AgBiS2 Semiconductor-Sensitized Solar Cells - American Chemical

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AgBiS2 Semiconductor-Sensitized Solar Cells Pen-Chi Huang, Wei-Chih Yang, and Ming-Way Lee* Institute of Nanoscience and Department of Physics, National Chung Hsing University, Taichung, 402, Taiwan S Supporting Information *

ABSTRACT: We present a new ternary semiconductor sensitizer-AgBiS2 for solar cells. AgBiS2 nanoparticles were grown using a two-stage successive ionic layer adsorption and reaction process. Post annealing transformed the double-layered structure into AgBiS2 nanoparticles of ∼16 nm in diameter. Liquid-junction semiconductor-sensitized solar cells were fabricated from the synthesized AgBiS2 semiconductor. The best cell exhibited a short-circuit current density Jsc of 7.61 mA/cm2, an open-circuit voltage of 0.18 V, a fill factor of 38.6%, and a power conversion efficiency η of 0.53% under 1 sun. The η increased to 0.76% at a reduced light intensity of 0.148 sun. The external quantum efficiency (EQE) spectrum covered the spectral range of 350−850 nm, with an average EQE of ∼54% over the main spectral region of 450−650 nm. The Jsc under 0.148 sun was equal to 1.69 mA/cm2, a respectable Jsc for a new sensitizer. media.13 Bulk AgBiS2 has an energy gap of Eg = ∼1.2 eV,14 which is close to the optimal gap (1.39 eV) for a solar absorber.15 It has a high absorption coefficient of α = ∼105 cm−1 (at λ = 600 nm).16 These two features give AgBiS2 the potential to be utilized for a high-efficiency solar absorber. AgBiS2 quantum dots (QDs) and nanostructured flowers have been synthesized using sonochemical and solvothermal reactions.17,18 In this work we have, for the first time, synthesized AgBiS2 nanoparticles using a sequential ionic layer adsorption reaction (SILAR) process. TiO2 photoelectrodes sensitized with AgBiS2 nanoparticles were fabricated to create liquid-junction solar cells. We investigate the dependence of the solar-cell performance on the number of SILAR cycles and solar power intensity.

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) are a promising candidate for the next generation of renewable photovoltaic sources.1 The key component of a DSSC is a photoelectrode consisting of a film of nanoporous TiO2 nanoparticles coated with a monolayer of organic dye. The most commonly used organic dyes are the ruthenium complexes, which have large optical absorption in the visible range of 300−700 nm, but weak absorption in the infrared spectrum. Search for new materials with absorption spectra extending into the infrared range is crucial for enhancing the performance of solar cells. A promising alternative for broad-band solar absorbers would be inorganic semiconductor sensitizers. There are two types of semiconductorsensitized solar cells (SSCs): extremely thin absorber (ETA) solar cells and liquid-junction semiconductor-sensitized solar cells. Remarkable progress in SSC performance has been achieved in 2013. The efficiency has been increased from 12.3 to 15% in ETA cells fabricated based on organic−inorganic hybrid perovskites.2,3 Inorganic semiconductor sensitizers have several advantages over organic dyes such as tunable absorption bands due to the quantum-size effect,4 high extinction coefficient and multielectron−hole pair generation by a single incident photon.5,6 Many semiconductor materials, including CdS, CdSe, PbS and Sb2S3 have been employed as sensitizers for SSCs.7−10 Most of these sensitizers belong to the binary metal chalcogenide systems. Ternary semiconductors are also important materials for solar cells. However, ternary metal chalcogenide SSCs have been much less explored because ternary semiconductors are more difficult to synthesize for there are three elements involved and the stoichiometry of the atoms must be correct. Recently, ternary AgSbS2 films and nanoparticles were successfully synthesized for use as a sensitizer for SSCs.11,12 This study extends the work to a new ternary systemAgBiS2. AgBiS2 belongs to a group of I−V−VI semiconductors which has important applications in linear and nonlinear optoelectronic devices, thermoelectric devices, and optical recording © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Preparation of TiO2 Photoelectrodes. AgBiS2 nanoparticles were grown on a TiO2 photoelectrode using the SILAR process. The TiO2 photoelectrode had a threelayered structure: a compact layer (thickness ≈ 80 nm), an ∼10−12 μm-thick, sensitizer-coated nanoporous TiO2 layer, and a TiO2 scattering layer (thickness ≈ 3 μm). The compact layer was prepared by spin-coating a 0.2 M titanium isopropoxide solution onto a clean fluorine-doped tin oxide (FTO) glass substrate (15Ω/◻, Nippon Sheet Glass), heated at 450 °C for 30 min. The TiO2 sensitizer layer was prepared by spreading TiO2 paste (Dyesol DSL-18NR-T, particle size ≈ 20 nm) onto the FTO glass using the doctor-blade technique, then heating it at 125 °C for 6 min. Finally, a scattering layer (particle size ≈ 300 nm) was coated on top of the sensitizer layer. The compact layer acted to reduce recombination between the electrons in the FTO and holes in the electrolyte. The scattering layer increased light harvesting by increasing the light paths in the solar cell. Received: May 9, 2013 Revised: August 14, 2013 Published: August 16, 2013 18308

