In the heat of the Sun: efficiency and temperature of solar cells

7 02 2009

With the renewable energy needs being widely recognized, a careful assessment of the viable photovoltaic technologies is needed. Solar energy is clean, regular and widely available, but it arrives in the Earth surface with a low concentration. Devices for producing electricity from sunlight on a  large scale must cover a broad area, and should be produced at a low cost. One critical number to feature the diffferent photovoltaic technologies is efficiency: which percent of the radiant energy in the light impacting on the device is converted to electrical energy. Doubling the efficiency immediately doubles the energy production for the same device size.

Monocrystalline silicon (MCS) solar cells are a mature technology that currently dominates the photovoltaic market. Solar cell modules of MCS with 15-17% efficiency are mass produced and installed. Younger technologies, such as thin film amorphous silicon, provide module efficiency of about 7%, which looks rather poor in comparison to MCS.

However, efficiencies are reported following the result of standard measurement protocols, including a fixed solar cell temperature. In operation on the field, the energy conversion process causes abundant heat, and the solar cell temperature raises. This is specially acute in sunny places like Spain, where the high ambient temperature slows down the heat evacuation from the device.

MCS efficiencies dramatically fall down when the temperature raises. This is due to the fact that MCS solar cells, based on a relatively thick, single piece of crystalline Si material, obey almost perfectly the ideal model of semiconductor physics, and this implies that the voltage must go down as temperature increases, exactly as

dV/dT = -0.00288 Volt/ºC

This leads to a decrease of about 0.5 points of efficiency per ºC increase (M. B. Prince, Journal of Applied Physics, 1955) — which is hugue. Our own monitoring of MCS pannels instaled in the roof of Universitat Jaume I shows that the top conversion efficiency remains around 9%.

MCS pannels installed in Castelló, Spain

MCS pannels installed in Castelló, Spain

Alternative technologies, with lower nominal efficiencies, may therefore become seriously competitive when considering real peformance versus cost. Dye-sensitized solar cells (DSC), where developed in the early 1990s by swiss scientist Michael Grätzel and coworkers. DSC have top efficiencies of 11%, but the efficiency-temperature curve is nearly flat, since by the principles of operation DSC employ a combination of nanoscale organic and inorganic materials instead of a single inorganic semiconductor. Sony has already produced top class DSC devices, and reported the different temperature behaviour which gives DSC a potential advantage over other technologies.

DSC also show advantages of performance by captation of diffuse light, and by maintaining the efficiency event at low light intensity. This gives a great deal of energy captation when the full daily cicle of sunlight is considered. A study led by Taro Toyoda in Japan (Journal of Photochemistry and Photobiology A: Chemistry 164, 203–207, 2004) also showed that DSC may outperform MCS pannels when the energy production is compared on the basis of the same power producing area.

While MCS provides the option prefered by many for photovoltaics, if would be desirable to have a variety of competing technologies that provide alternative pathways towards the central goal of establishing available benign solar energy for the future of humankind. Discussions of the promise of new photovoltaic technologies must look not only at efficiency, and cost, but also to performance in the real conditions in the heat of the sun.



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