High-Efficiency Planar Hybrid Perovskite Solar Cells Using Indium

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High Efficiency Planar Hybrid Perovskite Solar Cells Using Indium Sulfide as Electron Transport Layer Zhe Xu, Jihuai Wu, Yuqian Yang, Zhang Lan, and Jianming Lin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00726 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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ACS Applied Energy Materials

High Efficiency Planar Hybrid Perovskite Solar Cells Using Indium Sulfide as Electron Transport Layer

Zhe Xu, Jihuai Wu*, Yuqian Yang, Zhang Lan, Jianming Lin Eng. Res. Center of Environment-Friendly Functional Materials, Ministry of Education, Fujian Key Laboratory of Photoelectric Functional Materials (Huaqiao Univ.), Xiamen 361021, PR China.

Abstract: For a high performance perovskite solar cell (PSC), electron transport layer (ETL) play a prominent role in the transportation of photo-generated charge carriers from the perovskite layer to electrode. Herein, indium sulfide (In2S3) nanosheets are synthesized by solvent-thermal method, In2S3 ETL is prepared by spin-spraying technique, and planar PSC is thus fabricated. The morphology observation indicates that the as-prepared In2S3 ETL is compact, smooth, pinhole-free, and with an optimal thickness of 40 nm. The photoelectrical characterization reveals that, compared to TiO2 ETL, the In2S3 ETL has lower electron trap state density, lower potential barrier for electron injection from perovskite layer, higher electron mobility and electron extraction ratio at the In2S3/perovskite interface, smaller contact resistance and charge recombination. The planar device fabricated with CH3NH3PbI3 perovskite and In2S3 ETL gains an impressive power conversion efficiency (PCE) of 18.83%, and the photovoltaic device with TiO2 ETL gets a PCE of 15.88% under the same experimental condition. The results provide a novel, low-cost, efficient and alternative ETL material for PSCs.

Keywords: perovskite solar cells; indium sulfide; electron transport layers; solventthermal; spin-spraying.

* Corresponding author (E-mail address: [email protected] (J. Wu).

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1. INTRODUCTION In the last decade, photovoltaic devices based on lead amine halides with perovskite structures have been the most significant progresses in the solar cell field because of their many outstanding features, such as high light-to-electric conversion efficiency, good stability, low cost, and easy processing.1-4 Especially, the key active materials--lead amine halides-possess unique merits, including strong and broad optical absorption,5 high extinction coefficiency,6 long carrier lifetime and diffusion distance,7-9 the dual function of electron and hole transportation, and excellent semiconducting properties. The overspeed progress of perovskite photovoltaic devices surpasses most expectations, and the power conversion efficiencies of the devices have been boosted from 3.9% at beginning up to more than 22% at recently.10-18 As is well known, electron transport layer (ETL) is one vital component for perovskite solar cell (PSC), particularly for planar architectured device.19,20 To date, some ETL materials have been used in PSCs, such as polyoxometalates, transition metal compounds, metallic salts, organic macromolecules, and others.21-24 In these ETL materials, titania (TiO2) is the most frequently utilized due to its many merits.19 However, use of TiO2 ETL does entail some challenges: (i) The electron mobility of TiO2 is only about 10–4 cm2V–1s–1,25,26 which is relatively low compared to other ETL materials; (ii) There is an offset of conduction band minimum (CBM) between TiO2 and perovskite, and it may cause extra carrier recombination at the interface;27 (iii) High temperature annealing is required in general; (iv) Many defects such as oxygen vacancies and metal ion interstitials embed at the surface and particle boundaries of TiO2, and these defects will capture photogenerated charge carriers, resulting in high carrier recombination rates.28,29 Consequently, it is necessary to develop new efficient ETL materials for PSCs.


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Very recently, Hou et al. prepared indium sulfide (In2S3) nanoflakes by using the chemical bath deposition method and used them as ETL materials for PSCs.30 In2S3 is a nontoxic n-type semiconductor, it has high carrier mobility (~ 17.6 cm2V–1s–1), appropriate energy gap (2.0~2.8 eV), samll CBM and high stability.31-33 Different from Ref. 30, we synthesize In2S3 nanosheets by a hydrothermal reaction, and fabricate an In2S3 ETL by the spin-spraying method, thus obtaining a compact, smooth, and pinhole-free ETL. The PSC based on In2S3 ETL obtains a high power conversion efficiency (PCE) of 18.83%, while the device with TiO2 ETL gains a PCE of 15.88% under the same test condition.

