A.M. Chaparro

Morphological and compositional study of CBD-ZnSe thin films by microscopy techniques and angle resolved XPS

Abstract A morphological and compositional study is carried out on ZnSe thin films obtained by the chemical bath deposition (CBD) method. SEM, TEM and AFM images are shown for the morphological characterisation. Angle-resolved XPS (ARXPS) measurements are used for the study of surface and subsurface composition of the films. The composition underneath is studied with XPS analysis of films eroded by sputtering. It is found that films have a mixed ZnSe-ZnO (or Zn(OH)2) composition. The Zn/Se ratio in the film increases with depth, indicating that Zn is preferentially as ZnO-Zn(OH)2 close to the film-substrate interface, and that the ZnSe proportion increases above. Such composition inhomogeneity is attributed to a change in the deposition mechanism during film growth. At first, it proceeds via reaction of adsorbed Zn and Se precursors, and then by deposition of ZnSe clusters formed in the bulk of the solution. Apparently the first mechanism is less efficient for the formation of a pure ZnSe film, at least under the experimental conditions used here, hence gives rise to higher concentration of Zn oxides close to the film-substrate interface. At longer times during the deposition process, the cluster precipitation mechanism predominates and, consequentially, the top layers of the film become richer in ZnSe but less compact. Other compounds detected by ARXPS are Se0 occluded in the bulk of the film, and some SeO2 at the surface. Annealing at 300°C results in structural and compositional changes which involve compaction of the films, the loss of the occluded Se0, the increment of the SeO2 overlayer and the transformation of Zn(OH)2 into ZnO and/or Zn(O, Se) compounds.

Quartz-crystal microbalance study of the growth of Zn(Se,O) thin-films in a chemical bath. A sequential electroless-chemical process

Abstract The chemical bath deposition of Zn(Se,O) thin films is studied with a quartz crystal microbalance (QCM). The deposition velocity is measured with QCM under different experimental conditions, including substrate properties, bath temperature and bath composition. The plots of the growth velocity vs. time reflect the sequence of processes for the deposition of the film. At the beginning, an induction period takes place, which ends up with the formation of first ZnSe particles. These particles activate an electroless reaction induced by a reducer, hydrazine or hydroxide, which gives rise to deposition of a film with ZnO and ZnSe composition. This step is necessary to attain compact and thicker films with appropriate conditions for solar cell application. The electroless mechanism is decelerated after deposition of some nanometers of film, hence, a second growth mechanism starts to predominate consisting in the chemical reaction of soluble selenide with hydroxide and Zn2+ cations on the substrate surface. The layers resulting from this second mechanism have higher proportion of ZnSe than the previously electroless generated layers. At longer times, the predominant growth mechanism is the deposition of ZnSe clusters formed in the solution, which produces less compact and poorly adherent top layers. This sequence of mechanisms gives rise to films with heterogeneous morphology and composition in depth.

Bi influence on growth and physical properties of chemical deposited PbS films

AbstractPbS films were chemically deposited on glass substrates from chemical reducing bath doped with Bi. The growth and propertiesof PbS layers are investigated. The effect of the reducer and Bi ions on the deposition process was studied with a quartz- 3 q crystal microbalance technique. The reducer introduces a new deposition mechanism of electrochemical nature, which allows forthicker films. On the other hand, Bi ions introduce nucleation centers in the solution 3 q (Bi (OH )), which accelerate the 3 homogeneous precipitation and diminish the film thickness. The PbS grain size within the layer increases with increasing the Bicontent in the deposition bath. A considerable enhancement of the photoconductive signal was found in case of PbS filmsdeposited from Bi doped bath. The signal has nonlinear behavior with the Bi content in the chemical bath. There is an optimumquantity of Bi for which the photoconductive characteristic reaches maximum. 2003 Elsevier Science B.V. All rights reserved.

Study of the spontaneous growth of ZnO thin films from aqueous solutions

Abstract Spontaneous growth of ZnO thin films from Zn 2+ aqueous solution is promoted by oxygen dissolved in the solution in the presence of a reducing agent. A study of this process with the quartz-crystal microbalance (QCM) is presented. It is observed that the growth of ZnO is due to the reduction of oxygen. Mechanistics aspects are discussed. An electroless-chemical process is proposed, consisting in the formation of the superoxide radical (O 2 − ) followed by chemical reaction of two O 2 − with Zn(NH 3 ) 4 2+ cations. In this scheme, only one electron transfer per oxygen molecule is necessary. This is probably the reason of the easiness of the spontaneous growth of ZnO.

