Viorel Badescu (born in Bughea de Jos, Arges, Romania, 24 September 1953) is Professor of Engineering Thermodynamics and affiliated with Candida Oancea Institute at Polytechnic University of Bucharest. His mainstream scientific contributions consist of more than 250 papers and several books related to statistical physics and thermodynamics, the physics of semiconductors, and various aspects of terrestrial and space solar energy applications. Also, he has theorized on present-day Mars meteorology and Mars terraforming and on several macroengineering projects. He is Reviewer or Associate Editor of more than 50 international journals and member of eight scientific societies including the International Solar Energy Society and the European Astronomical Society. He received four awards including the Romanian Academy Prize for Physics in 1979. He is member of the Romanian Academy.
Title and Abstract of the Speech:
Research directions in third generation photovoltaics
Candida Oancea Institute, Polytechnic University of Bucharest,
Spl. Independentei 313, Bucharest 060042, Romania
The main limitation on photovoltaic efficiencies is caused by the polychromatic nature of the solar spectrum. A single band gap cell can use a small bang gap material. Then, many more photons are absorbed, resulting in a higher current, but with a small voltage, limited by the narrow band gap. Also, a single band gap device can use a large band gap semiconductor. This results in a large voltage but the number of photons absorbed is small and the current is small, too. The result is a (compromise) optimum band gap of around 1.3eV which maximises the product of current and voltage, the power from the device, and hence the efficiency. Several schemes (generically called ‘Third Generation approaches’) have been suggested and are being researched to increase the efficiency of photovoltaic cells above the limit for a single band gap device. They all involve use of more than one energy level in the device, with each level optimised to collect photons from one section of the solar spectrum. These include tandem cells, impurity photovoltaic, intermediate band, multiple carrier generation, hot carrier cells and cells in which quantum wells or quantum dots provide an intermediate level within the band gap. But one of the practical problems with all of these designs is that they require good optical properties as well as good electrical properties to reduce non-radiative recombination and to allow carrier transport – thus compounding the difficulty of making a good device. Separation of these functions into separate elements of a combined device would thus make a device much easier to optimize. The techniques based on up- and down-converters are examples of this category. They are briefly described in this talk.