Reducing the Schottky Barrier by SnS2 Underlayer

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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24789−24794

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Reducing the Schottky Barrier by SnS2 Underlayer Modification to Enhance Photoelectric Performance: The Case of Ag2S/FTO Longyan Gu,†,‡ Yan Lei,*,†,‡ Jie Luo,†,‡ Xiaogang Yang,†,‡ Tuo Cai,†,‡ and Zhi Zheng*,†,‡ †

Key Laboratory of Micro−Nano Materials for Energy Storage and Conversion of Henan Province, College of Advanced Materials and Energy, Institute of Surface Micro and Nano Materials and ‡Henan Joint International Research Laboratory of Nanomaterials for Energy and Catalysis, Xuchang University, 461000 Henan, P. R. China

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S Supporting Information *

ABSTRACT: The transfer and recombination of photoinduced charge carriers play the crucial roles in a photoelectric conversion system. In this work, the Ag2S/fluorine-doped tinoxide (FTO) was used as the platform to understand the photoinduced charge carrier transfer and recombination at the light absorber and electrode interface. SnS2 was evaporated onto the FTO surface to cooperate the Fermi level with Ag2S, which reduced the Schottky barrier at the Ag2S/FTO interface. Kelvin probe force microscopy measurements reveal that the Fermi level of FTO can be tuned from −4.93 to −4.75 eV by various SnS2 with different evaporation amounts. Transient surface photovoltage tests confirm that the recombination of the photogenerated charge carrier can be drastically suppressed. The photoelectric conversion efficiency of the resulting solar cell devices has been significantly improved. KEYWORDS: interface modification, charge carrier recombination, TSPV, solar cells, Ag2S

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INTRODUCTION Silver sulfide (Ag 2 S) is a stable I−VI group n-type chalcogenide compound which has attracted lots of attention for its extensive applications in photocatalysis,1−3 photoconductors,4 near-infrared (NIR) fluorescence,5−7 and solar cells.8,9 In addition, Ag2S has nontoxicity,10 polymorphous modifications within close temperature intervals,11 and mechanical ductility12 characteristics. In the application of solar cells, Ag2S thin films can absorb the sunlight from the ultraviolet (UV) to the NIR region because of its direct narrow band gap (Eg = 0.9−1.1 eV).5,7,13,14 For such a narrow band gap semiconductor, the related photoelectric conversion efficiency (PCE) can reach to about 30% in an ideal solar cell.15 It can be a promising semiconductor for next-generation solar cell applications, even the PCE is still low. Considering the photoelectric conversion process, the following steps will happen when the solar cell device converts light to electrons: (1) photon absorbing; (2) charge carrier generation; (3) charge carrier transfer to the electrode; (4) electron collected by cathode and hole collected by anode, respectively. To obtain high PCE, these steps must be highly efficient to realize high open circuit voltage (Voc), short circuit photocurrent density (Jsc), and fill factor (FF). Therefore, the photoinduced charge carrier recombination at the interface of heterojunction or at the electrode surface must be reduced. Some works have been reported addressing this consideration, such as the doping element to tune the Fermi level of Ag2S,15 using electron transfer layer between the electrode and Ag2S thin film8 and introducing an intermediate layer to form a cascade band structure.16−18 Despite these efforts, the Ag2S © 2019 American Chemical Society

thin-film-based solar cells are still featuring low PCE. One of the problems is how the electrode/Ag2S interface affecting the photoinduced charge carrier recombination has not been well studied. Therefore, we should further check out the photoinduced charge carrier transportation and recombination in mechanism. To our knowledge, the Schottky barrier will be generated at the interface of n-type semiconductor and electrode, once the Fermi levels are different.19,20 The band is bending upward at each side of n-type semiconductor and form a “V-type” band structure in the electrode/n-type semiconductor/p-type semiconductor structure.21,22 The photoinduced charge carrier recombination will happen at the inappropriate interface of electrode and n-type semiconductor. To achieve higher photoelectric performance, such recombination at the electrode surface should be suppressed. In this work, the fluorine-doped tinoxide (FTO) surface was modified by an SnS2 layer to tune the Fermi level of FTO, which was deposited by thermal evaporation and post thermal annealing treatment. A photoelectrochemical method was carried out to evaluate the variation of charge carrier transfer resistance between the Ag2S/FTO interface. The Kelvin probe force microscopy (KPFM) technique was used to study the Fermi level alignment of the Ag2S/TFO interface. The transient surface photovoltage (TSPV) technique has been employed to investigate the photoinduced charge carrier transportation and Received: April 26, 2019 Accepted: June 18, 2019 Published: June 18, 2019 24789

