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Seven factors limiting the performance of photovoltaic solar energy converters are listed and explained. They can be classified into basic and technology determined limitations. Possibilities for improvement on technology determined limitations are investigated for the silicon solar cell. Such possibilities are: heavier p-layer doping; change of geometry, possibly by application of grid structures; improvement of the material constants; and utilization of drift fields for improved collection. Discussed are materials other than silicon in regard to their potential for better performance than that obtainable from the silicon solar cell; and finally, new methods of approach, such as the multilayer and the multiple transition solar cell. Both of these methods yield theoretically large improvements, but realization depends on further advances in compound semiconductor technology and in knowledge about localized centers in the forbidden gap. Limit conversion efficiencies of 38.2 per cent for a 3-layer cell and of 51 per cent for a 3-transition cell, compared to 23.6 per cent for a single p-n junction, single transition cell, are obtained. Also discussed are the possible merits of the application of the graded energy gap to photovoltaic energy converters, and potential improvement in collection efficiency is found for certain cases.