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Supercritical synthesis and in situ deposition of PbS nanocrystals with oleic acid passivation for quantum dot solar cells
Materials Chemistry and Physics 156 (2015) 163e169
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Supercritical synthesis and in situ deposition of PbS nanocrystals with oleic acid passivation for quantum dot solar cells M.M. Tavakoli a, A. Simchi a, b, *, H. Aashuri a a b
Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran Institute for Nanoscience and Nanotechnology, Sharif University of Technology, 14588 Tehran, Iran
h i g h l i g h t s Supercritical fluid processing and in situ deposition of PbS QDs are presented. The prepared nanocrystals are mono-dispersed with an optical bandgap of 1.3 eV. Photovoltaic performance of the in situ deposited nanocrystals is reported. An improved PV performance compared to spin coated Schottky solar cells is shown.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2014 Received in revised form 23 February 2015 Accepted 26 February 2015 Available online 3 March 2015
Colloidal quantum dot solar cells have recently attracted significant attention due to their low-processing cost and surging photovoltaic performance. In this paper, a novel, reproducible, and simple solutionbased process based on supercritical fluid toluene is presented for in situ growth and deposition PbS nanocrystals with oleic-acid passivation. A lead precursor containing sulfur was mixed with oleic acid in toluene and processed in a supercritical fluid condition at different temperatures of 140, 270 and 330 C for 20 min. The quantum dots were deposited on a fluorine-doped tin oxide glass substrate inside the supercritical reactor. Transmission electron microscopy, X-ray diffraction, absorption and dynamic light scattering showed that the nanocrystals processed at the supercritical condition (330 C) are fully crystalline with a narrow size distribution of ~3 nm with an absorption wavelength of 915 nm (bandgap of 1.3 eV). Fourier transform infrared spectroscopy indicated that the PbS quantum dots are passivated by oleic acid molecules during the growth. Photovoltaic characteristics of Schottky junction solar cells showed an improvement over devices prepared by spin-coating. © 2015 Elsevier B.V. All rights reserved.
Keywords: Inorganic compounds Chalcogenides Electronic materials Chemical synthesis
1. Introduction Solution-processed photovoltaic devices have attracted increasing attention for solar harvesting because of their low processing cost, simplified procedure, and large-area coverage [1,2]. Inorganic nanocrystals with tunable bandgap have a great potential in thin film solar cells as they can easily be processed by straight forward and simple chemical routes [3,4]. Among various semiconductor nanocrystals, PbS quantum dots (QDs) are very promising for photovoltaic applications due to their high Bohr radius (~20 nm) and tunable bandgap to harvest the maximum part of
* Corresponding author. Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran. E-mail address: [email protected] (A. Simchi). http://dx.doi.org/10.1016/j.matchemphys.2015.02.043 0254-0584/© 2015 Elsevier B.V. All rights reserved.
solar spectrum in the near infrared region [5,6]. Therefore, synthesis and characterization of PbS nanocrystals by various chemical procedures have frequently been studied [7e10]. It is important to mention that the surface trap states of PbS nanocrystals must be passivated by using some agents such as organic molecules because the trap states cause recombination of charge carriers which reduces the photoelectrical performance [11]. The nanocrystals were then deposited on conductive substrates by dip-coating or spincoating. In order to improve photovoltaic (PV) efficiency, an additional ligand-exchange processing step by short thiol molecules [12,13] or halides [14,15] should be utilized. Recent studies [16,17] have shown that constructing uniform and compact films with short interparticle spacing is a key factor to obtain improved PV efficiency in devices based on PbS QDs. Since the ligand-exchange processing step affects the compactness and uniformity of the
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colloidal quantum dot films, direct deposition of colloidal inks was demonstrated and a PV efficiency of 2.1% was reported [18]. Supercritical fluid processing is a facile and promising method for the synthesis of various inorganic semiconductor nanocrystals [19e22]. Synthesis of QDs in supercritical fluid condition offers several advantages over conventional chemical routes including enhanced mass and heat transfer at elevated temperatures [23], reproducibility [24], low reagent consumption [25], and ability for automation and continuous production [26]. Studies on the supercritical synthesis of CdSe [23], CdS [24], PbS [25], and ZnS [25] nanocrystals have demonstrated the potential of the process in fine and precise control of parameters to attain nanocrystals with narrow size distribution. Recent advances in supercritical synthesis of inorganic semiconductor nanocrystals have dealt with in situ growth of nanocrystals on conducting substrates. Wang et al. [17] reported supercritical fluid (CO2) deposition of PbS QDs on glass substrates for energy transfer studies. Close-packed two-dimensional and three-dimensional arrays of the nanocrystals were attained. The potential of supercritical fluid to deposit nanoparticles in small structures or to form thin films of ordered arrays has also been demonstrated in other investigations [25e30]. In these studies, the nanocrystals were synthesized and then deposited on a substrate via different procedures. The unique feature of the present work is that, the synthesis and deposition of QDs on a conductive glass substrate were performed simultaneously. In other words, an in situ synthesis and deposition procedure based on one-step, reproducible and simple supercritical fluid was developed to prepare thin films of inorganic QDs for PV applications. The potential application of the method is shown for the preparation and deposition of oleic-acid capped PbS QDs on the surface of FTO glasses. To the best knowledge of the authors, this is a pioneer report on the fabrication of PbS QD solar cells by in situ supercritical deposition utilizing cheap precursors. During the supercritical process, the solvent, precursors, and ligand have a gas-like medium with a high miscibility and fast diffusion rates. Therefore, at the high temperatures, all of the molecules are broken to simple compounds or initial elements. As a result, the supersaturated medium of the supercritical fluid provides a proper situation for nucleation of PbS nanocrystals passivated by oleic acid. After deposition of the top conductive contact (gold), the PV efficiency of the device was measured on a Schottky junction cell. An improvement in PV efficiency of the in situ deposited films is shown.