DOI: 10.1021/acsami.9b07321 ACS Appl. Mater. Interfaces 2019, 11, 24789−24794

Research Article

ACS Applied Materials & Interfaces recombination between the Ag2S/TFO interface. Basing on these results, a more clear charge carrier recombination mechanism at the Ag2S/TFO interface is obtained, which may give more opportunities to further increase power conversion efficiency of the new type photoelectric system.

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EXPERIMENTS SECTION

Preparation of SnS2-Modified FTO Substrates. FTO was cleaned sequentially by ultrasonic treatment in detergent/water solution for about 30 min and NH3·H2O/H2O2/H2O (v/v 1:2:5) boiled at 80 °C for 30 min, and ultrasonic treatment was done with water for about 30 min. Finally, the FTO was dried at 80 °C for further use. SnS2 (5 N, Chengdu Alfa Metal Material Co. Ltd., China) was used without further purification. SnS2 was thermal evaporated to the FTO surface by a thermal evaporator (EMITECH K950X, England). The obtained SnS2-modified FTO substrates were thermal annealed at 350 °C for 30 min. Then, the SnS2/FTO samples cooled down to room temperature naturally for further use. Ag2S Thin Film Fabrication. First, Ag thin films were coated on the substrate by the magnetron sputtering method under 4 × 10−3 mbar pressure and 40 mA current (Quorum Q300T D, England). The thickness was controlled by a film thickness monitor. Second, the obtained Ag thin film was put into an ethanol/sulfur solution at 30 °C for about 4 h to convert into Ag2S thin films. The obtained Ag2S thin films were cleaned by ethanol and dried at 25 °C for further use. Solar Cell Device Assemble. Ag2S-based solar cell devices were assembled by spin coating the Spiro-OMeTAD 2,2′,7,7′-tetrakis(N,Ndi-p-methoxyphenyl-amine)-9,9′-spiro-bifluorene (Luminescence Technology Corp.) solution and then coating Au as the electrode through thermal evaporation. The Spiro-OMeTAD solution was fabricated by mixing 80 mg Spiro-OMeTAD, 28.5 μL of 4-tertbutylpyridine and 17.5 μL of lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) solution (520 mg of LiTFSI in 1 mL of acetonitrile) in 1 mL chlorobenzene. The solar cell devices fabrication was carried out in atmosphere condition. Characterization. X-ray diffraction (XRD) data were recorded by a Bruker D8 ADVANCE X-ray diffractometer with a scanning step of 0.02° per second, where the X-ray was generated by a Cu target tube with 40 kV and 40 mA condition. Scanning electron microscopy (SEM) images were collected on a FEI Nova NanoSEM 450 operating at 5 kV. Raman spectra were measured and recorded by Renishaw in Via using a 532 nm laser as exciting light. High-resolution transmission electron microscopy (HRTEM) and selected area diffraction (SAED) images were collected from JEM-2100F (JEOL, Japan). The Fermi levels of the samples were calculated by the contact potential difference (CPD) obtained from the atomic force microscope (AFM) equipped with a KPFM module (Bruker Icon, German). I−V curves were collected by a source meter (Keithley 2440) with a 1000 W xenon lamp equipped with a solar simulator (Newport). The electrochemical impedance spectroscopy (EIS) curves were collected by an electrochemical work station (CHI 660E, China) using a three-electrode method in 0.1 M Na2SO4 solution. TSPV is employed to understand the photoinduced charge carrier dynamic of the obtained thin films. The main corresponding setup consisted of FTO/SnS2/Ag2S, mica-flakes, and FTO. The photovoltage signals were collected by an oscilloscope (TDS 3054C 500 MHz, Tektronix). A 355 nm laser with a pulse width of 4 ns was used as the excited source.

