Nanoscale multilayer PbS thin films fabricated by liquid–liquid interface reaction technique for solar photovoltaic applications

Materials Science and Engineering B 132 (2006) 170–173

Nanoscale multilayer PbS thin films fabricated by liquid–liquid interface reaction technique for solar photovoltaic applications R.R. Hawaldar a , G.G. Umarji a , S.A. Ketkar a , S.D. Sathaye b , U.P. Mulik a , D.P. Amalnerkar a,∗ a

Center for Materials for Electronics Technology, Panchwati off Pashan Road, Pune 411008, India b Korea Research Institute of Chemical Technology (KRICT), Daejon, South Korea

Abstract In the present communication, the self-assembly of nanocrystalline PbS at the liquid–liquid interface is reported. The PbS nanocrystals were, subsequently, transformed in the form of thin films by dip coating. The resultant films were characterized by SEM-EDAX, TEM-SAED, XPS and UV–visible spectroscopy. Pyramidal features at the nanometer scale and a sharp excitonic peak at 656 nm are the salient aspects of this work. The band gap of the order of 1.8 eV (associated with the excitonic feature) is ideally suited for solar photovoltaic applications. © 2006 Elsevier B.V. All rights reserved. Keywords: PbS; Nanostructures; Liquid–liquid interface; Nanopyramid

1. Introduction Interest in nano-materials has fuelled up from the idea that they may boast superior electrical, chemical, mechanical or optical properties—at least in theory. Especially, nanocrystalline semiconducting materials possessing crystallite sizes comparable with Bohr radius exhibit quantum size effect (QSE), which leads to atom like discrete energy states within the nanocrystal that are, in turn, a function of the nanocrystal diameter. As many optical and electronic properties are dependent upon the energy and density of the electron states, engineering the size of the tiny structures can alter them. Many properties including onset of absorption (band gap), peak fluorescence wavelength, non-linear effects, electro- and magneto-optic effects can be tailored or enhanced. The absorption properties are of particular importance in photovoltaic applications. Semiconductor nanocrystals are potentially ideal for greatly enhancing the efficiency of solar cells, concomitantly, decreasing the cost of their fabrication. It is intriguing to think of photovoltaic (PV) devices based on nanocrystalline semiconducting films. Since nanocrystals can be engineered such that the band gap falls between 1.4 and 2 eV, they can be used to optimise single junction solar cells and therefore can offer a best chance of approaching the Shockley–Quessier efficiency limit of 31%. An encouraging

∗

Corresponding author. Tel.: +91 20 25898141; fax: +91 20 25898085. E-mail address: [email protected] (D.P. Amalnerkar).

0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.02.004

breakthrough in realizing the maximum attainable thermodynamic conversion efficiency of solar photon conversion past the Shockley–Quessier limit, upto 66% can be achieved for nanocrystal based solar cells by utilizing hot photo-generated carriers to produce higher photo-voltages or higher photocurrents or by creating multiple junction cells [1]. In this context, we foresee Q-PbS as a big chill pertaining to its large Bohr radius (20 nm), high dielectric constant (17.3) and a narrow band gap of 0.41 eV, ensuring strong quantum confinement. Most importantly, the band gap in Q-PbS can be tuned to any desired value between 0.41 and 5 eV [2]. In particular, this excellent band gap engineering amenability can make Q-PbS a forerunner in solar photovoltaic conversion which demands absorption between 1.4 and 2 eV [3]. With this application in mind, we have noted that till date, numerous reports related to the synthesis of Q-PbS involving stabilization in strong organic/inorganic matrix supports [4–8], polymers [9,10] forming nanocomposite, etc., are available. Other approaches using inverse micelle [11,12] as protecting media or microemulsion [13] systems as nanoreactors, microbial synthesis [14], syntheses at air/water interface using amphiphilic monolayers [15,16] as stabilizers or epitaxial formation on fatty acid monolayers [17] (using Langmuir–Blodgett method) have received considerable attention. It is well established that the stability of the nanocrystallites, particle size and consequent properties of nanoparticles strongly depend on the specific method and the experimental conditions of preparation. This can be related to the inherent property of the nanoparticles, namely the spontaneous aggrega-

