Synopsis: The Key to Thin-Film Solar-Cell Efficiency
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 by including a step during synthesis in which the material is treated with cadmium-chloride ()—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 treatment, Cl takes the place of a large fraction of Te atoms at the grain boundaries. This turns the boundaries into local - 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
Chen Li, Yelong Wu, Jonathan Poplawsky, Timothy J. Pennycook, Naba Paudel, Wanjian Yin, Sarah J. Haigh, Mark P. Oxley, Andrew R. Lupini, Mowafak Al-Jassim, Stephen J. Pennycook, and Yanfa Yan
Kai-Mei Fu studies the properties of atomic defects in materials with the goal of using these normally unwanted flaws to create quantum technologies for secure communication.
Researchers say they are close to simulating high-temperature superconductivity using a lattice of ultracold atoms, a step toward explaining this perfectly conducting state.
Experiments pressing two materials together show that static electricity accumulates when surface water lets ions move from one surface to another. Read More »
Nanomechanical resonators probe helium-4 at record low temperatures in a proof-of-concept experiment demonstrating the potential to explore fundamental properties of quantum fluids. Read More »