Figure 1. Schematics of band bending at FTO and Ag2S interface due to Fermi level alignment: (a) (ϕf, FTO < Ag2S) and (c) (ϕf, m-FTO = Ag2S) are FTO and Ag2S before contacting under dark conditions, respectively; (b,d) FTO with Ag2S after equilibrium. Rint is the charge recombination resistance at the FTO/Ag2S interface. m-FTO is the modified FTO substrate.

generation is due to the large different of the Fermi level of the n-type semiconductor and metal at equilibrium condition. A “V-type” band structure generates at each side of n-type Ag2S in the electrode/Ag2S/HTL structure, which will strongly influence on the charge carrier transportation and lead to serious charge recombination at the interface. Figure 1b shows the inappropriate band level bending at both sides of the Ag2S thin film connected with the FTO and HTL. Actually, the Fermi level of Ag2S is reported at about −4.6 eV (−0.11 eV lower than the Ecb level).25 After the charge equilibrates between FTO and the Ag2S, a formed Schottky barrier will also lead to the hole transfer to FTO and electrons gather at the FTO/Ag2S interface on light illumination. Next, the photoinduced holes and electrons recombination happen at this interface (Figure 1b). When the Fermi level of the FTO is equal to that of Ag2S (Figure 1c), a flat band (Ohmic contact) will be achieved without any band bending (Figure 1d). Obviously, electrons can be well collected by the FTO because of the Ohmic contact of the FTO/Ag2S interface (Figure 1d). In this case, photogenerated charge recombination can be efficiently suppressed with this appropriate band structure alignment. Therefore, the work function of FTO should be adjusted to match that of Ag2S. To adjust the Fermi level of FTO, SnS2 has been used to modify the FTO surface. Figure 2a shows the SnS2/FTO (mFTO) photograph; SnS2 were coated onto the FTO surface by the thermal evaporation method with a post thermal annealing treatment. Figure 2b−e illustrates the surface morphology of FTO which was measured with an AFM. SnS2 were evaporated with different quantities of 20 mg (Figure 2c), 30 mg (Figure 2d), and 40 mg (Figure 2e), respectively. Next, the roughnesses of SnS2/FTO samples have been checked. The roughness (Ra) of pure FTO is about 8.9 nm. The Ra of SnS2/ FTO thin films are 9.0 nm (c), 8.3 nm (d), and 8.2 nm (e), respectively. It indicates that the modifying of SnS2 will not significantly influence the morphology and roughness of the

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RESULTS AND DISCUSSION Considering the FTO (SnO2:F) and Ag2S interface band alignment, the general work function of FTO is around 5.0 eV,23 while the conduction band bottom (Ecb) and valence band top (Evb) of Ag2S are −4.5 and −5.4 eV,24 respectively. It seems that the photoinduced electrons of Ag2S can be transferred to the FTO (Figure 1a). In fact, the band level at Ag2S will be bended (supposing the Fermi level of Ag2S is higher than FTO), forming a Schottky barrier. This barrier 24790

DOI: 10.1021/acsami.9b07321 ACS Appl. Mater. Interfaces 2019, 11, 24789−24794

Research Article

ACS Applied Materials & Interfaces

Figure 2. SnS2 thin film fabricated by thermal evaporation with post thermal annealing. (a) Photograph of SnS2 thin film on FTO substrate; (b−e) AFM morphology of FTO and m-FTO with different SnS2 (the namely quantity are used for evaporation); (f) XRD patterns of SnS2 on the FTO substrate; (g) Raman curves of SnS2 thin film on the FTO substrate with different SnS2 evaporation quantities; (h) HRTEM image of a SnS2 thin film (inset is the SnS2 thin film on a Mo grid substrate); (i) SAED image of a SnS2 thin film.

