Highly Efficient and Stable Sn-Rich Perovskite Solar Cells by

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Highly Efficient and Stable Sn-rich Perovskite Solar Cells by Introducing Bromine Seojun Lee, and Dong-Won Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04011 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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ACS Applied Materials & Interfaces

Highly Efficient and Stable Sn-rich Perovskite Solar Cells by Introducing Bromine Seojun Lee1, and Dong-Won Kang1,* 1

Department of Solar & Energy Engineering, Cheongju University,

Cheongju, Chungbuk 28503, Republic of Korea

Abstract Compositional engineering of recently-arising methylammonium (MA) lead (Pb) halide based perovskites is an essential approach for finding better perovskite compositions to resolve still remaining issues of toxic Pb, and long-term instability, etc. In this work, we carried out crystallographic, morphological, optical, and photovoltaic characterization of compositional MASn0.6Pb0.4I3-xBrx by gradually introducing bromine (Br) into parental Pb-Sn binary perovskite (MASn0.6Pb0.4I3) to elucidate its function in Sn-rich (Sn : Pb = 6 : 4) perovskites. We found significant advances in crystallinity and dense coverage of the perovskite films by inserting the Br into Sn-rich perovskite lattice. Furthermore, light-intensity-dependent open circuit voltage (Voc) measurement revealed much suppressed trap-assisted recombination for proper Br-added (x=0.4) device. These contributed to attain unprecedented power conversion efficiency of 12.1% and Voc of 0.78 V, which are, to the best of our knowledge, the highest performance in the Sn-rich (≥60%) perovskite solar cells reported so far. In addition, impressive enhancement of photocurrent-output stability and little hysteresis

were

found,

which

paves

the

way

for

the

development

of

environmentally-benign (Pb-reduction), stable monolithic tandem cells using the developed low bandgap (1.24-1.26 eV) MASn0.6Pb0.4I3-xBrx with suggested composition (x=0.2-0.4).

Keywords: Perovskite, lead (Pb)-reduction, Sn-rich, bromine, solar cell

ACS Paragon Plus Environment


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*Corresponding Author: Dong-Won Kang, Ph.D. ([email protected], office: +82-43-229-8558, fax: +82-43-229-8523)


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Introduction Recently, perovskite solar cells (PVSCs) are getting more attentions due to their excellent power conversion efficiencies (PCEs) over 20% as well as potential advantages such as expected low manufacturing-cost and simple device fabrication employing solution processing.1-7 These strong benefits motivated researchers to proceed commercialization of PVSCs. However, there are some remaining issues that should be resolved before the industrialization: 1) lead (Pb)-free perovskites for environmentally-benign properties,8-15 2) long-term reliability,16, 17 and 3) large-area uniform coating for up-scaled photovoltaic (PV) module production.18 As for the Pb-free perovskites studied in this work, Sn is one

of

the

promising

representatives

for

substituting

Pb

since

its

environmentally-benign nature and similar properties with Pb in terms of similar ionic radii by relativistic effects (Sn2+ 1.35 Å and Pb2+ 1.49 Å), which can enable substitution without considerable lattice perturbation.8,10,11,19,20 However, Snbased perovskites without Pb is seriously unstable due to the rapid Sn-oxidation (Sn2+ → Sn4+), and the highest efficiency to date is limited to approximately 6%.10 As the alternatives to Pb, Copper (Cu)-based or bismuth (Bi)-based perovskites are also reported due to their stability against air-ambient in contrast to Sn.12-15 On the other hand, their PV performance reported is much lower than Sn-based devices, and further study should be made. From those research background and current status, we believe that Sn-Pb binary metal hybrid perovskites can be one of the promising candidate which practically reduces Pb utilization with sustaining suitable PCE of PVSCs in terms of a short-term roadmap. With replacing 25-50% of Pb by Sn in the Pb-Sn binary