sonicated in isopropanol for 30 min. After rinsing with DI water, the samples were sonicated in a DI bath for 30 min and finally dried by a nitrogen gas flow. A mixture of oleic acid and toluene (40 vol.%) was prepared by stirring for 60 min. Lead ethylxanthate (1:5 M ratio with respect to oleic acid) was added into 100 ml of the solution with continuous stirring for 5 min. This solution was transferred to a supercritical fluid reactor with 50 ml volume and the half of reactor was filled by the solution (25 ml). The pH was adjusted to 5.7 by adding NaOH. A cleaned FTO substrate was placed in the bottom of the vessel upward to in situ deposit the synthesized PbS nanocrystals. Three processing temperatures of 140, 270 and 330 C were examined. The reactor was put in the furnace, quickly heated (30 C/min) to the setting temperature, and held for 20 min. Afterward, the vessel was removed from the furnace and quenched in a cold water bath (~5 C) to decrease the temperature quickly. Finally, the substrate was removed from the reactor and washed several times with ethanol and isopropanol. To study the effect of the in situ deposition, the synthesized PbS QDs by supercritical method (without inserting the FTO substrate into the vessel) were separated from the solution by centrifugation
2. Experimental procedure 2.1. Materials Lead acetate, ethyl-xanthic acid, oleic acid, anhydrous toluene, ethanol, deionized water, isopropyl alcohol, and Triton X-100 were purchased from Merck (Germany) and used without purification. To prepare lead ethylxanthate, an aqueous solution of lead acetate was added dropwise to an aqueous solution of ethyl-xanthic acid (potassium salt) until the lead acetate to xanthate molar ratio reached 1:2. The lead ethylxanthate was filtered and washed several times with ethanol and dried at room temperature. 2.2. Supercritical fluid deposition of PbS nanocrystals Commercial FTO substrates with an ohmic resistance of 8 U were supplied from sigma Aldrich, USA. The plates were cut to squares with dimensions of 1.5 cm 1.5 cm and cleaned prior to the usage. At first, the samples were immersed in DI water (Milipore, 18 M U-cm) containing 3vol.% Triton X-100 and sonicated for 30 min. The specimens were then rinsed with DI water and
Fig. 1. (a) XRD patterns and (b) FTIR spectra of PbS QDs prepared at three different temperatures.
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(4000 rpm), washed several times by ethanol, and finally redispersed in octane to prepare a solution of PbS QDs (120 mg/ ml). The colloidal PbS nanocrystals were then deposited on the FTO substrate via a layer-by-layer procedure by employing the spin coating procedure at a rotating speed of 2500 rpm for 30 s. Eight layers were deposited to prepare the thin films. The prepared PbS QDs were characterized by high resolution transmission electron microscopy (HRTEM), UVevisible spectroscopy, and photoluminescence (PL) spectroscopy. 2.3. Device fabrication and photovoltaic measurement Gold contacts were deposited on the absorber layer by sputtering at a rate of 0.4 A /s. The thickness of the gold contact was about 50 nm. The solar spectrum at AM1.5G is simulated with a Xe lamp and filters with an intensity of 100 mWcm2. The currentevoltage (JeV) data were measured using a Keithley 2400
(USA) instrument. JeV sweeps were performed between 1 and þ1 V, with a step size of 0.02 V and a delay time of 200 ms at each point. 2.4. Characterization techniques Various analytical techniques were utilized to characterize the prepared nanocrystals, deposited films, and devices. Highresolution transmission electron microscopy (HRTEM, JOL, JEM2100, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS) was employed for structural studies and size distribution. Phase characterization was performed by X-ray diffraction method (XRD, Stone Sandi P, USA) utilizing a CuKa radiation. The thickness, morphology and roughness of the films were studied by field-emission scanning electron microscopy (FESEM, Hitachi S4160, Japan) and atomic force microscopy (AFM, JPK Co, Germany). Fourier transformed infrared spectroscopy (FTIR, PerkineElmer, Spectrum RX, USA) was utilized to study passivation of PbS QDs by the oleic acid molecules. The optical absorption (Varian Carry 500, USA) and photoluminescence quantum efficiency (PLQE at 670 nm) of the semiconductor nanocrystals were also measured.
