Band Gap Engineering Studies of PbTe, SnTe, and CdSeTe for CdTe Solar Cell Applications
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In an environment of growing energy consumption, the demand for cost effective and efficient approaches to renewable energy is on the rise. One of the leading possible alternatives is solar cell technology. Thin film solar cells made of cadmium telluride (CdTe) have gained in popularity recently since they offer a unique solution to both increase the efficiency of solar cells and decrease costs associated with manufacturing. However, CdTe solar cells have not yet reached their theoretical efficiency ceiling, mainly due to suboptimal engineering at the back contact of the device. To address this, we study the band gap using luminescence measurements, as well as the positions of the valence band and conduction band using X-ray photoemission spectroscopy (XPS) in three material systems: lead telluride (PbTe), tin telluride (SnTe) and cadmium selenium telluride (CdSeTe) alloys in the context of CdTe. In this work, PbTe and SnTe are introduced as back contact buffer layers in various CdTe structures. The valence band offset (VBO) of PbTe/CdTe and SnTe/CdTe is explored with XPS and found to be 0.34 eV and 1.25 eV, respectively. Effects of doping levels in the PbTe and SnTe layers are studied using Hall measurements, time-resolved photoluminescence (TRPL), and photoluminescence intensity measurements (PLI). Hall investigations led to the measurement of a two-dimensional electron gas in the PbTe films. Effects of the inclusion of the buffer layers on the overall CdTe solar cell device are evaluated using current-voltage (JV) analysis at room temperature. PbTe/CdTe band alignments were also calculated for various doping concentrations in the PbTe film and agreed well with all other measurement techniques. Potential CdTe solar cell device improvement with the inclusion of a PbTe back contact buffer layer is highly dependent on the carrier concentrations in both the PbTe layer and the CdTe layer. XPS measurements and JV measurements at different temperatures with SnTe layer unequivocally show that SnTe is not a suitable material for use in CdTe solar cells.
Some in industry have abandoned the traditional cadmium sulfide (CdS)/CdTe (n-p) junction of CdTe devices in favor of a CdSe_x Te_(1-x) layer where the Se content (x-value) varies from 1 to 0 through the absorber layer of the device. While these devices have shown improved performance, the dynamics of the valence and conduction band as the Se content changes is not well understood. Here, we explore these dynamics with the use of XPS, photoluminescence spectroscopy (PL spectroscopy), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD) on a series of uniform CdSe_x Te_(1-x) alloys, with varying x-values. PL spectroscopy and ellipsometry results consistently show a band gap of 1.67 eV for cubic CdSe. An alternative ternary VBO determination technique was developed for the CdTe/CdSeTe system which eliminated the need to accurately determine the valence band minimum – a significant source of error in more VBO determinations. XPS results predict a VBO between CdTe/CdSe of -0.487 eV.