2. EXPERIMENTAL 2.1 Materials Oleylamine (C18: 80-90%) was bought from Aladdin Industrial Corporation. Titanium diisopropoxide bis(acetylacetonate) (75.5%), N, N-dimethylformamide (DMF, 99.8%), dimethyl sulfoxide (DMSO, 99.8%), 1-butanol (99.8%), chlorobenzene (99.8%), acetonitrile (99.8%), 4-tert-butyl pyridine (96%), and bis(trifluoromethane) sulfonamide lithium salt (LiTFSI, 99.95%) were bought from Sigma-Aldrich, respectively. Anhydrous indium (III) chloride (InCl3, >99.0%) was bought from TCI Shanghai Corporation. Sulfur sublimed (99.5%) and lead iodide (PbI2, 99.9%) were bought from Alfa Aesar Corporation. CH3NH3I (MAI) was bought from Xi’an Polymer Light Technology Corporation, China. 2,2’9,7,70tetrakis(N,N-di-p-methoxyphenylanmine)-9,9-spirobifluorene (spiro-OMeTAD) was bought from Luminescence Technology Corporation, Taiwan. All chemicals were utilized without further purification. Gold (Au) with the purity of 99.999% was used for preparing counter electrode. 2.2 Synthesis of colloidal In2S3 In2S3 was synthesized by the solvent-thermal method. 3 mmol InCl3 were added into 5 3 ACS Paragon Plus Environment


mL oleylamine under stirring. Then, the solution was added into 20 mL cyclohexane, and stirred for 3 h, labeled as solution A. 3 mmol sulfur sublimed were added into the mixed solution of 2.5 mL oleylamine and 2.5 mL N-dodecyl mercaptan, and stirred for 30 min, labeled as solution B. Then, A was added to B dropwise, stirred for 30 min, then shifted into a 50 mL Teflon autoclave and maintained at 180 °C for 2 h. The resulted intermediates were mixed with 90 mL methyl alcohol and left to stand for 1 h to obtain suspension. The suspension was centrifugally separated at 9000 rpm for 10 min to get primrose yellow precipitate (In2S3). Then, the resulted precipitate was dispersed in 20 mL toluene, a stable In2S3 solution thus was obtained. 2.3 Preparation of In2S3 and TiO2 ETLs Fluorine-doped tin oxide-coated (FTO) conducting glass substrate (TEC-8, 8Ω·sq–1) was surface clean treated with UV-ozone for 15 min, followed by consecutively cleaning with ethanol, isopropanol, acetone, and ethanol. The In2S3 ETLs were spin-sprayed on FTO in 2000, 3000, 4000, 5000, and 6000 rpm for 30 s, respectively, and then heated at 80 °C for 5 min. For comparison, a TiO2 ETL was fabricated by spin-spraying 0.15 M titanium diisopropoxide bis(acetylacetonate) solution (1-butanol) on FTO, at 2000 rpm for 30 s, and then dried at 125 °C for 5 min. 2.4 Fabrication of perovskite solar cells CH3NH3PbI3 precursor solution was prepared by mixing MAI (1 mol), PbI2 (1 mol), DMSO (0.07 mL), and DMF (0.635 mL), referring to previously reported literatures,10,11,15,34 and stirred at room temperature for 1 h to form a fully dissolved perovskite precursor solution. 20 µL spiro-OMeTAD solution was prepared, which consisted of 72.3 mg spiroOMeTAD, 28.8 µL 4-tert-butyl pyridine and 17.5 µL Li-TFSI solution (520 mg Li-TSFI in 1 mL acetonitrile) in 1 mL chlorobenzene. Perovskite CH3NH3PbI3 and HTL (spiro-OMeTAD) were sequentially spayed on the ETL referring to the literatures 15,34. Finally, gold (Au)


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cathode with thickness of approximately 100 nm was covered on the HTL by thermal evaporation. The planar perovskite solar cell thus was fabricated. 2.5 Measurement and characterization (see Supporting Information)