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changing the fluid phase from subcritical to supercritical fluid yielded well crystalline PbS QDs. Results of atomic absorption spectroscopy (AAS, Varin, USA) of the remained liquid in the reactor showed that the concentration of residual lead was ~100 ppm (140 C), 25 ppm (for 270 C), and 1 ppm (for 330 C). This measurement indicated that under supercritical condition, the chemical reaction was completed; thereby, fully crystalline QDs were obtained. The unique properties of the supercritical medium including gas-like diffusivity, low viscosity, and the density closer to that of liquid are the advantages of the utilized procedure [25]. FTIR was employed to study the capping of PbS nanocrystals with the oleic acid molecules (Fig. 1b). The peaks at around 1398 cm1 and 1528 cm1 are ascribed to carboxylate group of oleic acid and the peaks around 2849 cm1 and 2918 cm1 are related to CH2 bonds [32]. The possible chemical reactions, which led to the formation of PbS nanocrystals, can be schematically presented as below:
HRTEM image and EDS spectrum of the processed nanocrystals under the supercritical fluid condition (330 C) are shown in Fig. 2a and b. Ultrafine PbS crystals with an average size of ~3 nm are seen. The interplanar spacing in the crystalline structure of PbS is 0.323 nm, which corresponds to the distance between two (111) planes of the cubic phase. A low-magnified TEM image of the nanocrystals is shown in Fig. 2c. The particle distribution was determined by image analyzing (CLEMEX-V1.4 software) of 3 different TEM images. As it is seen and supported by the size distribution graph (Fig. 2d), the nanocrystals are uniform with a narrow size distribution. We predicted the bandgap (Eg) of PbS QDs based on their average diameter (d, nm) by [33]: Eg ¼ 0.41 þ (0.0252d2 þ 0.283d)1 (eV)
(3)
The bandgap energy is about 1.3 eV that is well situated for sun harvesting in near-IR region [4]. Fig. 2e shows UVeVis spectrum of the prepared PbS colloidal nanocrystals. A well-defined excitonic peak at about 913 nm was seen. Photoluminescence quantum efficiency (PLQE) spectrum of the nanocrystals is shown in Fig. 2f. The high quantum efficiency of the nanocrystals (29%) can be attributed to their high crystallinity and good passivation by the organic molecules [32].
3. Results 3.2. Characteristics of the deposited films 3.1. Characterization of PbS nanocrystals XRD patterns of the prepared PbS nanocrystals are shown in Fig. 1a. Cubic lead sulfide (JCPDS 02-0699) without impurities of its oxides (within the detection limit of XRD) was obtained at three temperatures of 140, 270 and 330 C. Since the thickness of the films was almost equal (300 ± 30 mm, as determined by an upstepper equipment, Alpha-Step 200, USA), the intensity of XRD peaks demonstrates improved crystallinity of PbS nanocrystals with increasing the processing temperature. The supercritical fluid temperature of toluene is about 319 C [31]; hence, it appeared that
Cross-sectional SEM images of the in situ deposited film and spin-coated layer are shown in Fig. 3a and b. The films are uniform with a thickness of ~300 nm. AFM images of the top surfaces are shown in Fig. 3c and d. The surface roughness (RMS) is about 4 ± 1.5 nm and 7 ± 2 nm for the in situ deposited and spin-coated films, respectively. The height diagrams shown in Fig. 3e and f indicate that the supercritical fluid film is more uniform and compacted as compared to the spin-coated layer. The absorption spectra of the films are shown in Fig. 3g. The excitonic peak for the spin-coated and supercritical-processed films is located at 920 nm
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Fig. 2. Characterizations of nanocrystals prepared by supercritical fluid toluene at 330 C: (a) HRTEM image and (b) EDS spectrum indicate PbS QDs with an average size of about 3 nm were synthesized; (c) Low-magnified TEM image and (b) corresponding particle size distribution graph determine that the nanocrystals are uniform with narrow size distribution; (e) and (f) show UVevisible and PL spectrum of the nanocrystals, respectively.
and 945 nm, respectively. The slight red shift compared to the excitonic peak of PbS QDs (913 nm) can be attributed to the decreased interparticle spacing [32]. As it is seen, the supercritical film exhibits more shifts compared to the spin-coated layer, which can be an indicator of more compactness.
3.3. Photovoltaic performance Fig. 4 shows JeV plot for the prepared solar cells. The figure of merits for the prepared cells is reported in Table 1. The results of previous investigations [34e36] on oleic-acid capped PbS Schottky
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Fig. 3. (a, b) Cross-sectional FESEM and (c, d) AFM images of the films prepared by (a, c) spin-coating and (b,d) supercritical deposition. AFM height profiles of (e) spin-coated and (f) supercritical-deposited films. (g) UVevisible spectra of PbS films.
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Fig. 4. Photovoltaic performance of PbS quantum dot solar cells.
