Light harvesting for quantum solar energy conversion

Despite wide structural and functional differences, the laws that govern quantum solar energy conversion to chemical energy or electricity share many similarities. In the photosynthetic membrane, in common with semiconductor solar cells, the conversion process proceeds from the creation of electron–hole pairs by a photon of light, followed by charge separation to produce the required high-energy product. In many cases, however, mechanisms are needed to enhance the optical absorption cross-section and extend the spectral range of operation. A common way of achieving this is by light harvesting: light absorption by a specialised unit which transfers the energy to the conversion apparatus. This paper considers two examples of light harvesting — semiconductor solar cells and the photosynthetic apparatus — to illustrate the basic operation and principles that apply. The existence of a light harvesting unit in photosynthesis has been known since the early 1930ʹs but details of the process — relating, in particular, to the relationship between the structure and spectral properties — are still being unravelled. The excitation energy carriers are excitons but the precise nature of the transport — via the solid state Frenkel–Peierls variety or by Försterʹs resonant energy transfer — is still subject to debate. In semiconductor solar cells, the energy of the absorbed photon is collected by minority carriers but the broad principles remain the same. In both cases it is shown that the rate of energy conversion is described by a law which parallels the Shockleyʹs solar cell equation, and the light harvesting energy collection is subject to reciprocity relations which resemble Onsagerʹs reciprocity relations between coefficients which couple appropriate forces and flows in non-equilibrium thermodynamics. Differences in the basic atomic make-up in the two systems lead to different energy transport equations. In both cases, however, similar mathematical techniques based on Greenʹs functions can be used to advantage. The Greenʹs function provides a convenient vehicle for the determination of the probability of energy collection — known as the trapping probability in the photosynthetic unit. Using the reciprocity relation, both quantities are shown to be closely related to the distribution of the energy carriers in the dark. The collection probability can then be discussed in detail, by solving the semiconductor device equations in the case of solar cell, and by linking the Greenʹs function formalism to the random walk model in the case of the photosynthetic unit. The concept of resonant energy transfer is beginning to enter the arena of solid-state optoelectronics. It is an aim of this paper to show that similar phenomena — which exist in the domain of bioenergetics — can throw new light on a range of energy transfer and collection processes that are of considerable importance in many modern optoelectronic devices.