Co-reporter:Shaohua Huang, Guangye Zhang, Nicholas S. Knutson, Matthew T. Fontana, Rachel C. Huber, Amy S. Ferreira, Sarah H. Tolbert, Benjamin J. Schwartz and Yves Rubin
Journal of Materials Chemistry A 2016 vol. 4(Issue 2) pp:416-424
Publication Date(Web):05 Nov 2015
DOI:10.1039/C5TA07688A
Organic solar cells have been based mostly on conjugated polymers and the classic fullerene derivative PCBM and are characterized by modest open circuit voltages (Voc). Increasing Voc requires fullerene acceptors with higher LUMOs than PCBM. To date, most fullerene derivatives synthesized for this purpose either do not achieve the high photocurrent afforded by PCBM or show relatively poor compatibility with the next-generation low bandgap conjugated polymers used in high-efficiency organic solar cells. Here, we report the facile synthesis of methoxylated 1,4-bisbenzyl fullerene adducts and their application as efficient electron acceptors in conjugated polymer-based solar cells. The methoxy groups are found to be essential to increasing the LUMO levels, and accordingly the Voc, of the devices compared to the parent 1,4-bisbenzyl fullerene, and more importantly, to PCBM. The best fullerene 1,4-bisadduct provides a ∼20% enhancement in power conversion efficiency over PCBM when used with the classic crystalline polymer P3HT. When used in combination with a higher-performance low bandgap polymer, PTB7, the bisadduct both increases the device open-circuit voltage and maintains the high photocurrent provided by the more traditional PCBM. We also examine 10 different 1,4-fullerene bisadducts and show that the photovoltaic device performance is strongly influenced by the number and relative position of the methoxy substituents on the benzyl addends: moving a single methoxy substituent by one position on the benzyl rings can change the device efficiency by over a factor of 2.
Co-reporter:William J. Glover and Benjamin J. Schwartz
Journal of Chemical Theory and Computation 2016 Volume 12(Issue 10) pp:5117-5131
Publication Date(Web):August 30, 2016
DOI:10.1021/acs.jctc.6b00472
The hydrated electron, e–(aq), has often served as a model system to understand the influence of condensed-phase environments on electronic structure and dynamics. Despite over 50 years of study, however, the basic structure of e–(aq) is still the subject of controversy. In particular, the structure of e–(aq) was long assumed to be an electron localized within a solvent cavity, in a manner similar to halide solvation. Recently, however, we suggested that e–(aq) occupies a region of enhanced water density with little or no discernible cavity. The potential we developed was only subtly different from those that give rise to a cavity solvation motif, which suggests that the driving forces for noncavity solvation involve subtle electron-water attractive interactions at close distances. This leads to the question of how dispersion interactions are treated in simulations of the hydrated electron. Most dispersion potentials are ad hoc or are not designed to account for the type of close-contact electron-water overlap that might occur in the condensed phase, and where short-range dynamic electron correlation is important. To address this, in this paper we develop a procedure to calculate the potential energy surface between a single water molecule and an excess electron with high-level CCSD(T) electronic structure theory. By decomposing the electron-water potential into its constituent energetic contributions, we find that short-range electron correlation provides an attraction of comparable magnitude to the mean-field interactions between the electron and water. Furthermore, we find that by reoptimizing a popular cavity-forming one-electron model potential to better capture these attractive short-range interactions, the enhanced description of correlation predicts a noncavity e–(aq) with calculated properties in better agreement with experiment. Although much attention has been placed on the importance of long-range dispersion interactions in water cluster anions, our study reveals that largely unexplored short-range correlation effects are crucial in dictating the solvation structure of the condensed-phase hydrated electron.
Co-reporter:Chen-Chen Zho and Benjamin J. Schwartz
The Journal of Physical Chemistry B 2016 Volume 120(Issue 49) pp:12604-12614
Publication Date(Web):November 14, 2016
DOI:10.1021/acs.jpcb.6b07852
We use nonadiabatic mixed quantum/classical molecular dynamics to simulate recent time-resolved photoelectron spectroscopy (TRPES) experiments on the hydrated electron, and compare the results for both a cavity and a noncavity simulation model to experiment. We find that cavity-model hydrated electrons show an “adiabatic” relaxation mechanism, with ground-state cooling that is fast on the time scale of the internal conversion, a feature that is in contrast to the TRPES experiments. A noncavity hydrated electron model, however, displays a “nonadiabatic” relaxation mechanism, with rapid internal conversion followed by slower ground-state cooling, in good qualitative agreement with experiment. We also show that the experimentally observed early time red shift and loss of anisotropy of the excited-state TRPES peak are consistent with hydrated electron models with homogeneously broadened absorption spectra, but not with those with inhomogeneously broadened absorption spectra. Finally, we find that a decreasing photoionization cross section upon cooling causes the excited-state TRPES peak to decay faster than the underlying radiationless relaxation process, so that the experimentally observed 60–75 fs peak decay corresponds to an actual excited-state lifetime of the hydrated electron that is more likely ∼100 fs.
Co-reporter:Jennifer R. Casey, Benjamin J. Schwartz, and William J. Glover
The Journal of Physical Chemistry Letters 2016 Volume 7(Issue 16) pp:3192-3198
Publication Date(Web):August 1, 2016
DOI:10.1021/acs.jpclett.6b01150
The properties of the hydrated electron at the air/water interface are computed for both a cavity and a noncavity model using mixed quantum/classical molecular dynamics simulation. We take advantage of our recently developed formalism for umbrella sampling with a restrained quantum expectation value to calculate free-energy profiles of the hydrated electron’s position relative to the water surface. We show that it is critical to use an instantaneous description of the air/water interface rather than the Gibbs’ dividing surface to obtain accurate potentials of mean force. We find that noncavity electrons, which prefer to encompass several water molecules, avoid the interface where water molecules are scarce. In contrast, cavity models of the hydrated electron, which prefer to expel water, have a local free-energy minimum near the interface. When the cavity electron occupies this minimum, its absorption spectrum is quite red-shifted, its binding energy is significantly lowered, and its dynamics speed up quite a bit compared with the bulk, features that have not been found by experiment. The surface activity of the electron therefore serves as a useful test of cavity versus noncavity electron solvation.