3. RESULTS AND DISCUSSION 3.1 Morphology observation The transmission electron microscopy (TEM) image of the as-synthesized In2S3 is shown in Figure 1a, and the magnified TEM image is shown in Figure 1b. Atomic lattice fringes of In2S3 are obvious, indicating their high crystallinity. The lattice fringe with interplanar spacing (0.325 nm, inset in Figure 1b is consistent with the (311) crystal face of In2S3. The X-ray diﬀraction (XRD) pattern of the as-synthesized In2S3 is shown in Figure 1c. A series of narrow characteristic diffraction peaks are relative to the standard In2S3 (JCPDS Card No. 84-1385).



Figure 1. (a) TEM image, (b) HRTEM image and (c) X-ray diffraction spectrum of the as-prepared In2S3; (d) Structure schematic diagram of planar perovskite photovoltaic device, (e) Energy levels of each functional layer in the photovoltaic device.

The as-synthesized In2S3 was spin-sprayed on FTO substrate to form ETL, and thus a PSC with a structure of FTO/In2S3/pervoskite/HTL/Au was fabricated and the schematic diagram is displayed in Figure 1d. Figure 1e presents the schematic diagram of energy level of each function layer (relative to the vacuum level) in the device. The energy level of the lowest unoccupied molecular orbital (LUMO) of CH3NH3PbI3 is closer to the LUMO of In2S3 than TiO2, which means a smaller potential barrier for electron injection from the perovskite layer into the In2S3 ETL than that into the TiO2 ETL.27 Figure 2 shows vertical view SEM images of the bare FTO, In2S3 ETL covered FTO, and the traditional TiO2 ETL covered FTO, respectively. From Figure 2a, the bare FTO substrate has a rough surface, and the F-doped SnO2 particle sizes range from tens to hundreds of nanometers. After spin-spraying In2S3 on FTO shown in Figure 2b, the profile of the underlying FTO is still dimly visible. The In2S3 nanosheets uniformly cover on FTO and form a semitransparent, compact, smooth, and pinhole-free film, which not only holds blocking effect, but also contributes to the growth of perovskite on it.35 What's more, from Figure S1, the transmittance of In2S3 film is better than TiO2 film. Figure 2c displays the vertical view SEM image of traditional TiO2 ETL: the surface is rough, well-distributed and large specific area.


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Figure 2. FESEM images of (a) FTO, (b) In2S3 film and (c) TiO2 film.

Figure S2 shows the surface observations of the perovskite films covered on the In2S3 ETL and the TiO2 ETL, respectively. There are no obvious differences between both images in morphology: they both have level and smooth surfaces, and the perovskite particle sizes range from 100 to 300 nm. Similar morphology indicates that In2S3 layer, like TiO2 layer, is good basement for perovskite growth. The side-view FESEM images of In2S3 ETL prepared with distrinct spin-spaying speeds are shown in Figure S3. With the increase of the spin-spaying speed, the thickness of In2S3 layer decreases; when the spin-spaying speed is 5000, 4000, 3000, and 2000 rpm, the thickness of In2S3 layer is about 21, 40, 79, and 106 nm, respectively. 3.2 Photoelectric properties



Figure 3. (a) Dark I-V profiles of the electron-only device for VTFL behavior, the inset is the device schematic structure; (b) Steady-state photoluminescence spectra of In2S3/perovskite and TiO2/perovskite films (excitation: 507 nm); (c) Time-resolved photoluminescence spectra of perovskites deposited on In2S3 and TiO2 ETLs; (d) Tafel plots of ETLs (structure: FTO/In2S3/Au and FTO/TiO2/Au); (e) Nyquist plots of the PSCs with different ETLs (Insert is equivalent circuit diagram).