dx.doi.org/10.1021/jp4046337 | J. Phys. Chem. C 2013, 117, 18308−18314

The Journal of Physical Chemistry C

Article

Figure 1. TEM micrographs of (a) a bare TiO2 film, (b) an Ag2S-QD-coated TiO2 film, (c) AgBiS2(4) nanoparticles (after annealing at 150 °C), (d) a high-magnification image of AgBiS2(4) nanoparticles, and (e) lattice fringes corresponding to the (200) plane of the AgBiS2 phase.

produced Ag+ ions. The photoelectrode was then dipped into a 0.1 M Na2S methanol solution for 40 s, rinsed with methanol, then dried in air. The process produced S2− ions. This two-step process, referred to as one SILAR cycle, produced Ag2S QDs with diameters in the range of several nanometers, as has been reported previously.19 For the coating of the Bi−S layer, the Ag2S−coated photoelectrode was dipped into a 0.1 M Bi(NO3)3 ethanol/water solution (volume 1:1) for 20 s, rinsed with deionized water, and then dried in air. The photoelectrode was then dipped into a 0.1 M Na2S ethanol solution for 40 s,

2.2. Growth of AgBiS2 Nanoparticles. AgBiS2 nanoparticles were grown by means of a two-stage SILAR process. First, Ag2S quantum dots (QDs) were grown on the TiO2 photoelectode. Second, Bi2S3 QDs were grown on top of the Ag2S. Post annealing transformed the double-layered structure into the AgBiS2 phase. Details of the growth process are described below. For the growth of Ag2S QDs, a TiO2 photoelectode was dipped into a 25 °C, 0.1 M AgNO3 ethanol solution for 20 s, rinsed with ethanol, and then dried in air. This process 18309



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rinsed in ethanol, and dried in air. The number of Bi−S SILAR cycles was equal to that of Ag−S. After finishing the SILAR process, the Ag−S/Bi-S double-layered structure was annealed at 150 °C in air for 1 h. The heating treatment transformed the double-layered structure into the AgBiS2 phase. A sample that went through n SILAR cycles is referred to as AgBiS2(n). 2.3. Assembly of Solar Cells. The AgBiS2-coated TiO2 photoelectrode was assembled into solar cells by sandwiching it with an Au counterelectrode using a 190 μm-thick parafilm spacer. Au counterelectrodes of thickness ∼30 nm were prepared through sputtering deposition. We also compared the performance of cells using a Pt counterelectrode. The polysulfide electrolyte consisted of 0.5 M Na2S, 2 M S, 0.2 M KCl, and 0.5 M NaOH in ethanol/water (7:3, volume). 2.4. Material Characterization and Photovoltaic Measurements. Optical absorption spectra were measured using a Hitachi 2800A spectrophotometer. Transmission electron microscopy (TEM) was performed using a JEOL JEM-2010 transmission electron microscope. X-ray diffraction was performed using a PANalytical X’Pert Pro MRD diffractometer. Current−voltage (I-V) curves were recorded using a Keithley 2400 source meter under 100 mW/cm2 illuminated light intensity from a 150 W Oriel Xe lamp with an Oriel band-pass filter simulating the AM1.5 spectrum. The external quantum efficiency spectra (EQE) were measured using an Acton monochromator with a 250 W tungstenhalogen lamp. The active area of the cell, defined by a metal mask placed above the cell, was 3 ×3 mm2.

