.99 mA cm−2. This high but pseudo JSC has never been reported. As a consequence, PCEs measured under these two conditions were 6.30 % and 10.30 %, respectively. Moreover, no significant difference in VOC or FF was observed between the samples measured

Figure 5. Influence of scanning direction on those two kind

with or without a mask in PSCs fabricated with

of perovskite solar cells current voltage characteristics. Red

SnO2-ESL. Subsequent IPCE measurements exhibited

and black lines represent data measured with reverse scan and

similar regularity (Figure 6b). In the presence and

forward scan, respectively, where reverse scan defines a

absence of a mask, the maximum monochromatic light

measure from open-circuit to short-circuit and forward scan is

conversion efficiencies were 80 % and 130 %, respectively.

vice versa. The light intensity was AM 1.5G one sun

To

illumination.

disclose

the

underlying

cause

for

such

phenomenon, we designed and fabricated PSCs with Table 2 Solar Cell Parameters of SnO2-based and

different configurations. The schematic in Figure 7

TiO2-based solar cells in different scanning directions

shows the complete structure of PSCs, in which the

T-PSCs reverse scan

VOC (V)

JSC (mA/ cm2)

FF

PCE (%)

0.95

18.12

0.71

12.18

etched FTO was covered by ESL, mesoporous TiO2,

T-PSCs forward scan

0.91

19.18

0.60

10.54

Table 3 Solar Cell Parameters of S-PSCs in different

S-PSCs reverse scan

0.85

20.31

0.43

7.43

masking directions

S-PSCs forward scan

0.77

21.52

0.35

5.91

VOC (V)

JSC (mA/cm2)

FF

PCE (%)

S-PSCs w/o mask

0.86

30.99

0.39

10.30

S-PSCs w mask

0.82

19.22

0.40

6.30

performance. To obtain reliable PV data, we

T-PSCs w/o mask

0.93

16.49

0.71

10.97

incorporated some precautions, such as AM1.5G

T-PSCs w mask

0.92

17.28

0.70

11.18

Effect

of

measurement

conditions

on

PV

standard light source and a calibrated reference cell, in the measurement protocol.21 In general, solar cells used for

measurement

should

be

covered

with

a

non-transparent mask to expose the active area exclusively and eliminate additional contributions from the adjacent area. Previous studies indicated that different masking configurations affect DSSCs and polymer fullerene bulk heterojunction solar cells.21 Yang13 performed J-V measurements and found that the effect of using a mask is not significant in T-PSCs

Figure 6. (a) Current–voltage curves and (b) (IPCE) spectrum



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Page 6 of 9

of S-PSCs in the masking (circle) and no masking (square)

detailed PV parameters. In measurement without a

conditions.

mask (Figure 8a), JSC was as high as 28.30 mA cm−2 and resulted in a PCE of 9.83 %. In measurement with a mask (Figure 8b), JSC drastically reduced to 19.14 mA cm−2 with the corresponding PCE of only 6.39 %. In this study, this performance represented the true level of

S-PSCs

and

the

adjacent

regions

possibly

contributed to this widely accepted “active area” if a Figure 7. Schematic presentation of the device layout

mask was not used in the testing. Therefore, precisely identifying the type of adjacent regions that contribute

Table 4 Solar Cell Parameters of the S-PSCs under

mostly to the pseudo PV performance is important.

different testing configurations

After region II was disabled through scraping the active

Testing

layers (including HTM, perovskite, and mesoporous

VOC (V)

JSC (mA/cm2)

FF

PCE (%)

W/O Mask (a)

0.88

28.30

0.40

9.83

were 27.12 mA cm−2 and 9.51 %, respectively. These

W Mask (b)

0.82

19.14

0.41

6.39

values approximated to those in the case shown in

I + III (c)

0.89

27.12

0.40

9.51

Figure 8a. In this case, JSC and PCE returned to the

I (d)

0.84

19.50

0.40

6.47

level tested through standard method with a mask

III (e)

0.80

8.23

0.39

2.61

(Figure 8b) when region III was further covered (Figure

conditions

TiO2) with a blade (Figure 8c), the JSC and PCE values

8d). These experiments clearly showed that region III mostly contributed to the pseudo PV performance of S-PSCs. By contrast, region II almost provided no contribution to the PV performance. To prove our conjecture, we designed another cell as shown in Figure 8e; in this cell, region II was disabled and region I was Figure 8. Schematic presentation of the S-PSCs under

covered. Table 4 shows that only region III could

different testing configurations. (a) W/O mask, regions I, II,

achieve a JSC of 8.23 mA cm−2 and a PCE of 2.61 %.

