Publications [#66370] of Nan M. Jokerst

Papers Published

1. Rohatgi, A. and Chou, H.C. and Jokerst, N.M. and Thomas, E.W. and Ferekides, C. and Kamra, S. and Feng, Z.C. and Dugan, K.M., Effects of CdTe growth conditions and techniques on the efficiency limiting defects and mechanisms in CdTe solar cells, AIP Conf. Proc. (USA) no. 353 (1996), pp. 368 - 75, Lakewood, CO, USA
   (last updated on 2007/04/16)

   Abstract:
   CdTe solar cells were fabricated by depositing CdTe films on CdS/SnO₂/glass substrates using close-spaced sublimation (CSS) and metalorganic chemical vapor deposition (MOCVD). Te/Cd mole ratio was varied in the range of 0.02 to 6 in the MOCVD growth ambient in an attempt to vary the native defect concentration. Polycrystalline CdTe layers grown by MOCVD and CSS both showed average grain size of about 2 μm. However, the CdTe films grown by CSS were found to be less faceted and more dense compared to the CdTe grown by MOCVD. CdTe growth techniques and conditions had a significant impact on the electrical characteristics of the cells. The CdTe solar cells grown by MOCVD in the Te-rich growth condition and by the CSS technique gave high cell efficiencies of 11.5% and 12.4%, respectively, compared to 6.6% efficient MOCVD cells grown in Cd-rich conditions. This large difference in efficiency is explained on the basis of (a) XRD measurements which showed a higher degree of atomic interdiffusion at the CdS/CdTe interface in high performance devices, (b) Raman measurements which endorsed more uniform and preferred grain orientation by revealing a sharp CdTe TO mode in the high efficiency cells, and (c) carrier transport mechanism which switched from tunneling/interface recombination to depletion region recombination in the high efficiency cells. In this study, Cu/Au layers were evaporated on CdTe for the back contact. Lower efficiency of the Te-rich MOCVD cells, compared to the CSS cells, was attributed to contact related additional loss mechanisms, such as Cd pile-up near Cu/CdTe interface which can give rise to Cd-vacancy defects in the bulk, and higher Cu concentration in the CdTe layer which can cause shunts in the device. Finally, SIMS measurements on the CdTe films of different crystallinity and grain size confirmed that grain boundaries are the main conduits for Cu migration into the CdTe film. Thus larger CdTe grain size or lower grain boundary area per unit volume can mitigate the adverse effect of Cu on the cell performance

   Keywords:
   cadmium compounds;chemical interdiffusion;CVD coatings;grain boundaries;grain size;II-VI semiconductors;secondary ion mass spectroscopy;semiconductor growth;semiconductor thin films;solar cells;sublimation;vacancies (crystal);X-ray diffraction;