Photocell Optimization Using Dark State Protection

Abstract

Conventional photocells suffer a fundamental efficiency threshold imposed by the principle of detailed balance, reflecting the fact that good absorbers must necessarily also be fast emitters. This limitation can be overcome by "parking" the energy of an absorbed photon in a dark state which neither absorbs nor emits light. Here we argue that suitable dark states occur naturally as a consequence of the dipole-dipole interaction between two proximal optical dipoles for a wide range of realistic molecular dimers. We develop an intuitive model of a photocell comprising two light-absorbing molecules coupled to an idealized reaction center, showing asymmetric dimers are capable of providing a significant enhancement of light-to-current conversion under ambient conditions. We conclude by describing a road map for identifying suitable molecular dimers for demonstrating this effect by screening a very large set of possible candidate molecules.

Optimal power generation using dark states in dimers strongly coupled to their environment

“…The presence of large Stokes' shifts in candidate monomers for implementing dark state protection ideas, such as those found in [6], indicates that these molecules have strong coupling to vibrational modes. Therefore, we transform to the polaron frame which takes some of the phonon interaction Hamiltonian into the system, leaving a residual interaction that can be treated within a weak coupling theory.…”

“…The stronger the dipole-dipole coupling ¢ C is compared to the detuning of the monomers Δ in the polaron frame, the more delocalized the excitons are over the monomers [29]. The phase relationship between the monomer components of the eigenstate wave functions then determines their optical dipole matrix element with the ground state, and fine tuning the degree of delocalization leads to the formation of dark states for various monomers [6].…”

“…Since there is then no dark state advantage, and the benchmark has overall twice the extraction, the benchmark has a higher power output than the dimer. In appendix H we calculate the power output for candidate dimers found in [6].…”

“…However, the efficiency of these devices is much lower than those found in more conventional solar cells, with experimentally reported efficiencies reaching up to 13% after molecular optimization [3], but commonly much lower [1,4]. One way to improve these figures could be to exploit the results of recent theoretical studies, which show that if quantum interference can be harnessed in coupled organic molecules, then the efficiency of organic materials could be much higher [5][6][7][8][9].…”

“…Modeling has shown that careful optimization of the coupling of the monomers, through tuning their optical properties and relative position and orientation, leads to a hybridization and energy splitting of the single exciton eigenstates. The lower of these states can be optically inactive-the so-called dark state [5,6,10,11], whereas the higher lying state has an enhanced transition dipole-this is the bright state. The dark state is accessed by non-radiative, vibrational relaxation from the bright state.…”