R.R. Hawaldar et al. / Materials Science and Engineering B 132 (2006) 170–173

tion by minimizing the surface energy. Because of the possibility of using liquid phase and relatively low processing temperatures, it is possible to create junctions on inexpensive substrates such as coated glass, metal sheets, etc., and dispense with the costly microfabrication techniques used to make contemporary solar cells. In the present study, we used the liquid–liquid interface reaction technique (LLIRT) [18] that allowed us to produce thin nanocrystalline particulate films of Q-PbS. The important feature of the method is that it is free from capping/stabilizing agents. 2. Experimental 2.1. Materials The chemicals used in the present work were of analytical grade. Lead nitrate (99%, Aldrich), oleic acid (99%, Sigma), carbon tetrachloride (S.D. fine) were purchased. All the chemicals were used as received except carbon tetrachloride that was distilled and stored on molecular sieve type 4A. Deionised water was used for all experiments. 2.2. Method of preparation of film In this method, 0.2 ml solution of carbon tetrachloride (CCl4 ) saturated with H2 S was spread initially with the help of a syringe on a water surface containing 10−4 M Pb(NO3 )2 in polypropylene tray (15 cm × 15 cm × 2 cm). The reaction occurs at the interface resulting in a nanocrystalline PbS thin film. After all the CCl4 had evaporated, an oleic acid piston (pressure 30 dyn/cm) was used to compress the film slowly. The film was transferred onto a glass substrate (1 cm × 1 cm × 0.25 cm) by immersing

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vertically in the solution at a constant rate of 0.5 cm/min and lifting it vertically at the same rate so that the film covers the dipped area. This operation was repeated many times to get the desired film thickness. 2.3. Measurements The Q-PbS samples prepared by the LLIRT were characterized by transmission electron microscopy (TEM) along with selected area electron diffraction (SAED) followed by energy dispersive analysis by X-rays (EDAX), X-ray photoelectron spectroscopy (XPS) and optical spectroscopy. The optical absorbance measurements of the sample were conducted in the range 600–800 nm, with a resolution of 2 nm. All the measurements were done on single beam UV–vis diode array spectrophotometer (Hewlett-Packard 8452). Transmission electron micrographs and electron diffraction patterns of the virgin samples were obtained using JEOL, JEM-1200 EX electron microscope. For TEM examination, the film was deposited on a copper grid coated with collodion. The EDAX studies of the samples were carried out on JEOL (6360 LA) SEM instrument. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG Scientific (UK) ESCA-3-MK-II electron spectrometer with Mg K␣ (1253.6 eV) radiation. 3. Results and discussions The samples can be depicted as an assemblage of many freestanding nanopyramids by the transmission electron micrographs presented in Fig. 1 (black/grey contrasts in white background). Pyramids with a square base and a triangular top were clearly observed even in the low-magnification TEM image

Fig. 1. Transmission electron micrographs (a–d) of PbS Thin film featuring nanopyramids.

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Fig. 2. Size distribution of PbS nanopyramids.

(Fig. 1a). Relatively high-magnification TEM image (Fig. 1b) endorse the formation of pyramids of much smaller size presumably indicating the initial generation of large numbers of primary nuclei. From the still high magnification TEM image (Fig. 1d), the nanopyramids can be distinctly identified having a square base of 250 nm2 . However, nanopyramidal features with still smaller sizes can be located in TEM images (Fig. 1b and c). TEM further revealed that the samples contained substantial percentage of nanopyramids of same size and shape (monodispersity) (Fig. 1b and c). As TEM is a two-dimensional projection of the actual three-dimensional structure, the height of the pyramids cannot be ordinarily measured with TEM. However, by following the procedure employed by Tannenbaum et al. [19], the minimum height can be estimated to be about 5 nm, in turn, implying quantum confinement at least along the c-axis, irrespective of basal dimensions of the nanopyramids. The average particle size (height, in this case) is 16 ± 2 nm (in terms of V1/3, as denoted by Tannenbaum et al.) and the particle size distribution is shown in the histogram furnished in Fig. 2. The selected area diffraction (SAED) (inset of Fig. 1d) evinced that the PbS nanopyramids are polycrystals. Also, the spots seen in the diffraction pattern represent unagglomerated small crystals. The diffraction peaks can be indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0), (3 2 1), with an expanded lattice constant a = 6.22 for the cubic crystal structure. This lattice expansion is well within the limits theoretically predicted by Olkhovets et al. [20]. The energy levels in the QD’s being a function of size quantization and not lattice constant, the later has no effect on the energy gap [20]. In order to gain further insight into the growth planes leading to pyramidal crystal habit, we attempted the HRTEM analysis of the samples. Quite interestingly, our samples got evaporated at 200 KeV (400,000 magnifications). The only plausible reason that could be thought of responsible