Figure 3a presents the I-V profiles of the electron-only device utilizing In2S3 and TiO2 ETL in darkness. The linear relation at the low bias voltage demonstrates the ohmic response of the electron-only device, and when the bias voltage exceeds the kink point, the current density quickly nonlinearly increases with the increase of the voltage, meaning taht the trapstates are completed filled at this kink point. The trap-state density (nt) thus is determined by the trap-filled limit voltage (VTFL) according to Eq. (1).36

=

(1)

where e is the charge of electron, L is the thickness of perovskite film, ε is the relative permittivity of CH3NH3PbI3 (ε = 28.8),37 ε0 is the vacuum dielectric constant. From Figure 3a, the VTFL values of CH3NH3PbI3 on In2S3 and TiO2 ETLs are 0.72 and 1.28 V, respectively. Therefore, the trap-states density (nt) of CH3NH3PbI3 on In2S3 and TiO2 ETLs are 2.54×1016 cm–3 and 4.53×1016 cm–3, respectively. The reduction of nt indicates that the quality of CH3NH3PbI3 film is improved by utilizing the In2S3 ETL. The steady-state photoluminescence (PL) spectroscopy of the perovskite on In2S3 and TiO2 ETLs are displayed in Figure 3b. A lower PL intensity means a lower radiative recombination between the photogenerated electron and hole in the device, indicating that the In2S3 ETL is better for the transportation and collection of carriers than TiO2 ETL. It also can be seen that photoluminescence peaks of the perovskite on TiO2 and In2S3 ETLs have a small blue-shift from 774 for TiO2/perovskite to 766 nm for In2S3/perovskite, which can be ascribed to the passivation and reduction of the trap-states in perovskite film.38-40


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The time-resolved photoluminescence (TRPL) spectra of perovskites on In2S3 and TiO2 ETLs are shown in Figure 3c. The TRPL decay time and amplitude curves are fitted by using a bi-exponential Eq. (2).41

= ∑ A exp − ! (2) where Ai is the decay amplitude, τi is the decay time. The obtained data are collected in Table 1. For the device with TiO2 ETL, the TRPL decay time τ1 = 110.14 ns and τ2 = 15.42 ns; and the corresponding amplitudes A1 = 52.60% and A2 = 15.42%, respectively. For the device with In2S3 ETL, τ1 and τ2 decrease to 49.64 ns and 10.14 ns, and the corresponding amplitudes are 25.93% and 74.07%, respectively. The average decay time (τave) is estimated by using Eq. (3):42

τ$% = ∑ A τ

(3)

The τave value of the TiO2 ETL/perovskite is 65.24 ns, and the value is decreased to 20.38 ns when In2S3 ETL is used, suggesting a faster transfer of electrons from the perovskite layer to the In2S3 ETL. The faster electron injection rate and more efficient electron transformation from perovskite layer to In2S3 ETL is attributed to a good-match CBM between the perovskite layer and the In2S3 ETL, and high electron extraction of In2S3. The reduced TRPL decay time contributes to limite the charge recombination at the perovskite/ETL interface.43,44 Figure 3d displays the Tafel curves of In2S3 and TiO2 ETLs fabricated with the architecture of FTO/In2S3/Au and FTO/TiO2/Au. The substitution of TiO2 with In2S3 results in an obvious increase in current density from the Tafel curve. So, the conductivity of In2S3 ETL is higher than that of TiO2 ETL. The superior electrical conductivity of In2S3 ETL is propitious to the transportation of photo-generated charge carriers and the reduction of dark current.



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Table 1. TRPL parameters of the PSCs with TiO2 and In2S3 electron transport layers. Electron transport layers

A1 (%)

τ1 (ns)

A2 (%)

τ2 (ns)

τave (ns)

TiO2

52.60

110.14

47.40

15.42

65.24

In2S3

25.93

49.64

74.07

10.14

20.38

Figure 3e reveals the Nyquist plots of the devices with distrinct ETLs under standard one sun illumination (100 mW·cm–2, AM 1.5) at open circuit. The equivalent circuit consists of series resistance (Rs), recombination resistance (Rrec) and contact resistance (Rco) at ETL/perovakite or perovskite/HTL interfaces. Since the HTL is the same for both devices, and the interface of perovskite/HTL is identical for the two cells, the change of Rco is mainly caused by the ETL/perovakite interface, and the Rco value reflects the electron extractiontransport properties at ETL/perovakite interface. The Rco is relative to a high-frequency range and Rrec values is ascribed to the low-frequency in EIS spectra.26 As shown in Table 2, the Rco of the device with In2S3 ETL and TiO2 ETL is 70.78 Ω and 176.85 Ω, respectively. A smaller Rco indicates an efficient electron transportation and extraction at the In2S3 ETL/perovskite interface. Further, the Rrec values of the device fabricated with the In2S3 ETL and TiO2 ETL are 329.57 Ω and 135.90 Ω, respectively. A larger Rrec more efficiently suppresses the charge recombination.41 In other words, the In2S3 ETL can significantly improve electron injection, reduce transfer resistance and charge recombination, resulting in the increased JSC and FF. Table 2. EIS data of the PSCs with TiO2 and In2S3 electron transport layers. Electron transport layers

