Synopsis

The Key to Thin-Film Solar-Cell Efficiency

Physics 7, s46
The combination of atomic imaging and new calculations explains the large photovoltaic efficiency of thin-film cadmium-telluride solar cells.
C. Li et al., Phys. Rev. Lett. (2014)

Thin-film solar cells made their debut in pocket calculators, but they are now a serious competitor to silicon cells for power generation, with comparable efficiencies and rapidly decreasing costs. Cadmium telluride (CdTe) is one of the most promising thin-film materials. Its record efficiency was boosted to 20% by including a step during synthesis in which the material is treated with cadmium-chloride ( CdCl2)—a process meant to suppress or eliminate the detrimental effects from defects (i.e., to “passivate” them). But why passivation led to better efficiency has remained a mystery and progress has been driven by incremental trial and error. Now, as reported in Physical Review Letters, Chen Li and colleagues at Oak Ridge National Lab, Tennessee, the University of Toledo, Ohio, and the National Renewable Energy Laboratory, Colorado, have elucidated the atomic mechanisms of this phenomenon.

A CdTe thin film is a polycrystalline material made of many grains of CdTe single crystals. Compared to a perfect single crystal, one would expect that polycrystalline grain boundaries are detrimental, acting as recombination centers for the electron-hole pairs generated when a photon is absorbed. But through a combination of atomic-resolution electron microscopy and density-functional theory, the authors studied the atomic and electronic properties of grain boundaries and showed that they play an unexpected role: During CdCl2 treatment, Cl takes the place of a large fraction of Te atoms at the grain boundaries. This turns the boundaries into local p- n junctions, which separate photogenerated electrons from holes, protecting them from unwanted recombination. The results explain the benefits of the passivation treatment and may suggest new fabrication strategies for further efficiency improvements. – Matteo Rini


Subject Areas

NanophysicsMaterials ScienceEnergy Research

Related Articles

Driving a One-Way Flow with a Two-Way Field
Nanophysics

Driving a One-Way Flow with a Two-Way Field

A design for a nanopump that uses an alternating electric field could allow researchers greater control over nanoscale fluid flows. Read More »

Why Wetting a Surface Can Increase Friction
Materials Science

Why Wetting a Surface Can Increase Friction

Experiments suggest that hydrogen bonding explains why a wet surface can have nearly twice as much friction as a dry surface. Read More »

Cooking with Phason Gas
Condensed Matter Physics

Cooking with Phason Gas

Heat-transport measurements and neutron-scattering spectroscopy probe a form of thermal conduction based on excitations called phasons. Read More »

More Articles