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PVSCs, appropriate PCEs ranging 12-17% have been reported.19-23 In case of the increased Sn ratio over 50%, the previous studies found that Sn-oxidation and decomposition of perovskite were much severe compared to Pb-only perovskites, and thus it degraded surface morphologies and overall PCE only 2-6% for Sn-Pb binary PVSCs (Sn > 50%).19-21 Also, there have been very few studies reporting successful fabrication of Sn-rich (Sn>60%) PVSCs to date. However, a very recent work showed the best PCE about 10% with methlyammonium (MA) Pb-Sn binary (Pb : Sn = 4 : 6) iodide (MAPb0.4Sn0.6I3).24 On the other hand, the devices they made were processed at the temperature ranges of 180-240℃ which are much higher than the typical inverted PVSCs processing at 100-150℃.25 The low toxic Pb-Sn binary PVSCs (Pb < 50%) need also to be made at low temperatures due to essential merits of low-cost and flexible applications. In this work, we focus on the Sn-rich binary PVSCs (Sn : Pb = 6 : 4) processed at the typical low temperatures(50%) incorporation instead of Pb.21,29,30 With the Br incorporation into the Sn-rich perovskites, the absorption spectra showed blue-shifting due to the increase in their bandgap. Figure 3 (b) shows the bandgap (Eg, estimated by the onset absorption band) evolution with the different Br content in MAPb0.4Sn0.6I3-xBrx perovskites. The mixed halide Snrich perovskites have the Eg from 1.24 eV (x=0.2) to 1.32 eV (x=0.8), and the Eg difference (1.16 eV for x=0 and 1.32 eV for x=0.8) is not much pronounced



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even though the PbI2 was completely replaced by PbBr2 for maximum Brincorporation with maintaining Sn-rich Pb-Sn binary composition (Pb : Sn = 4 : 6). The observed Eg distribution which is still much lower than typical MAPbI3 (~1.56-1.6 eV) or MAPbI3-xBrx (1.6-2.3 eV) indicates that these Pb-reduced MASn0.6Pb0.4I3-xBrx perovskites can be also used for long-wavelength-absorber in combination

with

the

above

high

Eg

absorber

for

perovskite/perovskite

monolithic tandem architecture since we found that the proper Eg control could be realized with compositional engineering by the small amount of Brintroduction. In order to demonstrate an effectiveness of the developed Pb-reduced MASn0.6 Pb0.4I3-xBrx perovskites, low-temperature inverted planar device architecture was constructed by solution-processing, as shown in Fig. 4 (a). As for charge transport layers sandwiching the perovskites for efficient carrier extraction and collection, we applied PEDOT:PSS for HTL and PC61BM for ETL, which are identical to the structures used in state-of-the-art works.11,24 Hence, we could make a more meaningful comparison between our device characteristics consisting of the MASn0.6Pb0.4I3-xBrx perovskites and their results of the Sn-rich PVSCs.


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(a)

25 2

Current Density (mA/cm )

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20

x=0 x=0.2 x=0.3 x=0.4 x=0.5 x=0.6 x=0.7 x=0.8

15

10

5

0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Voltage (V) (b) Figure 4. (a) The inverted planar device architecture and (b) their J-V characteristics of the best-performing Sn-rich MASn0.6Pb0.4I3-xBrx (x = 00.8) PVSCs measured under AM 1.5 full illumination (100 mW/cm2)



(a)

0.85

32

0.80

Voc

30

Voc (V)

0.75 0.70

28

0.65

26

0.60 24 0.55

0.45

22

2

Jsc

0.50

Jsc(mA/cm )

20

0.40 18

0.35 0.30

16 0.0

0.2

0.4

0.6

0.8

Br-composition (x) 16

(b)

FF 0.7 14

0.6

FF

12

0.5

10

0.4

8

PCE 0.0

0.2

0.4

0.6

Power conversion efficiency, PCE (%)

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0.8

Br-composition (x)

Figure 5. Variation of the photovoltaic (PV) results of (a) Voc, Jsc, (b) FF, PCE and for the studied Sn-rich MASn0.6Pb0.4I3-xBrx with different Br content in the perovskite compositions (measured under AM 1.5 illumination).

