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