Hot injection processes in optically excited states: Molecular design for optimized photocapture

Design principles for efficient solar photocapture using a single molecule are presented. The proposed molecular model is composed of ground and excited bright and dark electronic states. Once photoexcited to the bright states, vibrational relaxation and dissipation to the ground vibrational level of the bright state commonly occur. This degrades a substantial amount of the incoming photon energy into heat, further reducing the efficiency of molecular photocells. One way to circumvent this energy flow from electronic excitation into heat is the hot injection process, by which the original excited bright state undergoes a rapid crossing to an acceptor dark state, with a higher potential energy minimum, and is trapped in the region of that minimum. By choosing an appropriate pair of vibrational modes, the overall energy gain can be increased substantially and the constraints on the bath behavior substantially simplified. We present calculations in a two-dimensional vibrational space, along with energy relaxation and transfer to the bath (using a Stochastic Surrogate Hamiltonian model). We find that the second degree of vibrational freedom, if carefully chosen, strongly increases the efficiency and the possibility of successful hot injection. In addition, the same molecular model can be designed to utilize the red part of the solar spectrum. Excited state absorption can recycle the wasted bright state population thus increasing the efficiency of solar capture.