Table 1 Figures of merit for the prepared Schottky devices. The data are average values of PV measurement on 50 contacts (spots) prepared on 4 separate substrates (devices). For comparison, photovoltaic performance of similar devices reported by others is presented. Solar cell
Voc (V)
Jsc (mA/cm2)
Fill factor
Efficiency (%)
Reference
Supercritical fluid Spin coating
0.32 ± 0.03
8.4 ± 0.8
0.34 ± 0.03
0.89 ± 0.15
0.31 ± 0.04
7.5 ± 1.2
0.33 ± 0.05
0.77 ± 0.11
0.4
0.13
0.38
0.02
Present work Present work [34]
0.2
8.99
0.39
0.7
[35]
0.17
11.01
0.48
0.9
[36]
Au/P3HT/ PbS/ITO ITO/PbS/ a-Si/Al ITO/PbS/Al
devices were also included in the table for comparison. The short circuit current and fill factor of the absorbing layer made by the supercritical process is higher than those prepared by the spin coating. As a result, the PV performance was improved by about 10%. It should be noted that the performance of QD solar devices would be higher if depleted bulk heterojunctiuon (DBH) cells were fabricated. This is because the n-type semiconductor layer (such as TiO2) enhances the depletion region and extraction of the charge carriers as the DBH architecture relies on a depletion region to provide field-driven charge separation and transport [37,38]. Doping and passivation of PbS QDs by transition metals (such as Cd) and/or utilizing short linker molecules also improve PV performance [4]. Anyway, in a comparative study between in situ synthesis and deposition with conventional chemical routes, the supercritical process seems to be promising approach, as reported in Table 1. The reported data in Table 1 are average values of PV measurement on 50 different contacts (spots) prepared on 4 separate devices. 4. Discussion A facile supercritical method was presented for synthesizing highly crystalline PbS QDs with narrow size distribution. Under the supercritical condition, the solvent, precursors, and ligand behave as a gas-like medium with a high miscibility and fast diffusion rates (typical of gasses) but with sufficiently high densities for solubilizing a wide range of compounds of low to medium polarity
(typical of liquids) [26]. Thus, the inherent limitations of conventional liquid solvents are prevented and well mixed constituents are attained [23]. At high temperatures, the large molecules of precursor (lead ethylxanthate) are broken to simple compounds or initial elements (see reaction 2). Meanwhile, the supersaturated medium of the supercritical fluid facilitates nucleation of PbS nanocrystals. Surfaces of the formed nuclei are passivated by the oleic acid molecules while the rapid crystal growth is retarded at the supercritical pressure. Upon the short residence time at the high temperature/pressure (20 min), the lead precursor is totally consumed (the residual lead concentration was 1 ppm as determined by AAS analysis); thereby, ultrafine and mono-dispersed nanocrystals are prepared. It is noteworthy to mention that the concentration of lead precursor, the temperature of supercritical fluid and the processing time were studied in this work via a series of experiments, which the details were not presented here. The in situ deposited PbS QD film was more uniform and compacted compared to the spin-coated layer. Supercritical fluid is known to have a near-zero surface tension that provides an ideal medium for deposition of small clusters to thin layers of ordered arrays [39]. In conventional methods such as spin coating, isolated and percolating domains are commonly formed. In supercritical fluid deposition, however, the nanocrystals are prone to deposit through a gas-antisolvent mechanism to compact and uniform films. The deposition mechanism of PbS nanocrystals on the FTO substrate is related to the reduced polarity of the toluene solvent [17], which decreases the colloidal stability. Since the nanocrystals are synthesized and deposited on the glass substrate at a high pressure, the film is very uniform and compact. Consequently, the current density of the Schottky device was enhanced compared to the spin-coated film (see Table 1). The improved transport of the charged carriers in PbS QDs is commonly linked to short interparticle distances (higher compactness) and better passivation of midband states while open voltage is mainly ascribed to the molecular length of the organic phase and passivation of deep trap states by transition metals [40]. Therefore, it seems that the better film formability of supercritical deposition is the main reason for the improved performance. 5. Conclusions A novel, one-step, and solution-process route is introduced to prepare uniform PbS films for solar cell applications. Supercritical fluid toluene was used to synthesize the nanocrystals and deposit them on FTO substrates simultaneously. The main findings are summarized below. Crystalline PbS QDs with narrow size distribution were successfully prepared by shifting the subcritical conditions to supercritical liquid. Fully crystalline and mono-dispersed PbS nanocrystals with an average size of 3 nm were prepared by supercritical fluid at 330 C for 20 min. In situ capping and passivation of the nanocrystals with oleic acid were attained during supercritical processing. Uniform films with a thickness of 300 nm were obtained by in situ supercritical deposition. As compared to the spin-coated layers, the films were denser and uniform as evidenced by AFM and optical absorption. Measurement of PV performance showed higher short circuit current and fill factor for the in situ deposited film. References [1] G. Wang, L. Wang, W. Xing, S. Zhuo, A novel counter electrode based on
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Supercritical synthesis and in situ deposition of PbS nanocrystals with oleic acid passivation for quantum dot solar cells M.M. Tavakoli a, A. Simchi a, b, *, H. Aashuri a a b
Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran Institute for Nanoscience and Nanotechnology, Sharif University of Technology, 14588 Tehran, Iran
