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Nanoscience and Technology in Solar Cells
Published in Kaufui V. Wong, Nanotechnology and Energy, 2017
Kaufui V. Wong, Nicholas Perilla, Patrick Andrew Paddon
Research in nanotechnology is also attempting to increase electron transfer. There is a better transfer of electrons in well- separated layers, but these tend to limit electron transfer to the interphase between the two materials. There is an attempt to solve this problem by having two interpenetrating layers of electron emitters and acceptors. This concept has been tried by a variety of methods, including two-phase blends of polymer/inorganic nanocrystals, dye/dye, polymer/fullerene, polymer/polymer, but the most exploited are polymer matrix with physically dispersed dye. Another possible method of efficiency enhancement has undergone extensive research and that is using dyes that are capable of singlet fission into two triplets, meaning that two electron–hole pairs could be produced from only one photon [11].
New Approaches: Testing the Limits in Organic Solar Cells
Published in Theodore Goodson, Solar Fuels, 2017
While many of the first reports utilized inorganic quantum dot structures, it wasn’t long before those interested in organic solar cell devices started to investigate the possibility of multiple exciton generation in organic semiconducting materials.12–20 In this case, the process most commonly invoked is that of single exciton fission. Singlet fission is a spin-allowed process in which two triplets match the energy of one singlet and subsequently lead to the formation of a triplet pair. It is often said that the two resulting triplet excitations produced from an excited singlet are born coupled into a pure singlet state. Singlet fission can therefore be viewed as a special case of internal conversion (radiationless transition between two electronic states of equal multiplicity).20 This process is similar to many other internal conversion processes. It can be very fast, particularly in molecular crystals. When this process is isoergic or slightly exoergic and the coupling is favorable, the transformation occurs on a ps or even sub-ps time scale.15
Using First-Principles Simulations to Understand Perovskite Solar Cells and the Underlying Opto-Electronic Mechanisms
Published in Carlito S. Ponseca, Emerging Photovoltaic Technologies, 2019
Another physical problem that falls beyond the capabilities of the approaches used in the previous sections deals with the relaxation of high-energy excited states, populated via the absorption of photons, towards the lowest lying excited state and then back to the ground state. Addressing this question is in general very important in the field of photovoltaic, since the possibility of using the excess of electronic energy, as for the case of singlet-fission or carrier multiplication mechanisms [40], is one of the route suggested to overcome the highest theoretical efficiency of a single junction cell, the so-called Shockley-Queisser limit (~34%) [108]. In the case of hybrid perovskites, the question is even more intriguing, in light of their “hybrid” crystalline structure. Electron cooling in direct inorganic semiconductor typically takes place in the ~ps time-scale and is mediated by acoustic phonons, while in organic materials this process is generally much faster (<100 fs) and is mediated by high-frequency vibrations (~1500 cm−1) associated to the motion of the molecular backbone [41, 113]. One could hence wonder how do hybrid perovskite configure within this picture? To simulate the electronic de-excitation processes, one should explicitly consider how the excess of electronic energy gets dissipated in the form of kinetic energy of the nuclei. In Section 8.3, it was shown that, within the Born–Oppenheimer approximation, the nuclear and electronic problems are decoupled, hence not allowing for the explicit energy exchange between the two subsystems. It is therefore immediately clear that the treatment of electronic de-excitation processes requires to go beyond the Born–Oppenheimer approximation. To this aim, the method of choice moves towards non-adiabatic simulations, where the coupled electronic and nuclear degrees of freedom are allowed to evolve together, with the possibility for the two subsystems to exchange energy.
Different routes towards triplet states in organic semiconductors: direct S0→T excitation probed by time-resolved EPR spectroscopy
Published in Molecular Physics, 2019
Clemens Matt, Deborah L. Meyer, Florian Lombeck, Michael Sommer, Till Biskup
In light of the long-standing debate about the role of triplet states in organic semiconductors, the additional route towards triplet excitons, namely direct excitation demonstrated here for TBT and previously even more convincingly for the somewhat extended molecule CbzTBT, may well be highly relevant in terms of efficiency of organic semiconductors. A summary of all the different pathways towards triplet states relevant or at least occurring in organic electronic devices is given in Figure 6. Whereas intersystem crossing from an excited singlet state into an excited triplet state predominates mainly in pristine polymers, back electron transfer from a CT state to a lower lying triplet state of one of the component involved mostly occurs in blends of donor and acceptor materials [38] and is generally seen as detrimental in OPV devices. Singlet fission, i.e. creating two excited (triplet) states from one excitation [39], opens the door towards quantum efficiencies above one, but normally needs a special arrangement of the energy levels of singlet and triplet states involved, namely the triplet level having about half the energy of the singlet level. Direct optical excitation of triplet states would open up the possibility in OPV devices to harvest low-energy radiation and hence increase the overall efficiency.
Singlet fission in tetracene: an excited state analysis
Published in Molecular Physics, 2020
Luis Enrique Aguilar Suarez, Maximilian F. S. J. Menger, Shirin Faraji
Predictions point out that if the temperature of the Earth raises above four degrees within the next years, around 43 to 58% of the current species inhabiting the planet will disappear [1]. Therefore, clean and renewable ways to generate electricity are needed. The conversion of sunlight into electricity seems to be one of the most promising ways to face the current environmental challenges [2]. Nevertheless, the artificial light-harvesting is still problematic due to the low quantum yields shown in solar cells [3]. Therefore, ways to improve their performance are needed. Singlet fission (SF) is a multiexcitonic generation process that occurs in organic solids, [4, 5] during which two coupled triplets are formed from a singlet photoexcited system in an overall spin-allowed process (see Figure 1) [4, 5]. SF has been explored as an alternative process to enhance the current performance of solar cells and break the so-called Shockley–Queisser theoretical limit of 34% [6, 7], since in principle two pairs of charge carriers can be generated per absorbed photon. It has been estimated that the inclusion of a SF material layer to a single-junction photovoltaic device could increase its efficiency from 34% to approximately 45% if the charge carriers are effectively harvested [8]. Furthermore, simulations have predicted that the addition of a SF material could increase the efficiency of a current silicon-based solar cell by up to 4.2% [9]. Nevertheless, the low number of molecules exhibiting SF and the poor understanding of how this process takes place have limited the development of SF-based solar cells. During recent years, experimental works have started to demonstrate the potential of SF materials for enhancing the performance of solar cells within a variety of architectures [10, 11].
The time-dependent density matrix renormalisation group method
Published in Molecular Physics, 2018
Singlet fission, the quantum mechanical process by which a singlet exciton splits into two distinct triplet excitons, has recently attracted lots of research interests due to its proposed use for carrier multiplication in organic solar cells (OSCs) [81]. However, the dissociation mechanism of the triplet exciton is still quite few, in contrast to the intense study for the dissociation of singlet excitons. In this part, we show our t-DMRG numerical simulations for the triplet exciton dissociation in a polymer chain.