May 31 2010

A promising future for solar PV cells

Published by under Student blog entries

Researchers in the Materials Sciences Division (MSD) of Lawrence Berkeley National Laboratory, working on conjunction with Cornell University and Japan’s Ritsumeikan University, made a promising discovery concerning the band gaps associated with solar cells made of the semiconductor indium nitride is much lower than initially thought.  For those not aware, band gaps of semiconductors correspond to the energy range in which no electron states can exist, and in the case of solar photovoltaic (PV) cells, represent the amount of energy needed to remove the outermost electron from its shell.  The color band gap associated with indium nitride semiconductors was previously believed to be 2 eV.

The most commonly used material to create solar cells is silicon:  it is the most inexpensive material available but loses a great percentage of its energy to heat.  The most desirous material for a PV cell are alloys (i.e. combination of metals) consisting elements from group III of the periodic table, like aluminum, gallium, and indium, with elements from group V, like nitrogen and arsenic.  Solar cells consist of two types of materials:  n-type (negatively doped) material and p-type (positively doped) material.  N-type materials are high in electrons, concentrated mostly in the conduction band, while p-type materials are low in electrons and have holes in their conduction bands.  Incoming photons of the right wavelength are able to wrestle electrons from the n-type material from the holes in the conduction band of the p-type material, creating a flow of electrons (i.e. a current).  The maximum efficiency for one-layer solar cells is 30%, although only an efficiency of 25% has been demonstrated, owed mostly to the band gaps of the materials used.  To combat this issue, stacked solar cells consisting of different alloys have been proposed, although these efficiencies can be at maximum 50% with only an efficiency of 30% has been shown.

Using the light-emitting properties of LED devices, researchers at the Berkeley labs, noticed that if some of the gallium in the PV cell was replaced by indium, wavelengths from most colors in the visible spectrum could be observed.  After obtaining the purest crystals of the alloy possible, thanks in large part to group at Cornell University headed by William Schaff, renowned for their expertise at molecular beam epitaxy (MBE), and also with a group at Ritsumeikan University headed by Yasushi Nanishi, the team at Berkeley noticed that if you look at the band gaps below 2 eV, the entire color spectrum could be observed.  After further study, it became clear to the researchers that the band gap of such a semiconductor was actually much lower than previously thought:  these studies showed that the band gap is only 0.7 eV.

This study offers great promise for the future of solar PV cells:  the indium nitrite alloy is much more resistant to radiation than most other materials and has a greater heat capacity than silicon, improving overall efficiency.  Although this study demonstrated the potential for n-type PV cells, it is believed that a p-type solar cell could be created using a similar methodology although the alloys may differ slightly in composition.  Experiments have been conducted with multilayer PV cells of the indium nitrite alloy, and although the two types of cells used could not be grown together (thus reducing the efficiency of this process if it were to be adopted today), they did demonstrate potential efficiencies of 70%.  Little information is currently available about acquiring the materials necessary for developing indium nitrite alloys.  Yet despite the need for increased research, this study offers a promising future for cheap solar energy around the world, and at the least, an added option for those looking outside of the silicon-paradigm.


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