h i g h l i g h t s Supercritical fluid processing and in situ deposition of PbS QDs are presented. The prepared nanocrystals are mono-dispersed with an optical bandgap of 1.3 eV. Photovoltaic performance of the in situ deposited nanocrystals is reported. An improved PV performance compared to spin coated Schottky solar cells is shown.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2014 Received in revised form 23 February 2015 Accepted 26 February 2015 Available online 3 March 2015
Colloidal quantum dot solar cells have recently attracted significant attention due to their low-processing cost and surging photovoltaic performance. In this paper, a novel, reproducible, and simple solutionbased process based on supercritical fluid toluene is presented for in situ growth and deposition PbS nanocrystals with oleic-acid passivation. A lead precursor containing sulfur was mixed with oleic acid in toluene and processed in a supercritical fluid condition at different temperatures of 140, 270 and 330 C for 20 min. The quantum dots were deposited on a fluorine-doped tin oxide glass substrate inside the supercritical reactor. Transmission electron microscopy, X-ray diffraction, absorption and dynamic light scattering showed that the nanocrystals processed at the supercritical condition (330 C) are fully crystalline with a narrow size distribution of ~3 nm with an absorption wavelength of 915 nm (bandgap of 1.3 eV). Fourier transform infrared spectroscopy indicated that the PbS quantum dots are passivated by oleic acid molecules during the growth. Photovoltaic characteristics of Schottky junction solar cells showed an improvement over devices prepared by spin-coating. © 2015 Elsevier B.V. All rights reserved.
Keywords: Inorganic compounds Chalcogenides Electronic materials Chemical synthesis
1. Introduction Solution-processed photovoltaic devices have attracted increasing attention for solar harvesting because of their low processing cost, simplified procedure, and large-area coverage [1,2]. Inorganic nanocrystals with tunable bandgap have a great potential in thin film solar cells as they can easily be processed by straight forward and simple chemical routes [3,4]. Among various semiconductor nanocrystals, PbS quantum dots (QDs) are very promising for photovoltaic applications due to their high Bohr radius (~20 nm) and tunable bandgap to harvest the maximum part of
* Corresponding author. Department of Materials Science and Engineering, Sharif University of Technology, 14588 Tehran, Iran. E-mail address: [email protected] (A. Simchi). http://dx.doi.org/10.1016/j.matchemphys.2015.02.043 0254-0584/© 2015 Elsevier B.V. All rights reserved.
solar spectrum in the near infrared region [5,6]. Therefore, synthesis and characterization of PbS nanocrystals by various chemical procedures have frequently been studied [7e10]. It is important to mention that the surface trap states of PbS nanocrystals must be passivated by using some agents such as organic molecules because the trap states cause recombination of charge carriers which reduces the photoelectrical performance [11]. The nanocrystals were then deposited on conductive substrates by dip-coating or spincoating. In order to improve photovoltaic (PV) efficiency, an additional ligand-exchange processing step by short thiol molecules [12,13] or halides [14,15] should be utilized. Recent studies [16,17] have shown that constructing uniform and compact films with short interparticle spacing is a key factor to obtain improved PV efficiency in devices based on PbS QDs. Since the ligand-exchange processing step affects the compactness and uniformity of the
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colloidal quantum dot films, direct deposition of colloidal inks was demonstrated and a PV efficiency of 2.1% was reported [18]. Supercritical fluid processing is a facile and promising method for the synthesis of various inorganic semiconductor nanocrystals [19e22]. Synthesis of QDs in supercritical fluid condition offers several advantages over conventional chemical routes including enhanced mass and heat transfer at elevated temperatures [23], reproducibility [24], low reagent consumption [25], and ability for automation and continuous production [26]. Studies on the supercritical synthesis of CdSe [23], CdS [24], PbS [25], and ZnS [25] nanocrystals have demonstrated the potential of the process in fine and precise control of parameters to attain nanocrystals with narrow size distribution. Recent advances in supercritical synthesis of inorganic semiconductor nanocrystals have dealt with in situ growth of nanocrystals on conducting substrates. Wang et al. [17] reported supercritical fluid (CO2) deposition of PbS QDs on glass substrates for energy transfer studies. Close-packed two-dimensional and three-dimensional arrays of the nanocrystals were attained. The potential of supercritical fluid to deposit nanoparticles in small structures or to form thin films of ordered arrays has also been demonstrated in other investigations [25e30]. In these studies, the nanocrystals were synthesized and then deposited on a substrate via different procedures. The unique feature of the present work is that, the synthesis and deposition of QDs on a conductive glass substrate were performed simultaneously. In other words, an in situ synthesis and deposition procedure based on one-step, reproducible and simple supercritical fluid was developed to prepare thin films of inorganic QDs for PV applications. The potential application of the method is shown for the preparation and deposition of oleic-acid capped PbS QDs on the surface of FTO glasses. To the best knowledge of the authors, this is a pioneer report on the fabrication of PbS QD solar cells by in situ supercritical deposition utilizing cheap precursors. During the supercritical process, the solvent, precursors, and ligand have a gas-like medium with a high miscibility and fast diffusion rates. Therefore, at the high temperatures, all of the molecules are broken to simple compounds or initial elements. As a result, the supersaturated medium of the supercritical fluid provides a proper situation for nucleation of PbS nanocrystals passivated by oleic acid. After deposition of the top conductive contact (gold), the PV efficiency of the device was measured on a Schottky junction cell. An improvement in PV efficiency of the in situ deposited films is shown.