Co-reporter:Amy S. Ferreira, Jordan C. Aguirre, Selvam Subramaniyan, Samson A. Jenekhe, Sarah H. Tolbert, and Benjamin J. Schwartz
The Journal of Physical Chemistry C 2016 Volume 120(Issue 39) pp:22115-22125
Publication Date(Web):August 29, 2016
DOI:10.1021/acs.jpcc.6b03300
The performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaics is highly sensitive to the morphology of the polymer within the active layer. To tune this morphology, we constructed both blend-cast and sequentially processed BHJ devices from the fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester (PCBM), in combination with a series of random poly(3-butylthiophene-co-3-octylthiophene)s with different fractions of each monomer, with the goal of controllably varying the average polymer side-chain length. What we found, however, was that the most important parameter for predicting device performance across this series of polymers was the regioregularity of the particular synthetic batch of polymer used, not the average side-chain length. Moreover, we found that regioregularity affected device performance in different ways depending on the processing route: lower regioregularity led to improved performance for sequentially processed devices, but was detrimental to the performance of blend-cast devices. We argue that the reason for this anticorrelation is that regioregularity is the single most important determinant of the relative crystalline of the polymer. The relative crystalline fraction, in turn, determines the ability of the polymer to swell in the presence of solvents. Polymer swelling is key to BHJ formation via sequential processing, but can lead to overly mixed systems using traditional blend-casting methods. As a result, we find that the best performing polymer for sequentially processed devices is the worst performer for blend-cast devices and vice versa, highlighting the importance of using both processing methods when exploring new materials for use in BHJ photovoltaics.
Co-reporter:Jordan C. Aguirre;Steven A. Hawks;Amy S. Ferreira;Patrick Yee;Selvam Subramaniyan;Samson A. Jenekhe;Sarah H. Tolbert
Advanced Energy Materials 2015 Volume 5( Issue 11) pp:
Publication Date(Web):
DOI:10.1002/aenm.201402020
Design rules are presented for significantly expanding sequential processing (SqP) into previously inaccessible polymer:fullerene systems by tailoring binary solvent blends for fullerene deposition. Starting with a base solvent that has high fullerene solubility, 2-chlorophenol (2-CP), ellipsometry-based swelling experiments are used to investigate different co-solvents for the fullerene-casting solution. By tuning the Flory-Huggins χ parameter of the 2-CP/co-solvent blend, it is possible to optimally swell the polymer of interest for fullerene interdiffusion without dissolution of the polymer underlayer. In this way solar cell power conversion efficiencies are obtained for the PTB7 (poly[(4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)(3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl)]) and PC61BM (phenyl-C61-butyric acid methyl ester) materials combination that match those of blend-cast films. Both semicrystalline (e.g., P3HT (poly(3-hexylthiophene-2,5-diyl)) and entirely amorphous (e.g., PSDTTT (poly[(4,8-di(2-butyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-alt-(2,5-bis(4,4′-bis(2-octyl)dithieno[3,2-b:2′3′-d]silole-2,6-diyl)thiazolo[5,4-d]thiazole)]) conjugated polymers can be processed into highly efficient photovoltaic devices using the solvent-blend SqP design rules. Grazing-incidence wide-angle x-ray diffraction experiments confirm that proper choice of the fullerene casting co-solvent yields well-ordered interdispersed bulk heterojunction (BHJ) morphologies without the need for subsequent thermal annealing or the use of trace solvent additives (e.g., diiodooctane). The results open SqP to polymer/fullerene systems that are currently incompatible with traditional methods of device fabrication, and make BHJ morphology control a more tractable problem.
Co-reporter:Guangye Zhang, Steven A. Hawks, Chilan Ngo, Laura T. Schelhas, D. Tyler Scholes, Hyeyeon Kang, Jordan C. Aguirre, Sarah H. Tolbert, and Benjamin J. Schwartz
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 45) pp:25247
Publication Date(Web):October 21, 2015
DOI:10.1021/acsami.5b06944
Although it is known that evaporated metals can penetrate into films of various organic molecules that are a few nanometers thick, there has been little work aimed at exploring the interaction of the common electrode metals used in devices with fullerene derivatives, such as organic photovoltaics (OPVs) or perovskite solar cells that use fullerenes as electron transport layers. In this paper, we show that when commonly used electrode metals (e.g., Au, Ag, Al, Ca, etc.) are evaporated onto films of fullerene derivatives (such as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)), the metal penetrates many tens of nanometers into the fullerene layer. This penetration decreases the effective electrical thickness of fullerene-based sandwich structure devices, as measured by the device’s geometric capacitance, and thus significantly alters the device physics. For the case of Au/PCBM, the metal penetrates a remarkable 70 nm into the fullerene, and we see penetration of similar magnitude in a wide variety of fullerene derivative/evaporated metal combinations. Moreover, using transmission electron microscopy to observed cross-sections of the films, we show that when gold is evaporated onto poly(3-hexylthiophene) (P3HT)/PCBM sequentially processed OPV quasi-bilayers, Au nanoparticles with diameters of ∼3–20 nm are formed and are dispersed entirely throughout the fullerene-rich overlayer. The plasmonic absorption and scattering from these nanoparticles are readily evident in the optical transmission spectrum, demonstrating that the interpenetrated metal significantly alters the optical properties of fullerene-rich active layers. This opens a number of possibilities in terms of contact engineering and light management so that metal penetration in devices that use fullerene derivatives could be used to advantage, making it critical that researchers are aware of the electronic and optical consequences of exposing fullerene-derivative films to evaporated electrode metals.Keywords: conjugated polymer; fullerene; gold nanoparticles; metal penetration; organic photovoltaic; PCBM; plasmonics; sequential processing
Co-reporter:D. Tyler Scholes; Steven A. Hawks; Patrick Y. Yee; Hao Wu; Jeffrey R. Lindemuth; Sarah H. Tolbert
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 23) pp:4786-4793
Publication Date(Web):November 10, 2015
DOI:10.1021/acs.jpclett.5b02332
We demonstrate that solution-sequential processing (SqP) can yield heavily doped pristine-quality films when used to infiltrate the molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) into pure poly(3-hexylthiophene) (P3HT) polymer layers. Profilometry measurements show that the SqP method produces doped films with essentially the same surface roughness as pristine films, and 2-D grazing-incidence wide-angle X-ray scattering (GIWAXS) confirms that SqP preserves both the size and orientation of the pristine polymer’s crystallites. Unlike traditional blend-cast F4TCNQ/P3HT doped films, our sequentially processed layers have tunable and reproducible conductivities reaching as high as 5.5 S/cm even when measured over macroscopic (>1 cm) distances. The high conductivity and superb film quality allow for meaningful Hall effect measurements, which reveal p-type conduction and carrier concentrations tunable from 1016 to 1020 cm–3 and hole mobilities ranging from ∼0.003 to 0.02 cm2 V–1 s–1 at room temperature over the doping levels examined.
Co-reporter:Rachel C. Huber;Daniel Kilbride;Nicholas S. Knutson;J. Reddy Challa;Amy S. Ferreira;Daniel B. Toso;Robert Thompson;Sarah H. Tolbert;Lekshmi Sudha Devi;Yves Rubin;Z. Hong Zhou
Science 2015 Volume 348(Issue 6241) pp:
Publication Date(Web):
DOI:10.1126/science.aaa6850
Photoinduction of long-lived polarons
Photosynthetic complexes and organic photovoltaics can rapidly create separated charges upon photoexcitation. However, unproductive charge recombination often occurs in the human-made system. This is in part because the charge acceptor and donor structures are much larger. Huber et al. created aqueous micelles that pair conjugated polyelectrolyte charge donors with fullerene acceptors at a much smaller interface. They observed the photoinduced formation of polarons—stable pairs of separated charges—with lifetimes of several days.
