Query: Sensitization of silicon by singlet exciton fission in tetracene

Candidate papers:

abstract: Silicon dominates contemporary solar cell technologies1. But when absorbing photons, silicon (like other semiconductors) wastes energy in excess of its bandgap2. Reducing these thermalization losses and enabling better sensitivity to light is possible by sensitizing the silicon solar cell using singlet exciton fission, in which two excited states with triplet spin character (triplet excitons) are generated from a photoexcited state of higher energy with singlet spin character (a singlet exciton)3 5. Singlet exciton fission in the molecular semiconductor tetracene is known to generate triplet excitons that are energetically matched to the silicon bandgap6 8. When the triplet excitons are transferred to silicon they create additional electron hole pairs, promising to increase cell efficiencies from the single junction limit of 29 per cent to as high as 35 per cent9. Here we reduce the thickness of the protective hafnium oxynitride layer at the surface of a silicon solar cell to just eight angstroms, using electric field effect passivation to enable the efficient energy transfer of the triplet excitons formed in the tetracene. The maximum combined yield of the fission in tetracene and the energy transfer to silicon is around 133 per cent, establishing the potential of singlet exciton fission to increase the efficiencies of silicon solar cells and reduce the cost of the energy that they generate. A silicon and tetracene solar cell employing singlet fission uses an eight angstrom thick hafnium oxynitride interlayer to promote efficient triplet transfer, increasing the efficiency of the cell.
author_names: Markus Einzinger

Tony C Wu

Julia F Kompalla

Hannah L Smith

Collin F Perkinson

Lea Nienhaus

Sarah Wieghold

Daniel N Congreve

Antoine Kahn

Moungi G Bawendi

Marcello Baldo
title: Sensitization of silicon by singlet exciton fission in tetracene
venue: Nature
year: 2019

abstract: Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low cost solution processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60) Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60) We measure a singlet fission efficiency of (71 18) demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.
author_names: Priya J Jadhav

Aseema Mohanty

Jason Sussman

Jiye Lee

Marcello Baldo
title: Singlet exciton fission in nanostructured organic solar cells.
venue: Nano letters
year: 2011

abstract: Singlet exciton fission, a process that generates two excitons from a single photon, is perhaps the most efficient of the various multiexciton generation processes studied to date, offering the potential to increase the efficiency of solar devices. But its unique characteristic, splitting a photogenerated singlet exciton into two dark triplet states, means that the empty absorption region between the singlet and triplet excitons must be filled by adding another material that captures low energy photons. This has required the development of specialized device architectures. In this Account, we review work to develop devices that harness the theoretical benefits of singlet exciton fission. First, we discuss singlet fission in the archetypal material, pentacene. Pentacene based photovoltaic devices typically show high external and internal quantum efficiencies. They have enabled researchers to characterize fission, including yield and the impact of competing loss processes, within functional devices. We review in situ probes of singlet fission that modulate the photocurrent using a magnetic field. We also summarize studies of the dissociation of triplet excitons into charge at the pentacene buckyball (C60) donor acceptor interface. Multiple independent measurements confirm that pentacene triplet excitons can dissociate at the C60 interface despite their relatively low energy. Because triplet excitons produced by singlet fission each have no more than half the energy of the original photoexcitation, they limit the potential open circuit voltage within a solar cell. Thus, if singlet fission is to increase the overall efficiency of a solar cell and not just double the photocurrent at the cost of halving the voltage, it is necessary to also harvest photons in the absorption gap between the singlet and triplet energies of the singlet fission material. We review two device architectures that attempt this using long wavelength materials: a three layer structure that uses long and short wavelength donors and an acceptor and a simpler, two layer combination of a singlet fission donor and a long wavelength acceptor. An example of the trilayer structure is singlet fission in tetracene with copper phthalocyanine inserted at the C60 interface. The bilayer approach includes pentacene photovoltaic cells with an acceptor of infrared absorbing lead sulfide or lead selenide nanocrystals. Lead selenide nanocrystals appear to be the most promising acceptors, exhibiting efficient triplet exciton dissociation and high power conversion efficiency. Finally, we review architectures that use singlet fission materials to sensitize other absorbers, thereby effectively converting conventional donor materials to singlet fission dyes. In these devices, photoexcitation occurs in a particular molecule and then energy is transferred to a singlet fission dye where the fission occurs. For example, rubrene inserted between a donor and an acceptor decouples the ability to perform singlet fission from other major photovoltaic properties such as light absorption.
author_names: Jiye Lee

