Co‐evaporated Cd_1–xMg_xTe thin films for applications in tandem solar cells

Abstract

For the last seven years the efficiency of CdTe solar cells is stagnated at 16.5% even though the practically achievable efficiency is 18‐20%. In order to advance further in CdTe technology, we pursue our efforts by developing alloys based on this material. In this work we have developed the ternary alloy material Cd1–xMgxTe by the co‐evaporation of CdTe and Mg, and obtained uniform thin films with band gap in a wide range (1.6 to 2 eV). In order to apply this ternary material as the absorber layer in a top cell, the required band gap is in the range 1.6 to 1.7 eV. Films with band gap close to 1.6 eV were deposited on both corning glass as well as CdS/Tec7 substrates depending on the end use. We have carried out studies on the effect of different post deposition treatments of the film on its structural and opto‐electronic properties. One of the important criteria in developing the top cell is the transmittance of the sub‐gap radiation through the device structure. The transmittance through our device structure is more than 80% demonstrating the promising nature of this material in developing tandem solar cells. Un‐optimized prototype devices showed open circuit voltage of 590 mV, short circuit current density of 12 mA/cm2, and efficiency more than 4%. The efficiency was low due to high series resistance and poor fill factor. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)