Science, this issue p. 1340
Co-reporter:Jordan C. Aguirre;Christopher Arntsen;Samuel Hernez;Rachel Huber;Alexre M. Nardes;Merissa Halim;Daniel Kilbride;Yves Rubin;Sarah H. Tolbert;Nikos Kopidakis;Daniel Neuhauser
Advanced Functional Materials 2014 Volume 24( Issue 6) pp:784-792
Publication Date(Web):
DOI:10.1002/adfm.201301757
The efficiency of bulk heterojunction (BHJ) organic photovoltaics is sensitive to the morphology of the fullerene network that transports electrons through the device. This sensitivity makes it difficult to distinguish the contrasting roles of local electron mobility (how easily electrons can transfer between neighboring fullerene molecules) and macroscopic electron mobility (how well-connected is the fullerene network on device length scales) in solar cell performance. In this work, a combination of density functional theory (DFT) calculations, flash-photolysis time-resolved microwave conductivity (TRMC) experiments, and space-charge-limit current (SCLC) mobility estimates are used to examine the roles of local and macroscopic electron mobility in conjugated polymer/fullerene BHJ photovoltaics. The local mobility of different pentaaryl fullerene derivatives (so-called ‘shuttlecock’ molecules) is similar, so that differences in solar cell efficiency and SCLC mobilities result directly from the different propensities of these molecules to self-assemble on macroscopic length scales. These experiments and calculations also demonstrate that the local mobility of phenyl-C60 butyl methyl ester (PCBM) is an order of magnitude higher than that of other fullerene derivatives, explaining why PCBM has been the acceptor of choice for conjugated polymer BHJ devices even though it does not form an optimal macroscopic network. The DFT calculations indicate that PCBM's superior local mobility comes from the near-spherical nature of its molecular orbitals, which allow strong electronic coupling between adjacent molecules. In combination, DFT and TRMC techniques provide a tool for screening new fullerene derivatives for good local mobility when designing new molecules that can improve on the macroscopic electron mobility offered by PCBM.
Co-reporter:William J. Glover, Jennifer R. Casey, and Benjamin J. Schwartz
Journal of Chemical Theory and Computation 2014 Volume 10(Issue 10) pp:4661-4671
Publication Date(Web):September 10, 2014
DOI:10.1021/ct500661t
We introduce a new simulation method called Coupled-Perturbed Quantum Umbrella Sampling that extends the classical umbrella sampling approach to reaction coordinates involving quantum mechanical degrees of freedom. The central idea in our method is to solve coupled-perturbed equations to find the response of the quantum system’s wave function along a reaction coordinate of interest. This allows for propagation of the system’s dynamics under the influence of a quantum biasing umbrella potential and provides a method to rigorously undo the effects of the bias to compute equilibrium ensemble averages. In this way, one can drag electrons into regions of high free energy where they would otherwise not go, thus enabling chemistry by fiat. We demonstrate the applicability of our method for two condensed-phase systems of interest. First, we consider the interaction of a hydrated electron with an aqueous sodium cation, and we calculate a potential of mean force that shows that an e–:Na+ contact pair is the thermodynamically favored product starting from either a neutral sodium atom or the separate cation and electron species. Second, we present the first determination of a hydrated electron’s free-energy profile relative to an air/water interface. For the particular model parameters used, we find that the hydrated electron is more thermodynamically stable in the bulk rather than at the interface. Our analysis suggests that the primary driving force keeping the electron away from the interface is the long-range electron–solvent polarization interaction rather than the short-range details of the chosen pseudopotential.
Co-reporter:Jordan C. Aguirre ; Amy Ferreira ; Hong Ding ; Samson A. Jenekhe ; Nikos Kopidakis ; Mark Asta ; Laurent Pilon ; Yves Rubin ; Sarah H. Tolbert ; Benjamin J. Schwartz ; Bruce Dunn @;Vidvuds Ozolins @
The Journal of Physical Chemistry C 2014 Volume 118(Issue 34) pp:19505-19523
Publication Date(Web):July 9, 2014
DOI:10.1021/jp501047j
Our program on capacitive energy storage is a comprehensive one that combines experimental and computational components to achieve a fundamental understanding of charge storage processes in redox-based materials, specifically transition metal oxides. Some of the highlights of this program are the identification of intercalation pseudocapacitance in Nb2O5, which enables high energy density to be achieved at high rates, and the development of a new route for synthesizing mesoporous films in which preformed nanocrystal building blocks are used in combination with polymer templating. The resulting material architectures have large surface areas and enable electrolyte access to the redox active pore walls, while the interconnected mesoporous film provides good electronic conductivity. Select first-principles density-functional theory studies of prototypical pseudocapacitor materials are reviewed, providing insight into the key physical and chemical features involved in charge transfer and ion diffusion. Rigorous multiscale physical models and numerical tools have been developed and used to reproduce electrochemical properties of carbon-based electrochemical capacitors with the ultimate objective of facilitating the optimization of electrode design. For the organic photovoltaic (OPV) program, our focus has been ongoing beyond the trial-and-error Edisonian approaches that have been responsible for the increase in power conversion efficiency of blend-cast (BC) bulk heterojunction blends of polymers and fullerenes. Our first approach has been to use molecular self-assembly to create the ideal nanometer-scale architecture using thermodynamics rather than relying on the kinetics of spontaneous phase segregation. We have created fullerenes that self-assemble into one-dimensional stacks and have shown that use of these self-assembled fullerenes lead to dramatically enhanced OPV performance relative to fullerenes that do not assemble. We also have created self-assembling conjugated polymers that form gels based on electrically continuous cross-linked micelles in solution, opening the possibility for water-processable “green” production of OPVs based on these materials. Our second approach has been to avoid kinetic control over phase separation by using a sequential processing (SqP) technique to deposit the polymer and fullerene materials in separate deposition steps. The polymer layer is deposited first, using solvents and deposition conditions that optimize the polymer crystallinity for swelling and hole mobility. The fullerene layer is then deposited in a second step from a solvent that swells the polymer but does not dissolve it, allowing the fullerene to penetrate into the polymer underlayer to the desired degree. Careful comparison of composition- and thickness-matched BC and SqP devices shows that SqP not only produces more efficient devices but also leads to devices that behave more consistently.
