Efficient Solar Cells and Quantum Dot

Photon–Matter Interaction & Application to Solid States

This framework can be applied to 2-dimensional systems excited at normal incidence which would be the basis of the experimental work. For bulk systems, we can consider a solid cylinder and quantize the electron states in cylindrical coordinates, and finally take the limit of large system. The bulk properties are independent of the geometry of the solid.  We use cylindrical rather than cartesian coordinates to decouple the Heisenberg equations of motion based on the electron's angular momentum. Using these generalized Block states, we can predict the kinetics of electrons, look for the electric currents and demonstrate the transfer of OAM from the light beam to the electrons.

This framework is applicable to many systems, from semiconductors to insulators having larger band gaps if the frequency of the twisted field is tuned above the energy bandgap. Therefore, twisted fields in the near infrared to UV spectrum can be used from this 2-level framework, and we can easily generalize to the case of more than one valence band. The excitation of solids by twisted light beams also produces a space-dependent carrier kinetics which requires local variables. We can potentially visualize the pattern of motion of the photo-excited electrons from the spatially inhomogeneous electric current density. The inter-band coherence contributions have fast oscillations similar to the inter-band polarization. However, intra-band coherence contributions are slower processes where we have a transfer of momentum from light to matter. The first quantization framework considers observables as operators and state as a wave function. In second quantization, we consider states as operators as well. Second quantization allows us to do multi-body dynamics. The second quantization allows the use of “big” Hilbert space with vectors that represent states of large numbers of particles because second quantization is a link between the quantum mechanics of many-particle systems and the relativistic quantum field theory. In structured beams, photons with OAM and SAM travel at a speed slower than velocity of light. This is due to a projection effect of the effective motion of the photon on the axis of propagation of the diverging beam and depends on the geometrical properties of the beam.

 

Structured Photons to Improve the Efficiency of Solar Cells

In this patent class we introduce an artificial photo conversion system using an amplitude or a phase mask which can enable the suppression of electron-hole recombination, thereby increasing or enhancing the efficiency of any PN junction or solar cell or any solid state used for energy harvesting or display. When a hologram is used in the path of a photon the orbital angular momentum generated by the photon can be transferred to the electron and a new quantum state can be created, and suppression of electron-hole recombination is supported. This suppression is due to the change in total angular momentum of the electron (spin + orbital) using a variety of methods to twist the photon field (passively using a hologram or actively using other methods).

In photovoltaic diodes, recombination of photogenerated electrons and holes is a major loss process. Biological light harvesting complexes (LHCs) prevent recombination via the use of cascade structures, which lead to spatial separation of charge carriers. Therefore, the twisted photons can be used in conjunction with organic photovoltaic cells (OPVs) to suppress recombination of electron-holes. The suppression of recombination can be engineered through the interplay between spin, OAM, and delocalization of electronic excitations in organic semiconductors. OPVs would have poor quantum efficiencies if every encounter leads to recombination, but state-of-the-art twisted photons with OPVs can demonstrate better quantum efficiencies.

Time-resolve spectroscopy can be used to study different engineered models with high efficiency systems in which the lowest lying molecular triplet exciton (T1) lies below the intermolecular charge transfer state (CT). Encounters of spin-uncorrelated electrons and holes generate CT states with both spin singlet (1CT) and spin triplet (3CT). Triplet exciton formation can be a major loss mechanism in OPVs. However, even when energetically favored, the relaxation of 3CT spin triplet to T1 can be strongly suppressed via, control over wave function delocalization allowing for disassociation of CT back to free charges (FC) thereby reducing recombination and enhancing performance. Extracted kinetics can demonstrate that triplets may grow as charges decay. One can consider that the primary decay channel for triplets is triplet-charge annihilation, due to the high charge density present, and model the time evolution with Langevin equation. The CT energy lies below T1 making relaxation from 3CT to T1 energetically favored, however, the more efficient 1:3 blend no triplet formation is possible at room temperature. But at low temperatures bi-molecular triplet formation can be observed in this blend. This suggests that there is a thermally activated process that competes with relaxation to T1. This process is disassociation of 3CT back to free charges. At high temperatures the disassociations of 3CT back to free charges with an associated time scale τ3, outcompetes relaxation of 3CT to T1. At lower temperatures this disassociation process is suppressed leading to a buildup of triplet excitons.

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