Elsevier

Superlattices and Microstructures

Volume 97, September 2016, Pages 519-528
Superlattices and Microstructures

Engineering of electronic and optical properties of PbS thin films via Cu doping

https://doi.org/10.1016/j.spmi.2016.07.025Get rights and content

Highlights

  • PbS:Cu thin films grown on glass substrate were obtained by CBD.

  • From Rietveld analysis, we note that experimental and refinement results match very well.

  • Copper incorporation has a strong effect on the surface morphology of the films.

  • Optical band gap was shifted from 0.72 eV to 1.69 eV by doping process.

  • An enhancement of the electrical properties was observed after doping.

Abstract

Copper-doped PbS polycrystalline thin films were deposited by chemical bath deposition by adding small amount of Cu (ysolution = [Cu2+]/[Pb2+]) between 0.5 and 2 at%. The composition, structure, morphology, optical and electrical properties of the films were investigated by means of X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS), UV–visible–near infrared (UV–Vis–NIR) spectrophotometry and Hall effect measurements. The XRD studies showed that the undoped films have PbS face centered cubic structure with (111) preferential orientation, while preferential orientation changes to (200) plane with increasing Cu doping concentration. The AFM and SEM measurements indicated that the film surfaces consisted of nanosized grains with pyramidal shape. Optical band gap was blue shifted from 0.72 eV to 1.69 eV with the increase in Cu doping concentration. The film obtained with the [Cu2+]/[Pb2+] ratio equal to 1.5 at% Cu showed the minimum resistivity of 0.16 Ω cm at room temperature and optimum value of optical band gap close to 1.5 eV. 1.5 at% Cu-doped PbS thin films exhibit the best optical and electrical properties, suitable for solar cells applications.

Introduction

Lead sulfide (PbS) is subject of intense research owing to its technological importance in the field of optoelectronics. PbS has important properties like small direct band-gap (0.4eV), p-type character and high absorption coefficient in the visible and infrared region (>105 cm−1), that make it a useful material as detectors for infrared radiation, gas sensors, solar control coatings, etc. [1], [2], [3]. Previous studies show the ability of blue shifting the band gap by doping. Then, it can be expected that the energy band gap (Eg) changes due to many effects like the quantum confinement (QC) and/or unit cell volume variation, etc. Proper electronic band alignment is critical for application of PbS based films as absorber material in solar cells devices, in which suitable electronic band structure is needed for the p-n junctions to operate properly under sunlight irradiation [4], [5].

There are numerous methods used to fabricate PbS films (either undoped or doped), such as spray pyrolysis, successive ionic layer adsorption (SILAR) and chemical bath deposition (CBD) [6], [7], [8]. Chemical bath deposition is a versatile method for thin film growth; it is relatively simple, capable of depositing large area coatings and, most importantly, cost effective and is a low temperature technique.

In order to improve the physical properties of PbS thin films, impurity doping is a useful approach. Typical dopants have been used to make PbS practically applicable belonging to the group 11 (Cu) and group 12 (Zn, Cd) [9], [10], [11], [12]. However, up to now, only few reports exist on Cu-doped PbS thin films using CBD [11] and [12]. They have been used other experimental conditions (precursors, temperature deposition and doping concentration) for improving the physical properties of PbS films. In our case, Cu:PbS have been elaborated with small amount of copper (0, 0.5, 1, 1.5 and 2 at.%) employing lead nitrate Pb(NO3)2 and copper nitrate (Cu(NO3)2) as chemical precursors and with successful low temperature deposition (T = 25 °C). Therefore, the coexistence of good surface morphology, optical and electrical properties is one of the prime concerns of solar cells devices, which is the motivation of this research.