Co-reporter:Guangye Zhang ; Rachel C. Huber ; Amy S. Ferreira ; Shane D. Boyd ; Christine K. Luscombe ; Sarah H. Tolbert
The Journal of Physical Chemistry C 2014 Volume 118(Issue 32) pp:18424-18435
Publication Date(Web):July 18, 2014
DOI:10.1021/jp5054315
Although most polymer/fullerene-based solar cells are cast from a blend of the components in solution, it is also possible to sequentially process the polymer and fullerene layers from quasi-orthogonal solvents. Sequential processing (SqP) not only produces photovoltaic devices with efficiencies comparable to the more traditional bulk heterojunction (BHJ) solar cells produced by blend casting (BC) but also offers the advantage that the polymer and fullerene layers can be optimized separately. In this paper, we explore the morphology produced when sequentially processing polymer/fullerene solar cells and compare it to the BC morphology. We find that increasing polymer regioregularity leads to the opposite effect in SqP and BC BHJ solar cells. We start by constructing a series of SqP and BC solar cells using different types of poly(3-hexylthiophene) (P3HT) that vary in regioregulary and polydispersity combined with [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM). We use grazing incidence wide-angle X-ray scattering to demonstrate how strongly changes in the P3HT and PCBM crystallinity upon thermal annealing of SqP and BC BHJ films depend on polymer regioregularity. For SqP devices, low regioregularity P3HT films that possess more amorphous regions allow for more PCBM crystallite growth and thus show better photovoltaic device efficiency. On the other hand, highly regioregular P3HT leads to a more favorable morphology and better device efficiency for BC BHJ films. Comparing the photovoltaic performance and structural characterization indicates that the mechanisms controlling morphology in the active layers are fundamentally different for BHJs formed via SqP and BC. Most importantly, we find that nanoscale morphology in both SqP and BC BHJs can be systematically controlled by tuning the amorphous fraction of polymer in the active layer.
Co-reporter:Steven A. Hawks ; Jordan C. Aguirre ; Laura T. Schelhas ; Robert J. Thompson ; Rachel C. Huber ; Amy S. Ferreira ; Guangye Zhang ; Andrew A. Herzing ; Sarah H. Tolbert
The Journal of Physical Chemistry C 2014 Volume 118(Issue 31) pp:17413-17425
Publication Date(Web):July 8, 2014
DOI:10.1021/jp504560r
Polymer:fullerene bulk heterojunction (BHJ) solar cell active layers can be created by traditional blend casting (BC), where the components are mixed together in solution before deposition, or by sequential processing (SqP), where the pure polymer and fullerene materials are cast sequentially from different solutions. Presently, however, the relative merits of SqP as compared to BC are not fully understood because there has yet to be an equivalent (composition- and thickness-matched layer) comparison between the two processing techniques. The main reason why matched SqP and BC devices have not been compared is because the composition of SqP active layers has not been accurately known. In this paper, we present a novel technique for accurately measuring the polymer:fullerene film composition in SqP active layers, which allows us to make the first comparisons between rigorously composition- and thickness-matched BHJ organic solar cells made by SqP and traditional BC. We discover that, in optimal photovoltaic devices, SqP active layers have a very similar composition as their optimized BC counterparts (≈44–50 mass % PCBM). We then present a thorough investigation of the morphological and device properties of thickness- and composition-matched P3HT:PCBM SqP and BC active layers in order to better understand the advantages and drawbacks of both processing approaches. For our matched devices, we find that small-area SqP cells perform better than BC cells due to both superior film quality and enhanced optical absorption from more crystalline P3HT. The enhanced film quality of SqP active layers also results in higher performance and significantly better reproducibility in larger-area devices, indicating that SqP is more amenable to scaling than the traditional BC approach. X-ray diffraction, UV–vis absorption, and energy-filtered transmission electron tomography collectively show that annealed SqP active layers have a finer-scale blend morphology and more crystalline polymer and fullerene domains when compared to equivalently processed BC active layers. Charge extraction by linearly increasing voltage (CELIV) measurements, combined with X-ray photoelectron spectroscopy, also show that the top (nonsubstrate) interface for SqP films is slightly richer in PCBM compared to matched BC active layers. Despite these clear differences in bulk and vertical morphology, transient photovoltage, transient photocurrent, and subgap external quantum efficiency measurements all indicate that the interfacial electronic processes occurring at P3HT:PCBM heterojunctions are essentially identical in matched-annealed SqP and BC active layers, suggesting that device physics are surprisingly robust with respect to the details of the BHJ morphology.
Co-reporter:Jennifer R. Casey, Argyris Kahros, and Benjamin J. Schwartz
The Journal of Physical Chemistry B 2013 Volume 117(Issue 46) pp:14173-14182
Publication Date(Web):October 4, 2013
DOI:10.1021/jp407912k
The hydrated electron—the species that results from the addition of a single excess electron to liquid water—has been the focus of much interest both because of its role in radiation chemistry and other chemical reactions, and because it provides for a deceptively simple system that can serve as a means to confront the predictions of quantum molecular dynamics simulations with experiment. Despite all this interest, there is still considerable debate over the molecular structure of the hydrated electron: does it occupy a cavity, have a significant number of interior water molecules, or have a structure somewhere in between? The reason for all this debate is that different computer simulations have produced each of these different structures, yet the predicted properties for these different structures are still in reasonable agreement with experiment. In this Feature Article, we explore the reasons underlying why different structures are produced when different pseudopotentials are used in quantum simulations of the hydrated electron. We also show that essentially all the different models for the hydrated electron, including those from fully ab initio calculations, have relatively little direct overlap of the electron’s wave function with the nearby water molecules. Thus, a non-cavity hydrated electron is better thought of as an “inverse plum pudding” model, with interior waters that locally expel the surrounding electron’s charge density. Finally, we also explore the agreement between different hydrated electron models and certain key experiments, such as resonance Raman spectroscopy and the temperature dependence and degree of homogeneous broadening of the optical absorption spectrum, in order to distinguish between the different simulated structures. Taken together, we conclude that the hydrated electron likely has a significant number of interior water molecules.
Co-reporter:Stephanie C. Doan and Benjamin J. Schwartz
The Journal of Physical Chemistry B 2013 Volume 117(Issue 16) pp:4216-4221
Publication Date(Web):July 5, 2012
DOI:10.1021/jp303591h
We examine the ultrafast relaxation dynamics of excess electrons injected into liquid acetonitrile using air- and water-free techniques and compare our results to previous work on this system [Xia, C. et al. J. Chem. Phys. 2002, 117, 8855]. Excess electrons in liquid acetonitrile take on two forms: a “traditional” solvated electron that absorbs in the near-IR, and a solvated molecular dimer anion that absorbs weakly in the visible. We find that excess electrons initially produced via charge-transfer-to-solvent excitation of iodide prefer to localize as solvated electrons, but that there is a subsequent equilibration to form the dimer anion on an ∼80 ps time scale. The spectral signature of this interconversion between the two forms of the excess electron is a clear isosbestic point. The presence of the isosbestic point makes it possible to fully deconvolute the spectra of the two species. We find that solvated molecular anion absorbs quite weakly, with a maximum extinction coefficient of ∼2000 M–1cm–1. With the extinction coefficient of the dimer anion in hand, we are also able to determine the equilibrium constant for the two forms of excess electron, and find that the molecular anion is favored by a factor of ∼4. We also find that relatively little geminate recombination takes place, and that the geminate recombination that does take place is essentially complete within the first 20 ps. Finally, we show that the presence of small amounts of water in the acetonitrile can have a fairly large effect on the observed spectral dynamics, explaining the differences between our results and those in previously published work.