FTO surface. The corresponding light absorbance spectra of the SnS2-modified FTO are illustrated in the Supporting Information as Figure S1. For understanding the SnS2 thin film, XRD analysis has been carried out (Figure 2f). Only one diffraction peak can be detected at 15.03°, which can be well indexed as the (001) plane of hexagonal phase SnS2 (JCPDS no. 23-0677). The other diffraction peaks all come from the FTO substrate. Hexagonal phase SnS2 consists of S−Sn−S triple layers, where these layers are stacked by weak van der Waals interactions just like the graphene crystal. This result indicates that the S−Sn−S triple layers are parallel to the FTO surface, and it may have advantage in the charge transfer. Raman measurements have been used for further studying the phase of deposited SnS2 thin films (Figure 2g). At 314 cm−1, a Raman shift peak can be detected, confirming the obtained SnS2 phase thin films.26,27 Moreover, it clearly shows that the relative intensity of Raman peaks (SnS2 and FTO) increases as the evaporation SnS2 precursor amount increases from 20 to 40 mg. This indicates that the thickness of SnS2 thin film is increasing. Figure 2h plots the HRTEM image of SnS2 thin films fabricated on a Mo grid substrate directly. The lattice distance has been measured to be 0.32 nm, that can be well indexed to the (100) plane of hexagonal SnS2. The inset shows a very continuous SnS2 thin film. Next, the SAED measurements have been carried out on this SnS2 thin film (Figure 2i). Diffraction rings have obtained on the SnS2 thin films, indicating a polycrystal feature. The radiuses of the rings are 0.32 nm (R1) and 0.18 nm (R2), respectively. This is also consisting with the above XRD result where the S−Sn−S triple layers of SnS2 are laying on the substrate. Ag2S was then fabricated on the FTO and SnS2/FTO substrates for further photoelectric conversion measurements. The thickness of the elemental Ag thin film precursor is about 230 nm (Figure S2). The related XRD pattern of Ag2S thin films fabricated at room temperature has been plotted in the Supporting Information as Figure S3. Figure 3a shows the photograph of Ag2S/SnS2/FTO thin films. The narrow band gap Ag2S can adsorb a broad sunlight spectrum from the UV to NIR region (UV−vis curve of Ag2S in the Supporting Information as Figure S4). Figure 3b illustrates the SEM image of Ag2S on the SnS2/FTO substrate. The Ag2S thin film

Figure 3. Photograph of the Ag2S/SnS2/FTO thin film (a); SEM image of Ag2S on SnS2/FTO (b); cross section of Ag2S/FTO (c) and Ag2S/SnS2/FTO (d).

is consisted of nanosheets perpendicular to the underlayer. The corresponding SEM of Ag2S on the FTO substrate without SnS2 modification can be found in the Supporting Information (Figure S5). Cross section SEM images of Ag2S/FTO and Ag2S/SnS2/FTO thin films are shown in Figure 3c,d. The thickness of Ag2S is about 1 μm, where a highly continuous Ag2S dense layer is observed behind the nanosheets. It was noted that the corresponding FTO used in Figure 3d was modified by evaporating 40 mg of SnS2 (evaporation quantity). The FTO modified with different SnS2 amounts were used for constructing solar cells (Figure 4).28 The thickness of Ag2S was kept at constant by sputtering the same thickness of elemental Ag thin films (e.g., 230 nm). Spiro-OMeTAD was used as hole transport material. Under simulated sunlight (100 mW/cm2, AM 1.5G filter), the performance parameters of champion devices have been listed in Table 1. Without SnS2, the solar cell device shows very low PCE (0.04%). When increasing the SnS2 evaporation quantity, the PCE increased. Among the devices, the 30 mg SnS2 evaporation quantity coated solar cell shows the best PCE at 1.12%. These results indicate that the SnS2 seriously influences on the charge carrier 24791