Rs(Ω)

Rco (Ω)

Rrec(Ω)

Cco (F)

CPEr-T(F)

CPEr-p(F)

TiO2

14.50

176.85

135.90

3.62×10–8

4.83×10–8

1.56

In2S3

12.71

70.78

329.57

6.02×10–8

1.79×10–8

2.03

3.3 Photovoltaic characteristics


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Figure 4a presents the current density-voltage (J-V) curves of the PSCs fabricated wtih In2S3 ETLs in different thicknesses, and the corresponding photovoltaic performances are collected in Table 3. With the spin-sparing speeds increase from 2000 to 4000 rpm, the thickness of In2S3 ETL decreases from 105 to 40 nm, and the PCE is improved from 16.75% to 18.83%, with a open-circuit voltage (VOC) of 1.10 V, short-circuit current density (JSC) of 22.98 (mA cm–2) and fill factor (FF) of 0.75. However, further increasing the spin-sparing speed to 5000 rpm, the thickness of In2S3 ETL decrease to 20 nm, resulting in a reduction of PCE to 16.19%. Therefore, the 4000 rpm is selected as the optimal spin-sparing speed to prepare the In2S3 ETL with a thickness of 40 nm. With regard to morphology, if the ETL layer is too thick, the distance between the perovskite layer and the FTO will increase, producing more recombination centers. If the ETL layer is too thin, the FTO will not be fully covered with the ETL.45 Thus, an appropriate thickness of 40 nm is optimized in our experimental condition. Table 3. Photovoltaic data of the PSCs with In2S3 ETL prepared in different spin-spraying speeds. Spin-spraying speed (rpm)

Thickness (nm)

VOC (V)

JSC (mA cm–2)

FF

PCE (%)

RS (Ω cm2)

2000

105 ± 2

1.08

22.69

0.69

16.75

4.83

3000

75 ± 2

1.09

22.73

0.73

17.60

3.34

4000

40 ± 2

1.10

22.98

0.75

18.83

3.07

5000

20 ± 2

1.08

21.21

0.71

16.19

4.84

Considering from the electrochemistry shown in Table 3, the JSC and VOC of the PSCs have not obviously changed with the thickness change of In2S3 ETL. So, the PCEs first increase and then decrease with the decrease of the thickness of In2S3 ETL, which is mainly due to the change of series resistance (Rs) and fill factor (FF) of the device. The thicker and more incomplete ETLs produce higher Rs, which leads to lower FF and PCE.



Figure 4. (a) J-V curves of the PSCs fabricated with In2S3 ETLs prepared in different spin-spraying speeds; (b) J-V curves of the PSCs fabricated with In2S3 and TiO2 ETLs, (c) IPCE curves and integrated JSC curves; (d) PCE value statistics of the PSCs (20 devices) based on TiO2 and In2S3 ETLs; Current density as a function of time measured at maximum output power point for the devices based on In2S3 ETL (e) and TiO2 ETL(f).

Figure 4b shows the J-V curves of the PSCs assembled with the as-synthesized In2S3 and traditional TiO2 ETLs. The corresponding photovoltaic data are gathered in Table 4. Compared the TiO2-based PSC with the In2S3-based PSC, the VOC increases from 1.06 to 1.10 V, FF increases from 0.69 to 0.75, and JSC enhances from 21.56 to 22.98 mA cm–2. The device with the In2S3 ETL achieves a PCE of 18.83%, while the device with TiO2 ETL gets an efficiency of 15.88%. The larger VOC is likely attributed to the better matched CBM between the ETL and perovskite layer.46 The higher FF and JSC stem from the higher conductivity, higher electron mobility, lower charge recombination and transport resistance of the device with In2S3 ETL,47 which is consistent with above mentioned demonstrations. Figure 4c shows the monochromatic incident photon-to-current efficiency (IPCE) curves and the integrated current density (JSC) curves for the PSCs fabricated with In2S3 and TiO2 ETLs, which is well consistent with the measured JSC. Compared to the JSC obtained from the


