Transport and charge separation are crucial steps in molecular or hybrid energy converters. We have developed a theoretical-numerical framework within the nonequilibrium Green’s function formalism to investigate the time-dependent transport of charges and energy, including the Coulomb interaction in the Hartree-Fock approximation. We thus analyze charge transport and separation in a donor-acceptor nanojunction illuminated with a femtosecond laser pulse. Our analysis conducts us to depart from the standard view of static density of states and driving energy, and to rather define and handle dynamical quantities on which relying to design ultrafast optoelectronics and highly efficient photovoltaics.
During the primary steps of photosynthesis, the light-harvesting complexes capture sunlight and transfer the associated energy to reaction centers where charge are separated. Surprisingly, optical spectroscopy has recently revealed manifestations of quantum coherence in the ultrafast dynamics of these natural nanosystems, that would be controlled by the interaction between excitations and the surrounding protein motion. Inspired by the architecture of a natural reaction center, we have designed a generic molecular nanodevice, and simulated the time-dependent photocurrent induced by a femtosecond laser pulse. In this analogue, a time-dependent external voltage is applied to the device in the picosecond timescale via a gate, in order to mimic the effects of protein vibrations. The voltage characteristics are the parameters of this study. The numerical investigation we propose aims at unraveling the conditions in which this external control may increase the photocurrent inside the nanodevice. To this aim, we have developed a combined theoretical/numerical framework to describe and understand the quantum transport of energy and charges, from the nonequilibrium Green's function formalism. Our findings show that such an external control may be beneficial for the integrated (dc) current owing through the interface. Indeed, this external control enables to prevent the back tunneling oscillations of the timedependent photocurrent, which globally enhances the dc current. This exploratory work paves the way towards smart biologically-inspired optoelectronics.
By means of nonequilibrium Green's functions using the Born approximation to treat the light-matter coupling, we numerically investigate impacts of competitive hybridization on the photocurrent of a quantum dot based optoelectronic device. The model of device is an absorbing quantum dot connected to two semiconducting electrodes through energy filtering quantum dots. Hybridization occurs between the absorber and the filter, via the inter-dot coupling β, and between the filter and the electrode, via the dot-lead coupling Γ. At the tunnel resonance between the absorber and the filter, the investigation reveals the existence of two operating regimes in the nanodevice characterized by opposite variations of the photocurrent depending on ratio β/ Γ.
Energetic and entropic issues are theoretically addressed in quantum optoelectronic nanodevices. We rely on the nonequilibrium Green's function methodology to provide a framework which combines optoelectronics and thermodynamics in a unified picture of energy conversion for nanoscaled optoelectronics. Indeed, we follow the self-consistent Born approximation to derive the formal expressions of energy and entropy currents owing inside a nanodevice only interacting with light. These expressions are numerically evaluated in a quantum-dot based nanodevice, where verification of the second law of thermodynamics raises questioning about the system model. We here put the focus on the spontaneous emission energy current to discuss the question.