Co-reporter:Jennifer R. Casey;Ross E. Larsen
PNAS 2013 Volume 110 (Issue 8 ) pp:2712-2717
Publication Date(Web):2013-02-19
DOI:10.1073/pnas.1219438110
Most of what is known about the structure of the hydrated electron comes from mixed quantum/classical simulations, which depend
on the pseudopotential that couples the quantum electron to the classical water molecules. These potentials usually are highly
repulsive, producing cavity-bound hydrated electrons that break the local water H-bonding structure. However, we recently
developed a more attractive potential, which produces a hydrated electron that encompasses a region of enhanced water density.
Both our noncavity and the various cavity models predict similar experimental observables. In this paper, we work to distinguish
between these models by studying both the temperature dependence of the optical absorption spectrum, which provides insight
into the balance of the attractive and repulsive terms in the potential, and the resonance Raman spectrum, which provides
a direct measure of the local H-bonding environment near the electron. We find that only our noncavity model can capture the
experimental red shift of the hydrated electron’s absorption spectrum with increasing temperature at constant density. Cavity
models of the hydrated electron predict a solvation structure similar to that of the larger aqueous halides, leading to a
Raman O–H stretching band that is blue-shifted and narrower than that of bulk water. In contrast, experiments show the hydrated
electron has a broader and red-shifted O–H stretching band compared with bulk water, a feature recovered by our noncavity
model. We conclude that although our noncavity model does not provide perfect quantitative agreement with experiment, the
hydrated electron must have a significant degree of noncavity character.
Co-reporter:Stephanie C. Doan and Benjamin J. Schwartz
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 9) pp:1471-1476
Publication Date(Web):April 12, 2013
DOI:10.1021/jz400621m
Unlike most polar liquids, excess electrons in liquid CH3CN take on two distinct forms – solvated electrons (esolv–) and solvated molecular anions – that are in equilibrium with each other. We find that excitation of esolv– leads to a short-lived excited state that has no effect on the equilibrium but that excitation of the molecular anion instantaneously leads to the production of new esolv–. We also find that esolv– produced by excitation of dimer anions relocalize to places far enough from their original location to alter their recombination dynamics. Finally, we show using polarized transient hole-burning that esolv– in liquid CH3CN have an inhomogeneously broadened spectrum, demonstrating that these electrons almost certainly reside in a cavity. Because there is no polarized hole-burning for esolv– in water or methanol, these results have important implications for the nature of excess electrons in all polar liquids.Keywords: acetonitrile anion; polarized transient hole-burning; solvated electron; ultrafast spectroscopy;
Co-reporter:Stephanie C. Doan, Gregory Kuzmanich, Matthew N. Gard, Miguel A. Garcia-Garibay, and Benjamin J. Schwartz
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 1) pp:81-86
Publication Date(Web):December 7, 2011
DOI:10.1021/jz201360w
Nanocrystalline diphenylcyclopropenone (DPCP) undergoes a photodecarbonylation reaction to form diphenylacetylene (DPA) with a quantum yield of ∼3, presumably the result of a quantum chain reaction (QCR). We have performed an ultrafast spectroscopic investigation of this process, and found direct evidence that, following photoexcitation of solid DPCP, DPA product molecules appear in a sequential manner over a ∼30 ps time scale. We also see spectroscopic evidence for an excited state of DPCP that is optically inaccessible from the ground state; this state can be populated via short-range energy transfer from excited DPA product molecules, providing a mechanistic explanation of the sequential re-excitation and decarbonylation of DPCP and constituting a direct observation of a QCR.Keywords: diphenylcyclopropenone; photodecarbonylation; quantum chain reaction; solid-state photochemistry; ultrafast spectroscopy;
Co-reporter:Alexander L. Ayzner, Stephanie C. Doan, Bertrand Tremolet de Villers, and Benjamin J. Schwartz
The Journal of Physical Chemistry Letters 2012 Volume 3(Issue 16) pp:2281-2287
Publication Date(Web):August 1, 2012
DOI:10.1021/jz300762c
We examine the ultrafast dynamics of exciton migration and polaron production in sequentially processed ‘quasi-bilayer’ and preblended ‘bulk heterojunction’ (BHJ) solar cells based on conjugated polymer films that contain the same total amount of fullerene. We find that even though the polaron yields are similar, the dynamics of polaron production are significantly slower in quasi-bilayers than BHJs. We argue that the different polaron production dynamics result from the fact that (1) there is significantly less fullerene inside the polymer in quasi-bilayers than in BHJs and (2) sequential processing yields polymer layers that are significantly more ordered than BHJs. We also argue that thermal annealing improves the performance of quasi-bilayer solar cells not because annealing drives additional fullerene into the polymer but because annealing improves the fullerene crystallinity. All of the results suggest that sequential processing remains a viable alternative for producing polymer/fullerene solar cells with a nanometer-scale architecture that differs from BHJs.Keywords: conjugated polymer; exciton; fullerene; organic photovoltaic; polaron; spectroscopy; ultrafast;
Co-reporter:Ross E. Larsen;William J. Glover
Science 2011 Vol 331(6023) pp:1387
Publication Date(Web):18 Mar 2011
DOI:10.1126/science.1197884
Abstract
Turi and Madarász and Jacobson and Herbert argue that the pseudopotential we derived for the hydrated electron contains inaccuracies that make it overly attractive. We show that our potential is derived correctly and argue that the criticisms presented are not relevant when evaluating a pseudopotential’s accuracy for condensed-phase simulation. Neither critique addresses our central result that the experimental properties of the hydrated electron are consistent with a noncavity picture.