Table 1. Photovoltaic parameters of the best-performing Sn-rich MASn0.6 Pb0.4I3-xBrx (x = 0-0.8) PVSCs under AM1.5-simulated full sunlight of 100mW/cm2, and averaged (avg.) parameters were also given based on estimating 20 devices.


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Voc (V)

Jsc (mA/cm2)

FF

η(%)

(avg.)

(avg.)

(avg.)

(avg.)

x=0

0.66 (0.62)

22.41 (21.66)

0.62 (0.60)

9.2 (8.1)

x=0.2

0.72 (0.71)

23.37 (22.79)

0.69 (0.64)

11.6 (10.4)

x=0.3

0.74 (0.73)

23.03 (22.92)

0.68 (0.64)

11.7 (10.7)

x=0.4

0.78 (0.75)

20.65 (22.11)

0.75 (0.69)

12.1 (11.4)

x=0.5

0.75 (0.70)

20.85 (19.73)

0.69 (0.62)

10.8 (8.6)

x=0.6

0.76 (0.74)

20.94 (19.43)

0.69 (0.66)

11.0 (9.5)

x=0.7

0.78 (0.77)

18.37 (16.69)

0.70 (0.66)

10.0 (8.5)

x=0.8

0.72 (0.72)

19.51 (18.95)

0.67 (0.66)

9.3 (9.0)

MAPb0.4Sn0.6I3-xBrx

Figure 4 (b) involves the J-V characteristics of the MASn0.6Pb0.4I3-xBrx PVSCs with the

Br-evolution

and

their

PV

parameter-variations

were

systematically

described in detail in Table 1, Fig. 5 (a), and 5(b). As for the control Sn-rich device without Br-addition (MASn0.6Pb0.4I3), it exhibited a PCE of 9.2% which is slightly lower than the highest PCE of 10.0% in the Sn-rich (60%) perovskites.24 With the increase in Br content from x=0 to x=0.4 in mixed halide composition, on the other hand, we observed remarkable improvements in the Voc and FF. The pronounced increase in the averaged Voc (0.62 → 0.75 V) and FF (0.60 →



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0.69) was essential to reinforce overall performance with introducing small amount of Br-evolution (x=0.2-0.4). At the proper Br-composition (x=0.4), we found very impressive PCE and Voc of 12.1 % and 0.78 V, respectively. This is, to the best of our knowledge, unprecedented result in the Sn-rich (≥ 60%) PVSCs reported to date.11,21,29,30 The trend of increased Voc and FF was not also much varied until the increased Br-composition up to 0.8, compared to the control perovskite without Br-addition (x=0). This improvement can be ascribed to the enhanced crystallinity of the perovskites as confirmed by XRD and FE-SEM analysis. The charge carrier transport and collection would be much feasible with less recombination in highly crystalline films as described later. On the other hand, the PCE began to decrease in the increment of the Br-composition (x≥0.5), and this is attributed to the dominant decrease in short circuit current (Jsc). The abrupt reduction in the Jsc is related to the increased Eg of the Sn-rich perovskite absorber according to increased Br-composition as confirmed in the absorption measurement above (Fig. 3). Figure S1 of Supporting Information displays EQE spectra of the fabricated MASn0.6Pb0.4I3-xBrx PVSCs with representative Br-content. The spectrum extends over 1000 nm without Br-addition (x=0), realized by reduced Eg from the Sn-rich composition. As the Br-composition increases, the edges of the spectrum at long-wavelength regime shows the blue-shifting phenomenon which is affected by the increased Eg with Br-addition. Nevertheless, we found the absorption edge over 930 nm in case of x=0.8, which covers much longer spectrum compared with that of MAPbI3 (780-800 nm). Recently, a perovskite/perovskite monolithic tandem cell with optimized bandgaps (1.8 eV from FA0.83Cs0.17Pb(I0.5Br0.5)3 of and 1.2 eV from FA0.75Cs0.25Sn0.5Pb0.5I3) was reported.23 In this point of view, we believe that MASn0.6Pb0.4I3-xBrx (x = 0.2-0.4)