Figure 2. XRD patterns of the Ag−S/Bi−S double-layered structure: (a) before annealing, (b) after annealing at 100 °C, (c) after annealing at 150 °C, (d) diffraction peaks of AgBiS2 based on the JCPDS database, (e) XRD pattern of Ag2S nanoparticles on TiO2, and (f) XRD pattern of Bi2S3. The peaks associated with TiO2 are marked with the symbol *.

3. RESULTS AND DISCUSSION 3.1. TEM. Figure 1 displays TEM micrographs obtained during various stages of growth. Figure 1(a) shows an image of a bare TiO2 film. Figure 1(b) shows an image of an Ag2S QDcoated TiO2 film. Many Ag2S QDs can be seen to be deposited randomly over the TiO2 surface. The QDs are round in shape with an average diameter of 7 nm. No obvious aggregation is observed. Figure 1(c) shows an image of AgBiS2 nanoparticles (after being annealed at 150 °C). Figure 1(d) shows a highmagnification TEM image of several individual AgBiS 2 nanoparticles. The nanoparticles have an average diameter of 16 nm and are also roughly round in shape. Some aggregation can be observed. Annealing at a lower temperature of 100 °C produced smaller nanoparticles, ∼13 nm in diameter (not shown). The fact that the diameter of AgBiS2 nanoparticles was significantly larger than that of Ag2S QDs (7 nm) can be attributed to two factors: (1) the AgBiS2 nanoparticles were produced from double-layered particles and (2) annealing increased the diameter. Figure 1(e) displays another image of AgBiS2 nanoparticles that shows resolved lattice fringes. The fringe spacing is 0.285 nm, which is assigned to the (200) plane of the AgBiS2 lattice. 3.2. XRD. Figure 2 shows the XRD patterns of the Ag−S/ Bi−S double-layered structure: (a) before annealing; (b) after annealing at 100 °C; and (c) after annealing at 150 °C. The peaks associated with TiO2 are marked with *. The XRD intensities of the AgBiS2 phase increase with increasing annealing temperature. The peaks of the 150 °C sample match the cubic AgBiS2 phase (JCPDS-04-0699, Figure 2(d)) well, with a lattice constant of 5.659 Ǻ . No other impurity phase is observed. Many peaks that are due to the TiO2 background can also be seen. For comparison, the XRD patterns of Ag2S (on TiO2) and Bi2S3 were measured and are displayed in Figure 2(e),(f). The two XRD patterns are clearly different from of the

AgBiS2 phase, indicating that the annealed AgBiS2 sample does not contain either the Ag2S or the Bi2S3 phase (note that some of the Ag2S or Bi2S3 peaks are at the same positions like that of TiO2, but an inspection of the other peaks supports the conclusion that the sample does not contain the Ag2S and Bi2S3 phases). Note that the Bi2S3 XRD pattern was measured with Bi2S3 powder obtained from SILAR. Bi2S3 nanoparticles coated on TiO2 did not produce discernible Bi2S3 XRD peaks because the amount of nanoparticle material is much less than that of the host TiO2. A similar phenomenon has been reported in other SSCs.10 It is interesting to note that the XRD pattern of the doublelayered structure before heating (Figure 2(a)) already showed characteristic peaks of the AgBiS2 phase even before annealing. Results indicate that the AgBiS2 structure formed spontaneously (although not completely) during the two-stage, room temperature SILAR process. The postdeposition annealing process completely transformed the double-layered material to the AgBiS2 phase. This finding reveals that the two-stage SILAR process is highly effective for the growth of AgBiS2 nanoparticles. 3.3. Optical Spectra. Figure 3 (a) displays optical transmission spectra T of 150 °C annealed AgBiS2(n) nanoparticles with various numbers of SILAR cycles n. The spectra were obtained by taking the ratio of the spectrum T(AgBiS2 coated TiO2) to T(TiO2). The transmission decreased with increasing n, indicating that as n increased, more nanoparticle material was grown on the photoelectrode, resulting in enhanced optical absorption. A notable result is the extremely low transmission (∼1−2%) in the visible range of 500−700 nm for samples n = 3,4. This indicates the nearly complete light harvesting by the nanoparticles, which is an essential 18310



