Dragica Vasileska1, I. Sankin2, Da Guo1, D. Brinkman3, C. Ringhofer4, A. Shaik1, Andrew Moore5 and James R. Sites5

1School of Electrical, Computer and Energy Engineering,

Arizona State University, Tempe, AZ, USA

2First Solar, Perrysburg, OH, USA

3Department of Mathematics, San Jose State University, CA, USA

4School of Mathematical and Statistical Sciences, Arizona State University, Tempe, AZ, USA

5College of Natural Sciences-Physics, Colorado State University, Fort Collins, CO, USA

The record efficiencies of thin-film CdTe technology are still ten absolute percent lower than the Shockley-Queisser limit. As short-circuit current density (JSC) is approaching the theoretical limit, both open-circuit voltage (VOC) and fill factor (FF) are far below the theoretical limits for most devices. Although VOC larger than 0.9V have been reported for single crystal (sx-) CdTe solar cells, low VOC still limits the performance of polycrystalline CdTe devices.

Since VOC is a strong function of the doping concentration in the absorber layer, better understanding of doping mechanisms and defects formation is a must. Like most common dopants in px-CdTe, Copper (Cu) forms multiple species of defects including interstitial donors (Cui), substitutional acceptors on Cd site (CuCd) and tightly-bounded complexes such as Cui-CuCd and Cdi-CuCd. Resulting amount of uncompensated acceptor impurities is usually three or four orders of magnitude smaller than the total atomic Cu concentrations, which limits the VOC of Cu-doped CdTe solar cells significantly. Low VOC of px-CdTe solar cells is also believed to be due to large defect density, and short minority carrier lifetime in the absorber layer. Not only carriers, the self-compensated active Cu dopants provide active defect (recombination centers) in CdTe material as well, which results in poor minority carrier lifetime, that once again connects Cu with the low VOC presented in px-CdTe PV cells.

Although total Cu concentration profiles can be measured by the secondary ion mass spectrometry (SIMS) technique, the concentration of different species of Cu (mainly Cui and CuCd) generally cannot be identified. Lacking a description of the transition process, theoretical concentrations of the related defects were estimated from charge neutrality equation with formation energies of defects obtained from First Principles calculations. Given this, gaining a better understanding of Cu migration in CdTe is of crucial importance in order to further enhance the performance of CdTe solar cells.

Moreover, PV modules (multiple solar cells electrically connected) are expected to function properly for more than 25 years, in order to provide electricity at proper cost. However, due to the fast diffusion rates of Cu atoms, the gentle balance between mutually compensating Cu impurities could be subject to temporal changes causing metastabilities observed in CdTe solar cells, which also makes the predictive simulation of device performance more important.

Thus, gaining a better understanding of mechanisms that govern formation and interactions between Cu-related defects is of crucial importance for further advancement of the CdTe photovoltaics.