Thermodynamics of light management in photovoltaic devices

This paper elaborates a comprehensive theory of the thermodynamics of light management in solar cells explicitly considering imperfect light trapping, parasitic absorption and nonradiative recombination losses. A quantitative description of the entropic losses that reduce the open-circuit voltage and the energy conversion efficiency from the radiative limit towards realistic situations is presented. The theory embraces the fundamental limits for idealized solar cell devices given by the Yablonovitch limit and the Shockley-Queisser limit. We discriminate between reversible and irreversible entropic loss processes for four fundamental light management concepts: (i) conventional light trapping as an integral part of the device, (ii) geometric concentration of incident light, (iii) angular restriction of incoming and outgoing light, and (iv) light concentration by luminescent solar collectors. Based on this discrimination, a comprehensive discussion of the interplay between the loss processes and light management is presented. As part of this analysis, a new figure of merit for efficient light trapping in solar cells is introduced as well as an example of a deterministic light trapping concept which induces almost optimal light trapping.