Co-reporter:William J. Glover, Ross E. Larsen, and Benjamin J. Schwartz
The Journal of Physical Chemistry A 2011 Volume 115(Issue 23) pp:5887-5894
Publication Date(Web):March 23, 2011
DOI:10.1021/jp1101434
The motions of solvent molecules during a chemical transformation often dictate both the dynamics and the outcome of solution-phase reactions. However, a microscopic picture of solvation dynamics is often obscured by the concerted motions of numerous solvent molecules that make up a condensed-phase environment. In this study, we use mixed quantum/classical molecular dynamics simulations to furnish the molecular details of the solvation dynamics that leads to the formation of a sodium cation-solvated electron contact pair, (Na+, e−), in liquid tetrahydrofuran following electron photodetachment from sodide (Na−). Our simulations reveal that the dominant solvent response is comprised of a series of discrete solvent molecular events that work sequentially to build up a shell of coordinating THF oxygen sites around the sodium cation end of the contact pair. With the solvent response described in terms of the sequential motion of single molecules, we are then able to compare the calculated transient absorption spectroscopy of the sodium species to experiment, providing a clear microscopic interpretation of ultrafast pump−probe experiments on this system. Our findings suggest that for solute−solvent interactions similar to the ones present in our study, the solvation dynamics is best understood as a series of kinetic events consisting of reactions between chemically distinct local structures in which key solvent molecules must be considered to be part of the identity of the reacting species.
Co-reporter:Arthur E. Bragg, Godwin U. Kanu, and Benjamin J. Schwartz
The Journal of Physical Chemistry Letters 2011 Volume 2(Issue 21) pp:2797-2804
Publication Date(Web):October 17, 2011
DOI:10.1021/jz201295a
We use the sensitivity of the charge-transfer-to-solvent dynamics of tetrabutylammonium iodide (t–ba+ I–) to local solution environment to explore the microscopic heterogeneity of binary solutions of tetrahydrofuran (THF) with water in low mole fraction. By tracking the spectrum of the nascent solvated electron, we show that the water is not homogeneously dispersed throughout the THF but rather is “pooled” on the nanometer length scales probed by electron diffusion. The sensitivity of the electron’s spectrum to the proximity of t–ba+ further reveals that the parent ion pair is differentially solvated, with t–ba+ and I– preferentially solvated by THF and water, respectively. Finally, we find that the hydration environment surrounding I– “turns on” geminate recombination between the electron and iodine atom, a process not observed in neat THF on the sub-nanosecond time scale. These findings all underscore the fact that microscopic heterogeneities direct the course of chemical transformations in mixed solution environments.Keywords: CTTS dynamics; preferential solvation; solvated electrons; solvent mixtures; THF−water mixtures;
Co-reporter:William J. Glover, Ross E. Larsen and Benjamin J. Schwartz
The Journal of Physical Chemistry Letters 2010 Volume 1(Issue 1) pp:165-169
Publication Date(Web):November 11, 2009
DOI:10.1021/jz9000938
Understanding how a solvent affects the quantum mechanics and reactivity of the chemical bonds of dissolved solutes is of fundamental importance to chemistry. To explore condensed-phase effects on a simple molecular solute, we have studied the six-dimensional two-electron wave function of the bonding electrons of the Na2 molecule in liquid argon via mixed quantum/classical simulation. We find that even though Ar is an apolar liquid, solvent interactions produce dipole moments on Na2 that can reach magnitudes over 1.4 D. These interactions also change the selection rules, induce significant motional-narrowing, and cause a large (26 cm−1) blue shift of the dimer’s vibrational spectrum relative to that in the gas phase. These effects cannot be captured via classical simulation, highlighting the importance of quantum many-body effects. Keywords (keywords): chemical bond; condensed phase; diatomic; molecular dynamics; quantum chemistry; vibrational spectrum;
Co-reporter:Ross E. Larsen;William J. Glover
Science 2010 Volume 329(Issue 5987) pp:65-69
Publication Date(Web):02 Jul 2010
DOI:10.1126/science.1189588
Co-reporter:William J. Glover, Ross E. Larsen and Benjamin J. Schwartz
The Journal of Physical Chemistry B 2010 Volume 114(Issue 35) pp:11535-11543
Publication Date(Web):August 12, 2010
DOI:10.1021/jp103961j
With no internal vibrational or rotational degrees of freedom, atomic solutes serve as the simplest possible probe of a condensed-phase environment’s influence on solute electronic structure. Of the various atomic species that can be formed in solution, the quasi-one-electron alkali atoms in ether solvents have been the most widely studied experimentally, primarily due to the convenient location of their absorption spectra at visible wavelengths. The nature of solvated alkali atoms, however, remains controversial: the consensus view is that solvated alkali atoms exist as (Na+, e−) tight-contact pairs (TCPs), species in which the alkali valence electron is significantly displaced from the alkali nucleus and confined primarily by the first solvent shell. Thus, to shed light on the nature of alkali atoms in solution and to further our understanding of condensed-phase effects on solutes’ electronic structure, we have performed mixed quantum/classical molecular dynamics simulations of sodium atoms in liquid tetrahydrofuran (Na0/THF). Our interest in this particular system stems from recent pump−probe experiments in our group, which found that the rate at which this species is solvated depends on how it was created ( Science 2008, 321, 1817); in other words, the solvation dynamics of this system do not obey linear response. Our simulations reproduce the experimental spectroscopy of this system and clearly indicate that neutral Na atoms exist as (Na+, e−) TCPs in solution. We find that the driving force for the displacement of sodium’s valence electron is the formation of a tight solvation shell around the partially exposed Na+. On average, four THF oxygens coordinate the cation end of the TCP; however, we also observe fluctuations to other solvent coordination numbers. Furthermore, we find that species with different solvent coordination numbers have unique absorption spectra and that interconversion between species with different solvent coordination numbers requires surmounting a free energy barrier of several kBT. Taken together, our results suggest that the Na0/THF species with different solvent coordination numbers may be viewed as chemically distinct. Thus, we can explain the kinetics of Na TCP formation as being dictated by changes in the Na+ solvent coordination number, and we can understand the dependence on initial conditions seen in the solvation dynamics of this system as resulting from the fact that the important solvent coordinate involves the motion of only a few molecules in the first solvation shell.
Co-reporter:Ian M. Craig, Hieu M. Duong, Fred Wudl, Benjamin J. Schwartz
Chemical Physics Letters 2009 Volume 477(4–6) pp:319-324
Publication Date(Web):6 August 2009
DOI:10.1016/j.cplett.2009.07.041
Abstract
The isoquinolinone derivative 2-methyl-1,4-diphenylbenzo[g]isoquinolin-3(2H)-one (MDP-BIQ) shows dual fluorescence emission with band positions and intensities that depend sensitively on the solvent. We show that this behavior arises from the fact that MDP-BIQ has two valence tautomers, one aromatic and one conjugated but non-aromatic, each of which are separately fluorescent. The aromatic tautomer, which has significant zwitterionic character, is stabilized by trace amounts of hydrogen-bond donors or Lewis acids. The relatively high fluorescence quantum yield of the aromatic tautomer (0.127 versus 2.4 × 10−3 for the non-aromatic tautomer) makes this and similar molecules outstanding candidates for use in sensors and other optoelectronic applications.