DOI: 10.1021/acsami.9b07321 ACS Appl. Mater. Interfaces 2019, 11, 24789−24794

Research Article

ACS Applied Materials & Interfaces

show the same trend for two impedance parts, one semicircle and one straight line; the semicircle comes from the Ag2S and FTO interface. Without SnS2 (blue curve), the radius is the largest among all of the samples, suggesting a high resistance at the Ag2S and FTO interface. Using SnS2 to modify the FTO and Ag2S interface, the radius of the semicircle becomes smaller in order (20 and 30 mg SnS2 samples), but the 40 mg SnS2-modified sample (green curve) increased. These results indicate that suitable SnS2 layer can reduce the charge carrier transfer resistance, but too much thicker SnS2 will increase it and lead to the photocurrent loss. Energy level of the heterojunction plays a crucial role in charge carrier transfer and recombination. The difference of Ag2S Ecb and FTO Ef is 0.43 eV, and the difference of Ag2S Evb and FTO Ef is 0.47 eV. The energy level differences are almost the same; thus, both the photoinduced holes and electrons can be collected by FTO, resulting in charge carrier recombination. To further understand the influence of the energy barrier at Ag2S and the FTO interface on charge carrier transfer and recombination, the KPFM and TSPV measurements have been carried out. Figure 6a shows the calculated Fermi level of the samples; the corresponding CPD measurements results can be found in the Supporting Information (Figure S6). We can find that the Fermi levels of FTO and Ag2S are −4.93 and −4.68 eV, respectively. This Fermi level difference can lead to a 0.25 eV Schottky barrier between the Ag2S and FTO interface at equilibrium. After modifying SnS2, the Fermi level of FTO can be increased from −4.93 to −4.75 eV. The mismatch of the Fermi level of Ag2S and FTO can be reduced to 0.11 eV (30 mg) and 0.07 eV (40 mg), respectively. Such a low barrier interface will enable the charge carrier transfer and suppress the recombination caused by the accumulation of photoinduced charge carrier at this interface. TSPV is a useful technique that can reveal the photoinduced charge carrier dynamics in photoelectric conversion and photocatalysis system.21,36−39 The positive and negative signals of TSPV measurements indicate photoinduced holes or electrons collected at the light illumination electrode side, respectively.40−42 For instance, the n-type semiconductor will be detected with a positive TSPV signal, suggesting the photoinduced hole signal that is caused by the band bending upward at its surface. The intensity of the TSPV signal means that the photoinduced charge carrier concentration gathered at the light illumination electrode side. In this work, the FTO is the electrode used for collecting electrons, and the TSPV signal should be negative (The TSPV result of Ag2S/FTO sample with 355 nm light illuminating from Ag2S surface was illustrated in the Supporting Information as Figure S7.). Figure 6b shows the TPSV curves of FTO/SnS2/Ag2S thin films. At about 3 × 10−7 s, a positive photovoltage peak has been recorded; when reaching about 9 × 10−7 s, the photovoltage becomes negative (the FTO/Ag2S sample). The FTO/SnS2 (20 mg)/Ag2S sample shows a similar TSPV curve, but the positive photovoltage peak becomes weaker. These results indicate that (1) the photoinduced holes gathered at the electrode first and then recombine with the electrons at the interface of Ag2S and FTO and22 (2) introducing SnS2 to the interface of Ag2S and FTO can reduce the hole concentration gathered by FTO, and the trend of suppressing recombination is also represented. As the SnS2 evaporation quantity reaches 30 mg, the positive photovoltage peak is eliminated, showing pure negative signal as expected on p-type semiconductors. The 40 mg sample shows a similar TSPV curves as the 30 mg

Figure 4. J−V curves of FTO/SnS2/Ag2S/Spiro-OMeTAD/Au solar cell devices with different SnS2 interface modifications. Inset shows the schematic of the solar cell device.