Priya J Jadhav

Philip David Reusswig

Shane R Yost

Nicholas J Thompson

Daniel N Congreve

Eric R Hontz

Troy Van Voorhis

Marcello Baldo
title: Singlet exciton fission photovoltaics.
venue: Accounts of chemical research
year: 2013

abstract: Singlet exciton fission is the process in conjugated organic molecules bywhich a photogenerated singlet exciton couples to a nearby chromophore in the ground state, creating a pair of triplet excitons. Researchers first reported this phenomenon in the 1960s, an event that sparked further studies in the following decade. These investigations used fluorescence spectroscopy to establish that exciton fission occurred in single crystals of several acenes. However, research interest has been recently rekindled by the possibility that singlet fission could be used as a carrier multiplication technique to enhance the efficiency of photovoltaic cells. The most successful architecture to date involves sensitizing a red absorbing photoactive layer with a blue absorbing material that undergoes fission, thereby generating additional photocurrent from higher energy photons. The quest for improved solar cells has spurred a drive to better understand the fission process, which has received timely aid from modern techniques for time resolved spectroscopy, quantum chemistry, and small molecule device fabrication. However, the consensus interpretation of the initial studies using ultrafast transient absorption spectroscopy was that exciton fission was suppressed in polycrystalline thin films of pentacene, a material that would be otherwise expected to be an ideal model system, as well as a viable candidate for fission sensitized photovoltaic devices. In this Account, we review the results of our recent transient absorption and device based studies of polycrystalline pentacene. We address the controversy surrounding the assignment of spectroscopic features in transient absorption data, and illustrate how a consistent interpretation is possible. This work underpins our conclusion that singlet fission in pentacene is extraordinarily rapid ~80 fs) and is thus the dominant decay channel for the photoexcited singlet exciton. Further, we discuss our demonstration that triplet excitons generated via singlet fission in pentacene can be dissociated at an interface with a suitable electron acceptor, such as fullerenes and infrared absorbing inorganic semiconducting quantum dots. We highlight our recent reports of a pentacene/PbSe hybrid solar cell with a power conversion efficiency of 4.7% and of a pentacene/PbSe/amorphous silicon photovoltaic device. Although substantive challenges remain, both to better our understanding of the mechanism of singlet exciton fission and to optimize device performance, this realization of a solar cell where photocurrent is simultaneously contributed from a blue absorbing fission capable material and an infrared absorbing conventional cell is an important step towards a dual bandgap, single junction, fission enhanced photovoltaic device, which could one day surpass the Shockley Queisser limit.
author_names: Mark W B Wilson

Akshay Rao

Bruno Ehrler

Richard H Friend
title: Singlet exciton fission in polycrystalline pentacene: from photophysics toward devices.
venue: Accounts of chemical research
year: 2013