Co-reporter:Alexander L. Ayzner, Christopher J. Tassone, Sarah H. Tolbert and Benjamin J. Schwartz
The Journal of Physical Chemistry C 2009 Volume 113(Issue 46) pp:20050-20060
Publication Date(Web):October 27, 2009
DOI:10.1021/jp9050897
The most efficient organic solar cells produced to date are bulk heterojunction (BHJ) photovoltaic devices based on blends of semiconducting polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) with fullerene derivatives such as [6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM). The need for blending the two components is based on the idea that the exciton diffusion length in polymers like P3HT is only ∼10 nm, so that the polymer and fullerene components must be mixed on this length scale to efficiently split the excitons into charge carriers. In this paper, we show that the BHJ geometry is not necessary for high efficiency, and that all-solution-processed P3HT/PCBM bilayer solar cells can be nearly as efficient as BHJ solar cells fabricated from the same materials. We demonstrate that o-dichlorobenzene (ODCB) and dichloromethane serve nicely as a pair of orthogonal solvents from which sequential layers of P3HT and PCBM, respectively, can be spin-cast. Atomic force microscopy, various optical spectroscopies, and electron microscopy all demonstrate that the act of spin-coating the PCBM overlayer does not affect the morphology of the P3HT underlayer, so that our spin-cast P3HT/PCBM bilayers have a well-defined planar interface. Our fluorescence quenching experiments find that there is still significant exciton splitting in P3HT/PCBM bilayers even when the P3HT layer is quite thick. When we fabricated photovoltaic devices from these bilayers, we obtained photovoltaic power conversion efficiencies in excess of 3.5%. Part of the reason for this high efficiency is that we were able to separately optimize the roles of each component of the bilayer; for example, we found that thermal annealing has relatively little effect on the nature of P3HT layers spin-cast from ODCB, but that it significantly increases the crystallinity and thus the mobility of electrons through PCBM. Because the carriers in bilayer devices are generated at the planar P3HT/PCBM interface, we also were able to systematically vary the distance the carriers have to travel to be extracted at the electrodes by changing the layer thicknesses without altering the bulk mobility of either component or the nature of the interfaces. We found that devices have the best fill-factors when the transit times of electrons and holes through the two layers are roughly balanced. In particular, we found that the most efficient devices are made with P3HT layers that are about four times thicker than the PCBM layers, demonstrating that it is the conduction and the extraction of electrons through the fullerene that ultimately limit the performance of both bilayer and BHJ devices based on the P3HT/PCBM material combination. Overall, we believe that polymer-fullerene bilayers provide several advantages over BHJ devices, including reduced carrier recombination and a much better degree of control over the properties of the individual components and interfaces during device fabrication.
Co-reporter:Bertrand Tremolet de Villers, Christopher J. Tassone, Sarah H. Tolbert and Benjamin J. Schwartz
The Journal of Physical Chemistry C 2009 Volume 113(Issue 44) pp:18978-18982
Publication Date(Web):October 13, 2009
DOI:10.1021/jp9082163
Plastic photovoltaic devices offer a real potential for making solar energy economically viable. Unfortunately, bulk heterojunction (BHJ) solar cells fabricated from blends of the commonly used materials poly(3-hexylthiophene), P3HT, and phenyl-C61-butyric acid methyl ester, PCBM, sometimes exhibit low efficiencies even when the procedures followed often produce solar cells with efficiencies exceeding 5%. In this Letter, we show that this irreproducibility is caused by subtleties in the film processing conditions that ultimately lead to poor electron extraction from the devices. For low-performing devices, photogeneration and charge extraction with a linearly increasing voltage ramp (photo−CELIV) measurements show an order-of-magnitude difference in the effective mobilities of the electrons and holes. Atomic force microscopy (AFM) experiments reveal that the top surface of these low-performing devices is nearly pure P3HT. We argue that small variations in the solvent evaporation kinetics during spin-coating of the BHJ active layer, which are difficult to control, cause PCBM to segregate toward the bottom of the P3HT film to different extents, explaining why electron extraction from the PCBM component of the BHJ is so difficult in poorly performing devices. Finally, we show that electron extraction can be greatly improved by spin-coating a thin PCBM layer on top of the BHJ before deposition of the cathode, allowing the reproducible fabrication of high-efficiency polymer solar cells.
Co-reporter:Arthur E. Bragg;Molly C. Cavanagh
Science 2008 Vol 321(5897) pp:1817-1822
Publication Date(Web):26 Sep 2008
DOI:10.1126/science.1161511
Abstract
The linear response (LR) approximation, which predicts identical relaxation rates from all nonequilibrium initial conditions that relax to the same equilibrium state, underlies dominant models of how solvation influences chemical reactivity. We experimentally tested the validity of LR for the solvation that accompanies partial electron transfer to and from a monatomic solute in solution. We photochemically prepared the species with stoichiometry Na0 in liquid tetrahydrofuran by both adding an electron to Na+ and removing an electron from Na–. Because atoms lack nuclear degrees of freedom, ultrafast changes in the Na0 absorption spectrum reflected the solvation that began from our two initial nonequilibrium conditions. We found that the solvation of Na0 occurs more rapidly from Na+ than Na–, constituting a breakdown of LR. This indicates that Marcus theory would fail to describe electron-transfer processes for this and related chemical systems.
Co-reporter:Alexander L. Ayzner, Darcy D. Wanger, Christopher J. Tassone, Sarah H. Tolbert and Benjamin J. Schwartz
The Journal of Physical Chemistry C 2008 Volume 112(Issue 48) pp:18711-18716
Publication Date(Web):2017-2-22
DOI:10.1021/jp8076497
We examine how thermal annealing affects the fullerene network in conjugated polymer bulk heterojunction (BHJ) solar cells. We begin by creating electron-only devices with a BHJ geometry by blending the fullerene derivative [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) with polystyrene (PS). These electron-only PS:PCBM diodes function even with a poly(ethylenedioxithiophene):poly(styrenesulfonate) (PEDOT:PSS) layer, indicating that PEDOT:PSS films do not serve as electron blocking layers. Atomic force microscopy shows that the degree of phase segregation in the PS:PCBM blend films is similar to that in the active layer of blends of PCBM with poly(3-hexylthiophene-2,5-diyl) (P3HT), so that the PS:PCBM blends provide a good model for the fullerene part of the BHJ network in P3HT:PCBM solar cells. We find that thermal annealing dramatically decreases the electron current that flows in the PS:PCBM diodes, suggesting that annealing leads to increased phase segregation that lowers the electron mobility on fullerenes in the BHJ geometry. We also find that annealing increases the photoluminesence of P3HT:PCBM blend films, indicating that thermal treatment produces increased phase segregation that leads to decreased exciton harvesting. The fact that annealing decreases both exciton harvesting and electron mobility implies that there is significant room to further improve polymer/fullerene photovoltaics by controlling the amount of phase segregation.
