|Fig. 1: Jablonski diagram showing the TTA up-conversion process in a sensitizer/annihilator mixture.|
In a previous report the concept of up-conversion and its effect in photovoltaics and solar cell efficiency enhancement has been studied.  There the rare-earth based systems were specifically studied as an interesting candidate because of their ladder like energy diagram matching with solar spectrum. Though being promising for solar applications the weak absorption of these materials lowers their up-conversion capability and the overall efficiency for practical purposes. In this report we continue our study on up-converting schemes with main focus on bi-molecular systems. The mechanism behind bi-molecular up-converters as well as their advantages and drawbacks and their potential for photovoltaic applications would be discussed in this report.
Among various up-conversion techniques two-photon absorption dyes, up-conversion with inorganic crystals and rare-earth materials are some of the most studied techniques. In spite of having very different natures all of these techniques suffer from required high excitation power, poor absorption of the visible light and low quantum efficiency. Unlike these techniques, triplet-triplet annihilation mechanism requires lower power sources. In addition the excitation source is not necessary to be coherent which makes this mechanism even more suitable for solar related applications. [2,3]
Also by utilizing different triplet annihilator- triplet emitter pairs the excitation and emission wavelengths can be changed in a wide range The TTA up-conversion occurs in a mixture of annihilators and emitters. The harvested energy by the sanitizers is transferred via a triplet-triplet energy transfer (TTET) process to the emitters in the mixture. The excited emitters then relax to their ground states by emitting a photon of higher energy, hence up-converting the exciting photon. Fig. 1 illustrates this process.
The process starts with exciting the sensitizer from its ground state, S0, to the singlet excited state S1. The excited atoms then populate the triplet excited state of T1 with an intersystem crossing (ISC). Note that the exciting photons cannot excite the sensitizers directly from S0 to T1 so the intermediate triplet state of S1 is necessary. The new populated state of T1 has a long lifetime so can transfer its energy to the triplet state of emitter via a triplet-triplet energy transfer (TTET) process. As the energy transfer between the triplet states is a Dexter process, requiring a contact between annihilator and the emitter, two types of molecules must be in the solution at the same time. Finally excited sensitizers in the triplet state collide each other and excite the atoms to their singlet state, following the spin statistic equation. The energy of this level is larger than the energy of the exciting photon hence an up-converted photon is emitted when the emitter relaxes to its ground state.
The sensitized triplet-triplet annihilation involves energy transfer between a sensitizer molecule called a donor and an acceptor. The sensitizer absorbs low energy photons in visible- to near IR region of the spectrum and excites to the triplet state with a long lived life time. In order to observe bimolecular quenching of the triplet excited state of the sensitizer the triplet acceptor energy must be lower than the triplet energy of the sensitizer. The greater the energy difference between the triplet sensitizer and triplet acceptor, the greater the driving force for this reaction and generally speaking, the more favorable the triplet energy transfer process. The sensitizer (donor) molecule is chosen so that its singlet excited state lies below that of the acceptor's singlet manifold while the sensitizer's triplet state lies above that of the acceptor. In essence, the singlet and triplet excited states of the sensitizer should be strategically nested between the singlet and triplet excited states of the acceptor/annihilator. As long as these specific energy criteria are met and the combined triplet energy from two acceptor molecules is greater than or equal to the acceptor's singlet state energy, then conditions are appropriate for the observation of up-converted fluorescence from the sample.
The spin statistical factors determine the probability of triplet-triplet annihilation and singlet generation. This probability on the other hand determines the efficiency of this process. When two excited emitter in triplet state of 3A1 interact, nine spin states will be produced with equal probability. The singlet state is only one of this process outcomes hence leading to the probability of 11.1%. However some recent results report efficiencies larger than this limit indicating that there might be some other states other than triplets leading to the singlet state. The following equation determines the up-conversion quantum yield for this process. φq is the energy transfer efficiency of TTET, φTTA is the probability of triplet to singlet conversion and φF is the quantum yield of fluorescing of the acceptor.
|Fig. 2: [Ru[dmb]3]+2 triplet sensitizer and DPA as triplet acceptor.|
Though lots of parameters involved in total efficiency enhancement of the process, the most important ones can be categorized in as: sensitizers must have high light harvesting capabilities as they are the first step of this process. The higher the cross section the more would be the number of the excited atoms. The probability of TTA increases with the number of excited atoms. The following TTET process for transferring the energy from the excited sensitizers to the emitters also is increased in probability with the number of the excited atoms. The triplet state must also have a long life time as well as a large quantum yield to lead to an efficient two-molecule quenching process via TTET. To have a strong up-converted signal from the excited emitter the quantum yield of the excited emitter must be high in the singlet state. Also the energy levels of the annihilator and emitter must be optimized in such a way that the transfer from triplet state of the sensitizer to the triplet state of the emitter can be maximized.
Ru(II) polyimine complexes have been subject of many investigations for photovoltaics and up-conversion applications. Their triplet states can be highly populated upon excitation with visible photons. Also their triplet states have long lifetime and the ISC process are very efficient in such systems hence making them proper candidates for TTA process and solar related applications. The first demonstration of this process was done in 2005 when Ru(dmb)3[PF6]2 and 10-diphenylanthracene (DPA) were used as sensitizers and acceptor pairs. (dmb = 4,4'-dimethyl-2,2'-bipyridine.) Fig. 2 shows the structure of Rubbery complex and its DPA acceptor pair.
The complex has its absorption peak at 450nm its triplet excited state has the energy of 600 nm photon, corresponding to a green one. The triplet state of acceptor, DPA, emits the photons of 700nm at its excited state. In the experiment the solution of sensitizer/annihilator mixture has been illuminated with a green laser at about 532nm wavelength and 5mW of power. The emitted light from the solution then looks blue corresponding to a photon of 430nm wavelength. Within this process the exciting photon was shifted about 100nm and up-converted to higher energy photon.
TTA up-conversion mechanism is a promising up-conversion approach because of the low power density required for excitation, high up-conversion quantum yield and tunable emission/excitation wavelength and strong absorption of excitation light. However there still lots of debate on the quantum yield and efficiencies of these systems for commercial applications such as solar cell or photovoltaices in general. Many of active researches these days are mainly focused to find and synthesize proper sensitizer/annihilator pairs for most efficient up-conversion efficiency.
© Hadiseh Alaeian. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 H. Alaeian, "Lanthanide Doped Up-Converters for Photovoltaic Applications," Physics 240, Stanford University, Fall 2012.
 T. N. Singh-Rachford and F. N. Castellano, "Photon Upconversion Based on Sensitized Triplet-Triplet Annihilation," Coord. Chem. Rev. 254, 2560 (2010).
 J. Zhao, S. Ji, and H. Guo, "Triplet-Triplet Annihilation Based Upconversion: From Triplet Sensitizers and Triplet Acceptors to Upconversion Quantum Yields," RSC Adv., 1, 937 (2011).