Fig. 3. EDAX spectra of nanoparticulate Pbs thin films.

for this type of behavior relates to the high effective temperature produced in the system due to high pressure required to generate high vacuum. It can be said that the complex transitory pyramidal morphology evolves due to irreversible evaporation of the solvent leading to high degree of local order and anisotropy during constant equilibrium fluctuations. The forces of surface tension hold the product thus formed. The high degree of supersaturation dictates the formation of large number of nuclei at the surface sub-phase; hence the exposed area of the particles will be much lower thus leading to formation of free nanoparticles. The film adheres to the substrate by Van der Waals forces, thus preventing ‘Oswald ripening’ [20,21]. PbS formation was confirmed by EDAX (Fig. 3). The formation of PbS has been unambiguously ascertained by XPS analysis of the samples. XP spectra revealed (i) doublet structure of S-2p with the peaks centered around 161.4 and 168.3 eV (Fig. 4b) and (ii) two peaks related to Pb-4f centered around 138.37 and 143.21 eV (Fig. 4a). These binding energy values are in close agreement with the values reported earlier for PbS [22]. The atomic ratio of Pb and S was calculated by considering the intensities of Pb-4f and S-2p levels and their corresponding photo-ionisation cross-sections. The ratio was found to be close to one suggesting the stoichiometry of PbS in the present case. The optical spectrum of our samples showed a very sharp peak (excitonic in nature) at 656 nm (Fig. 5). Such a sharp peak is hitherto unreported for Q-PbS. This peak is narrowed considerably and no satellite peaks of lower intensity are observed in the short and the long wavelength region of the spectrum. A red shift in the peak is noticed with increase in the number of dips.

Fig. 4. XP spectra of nanoparticulate PbS thin films: (a) Pb-4f and (b) S-2p.

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well-suited for photovoltaic applications (solid-state and photoelectrochemical routes). Further investigations in this direction are underway. To the best of our knowledge, this is the first report on the synthesis of PbS nanopyramids with average particle size 16 nm formed at liquid–liquid interface. Acknowledgements We are grateful to Mr. I.C. Rao (Director, CMET, Pune) for his kind interest and active support. We are indebted to Dr. U.P. Phadke (Advisor), Dr. K.S.K. Sai (Senior Director), Dr. U.C. Pandey (Director), Dr. Krishna Kumar (Director) and Dr. V.C. Sethi (Director) of the Department of Information Technology, Government of India for their affirmative support and critical suggestions. Generous Grant-in-Aid funding by the Department of Information Technology is gratefully acknowledged. References

Fig. 5. UV–visible spectra of nanoparticulate thin films of PbS corresponding to (a) 10 dips, (b) 30 dips, (c) 60 dips and (d) 100 dips, respectively.

This can be attributed to the increase in crystallite size with the increasing number of dips. In other words, it indicates transformation from cluster level to bulk state. The band gap calculated from the optical spectra amounts to be 1.8 eV. It is consistent with the fact that, in quantum dots, the energy of the lowest excitonic transition is the band gap [23]. The excitonic feature suggests that surface related charge separation and polarization effects are almost nil for the nanopyramids as no encapsulating agent was used to arrest the growth of nanoparticles. To date, many reports stating the fluorescence effect of PbS crystallites at room temperature or lower temperatures exists. No detectable fluorescence was observed for the nanopyramids. This indicates non-radiative relaxation of the photo-excited species [24]. The appearance of the excitonic peak undoubtedly confirms the superior quality of the nanopyramid clusters obtained in the present case. 4. Conclusion In the nutshell, the liquid–liquid interface reaction technique adopted by us leads to the formation of particulate film of QPbS exhibiting pyramidal features at nanometer scale as well as quite a pronounced excitonic peak at 656 nm. Interestingly, a band gap of 1.8 eV associated with this excitonic peak might be

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