Photoluminescence of Cu-doped CdTe and related stability issues in CdS/CdTe solar cells

Abstract

Effect of the composition on physical properties of CdTe absorber layer fabricated by chemical molecular beam deposition for use in thin film solar cellsWe explore Cu electronic states in CdTe using photoluminescence as the main investigative method. Our results are consistent with some Cu atoms occupying substitutional positions on the Cd sublattice and with others forming Frenkel pairs of the type Cu i ϩ -V Cd Ϫ involving an interstitial Cu and a Cd vacancy. In addition, we find that Cu-doped CdTe samples exhibit a significant ''aging'' behavior, attributable to the instability of Cu acceptor states as verified by our Hall measurements. The aging appears to be reversible by a 150-200°C anneal. Our results are used to explain efficiency degradation of some CdTe solar-cell devices which use Cu for the formation of a backcontact.

Interface Engineering of Inorganic Thin‐Film Solar Cells – Materials‐Science Challenges for Advanced Physical Concepts

“…Using Cu deposited onto CdTe films, it can by shown by XPS studies that a simple Cu overlayer is not formed, rather that the Cu, at least in part, diffuses into the substrate, with the formation of metallic Cd residues. [58] This interdiffusion, which is also evident in other investigations, [36,37,59,60] can be enhanced by increasing the substrate temperatures to about 300 8C. It is interesting to note that due to the temperature-activated interdiffusion, a change in the barrier height from the original pinning position of 0.9 eV above the valence-band maximum to a considerably lower value of 0.6 eV is also found, which is evidently related to the formation of Cu x Te y interface phases.…”

“…[35][36][37] As a consequence, the n-CdS/ p-CdTe heterojunction does not provide sufficient band bending (diffusion voltage) in the CdTe absorber layer, limiting the maximum photovoltage to be expected. In contrast to GaAs solar cells (V OC ¼ 1040 mV) with a similar bandgap, [8] CdTe solar cells provide only V OC values of 850 mV (depending on Cu content), [38] which is only slightly better than 50% of the band-gap value.…”

Carrier density and lifetime for different dopants in single-crystal and polycrystalline CdTe

“…32 Similarly, for pX and sX CdTe with ion-implanted or diffused Cu, resonantly excited luminescence, infrared absorption, and PL assign the Cu Cd level to 145-150 meV with a zero-phonon DAP transition at 1.45 eV. 26,27,32,[35][36][37][38][39][40] The emergence of the zero-phonon DAP peak at 1.453 eV and replicas for Cu + Te in Fig. 1(c) is consistent with the formation of 145-150-meV Cu Cd acceptors.…”

Modeling of current–voltage characteristics of CdS/CdTe solar cells

“…The performance of CdTe solar cells may degrade over time and the extent of the degradation depends on the absorber and contact materials which is examined by accelerated life testing under stress [39][40] [41] and considered as increase in recombination of carriers, resistance, and contact barriers [42]. Cu is used to form cheap and low resistance contact and easy deposition of CdTe but introducing some aspect of degradation [43] [44]. …”