sonicated in isopropanol for 30 min. After rinsing with DI water, the samples were sonicated in a DI bath for 30 min and finally dried by a nitrogen gas flow. A mixture of oleic acid and toluene (40 vol.%) was prepared by stirring for 60 min. Lead ethylxanthate (1:5 M ratio with respect to oleic acid) was added into 100 ml of the solution with continuous stirring for 5 min. This solution was transferred to a supercritical fluid reactor with 50 ml volume and the half of reactor was filled by the solution (25 ml). The pH was adjusted to 5.7 by adding NaOH. A cleaned FTO substrate was placed in the bottom of the vessel upward to in situ deposit the synthesized PbS nanocrystals. Three processing temperatures of 140, 270 and 330 C were examined. The reactor was put in the furnace, quickly heated (30 C/min) to the setting temperature, and held for 20 min. Afterward, the vessel was removed from the furnace and quenched in a cold water bath (~5 C) to decrease the temperature quickly. Finally, the substrate was removed from the reactor and washed several times with ethanol and isopropanol. To study the effect of the in situ deposition, the synthesized PbS QDs by supercritical method (without inserting the FTO substrate into the vessel) were separated from the solution by centrifugation
2. Experimental procedure 2.1. Materials Lead acetate, ethyl-xanthic acid, oleic acid, anhydrous toluene, ethanol, deionized water, isopropyl alcohol, and Triton X-100 were purchased from Merck (Germany) and used without purification. To prepare lead ethylxanthate, an aqueous solution of lead acetate was added dropwise to an aqueous solution of ethyl-xanthic acid (potassium salt) until the lead acetate to xanthate molar ratio reached 1:2. The lead ethylxanthate was filtered and washed several times with ethanol and dried at room temperature. 2.2. Supercritical fluid deposition of PbS nanocrystals Commercial FTO substrates with an ohmic resistance of 8 U were supplied from sigma Aldrich, USA. The plates were cut to squares with dimensions of 1.5 cm 1.5 cm and cleaned prior to the usage. At first, the samples were immersed in DI water (Milipore, 18 M U-cm) containing 3vol.% Triton X-100 and sonicated for 30 min. The specimens were then rinsed with DI water and
Fig. 1. (a) XRD patterns and (b) FTIR spectra of PbS QDs prepared at three different temperatures.
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(4000 rpm), washed several times by ethanol, and finally redispersed in octane to prepare a solution of PbS QDs (120 mg/ ml). The colloidal PbS nanocrystals were then deposited on the FTO substrate via a layer-by-layer procedure by employing the spin coating procedure at a rotating speed of 2500 rpm for 30 s. Eight layers were deposited to prepare the thin films. The prepared PbS QDs were characterized by high resolution transmission electron microscopy (HRTEM), UVevisible spectroscopy, and photoluminescence (PL) spectroscopy. 2.3. Device fabrication and photovoltaic measurement Gold contacts were deposited on the absorber layer by sputtering at a rate of 0.4 A /s. The thickness of the gold contact was about 50 nm. The solar spectrum at AM1.5G is simulated with a Xe lamp and filters with an intensity of 100 mWcm2. The currentevoltage (JeV) data were measured using a Keithley 2400
(USA) instrument. JeV sweeps were performed between 1 and þ1 V, with a step size of 0.02 V and a delay time of 200 ms at each point. 2.4. Characterization techniques Various analytical techniques were utilized to characterize the prepared nanocrystals, deposited films, and devices. Highresolution transmission electron microscopy (HRTEM, JOL, JEM2100, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS) was employed for structural studies and size distribution. Phase characterization was performed by X-ray diffraction method (XRD, Stone Sandi P, USA) utilizing a CuKa radiation. The thickness, morphology and roughness of the films were studied by field-emission scanning electron microscopy (FESEM, Hitachi S4160, Japan) and atomic force microscopy (AFM, JPK Co, Germany). Fourier transformed infrared spectroscopy (FTIR, PerkineElmer, Spectrum RX, USA) was utilized to study passivation of PbS QDs by the oleic acid molecules. The optical absorption (Varian Carry 500, USA) and photoluminescence quantum efficiency (PLQE at 670 nm) of the semiconductor nanocrystals were also measured.