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Figure 3. (a) Optical transmission spectra of AgBiS2(n) nanoparticles with various SILAR cycles n; (b) absorption spectra of AgBiS2(n) with various n; (c) transmission spectra after the first-stage growth (the Ag2S phase) and the second-stage growth (the AgBiS2 phase); and (d) absorption spectra after the first-stage and second-stage growth.

with n. The best performance was obtained at n = 4, which yielded Jsc = 5.76 mA/cm2, Voc = 0.16 V, and η = 0.36%, after that, the efficiency started to decrease again. The results show that the cell performance improved as more semiconductor material was deposited on the photoelectrode. When n ≥ 5, overloading of nanoparticles reduced the pore space within the nanoporous TiO2 particles, which probably impeded the electrolyte flow, hence, reduced the cell efficiency. After determining the optimal SILAR cycle to be n = 4, we proceeded to perform various treatments to improve the performance of this optimal cell AgBiS2(4). Figure 4(b) displays the I-V curves of AgBiS2(4) SSCs produced with various treatments. When an Au counterelectrode (Sample No. 5) was used to replace Pt in the cell assembly (Figure 4(b)), the efficiency η increased from 0.36 to 0.41%. When a scattering layer was added (sample no. 6), η increased to 0.46%. Finally, when a passivation ZnS layer was coated over the surface of the AgBiS2 nanoparticles (Sample No. 7), η further increased to 0.53%. The ZnS layer improved η by reducing the recombination between electrons in the conduction band of AgBiS2 (and TiO2) nanoparticles and holes in the electrolyte.21,22 The Au counterelectrode reduced the poisoning effect on the Pt counterelectrode caused by chemisorbed sulfur compounds.22,23 The above treatments improved the η of the untreated optimal sample (no. 3) from 0.36 to 0.53% (sample no. 7), an enhancement of 47%. Figure 5 displays the EQE spectrum of the best AgBiS2(4) SSC. The spectrum covers the spectral range of 350−850 nm, which includes the visible and part of the near-infrared ranges. The highest EQE is 57% at λ = 500 nm and the average EQE over the main spectral range of 450−650 nm is ∼54%. The upper cutoff wavelength is ∼850 nm (1.46 eV), which is

requirement for a good solar absorber. Figure 3(b) shows the absorption spectra (OD·hυ)2 plotted as a function of the photon energy, where h is the Planck constant, OD is the optical density defined as OD = αd and d is the sample thickness (since the nanoparticles are randomly distributed within the TiO2 photoelectrode, d is unknown for this sample, hence, α cannot be measured in this system. Thus, it is appropriate to use OD in the present case). The intercept of the curves can be used to estimate the energy gap Eg. The figure reveals that Eg decreases with increasing n. The result is attributed to the quantum-size effect: as n increases, particle size increases and Eg decreases. The Eg for n = 4 sample is ∼1.32 eV, which is slightly larger than that of bulk AgBiS2 (1.2 eV). It is interesting to note that this Eg is nearly equal to that of the optimal Eg (1.39 eV) of a solar absorber, a beneficial property for a sensitizer material. Figure 3(c) shows the transmission spectra of samples after the first-stage SILAR and after the second-stage SILAR. The former corresponds to the Ag2S phase and the latter corresponds to the AgBiS2 phase. The (OD·hυ)2 vs hυ curves are plotted in Figure 3(d). The intercepts yield Eg = 1.5 and 1.32 eV for Ag2S and AgBiS2, respectively. The Ag2S gap of Eg = 1.5 eV is higher than the bulk gap of 1.1 eV,20 which is also a consequence of the quantum-size effect. 3.4. Photovoltaic Performance. Figure 4(a) shows the IV curves of AgBiS2 SSCs with different SILAR cycles n = 2−5. The incident light intensity was 100 mW/cm2. Table 1 lists the photovoltaic data. The structure of the solar cells included a compact layer, a semiconductor sensitizer layer and a Pt counterelectrode; there was no scattering layer in these samples. Initially, the short-circuit current density Jsc and power conversion efficiency η increased with SILAR cycle n. The open-circuit voltage Voc and fill factor FF changed only slightly 18311



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Figure 5. EQE spectrum of an AgBiS2(4) SSC.