abstract: The Sun is the main source of renewable energy on Earth. Our planet receives about 174 PW of solar power. At the same time, global energy consumption from all energy sources is orders of magnitude lower and is equal to approximately 16 TW. Clearly, solar energy has a tremendous potential, as well as numerous advantages over conventional sources of energy. In recent years the photovoltaic industry exhibited significant growth, however the main obstacles that are preventing widespread introduction of solar cells are their still low efficiency and high cost. The major factors contributing to the low efficiency are non efficient conversion and lack of absorption of sunlight by the active layer material. Sunlight has a broad spectrum spanning over the UV, visible, and NIR optical ranges. Commonly, solar cells are constructed using a single junction, which leads to two fundamental loss mechanisms: i) for photons with a higher energy than the band gap the energy surplus is lost as heat and only the band gap energy harvested, ii) photons with an energy lower than the band gap are not harvested at all. Together, these losses constitute a limitation of the efficiency of a silicon solar cell by half. In the work described in this thesis we have explored ways to decrease these losses by using advanced materials that allow to efficiently down convert high energy photons, and up convert low energy photons to the energies close to the band gap energy. Both of these approaches can be achieved in certain classes of organic materials and are known as singlet exciton fission and photochemical upconversion. This thesis combines the results of experimental research of both singlet exciton fission and photochemical upconversion in organic materials by means of laser spectroscopy tools: transient absorption and time resolved luminescence. Singlet exciton fission is a process by which a singlet excited state is converted into a combination two triplet excited states with half the energy. The two triplet together constitute an overall singlet state and hence it is a spin allowed process that can in principle be very efficient. In this thesis, singlet fission has been studied in a range of perylenediimide (PDI) derivatives in the crystalline state. Substitution at imide nitrogen position allows to obtain different crystal structures in the solid state. In this way the electronic coupling between neighboring molecules in a crystal can be varied without significantly changing the energetics of their singlet and triplet levels. Singlet exciton fission was experimentally detected for a variety of different crystal structures of PDI. The formation of triplet excited states was found to occur on a sub picosecond time scale. The experimentally detected fission rates and triplet yields were significantly higher than predicted by earlier theoretical calculations and were found to depend only very weakly on the crystal structure. The latter can be explained by intermolecular vibrational modes that could significantly speed up fission in perylenediimides. Photochemical upconversion can be seen as the reverse process of singlet fission. In this case, two triplet excited states, formed through a triplet sensitizer, are combined into a single higher lying singet excited state by triplet triplet annihilation. Using this approach it is possible to convert low energy photons that are normally not absorbed in a solar cell into higher energy photons that can be converted efficiently. In this thesis, photochemical upconversion was studied in bi component mixtures of triplet sensitizer and triplet acceptor in solutions. Metal based porphyrins were used as triplet sensizers, and diphenylanthracene was used as triplet acceptor. The triplet sensitizer produces triplets by fast intersystem crossing due to spin orbit coupling. The triplets are subsequently transferred to the triplet acceptor by Dexter energy transfer. When two acceptor molecules in the triplet state encounter each other, triplet triplet annihilation occurs which results in emission from a singlet state. The overall process can be described as conversion of two low energy photons into one high energy photon. In this way, photons that are normally not absorbed by an active material in a solar cell can be converted into photons that can be absorbed, leading to a significant potential enhancement of the overall efficiency. In this work the dependence of the photochemical upconversion process on the metal in triplet sensitizer (porphyrin) was studied. Efficient upconversion was observed for platinum and palladium based porphyrins 25 30% while for the zinc porphyrin the efficiency was considerably less 12% For free base porphyrin, no upconversion was detected. The upconversion process is the result of a series of individual steps outlined above where most steps are the same for all the combinations. The differences in efficiency are traced back to the dependence of energy transfer efficiency on core metal of the porphyrin. Finally, we have made an attempt to achieve upconversion from the near infrared region. For this purpose, we have used porphyrin oligomers consisting of two or four porphyrin rings that have a red shifted absorption beyond 700 nm. It is shown that using the two ring oligomer as a sensitizer for a perylene bisimid, it is possible to convert 700 nm light to the 500 600 nm range. For the four ring oligomer, no noticeable upconversion was observed, possibly because the triplet of the porphyrin oligomer is just below that of the perylene bisimid in this case.
author_names: Yaroslav V Aulin
title: Singlet Exciton Fission and Photochemical Upconversion
year: 2016

abstract: Splitting Singlets Solar cell efficiency is limited because light at wavelengths shorter than the cell's absorption threshold does not channel any of its excess energy into the generated electricity. Congreve et al. (p. 334) have developed a method to harvest the excess energy in carbon based absorbers through a process termed "singlet fission." In this process, high energy photons propel two current carriers, rather than just one, by populating a singlet state that spontaneously divides into a pair of triplet states. Although it works in a functioning organic solar cell, the efficiency needs improving. Single photons are shown to propel more than one carrier in a carbon based solar cell. Singlet exciton fission transforms a molecular singlet excited state into two triplet states, each with half the energy of the original singlet. In solar cells, it could potentially double the photocurrent from high energy photons. We demonstrate organic solar cells that exploit singlet exciton fission in pentacene to generate more than one electron per incident photon in a portion of the visible spectrum. Using a fullerene acceptor, a poly(3 hexylthiophene) exciton confinement layer, and a conventional optical trapping scheme, we show a peak external quantum efficiency of (109 1) at wavelength l 670 nanometers for a 15 nanometer thick pentacene film. The corresponding internal quantum efficiency is (160 10) Analysis of the magnetic field effect on photocurrent suggests that the triplet yield approaches 200% for pentacene films thicker than 5 nanometers.
author_names: Daniel N Congreve

