First-principles Study of Back Contact Effects on CdTe Thin Film Solar Cells
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1268-EE02-04
First-Principles Study of Back Contact Effects on CdTe Thin Film Solar Cells Mao-Hua Du Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA ABSTRACT Forming a chemically stable low-resistance back contact for CdTe thin film solar cells is critically important to the cell performance. This paper reports theoretical study of the effects of the back contact material, Sb2Te3, on the performance of the CdTe solar cells. First-principles calculations show that Sb impurities in p-type CdTe are donors and can diffuse with low diffusion barrier. There properties are clearly detrimental to the solar cell performance. The Sb segregation into the grain boundaries may be required to explain the good efficiencies for the CdTe solar cells with Sb2Te3 back contacts. INTRODUCTION CdTe is an important thin-film solar cell material. It has a direct band gap of 1.5 eV, near the optimum for conversion efficiency in a single-junction solar cell under terrestrial irradiation. Its high light absorption coefficient allows efficient solar energy absorption within a thin film, reducing material cost of the solar cells. These basic material properties of CdTe are excellent for thin film photovoltaic applications, but many other factors also contribute to the overall solar cell performance. The back contact materials affect both efficiency and lifetime of the CdTe solar cells because the contact resistance reduces the carrier collection and the impurity diffusion from the back contact to the CdTe layer causes cell degradation.1 Therefore, forming a chemically stable low-resistance back contact is extremely important to the CdTe solar cell performance. The CdTe-based solar cells are heterojunction cells with polycrystalline n-type CdS and p-type CdTe thin films. Due to the high work function of the p-type CdTe (5.7 eV), it is difficult to find a good metal with higher work function to form an Ohmic contact with CdTe. The currently highest efficiency (16.5%) CdTe solar cells have Cu-containing compounds (such as Cu2Te) as the back contact,2 3 because Cu reduces contact resistance. However, Cu can diffuse into CdTe and CdS films, and thus play additional roles, such as accumulating in the grain boundaries (GBs) and the junction region,4 5 which may create conductive channels that shunt the solar cell. It has long been suspected that the degradation of the CdTe solar cells is related to the Cu diffusion.4 The instabililty of the Cu contact prompted the research on the alternative back contact materials. Recent studies show that using a Sb2Te3 back contact without Cu results in an ohmic contact and enhanced solar cell stability but slightly lower initial cell efficiency (15.8%) compared to those with Cu back contacts (16.5%).6, 7 However, there are also reports that show the existence of contact barrier8 and significant Sb diffusion into the CdTe layer (especially in the presence of oxygen)9 when the Sb2Te3 contact is used. The Sb impurity had been studied as an acceptor in CdTe by first-p
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