Figure 4. (a) I-V curves for AgBiS2 SSCs with various SILAR cycles. (b) I-V curves for AgBiS2(4) SSCs with various treatments.

consistent with the Eg (1.32 eV) obtained from optical measurements (Figure 3). Figure 6 shows the I-V curves of AgBiS2(4) SSCs under various sun intensities. Table 2 lists the photovoltaic data. The efficiency increases with decreasing light intensity. The efficiency at 14.8% sun is η = 0.76%, a 43% enhancement compared to η = 0.53% at 1 sun. The enhancement arises primarily from Jsc (1.69 mA/cm2 under 0.148 sun, which is equivalent to 11.4 mA/cm2 under 1 sun). The equivalent Jsc of 11.4 mA/cm2 is 50% larger than the Jsc (7.61 mA/cm2, 1 sun). The total photocurrent density Jph that a solar cell produces can be calculated from the EQE spectrum by the equation: Jph =

∫ Φ(λ)EQE(λ)dλ

Figure 6. I-V curves of an AgBiS2(4) SSC under various light intensities. Inset: Picture of a AgBiS2-coated TiO2 film.

where Φ(λ) is the solar photon flux, which can be found in the literature.24 Integrating eq 1 yields Jph = 11.7 mA/cm2, which is in good agreement with the equivalent Jsc of 11.4 mA/cm2 (0.148 sun). The result that the EQE-integrated Jph equals to the Jph (0.148 sun) can be explained by that the EQE measurements were performed using the single-wavelength dispersed light from the monochromator, which has low light intensity due to energy losses from the grating and mirrors within the monochromator. Our measurement indicated that the intensity of a single-wavelength light from the monochromator was approximately an order of magnitude smaller

(1)

Table 1. Photovoltaic Performance of AgSbS2 Sensitized Solar Cells Prepared with Various SILAR cycles n, Counterelectrodes and Passivation Coatingsa Jsc (mA/cm2)

Voc (V)

FF (%)

η (%)

AgBiS2(2)/Pt AgBiS2(3)/Pt AgBiS2(4)/Pt AgBiS2(5)/Pt

1.32 2.68 5.76 5.37

0.17 0.18 0.16 0.17

37.1 39.9 39.5 32.2

0.083 0.18 0.36 0.29

5 6 7

AgBiS2(4)/Au AgBiS2(4)/Au/scattering layer AgBiS2(4)/Au/scattering layer/ZnS

7.01 7.06 7.61

0.16 0.18 0.18

36.9 36.3 38.6

0.41 0.46 0.53

8 9

AgBiS2(4)/Pt/TiO2 (5 μm) AgBiS2(4)/Pt/TiO2 (12 μm)

7.08 6.57

0.16 0.17

38.4 35.0

0.44 0.39

Ag2S

5.02

0.23

36.2

0.40

sample no. 1 2 3 4

electrodes SILAR cycles n

10 a

2

The light intensity is 100 mW/cm . 18312



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Information, SI. The results are consistent with those shown in Figure 7 (a). We also compare the photovoltaic property of an Ag2S SSC. Figure 7(b) shows the I-V curve and Table 1 lists the photovoltaic data (sample no. 10). The I-V curve is in distinct difference from that of the AgBiS2 SSCs. The most important difference is that the Voc of the Ag2S cell (0.23 V) is significantly larger than that (∼0.17 V) of AgBiS2. The difference can be attributed to the different conduction band levels between Ag2S and AgBiS2. Ag2S has a smaller Eg of 1.1 eV and, hence, a broader absorption band. This also contributes to the different photovoltaic characteristics. The AgBiS2 SSCs have several notable features: (a) a high Jsc of 7.61 mA/cm2; (b) an Eg (1.32 eV) close to that for an optimal solar absorber. The efficiency of 0.76% (0.148 sun) is respectable, considering that this is the first time that the AgBiS2 SSCs is investigated. The open-circuit voltage Voc (