35 years ago, with the Solar One House at the University of Delaware, the first photovoltaic (PV) building-integrated house was created that, in piggyback, produced electric energy and heat collected from the back of the solar cells. That heat collection was optimized by a finned air collector surface that minimized the air resistance. The house exemplified a total systems approach, with molten salt heat storage for winter heating, and summer coolness storage from the night-cooled roof. It was a combined active and passive house with numerous energy conservation features, and it was publicized worldwide. The solar cells used were CdS/CuxS for reasons of simplicity of production and were consequently further developed to improve efficiency and life expectancy; the latter by replacing the top electrodes with graphite coated wires, and the elimination of most of the crevices that acted as nucleation centers for detrimental copper nodule formation. The cell research was stopped because of a principal efficiency limit to 10% that prohibited competition to the rapidly evolving thin-film solar cells. However, the CdS coatings, that prevented excessive high field in our cells, are still used in most of the thin-film solar cells that characteristically have a high trap density and would otherwise have high junction fields causing reduced fill factors and thereby reduced efficiencies. This is improved by the addition of CdS. The detrimental fields are sharply reduced by high-field domains that are characteristic of field-quenching and only stabilized in CdS that we investigated in the mid 1960s.
The Solar One house built in 1973.
The solar house that was pioneered in the early 1970s, is now well accepted with building-integrated solar collectors, and the future will also tend to a combined photovoltaic/thermal structure as soon as mass production permits better engineering and economics of such a combined system that has obvious advantages in area coverage and increased efficiencies by reducing the cell temperatures.
In general, the explosive growth of the solar cell market in the last decade which now exceeds several gigawatts per year is still mostly in crystalline silicon. But the market fraction of thin film PV is growing, mostly now in CdTe and amorphous Si, but more recently with ternary compounds emerging as serious partners. Embryonic ideas of completely new cell technologies should be nurtured, even though they may not appear to have critical near term potential.
As we expect photovoltaics to have a quantum jump up in demand motivated by a political climate change caused by avoiding a green house gas catastrophe, we will probably see a revolutionary growth in fully automated silicon production from the raw product stocks to the final solar panels in one factory building complex, at an initial investment exceeding $1 billion, and reducing the price to below $1.50 per installed watt, resulting in a consumer cost of less than 10 c/kWh.
To meet such ambitious goals, progress in photovoltaics has to be achieved on various fronts: improved materials with better optical and electronic properties, more efficient production techniques allowing the cost-effective, sustainable, and high yield fabrication of solar cells on the scale of manygigawatts per year, and better solar cell designs which lead to higher overall conversion efficiencies and energy harvesting factors.
The current issue of physica status solidi – Rapid Research Letters is dedicated to this endeavor. It contains cutting edge work from the leading institutions in Europe dealing with basic and applied research in photovoltaics for the next solar cell generation. Even small improvements in solar cell technology today have the potential to become major benefits in the future, as photovoltaics establishes itself more and more as an integral part of the energy mix which will power our planet in the 21st century.
Karl Wolfgang Böer, Distinguished Professor of Physics and Solar Energy, Fellow AAAS, APS, ASES, IEEE. Dr. 1952, Habilitation 1955, Professor and Director of the IV. Physics Department 1958–61, all Humboldt University Berlin, Founding Editor of physica status solidi 1961. Since 1962 Professor at the University of Delaware, Physics Department to 1971, Director and founder of the Institute of Energy Conversion 1972–75, also CEO of SES, Inc. 1973–79, President of the American Solar Energy Society 1978–79. Founder and Editor-in-Chief of Advances in Solar Energy 1984–2002. Editor of other journals and proceedings, author of two books, 312 papers and 18 patents, numerous awards.