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PVSCs are highly suitable as efficient sub-cell for covering long-wavelengthabsorption in terms of proper Eg (1.24-1.26 eV) and pronounced Voc (0.72-0.78 V) as well as much reduced Pb-composition (Pb=40%) in the Sn-Pb binary system.

x=0 x=0.4

0.8

1.17kT/q

0.7

Voc (V)

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0.6 0.5

1.59kT/q

0.4 0.3 1

10

100

2

Light Intensity (mW/cm ) Figure 6. Light-intensity-dependent Voc measurement of representative Sn-rich PVSCs consisting of the control MASn0.6Pb0.4I3 (x=0) and MASn0.6Pb0.4I3-xBrx (x=0.4)

In order to understand such improvements in Voc and overall PCE for the presented

mixed-halide

Sn-rich

PVSCs,

the

light-intensity-dependent

Voc

measurement was performed to investigate the recombination kinetics. The charge recombination process in PV cells can be estimated by extracting the diode ideality factor “n” determined by the slope of the Voc versus incident light intensity; n(kT/q) where k is the Boltzmann constant, T is the temperature, and



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q is the elementary charge.31 We measured the Voc-variation dependent on the light intensity for control device (x=0) and the representative cell with proper Br-composition (x=0.4) which is plotted on a linear-log scale as exhibited in Fig. 6. It was known that the ideality factor n approaches unity in case of bimolecular charge carrier recombination. On the other hand, the n approaches 2

when

Shockley-Read-Hall

(SRH)-type,

trap-assisted

recombination

dominates.31-33 The control Sn-rich device showed n value of 1.59 whereas the Br-added MASn0.6 Pb0.4I3-xBrx (x = 0.4) PV cell exhibited 1.17. This suggests that our control Sn-rich (≥ 60%) MASn0.6Pb0.4I3 PVSC is still affected by the trapassisted recombination process. As for the PVSC with Br-addition, however, it is likely that the influence of the trap-sites on the charge transport could be considerably suppressed and the bimolecular recombination process could dominate further. This improvement of electronic/structural disorder in the Bradded Sn-rich PVSCs can also be related with the crystallinity enhancement as found in the above XRD and FE-SEM analysis. On the basis of the above observations, we elucidated further the fabricated Bradded Sn-rich PVSCs in terms of operational stability. As displayed in Fig. S2 in Supporting Information, the J-V hysteresis was checked for all MASn0.6Pb0.4I3-xBrx (x = 0-0.8) PVSCs under scan rate at 0.01V/s. The Sn-rich control device (MASn0.6Pb0.4I3) showed slight hysteresis phenomenon by allowing for the PCE difference of 0.5% under forward-backward scan. On the other hand, the much suppressed hysteresis about only 0.1-0.2% PCE variations was observed for the all MASn0.6Pb0.4I3-xBrx (x = 0.2-0.8) PVSCs. In addition, the steady-state

photocurrent measurement was made for the representative Sn-rich PVSCs with and without Br-composition as depicted in Fig. 7.


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2

Current Density (mA/cm )

25

x=0 20

15

10

(a)

5

0 0

100

200

300

400

500

600

Time (s) 25

Current Density (mA/cm2)

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x=0.4 20

15

10

(b)

5

0 0

100

200

300

400

500

600

Time (s) Figure 7. The steady-state photocurrent measurement of representative Sn-rich PVSCs of (a) control MAPb0.4Sn0.6I3 and (b) MASn0.6Pb0.4I3-xBrx (x=0.4) for 600s under AM 1.5 full illumination.

