Development of wide band gap Cd1-XMgXTe/CdS top cells for tandem devices

In order to achieve advances in CdTe technology, we are continuing our efforts in developing wide band gap ternary alloy films based on CdTe. This approach will help in harnessing the already existing CdTe technology to develop high efficiency tandem devices. The band gap of Cd1-xMgxTe can be easily tuned from 1.48 to 1.8 eV (x=0 to 0.17). Close match of the lattice constant of MgTe with CdTe and the apparent complete miscibility of MgTe in CdTe and the rapid increase in band gap with Mg content gives the flexibility to prepare material with appropriate band gap for current- matching in a tandem solar cell. In this paper development of Cd1-xMgxTe films by vacuum co-evaporation and prototype Cd1-xMgxTe/CdS solar cells are reported. The co-evaporated films on large area substrates exhibited high spatial uniformity. We have used different experimental techniques such as XRD, Raman, PL, UV-VIS spectroscopy, and opto-electronic tools for studying the effect of Mg incorporation in to the lattice of CdTe. Evidences of lattice perturbations due to the incorporation of Mg were observed in XRD and Raman data, however no phase segregation was observed in the XRD spectra. We are continuing our investigation of the effect of post deposition chloride treatment and annealing on the band gap stability of the Cd1-xMgxTe films. Films chloride treated with MgCl2 exhibited better stability compared with those films annealed in CdCl2 vapor. However, our preliminary data on the CdS/Cd1-xMgxTe devices show that CdCl2 vapor treatment is beneficial for attaining better efficiency. Prototype devices of the configuration Tec7/CdS/Cd1-xMgxTe/Cu-Au was developed with Cd1-xMgxTe films with different x values. Efficiency varies in the range 2 to 4.6% depending on the band gap of the absorber.