III all expose to light; (b) a black mask covering the cell, only

We ascribed this phenomenon to the high conductivity

region I exposes to light; (c) region II has been scraped by a

of the SnO2 compact ESL because the 2D transport

knife (only compact ESL is left), region I and region III

channels along the parallel directions (to the substrate

expose to the light; (d) region III is covered by a black tape

or each functional layer) could be utilized to collect

and region I exposes to light; (e) region I is covered by a

electrons. Similar phenomena were also observed in

black tape and region III exposes to light.

bulk heterojunction and hybrid solar cells when highly conductive carrier transport materials were used.22

perovskite, HTM, and Ag electrode. Conventionally, only the region involving all of these function layers

■ CONCLUSION

worked as a true cell, namely, the “active area”. We

SnO2 compact ESL were prepared through a simple

speculated that all regions containing perovskite may

sol-gel technique and used for the first time as an ESL

potentially contribute to the PV performance.

in PSCs. The microstructures and PV properties of the

Theref-ore, three types of regions can be distinguished

fabricated

based on cell architecture (Figure 8a): the commonly

exhibited a higher JSC but lower VOC than conventional

accepted “active” area (overlap region containing all

T-PSCs. Consequently, the PCE of S-PSCs was inferior

function layers) (region I), region without covering

to that of T-PSCs. The PV performance of S-PSCs was

with Ag electrode (region II), and region with Ag

highly

electrode but without FTO (region III).

measurement without a mask, pseudo JSC exceeding 30

Figure 8 shows the schematic of different testing conditions for S-PSCs, and Table 4 summarizes the

S-PSCs

dependent

were

on

characterized.

measurement

S-PSCs

means.

In

mA cm−2 was achieved by S-PSCs. After careful examination,

the


apparent

“inactive”

regions

Page 7 of 9

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contributed to the final pseudo PV performance.

with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. 8.

■ AUTHOR INFORMATION

Etgar, L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.;

Liu, B.; Nazeeruddin, M. K.; Grätzel, M. Mesoscopic

Corresponding Authors

Ch3nh3pbi3/TiO2 Heterojunction Solar Cells. J. Am. Chem.

*T.M.: E-mail: [email protected]. Tel/Fax:

Soc. 2012, 134, 17396-17399.

+86-411-84986237.

9.

*Y.S.:

E-mail:

[email protected].

Tel/Fax:

Zhou, H.; Shi, Y.; Dong, Q.; Zhang, H.; Xing, Y.; Wang,

K.;

Du,

Y.;

Ma,

T.

Hole-Conductor-Free,

+86-411-84986237.

Metal-Electrode-Free TiO2/Ch3nh3pbi3 Heterojunction Solar

Notes

Cells Based on a Low-Temperature Carbon Electrode. J. Phys.

The authors declare no competing financial interest.

Chem. Lett. 2014, 5, 3241-3246. 10.

Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar

■ ACKNOWLEDGMENTS

Heterojunction Perovskite Solar Cells by Vapour Deposition.

This work was financially supported by the National

Nature 2013, 501, 395-8.

Natural Science Foundation of China (Grant No.

11.

51402036, 51273032 and 91333104), the International

Wu, M.; Ma, T. Low-Temperature and Solution-Processed

Science & Technology Cooperation Program of China

Amorphous Woxas Electron-Selective Layer for Perovskite

(Grant No. 2013DFA51000) and State Key Laboratory

Solar Cells. J. Phys. Chem. Lett. 2015, 755-759.

of fine chemicals (Panjin) (Grant No. JH2014009)

12.

project.

Seo, J.; Seok, S. I. Compositional Engineering of Perovskite

Wang, K.; Shi, Y.; Dong, Q.; Li, Y.; Wang, S.; Yu, X.;

Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.;

Materials for High-Performance Solar Cells. Nature 2015.

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Insight into Perovskite Solar Cells Based on SnO2 Compact Electron

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