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changing the fluid phase from subcritical to supercritical fluid yielded well crystalline PbS QDs. Results of atomic absorption spectroscopy (AAS, Varin, USA) of the remained liquid in the reactor showed that the concentration of residual lead was ~100 ppm (140 C), 25 ppm (for 270 C), and 1 ppm (for 330 C). This measurement indicated that under supercritical condition, the chemical reaction was completed; thereby, fully crystalline QDs were obtained. The unique properties of the supercritical medium including gas-like diffusivity, low viscosity, and the density closer to that of liquid are the advantages of the utilized procedure [25]. FTIR was employed to study the capping of PbS nanocrystals with the oleic acid molecules (Fig. 1b). The peaks at around 1398 cm1 and 1528 cm1 are ascribed to carboxylate group of oleic acid and the peaks around 2849 cm1 and 2918 cm1 are related to CH2 bonds [32]. The possible chemical reactions, which led to the formation of PbS nanocrystals, can be schematically presented as below:
HRTEM image and EDS spectrum of the processed nanocrystals under the supercritical fluid condition (330 C) are shown in Fig. 2a and b. Ultrafine PbS crystals with an average size of ~3 nm are seen. The interplanar spacing in the crystalline structure of PbS is 0.323 nm, which corresponds to the distance between two (111) planes of the cubic phase. A low-magnified TEM image of the nanocrystals is shown in Fig. 2c. The particle distribution was determined by image analyzing (CLEMEX-V1.4 software) of 3 different TEM images. As it is seen and supported by the size distribution graph (Fig. 2d), the nanocrystals are uniform with a narrow size distribution. We predicted the bandgap (Eg) of PbS QDs based on their average diameter (d, nm) by [33]: Eg ¼ 0.41 þ (0.0252d2 þ 0.283d)1 (eV)
(3)
The bandgap energy is about 1.3 eV that is well situated for sun harvesting in near-IR region [4]. Fig. 2e shows UVeVis spectrum of the prepared PbS colloidal nanocrystals. A well-defined excitonic peak at about 913 nm was seen. Photoluminescence quantum efficiency (PLQE) spectrum of the nanocrystals is shown in Fig. 2f. The high quantum efficiency of the nanocrystals (29%) can be attributed to their high crystallinity and good passivation by the organic molecules [32].
3. Results 3.2. Characteristics of the deposited films 3.1. Characterization of PbS nanocrystals XRD patterns of the prepared PbS nanocrystals are shown in Fig. 1a. Cubic lead sulfide (JCPDS 02-0699) without impurities of its oxides (within the detection limit of XRD) was obtained at three temperatures of 140, 270 and 330 C. Since the thickness of the films was almost equal (300 ± 30 mm, as determined by an upstepper equipment, Alpha-Step 200, USA), the intensity of XRD peaks demonstrates improved crystallinity of PbS nanocrystals with increasing the processing temperature. The supercritical fluid temperature of toluene is about 319 C [31]; hence, it appeared that
Cross-sectional SEM images of the in situ deposited film and spin-coated layer are shown in Fig. 3a and b. The films are uniform with a thickness of ~300 nm. AFM images of the top surfaces are shown in Fig. 3c and d. The surface roughness (RMS) is about 4 ± 1.5 nm and 7 ± 2 nm for the in situ deposited and spin-coated films, respectively. The height diagrams shown in Fig. 3e and f indicate that the supercritical fluid film is more uniform and compacted as compared to the spin-coated layer. The absorption spectra of the films are shown in Fig. 3g. The excitonic peak for the spin-coated and supercritical-processed films is located at 920 nm
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Fig. 2. Characterizations of nanocrystals prepared by supercritical fluid toluene at 330 C: (a) HRTEM image and (b) EDS spectrum indicate PbS QDs with an average size of about 3 nm were synthesized; (c) Low-magnified TEM image and (b) corresponding particle size distribution graph determine that the nanocrystals are uniform with narrow size distribution; (e) and (f) show UVevisible and PL spectrum of the nanocrystals, respectively.
and 945 nm, respectively. The slight red shift compared to the excitonic peak of PbS QDs (913 nm) can be attributed to the decreased interparticle spacing [32]. As it is seen, the supercritical film exhibits more shifts compared to the spin-coated layer, which can be an indicator of more compactness.
3.3. Photovoltaic performance Fig. 4 shows JeV plot for the prepared solar cells. The figure of merits for the prepared cells is reported in Table 1. The results of previous investigations [34e36] on oleic-acid capped PbS Schottky
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Fig. 3. (a, b) Cross-sectional FESEM and (c, d) AFM images of the films prepared by (a, c) spin-coating and (b,d) supercritical deposition. AFM height profiles of (e) spin-coated and (f) supercritical-deposited films. (g) UVevisible spectra of PbS films.
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Fig. 4. Photovoltaic performance of PbS quantum dot solar cells.
