Lead selenide (PbSe) is an important direct band gap IV-VI semiconductor material and have been attracting much interest among the researchers because of its interesting physical properties like high refractive index, narrow band gap (~0.27 eV), relatively large exciton Bohr radius (~46 nm), high mobility and high absorption coefficient (>10⁵ Cm-1). PbSe thin films in polycrystalline and nanocrystalline form have significant technological importance as detectors of infrared radiation, infrared emitters, photovoltaic devices and thermoelectric devices. Due to its larger exciton Bohr radius, quantum confinement effect could be observed to a fairly bigger particle size. Various deposition techniques have been reported to deposit PbSe thin films that include electrochemical, thermal evaporation, pulsed laser deposition, molecular beam epitaxy and chemical bath deposition (CBD). But, chemically deposited PbSe thin film appears to be most suitable as it is easy to handle, cost effective and has wide industrial applications. Most of the reported works on the synthesis of PbSe thin film by CBD technique involves the use of complexing agents like Trisodium citrate (TSC), triethanolamine (TEA), 1, 2-ethanedithiol (EDT) etc. The rapid use of these complexing agents may cause some environment issues due to their toxicity. Therefore, development of a simple and reliable synthetic technique free of toxic complexing agent is of great importance.

Nanocrystalline thin films of PbSe were deposited on glass substrates by simple chemical bath deposition technique at 60^oC. The effects of the deposition time on the thickness as well as structural, morphological and optical properties of the PbSe thin films were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis absorption spectroscopy. Band gaps of the films were found in the ranges 1.7 to 2.3 eV. XRD spectra reveals that as prepared PbSe films are FCC (face centered cubic) structured with preferred orientation along (2 0 0) direction. The SEM micrograph illustrates the homogeneous surface morphology without any visible pinholes and cracks. The room temperature conductivity of the as-deposited PbSe films was found in the range 0.27 x 10-5Ω^-1 cm^-1 to 3.32 x 10-5 Ω^-1 cm^-1. A set of three prototype Schottky solar cells with the structure ITO/PbSe/Ag has been fabricated by varying the thickness of PbSe layer from 230nm to 380nm. The cells exhibited power conversion efficiency in the range (0.26%-0.34%) under one sun illumination intensity (100mW/cm²). PbSe thin films were grown by continuous dip coating technique by reacting Lead nitrate[Pb (NO₃)₂] and sodium selenosulfate [Na₂SeSO₃] under ambient pressure at 60^oC. The synthetic technique is a modification of the method due to I. Grozdanov et al. [22]. A set of five PbSe thin films (S1, S2, S3, S4 and S5) were prepared during the deposition time 30, 40, 60, 90 and 120 min respectively.

Results and discussion

1. UV-Vis absorption spectroscopy

The optical absorption spectra of the PbSe thin films (S1, S2, and S3) are presented in Fig. 1(a). It is observed that the shapes of the curves are similar in nature.

1. XRD Analysis

The X-ray diffraction patterns of the PbSe thin films are shown in Fig. 2(a) and (b) at various deposition periods.

1. SEM Analysis

   The uniform pinhole-free surface morphology of PbSe thin films prepared for different deposition times are depicted in Fig. 3(a)-(c).

2. Photovoltaic performance:

   Fig 4 shows the I-V characteristics of ITO/PbSe/Ag schottky solar cells and photovoltaic parameters are shown in Table 1.

   Conclusions

   PbSe thin films with grain sizes ~ 8.8-12.7 nm in diameter, free of pinholes or cracks have been synthesized by a simple CBD technique. The quality of the PbSe thin film in terms of crystallinity, microstrain, dislocation density and roughness etc. was improved by optimizing the deposition time. A set of three prototype Schottky solar cells with the structure ITO/PbSe/Ag has been fabricated by varying the thickness of PbSe layer from 230nm to 380nm and as-prepared cells exhibited power conversion efficiency in the range (0.26%-0.34%).

   Acknowledgments

   The authors D. Saikia , Pallabi Phukan  acknowledges DeitY, New Delhi for the financial support under the Major research project to Sibsagar College, Joysagar, Assam, India [vide letter No. 12 (5)/2011-EMCD, Dated 01.02.2012] & SAIF, NEHU, Shillong and NEIST, Jorhat for providing the SEM and XRD results. The author R. Vaid acknowledges University Grants Commission (UGC) for providing financial assistance under Major research project (MRP- MAJOR-ELEC-2013-22797) under the 12^th plan period.