Physicists at the Netherlands Energy Research Foundation have developed a technique to measure imperfections in solar cells. That information points the way to more efficient and cost-effective solar cells. As light strikes a solar cell, it produces free electrons. The key to maximizing solar cell efficiency is to capture all those free electrons and put them to work. The problem is that because of imperfections in the cell itself, those electrons have a tendency to fall back, or recombine, into a bound state. The longer the electrons remain free, the greater the chance they will be captured. Thus, the mean free lifetime of electrons is an important parameter. The researchers used two means to determine recombination rates. Both measure conductivity and the number of free electrons: Microwave-detected photoconductance decay bounces microwaves off the surface of the cell. Modulated free-carrier absorption sends infrared light through the cell The former is better at low bias light levels, and the latter at high bias light levels. Using both techniques yielded a complete picture of free-electron production across varying light intensities. Economical solution The search for a more economical solar cell prompted the development of the measurement technique, according to Frank Schuurmans, a member of the research team, who now works for Shell Solar Energy B.V. in Helmond, the Netherlands. He noted that the record for solar cell efficiency of 24.4 percent -- not far below the theoretical maximum of 28.8 percent -- was achieved by the University of New South Wales in Australia last year. However, the record was set using relatively expensive silicon manufactured using ultraclean processes. Multicrystalline silicon is much cheaper, but the highest recorded efficiency, again from the University of New South Wales, is 19.8 percent. A key to improving this figure is thought to be the development of a better top coating. Silicon oxide has been favored, but now plasma-enhanced chemical vapor deposition (PECVD) of silicon nitride is a contender for an economical overcoat. Its performance has been the subject of research. "For the thermally grown oxides, the theory works fairly well," Schuurmans said. "For PECVD silicon nitride, we are coming close to understanding the physics."