Nicholas J Thompson

Eric R Hontz

Shane R Yost

Philip David Reusswig

Matthias Bahlke

Sebastian Reineke

Troy Van Voorhis

Marcello Baldo
title: External Quantum Efficiency Above 100% in a Singlet Exciton Fission Based Organic Photovoltaic Cell
venue: Science
year: 2013

abstract: Significance In the present work, we show compelling evidence for the unprecedented intramolecular singlet fission at room temperature and in dilute solutions within a set of three different regioisomeric pentacene dimers. Pump probe experiments, which were complemented by theoretical calculations using high level ab initio multireference perturbation theory methods, corroborate triplet quantum yields as high as 156 5% To this end, electronic couplings between the two pentacenes in the dimers, by virtue of through bond or through space interactions, are decisive in tuning the rates of singlet fission. Singlet fission (SF) has the potential to supersede the traditional solar energy conversion scheme by means of boosting the photon to current conversion efficiencies beyond the 30% Shockley Queisser limit. Here, we show unambiguous and compelling evidence for unprecedented intramolecular SF within regioisomeric pentacene dimers in room temperature solutions, with observed triplet quantum yields reaching as high as 156 5% Whereas previous studies have shown that the collision of a photoexcited chromophore with a ground state chromophore can give rise to SF, here we demonstrate that the proximity and sufficient coupling through bond or space in pentacene dimers is enough to induce intramolecular SF where two triplets are generated on one molecule.
author_names: Johannes Zirzlmeier

Dan Lehnherr

Pedro B Coto

Erin T Chernick

Ruben Casillas

Bettina S Basel

Michael Thoss

Rik R Tykwinski

Dirk Michael Guldi
title: Singlet fission in pentacene dimers
venue: Proceedings of the National Academy of Sciences
year: 2015

abstract: Triplet excitons are ubiquitous in organic optoelectronics, but they are often an undesirable energy sink because they are spin forbidden from emitting light and their high binding energy hinders the generation of free electron hole pairs. Harvesting their energy is consequently an important technological challenge. Here, we demonstrate direct excitonic energy transfer from 'dark' triplets in the organic semiconductor tetracene to colloidal PbS nanocrystals, thereby successfully harnessing molecular triplet excitons in the near infrared. Steady state excitation spectra, supported by transient photoluminescence studies, demonstrate that the transfer efficiency is at least (90 13) The mechanism is a Dexter hopping process consisting of the simultaneous exchange of two electrons. Triplet exciton transfer to nanocrystals is expected to be broadly applicable in solar and near infrared light emitting applications, where effective molecular phosphors are lacking at present. In particular, this route to 'brighten' low energy molecular triplet excitons may permit singlet exciton fission sensitization of conventional silicon solar cells.
author_names: Nicholas J Thompson

Mark W B Wilson

Daniel N Congreve

Patrick R Brown

Jennifer M Scherer

Thomas S Bischof

Mengfei Wu

Nadav Geva

Matthew Welborn

Troy A Van Voorhis

Vladimir Bulovic

Moungi G Bawendi

Marcello Baldo
title: Energy harvesting of non emissive triplet excitons in tetracene by emissive PbS nanocrystals.
venue: Nature materials
year: 2014

abstract: We demonstrate bilayer organic photovoltaic cells that incorporate a singlet exciton fission sensitizer layer to increase the external quantum efficiency (EQE) This solar cell architecture is realized by pairing the singlet exciton donor layer tris[4 (5 phenylthiophen 2 yl)phenyl]amine (TPTPA) with the singlet exciton fission layer 5,6,11,12 tetraphenylnaphthacene (rubrene) The presence of the rubrene layer at the donor acceptor interface allows for a singlet generated in TPTPA to undergo singlet exciton fission with a corresponding doubling in the TPTPA EQE from 12.8% to 27.6% This scheme de couples singlet exciton fission from photon absorption, exciton diffusion, and charge transport for very high EQE organic photovoltaic cells.
author_names: Philip David Reusswig

Daniel N Congreve

Nicholas J Thompson

Marcello Baldo
title: Enhanced external quantum efficiency in an organic photovoltaic cell via singlet fission exciton sensitizer
year: 2012

abstract: Magnetic field effects provide a convenient and specific probe of singlet exciton fission within optoelectronic devices. Here, we demonstrate that this tool may also be applied to screen potential fission material candidates in solution. We characterize the phenomenon in diphenyl tetracene (DPT) which shows strong fluorescence modulation and the expected field dependence in its transient decay as a function of concentration. Solution measurements may also be used to test for the presence of an intermediate charge transfer state, but we observe no changes to the field dependence of DPT singlet exciton fission in toluene relative to chloroform.
author_names: Nicholas J Thompson

Eric R Hontz

Wendi Chang

Troy Van Voorhis

Marcello Baldo
title: Magnetic field dependence of singlet fission in solutions of diphenyl tetracene
venue: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
year: 2015