Effects of Cu on CdTe / CdS Heterojunction Solar Cells with Au/Cu Contacts

Abstract

CdTe solar cells were fabricated by depositing an Au/Cu contact layer with various thicknesses and deposition conditions on polyerystalline CdTe/CdS/SnO2/glass structures. Cu has a dual effect on the cell performance; it helps in the formation of better ohmic contacts to CdTe and increases the acceptor doping concentration, but excess Cu could diffuse all the way to the CdTe/CdS interface to form recombination centers and shunt paths and degrade cell performance. Both secondary ion mass spectroscopy and capacitance-voltage measurements confirm the incorporation of Cu into the bulk of the CdTe films. Cd outdiffusion toward the surface of CdTe was observed during the Au/Cu deposition. The thickness of Cu plays a critical role in the CdTe solar cell performance because both the series and shunt resistances decrease with the increase in Cu thickness. Carrier transport analysis showed thai depletion region recombination dominates the current transport in the CdTe solar cells with an Au/Cu contact. The transport mechanism remains the same in spite of the amount of Cu incorporation into the bulk and interface. Higher Au/Cu deposition rates result in a greater excess of Cd at the CdTe surface, leaving more Cd vacant sites below the surface. This causes an increase in dopant concentration but also results in a higher defect density and reduced cell performance.

Self-consistent simulation of CdTe solar cells with active defects

“…In particular, there are two types of coupling in Eq. (18). The potential is the most involved of the couplings, with every (charged) defect appearing in Poisson's equation and the potential appearing in each of the (charged) defect equations.…”

“…[15][16][17] Evolution of Cu-related point defects and complexes in CdTe grains and at GBs is expected to cause pronounced effect on device performance leading to observed metastable phenomena. [18][19][20] The paper is organized as follows. In Section II, we discuss the drift-diffusion-Poisson system and the necessary extensions to the model for including active defects.…”

Numerical Simulation of Copper Migration in Single Crystal CdTe

“…The open-circuit voltage (V OC ), one of the key performance parameters of solar cells, is a strong function of doping concentration in the absorber [3]- [5]. As such, better understanding of mechanisms behind doping formation in CdTe is necessary to facilitate further improvements in efficiency and stability of this technology.…”

Transition metal nitride contacts for CdTe photovoltaics

“…Although there needs more research on the effects of Cu, it is generally accepted that Cu in CdTe solar cells is playing a dual role. One side Cu is a good p-type dopant by increasing carrier concentration in CdTe bulk and interface between back contact and CdTe [9,10]. On the other side Cu is a fast diffuser which migrates to the CdTe/CdS junction over time and then creates shunting paths [ll].…”