Direct Imaging of Cl‐ and Cu‐Induced Short‐Circuit Efficiency Changes in CdTe Solar Cells

Abstract

To achieve high‐efficiency polycrystalline CdTe‐based thin‐film solar cells, the CdTe absorbers must go through a post‐deposition CdCl2 heat treatment followed by a Cu diffusion step. To better understand the roles of each treatment with regard to improving grains, grain boundaries, and interfaces, CdTe solar cells with and without Cu diffusion and CdCl2 heat treatments are investigated using cross‐sectional electron beam induced current, electron backscatter diffraction, and scanning transmission electron microscope techniques. The evolution of the cross‐sectional carrier collection profile due to these treatments that cause an increase in short‐circuit current and higher open‐circuit voltage are identified. Additionally, an increased carrier collection in grain boundaries after either/both of these treatments is revealed. The increased current at the grain boundaries is shown to be due to the presence of a space charge region with an intrinsic carrier collection profile width of ≈350 nm. Scanning transmission electron microscope electron‐energy loss spectroscopy shows a decreased Te and increased Cl concentration in grain boundaries after treatment, which causes the inversion. Each treatment improves the overall carrier collection efficiency of the cell separately, and, therefore, the benefits realized by each treatment are shown to be independent of each other.

In-depth analysis of chloride treatments for thin-film CdTe solar cells

“…Following MgCl 2 treatment the junction is brought close to the interface, (as seen for CdCl 2 treatment2023), and the peak current is increased by approximately an order of magnitude relative to the as-grown case. Intermediate behaviour is seen for the ineffective chlorides—the junction is brought closer to the interface (NaCl ∼400 nm, KCl ∼590 nm), and the current peaks are increased—but the effects are much weaker for KCl than for NaCl, and neither is as effective as MgCl 2 .…”

S–Te Interdiffusion within Grains and Grain Boundaries in CdTe Solar Cells

“…EBIC is an ideal method to study the photovoltaic behavior of solar cells in short-circuit conditions [22], [23]. EBIC in an electron probe micro analyzer [24] and a scanning electron microscope (SEM) [8] has been used to detect the location of the homojunction in CdTe/CdS solar cells.…”

Controlled activation of ZnTe:Cu contacted CdTe solar cells using rapid thermal processing

“…In addition to limiting throughput, another drawback of this procedure is that deposition and diffusion occur simultaneously, making process control difficult and resulting in copper migration throughout the device [15]. In bulk CdTe a very small amount of copper may be beneficial [18,19], however excessive amounts lead to deep level defects and recombination centers [20][21][22].…”