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J-V measurements under simulated sunlight irradiation (AM 1.5 G), the current density calculated from IPCE is smaller, which results from the fact that the IPCE curve was measured under a weaker light intensity.48 Table 4. Photovoltaic parameters of the champion PSCs fabricated with TiO2 and In2S3 ETLs. Electron transport layers

VOC (V)

JSC (mA·cm–2)

FF

PCE (%)

RS (Ω cm2)

TiO2

1.06

21.56

0.69

15.88

4.02

In2S3

1.10

22.98

0.75

18.83

3.07

The PCE distribution histograms of the PSCs assembled with In2S3 and TiO2 ETLs are displayed in Figure 4d, and the statistical results are listed in Table S1 and Table S2, and the corresponding J-V curves are presented in Figure S4. The statistical results confirm that almost all the tested PSCs fabricated with In2S3 ETLs produce higher PCE values than that with TiO2 ETLs. The photocurrent density of In2S3 and TiO2 based devices at Vmp were measured and shown in Figure 4e and Figure 4f. Both devices assembled with the In2S3 ETL and the TiO2 ETL exhibit stable photocurrent density and PCE values throughout a 200 s period, the current density and PCE values for the device fabricated with In2S3 ETL can stabilize at 20.19 mA cm–2 and 18.44%, respectively. However, the device fabricated with TiO2 ETL displays lower stabilized current density (18.22 mA cm–2) and PCE (15.33%). The above results are consistent with the J-V curves.



Figure 5. (a) J-V curves of the PSC assembled with the In2S3 ETL under different scanning direction and different step times; (b) UV irradiation stability of the PSCs based on TiO2 or In2S3 ETLs (5 devices each).

Figure 5a shows the J-V curves of the PSC fabricated with the In2S3 ETL with different scanning directions and different scanning speeds. The device fabricated with In2S3 ETL shows the almost same photovoltaic behavior independent of scanning directions, and the scan speed has no significant effect on photovoltaic performance, implying that the J-V hysteresis is almost negligible, indicating good stability of the device based on In2S3 ETL. According to the literature 49, the UV stability of the devices were measured under UV light irrumination with an intensity of 10 mW·cm−2 for 50 h in ambient conditions without encapsulation. The result is displayed in Figure 5b, the PCE value of the device with In2S3 ETL is decayed by about 19% from its maximum value, while the device based on TiO2 ETL is decayed by 46% within the same test period. The result shows that In2S3 as an electron transport material can effectively improve the UV irradiation stability of the PSCs.

4. Conclusion In summary, a compact, smooth, and pinhole-free In2S3 ETL was prepared by hydrothermal reaction and spin-spraying. The planar CH3NH3PbI3 PSC based on In2S3 ETL gains a high power conversion efficiency of 18.83%, while the device based on TiO2 ETL obtains an efficiency of 15.88% under the same test condition. Excellent photovoltaic properties for In2S3 based device roots in the In2S3 ETL low potential barrier for electron injection, low electron trap-state density in perovskite, increased electron mobility and electron extraction ratio at the ETL/perovskite interface, reduced contact resistance and charge recombination. This study demonstrates a low-cost, efficient and alternative ETL material for PSCs.

ASSOCIATIED CONTENT


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S Supporting Information: Supporting Information is available free of charge on the ACS ○

website at DOI: 10.1021/acsaem.XXXXXXX. Experimental section: Measurement and characterization; Figure S1~S4, Table S1~S2.(PDF)

AUTHOR INFORMATION

Corresponding Author: Jihuai WU; E-mail: [email protected]; ORCID: 0000-0002-9820-1382 The authors declare no competing financial interest.

ACKNOWLEDGEMENT

The research was financially supported by the National Natural Science Foundation of China (Nos. U1705256, 51472094, 91422301, 61474047).

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High-Efficiency Planar Hybrid Perovskite Solar Cells Using Indium

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