Characterization of etching procedure in preparation of CdTe solar cells

doi:10.1016/0927-0248(96)00053-0

Solar Energy Materials and Solar Cells

Volume 44, Issue 2, 15 November 1996, Pages 177-190

https://doi.org/10.1016/0927-0248(96)00053-0 Get rights and content

Abstract

An etching procedure for forming a low resistance contact to polycrystalline CdTe thin films in CdS/CdTe solar cells was studied. The etching solution used was a mixture of HNO₃, H₃PO₄ and H₂O. X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and electric measurements revealed that the etching results in a formation of crystalline tellurium on the film surface, thereby increasing substantially the conductivity of the surface layer. The total process was found to consist of three steps: (i) immediately after an immersion into the etching solution there was a certain induction period with no discernible changes, (ii) a subsequent reaction step during which poorly crystallized elemental tellurium was formed, gaseous byproducts liberated and the surface changed its colour, and (iii) after taking out of the etching solution the tellurium crystallized causing a strong decrease in the sheet resistance. In situ XRD and electric measurements were carried out to follow the third step. The chemical aspects of the three steps as well as their contributions to the reproducibility and control of the overall etching procedure have been considered.

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Our study demonstrated that an oxide mixture layer made of CdTeOx, TeOx and CdOx is inevitably formed at the CdTe film surface. This oxide layer is usually etched away by chemical solution, such as bromine methanol solution or nitric-phosphoric acid (NP etching) (Häring et al., 1983; Konova et al., 1986; Kotina et al., 1998; Li et al., 1999; Rose et al., 1999; Sarlund et al., 1996). After chemical solution etching, a Te-rich layer is formed at CdTe film surface, as can be seen in Fig. 9.
The band alignment of CdTe/MoO_x hetero-interface contact at back electronic electrode in CdTe thin film solar cell was quantitatively characterized by photon electron emission spectroscopy. The experimental results showed that depending on the CdTe surface chemical state and the stoichiometric value of x in MoO_x, energy barrier with a value as low as 0.23–0.39 eV was developed at the CdTe/MoO_x contact interface for a MoO_x buffer layer with an x value of 2.86–3. CdTe solar cell with relatively high efficiency can be fabricated by using a MoO_x as a back contact buffer, which is allowed to have a relatively large stoichiometric x range of 2.86–3. The Te-rich CdTe surface obtained by chemical etching induced a reduction reaction upon the deposition of a MoO_x thin layer, leading to the formation of reduced Mo states and other defect states below the Fermi level, which assisted hole carrier transport.
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The role of chlorine has been previously studied but the results are sometimes contradictory [9]. The nitric-phosphoric (NP) wet etching process for forming a low resistance back contact to polycrystalline CdTe thin films in CdS/CdTe solar cells was studied [10,11]. Oxide layer on the surface of CdTe thin film was removed and cadmium was resolved into the NP etching solution.
Wet chemical etching process on as-deposited CdTe surface using nitric-phosphoric (NP) acid improved the efficiency of CdS/CdTe solar cells from 10.1% to 13.8%. Nitric-phosphoric (NP) acid solution etched native oxide (TeO₂) layer and resolved excess cadmium on CdTe surface. After the heat treatment process activated the CdCl₂, CdO layer which was believed to be a diffusion barrier of chlorine did not grow on the etched CdTe surface and new (V_Cd²⁻–2Cl_Te¹⁺)⁰ complexes was located at E_V + 0.045 eV. New (V_Cd²⁻–2Cl_Te¹⁺)⁰ complexes acted as a shallow acceptors and induced to improve V_oc and J_sc. The surface of CdTe thin film has been studied using Scanning Electron Microscope (SEM), X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and low temperature Photoluminescence (PL).
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A study was performed to reduce the CdS film thickness in CdTe thin film solar cells to minimize losses in quantum efficiency. Using close space sublimation deposition for CdS and CdTe a maximum efficiency of ~ 9.5% was obtained with the standard CdS film thickness of ~ 160 nm. Reduction of the film CdS thickness to less than 100 nm leads to poor cell performance with ~ 5% efficiency, mainly due to a lower open circuit voltage. An alternative approach has been tested to reduce the CdS film thickness (~ 80 nm) by depositing a CdS double layer. The first CdS layer was deposited at high substrate temperature in the range of 520–540 °C and the second CdS layer was deposited at low substrate temperature of ~ 250 °C. The cell prepared using a CdS double layer show better performance with cell efficiency over 10%. Quantum efficiency measurement confirmed that the improvement in the device performance is due to the reduction in CdS film thickness. The effect of double layer structure on cell performance is also observed with chemical bath deposited CdS using fluorine doped SnO₂ as substrate.

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PaperCharacterization of etching procedure in preparation of CdTe solar cells

Abstract

Solar Cells

J. Non.-Cryst. Solids

J. Non-Cryst. Solids

Adv. Sol. Energy

MRS Bull.

Annu. Rev. Mater. Sci.

Paper
Characterization of etching procedure in preparation of CdTe solar cells