Characterisation of CuInS2/ZnSe junctions by XPS and electroreflectance

The composition and solar cell behaviour of CuInS 2 /ZnSe junctions have been studied with XPS and electroreflectance (ER) techniques. Quite stoichiometric ZnSe thin films grow on CuInS 2 substrates by chemical bath deposition (CBD), even if using different concentration of Zn and Se precursors. This result contrasts with those of films grown on conducting SnO 2 , which present a higher concentration of Zn not bonded to Se, mainly as Zn(O,OH) and metallic Zn 0 . ER at different incidence angles (angle resolved electroreflectance, ARER) shows two signals from the CuInS 2 surface in contact with ZnSe. The signal with increasing intensity at lower incidence is assigned to an interfacial CuInS 2 phase with higher energy gap. A lower conversion efficiency was found on those cells where this signal is more intense. XPS results at the CuInS 2 /ZnSe interface seem to indicate that such interfacial ARER signal is associated with an interfacial In excess.

XPS analysis with depth resolution of chemical bath-deposited ZnSe thin films

X-ray photoelectron spectroscopy analysis of ZnSe thin films deposited by the chemical bath deposition method is carried out. The composition of the films at the surface and subsurface is determined by angle-resolved XPS detection of photoelectrons, and the elemental profile in the whole depth is determined by sputter-assisted XPS. In general, the films are composed of a mixture of ZnSe and ZnO (or Zn(OH)2 for non-annealed films), the relative proportions of which vary depending on the substrate properties and thermal treatments. Also, inhomogeneous depth composition is encountered, the films richer in ZnO being close to the film/substrate interface, with increasing ZnSe proportion above. This result is explained in relation to the mechanisms that contribute to film growth in the chemical bath. The characteristic composition of the films is expected to influence their behaviour as buffer layers for photovoltaic thin-film solar cells. Copyright

Influence of chemical bath deposition parameters on the formation of CuInS 2 / Zn(Se,O) junctions for thin film solar cells

Thin film solar cells of CuInS 2 /Zn(Se,O)/ZnO configuration have been studied as a function of the Zn(Se,O) buffer layer deposition parameters. Deposition of the buffer films was carried out by the chemical bath deposition (CBD) method, at different bath temperatures and compositions, and followed in situ with a quartz crystal microbalance. The CBD conditions were chosen to grow Zn(Se,O) buffer layers under different kinetic regimes but maintaining the same buffer thickness. The cells have been characterised with current-voltage and quantum efficiency measurements. Light soaking effects and medium term stability have been checked. It is found that Zn(Se,O) grown under predominant electroless kinetics gives rise to buffer films richer in oxygen, which allow for higher fill factors, higher efficiencies (around 10%) and stability of the cells. These cells show however lower open circuit potential. On the other hand, Zn(Se,O) buffers grown under chemical regime become richer in selenium, which gives rise to cells with higher open circuit potential, but lower fill factor, conversion efficiency and stability. Light soaking effects are also more important with the chemically grown buffers.

ZnSe Thin Films Deposited by the Chemical Bath Deposition Method, by XPS

Surface and bulk composition of ZnSe thin films deposited by the chemical bath deposition (CBD) method, are analyzed with x-ray photoelectron spectroscopy (XPS). In-depth composition information is obtained with angle-resolved XPS (ARXPS) and sputtering-assisted XPS. The films show excess Zn proportion (Zn/Se=1.1) due to an amount of ZnO (or OH−). ARXPS and sputtering assisted measurements show that the Zn excess is higher in the layers closer to the substrate, i.e., layers deposited at the beginning of the deposition process. A deposition mechanism most active at the beginning is responsible for the competitive deposition of ZnO. Other minority components in the films are surface SeO2, from ZnSe oxidation in air, bulk atomic Se, from oxidation of ZnSe and/or selenide bath precursors, and atomic Zn on layers closer to the substrate surface, from the initial CBD reaction.