Tin dioxide offers sufficient transparency for solar cells

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By studying the limits of transparency in conducting oxides, US researchers are hoping to design more efficient solar cells.

The team at the Computational Materials Group at the University of California, Santa Barbara (UCSB), has published its calculations on tin dioxide (SnO2) in the journal Applied Physics Letters.

Transparent conducting oxides are used as transparent contacts in a wide range of optoelectronic devices, such as photovoltaic cells, light-emitting diodes (LEDs) and touchscreens.

These materials are unique in that they can conduct electricity while being transparent to visible light. For optoelectronic devices to be able to emit or absorb light, it is important that the electrical contacts at the top of the device are optically transparent. Opaque metals and most transparent materials lack the balance between these two characteristics to be functional for use in such technology.

Conducting oxides strike an ideal balance between transparency and conductivity because their wide band gaps prevent the absorption of visible light by excitation of electrons across the gap, according to the researchers. At the same time, dopant atoms provide additional electrons in the conduction band that enable electrical conductivity. However, these free electrons can also absorb light by being excited to higher-conduction-band states.

The research team found that SnO2 only weakly absorbs visible light, thus letting most light pass through, so that it is still a useful transparent contact. In the study, the transparency of SnO2 declined when moving to other wavelength regions. Absorption was five times stronger for ultraviolet light and 20 times stronger for the infrared light used in telecommunications.

‘Every bit of light that gets absorbed reduces the efficiency of a solar cell or LED,’ said project lead Chris Van de Walle of the UCSB. ‘Understanding what causes the absorption is essential for engineering improved materials to be used in more efficient devices.’