Physical properties of pulse electrodeposited lead sulfide thin films
Introduction
Lead sulfide (PbS) is a narrow band gap IV–VI semiconductor material widely investigated for its technological applications such as infrared detector [1], [2], [3], solar control coatings [4], optoelectronic [5], [6], and photovoltaic devices [7]. The optical and electrical properties of this material are related strongly with the grain size and have been related with the growth conditions. The band gap (Eg) of PbS can be varied significantly by changing the grain size, and there are reports of opening the band gap to as high as 2.5 eV from its bulk value of 0.41 eV by decreasing the grain size to the order of 2 nm [8]. This notable change in band gap is a result of the small effective masses of electrons (me) and holes (mh); me = mh = 0.09 mo, and the large exciton Bohr radius (20 nm) [9].
PbS thin films can be obtained by different deposition techniques such as vacuum evaporation [10], successive ionic layer adsorption and reaction (SILAR) [11], chemical vapor deposition (CVD) [12], electrodeposition (ED) [13], [14], [15], [16], and chemical bath deposition (CBD) [17], [18]. Among the above mentioned methods CBD and ED are mostly used as low cost film growth techniques. The pulse electrodeposition (PED) where the excess metal ions and weak bonds can be removed by applying periodic oxidation (E2) and reduction (E1) potentials has been reported in the case of semiconductor thin films [19], [20]. Recently Thirumoorthy and Murali reported pulse electrodeposition of PbS thin films [14].
Photovoltaic devices based on PbS have been fabricated since 1970 and the photovoltaic effect has been demonstrated in CdS/PbS [21] and ZnxCd1−x S/PbS [22]. Watanabe and Mita reported CdS/PbS solar cells with open-circuit voltage (Voc) 400 mV and short circuit current density (Jsc) 0.04 mA/cm2, under 3000 W/m2 illumination from a tungsten–halogen source [21]. Recently Hernandez-Borja et al. has reported prototype PbS/CdS solar cells with 1.6% efficiency where both CdS and PbS are deposited by chemical bath method [7]. A heterojunction of PbS nanocrystals and C60 demonstrated a Voc of 390 mV, Jsc of 10.5 mA/cm2 and η of 2.2% [23]. A stable Bi2S3/PbS solar cell has shown an open circuit voltage (Voc) of 280 mV, Jsc of 6 mA/cm2 and a solar-to-electric energy conversion efficiency of 0.5% [24]. Further, the multiple carrier generation reported in nanostructured PbS makes it a possible candidate for next generation solar cells [25]. The possibility to tune the band gap of PbS may open the opportunity to explore this material for applications in tandem solar cells or graded band gap devices where the band gap of the absorber decreases toward the metallic back contact [26].
In this work, we have reported a detailed characterization including structural, spectroscopic, optical and optoelectronic properties of PbS thin films prepared by the PED method on conducting glass substrates. Polycrystalline PbS thin films with good adherence and uniformity were obtained by optimizing the deposition parameters. The results presented in this paper will update the knowledge on this technologically important material.
Section snippets
PED of PbS thin films
A three electrode cell was utilized for the PED of PbS thin films. The SnO2:F-coated glass (Tec 15) from Pilkington was used as the working electrode (the cathode). The reference electrode was saturated calomel electrode (SCE) and a platinum mesh was used as counter electrode. The deposition bath contained an aqueous solution of 1 × 10−3 M Pb(NO3)2 and 2.7 × 10−3 M Na2S2O3, the pH was adjusted to 2.7 by adding dil. HCl. The area of deposition was 5 cm2. Prior to the deposition N2 gas was bubbled
Structural and morphological studies
The XRD pattern of a PbS thin film prepared by PED is shown in Fig. 3. The reference patterns of the cubic PbS of mineral Galena (PDF 5-592) and the substrate SnO2 (PDF no. 41-1445) are also included in the figure. All the peaks observed in the XRD pattern can be assigned to the diffraction patterns of cubic PbS. The peaks with 2 theta values 25.82°, 29.96°, 42.96°, 50.94°, 53.36°, 62.48°, 68.90° correspond to the crystalline planes (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), and (3 3 1)
Conclusions
Stoichiometric and highly adherent PbS nanostructured thin films were deposited by pulse electrodeposition and studied for its structural, morphologic, optical and opto-electronic properties. The size of the nanograins in the film determined by HRTEM ranges from 2 to 20 nm with more than 63% of the grains below 7 nm. An atomic model with cuboctahedral morphology was proposed for the PbS crystals and verified with the atomic resolution TEM images. The stability of the PbS nanoparticles was
Acknowledgments
The authors wish to thank Maria Luisa Ramón Garcia for the XRD analysis; O. Gomez Daza for general assistance in the chemical laboratory, José Campos for technical assistance in electrical studies and Rogelio Morán Elvira for SEM measurements. Authors acknowledge the financial support received through the project PAPIIT–UNAM (IA100712-2), and CONACyT 129169.
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