Investigation of ZnSe and ZnSxSe1-x for application in thin film solar cells

Armstrong, Stephen (2005) Investigation of ZnSe and ZnSxSe1-x for application in thin film solar cells. Doctoral thesis, Northumbria University.

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Abstract

Abstract Successful deposition of ZnSe and ZnS„Sei_x layers has been performed with close-spaced thermal evaporation (CSTE). ZnSe (Eg = 2.67eV) and ZnS,Sei, (Eg to 3.6eV) have the potential to replace CdS (Eg = 2.42 eV) as a buffer layer in solar cell applications, giving a two-fold benefit: (i) increased blue response of the cell, potentially allowing more light to reach the pn junction and contribute to photogeneration and (ii) reduce the toxic Cd element of the buffer layer. CSTE has produced films in which the deposition parameters can be controlled to alter the morphology of the as-deposited coatings. SEM and AFM investigations have shown that pinhole free ZnSe and ZnS„Sei_x films can be produced with this deposition process. In addition, the ZnSxSei, layers show a linear shift in lattice constant and a systematic shift in energy bandgap with alloy composition. XRD data and the steep absorption edges in the transmittance data confirm the good crystallinity of the layers. To partner the ZnSe and ZnS,,Sei_x buffer layers in a thin film heterojunction, CdTe absorber layers were grown in the superstrate configuration. These CdTe layers were deposited in the same deposition chamber, without breaking vacuum, to reduce the risk of interfacial contamination. ZnSe and ZnSxSei_x / CdTe solar cells were fabricated with the best cell producing PV characteristics of: short circuit current 17mAcm-2, open circuit voltage 460mV and efficiency approaching 3%. The spectral response of all ZnSe and ZnSxSei_x / CdTe devices demonstrated a systematic shift to shorter wavelengths with increasing alloy composition, therefore showing the potential of these materials to increase solar cell efficiency. This low cost deposition process has shown excellent potential to be scaled up for commercial applications.

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