In the present work, our aim is to develop Cu doped PbS thin films with chemical bath deposition method in order to improve optoelectronic properties of PbS nanomaterial. Therefore, we are committed to a systematic study in order to investigate Cu doping effect on structural, morphological, optical and electrical properties of PbS thin films. To date, the use of small amount of copper, as a selective doping agent, in PbS binary compound with ‘p’ character to obtain optical band gap close to 1.5 eV, low resistivity of 0.16 Ω cm and relatively smooth surface, simultaneously, has not yet been achieved. Our results demonstrate the possibility of obtaining PbS thin films with Eg and electronic conduction suitable for application in p-n junction solar cells, close to the optimum ideal condition to maximize the photoconversion efficiency, according to the Shockley–Queisser limit [13].

Section snippets

Growth of thin films by CBD

The chemical bath contained lead nitrate [Pb(NO3)2], copper nitrate [Cu(NO3)2], sodium hydroxide [NaOH] and thiourea [SC(NH2)2], in appropriate concentrations and double distilled water. The reactive solution was prepared by the sequential addition of: 0.17 M lead nitrate, 0.57 M sodium hydroxide and 0.1 M thiourea. In order to get the Cu-doped PbS films, the copper nitrate was then added in desired amounts (ysol = ([Cu2+]/[Pb2+])sol) was varied from 0.5 to 2 at%. Double distilled water was

RBS analysis

The purpose of the RBS analysis is to determine the thickness and the composition of the films. Selected RBS spectra recorded at different Cu nominal content are shown in Fig. 1a. An example of spectrum simulation obtained through RUMP code is reported in the inset for the Pb signal for the sample at 2 at% Cu. A good agreement is obtained between experimental and simulated curves for all samples. The atomic percentage of Cu in the films is below 1% for all the samples. The thickness t of the

Conclusion

The evolution of physical properties of PbS material by Cu doping is studied. The structural study shows that the copper ions are incorporated in the lattice of PbS. All films of the Cu:PbS system are found to be polycrystalline in nature as confirmed by XRD patterns. The decreases of crystallite size is observed, accompanied with reducing of surface roughness and change in crystalline (111) orientation. The carrier concentration and Hall mobility are increased with Cu-incorporation level

References (31)

  • J.H. Warner et al.

    Mater. Lett.

    (2006)
  • T. Fu

    Sens. Actuators B

    (2009)
  • I. Pop et al.

    Thin Solid Films

    (1997)
  • Dulen Saikia et al.

    Thin Solid Films

    (2014)
  • K.C. Preetha et al.

    Mater. Semicond. Process.

    (2014)
  • J. Hernández-Borja et al.

    Sol. Energy Mater. Sol. C

    (2011)
  • B. Touati et al.

    Mater. Sci. Semicond. Process.

    (2015)
  • S. Thangavel et al.

    Thin Solid Films

    (2012)
  • Xiaoyao Zheng et al.

    Mater. Lett.

    (2016)
  • L.R. Doolittle

    Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms

    (1985)
  • L. Beddek et al.

    J. Alloys Compd.

    (2016)
  • Yu Jun Yang et al.

    Thin Solid Films

    (2008)
  • M. Muthusamy et al.

    Optik Int. J. Light Electron Opt.

    (2015)
  • A. Kariper et al.

    Mater. Chem. Phys.

    (2011)
  • S. Louidi et al.

    J. Non-Crystalline Solids

    (2010)
  • Cited by (25)

    • Morphological, structural, and electrical properties of PbS thin films deposited on HfO<inf>2</inf>, SiO<inf>2</inf>, and Al<inf>2</inf>O<inf>3</inf> for TFTs applications

      2021, Ceramics International
      Citation Excerpt :

      Fig. 4b shows the Pb 4f core level, which was fitted using two doublet peaks with a spin-orbit splitting of 4.87 eV. The doublets located at 137.25 eV and 138.28 eV correspond to Pb in PbS and Pb in PbSO4 [20]. Fig. 4c shows the S 2p core level which was fitted with six peaks.

    • Physical properties of Pb doped ZnS thin films prepared by ultrasonic spray technique

      2020, Physics Letters, Section A: General, Atomic and Solid State Physics
    View all citing articles on Scopus
    View full text