The control device revealed gradual decrease in the photocurrent with



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evolving the measurement time. However, the reliable output of the photocurrent was observed for the MASn0.6Pb0.4I3-xBrx (x = 0.4) PVSCs as confirmed in Fig. 7 (b). It should be also noted here that above J-V and steadystate photocurrent characteristics were measured under atmospheric-air, which is different from previous works that performed such measurements under inertgas ambient (i.e. nitrogen)21,24,29 due to the instability of Sn-rich PVSCs caused by the oxidation. Our MASn0.6Pb0.4I3-xBrx (x = 0.4) PVSC showed the exceptional stability without any encapsulation under air-ambient in spite of Sn-rich composition (≥60%). Since all the PV cells were fabricated with identical PEDOT:PSS and PCBM for charge transport layers, the remarkable advances in the device performance and reliable operation are attributed to the employed Bradded Sn-rich perovskite. It is believed that the densely-packed crystalline structure combined with suppressed trap-assisted recombination process could support such improvements of the device stability as well as increment in Voc and FF. The Sn-rich PVSCs with introduction of the small amount of Br (x=0.4) could offer the highest Voc (~0.78 V) and PCE (12.1%) in Pb-Sn binary PVSCs by realizing Pb-reduction (40%). In addition, the proper Br-composition allowed for the Eg about 1.24-1.26 eV which are also strongly compatible with utilization of long-wavelength-absorber for monolithic tandem PVSCs. Recent studies showed cesium (Cs) addition into the cation site (A) of ABX3 perovskite, which made great improvements in PCE of PVSCs (i.e. 10.9% → 14.8%).16,20,23 We are currently in progress to develop Cs1-xMAxSn0.6Pb0.4I3-yBry which is expected to achieve a new record efficiency again in the Sn-rich (≥60%) Pb-Sn binary PVSCs.

Conclusions


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We presented the compositional engineering of bromine (Br) introduction into Sn-rich perovskites (MASn0.6Pb0.4I3) and mixed halide Sn-rich MASn0.6Pb0.4I3-xBrx (x = 0-0.8) were newly studied in terms of structural, morphological, and optical properties by realizing Pb-reduction down to 40% in Pb-Sn binary PVSCs. With the Br-introduction, the crystallinity of Sn-rich perovskites was improved as checked by XRD and densely-packed crystallite domains was observed by FESEM. The MASn0.6Pb0.4I3-xBrx perovskites fabricated with low temperature solution processing revealed pinhole-free and compact structures suitable for inverted planar PVSC architecture. In addition, MASn0.6Pb0.4I3-xBrx perovskites with proper Br-composition (x=0.2-0.4) showed the Eg about 1.24-1.26 eV which can harvest long spectral-wavelengths over 1000 nm. The planar PVSCs with employing suggested MASn0.6Pb0.4I3-xBrx (x=0.4) showed a high Voc of 0.78 V and a PCE of 12.1%, the highest performance among the Sn-rich (≥60%) Pb-Sn binary PVSCs reported to date. Furthermore, the exceptional stability such as steady-state photocurrent and J-V hysteresis was confirmed for the Sn-rich perovskites with Br-addition. These outstanding results not only pave the way for the successful development of environmentally-benign and highly efficient PVSCs without toxic Pb-utilization but also offer a promising opportunity for high-performance homogeneous perovskite tandem cells using the low bandgap PVSCs with Pb-reduction as the bottom sub-cells.

Supporting Information The EQE spectra and J-V hysteresis characteristics of the MASn0.6Pb0.4I3xBrx PVSCs with various Br-compositions were included in Supporting Information.



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Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2015R1C1A1A01053624).


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Highly Efficient and Stable Sn-Rich Perovskite Solar Cells by

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