PHOTOVOLTAIC SOLAR CELLS: LIMITS AND OPPORTUNITIES

ABSTRACT:   
	
	We know rather well how common solar cells work (and I will explain this) and we have a good guide for their maximal efficiencies, using the so-called detailed balance (“what comes in, does go out”) model of Schockley and Queisser. While much effort is directed towards circumventing these limits, for newer types of cells things are not so simple. I will explain why, using some straightforward criteria that help evaluate and compare the various types of solar cells (and in some cases allow including natural photosynthesis in the comparison). Using these and other criteria I will get to the issue if differences between cell types are just a matter of “not trying hard enough” or if basic scientific bounds, beyond those known today, exist.

	This analysis suggests the existence of additional limits for newer types of solar cells, including Organic Photovoltaic and Dye-sensitized cells. Because of the molecular/polymeric nature of organic materials, effects of tail and of vibronic states are pronounced in these cells. Identifying limits, due to such states leads to more realistic predictions for future progress than what (over)hype exposes us to, and stimulates efforts to find ways to circumvent such limits.

* work done with P. K. Nayak (Weizmann Inst.), & with  
   J. Bisquert (Un. Jaume, Spain), A. Kahn (Princeton Univ.) and K. L. Narasimhan (IIT Bombay).