Table 1 Figures of merit for the prepared Schottky devices. The data are average values of PV measurement on 50 contacts (spots) prepared on 4 separate substrates (devices). For comparison, photovoltaic performance of similar devices reported by others is presented. Solar cell
Voc (V)
Jsc (mA/cm2)
Fill factor
Efficiency (%)
Reference
Supercritical fluid Spin coating
0.32 ± 0.03
8.4 ± 0.8
0.34 ± 0.03
0.89 ± 0.15
0.31 ± 0.04
7.5 ± 1.2
0.33 ± 0.05
0.77 ± 0.11
0.4
0.13
0.38
0.02
Present work Present work [34]
0.2
8.99
0.39
0.7
[35]
0.17
11.01
0.48
0.9
[36]
Au/P3HT/ PbS/ITO ITO/PbS/ a-Si/Al ITO/PbS/Al
devices were also included in the table for comparison. The short circuit current and fill factor of the absorbing layer made by the supercritical process is higher than those prepared by the spin coating. As a result, the PV performance was improved by about 10%. It should be noted that the performance of QD solar devices would be higher if depleted bulk heterojunctiuon (DBH) cells were fabricated. This is because the n-type semiconductor layer (such as TiO2) enhances the depletion region and extraction of the charge carriers as the DBH architecture relies on a depletion region to provide field-driven charge separation and transport [37,38]. Doping and passivation of PbS QDs by transition metals (such as Cd) and/or utilizing short linker molecules also improve PV performance [4]. Anyway, in a comparative study between in situ synthesis and deposition with conventional chemical routes, the supercritical process seems to be promising approach, as reported in Table 1. The reported data in Table 1 are average values of PV measurement on 50 different contacts (spots) prepared on 4 separate devices. 4. Discussion A facile supercritical method was presented for synthesizing highly crystalline PbS QDs with narrow size distribution. Under the supercritical condition, the solvent, precursors, and ligand behave as a gas-like medium with a high miscibility and fast diffusion rates (typical of gasses) but with sufficiently high densities for solubilizing a wide range of compounds of low to medium polarity
(typical of liquids) [26]. Thus, the inherent limitations of conventional liquid solvents are prevented and well mixed constituents are attained [23]. At high temperatures, the large molecules of precursor (lead ethylxanthate) are broken to simple compounds or initial elements (see reaction 2). Meanwhile, the supersaturated medium of the supercritical fluid facilitates nucleation of PbS nanocrystals. Surfaces of the formed nuclei are passivated by the oleic acid molecules while the rapid crystal growth is retarded at the supercritical pressure. Upon the short residence time at the high temperature/pressure (20 min), the lead precursor is totally consumed (the residual lead concentration was 1 ppm as determined by AAS analysis); thereby, ultrafine and mono-dispersed nanocrystals are prepared. It is noteworthy to mention that the concentration of lead precursor, the temperature of supercritical fluid and the processing time were studied in this work via a series of experiments, which the details were not presented here. The in situ deposited PbS QD film was more uniform and compacted compared to the spin-coated layer. Supercritical fluid is known to have a near-zero surface tension that provides an ideal medium for deposition of small clusters to thin layers of ordered arrays [39]. In conventional methods such as spin coating, isolated and percolating domains are commonly formed. In supercritical fluid deposition, however, the nanocrystals are prone to deposit through a gas-antisolvent mechanism to compact and uniform films. The deposition mechanism of PbS nanocrystals on the FTO substrate is related to the reduced polarity of the toluene solvent [17], which decreases the colloidal stability. Since the nanocrystals are synthesized and deposited on the glass substrate at a high pressure, the film is very uniform and compact. Consequently, the current density of the Schottky device was enhanced compared to the spin-coated film (see Table 1). The improved transport of the charged carriers in PbS QDs is commonly linked to short interparticle distances (higher compactness) and better passivation of midband states while open voltage is mainly ascribed to the molecular length of the organic phase and passivation of deep trap states by transition metals [40]. Therefore, it seems that the better film formability of supercritical deposition is the main reason for the improved performance. 5. Conclusions A novel, one-step, and solution-process route is introduced to prepare uniform PbS films for solar cell applications. Supercritical fluid toluene was used to synthesize the nanocrystals and deposit them on FTO substrates simultaneously. The main findings are summarized below. Crystalline PbS QDs with narrow size distribution were successfully prepared by shifting the subcritical conditions to supercritical liquid. Fully crystalline and mono-dispersed PbS nanocrystals with an average size of 3 nm were prepared by supercritical fluid at 330 C for 20 min. In situ capping and passivation of the nanocrystals with oleic acid were attained during supercritical processing. Uniform films with a thickness of 300 nm were obtained by in situ supercritical deposition. As compared to the spin-coated layers, the films were denser and uniform as evidenced by AFM and optical absorption. Measurement of PV performance showed higher short circuit current and fill factor for the in situ deposited film. References [1] G. Wang, L. Wang, W. Xing, S. Zhuo, A novel counter electrode based on
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