Table 1. Performances of FTO/SnS2/Ag2S/SpiroOMeTAD/Au Solar Cell Devices SnS2 evaporation quantity (mg)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0 20 30 40

0.13 0.14 0.27 0.30

1.58 2.68 11.86 9.54

21.50 29.47 34.95 31.08

0.04 0.11 1.12 0.89

transfer between the FTO and Ag2S interface. Suitable SnS2modified interface can suppress the photoinduced charge carrier recombination and enhance Voc and Jsc at the same time. But excess SnS2 (40 mg one) may lead to a relative higher charge carrier transfer resistance at the interface of Ag2S and FTO and reducing Jsc. The influences of charge carrier transfer recombination on photoelectron generation were studied by using an electrochemical method. Figure 5 shows the EIS measurements of

Figure 5. EIS curves (the inset is the simulated circuit) of FTO/ SnS2/Ag2S with different evaporation quantity of SnS2.

FTO/SnS2/Ag2S with different evaporation quantities of SnS2. Here, we used a three-electrode method to evaluate the charge carrier transfer resistance in aqueous electrolytes (0.1 M Na2SO4 solution). The Nyquist EIS curves show very different characteristics for the solid−solid interface and solution−solid interface.29−31 For instance, the impedance property of the solid−solid interface always presents semicircles corresponding to the process controlled by charge carrier transfer,32,33 but a straight line will appear for the solution−solid interface because of the determination of semi-infinite diffusion (Warburg impedance).34,35 It is clear that the EIS curves 24792

DOI: 10.1021/acsami.9b07321 ACS Appl. Mater. Interfaces 2019, 11, 24789−24794

Research Article

ACS Applied Materials & Interfaces

Figure 6. Fermi level values of different SnS2-modified FTO (a); TSPV curves of FTO/SnS2/Ag2S (b) (the inset is the schematic of TSPV setup).

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SnS2 sample, but the photovoltage signal is weaker. These results indicate that modifying SnS2 with a suitable amount between the FTO and Ag2S interface can efficiently suppress the recombination of photoinduced charges, but too much SnS2 will reduce the concentration of photoinduced charge carriers. Combing the Fermi level measurements with the EIS results, we can find that the Fermi levels of Ag2S and FTO are brought to the same level by modifying SnS2, and the Schottky barrier is removed. The photoinduced electron can be transferred to FTO efficiently without recombination at Ag2S and the modified FTO interface.

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*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (Z.Z.). ORCID

Yan Lei: 0000-0003-2906-0050 Xiaogang Yang: 0000-0002-1142-3100 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (grant nos. 61504117, U1604121, 21673200); Key Research and Development Project of Henan Province (no. 192102210183); Youth Talents Lifting Project of Henan Province.

CONCLUSIONS

Understanding the photoinduced charge carrier transfer and recombination is important for the photoelectric conversion device. In this work, we use the Ag2S/FTO as the case to study this topic by adjusting the Fermi level of FTO with a uniform SnS2 layer. The Fermi level of FTO can be adjusted from −4.93 to −4.75 eV, which can be well matched with the Fermi level of Ag2S (−4.68 eV). Therefore, the Schottky barrier between FTO and Ag2S is strongly reduced. As the same time, the charge carrier transfer resistances have also been discussed by EIS measurements, which can be reduced by the SnS2 modification on FTO. TSPV measurements show that the photoinduced charge carrier recombination between Ag2S and the FTO interface has been efficient suppressed by SnS2 modification. Jsc of the resulting solar cell devices have increased from 1.58 to 11.86 mA/cm2. Voc has been increased from 0.13 to 0.30 V. In summary, the Fermi levels of Ag2S and FTO have been aligned appropriately, and the Schottky barrier at the interface was reduced. Therefore, the photoinduced charge carrier can be collected by the electrode more efficiently. A clearer photoinduced charge carrier transfer and recombination mechanism between the electrode and light absorber is given in this work, which may also help the optimization of other photoelectric conversion systems.

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AUTHOR INFORMATION

Corresponding Authors

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07321. UV−vis curve of Ag2S and SnS2 thin film, XRD pattern of Ag2S, SEM image of Ag2S thin film, and CPD measurements of related samples and TSPV curve of Ag2S (PDF) 24793

DOI: 10.1021/acsami.9b07321 ACS Appl. Mater. Interfaces 2019, 11, 24789−24794

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.9b07321 ACS Appl. Mater. Interfaces 2019, 11, 24789−24794