Co-reporter:Alex D. Smith;Clifton Kwang-Fu Shen;Sean T. Roberts
Research on Chemical Intermediates 2007 Volume 33( Issue 1-2) pp:125-142
Publication Date(Web):2007 January
DOI:10.1163/156856707779160762
In this paper, we investigate the photophysical properties of the conjugated poly electrolyte poly(2-methoxy-5-propyloxy sulfonate phenylene vinylene) (MPS-PPV), dissolved in both water and DMSO as a function of the solution ionic strength. Dynamic light scattering indicates that MPS-PPV chains exist in a highly agglomerated conformation in both solvents, and that the size of the agglomerates depends on both the ionic strength and the charge of the counter-ion. Even though the degree of agglomeration is similar in the two solvents, we find that the fluorescence quantum yield of MPS-PPV in DMSO is nearly 100-times greater than that in water. Moreover, intensity-dependent femtosecond pump-probe experiments show that there is a significant degree of exciton-exciton annihilation in water but not in DMSO, suggesting that the MPS-PPV chromophores interact to form interchain electronic species that quench the emission in water. Given that the emission quenching properties depend sensitively on the chain conformation and degree of chromophore contact, we also explore the superquenching may be either enhanced or diminished in either of the solvents via addition of simple salts, and we present a molecular picture to rationalize how the conformational properties of conjugated polyelectrolytes can be tuned to enhance their emissive behavior for sensing applications.
Co-reporter:T.-Q. Nguyen;J. Wu;S. H. Tolbert;B. J. Schwartz
Advanced Materials 2001 Volume 13(Issue 8) pp:
Publication Date(Web):18 APR 2001
DOI:10.1002/1521-4095(200104)13:8<609::AID-ADMA609>3.0.CO;2-#
How fast does energy transfer along a conjugated polymer chain, and how fast between chains? A semiconducting polymer/mesoporous silica composite, which provides just enough space for one polymer chain per pore (see Figure), provides the answer through investigations of the polymer's luminescence anisotropy, with remarkable results.
Co-reporter:Benjamin J Schwartz, Thuc-Quyen Nguyen, Junjun Wu, Sarah H Tolbert
Synthetic Metals 2001 Volume 116(1–3) pp:35-40
Publication Date(Web):1 January 2001
DOI:10.1016/S0379-6779(00)00510-5
In this paper, we show how composite samples consisting of chains of the semiconducting polymer MEH-PPV embedded into the channels of oriented, hexagonal nanoporous silica glass allow control over energy transfer and exciton migration in the polymer. The composite samples are characterized by two polymer environments: randomly oriented and film-like segments with short conjugation-length outside the channels, and well aligned, long conjugation segments that are isolated by encapsulation within the porous glass. Ultrafast emission anisotropy measurements show that excitons migrate unidirectionally from the polymer segments outside the pores to the oriented chains within the pores, leading to a spontaneous increase in emission polarization with time. Because the chains in the pores are isolated, the observed increase in polarization can take place only by exciton migration along the polymer backbone. The anisotropy measurements show that energy migration along the backbone occurs more slowly than Förster energy transfer between polymer chains; transfer along the chain likely takes place by a thermally-activated hopping mechanism. Similar time scales for intra- and interchain energy transfer are also observed for MEH-PPV chains in solution. All the results provide new insights for optimizing the use of conjugated polymers in optoelectronic devices.
Co-reporter:Thuc-Quyen Nguyen, Rena Y. Yee, Benjamin J. Schwartz
Journal of Photochemistry and Photobiology A: Chemistry 2001 Volume 144(Issue 1) pp:21-30
Publication Date(Web):31 October 2001
DOI:10.1016/S1010-6030(01)00377-X
It is becoming increasingly clear that the electronic properties of conjugated polymers are controlled by the way the films are cast: changing the solvent, spin speed or concentration changes the film morphology and thus the performance of devices based on these materials. In this paper, we show that the way a conjugated polymer is dissolved into solution also affects the interchain interactions and electronic behavior in the resulting film. Light scattering shows that even low molecular weight samples of poly(2,5-bis[N-methyl-N-hexylamino]phenylene vinylene) (BAMH-PPV) do not completely dissolve in the good solvent o-xylene, even after stirring for 2 days. The solutions behave more as a suspension of small pieces of polymer film, showing solid-state effects such as exciton–exciton annihilation; the corresponding cast films have a rough, agglomerated morphology. Complete dissolution of the polymer can be achieved either by heating the solutions while stirring for 2 days, or by stirring at room temperature for 2 weeks. In addition to aiding dissolution, heating is found to promote interactions between conjugated polymer chains, leading to films with a higher degree of exciton–exciton annihilation and devices with higher operating currents but lower electroluminescence quantum efficiencies than films cast from solutions that were fully dissolved but not heated. All the results suggest that understanding the details of how a conjugated polymer is dissolved into solution is critical to being able to reproducibly fabricate and optimize conjugated polymer-based devices.
Co-reporter:Shaohua Huang, Guangye Zhang, Nicholas S. Knutson, Matthew T. Fontana, Rachel C. Huber, Amy S. Ferreira, Sarah H. Tolbert, Benjamin J. Schwartz and Yves Rubin
Journal of Materials Chemistry A 2016 - vol. 4(Issue 2) pp:NaN424-424
Publication Date(Web):2015/11/05
DOI:10.1039/C5TA07688A
Organic solar cells have been based mostly on conjugated polymers and the classic fullerene derivative PCBM and are characterized by modest open circuit voltages (Voc). Increasing Voc requires fullerene acceptors with higher LUMOs than PCBM. To date, most fullerene derivatives synthesized for this purpose either do not achieve the high photocurrent afforded by PCBM or show relatively poor compatibility with the next-generation low bandgap conjugated polymers used in high-efficiency organic solar cells. Here, we report the facile synthesis of methoxylated 1,4-bisbenzyl fullerene adducts and their application as efficient electron acceptors in conjugated polymer-based solar cells. The methoxy groups are found to be essential to increasing the LUMO levels, and accordingly the Voc, of the devices compared to the parent 1,4-bisbenzyl fullerene, and more importantly, to PCBM. The best fullerene 1,4-bisadduct provides a ∼20% enhancement in power conversion efficiency over PCBM when used with the classic crystalline polymer P3HT. When used in combination with a higher-performance low bandgap polymer, PTB7, the bisadduct both increases the device open-circuit voltage and maintains the high photocurrent provided by the more traditional PCBM. We also examine 10 different 1,4-fullerene bisadducts and show that the photovoltaic device performance is strongly influenced by the number and relative position of the methoxy substituents on the benzyl addends: moving a single methoxy substituent by one position on the benzyl rings can change the device efficiency by over a factor of 2.
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