Study of ZnTe:Cu/Metal Interfaces in CdS/CdTe Phovoltaic Solar Cells
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Study of ZnTe:Cu/Metal Interfaces in CdS/CdTe Phovoltaic Solar Cells T.A. Gessert, C.L. Perkins, S.E. Asher, A. Duda, and M.R. Young National Renewable Energy Laboratory (NREL), Golden, CO, USA 80401 ABSTRACT The present model for current transport at the CdTe/p-ZnTe:Cu/metal back contact assumes that the CdTe and ZnTe valence bands align, while current transport at a highly doped ZnTe and a metal interface proceeds by tunneling. To test part of this model, we have investigated the electrical and material properties of CdS/CdTe devices where the outer metal is either Ti or Ni. Our results show that differences in device series resistance are not linked simply to metal/ZnTe:Cu interfacial contact resistance, but that metallization-induced diffusion remains a more likely cause of significant performance distinctions. INTRODUCTION The demonstration of a high-performance and stable contact system for CdS/CdTe thin-film photovoltaic (PV) devices remains an important goal of the CdTe community. Many studies have suggested that if a contact process combines a pre-contact chemical etch with thermal diffusion of Cu from the contact, a ~0.3-0.4 eV barrier forms within the CdTe (see Figure 1a) [1]. Because CdTe is difficult to dope to high acceptor levels [2], current transport occurs by thermal excitation of carriers over the barrier, rather than by tunneling. Effects of this type of barrier are believed to manifest as crossover and/or rollover of the first-quadrant current-voltage (IV) characteristics at room temperature, and these effects become more pronounced at reduced temperature [1,3]. Although transport by diffusion over a barrier is not expected to be a lowresistance pathway, it is widely accepted that the occurrence of rollover/crossover does not preclude fabrication of devices with performance that is high enough for economic viability. In contrast to the type of contact described ZnTe:Cu A B above, hole transport between CdTe and ZnTe is Interface enabled by nearly perfect alignment of the CdTe CdTe CdTe and ZnTe valence bands [4,5]. Further, because ZnTe can be doped to very high acceptor levels Metal Metal [6,7], transport into the metal can occur by Wide Narrow tunneling (see Figure 1b). The combination of Barrier Barrier these attributes should produce a contact with lower resistance and enhanced stability (i.e., Figure 1. Band diagrams of contact region for stability should improve because the critical (A) contact formed by band bending in CdTe and contact barrier relies on stable doping in ZnTe (B) p-ZnTe:Cu contact interface. rather than doping in CdTe). Indeed, devices incorporating this contact have demonstrated fill factors of 77% and good stability [8]. Although the high level of device performance achieved by ZnTe-based contacts implies that contact operation is reasonably understood, important questions remain. For example, it has been shown that when Cu-doped ZnTe layers (ZnTe:Cu) are deposited onto actual polycrystalline CdTe films, the carrier concentration in the ZnTe:Cu layer is much lowe
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